4 Reaction Mechanisms Part (i) Pericyclic Reactions By G. B. GILL Department of Chemistry University of Nottingham Nottingham NG7 2RD 1 Introduction The ene-type reactions of singlet oxygen with olefins have continued to attract attention with the main interest focussed upon attempts to delineate mechanisms. Recent advances in the area have been reviewed.' Results of a theoretical treatment based on ab initio calculations for the reaction between ethylene and '02combined with thermochemical methods for estimating the effects of substituents to distinguish the pathways involving peroxy diradical open 1,4-zwitterion and perepoxide intermediates have been reported.* It was concluded that the major reaction pathway involves a diradical and that solvent and substituent effects can be accounted for by assuming variable diradical-zwitterionic character.The reaction of 2-methyoxybut-2-ene is predicted to proceed via the diradical (1)in non-polar solvents whereas in polar solvents the opposite regiochemistry should be favoured leading to the formation of the zwitterion (2); this prediction should be verifiable. Stephenson3 has suggested that as activation barriers in the olefin-'02 reactions are very low future developments may necessarily lean towards theoretical treatments. He proposes that the interactions of frontier orbitals between highly electrophilic singlet oxygen (LUMO) and both the olefinic 7r-orbitals and C-H bonds (HOMO) lead to complex formation. In the case of cis-2-butene the complex can be pictured as in (3) in which the pseudo-.rr-orbitals of CH3 contribute to the .rr-HOMO.The interaction is expected to be stronger than for trans-2-butene where only one carbon Meope Meo8Me Me H (2) ' L. M. Stephenson M. J. Grdina and M. Orfanopoulos,Acc. Chem. Res. 1980.13,419. 'L. B. Harding and W. A. Goddard J. Am. Chem. SOC.,1980,102,439. 'L.M.Stephenson Tetrahedron Lett. 1980,21,1005. 28 G.B. Gill pseudo-rr-orbital can be involved but weaker than in trisubstituted olefins or with CH30-substitution (CH30 is a strong cis-director). The attraction of this model is that it does not conflict with the present (isotope-effect) studies or with previously established data and allows for the possibility of relaxation to perepoxide diradical or zwitterionic species depending upon the nature of the alkene undergoing reaction.The reaction of lo2with the optically active olefin (4) followed by reduction afforded the alcohols (R)-(5)and (S)-(6)in 1:1 ratio; while the process is highly stereospecific the lack of stereoselectivity (isotope effect) is not what would be anticipated for a normal ene reaction but is consistent with for example the perepoxide me~hanism.~ The finding5 of an isotope effect in the ene reaction of N-phenyltriazolinedione with (E)-2,3-bis(trideuteriomethyl)-2-butene (the Stephenson model3) but not with the (2)-olefin has led to the suggestion that an aziridinium imide (cf.perepoxide intermediate) is intermediate in the change. The formation of a diazetidine from adamantylideneadamantane and the triazolinedione implies from steric considerations that orthogonal approach does occur.A number of papers have been concerned with the stereoelectronic effects in the cycloaadition reactions of norbornene and related molecules. A field effect rather than through-space orbital mixing has been suggested as the origin of the polariz- ation of the rr-orbitals of 7-substituted norbornadienes and provides a satisfactory rationalization of the deactivation toward exo-anti-attack by electronegative sub- stituents. However the more subtle rate effects of syn-endo- and anti-endo-attack are less well understood.6 Comparison of the dipolar and Diels-Alder cycloaddition rates of norbornene bicyclo[2.2.2]octene (7) and (8)reveal that the rate constants for (7) and (8) are similar to or smaller than those for n~rbornene.