The course of cyclic ethylene acetal formation from 7a-methyl-2,3,7,7a-tetrahydro-1 H-indene-l,5(6H)-dione

Abstract

All attempts to convert the dione (1a) [7a-methyl-2,3,7,7a-tetrahydro-1H-indene-1,5(6H)-dione] into the 5-monoacetal (8) [5,5-ethylenedioxy-7a-methyl-4,5,6,7-tetrahydro-2H-inden-l(7aH)-one] by direct ethylene acetal formation under mild conditions have led to complex mixtures usually containing the required monoacetal (8),its conjugated enone isomer (9), the diacetal (5) and the 1-monoacetal (2). The diacetal (5) readily forms a dimer (4) in the presence of anhydrous acid, but in dioxan containing aqueous sulfuric acid it is hydrolysed to the 5-monoacetal (S), isolated in 68% yield after chromatographic separation from the dione (la). For preparative purposes the most convenient and reliable method for the preparation of (8) is conversion of (la) into the 1-cyanohydrin (34), formation of the corresponding 5-acetal (36), and elimination of HCN from the 1-cyanohydrin group with pyridine; this gave the 5-monoacetal(8) in 66% overall yield.