The hydration of o-ethynylbenzoic acid derivatives, such as ethyl o-ethynylbenzoate, o-ethynylbenzonitrile, and o-ethynylbenzamide, in sulfuric acid in the presence of marcuric sulfate, afforded 3-substituted isocoumarins. The palladium-catalyzed reaction of o-halobenzoic acid derivatives with terminal acetylenes to obstain the starting materials for the cyclization is also described.
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Sakamoto et al. (1986) studied this question.