Stereoselective titanium-mediated aldol reactions of (S)-2-tert-butyldimethylsilyloxy-3-pentanone
摘要:
Titanium-mediated aldol reactions based on (S)-2-tert-butyldimethylsilyloxy-3-pentanone, a lactate-derived chiral ketone, provide the corresponding 2,4-syn-4,5-syn adducts in high yields and diastereomeric ratios with a wide array of achiral and chiral aldehydes. Furthermore, spectroscopic studies of intermediates involved in the process have permitted to propose a mechanism that accounts for the experimental results. (c) 2006 Elsevier Ltd. All rights reserved.
[reaction: see text]Comprehensive analysis of the enolization of alpha-silyloxyketones by Chx2BCl/R3N has allowed us to design stereoselective Chx2BCl-mediated aldol processes that afford syn or anti aldol products and to disclose a hypothesis that accounts for the subtle effects that determine their enolization.
Aldol-like reactions of titanium enolates derived from α-OH, α-OBn, and α-OTBS ketones with a series of aldehydes have been studied. In sharp contrast to the O-benzyl derivatives, the TBS-protected ketones lead to excellent yields and selectivities ( ratios from 30:1 to >95:1) even for the lactate-derived substrate (2-tert-butyldimethylsilyloxy-3-pentanone).