Pericyclic additions

 Hi, every one
There are two recent papers of contradictory opinions on the mechanism of Diels-Alder reactions if you are interested:
1. Tetrahedron Lett. 2001, 42, 5077. ?Merging of 4+2 and 2+4 cycloadditon paths in the regiospecific dimerization of methacrolein. A case of concerted crypto-diradical cycloaddition?
?Collision-induced electron reorganization as the general mechanism of concerted cycloaddition reactions?.
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