Guidelines for β-Lactam Synthesis: Glycal Protecting Groups Dictate Stereoelectronics and [2+2] Cycloaddition Kinetics
作者:Anant S. Balijepalli、James H. McNeely、Aladin Hamoud、Mark W. Grinstaff
DOI:10.1021/acs.joc.0c00510
日期:2020.10.2
The electron-withdrawing ability of the protecting groups, but not bulk, impacts the electron density of the glycal allyloxocarbenium system when oriented pseudo-axially (i.e., stereoelectronics). In this conformation, ring σC–O* orbitals oriented antiperiplanar to the allyloxocarbenium system decrease glycal reactivity via negative hyperconjugation as protecting group electron withdrawal increases. Transition-state
The Lewis acid-mediated addition of the titanium enolate from the pivaloyl-protected (S) N-glycolyl-4-isopropyl-1,3-thiazolidine-2-thione to glycals affords the corresponding syn C-alpha-glycosides in high yields and excellent diastereomeric ratios. Further removal of the chiral auxiliary permits access to enantiomerically pure C1'-pivaloyloxy pyrans in a straightforward manner. (C) 2013 Elsevier Ltd. All rights reserved.
Stereoselective Synthesis of α- and β-C-Glycosides by Addition of Titanium Enolates to Glycals
Both α- and β-C-glycosides can be prepared by SnCl4-mediated addition of titanium enolates from N-acetylthiazolidinethiones to glycals. Subsequent removal of the chiral auxiliary provides enantiomerically pure fragments that are useful for the synthesis of natural products.