17 Histaminic and Cholinergic Agonists and Antagonists By A. F. CASY School of Pharmacy Liverpool Polytechnic Byrom Street Liverpool L3 3AF 1 Introduction Ligands which bind to pharmacological receptors are of two chief types ; those which induce a physiological response are termed agonists while those which block the receptor and make it insensitive to an agonist are termed antagonists. Agonists and antagonists in the histaminic and cholinergic fields form the theme of this review. These areas concern the naturally endogenous agonists histamine and acetylcholine (ACh) respectively; the former is released as the result of various forms of bodily maltreatment and is responsible for allergic diseases in man while the chief role of the latter is the transmission of nerve impulses across the gaps (synapses) that link neuron to neuron neuron to muscle cell and neuron to secretory cell.Emphasis in the review is placed on the literature of the past 5 years. With certain exceptions e.g. dexetimide (73) and pancuronium (80) few novel clinical agents have been introduced as a result of the work described although substances of potential value in medicine have been discovered notably the selective H,-receptor histamine antagonists. Most of the work reviewed has been undertaken in a search for clues about the nature of histaminic and cholin- ergic receptors and modes of ligand-receptor interactions. 2 Histamine Agonists and Antagonists Interest in the naturally occurring amine histamine (1) [4(5)-(2-aminoethyl)-imidazole] has been stimulated by the need to define at least two types of histamine CH,CH,NH fi (and tautomer) HN N v receptor.Receptors blocked by mepyramine and other compounds clinically recognized as antihistaminics are termed H while mepyramine-insensitive types are designated H2.1 Thus the actions of histamine in stimulating secretion of ' A. S. F. Ash and H. 0.Schild Brit. J. Pharmacoi. 1966 27 427. 477 478 A. F. Cusy acid by the stomach,2 accelerating the heart,3 and inhibiting contractions of rat uterus4 are not antagonized by mepyramine and related drugs and are thus considered to be mediated at H receptors. This sophistication of histamine receptor theory has its counterparts in the catecholamine (a- P-receptors) and cholinergic (muscarinic nicotinic receptors) fields and has resulted in a search for specific agonists and antagonists.Kier's proposal5 that the drug-receptor interactions of histamine involve two distinct conformations [H trans-+NH3/Ar (2); H gauche (3)] of histamine monocation (population =-96% at Ar (2) (3) physiological pH) has prompted several conformational studies by M0,697uand 'H n.m.r. technique^.^ In general the data concur in showing that histamine has only a small preference for the trans-rotamer (2). In contrast crystalline histamine acid phosphate (dication) exists entirely in the truns-conformation.8 From differences between the pK values of N"-and N'-methyl(and -benzyl)- histamines [see (4)] it is calculated that the tautomeric ratio for histamine mono- cation in water is 4(N'-H) to l(N"-H).9 A similar value is reported for histidine monocation based on a I3C n.m.r.study." hCH ,6H2 N : 2CH I4H3 4d:H 'NUN" HN N' Y (4) (5) All possible monomethylhistamines (5) have been prepared and tested as H and H agonists.",'2 Full details of synthesis are not available but a convenient E. R. Loew and 0.Chickering Proc. SOC.Exp. Biol. Med. 1941 48 65. ' U. Trendelenburg J. Pharmacol. Exp. Therap. 1960 130,450. ' P. B. Dews and J. D. P. Graham Brit. J. Pharmacol. 1946 1,278. L. B. Kier J. Medicin. Chem. 1968 11 441. P. F. Periti Pharmacol. Res. Comm. 1970 2 309; J.-L. Coubeils P. Courriere and B. Pullman Compt. rend. 1971 272 D 1813; S. Margolis S. Kang and J. P. Green Internat.J. Clin. Pharmacol. 1971,5 279. (a) C. R. Ganellin E. S. Pepper G. N. J. Port and W. G. Richards J. Medicin. Chem. 1973 16 610; (6)A F. Casy R. R. Ison and N. S. Ham Chem. Comm. 1970 1343; (c) N. S. Ham A. F. Casy and R. R. Ison J. Medicin. Chem. 1973 16,470. * M. V. Veidis and G. J. Palenik Chem. Comm. 1969 196; M. V. Veidis G. J. Palenik R. Schaffrin and J. Trotter J. Chem. SOC.(A) 1969 2659. C. R. Ganellin J. Pharm. Pharmacol. 1973 25 787. lo W. F. Reynolds I. R. Peat M. H. Freedman and J. R.Lyerla J. Amer. Chem. SOC. 1973,95 328. J. W. Black W. A. M. Duncan C. J. Durant C. R. Ganellin and E. M. Parsons Nature 1972 236 385. ' la K. Kowalewski and A. Kolodeg Pharmacology 1974 11 207. C. R. Ganellin G. N. J. Port and W.G. Richards J. Medicin. Chem. 1973 16 616; C. R. Ganellin ibid. p. 620. Histaminic and Cholinergic Agonists and Antagonists 479 CH,CH(CH,OH)N H CH,CH( Me)NH Reagents i HBr-AcOH; ii H,-Pd/C AcOH,H,O Scheme 1 conversion of L-histidinol(6) into a-methylhistamine (7) (Scheme 1)is reported.' While methyl substituents generally depress the stimulant activity of histamine the 4-Me derivative is noteworthy in being a good H (43"/ of parent) but a feeble H agonist (H1/H2 ratio = 0.005),12a result confirmed by others.I4 The con- formationally restrained