摘要:
2136 J.C.S. Perkin IWithanolides Q and R, Two New 23-Hydroxy-steroidal LactonesBy Isaac Kirson and Ariela Cohen, Department of Organic Chemistry, The Weizmann Institute of Science,Rehovot, Israel4rieh Abraham, Agricultural Research Organization, Volcani Center, Bet- Dagan, IsraelTwo new steroidal lactones of the withanolide type, isolated from Witbania somnifera Dun (Solanaceae), areCharacterized by the unusual presence of a C-23 hydroxy-group. Withanolide Q (1 a) is (22S,23R)-17a,23,27-trihydroxy-I -oxowitha-2,5.24-trienolide and withanolide R (2a) is (22S,23R)-6a,7a-epoxy-5a,23-dihydroxy-1 -oxowitha-2.24-dienolide.IN a previous paper1 we dealt with the structures ofeleven steroidal lactones of the withanolide series, isolatedfrom the ' Indian I ' chemotype of Withania somniferaDun.The twelfth compound of this group, C,H,O,,isolated in minute quantities, was not at that timeidentified completely.We have investigated recently the steroidal lactonesproduced in the offspring obtained by cross-pollinationof the various chemotypes of Withania somnqera.2Crossing chemotypes I and 111 (both of which grownaturally in Israel) resulted in offspring F,, F,, and F3,each containing an array of withanolides. Chromato-graphy of the crude extract obtained from the leaves ofone of the offspring F3 afforded, inter alia, a new witha-nolide which proved to be structurally related to thetwelfth compound of the Indian chemotype. Thepresent work is concerned with the structures of thesetwo compounds, designated withanolides Q and R.Withanolide Q (la) exhibits two i.r.bands in thecarbonyl region at 1 650 (ap-unsaturated ketone) and1 705 cm-l (ap-unsaturated blactone) and a U.V. maxi-mum absorption at 218 nm (E 17900) attributable tothe same unsaturated carbonyl chromophores. N.m.r.signals in the low-field region for three vinylic protonsagree exactly with those displayed by withanolide G,Sthus allowing the assignment of a 2,5-dien-l-one structuret o the AB ring system. An unresolved band at 6 4.42accounting for four protons and a singlet at 6 2.11 for avinylic methyl group are assigned to the side chain.I. Kirson, E. Glotter, D. Lavie, and A. Abraham, J . Chem.SOC. ( C ) , 1971, 2032.a D. Lavie. Nobel Symposium 25, Chemistry in BotanicalClassification, Academic Press, New York and London, 1973,p.181.Acetylation of (la) gave the diacetate (lb), in the n.m.r.spectrum of which the 6 4.42 band was completelyresolved into a two-proton singlet at 6 4.95 (27-H,), aone-proton double doublet at 6 4.80 (22-H), and a one-proton doublet at 6 5.8 (23-H). The 6 4.95 singlet, inH I H I( 1 ) a ; R = H (2) a ; R = Hb; R = AC b ; R = CICconjunction with the single vinylic methyl signal (6 2.11)defines the substituents at C-24 and C-25 as CH, andCH,*OAc. The double doublet at 6 4.80 points to twoprotons only in the neighbourhood of the 22-H; however,in contrast to the 22-H in the 20-hydroxywithanolides 1 9 3the signal of which is split by the two C-23 protons withJ 12 and 5 Hz, in the present case the coupling constantsare only 3.5 and 2 Hz, excluding any axial-axial inter-action.Deuteriation experiments on deoxydihydro-withaferin A 4 led to the exchange of the two C-23protons and resulted in the appearance of the 22-Hsignal as a doublet (J 4 Hz). The only possible con-s E. Glotter, I. Kirson, A. Abraham, and D. Lavie, Tetra-D. Lavie, E. Glotter, and Y. Shvo, J . Chem. SOC., 1966, 7517.hedron. 1973, 29, 13631975clusion is that in (lb) the axial proton at C-23 is replacedby an acetoxy-group, thus explaining also the doubletat 6 5.81 (J 2 Hz) for the 23-H.Double-resonance measurements confirmed the aboveinterpretation: irradiation at 6 2.34. (20-H) leads todecoupling of the 21-H (singlet instead of doublet) andof the 22-H (doublet, J 2 Hz, instead of double doublet) ;conversely irradiation at 6 4.80 (22-H) leads to decoup-ling of the 23-H (singlet instead of doublet) and of the2137protons (Zl-H, and 22-H) which are near the 17-OH areshifted downfield. The Cotton effect in the c.d.spectrumof the lactone ring in (la) is positive, as in all otherwithanolides (260.5 nm; A& + 3.85). Consequently(la) is (22S,23R)-17cc,23,27-trihydroxy-l-oxowitha-2,5,-24trienolide.On the basis of the above data we can now assign asimilar structure to the side chain of (2b) [the naturallyoccurring alcohol (2a) could not be isolated as such, butN.m.r. signals * of withanolides Q and R and their acetatesCompd. 