Mild Dehydrating Reagents in the Carbon Dioxide-Based Synthesis of Cyclic Carbamates from Amino Alcohols.

Conventional CO2-based carboxylation of amino alcohols to cyclic carbamates is often hindered by poor chemo- and stereoselectivity, the need for high pressure or subzero temperatures, reliance on expensive or toxic reagents and catalysts, and the formation of hazardous waste. To address these limitations, we evaluated mild dehydrating reagents for their ability to promote the cyclization. Among the reagents tested, n-butyl chloride emerged as the optimal choice, offering a desirable combination of low cost, low toxicity, and high selectivity. In contrast to conventional highly reactive dehydrating reagents, n-BuCl provides high selectivity toward carbamate anion alkylation. Upon heating, the resulting linear carbamate intermediate cyclizes, liberating benign butanol and yielding almost exclusively stereoretentive five- and six-membered cyclic carbamates in 17-98% yields.