~ Molecular mechanics MM2 calculations of strain energies and heats of formation (which provide quite good agreement between calculated AH,“values and experimental values where available) reveal that loss of strain in (7) and (8) in becoming dihydro-(7) or -(8) should provide a relative rate acceleration upon cycloaddition of 103-104.The high rate constants for norbornene therefore originate from another source -possibly from non-equivalent orbital extension (Fukui’s concept).Electronic control of stereoselectivity in cycloadditions to molecules of the type (9) and (10) has been examined in detail.’ Whereas (9) undergoes [4+2] rr-M. Orfanopoulos and L. M. Stephenson J. Am.Chem. SOC.,1980,102,1417. C. A.Seymour and F. D. Greene J. Am. Chem. SOC.,1980,102,6384. P. H. Mazzocchi B. Stahly J. Dodd N. G. Rondan L. N. Domelsmith M. D. Rozeboom P. Caramella and K. N. Houk J. Am. Chem. SOC.,1980,102,6482. ’R. Huisgen P.H. J. Ooms M. Mingin and N. L. Allinger J. Am. Chem. SOC.,1980 102 3951. (a) L. A. Paquette V. C. Carr M. C. Bohm and R. Gleiter J. Am. Chem. SOC.,1980,102,1186; (6)hi. C. Bohm R. V. C. Cam R. Gleiter and L. A. Paquette ibid.,p. 7218;(c) L.A.Paquette R. V. C. Carr E. Arnold and J. Clardy J. Org. Chem. 1980 45,4907; (d)L.A. Paquette F. Bellamy M. C. Bohm and R. Gleiter ibid.,p. 4913;(e) L.A.Paquette R. V. C. Carr P. Charumilind and J. F. Blount ibid. p. 4922. Reaction Mechanisms -Part (i) Pericyclic Reactions OH (10) X = (CHA (11) cycloadditions with typical dienophiles entirely by endo-attack both endo and em products are obtained from (10)in most cases.sa*b*e Since steric factors are ruled out in determining the endo stereoselectivity in (9) the results have been rationalized in terms of cT-orbital mixing with the T diene level leading to a disrotatory tilt of the diene orbitals thereby minimizing antibonding interactions on the endo face in the interaction with the enophile T-level.Only moderate endo stereoselectivity was observed in the case of the cycloadditions of 102,8cTd and this is presumably the result of the very low energy of the 7rl(S) level for lo2,and hence a much smaller interaction with the diene nl(S)level so that differences between endo and ex0 approach are not large.Diels-Alder reactions of the fused cyclopentadiene (11) occur exclusively to the face bearing the OH group in accord with the bias of the T-HOMO in the syn-direction (Fukui et ~21.)~~ 2 Cycloadditions and Cycloreversions An algorithm of the form y(rc)= (E$ +E; -E:)/C*,based on the Michl model has been derived for prediction of reactivity in allowed [2 + 21 and [4 + 41 singlet-state photo-cycloadditions.'O Favourable features are high singlet-state energy (E is that for excited reactant A) low triplet energy (E$ and E:! are triplet energies for reactants A and B) and a high frontier-orbital density at the reacting positions (c2 is the sum of products of HOMO and LUMO orbital coefficients over reacting positions); y(rJ is the resonance integral for end-on interaction of carbon 2p orbitals at distance r,.There is reasonable correlation of the algorithm with known photo- reactivities which allows segregation into three categories which are highly reactive [y(rc)f 20 kcalmol-')I moderately reactive [20 s y(rc)s 251 and unreactive [y(rc)b 25 kcal mol-'1. The electronic structure of the first excited states of hypostrophene (12) deduced from analysis of the first bands in the He(1) photoelectron spectra of (12) and of its di- and tetra-hydro- and its homo- and bis-homo-derivatives in combination with semi-empirical calculations has been discussed in the light of its inability to undergo [,2 + ,2,] photo-cylcoaddition." The results confirm that for (12) T+(the in-phase combination of the two ethylenic orbitals) is the HOMO arising from the mixing in (12) D.W. Jones J. Chem. Soc. Chem. Commun. 1980,739. lo R. A. Caldwell J. Am. Chem. SOC., 1980 102 4004. J. Spanget-Larsen R. Gleiter G. Klein C. W. Doecke and L. A. Paqiiette Chern. Ber. 1980,113,2120. 