E-isomer 2-isomer confirmation for the structure of 6a derives from its hydrolysis with 10% HC1 to the corresponding knownll ester 7. The condensation of ortho ester 4a with N- benzyloxycarbonyl-L-phenylalanine p-nitrophenyl ester12 (8) in the presence of triethylamine in CH2Clz gave the dipeptide ortho ester 9 in 63% yield. The latter was characterized by spectral (ir and NMR) data which confirmed the presence of urethane ...