Synthesis of 1-(alkoxycarbonyl)methylene-1,3-dihydroisobenzofurans and 4-(alkoxycarbonyl)benzo[c]pyrans by palladium-catalysed oxidative carbonylation of 2-alkynylbenzyl alcohols, 2-alkynylbenzaldehydes and 2-alkynylphenyl ketones

A direct synthesis of 1-(alkoxycarbonyl)methylene-1,3-dihydroisobenzofurans 2 and 5 and 4-(alkoxycarbonyl)benzo[c]-pyrans 3 and 6 by oxidative Pd-catalysed cyclization/alkoxy-carbonylation of 2-alkynylbenzyl alcohols 1, and of 2-alkynylbenzaldehydes or 2-alkynylphenyl ketones 4 is reported. Reactions were carried out in ROH or CH3CN/ROH (R = Me, iPr) as the solvent at 70-105 degreesC in the presence of catalytic amounts of PdI2 in conjunction with KI under a 4:1 or 3:1 CO/air mixture (2.0 or 3.2 MPa total pressure at 25 degreesC). The reaction occurs through intramolecular attack by the nucleophilic oxygen atom (either already present in the starting material, as in 1, or generated in situ by ROH attack on carbonyl group, as in 4) on the triple bond coordinated to PDII, followed by alkoxycarbonylation. The presence of substituents at the alkyne terminal position and at the carbon atom a to the hydroxy group play a key role in the selectivity of the process towards the formation of a five- or six-membered ring. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004).