Dienes-cyclobutene requirements

A pathway for the formation of (66) via ring-opening of (67) is favoured by most authors.142 However, the formation of dienes (68) requires a different mechanistic rationalization and the authors of this study suggest that cyclobutenes (67) are not necessary intermediates in the skeletal rearrangement of enynes (64). 

We can consolidate the above two analyses In the transformation of a 1,3-diene into a cyclobutene or vice versa, the reaction must be accomplished photochemically if the desire is to effect a disrotatory ring closure or thermally to allow a conrotatory ring closure. The choice of one rotation over the other is dependent purely on the stereochemical requirement of the product. This notion becomes amply clear from the following transformations. 

An electrocyclic reaction is as easy to analyze as that. Identify the HOMO, and then see whether conrotatory or disrotatory motion is demanded of the end carbons by the lobes of that molecular orbital. All electrocyclic reactions can be understood in this same simple way. The theory tells us that the thermal interconversion of cyclobutene and 1,3-butadiene must take place in a conrotatory way. For the cyclobutene studied by Vogel, conrotation requires the stereochemical relationship that he observed. The cis 3,4-disubstituted cyclobutene can only open in conrotatory fashion, and conrotation forces the formation of the cis,trans diene. Note that there are always two possible conrotatory modes (Fig. 20.12), either one giving the same product in this case. 

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