AN INVESTIGATION OF THE DIASTEREOSELECTIVITY OF NUCLEOPHILIC ADDITIONS TO HYL-1-OXA-4-THIASPIRO[4.5]DEC-6-ENE-7-CARBALDEHYDE - HYBRIDIZATION OF THE NUCLEOPHILE ALTERS THE DIASTEREOSELECTIVITY

hyl-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-dias tereoselectivity observed with sp(3) nucleophiles was more or less 1.0 :1.1 in favor of attack anti to sulfur, sp(2) and sp nucleophiles exhi bited relatively much improved but reversed pi-selection under the sam e 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 degrees C . This selectivity was reduced to 1.6:1.0 when the reaction was conduc ted 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 reactio n 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 Wip f and Kim. The syn to S selectivity in reactions with sp(2) and sp nuc leophiles may be a result of significant and yet specific electrostati c attraction of S for these nucleophilic species which have their nega tive charges concentrated largely on the carbon for their known signif icant polar characters.