Furans 2,5-adduct from nitration

The high reactivity of pyrroles to electrophiles is similar to that of arylamines and is a reflection of the mesomeric release of electrons from nitrogen to ring carbons. Reactions with electrophilic reagents may result in addition rather than substitution. Thus furan reacts with acetyl nitrate to give a 2,5-adduct (33) and in a similar fashion an adduct (34) is obtained from the reaction of ethyl vinyl ether with hydrogen bromide. 

An attempt to detect the aryne 542 from the in situ diazotization of the amino acid 543 with isoamyl nitrite in the presence of furan (146a) or tetracyclone (151) failed to yield any of the aryne adducts 544 or 545, respectively. The only identifiable products isolated in low yield were the acids 546 and 547 and their corresponding nitro compounds 548 and 549. Although it was considered that the latter two compounds might arise via the unlikely dipolar form (542b) of the aryne, it was demonstrated that they were actually transformation products of the former two compounds, 546 and 547, under the diazotization conditions. Similar decarboxylative nitrations with isoamyl nitrite and nitric acid have been observed in related systems. The acids 546 and 547 were proposed to arise from the dipolar species 550,... 

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