20-Oxo-5β-[9, 12, 12-2H3] pregnan-3α-yl-l-glutamyl 1-ester 11 was synthesized as an internal standard for quantification of a neuroprotective NMDA receptor ligand, 20-oxo-5β- pregnan-3α-yl-l-glutamyl 1-ester 18 and its metabolites, in plasma and tissue. 11α-Hydroxy- progesterone (1) was reduced under basic conditions to yield the corresponding 5β-steroid. Protection of the 3-and 20-oxo groups and oxidation of the 11α-hydroxy group was then ...