Conjugate Additions of Me<sub>2</sub>CuLi to Enantiopure γ-Hydroxy-δ-sulfinyl and Sulfonyl Pentenoates
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A systematic study of dimethyl cuprate conjugate additions to diastereoisomeric ethyl γ-hydroxy (or tert-butyldimethylsilyloxy)-δ-p-tolylsulfinyl-2-pentenoates and the analoguous sulfones showed mainly 3,4-anti diastereoselectivity when the reaction occurred in the presence of TMSCl. The π-facial diastereoselection is mainly governed by the γ-hydroxy or silyloxy group, whereas the role of the sulfur functionality is to increase the reactivity of the pentenoate system, probably by assisting the transfer of the alkyl group from the cuprate. This was evidenced by the reactions on similar systems that lack the sulfur functions. The appropriate choice of NH4OH or HCl hydrolysis in the workup allowed direct access to the open chain products or the lactones.
