Patent ID: 8461313

Claim:
A compound having formula XXV: wherein each of R 5 and R 7 is independently selected from the group consisting of hydrogen, alkyl and a nitrogen protecting group; each of R 15 , R 16 , R 17 and R 18 is independently selected from the group consisting of O, S, alkyl, substituted alkyl, aryl, substituted aryl and halogen; X 1 is —CH(X 1′ ) or a group of formula XXVI: wherein R 4 is a protecting group and n is an integer from 0 to 200; X 1′ is a protected or unprotected side chain of a naturally occurring or non-naturally occurring amino acid; X 2 is selected from the group consisting of amide, alkyl, or carbonyl containing linker and a bond; and X 3 is a degradable linker which is optionally absent.