Retro-Diels-Alder type cleavage

These data show that villamine is a 2,7-dihydropleiocarpamine derivative. The thermal cleavage to pleiocarpamine (196) and macroline (215) is considered to be of the retro-Diels-Alder type. 

In this structure type the cleavage of the labile bonds in ring C by a retro Diels-Alder reaction is dominant, generating two fragments one, the most characteristic, represents the pyrrolidine ring (plus substituents in position 2), and the other (a less abundant fragment) encompasses the aromatic lactone or hemilactone moiety (Fig. 14). A further general and noteworthy feature is the low abundance of the molecular ion in all compounds with a double bond (211). 

The type of pictorial, back-of-the-envelope, application of theory exemplified by the 4n + 2 rule would soon be applied toward understanding a wide range of no-mechanism reactions. The Diels-Alder reaction (see chapter 3) is one example. The reverse reaction ( retro-Diels-Alder ) also occurs readily. In one step (one transition state), three n bonds are broken and two new o bonds and one n bond are created. The simultaneous cleavage and formation of all bonds in one step is referred to as concerted and such reactions often occur under mild conditions and form specific stereoisomers. Since the bonds made and broken form a continuous cycle in the transition state, the Diels-Alder is an example of an electrocyclic reaction. In contrast to the Diels-Alder reaction, dimerizations of two alkene molecules (R2C=CR2) to cyclobutanes typically fail (as do the reverse thermal decompositions of cyclobutanes). These reactions often succeed when ultraviolet light (photochemistry) is used in place of heat (thermal chemistry). 

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