Theoretical evidence for pyramidalized bicyclic serine enolates in highly diastereoselective alkylations.

Carlos Aydillo, Gonzalo Jiménez-Osés, Jesús H Busto, Jesús M Peregrina, María M Zurbano, Alberto Avenoza

Index: Chemistry 13(17) , 4840-8, (2007)

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Abstract

A new chiral serine equivalent and its enantiomer have been synthesized from (S)- and (R)-N-Boc-serine methyl esters (Boc: tert-butyloxycarbonyl). The use of these compounds as chiral building blocks has been demonstrated in the synthesis of alpha-alkyl alpha-amino acids by diastereoselective potassium enolate alkylation reactions and subsequent acid hydrolyses. Theoretical studies were performed to elucidate the stereochemical outcome of both the formation of five-membered cyclic N,O-acetals and the subsequent alkylation process, which occurs with total retention of configuration. This feature could be explained in terms of the high degree of pyramidalization of enolate intermediates.