Tetronic acid derivatives: Synthesis of an indolone-spiro-hydroxytetronimide

Abstract

The synthesis of l-methylindol-2(3H)-one-3-spiro-2'-3',4'-dihydroxy5'-imino-2',5'-dihydrofuran (4; R1 = R2 H) is described. The compound is readily oxidized to yield 1-methylindole-2,3-dione and 2-oxomalonamic acid, which was isolated as the phenylhydrazone, and on acetylation it forms a mono- or di-acetate. It reacts with phenylhydrazine with opening of the tetronimide ring to form 3-(2',3'-dihydro-l'-methyl-2'-oxoindo1-3'-yl)-3-oxo-2-phenylhydrazonopropanamide (5). The latter compound can be cyclized to yield a new pyranoindole (6).