Trans-cyclooctene amino and hydroxy acids and their use in multiple cycloaddition reactions for labeling of molecules

A compound of the formula:

• or an acid addition salt or base addition salt thereof,
• wherein:
• X1 is a group of the formula:
• X2 is —CH2—or —O—;
• X3 is a single bond or —(CH2CH2O)m—;
• X4 is —C(O)NH— or —NHC(O)—;
• X5 is —(CH2)n—;
• X6 is hydrogen, C1-C6 alkyl, C1-C6-alkoxy-C1-C2-alkyl, C2-C7-alkanoyloxy-C1-C2-alkyl or C2-C7-alkanoylsulfanyl-C1-C2-alkyl;
• R1 is hydrogen, halogen, C1-C4-alkyl, (RaO)2P(O)OC1-C4-alkyl, (RbO)2P(O)C1-C4-alkyl, CF3, CN, OH, C1-C4-alkoxy, OCF3, C2-C5-alkenoxy, C2-C5-alkanoyloxy, C1-C4-alkylaminocarbonyloxy or C1-C4-alkylthio, C1-C4-alkylamino, di-(C1-C4-alkyl)amino, C2-C5-alkenylamino, C2-C5-alkenyl-C1-C4-alkylamino or di-(C2-C5-alkenyl)amino;
• R4 is —OH or —NH2;
• Ra is hydrogen or C2-C5-alkanoyloxymethyl;
• Rb is hydrogen or C2-C5-alkanoyloxymethyl;
• m is 1;
• and
• n is 1, 2, 3 or 4.