analogue of histamine 2-(4-imidazoyl)cyclopropylamine (lo) in which imidazoyl and amino functions are approximately anticlinal is a feeble agonist at both H and H sites;" the cyclopropane ring was constructed by the reaction of the trans-urocanic acid derivative (8) with dimethylsulphoxo- nium methylide and conversion of the alkoxycarbonyl group of (9)into an amino- group by a Curtius degradation (Scheme 2).H \ ,co2Bus ,dNH2 \ " * -+II IV II m hc==Lr-7do2Bus TrN N TrN N HN wN --& (81 (9) (10) Tr = CPh Reagents i Me,S(O)=CH,; ii KOH; iii HCI; iv Et,N-CIC0,Et-NaN Scheme 2 Most small-ring heterocycles with a P-aminoethyl side-chain are histamine agonists.' The anomalous report' that the claimed 2-positional isomer (1l) isohistamine is without histamine activity has been studied ;in reinvestigations of Jones' synthesis,' * the first step (reaction between 2-chloromethylimidazole and cyanide) was shown to involve nuclear rather than side-chain substitution giving l3 R.R. Ison and A. F. Casy J. Medicin. Chem. 1970 13 1027. l4 G. Bertaccini M. Impicciatore T. Vitali and V. Plazzi Farmaco Ed. Sci.,1972 27 680 l5 A. Burger M. Bernabe and P. W. Collins J. Medicin. Chem. 1970 13 33. l6 R. G. Jones in 'Handbook of Experimental Pharmacology' Vol. XVIII/I Springer- Verlag Berlin 1966 Ch. 1. " R. G. Jones J. Amer. Chem. Soc. 1949,71 383. '* (a) G. J. Durant M. E. Foottit C. R. Ganellin J. M. Loynes E. S. Pepper and A. M. Roe Chem. Comm. 1968 108; (b) E. C. Kornfeld L. Wolf T. M. Lin and I. H. Slater J. Medicin. Chem. 1968 11 1028. 480 A. F. Casy H,NCH k f=l HN HNYNCfi% HNyN 3 -HNYN (CH2)2NH CHACI Me Me (11) (12) (13) (12)('H n.m.r. evidence) hence Jones' product is an aminomethyl derivative (13).Reaction between 1-benzyl-2-chloromethylimidazoleand cyanide in aqueous ethanol (as Jones' ') gave a mixture of the two isomeric cyanides but reaction in Fl /=I / H,NCH,CH(OEt) HNyN HNYNcH2ph OEt PhCONHCH,CH,C ______+ CH,CN CH2CHz NHCOPh 1HCI (14) (15) (1 1) DMSO gave the 2-cyanomethyl compound (14) exclusively.18' Authentic iso-histamine obtained from (14) and also from the ethoxyimine (1 5) and 2-amino- acetaldehyde diethylacetal proved to be a weak histamine-like agonist. Several compounds are reported that are active in inhibiting gastric acid secretion e.g.(16),' (17),20and 2,2'-bipyridyl,' but only N-(4-imidazol-4-ylbutyl)-Ph S \ 4 CH-C\ NH PhCH,CMe,NHCH,COMe CN \ (17) (16) S ICH,.).NHC// CH,.SCH.CH .NHC \ NHMe \-/ HN VN "-methylthiourea (18) (burimamide) and the 4-methylimidazole derivative (19) (metiamide) prevent gastric secretion stimulated by histamine thereby being defined as specific H,-receptor antagonists.' '*'In Burimamide behaves as a competitive antagonist at H sites (PA 5 rat uterus and guinea pig atria)* and l9 R.G. Bianchi and D. L. Cook Fed. Proc. 1968,27 1331. 'O W. Lippmann Experientia 1968 24 1153. ' D. E. Butler P. Bass I. C. Nordin F. P. Hauck and Y. J. L'ltalien J.Medicin. Chem. 1971 14 575. * The parameter PA is defined as the negative logarithm ofthe concentration ofantagon- 1st which reduces the effects of a double dose of agonist to those of a single dose and it is a convenient means of comparing the potencies of different antagonists.It is also equivalent to the logarithm of the affinity constant of the antagonist in the equilibrium ligand + receptor complex. Histaminic and Cholinergic Agonists and Antagonists 481 does not interact significantly with H receptors as present in guinea pig ileum; its crystal and molecular structures have been reported,22 and it may be synthe- sized (Scheme 3) from lysine ethyl ester (19a).22a,b Metiamide2,' (cJ work on 4-methylhistamine described above) also inhibits bethanechol-chloride-induced (cholinergic) secretion of hydrochloric acid.' lo 2)4NH Et02CCH(CH2),NH2 HN/__((CH2)4NH2 % HN wN % (18) I YNH NH2 S (194 Reagents i Na-Hg; ii KCNS; iii FeCI,; iv MeNCS Scheme 3 CH(OH)CH,NHPr' (20) Since pronethal (20) a bicyclic analogue of isoprenaline is a /?-adrenergic antagonist aminoethylimidazo-[ 1,2-a]-and-[ 1,5-a]-pyridines have been examined as histamine antagonist^.^^ The derivative (22) was made (see Scheme 4) from + BrCH2COCH2CH2N=Phth2 vccH2CH2NH2 ONH2 (21) N=Phth = phthalimido Reagents i DMF HCO,-; ii 6M-HCI Scheme 4 2-aminopyridine and the a-halogeno-ketone (21) and the isomer (23) by a route starting with a Friedel-Crafts acylation of imidaz0[1,5-a]pyridine.~~ Neither (22) nor (23) had H agonist or antagonist activities but (22) was a moderately active H agonist.'' B. Kamenar K. Prout and C. R. Ganellin J.C.S. Perkin 11 1973 1734. 220G. J. Durant J. C. Emmett C. R. Ganellin and G.R. White B.P. 1 307 539 21 Feb. 1973. '"S. Akabori and T. Kaneko Bull. Chem. SOC. Japan 1936 11 208. '*'G.J. Durant J. C. Emmett and C. R. Ganellin B.P. 1 338 169 21 Nov. 1973. 23 G. J. Durant J. M. Loynes and S. H. B. Wright J. Medicin. Chem. 1973 16 1272. J. D. Bower and G. R. Ramage J. Chem. SOC.,1955,2834. 