Exploitation of Mechanistic Product Selectivity for the Two‐Step Synthesis of Optically Active Bio‐Derived Cyclic Carbonates Incorporating Amino Acids

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.

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