A new synthesis of $(\pm)$- and (+)-2,6,7,7a-tetrahydro-1$\beta$-hydroxy-4-formyl-7a$\beta$-methylindenes

A new method for the preparation of the synthon $(\pm)$-2,6,7,7a-tetrahydro-1$\beta$ -hydroxy-4-formyl-7a$\beta$-methylindene (1,a) for the total synthesis of steroids in both $(\pm)$ and (+)forms, starting from the known $\beta$-ketoester, $(\pm)$-methyl $1\beta$-t-butoxy-5,6,7,7a-tetrahydro-7a$\beta$-methyl-5-keto-4-indancarboxylate (2,a) has been described, An alternative route to (1,a) has been investigated. Although the compound, $(\pm)$-1$\beta$-hydroxy-5,6,7,7a-tetrahydro-7a$\beta$methyl-5-keto-4-methoxymethylindan (2,b) could not be prepared, interesting pathways leading to two unexpected products, $(\pm)$-5,6,7,7a-tetrahydro-4,7a-dimethyl-5H-indene-1,5-dione and $(\pm)$-2,6-diketo-3-methyltricyclo-(5,2,1,0)decan-8-ol (3 and 4), were encountered during an attempted annelation reaction of the ketone, N-diethylamino-5-methoxypentan-3-one (6), with 2-methylcyclopentan-1,3-dione (5). Trapping of the intermediate, $(\pm)$-3a,4,5,6,7,7a-hexahydro-3a-hydroxy-4-methylene-7a-methylindene-l,5-dione (7), through the formation of the adduct, $(\pm)$-3a,4,5,6,7,7a-hexahydro-3a-hydroxy-4-($1^{\prime}$,$3^{\prime}$-diketo-$2^{\prime}$-methylcyclopentano-$2^{\prime}$-methyl)-7a-methylindene-1,5-dione (8), established the mechanism of the formation of the products (3 and 4).