We then studied the effects of the exocyclic sulfur substituent (R 1 ) of the SIA on imparting enantioselectivity in the reaction. Electrophiles E2 and E4 were selected to probe the selectivity. In general, all the products resulting from E2 reacting with R 1 substituted SIAs are highly enantiomerically enriched yielding products 97:3 e.r. or greater. However, enantioselectivities with substituted R 1 SIA heterocycles react with E4 in either comparable selectivity to 5 or worse. The enantioselectivity reacting E4 and the p-biphenyl substituted SIA was essentially the same (88:12 e.r.) compared to 5. The alkyl isopropyl SIA reacts with lower selectivity (75:25 e.r.) compared to 5. A small decrease in the enantioselectivity was observed with the reaction of m-fluoro-o-methyl substituted SIA, product 16 (84:16 e.r.). The lowest enantioselectivity observed was obrosyl SIA reacting with E4, product 25 (69:31 e.r.). It is clear that ortho bromine substitution has a significant effect on the desymmetrization process compared to ortho methyl substitution. Overall, para substituted aryl groups do not affect the reaction, while ortho substituted aryl groups and sterically