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| - New Substituent Effects of the Trimethylsilyl Group: Photochemistry of 3-Trimethylsilyl-2,5-cyclohexadienones andPreparation of 4-Alkylidenecyclopentenones
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| - The 4-acetoxymethyl-4-alkyl-3-trimethylsilyl-2,5-cyclohexadien-1-ones 9a−g were prepared frommethyl 2-trimethylsilylbenzoate by the Birch reduction−alkylation reaction. Type A photorearrangements of 9a−g were regiospecific to give mixtures of two diastereomers of the corresponding5-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 givethe 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 ratherthan 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 photobehaviorat 366 and 300 nm, while the 4-(3‘-butenyl) analogue 30b gave the intramolecular 2 + 2 cycloadduct31b. The effects of the trimethylsilyl and methoxy substituents on the photochemical reactivity of2,5-cyclohexadien-1-ones are discussed from the perspective of n → p* vs π → p* character of thetriplet states of the dienones.
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