Patent ID: 7964181

Claim:
A compound having a formula of structure I: or a synthetically acceptable salt thereof, wherein: R 1 is a group of a formula: R 2 is H or alkyl; R 3 is H or a first nitrogen protecting group; R 4 is H, alkyl, (CH 2 ) m C(═O)OH, (CH 2 ) m C(═O)N(H)R 11 , (CH 2 ) m C(═O)OR 11 , (CH 2 ) q OH, (CH 2 ) q OBn, (CH 2 ) q Oallyl, (CH 2 ) m C(═O)N(R 8 ) 2 , or (CH 2 ) m C(═O)N(R 8 )(CH 2 ) p N(R 8 ) 2 ; R 5 is H or alkyl; R 6a is H, alkyl, (CH 2 ) m C(═O)OH, (CH 2 ) m C(═O)N(H)R 11 , (CH 2 ) m C(═O)OR 11 , (CH 2 ) q OH, (CH 2 ) q OBn, (CH 2 ) q Oallyl, (CH 2 ) m C(═O)N(R 8 ) 2 , or (CH 2 ) m C(═O)N(R 8 )(CH 2 ) p N(R 8 ) 2 ; R 6b is H or alkyl; provided that one of R 4 and R 6a is H or alkyl and the other of R 4 and R 6a is (CH 2 ) m C(═O)OH, (CH 2 ) m C(═O)N(H)R 11 , (CH 2 ) m C(═O)OR 11 , (CH 2 ) q OH, (CH 2 ) q OBn, (CH 2 ) q Oallyl, (CH 2 ) m C(═O)N(R 8 ) 2 , or (CH 2 ) m C(═O)N(R 8 )(CH 2 ) p N(R 8 ) 2 ; R 7 is H, C(═O)alkyl, C(═O)(CH 2 ) m (NR 8 ) 2 , alkyl, aralkyl, or aryl; each occurrence of R 8 is independently H, aryl, or alkyl; R 11 is a peptide solid support; R 12 is H or a second nitrogen protecting group; each occurrence of m is an independent integer having a value between 0 and 6; each occurrence of q is an independent integer having a value between 1 and 6; p is an integer having a value between 1 and 10; and y is 0 or 1.