30 G. B. Gill of several totally symmetric bicyclohexane orbitals thus placing it above *-(the out-of-phase combination). However the LUMO previously assigned to d,is not so readily deduced because of the closeness in energy of rrT and T?.Possibly therefore derivatives of (12) may be ‘allowed’ to follow the proscribed [,2 + ,2,] pathway. The regioselectivities of the [,4 + ,2,] cycloadditions of alkyl-allenes to tetraphenylcyclopentadienone differ markedly from those for the formal [2 + 21 additions of maleic anhydride.Coupled with PMO predictions the results have been interpreted in favour of the [,2 + (,2 + ,2,)] ,or diradical pathway rather than the [,2 + ,2,] mechanism.12 Experimental and theoretical criteria are thoroughly examined in a review which attempts to draw a line of distinction between concerted and multi-step mechanistic alternatives in Diels-Alder reaction^.'^ The [4 + 21 cyclo-dimerization of 2,3-dimethyl-1,3-butadiene has been examined at high pre~sure.’~ Study of the pressure dependence of the rate constant reveals that A V*represents only about 70-75% of A V (the reaction volume) indicating that interatomic distances between atoms that are participating in bond formation are not as close as those distances in the adduct.Both concerted and diradical mechanisms are considered to compete; however the A V* criterion (if applicable) assumes rate-determining formation of the inter- mediate. The considerable recent interest in intramolecular Diels-Alder reactions con- tinues unabated; reviews” covering various features of such processes and methods for the generation of the necessary intermediates are particularly welcome. Such has been the number and diversity of papers in this area (estrone has been a favoured target) that only a few representatives can be considered here. An innovative one-step synthesis of polycycles including (f)-estrone has been reported which utilizes a [CpC~(CO)~]-catalysed co-oligomerization of substituted 1,5-hexadiynes with bis(trimethylsily1)acetylene solvent to produce the requisite benzocyclo- butenes.16 An example of the overall procedure is shown by the transformation (13) -+ (14) -+ (15) in 60% yield.A heteroatom variant with inverse electron demand of the Diels-Alder reaction has been used in the preparation of (16) (33’/0) which is formed stereo~electively.~~ The ene-diene precursor of (16) was formed simply by condensation of (R)-citronella1 with cyclohexane-1,3-dione the cyclo- addition occurring at room temperature. ’’ D. J. Pasto Tetrahedron Lett. 1980,21 4787. l3 J. Sauer and R. Sustmann,Angew. Chem. Int. Ed. Engl. 1980,19 779. l4 G.Jenner and J. Rimmelin Tetrahedron Lett. 1980 21 3039. Is G.Brieger and J.N. Bennett Chem. Rev. 1980,80,63;R.L.Funk and K. P. C. Vollhardt Chem. SOC. Rev. 1980 9 41;J. J. McCullough,Acc. Chem. Res. 1980,13,270. l6 R. L. Funk and K. P. C. Vollhardt J. Am. Chem. SOC.,1980,102 5245,5253. l7 L.-F. Tietze G. von Kiedrowski K. Harms W. Clegg and G. Sheldrick Angew. Chem. Znt. Ed. Engl. 1980 19 134. Reaction Mechanisms -Part (i) Pericyclic Reactions (16) (17) Interesting use has been made of bridgehead alkenes (formed by way of intramolecular Diels-Alder reactions) of the type (17) to control the relative configurations of a number of asymmetric centres.” Additions to the bridgehead double-bond occur with almost complete syn -stereoselectivity as reagents are denied access to the back side of the molecule.Pyrolysis of the diene (18)afforded the single cyclo-adduct (19); if the (a-acylimine is intermediate then there is a preference for endo placement of the non-terminal unsaturated group (N-acyl) in contrast to the analogous all-carbon systems.” The severe limitation in the use of (a-dienes in that two relatively easily accessible transition states are available for intramolecular cycloaddition has been discussed. Differences due to geometric factors are highlighted in the synthesis of optically active (20) by refluxing the (2)-amide in toluene (95O/O yield).20 The (E)-amide remained largely