Studies on the Pungent Constituents in Black Pepper (Piper nigrum L.)

Part III. Piperine isomers

Abstract

The photochemical isomerization of piperine (2-trans-4-trans) and synthesis of piperine isomers (2-cis-4-trans, 2-trans-cis and 2-cis-4-cis piperine) were investigated. When piperine was irradiated in an alcoholic solution under a high pressure mercury lamp, there occurred cis-trans isomerization and this photoisomerized piperine was found to be a mixture of isopiperine (2-cis-4-trans) and iso-chavicine (2-trans-4-cis) from the data of UV, IR, NMR and chemical synthesis, and the former to be the major product. This was also certified by piperine homolog, cinnamacrylic acid piperidide which was photoisomerized in the same manner. 2-cis-4-trans piperine was easily synthesized by the method of E. Ott and F. Eichler, but 2-trans-4-cis and 2-cis-4-cis piperine were synthesized as follows. Dehydrobromination of brominated piperic acid gave trans-2-en-4-yn carboxylic acid in good yield, which was converted to its piperidide, following partial hydrogenation gave 2-trans-4-cis piperine, while partial hydrogenation after trans-2-en-4-yn carboxylic acid piperidide had been transformed to its cis form by irradiation under a high pressure mercury lamp, gave 2-cis-4-cis piperine.
The piperine isomers were less pungent than piperine and could not be detected in black pepper.