Enhancement of Optical Rotation of Levodopa by Cyclization

Abstract

The optical rotation of levodopa is an important purity criterion, since it provides the only means to control the presence of the allegedly toxic and therapeutically inactive D-isomer. The specific rotation, [α]D25, of levodopa is only about −12°, which is insufficient to control optical purity. Barooshian et al. (1) proposed a TLC limit test for d-dopa in levodopa, involving conversion of the enantiomers to diastereomeric dipeptides. Jouin and Saias (2) and Coppi et al. (3) described methods based on stereo- specific enzymatic destruction of levodopa. The BP 1973 (4) provides requirements of [α]36520 -119 to −125° for levodopa drug substance and [α]D20 -38.5 to −41.5° for its dosage forms, the rotation being increased by addition of aluminum sulfate and sodium acetate.