Patent ID: 8399614

Claim:
A compound or salt thereof having the structure of Q is selected from the group consisting of a bond, —C(O)—, or lower alkylene; R 1 is H, an amino protecting group, or at least one amino acid; and R 2 is OH, an ester protecting group, or at least one amino acid; each of R 3 and R 4 is independently H, halogen, lower alkyl, or substituted lower alkyl, or R 3 and R 4 or two R 3 groups optionally form a cycloalkyl or a heterocycloalkyl; each R a is independently selected from the group consisting of H, halogen, alkyl, —NO 2 , —CN, substituted alkyl, —N(R′) 2 , —C(O) k R′, —C(O)N(R′) 2 , —OR′, and —S(O) k R′, where k is 1, 2, or 3; and each R′ and R is independently H, alkyl, substituted alkyl or —C(O)-alkyl; wherein when Q is a bond: (i) R′ is C 3 -C 6 alkyl, substituted C 2 -C 6 alkyl or —C(O)-C 3 -C 6 alkyl when R is H or (ii) R is C 3 -C 6 alkyl, substituted C 2 -C 6 alkyl or —C(O)-C 3 -C 6 alkyl when R′ is H; or (iii) R′ and R are each substituted alkyl.