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2. a For detailed discussion see “Heterocyclic Compounds, Chromenes, Chromanones and Chromones”, G. P.Ellis, ed, Wiley-Interscience, New York, 1977, pp 511, 611, 635, 639, 815 and references therein;
3. b G. J. P. Becket, G. P. Ellis and M. I. Trindade, J. Chem. Res. (S), 47 (1978).
4. R. M. Moriarty, H. Hu and S. C. Gupta, Tetrahedron Letters, 1283 (1981);
5. R. M. Moriarty, L. S. John and P. C. Du, J. Chem. Soc., Chem. Commun., 641 (1981);