2,3-DIOXO-2,3-DIHYDROFURANS .2. VERSATILE SYNTHESIS OF 2,3-DIOXO-2,3-DIHYDROFURANS AND ALKYLIDENEBUTENOLIDES - CRYSTAL AND MOLECULAR-STRUCTURE OF 5-(4-CHLOROPHENYL)-4-METHOXYCARBONYL-2,3-DIOXO-2,3-DIHYDROFURAN

Reaction of the 1,3-dicarbonyl compounds 5a-f, containing at least one alpha-hydrogen atom in R1 and/or R2, with oxalyl halides 2 in the presence of magnesium chloride yields regioand on the whole stereospecifically the previously unknown (Z)-alkylidenebutenolides 7a-f. Similarly, the 4-alkoxycarbonyl-5-aryl-2,3-dioxo-2,3-dihydrofurans 6g-p are obtained (X-ray structure of 6i) starting from 3-aryl-3-oxo-propanoates 5g-p. Correspondingly, malondiamides 8a,b,e,f react with oxalyl chloride (2a) to give the 2,3-dioxo-2,3-dihydrofurans 9a,b and 11a,b. Having mono(alkyl/aryl)amino substituents in position 5, compounds 11a,b spontaneously tautomerize to give imino enols 12a,b. Hydrochlorides 10a,b are formed on treatment of malondiamides 8c,d with oxalyl chloride (2a). In the case of the beta-oxocarboxamides 13/14 the substituents at the amide nitrogen atom control the regiochemistry. Starting from 13 (R2 = Ph) and oxalyl chloride (2a), 4-acetyl-5-diphenylamino-2,3-dioxo-2,3-dihydrofuran (15) is formed regiospecifically, whereas under the same conditions 13/14 (R2 = CH2Ph) afford 4-dibenzylcarbamoyl-5-methyl/phenyl-2,3-dioxo-2,3-dihydrofurans 16/17. Furanone 16 spontaneously enolizes to give 4-dibenzylcarbamoyl-3-hydroxy-5-methylene-2(5H)-furanone (18). Similarly, the N-monosubstituted beta-oxocarboxamides 19a,b readily react regiospecifically with oxalyl chloride (2a) to give via the 2,3-dioxo-2,3-dihydrofurans 20a,b the imino enols 21a,b.