Regio- and stereoselectivity in the Lewis acid- and NaH-induced reactions of thiocamphor with (R)-2-vinyloxirane

Abstract

The reaction of the enolizable thioketone (1R,4R)-thiocamphor (=(1R,4R)-1,7,7-trimethylbicyclo[2.2.1]heptane-2-thione; 1) with (R)-2-vinyloxirane (2) in the presence of a Lewis acid such as SnCl4 or SiO2 in anhydrous CH2Cl2 gave the spirocyclic 1,3-oxathiolane 3 with the vinyl group at C(4’), as well as the isomeric enesulfanyl alcohol 4. In the case of SnCl4, an allylic alcohol 5 was obtained in low yield in addition to 3 and 4 (Scheme 2). Repetition of the reaction in the presence of ZnCl2 yielded two diastereoisomeric 4-vinyl-1,3-oxathiolanes 3 and 7 together with an alcohol 4, and a ‘1 :2 adduct’ 8 (Scheme 3). The reaction of 1 and 2 in the presence of NaH afforded regioselectively two enesulfanyl alcohols 4 and 9, which, in CDCl3 , cyclized smoothly to give the corresponding spirocyclic 1,3-oxathiolanes 3, 10, and 11, respectively (Scheme 4). In the presence of HCl, epimerization of 3 and 10 occurred to yield the corresponding epimers 7 and 11, respectively (Scheme 5). The thio-Claisen rearrangement of 4 in boiling mesitylene led to the allylic alcohol 12, and the analogous [3,3]-sigmatropic rearrangement of the intermediate xanthate 13, which was formed by treatment of the allylic alcohol 9 with CS2 and MeI under basic conditions, occurred already at room temperature to give the dithiocarbonate 14 (Schemes 6 and 7). The presented results show that the Lewis acid-catalyzed as well as the NaH-induced addition of (R)-vinyloxirane (2) to the enolizable thiocamphor (1) proceeds stereoselectively via an SN2-type mechanism, but with different regioselectivity.