Wrong Diels-Alder Adduct

Atom and bond budgets of starting material and product indicate that this transformation is simply an isomerization where two new C-C bonds have been created at the expense of one C=C bond. In the absence of an efficient leaving group, which is the case in I, this cannot be achieved. Therefore, the reaction mechanism must not be underestimated. 

It also becomes apparent from this analysis that the two tetragonal carbons of cyclobutene in I must have been activated at some point for them to become the complementary centers of C-C bond construction. This activation occurs by means of the cyclobutene ring disconnection. This fragmentation follows 

Intermediate III features the familiar combination diene-dienophile (the n-butylvinyl sulfide fragment). Consequently, it is prone to undergo a Diels-Alder cycloaddition in an intramolecular fashion.3 As a consequence, tetracyclic structure II would be built in essentially one step. 

Although the relative stereochemistry of the B,C ring fusion of compound II was not discussed in the original paper, it is always instructive to treat this point in cycloaddition reactions, and some valid predictions may be advanced on theoretical grounds only. Assuming that a transoid C=C bond is produced during the destruction of the cyclobutane fragment, the only model of inter- 

A molecular model of IX, unfortunately, is quite difficult—if not impossible—to construct for no other reason that, in spite of an artful drawing (ours here reproduces the original proposition closely), it cannot possibly achieve this precise conformation. 

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Problem 32. Isolation of the Wrong Diels-Alder Adduct 176... 

The reaction is clearly a cycloaddition but at first sight the selectivity is all wrong. The puzzle is solved when we reaUze that this is a reverse electron demand Diels-Alder. The diene is very electron-deficient with its two conjugated carbonyl groups so the dienophile needs to be electron-rich. It is not very electron rich as drawn, but its enol is. The first formed adduct loses carbon dioxide in a reverse cycloaddition. 

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