A new synthetic method for functionalized cyclopentanones was developed on the basis of a [3+2] cycloaddition reaction of a 1-(methylthio)-2-siloxyallyl cationic species and olefins. Allyl acetates 1a and 1b, which are the precursors of the allyl cationic species, are easily prepared in three or four steps from commercially available compounds. Under the influence of EtAlCl2 or AlCl3, 1a or 1b reacted with various kinds of olefins such as enol ethers, vinyl sulfides, styrenes, and trialkylolefins to afford the corresponding cyclopentanones in good yields. It is noteworthy that the sterically more hindered regioisomer was predominantly formed in every case. Furthermore, the reactions of 1b with vinyl sulfides exhibited surprisingly high stereoselectivity, which can be rationalized by the six-membered transition state models involving an orbital interaction between the sulfur atom of the vinyl sulfide and the α-carbon of the allyl cation.

An efficient synthetic method for (-)-coriolin has been developed on the basis of a [3+2] cycloaddition reaction of a 1-(methylthio)-2-siloxyallyl cationic species and vinylsulfides. An enantiomerically pure C-ring unit was prepared through optical resolution of five-membered allyl ester 6b using a lipase. The first [3+2] cycloaddition reaction of C-ring unit (S)-7 gave bicyclic ketones 8 and 9, which were easily converted into vinyl sulfide 11. Stereoselective construction of the A-ring was achieved by the second [3+2] cycloaddition reaction of the BC-ring unit. New methods for introduction of the oxygen functional groups to the triquinane skeleton were also developed for the last stages of the total synthesis. Thus, the C7 hydroxyl group was introduced by epoxidation of dienol silyl ether 17, and stereocontrolled construction of the spiro epoxide moiety was accomplished on the basis of a Darzens-type reaction.

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A New Synthetic Method for Cyclopentanones via Formal (3 + 2) Cycloaddition Reaction.-Alkenyl sulfides (II) react with AlCl3 or EtAlCl2 to give the corresponding 2-oxyallyl cations, which undergo a formal (3 + 2) cycloaddition with alkenes such as (I), (IV), and (VI) to give cyclopentanone derivatives. The reaction is highly regioselective to yield the sterically more hindered regioisomer. -(MASUYA, K.; DOMON, K.; TANINO, K.; KUWAJIMA, I.; Synlett (1996) 2, 157-158; Dep. Chem., Tokyo Inst. Technol., Meguro, Tokyo 152, Japan; EN)