Route evaluation and development of a practical synthesis of methyl (S)-2-chloro-5,6,7,8-tetrahydropyrido[4,3-d]pyrimidine-7-carboxylate

A rapid and reliable route to methyl (S)-2-chloro-5,6,7,8-tetrahydropyrido[4,3-d]pyrimidine-7-carboxylate moiety that is useful as a synthetic scaffold is described. Previously, this Boc-protected entity was prepared in 10 chemical steps starting with L-hydroxyproline with an overall yield of 0.67%. The newly developed synthetic route provided the desired target in seven chemical steps with an overall yield up to 38%. Three main issues that needed to be addressed with the previous route were; first, the ring expansion of the L-hydroxyproline that generated an inseparable regioisomeric mixture (1.5:1) by flash chromatography; secondly, the low yielding condensation step between the keto ester and urea; thirdly, the low yielding chlorination of the desired isomer. Starting with commercially available (2-chloropyrimidin-5-yl)methanol, the new route incorporates a Knochel iodination, a Negishi cross-coupling, and a ring closure as the key steps. This new route afforded us the opportunity to deliver enantiomerically pure intermediate in support of drug discovery efforts. (C) 2020 Elsevier Ltd. All rights reserved.