482 A. F. Casy Recent developments in conventional (H') histamine antagonists have empha- sized the importance of stereochemical relationships. Dextrorotatory isomers of the pheniramines all have the same configuration [S,(24)] since reduction of H (24) a; Ar = Ph b; Ar = C,H,-p-CI c; Ar = C,H,-p-Br ( +)-bromo- and (+)-chloro-pheniramine catalysed by palladium gives ( +)-pheniramine (24a).25 [( +)-(24b) is a more effective antihistaminic than the laevo-i~omer].~~ The configuration of dextrochlorpheniramine was correlated with that of the amide (25) of known stereochemistry by the sequence shown in Scheme 5.The more potent laevo-isomer of carbinoxamine tartrate (26) was qph -$ (+) -(24b) -+ +i ii CH,CH,NMe iii iv CH=CH Ph COPh COPh ' (isomers separated) 1v. vi OCH,CH,NMe H H Ph I H (26) (25) Reagents i H2-PtOz; ii PhCOCI; iii Mel; iv Ag,O-D,O on solid benzamide; v 0,; vi 12M-HC1 Scheme 5 shown to be sterically related to dextropheniramine by similar chemical inter- conversion^.^ ' Further studies of antihistamines that exhibit geometrical isomerism have been made. All four isomeric butenes (E- and Z-but-1-ene and but-2-ene pairs) that result when the tertiary alcohol (27) is dehydrated have been isolated and characterized by U.V.and 'H n.m.r. data;28 the most potent was the E-but-2-ene (28) (PA = 10.3),29which corresponds with the base of pyrrobutamine diphos- phate (Pyronil) a clinical agent of high potency. The more potent forms of 25 A. Shafi'ee and G. Hite J. Medicin. Chem. 1969 12 266. 26 R. T. Brittain P. F. D'Arcy and J. H. Hunt Nature 1959 183 734. 27 V. Barouh H. Dall D. Patel and G. Hite J. Medicin. Chem. 1971 14 834. 20 A. F. Casy and R. R. Ison J. Pharm. Pharmacol. 1970 22 270. 29 R. R. Ison F. M. Franks and K. S. Soh J. Pharm. Pharmacol. 1973 25 887 Histaminic and Cholinergic Agonists and Antagonists 483 OH Ph I \ /H PhCCH,CH /c=c\ Na I CH2 (27) Further related but-2-ene pairs all have an E(Ar/CH,N') config~ration.~~.~~ E-2 pairs related to triprolidine (29)have been obtained by the route (30)-(31):' v11 QH / c,x OH Y /H ArCO(CH,),NR 5 Ark.H,CH,NR c=c /\/---I Mannirh Mannich base (30) IJ-JI / Me (29) Ar \ C=CHCH,NR (a) R = Ph; (b) R = a; (c)R = CHPh -3 AcNHICqzH AcO RNH + NHR C 0 IIR or S 0 (32) AcNHGNR EtNHcNR 01LIAIH \ / (33) In all cases the E(2-py/CH2NR2)isomer had the higher potency with receptor affinity ratios near 10except for triprolidine and its isomer (> 1000).29Structural requirements for HI-receptor blockade have been discussed in the light of these data.28-31 30 A.F. Casy and A. P. Parulkar Canad.J. Chem. 1969,47 423. R. R. Ison and A. F. Casy J. Pharm. Pharmacol. 1971 23 848. 484 A. F. casy Several conformational studies of antihistaminics have now been made both in the solid32,33 and solute state;34 one of these33" confirms the S-configuration of ( + )-chlorpheniramine. The R-and S-3-ethylaminopyrrolidineunit [as in (33)] has been employed as a probe for the study of the stereoselective behaviour of a variety of pharmaco- logical receptors including those of histamine. Potential antihistaminics were made by treating enantiomorphic forms of N-acetylaspartic anhydride with various amines as shown [(32) +(33)]. The N-phenyl derivatives (33a) were weak antagonists of histamine at guinea-pig ileum sites with the S-ten times more active than the R-f~rm.~" Attachment of larger aromatic substituents to ring nitrogen led to compounds of far higher potency e.g.(33b) (molar ED,* -7 x lo-*) and (33c) (molar ED, -9 x lop9),but little difference in activity was found between isomeric pairs.35b The lack of stereoselectivity found is in accord with other studies employing isomers that have the asymmetric centres close to the basic nitrogen rather than an aromatic feature of the molecule.36 Analogues of (33a) with an additional phenyl substituent in the ring (34) are n-NMe CNh (34) (35) also much more potent than the parent and may be regarded as cyclic forms of phenbenzamine (35).3' The trans-isomer prevented histamine-induced contrac- tions of guinea-pig ileum at a concentration as low as 2 x lo-' mol kg-' and its action could not be readily reversed while the cis-form acted competitively Ph -+ cis-(34) Ph Reagents i NH,OH; ii Ac,O; iii B,H,; iv NaBH,; v TsCl; vi NaN,- ;vii Mel-LiAlH Scheme 6 32 G.R. Clarke and G. J. Palenik J. Amer. Chem. Soc. 1972,94 4005. 33 M. N. G. James and G. J. B. Williams (a)J. Medicin.Chem. 1971 14 670; (b)Cunud. J. Chem. 1974 52 1872; (c)ibid. p. 1880. 34 N. S. Ham J. Pharm. Sci. 1971,60 1764. 35 (a) D. T. Witiak Z. Muhi-Eldeen N. Mahishi 0. P. Sethi and M. C. Gerald J. Medicin.Chem. 1971,14,24; (6)D. T. Witiak S. Y. Hsu J. E. Ollmann R. K. Griffith S. K. Seth and M. C. Gerald ibid. 1974 17 690. 36 M. J. Jarrousse and M. T. Regnier 1951 Ann. pharm. franc. 1951 9 321 ; A. P. Roszkowski and W.M. Govier Pharmacologist 1959 1 60; F. E. Roth Chemo-therapiu 1961 3 120. 37 P. E. Hanna and A. E. Ahmed J. Medicin. Chem. 1973 16,963. Histaminic and Cholinergic Agonists and Antagonists 485 (PA 7.97). Both isomers were obtained from 1,5-diphenyl-3-pyrrolidone(36) (Scheme 6). and configurational assignments were made from ‘H n.m.r. data. Novel antihistaminics reported recently are mostly variants of known structural types (reviewed by Witiak).3B Thus the indole (37) is related to ~lernizole,~~ the phenothiazine (38) to pr~methazine,~’ and azatadine (39) to ~yproheptadine.