Patent ID: 6991911

Claim:
A compound of the formula: wherein: R 1 is H, lower alkyl, a protecting group, or is taken together with R 2 to form a ring, R 2 is H, lower alkyl, a protecting group, —(CH 2 ) n SCH 2 C(O)R 6 or —(CH 2 ) n C(SO 2 R 6 )═CH 2 , or is taken together with R 1 to form a ring, R 3 and R 4 are independently H or lower alkyl or a protecting group, or, when R 1 is taken together with R 2 to form a ring, at least one of R 3 or R 4 is —C(O)(CH 2 ) n R 5 , —C(O)(CH 2 ) n NHC(O)R 5 , —C(O)(CH 2 ) n NHC(O)(CH 2 ) n SR 5 , —(CH 2 ) n C(SO 2 R 5 )═CH 2 , —(CH 2 ) n SCH 2 C(O)R 5 , or —(CH 2 ) n C(SO 2 R 5 )═CH 2 , or when R 1 is not taken together with R 2 to form a ring, R 2 is not H or lower alkyl or a protecting group, R 5 is H, —OH, —SH, —O-lower alkyl, halogen, NH 2 , -succinimidyl, -maleimidyl, immunogenic carrier, or label, R 6 is H,—OH, —SH, —O-lower alkyl, halogen, NH 2 , -succinimidyl, -maleimidyl, immunogenic carrier, or label, and n is an integer from 1 to 5, and including acid salts thereof.