Knoevenagel reaction electrocyclization

Knoevenagel reactions followed by sigmatropic rearrangements or electrocyclizations ... 

Both natural and non-natural compounds with a 2ff,5ff-pyrano[4,3-fc]pyran-5-one skeleton are of interest in medicinal chemistry. Several natural products, such as the pyripyropenes, incorporate this bicyclic ring system. The group of Beifuss has described an efficient microwave-promoted domino synthesis of the 2ff,5H-pyr-ano[4,3-fo]pyran-5-one skeleton by condensation of a,/3-unsaturated aldehydes with 4-hydroxy-6-methyl-2]-f-pyran-2-one (Scheme 6.244) [428]. It is assumed that in the presence of an amino acid catalyst a Knoevenagel condensation occurs first, which is then followed by a 6jr-electron electrocyclization to the pyran ring. While the conventional thermal protocol required a reaction time of up to 25 h (refluxing ethyl... 

Tandem sequences involving Knoevenagel condensation followed by electrocyclization have not been employed as extensively as other tandem reactions (such as the tandem Knoevenagel-Diels-Alder reaction), but have been used in a few cases with great success, rapidly constructing complex polycyclic cores. 

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