Condensed cyclic and bridged-ring systems. Part 10. Alumina-induced regioselective intramolecular alkylations of cis- and trans-4a-(2-methylsulphonyloxyethyl)-3,4,4a,9,10,10a-hexahydrophenanthren-2(1H)-ones

Abstract

The synthesis and intramolecular alkylations of cis- and trans-4a-(2-methylsulphonyloxyethyl)-3,4,4a,9,10,10a-hexahydrophenanthren-2(1H)-ones (5a) and (6a), and the respective 6- and 5-methoxy analogues (5b) and (5c), and (6b) and (6c), are described. While alumina-induced alkylation of the cis-isomers (5a) and (5b) is shown to give, in each case, a mixture of the corresponding regioisomeric bridged-ketones, cis-3,4,4a,9,10,10a-hexahydro-1,4a-ethanophenanthren-2(1H)-one (23a) and cis-3,4,4a,9,10,10a-hexahydro-3,4a-ethanophenanthren-2(1H)-one (24a), and the 6-methoxy derivatives (23b) and (24b) in a ratio of ca. 3-4 : 1, the trans-isomers (6a-c) produce exclusively the respective trans-hexahydro-3,4a-ethanophenanthren-2(1H)-ones (26a-c). Alkylation of the cis-ketone (5a) with sodium hydride in benzene gives a 3 : 1 mixture of isomers (23a) and (24a), whereas with potassium t-butoxide in t-butyl alcohol the only isolable product is cis-3-hydroxy-4a,9,10,10a-tetrahydro-1,4a-ethanophenanthren-2(1H)-one (25)[arising from the concomitant autoxidation of the major alkylated product (23a)]. Using sodium hydride or potassium t-butoxide as the base, the trans-ketone (6a) gives trans-3,4,4a,9,10,10a-hexahydro-3,4a-ethanophenanthren-2(1H)-one (26a) as the sole product. The attempted preparation of the 5-methoxy-methanesulphonate (5c) from the corresponding cis-keto-alcohol (21c) led to cis-3,4,4a,9,10,10a-hexahydro-5,4a-(epoxy-ethano)phenanthren-2(1H)-one (27)via elimination of the aromatic methoxy methyl group.