Diastereoselectivity torsional effects

These results have been interpreted in terms of the stereodivergent nature of the two fast-equilibrating and differently reactive envelope conformations of such cyclopentenes. Specifically, torsional effects may dictate the ul topicity of the former substrates, whereas normal steric factors may account for the opposite diastereoselection of the latter cases. A similar syn selectivity has been observed in the osmylation of other cyclopentene derivatives103 and of f/.v-disubstituted cyclobutenes102. 

The force-field calculation confirms complete exo diastereoselection in both cases especially in the [3.2.1] system 7, enhanced conformational rigidity and torsional effects ( norbor-nene effect ) are responsible for the calculated extremely high d.r. (99.9998 0.0002 at 0°C). 

The orbital tilting argument implies that the size of the dienophile should have no influence on the diastereoselectivity, hence variations in stereoselectivity with different dienophiles could not be explained. More recently an explanation based on torsional and steric effects has been advanced that permits predictions of stereoselectivities for as yet unstudied cases. A computational model has also been developed to provide semiquantitative predictions of stereoisomer ratios in reactions of substituted species118. 

One of the most important lessons taken from this body of work has been the excellence of carbohydrates as a teaching ground for many of the fundamental concepts of modem organic chemistry, including the concepts of ion pairs, of transient intermediates, of neighboring group participation and of its sister anchi-meric assistance, of torsional strain and interactions, of so-called stereoelectronic effects, and of double diastereoselection. 

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