Abstract
The stereochemistry of Diels-Alder (D.A.) adducts of 2,5-dialkylcyclohexenones with butadiene has been chemically clarified. The key intermediate (21) for synthesis of 8-deoxyserratinine type of alkaloids and dl-chamaecynone (29) have been synthesized by taking advantage of this type of D.A. reaction. The study on the stereochemistry of the D.A. adduct (2) of dienophile (1) with butadiene in the presence of AlCl_3 suggested that butadiene approached the dienophile (1) from the side opposite to the C-5 methyl group. The result of re-investigation of the reported reaction of carvone with butadiene by heating method showed that reported structures should be revised in such a way that the structure (7) for the major product is replaced by the formula (8) and the structure (8) for the minor product by the formula (7). Moreover, addition of Lewis acid much improved the yield and stereoselectivity of this reaction. In connection with synthetic study on 8-deoxyserratinine type of alkaloids, the D.A. reaction of dienophile (22) obtained from (23) in five steps with butadiene provided the desired key intermediate (21). The D.A. reaction of a new dienophile (31) with a new diene (30) gave the main product (32), which has the same four chiral centers as those of chamaecynone (29), accompanied with the compounds (37) and (38). The compound (32) was converted in four steps into dl-chamaecynone (29).
