wherein R is a member selected from the group consisting of hydrogen, alkyl and acyl; R1 is a member selected from the group consisting of hydrogen and alkyl; or optionally, R and R1 and the nitrogen to which they are bound form an azido moiety; R2 is a member of the group selected from hydrogen, optionally substituted alkyl, optionally substituted aryl and a side chain of an amino acid; R3 is a m