Vinylallenes racemization

Initial attempts to perform the 1,5-substitution enantioselectively with chiral enyne acetates proceeded disappointingly. For example, treatment of the enantio-merically pure substrate 51 with the cyano-Gilman cuprate tBu2CuLi LiCN at -90 °C provided vinylallene 52 as a 1 3 mixture of E and Z isomers with 20 and 74% ee, respectively (Scheme 2.19) [28], As previously described for the corresponding Sn2 substitution of propargylic electrophiles, this unsatisfactory stereoselection may be attributed to a racemization of the allene by the cuprate or other organome-... 

A diastereoselective synthesis of the racemic vinylallene 96 was described by Gordon and Tabacchi [96] in 1992 (Scheme 18.31). Here, the exocyclic allene was formed via anti-SN2 -substitution of the propargylic sulfmate 98 with lithium diisopropenyl-cyanocuprate, which, however, was also found to undergo side reactions such as reduction and direct attack of the nucleophile at the sulfmate. Deprotection of the substitution product 99 (which was obtained with only 35% yield) finally provided the desired target molecule 96. 

Highly enantioselective 1,5-substitution reactions of enyne acetates are also possible under carefully controlled conditions (Eq. 4.31) [46]. For example, treatment of enantiomerically pure substrate 70 with the cyano-Gilman reagent tBu2CuLi-LiCN at —90 °C provided vinylallene 71 as a 1 3 mixture of E and 2 isomers with 20% and 74% ee, respectively. This mediocre selectivity might be attributable to race-mization of the allene by the cuprate or other reactive copper species formed in the reaction mixture. The use of phosphines as additives, however, can effectively prevent such racemizations (which probably occur by one-electron transfer steps) [47]. Indeed, vinylallene 71 was obtained with an ee of 92% for the E isomer and of 93% for the 2 isomer if the substitution was performed at —80 °C in the presence of 4 eq. of nBusP. Use of this method enabled various substituted vinylallenes (which are interesting substrates for subsequent Diels-Alder reactions Sect. 4.2.2) to be prepared with >90% ee. 

In 1988 Gibbs and Okamura [128] described the intramolecular Diels-Alder reaction of the non-racemic vinylallene 150 to afford the adduct 151 (as a mixture of epimers at sulfur) in a completely exo-selective manner, due to the topographical and steric arrangement of the starting vinylallene (Scheme 74). Compound 150 was obtained as a mixture of epimers at sulfur from the optically pure propargylic alcohol 149 (this transformation involved a sulfoxide-sulfenate rearrangement). Compound 151 was used to synthesize (-l-)-sterpurene. 

Asymmetric epoxidation has been performed with optically active peracid. " and racemic oxiranes have been resolved on a glc column containing an optically active complex. The epoxidation of allenes has been examined, as have the reactive intermediates formed in the epoxidation of simpler allenes. Such intermediates have been isolated from sterically hindered allenes. PNPBA epoxidation of a series of vinylallenes results in a-allenoxiranes and, as the main products, conjugated cyclopentenones. ... 

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