Treatment of aldehydes and an α,β-unsaturated thioester with a catalytic amount of Me2S in the presence of TiCl4 followed by Ti(O-i-Pr)4 or DBU gave α-methylene-β-hydroxy esters or thioesters, respectively, in moderate to good yields. DABCO and Bu3P were ineffective for the Baylis-Hillman reaction of a thioacrylate.
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Kataoka et al. (1999) studied this question.