Intramolecular cyclization of bis(2,4,6-trialkylphenyl) ketenes to isochromenes

Abstract

The crowded ketenes, (2,4,6-R₃C₆H₂)₂C[double bond, length half m-dash]C[double bond, length half m-dash]O, R = Prⁱ, Me, undergo intramolecular cyclization of the ketene oxygen on an o-alkyl group to give substituted isochromenes. The reaction is two orders of magnitude faster at 170 °C for R = Prⁱ than for R = Me. A stepwise reaction involving a 1,5-sigmatropic rearrangement to a conjugated tetraenal which does not accumulate and rapidly cyclizes to the isochromene is suggested.

References

1. T. T. Tidwell, Ketenes, Wiley, New York, 1995.
2. H. Heimgartner, J. Zsindely, H.-J. Hansen and H. Schmid, Helv. Chim. Acta, 1970, 53, 1212.
3. (a) R. F. C. Brown and M. Butcher, Aust. J. Chem., 1969, 22, 1457; (b) P. de Champlain, J.-L. Luche, R. A. Marty and P. De Mayo, Can. J. Chem., 1976, 54, 3749.
4. (a) R. Hug, H.-J. Jansen and H. Schmid, Helv. Chim. Acta, 1972, 55, 10; (b) This work quotes the source of this reaction as P. Gilgen, Diplomarbeit, University of Zurich, 1971.
5. P. Schiess and P. Radimerski, Helv. Chim. Acta, 1974, 57, 2583.
6. e.g. J. N. Chatterjea, Chem. Ber., 1958, 91, 2636; J. N. Chatterjea and H. Mukherjee, J. Indian Chem. Soc., 1960, 37, 443; H. C. Kristinsson, R. A. Mateer and G. W. Griffin, J. Chem. Soc., Chem. Commun., 1966, 413; C. Normant-Chefnay, Bull. Soc. Chim. Fr., 1971, 1362.
7. J. Frey and Z. Rappoport, J. Am. Chem. Soc., 1996, 118, 5169.
8. Atomic coordinates, bond lengths and angles, and thermal parameters have been deposited at the Cambridge Crystallographic Data Centre (CCDC). For details of the deposition scheme, see ‘Instructions for Authors’, J. Chem. Soc., Perkin Trans. 2, 1997, Issue 1. Any request to the CCDC for this material should quote the full literature citation and the reference number 188/88.
9. R. C. Fuson, D. H. Chadwick and M. L. Ward, J. Am. Chem. Soc., 1946, 68, 389.