The biosynthesis of cocaine has long attracted the attention of biochemists and organic chemists. This interest is partly motivated by the strong physiological effects of cocaine, but a further incentive was the unusual bicyclic structure of the molecule. The biosynthesis can be viewed as occurring in two phases, one phase leading to the N-methylpyrrolinium ring, which is preserved in the final product. The second phase incorporates a C4 unit with formation of the bicyclic tropane core.[1]
The biosynthesis begins with L-glutamine, which is derived from L-ornithine in plants. The roles of L-ornithine and L-arginine was confirmed by Edward Leete.[2] Ornithine then undergoes a PLP-dependent decarboxylation to form putrescine. In animals, however, the urea cycle derives putrescine from ornithine. L-Ornithine is converted to L-arginine,[3] which is then decarboxylated via PLP to form agmatine. Hydrolysis of the imine derives N-carbamoylputrescine followed with hydrolysis of the urea to form putrescine. The separate pathways of converting ornithine to putrescine in plants and animals have converged. A SAM-dependent N-methylation of putrescine gives the N-methylputrescine, which then undergoes oxidative deamination by the action of diamine oxidase to yield the aminoaldehyde, which spontaneously cyclizes to N-methyl-Δ1-pyrrolinium cation.
Beyond its role in cocaine, the N-methyl-pyrrolinium cation is a precursor to nicotine, hygrine, cuscohygrine, and other natural products.[1]
The additional carbon atoms required for the synthesis of cocaine are derived from acetyl-CoA, by addition of two acetyl-CoA units to the N-methyl-Δ1-pyrrolinium cation.[4] The first addition is a Mannich-like reaction with the enolate anion from acetyl-CoA acting as a nucleophile towards the pyrrolinium cation. The second addition occurs through a Claisen condensation. This produces a racemic mixture of the 2-substituted pyrrolidine, with the retention of the thioester from the Claisen condensation. In formation of tropinone from racemic ethyl [2,3-13C2]4(Nmethyl- 2-pyrrolidinyl)-3-oxobutanoate there is no preference for either stereoisomer.[5] In the biosynthesis of cocaine, however, only the (S)-enantiomer can cyclize to form the tropane ring system of cocaine. The stereoselectivity of this reaction was further investigated through study of prochiral methylene hydrogen discrimination.[6] This is due to the extra chiral center at C-2.[7] This process occurs through an oxidation, which regenerates the pyrrolinium cation and formation of an enolate anion, and an intramolecular Mannich reaction. The tropane ring system undergoes hydrolysis, SAM-dependent methylation, and reduction via NADPH for the formation of methylecgonine. The benzoyl moiety required for the formation of the cocaine diester is synthesized from phenylalanine via cinnamic acid.[8] Benzoyl-CoA then combines the two units to form cocaine.
The synthesis and structure elucidation of cocaine was reported by Richard Willstätter in 1898.[9] Willstätter's synthesis derived cocaine from tropinone. Robert Robinson and Edward Leete also made significant contributions.[10]