Cyclohexenones, bicyclic lactonic

The prediction of product structure is difficult for such cycloadditions. The difficulty arises from the fact that either the ketene carbonyl group or olefinic linkage can act as the two-electron component. Indeed, in such cycloadditions, both modes of addition can be observed. When 3,3,4,4,5,5-hexamethyl-l,2-bis(methylene)cyclopentane 9 and diphenylke-tene were reacted in cyclohexane at 55 °C for long period of time, virtually equal amounts (70%) of dihydropyran 10 and the cyclohexenone 11 were obtained (Scheme 5). Compound 10 can be readily isomerized in acidic media to afford the conjugated diene 12. When the reaction of 1 with 9 was performed in benzonitrile instead of cyclohexane, the ratio of 10 11 rose from 1 1 to 3 1 (1987JCS(CC)1804). Interestingly, when the reaction was carried out in the presence of a Lewis acid such as ZnCla, two equivalents of diphenylketene was consumed which reacted with the diene to furnish a bicyclic lactone. 

The plan was to assemble the first carbocyclic ring of 3 by intramolecular aldol condensation of the keto aldehyde 15. The enantiomericaUy-pure secondary methyl substituent of 15 derived from the commercial monoester 10. Activation as the acid fluoride followed by selective reduction led to the volatile lactone 11. Opening of the lactone with HjCONHCHj HCl gave, after protection, the Weinreb amide 12. Alkylation of the derived hydrazone 13, selectively on the methyl group, led, after deprotection, to 15. The intramolecular aldol condensation of 15 did dehver the imstable cyclohexenone 1. Under the acidic conditions of the aldol condensation, the enol derived from the piperidone added in a Michael sense, from the axial direction on the newly-formed ring, to give the frans-fused bicyclic diketone 2. 

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