Asymmetric addition of phenylzinc reagents to C-alkynyl nitrones. Enantiomeric enhancement by a product-like additive

Asymmetric addition of diphenylzinc to C-alkynyl nitrones was achieved by utilizing di(t-butyl) (R,R)-tartrate as a chiral auxiliary to afford the corresponding optically active (S)-N-(1-phenyl-3-substituted prop-2-ynyl)hydroxylamines. By the addition of a product-like additive, enantiomeric enhancement was observed. A mixed zinc reagent, PhZnMe, improved the enantioselection to afford hydroxylamines in up to 92% ee.

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(S)-N-Benzyl-N-(1,3-diphenylprop-2-ynyl)hydroxylamineC22H19NOEe = 92%[α]D25=-42 (c 1.23, EtOH)Source of chirality: (R,R)-DTBTAbsolute configuration: (S)

(S)-N-Benzyl-N-(1-phenyl-3-p-tolylprop-2-ynyl)hydroxylamineC23H21NOEe = 87%[α]D25=-45 (c 1.23, EtOH)Source of chirality: (R,R)-DTBTAbsolute configuration: (S)

(S)-N-Benzyl-N-[3-(4-bromophenyl)-1-phenylprop-2-ynyl]hydroxylamineC22H18NOBrEe = 90%[α]D25=-69 (c 0.50, EtOH)Source of chirality: (R,R)-DTBTAbsolute configuration: (S)

(S)-N-Benzyl-N-(1-phenylnon-2-ynyl)hydroxylamineC22H27NOEe = 87%[α]D25=-43 (c 1.14, EtOH)Source of chirality: (R,R)-DTBTAbsolute configuration: (S)

(R,E)-N-Benzyl-N-(1,3-diphenylallyl)hydroxylamineC22H21NOEe = 49%[α]D25=+7 (c 0.73, EtOH)Source of chirality: (R,R)-DTBTAbsolute configuration: (R)