~~ The last named a potent antihistaminic in animals and also effective ~linically,~~ is made by dehydration of the tertiary alcohol formed from the ketone (40)43 and Grignard reagent (41).The 4-benzamidopiperidine (42) (PA 9.6 cJ:chlor pheniramine 8.6)44 is of greater structural interest since it does not possess the geminal aromatic feature linked through a linear chain to basic nitrogen (Ar-Ar’X . . N) common to most other classes of antihistaminic. 3 Acetylcholine (ACh) Agonists and Antagonists Much of the work reviewed here concerns efforts to define the pharmacophoric or ‘active’ conformation of cholinergic molecules in the search for clues about the molecular nature of cholinergic receptors and the ways in which ligand- receptor combinations are achieved. There is promise from work on electric 38 D. T. Witiak in ‘Medicinal Chemistry’ ed. A. Burger 3rd edn. Wiley-Interscience New York,1970. 39 R. N. Schut F.E. Ward 0.J. Lorenzetti and E. Hong J. Medicin. Chem. 1970,13 394. 40 C. Kaiser D. H. Tedeschi P. J. Fowler A. M. Pavloff B. M. Lester and C. L. Zirkle J. Medicin. Chem. 1971 14 179. 41 F. J. Villani P. J. L. Daniels C. A. Ellis T. A. Mann K.-C. Wang and E. A. Wefer J. Medicin. Chem. 1972 15 750. 42 A. Sabbah La Vie Mkdicale 1969 41 5401; B. Sigal and M. Herblot Gar. Med. Fr. 1970 77 364. 43 F. J. Villani P. J. L. Daniels C. A. Ellis T. A. Mann and K.-C. Wang J. Heterocyclic Chem. 1971 8 73. 44 J. L. Archibald P.Fairbrother and J. L. Jackson J. Medicin. Chem. 1974 17 739. 486 A.F. casy organ tissues (electric eel and ray)45 and brain46 that direct study of the receptors themselves may eventually be possible but at present any receptor information must largely be inferred from ligand characteristics.Several groups notably that of Pa~ling,~~ have provided information about the solid-state conformation of a wide range of ACh agonists and antagonists. Torsion angles 72 and 73 are of most value in defining the conformations of ACh and its congeners (43). In most cases 73 values fall in the range 180’ & 36’ (44) which means that the acetyl portion of the molecule is set well away from the quaternary head. The torsion angle relating ‘NMe to OCOMe (72) commonly has a value in the range 73-94’ so the N and 0 functions are more or less synclinal (44). It turns out that most compounds with the molecular feature 0-C-C-N+ where the charged group is quaternary nitrogen or a protonated base and the oxygen function is OH or acyloxy prefer the synclinal N/O arrange- ment in the solid and evidence for a stabilizing electrostatic 0-N+ interaction has been ad~anced.~’ Antiplanar-anticlinal conformations are preferred by the potent agonists carbarnoylcholine (stabilized by hydrogen bonds)” and (+)-trans ACTM” (72 fixed by ring rigidity) and the weakly active thio- and seleno-analogues of AChS2 in which the ether oxygen is replaced by a bulkier and less electronegative atom.N/O conformations are also synclinal for the cyclic muscarinic agents L-( +kmuscarine iodide (45),’ the cis-dioxolan (46),54 and 5-methylfurmethide (47)47 but antiplanar for L-( +)-muscarone (48).” The crystal structures of nicotine and other nicotinic agonists e.g.a-methyl-ACh ” R. D. O’Brien M. E. Eldefrawi and A. T. Eldefrawi Ann. Rev. Pharmacol. 1972,12 19; G. Biesecker Biochemistry 1973 12 4403. 46 H. B. Bosmann J. Biol. Chem. 1972,247 130. *’ R. W. Baker C. H. Chotia P. Pauling and T. J. Petcher Nature 1971,230,439. ‘* M. Sundaralingam Nature 1968 217 85; C. Chotia and P. Pauling ibid. 1968 219 1156. 49 (a) F. G. Canepa and E. F. Mooney Nature 1965,207 78; (b)A. F. Casy M. M. A. Hassan and E. C. Wu J. Phurm. Sci. 1971,60 67. 50 Y.Barrans and J. Clastre 1970 Compt. rend. 1970 270 C 306. ’ C. Chotia and P. Pauling Nature 1970 226 541. 52 E. Shefter and H. G. Mautner Proc. Nut. Acad. Sci. U.S.A. 1969 63 1253. 53 F. Jellinek Acta Cryst. 1957 10 277. ’* P. Pauling and T. J. Petcher Chem. Comm.1969 1258. ” P. Pauling and T. J. Petcher Nature New Biol. 1972 236 112. Histaminic and Cholinergic Agonists and Antagonists + 487 NMe IVLt: M'e (45) (46) (47) (48) lactoylcholine and 1,l-dimethyl-4phenylpiprazine,have torsion-angle features similar to those of most muscarinic agentss6 Conformational studies by n.m.r. (chiefly 1H)49b*s7 and MO theory" are also extensive and by and large complement the results of X-ray diffraction. Thus n.m.r. analyses establish strong preferences for synclinal +N/O conformers in the cases of ACh B-methyl ACh and also carbamoylcholine (see above) as solutes in D,O ;acetylthio-and acetylseleno-choline are chiefly antiplanar in solution while a-methyl ACh (which exists in 2 crystalline forms)'' displays little conformational preference.It is clear that the preferred conformations of ACh and many of its active analogues are those with synclinal N and 0 functions but there is no guarantee that such forms represent the conformation adopted by the agonist at the cholin-ergic receptor especially as barriers to rotation in molecules such as ACh are low. Data on conformationally restrained analogues with onium and acetate functions relatively 'frozen' may thus provide clearer information about the active conformation. This approach has been followed but has the drawback of requiring molecules larger than the parent with reduced receptor affinity a likely consequence. If meaningful conclusions are to be made the mode of action of the analogue must be established (muscarinic nicotinic direct or indirect influ-ence of AChE) and regrettably evidence on this point is often in~omplete.