Competing Diels–Alder intramolecular pathways from cross-conjugated trienes: Fused hydrindenones (bicyclo[4.3.0]nonenones) vs. bridged-ring (bicyclo[3.3.1]nonenones) adducts from a diene-transmissive precursor

D-Isoascorbic acid was converted to the tetraenone 12 to examine its behaviour in the intramolecular Diels–Alder reaction with pendant dienes in a diene-transmissive environment. These studies were directed toward the synthesis of the hydrindenone nucleus via a tether-controlled Diels–Alder reaction. It was anticipated that this would lead to a diene-transmissive strategy for steroids such as Desogestrel®. In contrast to our previous studies with decalins, the dominant isomer (150 °C) was the bicyclo[3.3.1]nonadieneone adduct 23, and none of fused adduct 27 was detected. These products were accompanied by two minor hydrindenone adducts 24 and 25. The isomer ratio was dependent on the temperature of the reaction.Key words: Diels–Alder, hydrindane, pentadienylation, intramolecular cycloaddition, bicyclo[3.3.1]nonenone.