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Evaluation of camphor derivatives in terpenoid synthesis Palme, Monica H.

Abstract

New enantiospecific syntheses of 5,6-dehydrocamphor (36) and 5-methy1-5,6-dehydrocamphor (178) are reported, and these two compounds were evaluated as intermediates in terpenoid synthesis. Addition of an alkenyl unit to (+)-5,6-dehydro-camphor (36) and subsequent anionic oxy-Cope rearrangement of the resulting 1,5-diene (64) provided hydrindenone 66. Ring expansion of 66 provided decalin intermediate 69 which contains the A/B ring system common to many terpenoids; however, the stereo-specific introduction of an angular methyl group to provide a system such as 40 eludedus. Therefore, a similar synthetic strategy using (+5-methyl-5,6-dehydrocamphor (178)was investigated. Isopropenyl addition to 178 and subsequent anionic oxy-Copere arrangement provided hydrindenone 190 which contained the desired angular methyl group. (-)-5-Methyl-5,6-dehydrocamphor (178) was also converted to enone 204, but stereoselective conjugate addition to this enone (204) was not satisfactory. Addition of amore complex alkenyl unit to (-)-5-methyl-5,6-dehydrocamphor (178) provided 1,5-diene179, however, anionic oxy-Cope rearrangement to provide hydrindenone 180 did not occur, presumably due to steric effects. The first enantio selective synthesis of (-)-4-methylcamphor (229) is also reported. It is expected that 229 will undergo reactions analogous to those reported for camphor(25) and therefore would provide a route to trans hydrindenone 232. The latter compound is a potentially useful intermediate in the synthesis of the lanostane group of triterpenoids whereas its enantiomer (ent-232) derived from (+)-4-methylcamphor (ent-178) could gain access to the euphane group of triterpenoids.

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