i) When Y=NH2 and P2 is a protecting group such as benzyl, and when Z is for example ???(CH2)n???CO2H (n=1-4) then when n=1, dibenzyl L-aspartic acid is used; when n=2, L-glutamic acid dibenzyl ester is used and; when n=3, L-2-amino adipic acid dibenzyl ester is used.