unaffected under similar conditions. Me Me .CO,Me heat 0 CHiPh Two ap-unsaturated aryl sulphones are reported to be fairly reactive dienophiles and the ready replacement of the ArS02 grouping is likely to recommend their further use in synthesis.Thus phenyl vinyl sulphone is an equivalent of ethylene and of terminal olefin,’l and ethynyl p-tolyl sulphone is an acetylene and trimethyl- silylacetylene equivalent.22 It appears however at least for phenyl vinyl sulphone that Lewis-acid catalysis is not effective in promoting reaction. Following upon domino Diels-Alder reactions which found favour in recent years (notably in the construction of sub-units of dodecahedrane) further development may be anticipated for ‘timed Diels-Alder reaction~’.~~ Thus the reaction of a bis-diene e.g. (21) with a bis-dienophile e.g. (22) results in intermolecular [4+2] reaction e.g. to give (23) followed by slower intramolecular [4 +21 cycloaddition e.g.to give (24). Regiochemical control of course is all-important in the first addition and requires careful consideration of the structures of the bis-dienes and bis-dienophiles. K. J. Shea P. S. Beauchamp and R. S. Lind J. Am. Chem. SOC.,1980,102,4544. l9 B.Nader R. W. Franck and S. M. Weinreb J. Am. Chem. SOC.,1980,102,1153. 2o S.G.Pyne M. J. Hensel S. R. Bryn A. T. McKenzie and P. L. Fuchs J. Am. Chem. SOC.,1980,102 5960. 21 R. V. C. Carr and L. A. Paquette J. Am. Chem. SOC.,1980,102 854. 22 A. P.Davis and G. H. Whitham J. Chem. SOC.,Chem. Commun. 1980,639. 23 G. A. Kraus and M. J. Taschner J. Am. Chem. SOC.,1980,102 1974. 32 G.B. Gill R Me J-Ld [77 "CI __* (23) (R = OSiMe2Bu') Studies of thermal [4 +21 cycloreversions of (25)-(27) have revealed that AGS decreases in the series respectively 29 26 and ~20 kcal mol-'.The rate acceler- ation of at least lo6by the 1-alkoxide substituent in (27) has been ascribed to the loss in basicity of ca 8 pKb units in the conversion of alcoholate into phenolate ion adding -11 kcal mol-' [i.e. -2.303RTA(pKb)] to the negative free enthalpy of reaction of the parent system. Since the transition state of a [4 +21 cycloreversion is product-like it should participate appreciably in the gain in free energy.24 (25) X=H (26) X =OSiMe (27) X=O-NMe4 There are few recorded instances of photochemical [2 +2 +2 +21 processes involving bis-terminal addition to an acetylene component. Nevertheless this procedure has been employed in a new route to the elassovalene system (28).25 Further developments have been reported this year in [4 +3'1 cycloadditions of ally1 carbonium ions to dienes.An alternative method for the construction of seven-membered rings involves cycloaddition of enophiles to the pyrylium zwitterion (29) affording adducts (30). Reactions were generally regiospecific but variable stereoselectivity was observed; however em-addition was normally favoured.26 A concerted mechanism appears to be involved in the [6+3-1 reactions of cyclohep- tatriene with 1,l-diphenyl- and 1,3-diphenyl-2-a~a-allyl-lithium.~~ 3 Ene Reactions Full details have now appeared of Lewis-acid-catalysed ene reactions of alkenes with acetylenic esters28a and with methyl a-chloroacrylate,286 and recent develop- ments in this area have been reviewed.28c Initial studies on the ene additions to alkenes of formaldehyde and higher aldehydes2'" and of terminal alkynes to formal- 24 0.Papies and W. Grimme Tetrahedron Lett. 1980,21,2799. R. Askani and B. Pelech Tetrahedron Lett. 1980,21,1841. 25 26 J. B. Hendrickson and J. S. Farina J. Org. Chem. 1980 45 3359 3361. 2'7 D. J. Bower and M. E. H. Howden J. Chem. SOC.,Perkin Trans. 1 1980 672. 28 (a)B. B. Snider D. M. Roush D. J. Rodini D. Gonzalez and D. Spindell J. Org. Chem. 1980 45 2773; (b)B. B. Snider and J. V. DunEia J. Am. Chem. Soc. 1980 102 5926; (c) B. B. Snider Acc. Chem. Res. 1980 13,426. 