Synthesis utilising the β-carbonyl system. Part 5. A synthesis directed towards the fungal xanthone bikaverin
Abstract
2-[3-(1-Oxo-3-aminobut-2-enyl)-7-methoxy-5-methyl-4-oxo-4H-benzopyran-2-ylmethyl]dioxolans, (7a and b), with an ethoxycarbonylmethyl or methyl substituent on the 2-position, have been synthesised starting with three precursors corresponding to β-tetra-, β-tri-, and β-di-carbonyl compounds. Acid treatment of the 2-ethoxycarbonylmethyl compound (7a) afforded ethyl 3-(1-hydroxy-6-methoxy-3,8-dimethyl-9-oxoxanthen-2-yl)-3-oxopropanoate (13a) as the major product together with three minor products, including ethyl 2-acetyl-1-hydroxy-6-methoxy-8-methyl-9-oxoxanthen-3-ylacetate (11); whereas the 2-methyl compound (7b) yielded 2-acetyl1-hydroxy-6-methoxy-3,8-dimethylxanthen-9-one (13b), exclusively. The structures of (13a) and (17) were cosnfirmed by 13C n.m.r. Spectroscopy using the long-range selective proton-decoupling (LSPD) method. Formation of (13a) from (7a) is explained in terms of a Wessely–Moser rearrangement of the intermediary benzopyranone (9a) or the xanthone (10a) under mild acidic conditions. Base-catalysed cyclisation of the xanthone3-acetate (11) gave 8,10,11-trihydroxy-3-methoxy-1-methyl-12H-benzo[b]xanthen-12-one (20) in excellent yield.