Efficient route to (S)-azetidine-2-carboxylic acid
Futamura, Y.(Keio Univ., Tokyo (Japan)) | Kurokawa, M. | Obata, R. | Nishiyama, S. | Sugai, T.
A new and efficient route to (S)-azetidine-2-carboxylic acid (99.9% ee) in five steps and total yield of 48% via malonic ester intermediates was established. As the key step, efficient four-membered ring formation (99 %) was achieved from dimethyl (S)-(1'-methyl)benzylaminomalonate by treating with 1,2-dibromoethane (1.5 eq) and cesium carbonate (2 eq) in DMF. Krapcho dealkoxycarbonylation of dimethyl (1'S)-1-(1'-methyl)benzylazetidine-2,2-dicarboxylate, the product of this cyclization procedure, proceeded with preferential formation (2.7:1, 78% total yield) of the desired (2S,1'S)-monoester, with the help of a chiral auxiliary which was introduced on the nitrogen atom. The undesired (2R,1'S)-isomer could be converted to that with proper stereochemistry, by a deprotonation and subsequent reprotonation step. Finally, lipase-catalyzed preferential hydrolysis of the (2S,1'S)-monoester and subsequent deprotection provided enantiomerically pure (S)-azetidine-2-carboxylic acid in a 91% yield from the mixture of (2S,1'S)- and (2R,1'S)-isomers.
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