29 (a)B. B. Snider and D. J. Rodini Tetrahedron Lett.1980,21 1815; (b)D. J. Rodini and B. B. Snider ibid. p. 3857. Reaction Mechanisms -Part (i)Pericyclic Reaction dehyde2" have been reported. In both cases the proton-scavenging properties of Me2AlCl are put to good effect. The formation of a-allenic alcohols and (2)-3-chloroallylic alcohols in the terminal alkyne-formaldehyde reactions indicates the probable intermediacy of the dipolar species (31). Large rate accelerations coupled with greatly improved diastereoselectivities have been reported for the Et2AlC1-promoted intramolecular ene reactions of the (E)-and (2)-dienes (32) and (33) re~pectively.~'In the related reaction of the (-)-8-phenylmenthyl ester analogue of (33; Y = chiral carboxylate) cyclization afforded the trans-pyrrolidine (34) enantioselectively; a total synthesis of (+)-a -allokainic acid the natural epimer has therefore been achieved./COCF3 /COCF3 C02Et "Rf"_) H c1. -0 \=-C0,Et /\ (34) 7 Me Me XY (3 1) (32) X = C02Et Y =H (33) X = H Y = COZEt Quantitative studies of the reaction of dimethyl mesoxalate with alkenes which include the measurement of ahvation parameters and primary and secondary kinetic isotope effects the illustration that there is little variation of rate with change in solvent polarity and the establishment of the preponderant formation of (E)-adducts have been interpreted in favour of a late (product-like) transition state utilizing an exo arrangement in which the transfer of H occurs n~n-linearly.~~ Hammett correlations for thermal and Lewis-acid-catalysed ene additions of diethyl mesoxalate to 1-aryl-cyclopentenes have revealed a strong electronic bias in the catalysed process (p = -3.9) relative to the thermal reaction (p = -1.2) which is manifested in striking differences in regioselectivity in additions to other 01efins.~~ Hydrolysis and oxidative decarboxylation of the ene-adducts gives allylcarboxylic acids; hence oxomalonate is a C02synthon in ene reactions.Inability to achieve a planar arrangement of reaction centres in the transition states of acetylenic retro-ene processes prevents reaction. Hence vapour-phase thermolysis of (39 (36) and (38) afforded the retro-ene products whereas the alkyne (37) was recovered ~nchanged.~~ Thermolysis of (R)-(+)-laurolenal (39) at 297 "Cyields CO and (R)-(+)-(42)in high optical purity by a formal retro-nor-ene process; the acid (R)-(+)-(40)likewise was cleanly converted into (R)-(+)-(42)by retro-ene cleavage.34 In contrast the alcohol (R)-(+)-(41) initially underwent ene-retrogression to give (42) and CH20 slowly but oxidation of (41) by formal- dehyde yielded (39) and CH30H in the later phases of the thermolysis leading to a more rapid formation of (R)-(+)-(42).Relative rates were approximately 9 :2 :1 for (40) (39) and (41).30 W. Oppolzer and C. Robbiani Helv. Chim. Acta 1980,63 2010;W.Oppolzer C. Robbiani and K. Battig ibid. p. 2015;for related thermal studies see P. D. Kennewell S. S. Matharu J. B. Taylor and P. G. Sammes J. Chem. SOC.,Perkin Trans. 1 1980 2542.31 0.Achmatowicz and J. Szymoniak J. Org. Chem. 1980,45 1228,4774. 32 M.F.Salomon S. N. Pardo and R. G. Salomon J. Am. Chem. SOC.,1980,102,2473. 33 A.Viola and J. J. Collins J. Chem. SOC.,Chem. Commun. 1980 1247. 34 R. J. Crawford and €4. Tokunaga Can. J. Chem. 1980,58,463. 34 G.B. Gill H H&n U X Me Me Me% Me Me ri (42) (35) n = 2 X = CH=CH2 (39) x=co (36)n = 3,X = CH=CH2 (40)X=COO (37) n = 2 X = CGCH (41) X=CH20 (38) n = 3 X = CrCH N-Sulphinylnonafluorobutanesulphonamide(n-C4F9SO2N=S=O) is a 'super- enophile' being ca. 103-1O4 times more reactive than N-sulphinyltoluene-p- sulphonamide; reactions with typical olefins have tl, = 1-5 min at 20 "C and hence even electron-deficient alkenes can be converted into the ene adducts.Enolizable dialkyl ketones react by way of the enol in an autocatalytic process.35 The high ene reactivity of SO is revealed by the production of adducts (43; X = H or Me) at -78 "C from 1,l-dimethylallene and tetramethylallene; unusually the products have stability with respect to the starting materials although decomposition