A new series of non‐classical type C heteropentalenes: 2H‐pyrrolo[2,1‐c][1,2,4]triazoles

Abstract

7‐Functionalized title compounds 5 are obtained by cyclization of 3‐acetonyl‐ or 3‐[(alkoxycarbonyl)‐methyl]‐4‐phenacyl‐1,2,4‐triazolium salts 2 having methyl at C(5) the process can be effected in an acetate buffer or by base, irrespective of the function at C(3). 5‐Unsubstituted salts 2 do not react unless the side chain at C(3) is an acetonyl group. Cyclization of 2 with acetic anhydride‐base gives rise to 5,7‐difunctionalized compounds 8; again methyl at C(5) of 2 is compulsory, but here the reaction can be extended to salts having an (alkoxycarbonyl)methyl group at C(4). Regarding defunctionalization, acetyl groups can be split from C(5) only, whereas ester functions are removable also from C(7). Title compounds devoid of acceptor groups (XIII) are unstable but can be trapped by electrophilic reagents (DMAD, acetic anhydride, and phenyl isocyanate) to give the derivatives 10 and 12. The 7‐functionalized products 5 are likewise susceptible to SE‐reactions. By comparison, all title compounds appear to be more reactive toward this kind of reagents than the isomeric 1H‐pyrrolotriazoles (13) including 2H‐pyrrolotetrazoles (III). This is consistent with B3LYP‐DFT calculations using appropriate models.