~~" Smissman and co-workers* used trans-decalin as the restraining framework and prepared all four RS-isomers of (49);low orders of muscarinic activity were OCOMe hoCoMe +I -&Mel &Me3 NMe3 H (TIocoMe H (49) (50) (49) 56 C.Chotia and P. Pauling Proc. Nut. Acad. Sci. U.S.A. 1970 65 477 and references there cited. 57 C. C. J. Culvenor and N. S. Ham Chem. Comm. 1966 537; ibid. 1970 1242; R. J. . Cushley and H. G. Mautner Tetrahedron 1970 26 2151 ;P. Partington J. Feeney, and A. S. V. Burgen Mol. Pharmacol. 1972 8 269; T. D. Inch R. A. Chittenden and C. Dean J. Pharm. Pharmacol. 1970,22,956.58 L. B. Kier Mol. Pharmacol. 1967,3,487; B. Pullman,Ph. Courritre,and J. L. Coubeils ibid. 1971,7,397; M. Froimowitzand P. J. Gans J. Amer. Chem. SOC.,1972,94,8020; R. J. Radna D. L. Bevetidge and A. L. Bender ibid. 1973,95 3831. 59 C. Chotia and P. Pauling Chem. Comm. 1969,626. 59 (a) A. F. Casy Progr. Med. Chem. 1975 11 1 and references cited therein. *The untimely death of Professor Smissman in Summer 1974 has deprived medicinal chemistry of one of its leading exponents. 488 A. F. Casy shown by the NaOa(50) and NaOeisomers the former being the more potent.60 There is evidence that (50) maintains an anticlinal-antiplanar conformation in both solute ('H n.m.r.)60 and solid (X-ray)61 states. Analogues of (50) with methyl substituents at C-2and/or C-3 all had significant ACh-like activity.62 An example of the synthetic procedures used is shown (Scheme 7).The 4-t-butyl derivatives +M~I e &:ye meM OCOh rriins opening Me N3 NMe3 Reagents i NaN,; ii H,-PtO,; iii CH,O-H,; iv MeCOCl; v Me1 Scheme 7 (51) represent a quartet similar to (49),and of these the NaOaisomer again proved the more potent (but weak) muscarinic agent.63 More satisfying results in terms of'the potency were obtained with the cyclopropyl analogues (52).64 The (+)-OCOMe Bu' (51) trans-isomer (ACTM) equalled or surpassed ACh itself in two test systems while the (-)-enantiomer and RS-cis-form were feeble or inactive. From an X-ray crystallographic study ( +)-trans-ACTM has a 1S,2S configuration [as ( +)/3-methyl ACh and ( +)-muscarinel and an NCCO torsion angle of 137"(anticlinal)?l trans-ACTM was made from the trans-amide (54) itself derived from the ester (53) formed along with some cis-isomer during the copper-catalysed reaction between ethyl diazoacetate and 2-vinyloxytetrahydropyran (THP used as a protective 6o E.E. Smissman W. L. Nelson J. B. Lapidus and J. L. Day J. Medicin. Chem. 1966 9 458. 61 E. Shefter and E. E. Smissman J. Pharm. Sci. 1971,60 1364. 62 E. E. Smissman and G. R. Parker J. Medicin. Chem. 1973 16 23. 63 A. F. Casy E. S. C. Wu and B. D. Whelton Canad. J. Chem. 1972 50 3998; D. F. Biggs I. Chu E. S. C. Wu and A. F. Casy J. Pharm. Pharmacol. 1973,25 153P. 6* P. D. Armstrong J. G. Cannon and J. P. Long Nature 1968 220 65; C.Y. Chiou J. P. Long J. G. Cannon and P. D. Armstrong J. Pharmacol. Exp. Ther. 1969 166 243. 65 P. D. Armstrong and J. G. Cannon J. Medicin. Chem. 1970 13 1037. Histaminic and Cholinergic Agonists and Antagonists Other frameworks used to prepare rigid ACh analogues include bicyclo[2,2,2]- octane [anticlinal form (55) has significant muscarinic activity]66 and 2- and 3-aminobornanes (56).67Much chemical detail on the bornanes has been reported but little pharmacology. A A = OCOMe B = +NMe and vice versa OCOMe (55) (56) Analogues of muscarine (45)also provide evidence about the active conforma- tion of ACh. The difficult access of (+)-muscarine from natural sources (Fly Agaric) has been overcome by a convenient synthesis from 2-acetamido-4,5- dihydroxyhexanoic acid (from N-acetylcrotonylglycine and performic acid).68 The diol(57) was resolved enzymatically at the or-carbon and the resultant a-L(+)-amino-acid diol (58) was deaminated with nitrous acid-methanol to give the tetrahydrofuran (59) with retention of configuration at C-2.The reaction of (59) MeCH-CH-CH2eHC0,H MeCH-CHCH,CHC02 -II I II I OH OH NHCOMe OH OH +NH3 (57) selectively de-acetylated by hog-kidney acylase (59) (60) with dimethylamine gave two amides which were separated and transformed into L-( +)-muscarine (45) and L( +)-allomuscarine (60) respectively. The RS-deoxy- analogue of muscarine (45; cyclic 0 replaced by CH,) is reported69 to be 5-10 times more potent a spasmogen on guinea-pig ileum than the parent racemic mixture.Other analogues of (45) include 7-oxabicyclo[2,3,l]heptanes (61)" and of special importance the 1,3-dioxolan (62) which is closely related sterically to b6 W. L. Nelson and R. S. Wilson J. Medicin. Chem. 1971 14 169; A. H. Beckett and R. E. Reid Tetrahedron 1972 28 5555. 