Synthesis of deuterium labeled NMDA receptor inhibitor – 20-Oxo-5β-[9,12,12-2H3]pregnan-3α-yl-l-glutamyl 1-ester

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 followed by a deuterium exchange, conducted under basic conditions using deuterated methanol. Next, the carbonyl moiety at C-11 was reduced and the 11α-hydroxyl group removed through utilization of the Barton–McCombie reaction. Subsequent deprotection of the 3- and 20-acetals and stereoselective reduction of the 3-oxo group gave the desired trideuterated pregnanolone (8). This was coupled with protected glutamic acid, which was then deprotected to yield [9,12,12-2H3]-pregnanolone glutamate (11) with >99% isotopic purity.

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► Synthesis of the deuterated inhibitor of NMDA receptor.
► Efficient method for introduction of three deuterium atoms into positions 9 and 12 of steroid skeleton.
► Deoxygenation of 11-hydroxy substituent via the Barton–McCombie reaction.