New substituent effects of the trimethylsilyl group: photochemistry of 3-trimethylsilyl-2,5-cyclohexadienones and preparation of 4-alkylidenecyclopentenones

The 4-acetoxymethyl-4-alkyl-3-trimethylsilyl-2,5-cyclohexadien-1-ones 9a-g were prepared from methyl 2-trimethylsilylbenzoate by the Birch reduction-alkylation reaction. Type A photorearrangements of 9a-g were regiospecific to give mixtures of two diastereomers of the corresponding 5-trimethylsilylbicyclo[3.1.0]hex-3-en-2-ones 11a-g. These bicyclohexenones are uniquely photostable; the diastereomers do not photointerconvert nor do they undergo the type B photorearrangement. Bicyclohexenones 11a-g undergo acid-catalyzed protiodesilylative rearrangement to give the 4-alkylidene-2-cyclopenten-1-ones 25a-g. It was of interest to find that the 4-(3'-butenyl)-2,5-cyclohexadienone 9e photorearranged to the 5-trimethylsilylbicyclo[3.1.0]hex-3-en-2-one 11e rather than undergoing the intramolecular 2 + 2 photocycloaddition. Furthermore, the 4-acetoxymethyl-3-methoxy-4-methyl-5-trimethylsilyl-2,5-cyclohexadienone 30a did not show type A photobehavior at 366 and 300 nm, while the 4-(3'-butenyl) analogue 30b gave the intramolecular 2 + 2 cycloadduct 31b. The effects of the trimethylsilyl and methoxy substituents on the photochemical reactivity of 2,5-cyclohexadien-1-ones are discussed from the perspective of n --> p* vs pi --> p* character of the triplet states of the dienones.