2-33 3-H 6-H 7-H 22-H 23-H 19-H, 18-H3 21-H, 0 ther(la) 5.92dq 6.86dq 5.63 4.42 4.42 1.26s 0.87s 1.13d 32-H, 23-H, 27-H,: broad(10, 3, 1) (10, 5, 2.5) W& 8 (7) band at 4.42; 28-H:[4.83] [1.22] [0.78] [1.26] 2.11s [2.31](lb) 5.90dq 6.85dq 5.63 4.80dd 5.81d 1.24s 0.79s 0.95d 27-H,: 4.96s; 28-H, and(10, 3, 1) (10, 5, 2.5) W& 8 (2.6, 2) (2) (7) acetates : 2.06, 2.09,C6.211 [1.21] [0.78] [1.21] 2.16(2b) 5.85dq 6.56dq 3.05d 3.33dd 4.73dd 5.63d 1.17s 0.80s 0.93d 27- and 28-H3: 1.97; 14c0:(10,3,1) (10, 4.5, 3) (4) (4, 1) (3.5, 2) (2) (7) 2.15* 6 Values: J/Hz in parentheses; data for solutions in C,D,N in square brackets.21-Me; finally, irradiation at 6 5.81 (23-H) induces thedecoupling of the 22-H (now doublet, J 3.5 Hz).Treatment of (lb) with basic alumina a t room tem-perature caused elimination of the 23-acetoxy-group,to give the corresponding pyrone, A,, 307 nm.On the basis of the molecular weight of (la), thecompound must possess a third hydroxy-group which,in the absence of any relevant n.m.r.signal should betertiary. It is assigned the l7a-position by analogy withother withanolides and in agreement with pyridine-induced n.m.r. shifts A(CDC1, - C5D5y) of H-H, +0.01,21-H, -0.26, and 22-H -0.41 p.p.m.Comparison of the n.m.r. spectra of (la) and itsdiacetate (Ib) (in CDC1,) reveals two relevant aspectsconcerning the protons near to the 23-substituent. In(lb), the 18-H3 and the 2l-H, sipals are shifted upfield,whereas the 22-H signal is shifted downfield by 0.40p.p.m. A model shows that the 23-acetate grouprestricts rotation about the C(20)-C(22) bond; as aresult, in the most preferred conformation, the 18- andthe 21-protons are more distant from the 23-substituentthan in (la), whereas the 22-H is held closer to the 17cc-OH, thus inducing deshielding of the latter and shieldingof the former.The spectra of (la) and (lb) in C,D,Nsupport such an interpretation. Whereas in (la)A(CDC1, - C,D,N) is positive for 18-H3 and negativefor 21-H,, 22-H, and 28-H,, in (lb) 18-H3 and 28-H,are practically unaffected, whereas the 21-H, and the22-H signals are shifted downfield. It is known5 thatthe effect of pyridine is due to hydrogen bonding withthe hydroxylic oxygens. In (la) the pyridine co-ordinated to the 23-OH restricts rotation about theC(20)-C(22) bond in such a way that the distance betweenthe 23-OH and the 18-H3 is increased (upfield shift of theWH, signal) ; concomitantly, the protons which areclose to the 23-OH and the 17-OH are significantlydeshielded. In (lb), where pyridine does not co-ordinatearound the 23-acetoxy-group, only the signals of thoseP.V. Demarco, E. Farkas, D. Doddrell, B. L. Mylari, andE. Wenkert, J . Amer. Chem. SOC., 1968, 90, 5840.only as the 23-monoacetate]. Comparison of n.m.r.signals confirms that the only difference between the sidechains in (lb) and (2b) is the absence of the 27-OH in thelatter. As to the substitution pattern of rings A and B,and the similarity to (22R)-6~,7cc-epoxy-5a,17a-dihydr-oxy-l-oxowitha-2,24-dienolide leaves no doubt, aboutthe structure (2b).Since the molecular weight of (2b)indicates the presence of only seven oxygen atoms, thecompound,possesses only one tertiary OH (5cc-OH).The mass spectral fragmentation pattern of (la)(M+ 470) supports the assigned structure. The basepeak (m/e 267) is due to the cleavage of the C(17)-C(20)bond, accompanied by elimination of a molecule of water.Cleavage of the C(20)-C(22) bond, which is the dominantprocess in the fragmentation of withanolides unsub-stituted a t