Electron-withdrawing groups Dimroth rearrangement

As can be expected from the results mentioned in Section IV,B,2,b, the Dimroth rearrangements in pteridines are facilitated by the presence of electron-withdrawing groups. This can be exemplified by the observed lower rate of the rearrangement of the dianion of 2-imino-3-j8-carboxyethyl-2,3-dihydro-4-oxopteridine compared to that of the compound containing a carbalkoxy substituent in position 6 (9 hours vs 4 hours for complete conversion) (Scheme IV.28) (60JCS539). 

The acid treatment of 1-aminopyridine derivatives possessing strong electron-withdrawing groups at C4 as in 97 gave pyrazolopyridines 98 Dimroth rearrangement had taken place followed by cyclization (84PJC85) (Scheme 30). 

Heating imines 336 derived from 1,5-diaminotetrazole in DMSO provided hydrazones 337 via Dimroth rearrangement. The reaction was greatly favored due to the electron-withdrawing groups attached to the imine carbon atom (90CB1575) (Scheme 103). 

A method for activation is the introduction of an additional nitrogen atom into the six-membered ring, as, for example, in the imidazo[l,2-fljpyrazine, imidazo[l,2-a]pyrimidine (7), or imidazo[l,2-c]pyrimidine (10) systems. Because of its electron-withdrawing property, the extra aza group of the last two systems activates the 5-position for base-catalyzed Dimroth rearrangement. 

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