Synthesis of 4,4-disubstituted cyclohexenones. Part 2. Cycloaddition of 2-chloroacrylonitrile to 5-substituted 1,3-dimethoxycyclohexa-1,4-dienes

Abstract

The cycloaddition of 2-chloroacrylonitrile to 1,3-dimethoxycyclohexadienes (3; R1= OMe, R2= H), derived by in situ conjugation of the Birch reduction products (12) produced from aromatic precursors (11) gave after acid work-up mainly bicyclo[2.2.2]octanone derivatives (5) and rearranged products(13). There is a strong preference for addition to the less hindered face of (3; R1= OMe, R2= H)[to give product (5) in which R3 is ‘syn’ to the carbonyl group], especially when the R3 substituent is large, and there is also a preference for the CN group in (5) to be ‘endo’. This is illustrated most favourably in the formation of the cyclic acetals (24) produced by fluoride desilylation of the adducts (5ix) and (5iy) followed by acid cyclisation. Although the chloronitrile function in (5) could not be solvolysed (aqueous Na2S) to the corresponding ketone, this conversion could be effected after the carbonyl group in (5) had been reduced to alcohol. Lithium t-butoxyaluminium hydride reduction of ketones (5ax) and (5ay) gave diols (6ax) and (6ay) respectively whereas borohydride reduction of ketone (5bx) gave the alcohols (19a) and (19b). Alcohols (19a), (19b), (6ax), and (6ay) were then converted respectively into ketones (20a) and (20b)[for (19)] and (7a)[for (6)]. Novel by-products in the sodium sulphide solvolysis of (6ax), (6ay) were the diols (21aa) and (21ab) which were shown to arise by reduction of (7a) in the presence of sodium sulphide. Jones oxidation of keto alcohol (7a) gave (18a) and oxidation of either keto alcohol (20a) or (20b) gave the same diketone (18b). Beckmann fragmentation of the corresponding oxime gave the cyclohexenone (22) thus establishing a formal route to cyclohexenones.