An investigation of the diastereoselectivity of nucleophilic additions to 6-methyl-1-oxa-4-thiaspiro[4.5]dec-6-ene-7-carbaldehyde. Hybridization of the nucleophile alters the diastereoselectivity

Abstract

6-Methyl-1-oxa-4-thiaspiro[4.5]dec-6-ene-7-carbaldehyde was treated with a variety of nucleophiles under varying conditions. Whereas the 1,4-diastereoselectivity observed with sp³ nucleophiles was more or less 1.0:1.1 in favor of attack anti to sulfur, sp² and sp nucleophiles exhibited relatively much improved but reversed π-selection under the same conditions. The highest selectivity observed was 3.5:1.0 in favor of attack syn to sulfur in reaction with PhMgBr in Et₂O at −80 °C. This selectivity was reduced to 1.6:1.0 when the reaction was conducted in a 9:1 mixture of Et₂O and HMPA but enhanced to 7.2:1.0 when the polarity of the reaction medium was reduced by conducting the reaction in a 1:1 mixture of Et₂O and n-hexane. Neither the anti to S nor the syn to S diastereoselectivity obeyed the dipole model reported by Wipf and Kim. The syn to S selectivity in reactions with sp² and sp nucleophiles may be a result of significant and yet specific electrostatic attraction of S for these nucleophilic species which have their negative charges concentrated largely on the carbon for their known significant polar characters.