A detailed ab initio MO investigation of the diastereoselectivities of five- and six-membered ring ketones bearing O and S, C and S, and C and O substituents at the α-carbon

The steric effects in the geometry on cation-chelation predict the experimental π-selectivity of 1-oxa-4-thiaspiro[4.5]decan-6-one, 1-oxa-4-thiaspiro[4.4]nonan-6-one, 1-thiaspiro[4.4] nonan-6-one, and 1-oxaspiro[4.4]nonan-6-one. The reversal in the selectivity of 1-oxa-4-thiaspiro[4.4]nonan-6-one on reduction with (i-Bu)2AlH appears to be a direct consequence of the steric interactions arising from the large i-Bu substituents. The antiperiplanar effects are not as significant as the steric effects.