Synthesis of 4,4-disubstituted cyclohexenones. Part 3. The reaction of 1,3-bis(trimethylsilyloxy)cyclohexa-1,3-dienes with dienophiles. An unexpected rearrangement of the adducts from the reaction with 2-chloroacrylonitrile

Abstract

The cycloaddition of 2-chloroacrylonitrile to 1,3-bis(trimethylsilyloxy)cyclohexa-1,3-dienes (3) occurs in good yield to give after base-catalysed silyl enol ether cleavage the adducts (4). Cycloaddition with acrylonitrile occurs in moderate yield, while more reactive dienophiles such as nitroethylene and E-nitroacrylate lead to adducts which rapidly rearrange or decompose. The cycloaddition of 2-chloroacrylonitrile to diene (3b) in toluene, hexane, or dichloromethane in the presence of Lewis acids such as Me₂AlCl or TiCl₄, occurs in high yield at –78 °C with great syn-selectivity and a slight preference for the exo-nitrile stereochemistry. Desilylation of adducts (4a) and (4b) gave respectively the alcohols (8a) and (8b) and the novel bicyclic ether rearrangement products (9a) and (9b) whose yield could be maximised by carrying out the desilylation with Bu₄NF in the presence of 4 Å molecular sieves. The diols (14a) and (14b) prepared by borohydride reduction of ketones (4) did not undergo this rearrangement thereby confirming that the presence of the carbonyl group in (4) was essential. Borane–THF reduction of the alkoxyenones (9) caused the unexpected formation of the cyclohexenes (15). Lewis acid-catalysed cycloaddition of 2-chloronitrile to the methoxysilyloxydiene (16a) gave the adduct (17) in moderate yield with concomitant formation of rearrangement products (18) and (19). The corresponding addition to (16b) was not promising.