Fluorescent Pigment and Phenol Glucosides from the Heartwood of <italic>Pterocarpus marsupium</italic>

The fluorescence shown by extracts of the heartwood of Pterocarpus marsupium is attributed to salts of the new compound 1, whose structure was elaborated using detailed spectroscopic/ spectrometric studies. The plant material also contains the nonfluorescent compounds 2 and 3. The absolute configuration of 1 was determined by experimental and theoretically calculated electronic CD spectra, while that of 3 was deduced from ECD comparison with reported results in the α-hydroxydihydrochalcone series. T heartwood of Pterocarpus marsupium Roxb. (Fabaceae) is endowed with various medicinal properties, and these features are revealed in recent patents for herbal preparations useful in diabetes. The known constituents represent various flavonoids, diphenylpropane derivatives, and sesquiterpenes. Since flavone glycosides and plant extracts rich in this class of compounds are suggested to be useful for therapy and prophylaxis of diabetes, similar constituents of this plant have been under intense investigation. Our primary interest was, however, in the polar constituents of the plant, as the heartwood has been reported to contain an uncharacterized water-soluble fluorescent material. Fluorescence spectroscopy is regarded as an important tool for the identification of timbers. Indeed a study on the fluorescence characteristics of the dry wood of this plant and its methanolic extract showed distinctive differences, suggesting a detailed examination of the pure constituent responsible for the fluorescence phenomenon. However, isolation of the fluorescent constituent has not been reported by earlier investigators, except for a preliminary finding. Purification through a combination of Biogel P-2 chromatography and preparative paper chromatography of the crude extract yielded a sample exhibiting an emission maximum at 467 nm with an excitation maximum at 428 nm. Its mass and NMR spectra suggested it to be a phenolic glycoside of molecular weight 466 having an aliphatic appendage. It was, therefore, necessary to develop a more convenient method of purification to obtain the material in sufficient quantities for proper structural investigation. The details of this study are described herein. ■ RESULTS AND DISCUSSION The heartwood of P. marsupium was extracted with MeOH. The dried MeOH extract was washed with EtOH and then re-extracted with MeOH to concentrate the fluorescent compound(s). Repeated chromatography of this over Diaion HP-20 yielded nonfluorescent material in the initial fractions eluted with H2O and the fluorescent material in subsequent fractions. Further elution with 20−30% MeOH in H2O produced a brown solution showing the same fluorescent spot on TLC and containing 1. Rechromatography of the initial fractions over silica gel afforded compound 2. Fractions eluted with 50% aqueous MeOH from the Diaion column, after further chromatography on silica gel, yielded compound 3. Received: November 9, 2011 Published: February 29, 2012 Article