Domino annulation reaction under

Example 3, A domino annulation reaction under Willgerodt-Kindler conditions ... 

In order to study any chemoselectivity influences of microwave irradiation on the domino Knoevenagel/hetero-Diels-Alder process (the so-called Tietze reaction), Raghunathan and coworkers [31a] investigated the transformation of 4-hydroxy coumarins (10-85) with benzaldehydes 10-86 in EtOH to afford pyrano[2,3-c]cou-marin 10-87 and pyrano[2,3-b]chromone derivatives 10-88. Normal heating of 10-85a and 10-86a at reflux for 4h gave a 68 32 mixture of 10-87a and 10-88 in 57% yield, whereas under microwave irradiation a 97 7 mixture in 82% yield was obtained. Similar results were found using the benzo-annulated substrates 10-85b and 10-86b. 

Dihydrobenzofurans belong to an important class of heterocycles, widely embedded in many biologically interesting natural products [33, 34] and have shown important biological activities [35]. Perumal and coworkers developed a facile stereoselective protocol for the synthesis of tra s-2,3-dihydrobenzofurans 41 in moderate yields (Scheme 12.15). The domino reaction of cyclic 1,3-diketones 33 with 2,2 -sulfonylbis(l,3-diarylprop-2-en-l-ones) 40 in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in ethanol was performed under microwave irradiation as well as conventional thermal conditions. This transformation involved a domino Michael addition/proton exchange/annulation sequence [36]. 

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