Synthetic approaches to phomactins: on the stereoselectivity of some [2,3]-Wittig rearrangements

On treatment with n-butyllithium, 4-alkoxy- and 4-silyloxy-2-(tributylstannylmethoxymethyl)-1,6-dimethyl-1-(phenylsulfonylmethyl)cyclohex-2-enes undergo tin-lithium exchange followed by [2,3]-Wittig rearrangements to give 3-alkoxy- and 3-silyloxy-2-hydroxymethyl-5,6-dimethyl-1-methylene-6-(phenylsulfonylmethyl)cyclohexanes in which the 2-hydroxymethyl and 6-phenylsulfonylmethyl residues are cis-disposed about the six-membered ring. In contrast, the corresponding 1-(phenylsulfanylmethyl)cyclohexenes give mainly methylenecyclohexanes with the 2-hydroxymethyl and 6-phenylsulfanylmethyl groups trans-disposed about the six-membered ring. This stereoselectivity is independent of the nature of the alkoxy- or silyloxy-substituent and configuration at C4. The 3-tert-butyldiphenylsilyloxy-1-methylene-6-(phenylsulfonylmethyl)cyclohexane was converted into a macrocyclic precursor of the phomactins.