Retro-cycloadditions cheletropic

There are other kinds of cycloadditions, too. The [4 + 1] cycloaddition, a cheletropic reaction, usually goes in the retro direction for entropic reasons. 3-Sulfolene (butadiene sulfone, 2,5-dihydrothiophene 1,1-dioxide) undergoes a [4 + 1] retro-cycloaddition to generate S02 and 1,3-butadiene, which can undergo a Diels-Alder reaction with a dienophile. It is much more convenient to... 

Not all cheletropic reactions proceed suprafacially with respect to the larger component. In the following [6 + 1] retro-cycloaddition, the cis Me groups become out and in in the product. Results such as these are difficult to rationalize or predict a priori. 

---