C-23, is less important in the present case andleads to the signal m/e 295 (M+ - 157 - 18) ; the secondpossible fragment from this rupture, which wouldcorrespond to the lactone ring, is of low abundance.Nevertheless, there are two significant signals (m/e 342and 324) related to the cleavage of the lactone itselfalong the C(22)-C(23) and the 0-C:O bonds.In the spectrum of the diacetate (lb) (Mf 554) themain signals correspond to the trivial elimination of waterand acetic acid (m/e 536, 494, 476, 434, and 416).Lossof the whole side chain (accompanied by elimination ofwater) leads to the same fragment, in/e 267, as in (la).EXPERIMENTALOpticalrotations were recorded with an automatic Perkin-Elmer141 polarimeter and refer to solutions in chloroform. 1.r.spectra were recorded on a Perkin-Elmer Infracord 137spectropliotometer equipped with a sodium chloride prism,and refer to KBr pellets; U.V. spectra were recorded on aCary 14 instrument (ethanol as solvent) ; n.ni.r. spectrawere determined on a Varian A-60 spectrometer for 5-10%solutions in deuteriochloroform, containing tetramethyl-silane as internal standard.T.1.c. was carried out onchromatoplates of silica gel G (Merck) and spots weredeveloped with iodine vapour. Mass spectra were takenM.p.s. were taken on a Fisher-Johns apparatusJ.C.S. Perkin Iunder the direction of Dr. 2. Zaretskii with an Atlas CH4instrument. Analyses were performed in the micro-analytical laboratory of the Weizmann Institute, under thedirection of Mr. R. Heller.Plant Material.-Reciprocal cross-pollination of Withaniasomnifera Dun., chemotypes I and 111, resulted in theoffspring F,. Self-pollination of F, afforded several off-spring F, in which recombination of the chromosomes hadtaken place. One of these, which proved to be a new,artificial chemotype, remained unchanged when it was self-pollinated again.The offspring F, thus obtainedconstituted the plant material of the present work.Isolation.-Crushed air-dried leaves (2 kg) were exhaust-ively extracted with methanol and the extract was workedup as previously de~cribed.~ The crude product (27 g) waschromatographed on a silica gel column (Woelm 0.063-0.1mm; 3 kg). Elution with benzene-ethyl acetate (4.5 : 5.5)afforded crude withanolide Q (400 mg), which was purifiedon preparative chromatoplates (Woelm 60 Fzsa; 2 mmthick). Withanolide Q (la) (120 mg) (22S,23R)-17aj23,27-trihydroxy- l-oxowitha-2,5,24-trienolide] had m.p. 200-202"(from ethyl acetate), [.ID -6.6" (c 0.12); vmY. 1650 and1 705 cm-l, Am,.218 nm (E 17 950); c.d. (ethanol; c 0.63)340 (Ac -3.14) and 260.5 nm (f3.85) (Found: C, 71.45;H, 8.2%; M+, 470. C,,H,,O,requires C, 71.45; H, 8.15%;nrl, 470).Acetylation of (la) (50 mg) with acetic anhydride inpyridine, overnight a t room temperature, gave the 23,27-diacetate (lb), m.p. 160-162" (from acetone), [.ID -88.8"(G 0.08); vmX. 1655, 1715, and 1 745 cm-l, A,x. 220 nm( E 18 500) (Found: C, 69.3; H, 7.7%; M+, 554. C32H4208requires C, 69.3; H, 7.65%; M, 554).Withanolide R Monoacetate (2b) .-Chromatography of thecrude extract from leaves of W. sowznzfera ' Indian I 'chemotype resulted in the separation of several purewithanolides and in two inseparable mixtures designated Aand B. Mixture A was acetylated with acetic anhydride-pyridine overnight a t room temperature and the crudemixture of acetates (0.5 g) was separated by thick-layerchromatography on silica gel PF,,, (1 mm thick in benzene-ethyl acetate (2 : 8). Extraction of the third band from thetop afforded (22S,23R)-23-acetoxy-6a,7a-e~oxy-5cc-hydroxy-l-oxowitha-2,24-dienoZide (2b) (22 mg), m.p. 182-184"(from acetone-hexane), [a], -64" (c 0.09), v,, 1698,1 712, and 1 745 cm-l, LX. 221.5 nm (E 17 400) (Found:C, 70.6; H, 7.4%; M', 510. C,,H,@, requires C, 70.55;H, 7.5%; M, 510).We thank Professor D. Lavie and Dr. E. Glotter fordiscussions and Mrs. B. Romano for the c.d. measurements.[5/579 Received, 25th March, 1975
ISSN:1472-7781
DOI:10.1039/P19750002136
出版商:RSC
年代:1975
数据来源: RSC