Xylocarpins A and B, two new mexicanolides from the seeds of a Chinese mangrove Xylocarpus granatum : NMR investigation in mixture

Xylocarpins A and B, two new mexicanolides with a tiglate group at C‐3, have been identified in the mixture using NMR spectroscopy. Both compounds were isolated in the mixture from the seeds of a Chinese mangrove Xylocarpus granatum . The first complete assignments of 1 H and 13 C NMR data for these mexicanolides were achieved by means of 2D NMR techniques, including 1 H 1 H COSY, HSQC, HMBC and NOESY spectra. In order to separate xylocarpins A (1) and B (2) by chemical method, the mixture of two compounds was reduced with sodium borohydride in anhydrous methanol. However, the reduction led to the opening of the δ‐lactone ring in xylocarpin B and afforded compound 3 as the main product. The complete NMR assignments of compound 3 were also achieved by means of the above 2D NMR techniques. Moreover, xylocarpin A was easily transformed into xylocarpin B during our normal liquid column chromatography. From this point of view, xylocarpin A was deemed to be the genuine natural product and xylocarpin B might be an artifact. Copyright © 2007 John Wiley & Sons, Ltd.