Isolation, Characterization and Synthesis of a Phenyl-substituted Pyrrole Isolated from the Flavonoid Fraction of Mānuka Honey

Abstract

Compound 1, which occurs in the flavonoid fraction of mānuka honey and showed a statistical correlation with the non-peroxide antibacterial activity of the honeys, was extracted from fifteen kilograms of mānuka honey using Amberlite XAD-2 resin and liquid-liquid extraction, and isolated by a combination of Sephadex-LH20 column chromatography and HPLC. Characterization of 1 was achieved by one and 2D- 1H and 13C NMR spectroscopy and GC-MS and 1 was identified as 2-formyl-5-(2-methoxyphenyl)-pyrrole. In addition to 1, two other non-flavonoids were isolated from the flavonoid fraction and their identities confirmed as caffeic acid and p-coumaric acid. Synthesis of 9 (3-hydroxy-1-(2-methoxyphenyl)-3-(oxazol-4-yl) propan-1-one), an intermediate in the route to 1, gave a yield of 67.5% as a pale yellow crystals after crystallization from CH2Cl2/hexane. Synthesis of 1 from 9 only resulted in barely traceable amount of 1. The dominant product after recrystallization from CH2Cl2/hexane was 10 ((E)-1-(2-methoxyphenyl)-3-(oxazol-4-yl)prop-2-en-1-one) which was the dehydrated analogue of 9. The synthesis of 1 was repeated. The product mixture was fractionated on a silica gel column, followed by two cycles of preparative layer chromatography applied to the fractions which contained 1 and yield 0.36 mg of 1 (0.00179 mmol, 0.2%).