Electrophilic cyclizations of 2,3-dialkynylquinoxalines and 1,2-dialkynylbenzenes: a comparative study

The reactivity of 2,3-dialkynylquinoxalines towards electrophiles (Br2, I2, ICl, NBS, HBr) has been studied. All tested reactions, except one with HBr, start with the addition of an electrophile to the carbon–carbon triple bond that promotes further 5-exo-dig carbocyclization ultimately yielding a mixture of stereoisomeric cyclopenta[b]quinoxaline derivatives. The nature of substituents on the Ctriple bond; length of mdashC bonds of the starting molecule influence the stereochemical result of the reaction. The ratio of isomeric cyclization products also depends on the electrophile used. ortho-Dialkynylbenzenes and ortho-dialkynylquinoxalines demonstrate rather similar reactivity towards halogen electrophiles, but differ in their reactions with hydrobromic acid, which is caused by the basic nature of the aza group of the quinoxaline substrate.