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Free keywords:
DIELS-ALDER REACTIONS; (S)-PROLINE BENZYL ESTER; CARBONYL-COMPOUNDS; RADICAL ADDITIONS; LEWIS ACID; CONJUGATE ADDITIONS; ORGANIC-SYNTHESIS; ASYMMETRIC SYNTHESIS; RADICAL REACTIONS; AMINO ACID ESTERS; PROLINE;
Abstract:
Valine and proline benzyl ester are converted into the aliphatic and aromatic alpha-oxo amides 5 by activation of the respective alpha-oxo acid by means of N,N'-diisopropylcarbodiimide. The alpha-oxo amides 5 undergo Barbier-type reactions with differently substituted allyl halides in the presence of zinc and pyridinium toluenesulfonate in water/THF (2:1, v/v) at 4-degrees-C. Thereby, the alpha-hydroxy amides 8/9 are formed in high yields and, in the case of the proline ester amides, with diastereomer ratios of 4 - 6:1. In the absence of any allyl halide, or if the addition of the allyl moiety to the carbonyl groups is only slow, the oxo groups are reduced to secondary alcohols, thus giving rise to the alpha-hydroxy amides 12/13. During the reduction of the pyruvamide 5d also the competitive formation of the pinacol adduct 15 is observed. These results indicate that in the Barbier reactions in aqueous solution radicals are involved as intermediates.
