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Issue 0, 1983
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Synthesis and reactions of 4-aryloxyflavans

Abstract

4α-Aryloxyflavans unsubstituted in ring A have been synthesised by the reaction of phenols with flavan-4β-ols in the presence of boron trifluoride in ether. If reaction times are prolonged beyond disappearance of the starting 4β-ols, thermodynamic control leads to 4-arylflavans and the yields of 4-aryloxyflavans are negligible. 4-Arylflavans are the sole products when the catalyst is toluene-p-sulphonic acid. Thermal decomposition of flavan-4β-yl phenol carbonate in the presence of phenols affords a synthesis of 4α-aryloxyflavans free from 4-arylflavans. Syntheses of 7-methoxy-4α-aryloxyflavans have not been successful, nor can 4-aryloxyflavan-3-ols be obtained from 3,4-diols by these methods. The 4-aryloxyflavans react rapidly with acids to yield 4-carbocations which can be trapped by a variety of nucleophiles yielding 4α-ols with water, 4α-alkoxyflavans with alcohols, 4α-arylflavans with phenols, and 4α-sulphides with thiols.

2,3-cis-Flav-3-ene epoxide reacts with phenols to give 2,3-cis-3,4-cis-4-aryloxyflavan-3-ols and with sodium salts of phenols to give 2,3-cis-3,4-trans-4-aryloxyflavan-3-ols. The 2,3-trans-epoxide similarly gives the 2,3-trans-4-aryloxyflavan-3-ols. A biflavonoid containing a 4-aryloxy link has been synthesised from the 2,3-trans-epoxide and 7-hydroxyflavan-4-one. A series of 2,3-trans-4′-methoxy-4-aryloxyflavan-3-ols has been synthesised from crude 2,3-trans-4′-methoxyflav-3-ene epoxide. The substitution of a methoxy group into position 7 of the flavonoid A-ring prevented the preparation of the epoxides, but the action of N-bromosuccinimide and sodium acetate in acetic anhydride and acetic acid on 7,4′-dimethoxyflav-3-ene gave 2,3-cis- and 2,3-trans-4-acetoxy-3-bromo-7,4′-dimethoxy-flavans, the latter of which was converted by the action of sodium salts of phenols into the expected 4-aryloxyflavan-3-ols. 5,7,3′,4′-Tetramethoxyflav-3-ene gave nuclear brominated products even with N-bromosuccinimide; thus the synthesis of 5,7,3′,4′-tetramethoxy-4-aryloxyflavan-3-ols has not been possible by this method.

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Article type: Paper
DOI: 10.1039/P19830002903
Citation: J. Chem. Soc., Perkin Trans. 1, 1983,0, 2903-2912

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    Synthesis and reactions of 4-aryloxyflavans

    G. Bateman, B. R. Brown, J. B. Campbell, C. A. Cotton, P. Johnson, P. Mulqueen, D. O'Neill, M. R. Shaw, R. A. Smith and J. A. Troke, J. Chem. Soc., Perkin Trans. 1, 1983, 0, 2903
    DOI: 10.1039/P19830002903

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