Stereochemical research on the hydrolysis of optically pure spirosulfuranes: Efficient synthesis of chiral sulfoxides with completely opposite stereochemistry

The synthesis of optically pure alkoxy(acyloxy)spirosulfuranes 5a-e, using the 2-exo-hydroxy-10-bornyl group as a chiral ligand, has been developed in high yield and with excellent diastereoselectivity. The X-ray analyses of 5a and 5b indicated that the spirosulfuranes have the trigonal bipyramidal (TBP) structures around the sulfur atom. Recrystallization of 5c,d from moi st solvent (95% EtOH aq) gave sulfoxides 7c,d, respectively, as single prod ucts with an S-s absolute configuration at the sulfur atom. In contrast, hy drolysis of sulfuranes 5b,c,d under the basic conditions afforded the sulfo xides 8b,c,d, with an R-s absolute configuration at the sulfur atom, in hig h yield and with excellent diastereoselectivity. The stereochemical and mec hanistic study of the hydrolysis of spirosulfuranes was performed by using spirosulfurane 5a. Hydrolysis of 5a under acidic and basic conditions gave, diastereoselectively, the corresponding sulfoxides 7a and 8a with the oppo site absolute configuration at the sulfur atom. The structures of the sulfo xides 7a,c and 8a,b were confirmed by X-ray crystallographic analyses. The mass spectral and O-17 NMR studies of the sulfoxides 7a-(18(17))O and 8a-(1 8(17))O, which were prepared by the hydrolysis of 5a with isotopically labe led water, revealed definitely that the oxygen atom bound to the sulfur ato m in these compounds derives from water. The possible mechanisms of the rea ctions which account for the observed stereochemical results have been sugg ested.