O-propargylic salicylaldehydes

Acyl-2//-chromenes can be synthesized from O-propargylic salicylaldehydes in the presence of a Bransted acid (HBF4). The reaction proceeds via a formal [2+2]-cycloaddition of the alkyne and carbonyl group to form intermediate oxete 38, which undergoes cycloreversion to afford 3-acyl-2//-chromenes in excellent yield (Scheme 12) <2005OL2493>. 

Bashiardes et al. reported a study involving cydoaddition of O-allylic or O-propargylic salicylaldehydes 152 or 153 to a-amino esters 154 treated under different conditions toluene at reflux (A) conventional heating without solvent (B) or microwave irradiation under solvent-free conditions (C) (Scheme 11.38) [lOOj. The cydoaddition reaction involves polar 1,3-dipolar azomethine ylide intermediates 155 and 156. The best results were obtained in a one-pot procedure under micro-wave conditions providing pyrrolidines 157 or pyrroles 158 in excellent yields ranging from 70 to 98% at 130 °C for 5-30 min. 

The synthesis of pentacyclic indole derivatives, 9-phenyl-9,13c-dihydro-6//-chromeno[4, 3 4,5]-thiopyrano[2,3-( ]indoles (Scheme 2.3), has been achieved via domino Knoevenagel-hetero-Diels-Alder reactions of indolin-2-thiones and O-propargylated salicylaldehyde derivatives in the presence of 10 mol% of ZnO as a heterogeneous catalyst in CH3CN (Kiamehr and Moghaddam 2009). When the... 

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