Mechanistic Studies of the Deslongchamps Annulation

The
Cs2CO3-mediated annulations (“Deslongchamps
annulations”) of three spirocyclic benzoquinone monoketals 5b–d with an ester or acyl substituent
at C-2 to two tert-butyl esters of γ,δ-unsaturated
β-ketocarboxyl acids (“Nazarov reagents”; 2a,b) were monitored 1H NMR spectroscopically.
This revealed that a primary product, by all likelihood the Michael
adduct, forms fast and prior to the appearance of the Deslongchamps
adduct. These primary products form reversibly. This was proved by
two crossover and four scavenging experiments. Therein, components
already incorporated in one of the mentioned primary products ended
up in Deslongchamps adducts different from the one, which would have
resulted if the respective primary product had reacted alone. However,
these experiments leave open whether our primary products are intermediates
en route to Deslongchamps products or whether they represent dead
ends.