Science of Synthesis

Suffert, J., Science of Synthesis, (2005) 26, 937.

The catalytic enantioselective introduction of an α-hydroxy group to a tin ketone enolate can be efficiently realized using nitrosobenzene in the presence of a Lewis acid catalyst (‌Scheme 98‌).[‌200‌] Chiral R-hydroxy ketones in up to 97% ee are obtained in high yields (64–95%). The reaction is performed in the presence of 2,2′-bis(diphenylphosphino)-1,1′-binaphthyl or 2,2′-bis(di-4-tolylphosphino)-1,1′-binaphthyl [(R)-Tol-BINAP] and silver(I) trifluoromethanesulfonate to furnish the aminooxy ketone ‌211‌ with high O-selectivity. A small amount of the N-adduct is observed in some cases (1–19%) but generally the reaction is very selective, giving more than 99% of the O-adduct. Of the various silver salts surveyed, the trifluoromethanesulfonate and perchlorate provide superior levels in both asymmetric induction and regioselection. The transformation of R-aminooxy ketones ‌211‌ into the corresponding R-hydroxy ketones ‌212‌ is smooth and facile. For example, after reaction of nitrosobenzene with a cyclohexanone-derived tin enol ether, and the cleavage of the N—O bond using a catalytic amount of copper(II) sulfate and excess methanol, (R)-2-hydroxycyclohexanone is obtained in 93% yield and 97% ee.

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