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Chiral Vinyl Anions for “Carbonyl Umpolung”. Highly Stereoselective Addition of a Novel Enantiomerically Pure Vinyllithium Reagent to Aldehydes
Author(s) -
Mahler Hellmut,
Braun Manfred
Publication year - 1991
Publication title -
chemische berichte
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 0009-2940
DOI - 10.1002/cber.19911240612
Subject(s) - chemistry , stereoselectivity , umpolung , synthon , enantiomer , ozonolysis , reagent , diastereomer , aldehyde , organic chemistry , medicinal chemistry , catalysis , nucleophile
The vinyllithium reagent 13 and its enantiomer are generated by a bromine/lithium exchange reaction starting from dibromoalkenes 11 and 12 , both available from the corresponding enantiomer of alkyl lactate. When 13 is allowed to react with aldehydes or with acetophenone, a highly stereoselective addition to the Re face of the carbonyl compounds occurs to give predominantly the diastereomers 15 . Alkenes 25a,c , accessible by another bromine/lithium exchange reaction of 15a,d and subsequent protonation, can be cleaved by ozonolysis followed by reduction to afford carbinols 27a, b in > 98% ee. The sequence corresponds to a stereoselective introduction of a methanol d 1 synthon ( ⊖ CH 2 OH) or, as shown by other examples, of acyl and formyl d 1 synthons ( ⊖ CRO and ⊕ CHO) into prochiral carbonyl compounds. As a consequence, 13 and its enantiomer may be regarded as highly stereoselective reagents for "carbonyl umpolung". A series of further vinyllithium reagents, 38a ‐ g and 54/55 , is treated with benzaldehyde, but none of those displays comparable enantiofacial selectivity. The prerequisites to the highly stereoselective reactions of 13 and its enantiomer are briefly discussed.