When II or Va was heated above 100°, equilibrium between tropone, I, II, and Va was rapidly attained before formation of XI was noted, so that no information is available as to the origin of XI. However, the presence of XVII was never detected by nmr, and the direct Cope rearrangement of II to XI is sterically impossible, so that the concerted formation of XI is implied.A second example now exists of competing exo [6 + 4] and endo [4 + 2] cycloadditions, lending further support to the Woodward-Hoffmann explanation of exo-endo phenomena.2,14Acknowledgment.The financial support of the National Institutes of Health and the National Science Foundation (Harvard) and the donors of the Petroleum Research Fund, administered by the American Chemical Society (L. S. U.), is gratefully acknowledged.
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