Studies of heterocyclic compounds. Part 26. Synthesis of 1,6,6aλ4-triheterapentalenes from isothiazole-5-carbaldehyde

Abstract

1,6,6aλ⁴-Triheterapentalenes have been synthesised from isothiazole-5-carbaldehyde. Methylation of isothiazole-5-carbaldehyde phenylhydrazone, p-nitrophenylhydrazone, and methylhydrazone with methyl fluorosulphonate, and treatment of the resulting 2-methylisothiazolium-5-carbaldehyde hydrazone fluorosulphonates with aqueous sodium carbonate gave 6-methyl-1-phenyl-, 6-methyl-1-p-nitrophenyl-, and 1,6-dimethyl-6aλ⁴-thia-1,2,6-triazapentalene, respectively. Methylation of isothiazole-5-carbaldehyde oxime with methyl toluene-p-sulphonate gave a mixture of 5-hydroxyiminomethyl-2-methylisothiazolium toluene-p-sulphonate and 5-(2-hydroxy-2-methyliminiomethyl)isothiazole toluene-p-sulphonate. Deprotonation of 5-hydroxyiminomethyl-2-methylisothiazolium toluene-p-sulphonate with aqueous sodium carbonate gave 6-methyl-1-oxa-6aλ⁴-thia-2,6-diazapentalene, the first derivative of the 1-oxa-6aλ⁴-thia-2.6-diazapentalene system to be reported. Deprotonation of 5-(2-hydroxy-2-methyliminiomethyl)isothiazole toluene-p-sulphonate gave 5-[methyl(oxido)iminiomethyl]isothiazole, which was also obtained by the reaction of isothiazole-5-carbaldehyde with N-methylhydroxylamine. Methylation of 5-[methyl(oxido) iminiomethyl] isothiazole with methyl fluorosulphonate gave 2-methyl-5-[methyl (oxido)iminiomethyl]isothiazolium fluorosulphonate, for which an alternative four-electron three-centre bonded structure is considered. The Wittig reaction of isothiazole-5-carbaldehyde with methoxymethylenetriphenylphosphorane gave a mixture of (E)- and (Z)-5-(2-methoxyvinyl)isothiazole which, when methylated with methyl fluorosulphonate, gave a mixture of (E)- and (Z)-2-methyl-5-(2-methoxyvinyl)isothiazolium fluorosulphonate. The fluorosulphonate reacted with methylamine to give 1,6-dimethyl-6aλ⁴-thia-1,6-diazapentalene, with sodium hydrogen sulphide to give mainly 6-methyl-1,6aλ⁴-dithia-6-azapentalene, and with sodium hydroxide to give 6-methyl-1-oxa-6aλ⁴-thia-6-azapentalene.