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/alkoxycarbonylation of 2‐alkynylbenzyl alcohols 1 , and of 2‐alkynylbenzaldehydes or 2‐alkynylphenyl ketones 4 is reported. Reactions were carried out in ROH or CH 3 CN/ROH (R = Me, i Pr) as the solvent at 70−105 °C in the presence of catalytic amounts of PdI 2 in conjunction with KI under a 4:1 or 3:1 CO/air mixture (2.0 or 3.2 MPa total pressure at 25 °C). 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 Pd II , followed by alkoxycarbonylation. The presence of substituents at the alkyne terminal position and at the carbon atom α to the hydroxy group play a key role in the selectivity of the process towards the formation of a five‐ or six‐membered ring. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)