C-Galactosylceramide diastereomers via Sharpless asymmetric epoxidation chemistry

C-Glycoside analogs of α-galactosylceramide (KRN7000) were synthesized in 19 linear steps with Sharpless asymmetric epoxidation as a key reaction. Opening of a hydroxy epoxide with sodium azide provided an anti vicinal azido diol with inversion of configuration at the azide-bearing carbon while opening with Ti(O-i-Pr)2(N3)2 gave syn vicinal azido diol with retention. The latter, unusual outcome could be rationalized either by invoking Ti-catalyzed intramolecular double SN2 inversion or by epoxide opening/intramolecular delivery of azide from the Ti complex.