Synthesis of cyclic carbonates bearing amino acid functionality is described. The use of enantiopure amino acids and glycidol, in the initial formation of the substrate, combined with the stereo-retentive mechanism of the cycloaddition with CO₂ furnishes bio-derived cyclic carbonates which display a range of optical activities. A DFT study provides important insights into the operative mechanism.

Abstract

The synthesis of bio-derived cyclic carbonates is attracting a lot of attention as the incorporation of bio-derived functionality into these compounds provides the opportunity to prepare previously unknown structures, whilst also improving their sustainability profiles. This study presents a facile preparation of diastereomerically pure bio-derived cyclic carbonates displaying a range of optical rotation values. These compounds are obtained from glycidol, amino acids and CO₂ in a facile two-step approach. Initially, the diastereomerically pure amino acid functionalised epoxides are prepared through a robust Steglich esterification of enantiopure glycidol (R or S) and an amino acid (D or L). Thereafter, in a second step, cycloaddition of the epoxide with CO₂ results in the retention of the initial stereochemistry of the epoxide, furnishing novel diastereomerically pure and optically active cyclic carbonate products. A DFT study has explained the basis of this observed retention of configuration for these compounds. Further, results from this DFT study also provide new mechanistic information concerning a co-catalyst-free cycloaddition reaction starting from glycidol when using the gallium-catalyst, which is found to operate through metal-ligand cooperativity.