Synthesis of Xyloketals, Natural Products from the Mangrove Fungus Xylaria sp

The xyloketal family of compounds was synthesized by condensation of phloroglucinol ( 7 ) or 2,4‐dihydroxyacetophenone ( 15 ) with the enone 8a or 14 in multistep, one‐pot, domino reactions, leading to the xyloketals and 5‐demethylxyloketals, respectively. In the case of condensation with phloroglucinol ( 7 ), the mono‐, bis, and tris adducts 6 / 18 , 2 / 19 , 1 / 20 are formed; their ratios depend on the ratio of stating materials and reaction time. The ring junction of the pyran and furan rings B and C (at C‐2 and C‐6) is always cis . The cis orientation of the methyl groups at C‐2 and C‐5 is also predominant (8.5:1.5), as found in the natural products. In the bis and tris adducts ( 2 / 19 , 1 / 20 ), the relative orientation of the rings B and C can be syn and anti , with the sterically less demanding anti orientation slightly favored statistically. Xyloketal D ( 4a ) was obtained in the condensation of 15 with enantiomerically enriched enone 8a , confirming the absolute configuration of the natural product. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)