The reaction of FK-506 (I) with Tbdms-Cl and imidazole gives the bis silylated compound (II), which is regioselectively monodesilylated by means of aq. HF in acetonitrile to yield the monosilylated compound (III). The silylation of (III) with 2-bromophenyldimethylsilyl chloride (IV) and imidazole affords the new disilylated compound (V), which is submitted to radical translocating/reducing conditions by means of Et3N/O2 or AIBN as radical initiator and Bu3Sn2H as deuterium source. Under these conditions a mixture of labeled compounds (VI) and (VII) is obtained.

This mixture is desilylated by means of aq. HOAc in THF to yield from a 41:59 up to a 68:32 mixture of labeled and unlabeled FK-506.

The reaction of ascomycin (I) with Tbdms-Cl and imidazole gives the bis silylated compound (II), which is regioselectively monodesilylated by means of aq. HF in acetonitrile to yield the monosilylated compound (III). The silylation of (III) with 2-bromophenyldimethylsilyl chloride (IV) and imidazole affords the new disilylated compound (V), which is submitted to radical translocating/reducing conditions by means of Et3N/O2 as radical initiator and Bu3Sn2H as deuterium source. Under these conditions a mixture of labeled compounds (VI) and (VII) is obtained where about 70% is the ascomycin labeled compound (VI) and 30% the aromatic labeled compound (VII).

This mixture is desilylated by means of aq. HOAc in THF to yield a 53:47 mixture of labeled and unlabeled ascomycin.