occurs at 25 0C.36The limited stereoselectivity in allylic oxidation of the isotopomeric olefins (E)-and (Z)-RCH=C(CH3)14CH3 (R = 2-cyclopentylethyl) by Se02 can be traced to differences in steric interactions in the chair-like transition states there being a 7 :3 preference for pseudo-equatorial R.37 Me =$q HO,S X (43) (44) The intramolecular ene reactions of 1,2-diallylcylohexanes have been investi- gated with a view to ring expansion by six carbon atoms.However the ene adducts (44) have yet to be persuaded to undergo the indicated further retro-ene cleavage that is required for the ring-expan~ion.~~ Transannular ene reactions have been used in an elegant approach to the synthesis of eudesmane-type corn pound^.^^ Retro-ene reactions may be involved in the ammonolysis of a-methoxyvinyl carbonates40a and in hydroxyammonolysis of enol ester~,~" and an ene-type reaction has been proposed involving acyl-group transfer in the addition of N-phenyltriazolinedione to a-angelica la~tone.~~ However a dipolar mechanism seems to be more likely for the acyl-transfer. 4 Sigmatropic Rearrangements Applications of sigmatropic processes in stereo-controlled syntheses of acyclic systems have been reviewed,42 as have energy-surface considerations in sigmatropic 3s R.Bussas and G. Kresze Angew. Chem. Znt. Ed. Engl. 1980,19,732; G.Kresze and R. Bussas ibid. p. 737. G. Capozzi V. Lucchini F. Marcuzzi and G. Melloni Tetrahedron Lett. 1980,21,3289. 37 W.-D. Woggon F. Ruther and H. Egli J. Chem. SOC.,Chem. Commun. 1980,706. 38 E. N. Marvel1 and J. C.-P. Cheng J. Org. Chem. 1980 45,4511. 39 P. A. Wender and J. C. Hubbs J. Org. Chem. 1980,45 365; P. A. Wender and L. J. Letendre ibid. p. 367. 40 (a) Y. Kita J.4. Haruta H. Tagawa and Y. Tamura J. Org. Chem. 1980 45 4519; (b) F. W. Lichtenthaler and P. Jarglis Tetrahedron Lett. 1980,21 1425 1429. 41 W. E. Bottomley G. V. Boyd and R.L. Monteil J. Chem. SOC.,Perkins Trans. 1 1980 843. 42 P. A. Bartlett Tetrahedron 1980 36 3. Reaction Mechanisms -Putt (i) Pericyclic Reactions shifts.43 The claim of base-assisted carbon-Claisen rearrangement of 4-phenyl-l- butene could not be ~ubstantiated.~~ Hence the all-carbon [3,3] rearrangement must be a high-energy process. Competitive formation of tricyclohexanes and Cope products in the thermal isomerization of allyl-substituted cyclopropenes indicates a non-concerted mechanism passing through intermediates analogous to the 1,4- cyclohexylene diradi~al.~' Relative rates and energy differences between the diastereoisomeric transition states in the Cope rearrangements of (*)-(45) and (&)-(46) involving a chair-like geometry (of local C2hsymmetry) and meso -(47) and meso-(48) involving a boat-like geometry (local Cz,symmetry) have been dete~mined.~~ The sizeable AASS terms (11-13 cal mol-' deg-') for (45) (47) (i.e.-11.4 -0.4 cal mol-' deg-') and (46) (48) (i.e. -8.3 +5.0 cal mol-' deg-') are puzzling. Since it appears that a dissociation-recombination mechanism can be discounted for the rneso-compound (47) (48) reactions the AS*values do not provide a valid criterion of concertedness as these Cope reactions satisfy other mechanistic criteria for concerted rearrangements. The [1,3] rearrangements of 5 -X-bicyclo[2.1 .O]pent-2-enes have been examined by ab initio calculations; no evidence for participation of lone pairs as required by the pseudopericyclic concept was Thermal rearrangement of (+)-(49) by the norcaradiene walk mechanism occurs predominantly (295%) with inversion at the migrating carbon C-7.48" Analogous but non-degenerate rearrangement of (50) proceeds with ca 80% inversion of configuration at the migrating carbon under thermal control; predominant inversion of configuration also occurs in the photo-induced [1,5] sigmatropic reaction.The relevance of orbital symmetry considerations of the Woodward-Hoffmann type to either process is therefore open to Details have been reported of the stereochemistry of and migratory aptitudes in [1,5] sigmatropic shifts of acyl and vinyl groups in 1,3-dimethylindene~.