Asymmetric Synthesis of Benzoin by SmI2-Mediated Enantioselective Protonation

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Abstract
Samarium (Z)-1,2-diphenylethen-1,2-diolate, which is considered to be an intermediate of a SmI2-mediated reduction of benzil, was protonated enantioselectively by quinidine to afford (R)-benzoin in 91%ee. It was found that quenching the unreacted enediolate with oxygen was crucial to obtain high enantioselectivity. The configuration of the samarium enediolate was confirmed by isolating (Z)-O,O′-diacetyl-1,2-diphenylethen-1,2-diol.