67 R. A. Chittenden and G. H. Cooper J. Chem. SOC.(0,1970 49; T. Ahmad M. N. Anwar M. Martin-Smith R. T. Parfitt and G. A. Smail ibid. 1971 847; A. H. Beckett N. T. Lan and A. Q. Khoklar J. Pharm. Pharmacol. 1971 23 528; G. H. Cooper D. M. Green R. L. Rickard and P. B. Thompson ibid. 1971 23 662. '* J. Whiting Y.-K. Au-Young and B. Belleau Cunad. J. Chem. 1972 50 3322. 69 K. G. R. Sundelin R. A. Wiley R. S. Givens and D. R. Rademacher J. Medicin. Chem.1973 16 235. 70 W. L. Nelson D. R. Allen and F. F. Vincenzi J. Medicin. Chem. 1971 14 698. 490 A. F. Cusy the natural agent.71 Variants of (62) restricted about the C-4-C-6 bond have been made e.g. (63)-(66).’* The spiran (66),with an anticlinal NCCO torsion angle was the most potent [about 0.1 x potency of parent (62)]; in contrast the open form (67) in which the 4-methyl group opposes an anticlinal22 was a feeble agonist. (66) (67) Several proposals about the conformational requirements of cholinergic receptors based on the one hand on preferred agonist conformation^^^.'^ and on the other on multiple modes of ligand binding,74 have been advanced and the. matter remains controversial. Turning to anticholinergic agents and especially the question of whether or not muscarinic agonists and antagonists occupy common receptor~,~~.’’ more evidence has appeared that underplays the influence of the nitrogen moiety in antagonist molecules.Earlier it was found that whereas the agonist properties of P-methyl ACh depend critically on the configuration of the p-centre enantio- mers of analogues that are antagonists show only modest differences in potency [see (68)]. In contrast the configuration of the acyloxy centre (when chiral) has a pronounced influence on blocking activity [see (69)].’6 Analogues of the ” B. Belleau and J. Puranen J. Medicin. Chem. 1963 6 325. 72 M. May and D. J. Triggle J. Pharm. Sci. 1968 57 51 1 ; D. R. Garrison M. May H. F. Ridley and D. J. Triggle J. Medicin.Chem. 1969 12 130; H. F. Ridley S. S. Chatterjee J. F. Moran and D. J. Triggle ibid. p. 931. 73 L. B. Kier in ‘Fundamental Concepts in Drug-Receptor Interactions’ ed. J. F. Danielli J. F. Moran and D. J. Triggle Academic Press New York 1970 p. 15; L. B. Kier ‘Molecular Orbital Theory in Drug Research’ Academic Press New York 1971. 74 D. J. Triggle ‘Neurotransmitter-Receptor Interactions’ Academic Press London 1971; J. F. Moran and D. J. Triggle in ‘Cholinergic Ligand Interactions’ ed. D. J. Triggle J. F. Moran and E. A. Barnard Academic Press New York 1971 p. 119. 7s E. J. Ariens and A. M. Simonis Ann. New York Acad. Sci.,1967 144 842; R.W. Brimblecombe D. Green and T. D. Inch J. Pharm. Pharmacol. 1970 22 951. 76 B. W. J. Ellenbroek R. J. F. Nivard J.M. Van Rossum and E. J. Ariens J. Pharm. Pharrnacol. 1965 17 393. Histaminic and Chotinergic Agonists and Antagonists 49 1 X Affinity ratio SIR + Me,NCH,CHMeOCOX Me 320 Ph,CH 0.2 (48) Ph,C(OH) 0.8 X AfJinity ratio ( -)/( +) + Me,NCH,CH20COX Ph(Me)C(CH,OH) 300 Ph(C,Hll)CH 100 (69) 1,3-dioxolan(62)with antagonistic properties have now been made with 2-methyl replaced by a bulky aromatic group. In especially painstaking work," all eight isomers of the dioxolan (71) were isolated (see Scheme 8) and tested ;configura-/-TcH2NMe3 Ph CH,OTs I I I .. . I1 C,H,,C-CHO + CHOH + rCCH20Ts I I OH CHzOH "Yo -"YO R or S R or S Ph/C\OH Ph/\\OH (70) I I (separate c and t) (71) Reagents i HNMe,; ii Me1 Scheme 8 tional priorities for activity were in the order (i) an R-benzylic centre ;(ii) a 2-S-centre (adjacent to benzylic carbon); and (iii) a 4-R-centre (adjacent to CH,&Me,).Similar conclusions of stereochemical influence were reached from studies on the dioxolans (72);thus R- and S-(72; Ar = Ph) were almost equally p~tent.~' rCCHzkMe3 00 PhCHZ, Y Ar Ph (72) Ar = PhorC,H, (73) Benzetimide (73),an anticholinergic agent as active as atropine,79 also contains an asymmetric benzylic centre and its dextro-isomer (dexetimide) is over 1000 times as active as the laevo-form from PA, values (guinea-pig ileum)." " R. W. Brimblecombe T. D. Inch J. Wetherell and N. Williams J. Pharm. Pharmacol. 1971 23 649. R. W. Brimblecombe and T. D. Inch J.Pharm. Pharmacol. 1970,22,881; K. J. Chang R. C. Deth and D. J. Triggle J. Medicin. Chem. 1972 15 243. 79 B. Hermans P. Van Daele C. Van de Westeringh C. Van der Eycken J. Boey J. Dockx and P. A. J. Janssen J. Medicin. Chem. 1968 11 797. P. A. J. Janssen C. J. E. Niemegeers K. H. L. Schellekens P. Demoen F.M. Lenaerts J. M. van Nueten I. van Wijngaarden and J. Brugmans Arzneim.-Forsch. 1971 21 1365. 492 A. F. Casy Dexetimide has a configuration (S from X-ray study)81 that is comparable with that of hexahydrobenzilates such as (74) if the sequence of aromatic hydrogen- + bonding donor and C..