Pericyclic additions

["Yuan-Yuan chen" <yuan_yuanchen' at \`hotmail.com>]

 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?
 2. http://preprint.chemweb.com/orgchem/0107002
	?Collision-induced electron reorganization as the general mechanism
 of
 concerted cycloaddition reactions?.
 Yuan-Yuan
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