In the case of terminal alkynoic acids (R = H), the metal-catalysed cyclization reaction yields exclusively exocyclic enol lactones. This can be interpreted in terms of the regioselective Markovnikov addition of the carboxylic acid to the terminal alkyne. 12 Alternatively the anti-Markovnikov addition would produce the corresponding endocyclic enol lactone. However, anti-Markovnikov addition products have never been observed to any significant extent, hence the use of this ...
