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The palladium-catalyzed annulation of internal alkynes by o-iodophenols in the presence of carbon monoxide results in the exclusive formation of coumarins 12. No isomeric chromones 13 have been observed. One atmosphere of carbon monoxide is sufficient for this process. The use of a sterically unhindered pyridine base is essential to achieve high yields. The best reaction conditions utilize 0.5 mmol of o-iodophenol, 5 equiv of alkyne, 5 mol % of Pd(OAc)i, 2 equiv of pyridine, 1 equiv of w-BmNCl in 5 ml of DMF at 120 °C. A wide variety of 3,4-disubstituted coumarins containing alkyl, aryl, silyl, alkoxy, acyl, and ester groups have been prepared in moderate to good yields. However, mixtures of regioisomers have been obtained when unsymmetrical alkynes are employed. Iodophenols with electron-withdrawing and electron-donating substituents and 3 -iodo-2-pyridone are effective in this annulation process.

Synthesis of heterocycles via palladium-catalyzed carbonylative annulation of internal and terminal alkynes