~' Preference for the exo alignment of the unsaturated terminus of the migrating group has been established and rationalized as an avoidance of antibonding secondary interactions in the endo transition-state geometry.Details of the stereochemical course in the intramolecular [1,7] antarafacial H shift leading to vitamin D3 have been unravelled by labelling studies. Thus the triene precursor adopts a left-handed twist of the reaction centres placing the A ring above the C/D rings and resulting in 9p transfer of the pro-R hydrogen atom.50 43 J. J. Gajewski Acc. Chem. Res. 1980 13 142. 44 M. Newcomb and R. S. Vieta J. Org. Chem. 1980,45,4793. 45 A. Padwa and T. J. Blacklock J. Am. Chem. SOC., 1980,102,2797. " K. J. Shea and R. B. Phillips J. Am. Chem. SOC.,1980,102 3156. 47 J. P. Snyder and T. A. Halgren J. Am. Chem. SOC.,1980,102,2861. 48 (a)F.-G.Klarner and B. Brassel J. Am. Chem. SOC., 1980 102 2469; (6) W. T. Borden J. G. Lee and S. D. Young,ibid. p. 4841; for related [1,6] shifts see R. F. Childs and C. V. Rogerson ibid.,p. 4159. 49 D. J. Field and D. W. Jones J. Chem. SOC.,Perkin Trans. 1 1980,714 1909. R. M. Moriarty and H. E. Paaren Tetrahedron Lett. 1980 21 2389. G.B. Gill Cope rearrangements of acyclic 1,Sdienes can be accelerated in appropriate cases by Pd” catalysts. The presence of an alkyl group at either C-2 or C-5 (not both) appears to be necessary presumably because of the need to provide stability in an intermediate species such as (5l)? Preference for chair-like transition states in anion-accelerated oxy-Cope rearrangements follows from the results of the rearrangements of the potassium salts of the four individual dienols of gross structure (52).52Application to the stereoselective synthesis of (+)-erythru-juvabione is also reported.Although formation of the oxy-anion from ~is~-2,4,7-~yclononatrienol results in enhancement of both [1,5] hydrogen shift and oxy-Cope rearrangement there is a bias of ca 104-105in favour of the [3,3] process resulting in quantitative conversion (following work-up) into (53).53 The rearrangement of the lithium salts of 2-vinylcyclopropanols to cyclopentenols apparently occurs by concerted [1,3] rearrangement. 54 Concerted [5,4] rearrangements proceeding uiu a nine-membered ten-electron transition state have been demonstrated in the base-catalysed transformations of allyl(pentadieny1)ammoniumcations.Analogous rearrangements of the propynyl- (pentadieny1)ammonium ylides however afforded only products resulting from [1,2] [3,2] and [5,2] sigmatropic shifts.55 5 Electrocyclic Reactions The chemistry of allene oxides has been reviewed.56 Simple Woodward-Hoff mann predictions are at variance with the conclusions based on CNDO/B potential-energy surfaces for ring-closure of 1-and 2-heterosubstituted butadienes and can be rationalized in terms of nodal shifts within the T moleculai orbitals as ring-closure sets in.57 Whereas photochemical (conrotatory) ring-opening of the trans-bicyclic dienes (55) and (56) affords trans,cis,truns-cyclo-deca-and -undeca-1,3,5-trienes by the least-motion pathway ‘discordant’ ring-opening of the lower homologue (54) occurs since only cis,cis,cis-cyclonona-l,3,5-triene is formed.Relief of strain is responsible. Thermal (disrotatory) cyclization of the trienes afforded the cis-fused isomers of (54)-(56)? ” L. E. Overman and F. M. Knoll J. Am. Chem. SOC.,1980,102,865. ” D. A.Evans and J. V. Nelson J. Am. Chem. SOC., 1980,102,774. 53 L.A.Paquette G. D. Crouse and.A. K. Sharma J. Am. Chem. SOC.,1980,102,3972. 54 R. L. Danheiser C. Martinez-Davila and J. M. Morin J. Org. Chem. 1980 45 1340. ” T.Laird W. D. Ollis and I. 0.Sutherland J. Chem. SOC.,Perkins Trans. 1,1980,2033;see also previous papers in this series. T. H. Chan and B. S. Ong Tetrahedron 1980,36,2269. ” J. P. Snyder J. Org. Chem. 1980,45 1341. ’* W.G.Dauben and E. G. Olsen J. Org. Chem. 1980,45,3377.