Trimethyl orthoformate → methyl L-prolinate hydrochloride

To a solution of L-Proline (115 g,1 mol) in methanol (1.2 L) at 0° C. was added sulfuryl chloride (45 mL, 0.55 mol) slowly during ~15-20 min, then the reaction mixture stirred overnight at room temperature. After 16 hours at room temperature and another 4 hours at reflux, trimethyl orthoformate (120 mL, 1.1 mol) was added and heating at reflux continued for another 24 hr. The reaction mixture was then concentrated and the resultant oil was placed under high vacuum for several days to afford the product as a clear oil (205 g).