Synthesis of the four stereoisomers of cyclobutane analogues of phenylalanine in enantiomerically pure form

All stereoisomers of 1-amino-2-phenylcyclobutanecarboxylic acid-c(4)Phe-have been synthesized and the series c(n)Phe has thus been completed. The use of two different strategies based on a cyclization reaction, starting from ethyl isocyanoacetate and dialkyl malonate, respectively, gave both cis-c(4)Phe and trans-c(4)Phe in racemic form. HPLC resolution of one of the intermediates using a cellulose-derived chiral stationary phase allowed the isolation of the corresponding enantiomerically pure N-protected amino acids, prepared for incorporation into peptides. The relative stereochemistry of enantiopure compounds has been unambiguously assigned. (c) 2005 Elsevier Ltd. All rights reserved.