An Unplanned Diels-Alder Cycloaddition

Although one such group is observed in II, in principle it would not find the complementary dienophile in compound I since the latter obviously plays the role of dienophile. However, that this is not always the case is exemplified 

As a consequence, the way to a [4 + 2] cycloaddition would be open, with the linking point being this carbonyl group. If dihydropyran X were formed (note the position of the C=C bond as compared to IX) and no other reagents were to be added, then compound X should contain all the structural elements 

The participation of an aldehyde intermediate was suggested by the detection of a CHO proton in the nmr spectrum of a crude distillate in a related experiment.1 This observation suggests that dienyl aldehyde XIII may be on the way to compound III. This should not be surprising since every reaction pathway of those just postulated, converge towards this aldehyde. 

As for the remaining 10% of the audience, they would not know what to do  

See also E. N. Marvel, T, Chadwick, G. Caple, T. Gosink, and G. Zimer, J. Org, Chem., 37, 2992 (1972) and references cited therein. 

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