An efficient stereoselective synthesis of 6-alpha-aminoestradiol: preparation of estradiol fluorescent probes.

6-Oxoestradiol (2) was protected as its bis[(2-trimethylsilylethoxy)methyl] ether (4) and converted to the corresponding oxime (4). The oxime (4) on reduction with zinc in ethanol afforded the bis-SEM ether of 6-alpha-aminoestradiol (5) in 96% epimeric excess. Subsequently, 5 was hydrolyzed with HF to 6-alpha-aminoestradiol (6) in good yield. The absolute stereochemistry of the amino group in 6 was established by NMR and confirmed by X-ray crystallography on the corresponding 4-bromobenzamide derivative (9). Treatment of amine (6) with 6-(t-butoxycarbonylamino)hexanoic acid succinimidyl ester (10) followed by hydrolysis produced the amine (12) with a C-6 linker. The fluorescent probes (7 and 13) were prepared from 6 and 12 respectively, in 54-60% yield and > 99% purity.