2-Alkynylbenzoic acids

CycUzation. 2-Alkynylbenzoic acids afford lactones by the action of AgNO. 3-Substituted isocoumarins are the major products. 

Alkylidene phthalides are produced from 5-exo-dig cyclization of o-alkynylbenzoic acids. However, concomitant generation of isocoumarins via 6-endo-dig cyclization is normally problematic [63]. 

Alkyne metathesis is generally restricted to internal alkynes, and is driven to completion by the evolution of gaseous 2-butyne. Representative examples of alkyne cross-metathesis are depicted in Scheme 33, and include (i) dimerization of alkynylbenzoic acid derivative 287, (ii) alkyne cross-metathesis of 289 and bis(trimethylsilyl)acetylene, (iii) cross-metathesis of enyne 291 and various alkynes and metathesis dimerization of 291, and (iv) formation of high molecular weight alkyne-containing polymers through acyclic diyne metathesis (ADIMET) polymerization of acyclic diynes (e.g., 293) " using the molybdenum hexacarbonyl/2-chlorophe-nol system. 

Disubstituted isocoumarins arise from the copper(II)-catalyzed addition of o-halobenzoic acids to active internal alkynes (13JOC1660), rhodium(III)-mediated oxidative coupling ofbenzoic acids with disubsti-tuted alkynes (13T4454), palladium(II)-catalyzed tandem annulation reaction of o-alkynylbenzoates with methyl vinyl ketone (13T8626), and nickel(II)-promoted t-butyl isocyanide insertion in 2-(o-bromophenyl)-1-ethanones followed by hydrolysis (Scheme 69) (13SC3262). 

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