Cyclohexene carboxaldehydes, synthesis

Jorgensen showed that the asymmetric synthesis of cyclohexene derivatives can also be accomplished by reaction of two equivalents of an enal with an active methylene compound. The sequence comprising two iminium-activated Michael additions followed by enamine-mediated aldol reaction afforded various cyclohexene carboxaldehydes (Scheme 8.20). 

Cyclopentene-l-carboxaldehydes are obtained from cyclohexene precursors by the sequence cyclohexene - cyclohexane-1,2-diol -> open-chain dialdehyde - cyclopentane aldol. The main advantage of this ring contraction procedure is, that the regio-and stereoselectivity of the Diels-Alder synthesis of cyclohexene derivatives can be transferred to cyclopentane synthesis (G. Stork, 1953 G. BUchi, 1968). 

---