The mechanism is not yet known but the modified biscinchona alkaloid shown here is an excellent catalyst for the general and highly enantioselective 1,4-addition of thiols to cyclic enones. Selectivities ranging from 93 to >99 % ee were achieved for the addition of 2-thionaphthol to six- to nine-membered enones.
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McDaid et al. (2002) studied this question.