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Synthesis of [N-methyl-11C]-3-[(6-dimethylamino)pyridin-3-yl]-2,5-dimethyl-N, N-dipropylpyrazolo[1,5-a]pyrimidine-7-amine: A potential PET ligand for in vivo imaging of CRF1 receptors

✍ Scribed by J.S. Dileep Kumar; Vattoly J. Majo; Jaya Prabhakaran; Norman R. Simpson; Ronald L. Van Heertum; J. John Mann

Publisher
John Wiley and Sons
Year
2003
Tongue
French
Weight
138 KB
Volume
46
Category
Article
ISSN
0022-2135

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✦ Synopsis

Abstract

A convenient synthesis of [N‐methyl‐^11^C]‐3‐[(6‐dimethylamino)pyridin‐3‐yl]‐2,5‐dimethyl‐N,N‐dipropylpyrazolo[1,5‐a]pyrimidine‐7‐amine (R121920), a highly selective CRF~1~ antagonist has been developed as a potential PET ligand. 3 ‐ [(6 ‐ methylamino)pyridin ‐ 3 ‐ yl]‐2,5‐dimethyl‐N,N‐dipropylpyrazolo [1,5‐a]pyrimidine‐7‐amine (7), the precursor for radiolabelling was synthesized through a novel palladium catalyzed Suzuki coupling of aryl bromide 5 with heteroaryl boronate ester 4. The requisite boronate ester 4 was synthesized in four steps from 2‐amino‐4‐bromopyridine in 50% overall yield. Although the synthesis of cold R121920 proceeded in 93% yield by sodium hexamethyl‐disilazide (NaHMDS) mediated N‐methylation of the desmethylamine 7 at −78°C, the attempted radiosynthesis under various conditions using conventional bases were not successful. However, the radiolabeling of [^11^C]R121920 was successfully carried out with [^11^C]MeOTf in acetone at −20°C in the absence of added basic reagents. The radiotracer was purified by RP‐HPLC followed by RP‐solid phase extraction. The yield of the reaction was 5% (at EOB) and the specific activity was >1000 Ci/mmol (at EOB) with a radiochemical purity >99%. Copyright © 2003 John Wiley & Sons, Ltd.

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