-Nfeatures be compared. The binding of R-and S-[3H]benzetimide to subcellular fractions of rat brain rich in membranes is stereospecific (R-isomer displaced by atropine but not by S-benzetimide or tubocurarine) and appears to involve muscarinic receptors.82 This use of dexetimide provides a further example of the use of optically active ligands for receptor localization (cf.work on opiate receptor^).^^ log K, MeND OCOCPh(C6H,,)OH HCI 10.92 Me1 11.08 R-(74) Bar10w~~ has emphasized the importance of optical purity in assessing the significance of enantiomeric potency ratios in anticholinergic drugs (considera- tions which apply equally well to other areas of medicinal chemistry). If isomers are only 95 % resolved the highest observed stereospecificity theoretically possible would be 19 while values of 100 and 1000 correspond to optical purities of 99.01 and 99.90 respectively. Indeed biological assays are capable of assessing purity more precisely than any physical technique (e.g.n.m.r.) presently available.R- and S-Hexahydrobenzilic acids may be obtained in high states of optical purity from arabin~se,~~ and a study of potency ratios of their esters with 2-dimethyl- aminoethanol and 1-methyl-4-piperidinol has been made by whole-animal and isolated-tissue methods.86 A divergence between in viuo and in uitro ratios was noted which became greater the higher the affinity constant of the ester of R-configuration and it was proposed that there is a minimum dose below which maximum anticholinergic effects cannot be obtained in uiuo. Thus with excep- tionally potent agents such as (74) the advantages of a high affinity for the receptor are offset by the need to administer enough drug to allow for its uptake at sites of loss.mocox N (75) a; X = CHPh b; X = C(OH)Ph c; X = Me Anticholinergics based on 3-quinuclidinol(75a,b) are unusual in having poten- cies that are markedly dependent on the configuration of the amino-alcohol 81 A. L. Spek and A. F. Peerdeman Nature 1971 232 575. 82 w. Soudijn I. van Wijngaarden and E. J. Ariens European J. Pharmacol. 1973 24 43; A. J. Beld and E. J. Ariens ibid. 1974 25 203. 83 C. B. Pert and S. H. Snyder Proc. Nut. Acad. Sci. U.S.A. 1973 70 2243. 84 R. B. Barlow J. Pharm. Pharmacol. 1971 23 90; R. B. Barlow F. M. Franks and J. D. M. Pearson ibid. 1972 24 753. 85 T. D. Inch R. V. Ley and P. Rich J. Chem. Soc. (0,1968 1693. 86 R. W. Brimblecombe D.M. Green T. D. Inch and P. B. J. Thompson J. Pharm. Pharmacol. 1971 23 745; T. D. Inch D. M. Green and P. B. J. Thompson ibid. 1973 25 359. Histaminic and Cholinergic Agonists and Antagonists centre.87 It is strange as well that the protonated base rather than the metho- salt form of the agonist (7%) is distinctly the more active form,** a fact that may be related to the rigid nature of the quaternary head compared with the situation in ACh and its close relatives. The more potent enantiomer of (75c) methiodide [R-(-); configuration by X-ray analysis]89 has an inverse steric relationship to agonists such as S-P-methyl ACh and (+)-muscarine. With hydrochlorides (75c) however receptor selectivity is reversed with the S-isomer the more potent agent (S :R ratio at least 13 :1); N-methylation drastically reduces the activity of the S-isomer (75c) (1100-fold) but has little effect on that of the R-isomer.” Finally a few developments in neuromuscular blocking agents are mentioned.In 1970 (+)-tubocurarine long accepted as a bisquaternary compound was shown by a ‘H n.m.r. study to possess in fact one quaternary and one protonated tertiary nitrogen centre ;” only three N-methyl resonance signals could be assigned one of which moved upfield when the pH was raised (typical of de- protonation of an +NHMe gro~p).~’ Cleavage experiments established the location of these centres and the structure (76) has now been confirmed by an X-ray diffraction The NN-dimethyl quaternary salt corresponding with (76) is actually (+)-chondrocurarine chloride earlier held to differ from tubocurarine in the location of a methyl ester residue.A solute conformation of (76) has been proposed based on a variable-temperature ‘H n.m.r. The neuromuscular blocking potency of ( +)-isotubocurarine (76; quaternary and tertiary N features interchanged) is almost twice that of (+)-tubocurarine in the cat.94a (76) 87 L. H. Sternbach and S. Kaiser J. Amer. Chem. SOC.,1952,74 2215 2219; A. Meyer- hoffer J. Medicin. Chem. 1972 15 994. 88 M. D. Mashkovsky in Proc. First International Pharmacological Meeting Stockholm 1961 Vol. 7 Pergamon Oxford 1962 p. 359; M. D. Mashkovsky and C. A. Zaitseva Arzneim.-Forsch. 1968 18 320. 89 R. W. Baker and P. Pauling J.C.S.Perkin IZ 1972 2340. 90 R. B. Barlow and A. F. Casy Mol. Pharmacol. in press. 91 A. J. Everett L. A. Lowe and S. Wilkinson Chem. Comm. 1970 1020. 92 A. F. Casy ‘PMR Spectroscopy in Medicinal and Biological Chemistry’ Academic Press London 1971 p. 58. 93 P. W. Codding and M. N. G. James J.C.S. Chem. Comm. 1972 1174; Acta Cryst. 1973 B29 935. 94 R. S. Egan R. S. Stanaszek and D. E. Williamson J.C.S. Perkin 11 1973 716. 