Asymmetric synthesis using sulfimides
Asymmetric synthesis, the synthesis of chiral molecules, has developed into one of the most important areas of chemistry. Numerous methods are used to prepare chiral compounds, one of which involves using chiral acyl anion equivalents. The potential of imides of cyclic sulfimides 148 as chiral acyl anion equivalents was found to be limited to simple alkylations using sodium hydride and an alkyl iodide in DMF. Alkylated adducts 154,158 and 159 were prepared with good diastereoselectivity, with the anti and anti-anti geometries being preferred for 154 and 158, and 159, respectively. The conformations of the parent sulfimides 148 were investigated. We found that cyclic sulfimides (1,3,4-oxathiazines) 132 were inaccessible, which precluded our investigation into the potential of this new class of compound as chiral acyl anion equivalents. In the course of this work, the BPTM group was developed as a replacement for the troublesome PTM group as a protecting group for primary, secondary and benzylic alcohols. Vinyl sulfimides 186 were prepared using a modified Wadsworth-Emmons reaction, with good E selectivity. Additions of alcohols to give adducts 185 proceeded with good diastereoselectivity. The attempted deprotection of adduct 185b using hydrogenolysis resulted in reduction of the sulfimide group to yield protected ß-hydroxy sulfide 192. Radical additions to vinyl sulfimides 186 resulted in 2-vinyl oxa-heterocycles 202 and 210, with THE and THP as solvent, respectively. A radical addition mechanism has been proposed, but uncertainty still exists as this mechanism can not explain both triethylborane and benzoyl peroxide mediated reactions as the E/Z selectivities are different. At this stage, an ionic mechanism can not be ruled out. 2-Vinyl oxa-heterocycles 202 and 210 have been converted, using Taylor's variant of the Malherbe-Bellus reaction, into 9- and 10-membered lactones 220 and 221, respectively, which are closely related to a number of important natural products. Considerable progress has been made in developing a new asymmetric sulfimidation procedure. Promising enantioselectivites have been observed using a copper-catalysed decompsition of tosyl azide or PhI=NTs 224 into nitrenes. Interception of the nitrenes by sulfide within the chiral influence of C-2 symmetric chiral ligands 225 or 231 yielded sulfimide 65. A discrete copper-nitrene species is thought to be an intermediate in the catalytic cycle.