Practical syntheses of both enantiomers of cyclopropylglycine and of methyl 2-cyclopropyl-2-N-Boc-iminoacetate

A facile three-step synthesis of racemic cyclopropylglycine in multigram quantities from inexpensive cyclopropyl methyl ketone has been elaborated. Enzymatic hydrolysis of the N-Boc-protected methyl ester of cyclopropylglycine 9 with the inexpensive enzyme papain from Carica papaya affords both enantiomers of cyclopropylglycine (8) with enantiomeric excesses of 99% or better after deprotection under acidic conditions. Furthermore, the new cyclopropyl group-containing building block methyl 2-cyclopropyl-2-N-Boc-iminoacetate (13) was prepared by N-chlorination and subsequent dehydrochlorination with 1.8-diazabicyclo[5.4.0]undec-7-ene (DBU). Addition of nucleophiles to 13 offers a ready access to an unusual. orthogonally bisprotected alpha,alpha-diamino acid derivative and interesting components of rigid peptide backbones.