Synthesis of Deuterium-Labeled Zaleplon-d5

Zaleplon is a pyrazolopyrimidine sedative-hypnotic agent licensed for the short-term treatment of insomnia. Due to its potential for abuse and addiction, zaleplon is classified as a controlled substance in some countries. Accurate analysis and identification of zaleplon in drug abuse cases requires the use of isotopically labeled internal standards, such as zaleplon-d5.

Here, we present the synthesis of deuterium-labeled zaleplon-d5, which can serve as an internal standard for the GC-MS analysis of zaleplon. Synthesis of Zaleplon-d5 by Scheme 1

Our first route to zaleplon-d5 shown in Scheme 1.

• Initially, 3nitroacetophenone 1 was treated with N,N-dimethylformamide dimethylacetal under reflux to yield the intermediate, enamide 2.

• Another key intermediate, 5-amino-1H-pyrazole-4-carbonitrile (4) was obtained by refluxing ethoxymethylenemalononitrile (3) and hydrazine hydrate in ethanol.

• In the following step, compounds 4 and 2 underwent cyclization under mild acidic condition at reflux to yield 7-(3-nitro-phenyl)-pyrazolo[1,5-a]pyrimidine-3-carbonitrile (5).

• An efficient reduction of 5 using 10% Pd/C catalyst at an H pressure of 60 psi gave 7-(3-amino-phenyl)-pyrazolo[1,5-a]pyrimidine-3-carbonitrile (6).

• The 3-amino phenyl pyrazolopyrimidine 6 was treated with an acetic anhydride and pyridine to produce acetamide 7.

• Introduction of the isotopic label was attempted by treating acetamide 7 with ethyl iodide-din the presence of sodium hydride, under an inert atmosphere at 50℃.

Unfortunately, the yield of zaleplon-d5 obtained was negligible. Various alternative conditions were tried, but the maximum yield obtained was only 20%. The purification of zaleplon-d5 was also problematic, as zaleplon-d5 could not be successfully crystallized and was difficult to elute from a silica gel column. Therefore, this route was abandoned.

Synthesis of Zaleplon-d5 by Scheme 2

• The synthesis started from compound 1, which is readily available and inexpensive. 3'-Nitroacetophenone 1 was reduced to 3'-aminoacetophenone 9.

• Subsequently, Acylation to generate acetamide 10, which in turn was treated with N,N-dimethylformamide dimethylacetal to produce enamide 11.

• Enamide 11 was alkylated using ethyl iodide-d5 and sodium hydride as base at room temperature to yield the N-ethylated enamide 12.

• In the final step, N-ethylenamide 12 was coupled with 5-aminopyrazole 4 in aqueous acid at 50℃ to produce zaleplon-d5 8 in 85% yield.

• Further purification by recrystallization of the crude product from 30% aqueous acetic acid yielded colorless crystals of zaleplon-d5 8.

In conclusion, this work provides an elegant route to zaleplon-d5, an internal standard for zaleplon analysis. This synthesis paves the way for the quantitative detection of zaleplon in drug abusers.