alpha-Chloro- and alpha-bromoalkyl Grignard reagents 11 and 30 with > 97% ee (enantiomeric excess) were generated by a sulfoxide/magnesium exchange reaction from the enantiomerically and diastereomerically pure sulfoxides 25 and 27. The resulting alpha-haloalkyl Grignard reagents are configurationally stable at -78 degrees C. Racemization sets in at or above -60 degrees C, especially when the solution
Iterative Stereospecific Reagent-Controlled Homologation of Pinacol Boronates by Enantioenriched α-Chloroalkyllithium Reagents
作者:Paul R. Blakemore、Matthew S. Burge
DOI:10.1021/ja068808s
日期:2007.3.1
Reaction of pinacol boronates with putative enantioenriched alpha-chloroalkyllithium species, generated in situ from homochiral alpha-chloroalkylsulfoxides by sulfoxide ligand exchange with t-BuLi in PhMe at -78 degrees C, gave chain-extended boronic ester products with generally excellent stereochemical fidelity. Iteration of this stereospecific reagent-controlled homologation (StReCH) process enabled the programmed synthesis of all four stereoisomers of a stereodiad-containing model system (4-benzyl-1,6-diphenylhexan-2-ol) with er >= 97:3 in all cases.