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This a [4+6] thermal cycloaddition of cyclopentadiene with cyclohepta-2,4,6-trienone. There are six atoms in the trieneone and four atoms involved on the cyclopentadiene, making it a 4+6 cycloaddition. This reaction produces 100% endo product.
J. H. Rigby, S. D. Rege, V. P. Sandanayaka and M. Kirova, J. Org. Chem., 1996, 61, 842–850.