Cyclopropene 2 + 2 Cycloreversion

In contrast to cyclopropenones, most methylene cyclopropene derivatives do not undergo clearly defined thermal transformations neither cycloreversion analogous... 

Since the norcarene intermediate 34 has a double bond in the 6-membered ring, a Diels-Alder cycloreversion leading to cyclopropene (35) and butadiene is also a possible disconnection. The corresponding synthetic sequence has been carried out in the laboratory in 37% yield [32] ... 

Calculations have indicated that direct thermal extrusion of nitrogen from 3//-pyrazole to give cyclopropene is an unfavorable process, though more favorable in terms of heat of reaction than cycloreversion to diazomethane... 

A final example of a rDA cycloreversion of a cycloadduct that acts as a protected butadiene illustrates the generation of extended conjugation in a non-aromatic cyclic system. Equation (65) shows that the addition of cyclopropene (147) to (146) can yield either (148) or (149), and both adducts undergo rDA extrusion of carbon dioxide to produce a triene. Hydrolysis of the triene ketals gives access to substituted tropones (150). 

There is a remarkable difference in the reaction modes of the two epimers of methyl 2,4-diphenyl-ejco-8-oxatricyclo[3.2.1.0 ]oct-6-ene-3-carboxylate (25). The jj -compound 25A isomerizes via [2n+2Diels-Alder cycloreversion at 160°C to give furan and the corresponding cyclopropene 28. 

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