Synthesis and reactivity of isoindigo: A revisit

Abstract

Isatin (1) and 2-oxoindole (2) mixed in acid-catalyzed conditions gave a 3-substituted indole-2-one (isoindigo) 3
and not a 2-substituted indole-3-one (indirubin) 4. The
bioxindole afforded exclusively the thermodynamically
stable cis-diastereoisomerically pure d-lactam of benzonaphthyridine-5,11-dione 6 via two-fold acid-catalyzed
rearrangements, and was subsequently reduced into
diazachrysene (7). The treatment of isoindigo (3) with
hydrazine hydrate gave pyrazines in tautomeric form 8a
↔8b. Isatin (1) was treated by Witting reaction to give 3-
aylidene-1,3-dihydroindol-2-ones 10a-C/11a-c in good
yields. This dichotomy of behaviour of isatin is documented
and rationalized.