940 T. 0.Soine and J. Naghaway J. Pharm. Sci. 1974,63 1643. 494 A. F. Casy Recent developments in novel curare-like agents have centred on steroid derivatives prompted perhaps by disclosure of the neuromuscular blocking activity of alkaloids related to connesine and of mal~uktine.~~.~~ The steroid skeleton also allows the synthesis of agents with varying but in each instance fixed +N to +N distance.In a series of Sa-and SP-androstanes (77) potency was most sensitive to the configuration of the 3-substituent (p-gave optimal a~tivity).~’ in Sa-derivatives a 3/I-quaternary nitrogen atom was essential but a /I-quaternary nitrogen atom could be replaced by a P-tertiary nitrogen at C-17 without change in potency.98 No critical relationship between inter-onium distance and potency was found-the +N to +N distance was close to 10% for both potent and weakly active derivatives. In the solid state the +N to +N distance for tubocurarine dichloride is 8.97 A,93 significantly shorter than that generally accepted as essential for neuromuscular blockade.99 The distance increases to 10.7 A when both nitrogens are quaternary as in 00”-trimethyltubocurarine di-iodide.loo Combination of knowledge about the monoquaternary compound (78) (1/16 x Br 0 (78) tubocurarine) lo’ which includes an ACh-like moiety in ring A and diquaternary salts like malouktine led to the synthesis of pancuronium (80)(+Nto +Ndistance 11.08 A) now established as a potent clinical agent of medium duration of acti~n.’~**’~~ For high potency two nitrogen atoms are needed at least one of which should be quaternary [the dihydrochloride form of (80) is inactive] but there are no data on the 2-N9 16-N‘ type of salt.Potency falls when the size of the quaternary groups is reduced and the diol corresponding with (80) is only about a tenth as active as the parent.With monoacetates deacetylation at 95 A. Quevauviller and F. Laine Ann. pharm. franc. 1960 18 678. 96 D Busfield K. J. Child A. J. Clarke B. Davies and M. G. Dodds Brit. J. Pharmacof. 1968 33 609. 97 D. G. Bamford D. F. Briggs M. Davis and E. W. Parnell Brit. J. Pharmacol, 1967 30,194. 98 D. G. Bamford D. F. Briggs M. Davis and E. W. Parnell J. Pharm. Pharmacof. 1971 23 595. 99 M. Martin-Smith in ‘Drug Design’ ed. E. J. Ariens Academic Press New York and London 1971 Vol. 11 p. 503. loo H. M. Sobell T. D. Sakore S. S. Tavale F. G. Canepa P. Pauling and T. J. Petcher Proc. Nat. Acad. Sci. U.S.A. 1972 69 2212. J. J. Lewis M. Martin-Smith T. C. Muir and H. H. Ross J. Pharm. Pharmacol. 1964 19 502. lo* W. R. Buckett C. E.B. Marjoribanks F. A. Marwick and M. B. Morton Brit. J. Pharmacof. 1968 32 67 1. ‘03 W. R. Buckett C. L. Hewett and D. S.Savage J. Medicin. Chem. 1973 16 1116. Histaminic and Cholinergic Agonists and Antagonists C-17 (dacuronium) causes the greater potency fall. In the solid state ring A of pancuronium adopts a skew-boat conformation (avoiding the 1,3-diaxial inter- actions of the chair),Io4 and there is 'H n.m.r. evidence that similar conformations obtain for the Pancuronium is made from the diepoxyandrostane (79)lo5 as shown in Scheme 9. (79) cN@,As 2 J.+ some 1b-epimer Me AcO" H (80) Reagents i NH, H,O; ii NaBH,; iii Ac,O-py; iv Me1 Scheme 9 Bisquaternary steroids of type (81) with an inter-onium separation of 5.8 A had their main action at an autonomic ganglion rather than the neuromuscular function,' O6 a result supporting experience with or,m-bisquaternary alkanes.lo7 (82) Y ED, (paralysing activity in mice) 0.16mg kg-' +NR3 (81) (+)-tubocurarine 0.05mg kg-' Some short-acting competitive neuromuscular blocking agents with the quaternary nitrogens linked by a nitrogen-rich chain have been described e.g.(82) I04 D. S. Savage A. F. Cameron G. Ferguson C. Hannaway and I. R. Mackay J. Chem. SOC.(B) 1971,410. lo' C. L. Hewett and D. S. Savage J. Chem. SOC.(0,1968 1134; A. Hassner and P. Catsoulacos J. Org. Chem. 1967 32 549. '06 I. G. Marshall and M. Martin-Smith European J. Pharmacol. 1972 17 39. lo' R. B. Barlow 'Introduction to Chemical Pharmacology' Methuen London 2nd end.1964 p. 165. 496 A. F. casy with an inter-onium distance of 7.5~.'08*'09The brief action of (82) may be due to its rapid metabolism to 3-methyl-2-phenylimidazo[ 1,2-a]pyridine. Reasonably potent monoquaternary neuromuscular blockers that lack a second basic nitrogen atom are rare and recent examples are carbonyl-functional- ized derivatives of alkyl and cycloalkyl aryl ketones e.g. (83),which is as active as suxamethonium in some species."' The hydrazones (84) and (85) also ap- (83) (84) (85) proach suxamethonium in potency and activity is lost when the position of the trimethylammonium substituent is changed. lo* L. Bolger R. T. Brittain D. Jack M. R. Jackson L. E. Martin J. Mills D. Poynter and M. B. Tyers Nature 1972 238 354.lo9 D. J. Pointer J. B. Wilford and D. C. Bishop Nature 1972 239 332. ' lo D. G. Bamford D. F. Biggs P. Chaplen M. Davis and J. Maconochie Experientia ' '' 1972 28 1069. I. Chu Ph.D. Thesis University of Alberta 1973.