Studies of heterocyclic compounds. Part 22. Reactions of 1,6-dioxa-6aλ4-thiapentalenes

Abstract

Electrophilic substitution of 1,6-dioxa-6aλ⁴-thiapentalene involves attack at position(s) 3 (and 4) and gives either normal substitution products or products of substitution with rearrangement. Bromination of 1,6-dioxa-6aλ⁴-thiapentalene invariably gave 3,4-dibromo-1,6-dioxa-6aλ⁴-thiapentalene. Iodination with iodine and silver acetate could be controlled to give either 3,4-di-iodo- or 3-iodo-1,6-dioxa-5aλ⁴-thiapentalene. Tritylation with trityl perchlorate in the presence of calcium carbonate gave 3-trityl-1,6-dioxa-6aλ⁴-thiapentalene. Acetoxymercuration in acetic acid afforded an insoluble bisacetoxymercurio-derivative quantitatively. The position of substitution was established by comparative ¹H n.m.r. spectral studies. Attempts to formylate, acetylate, and nitrate 1,6-dioxa-6aλ⁴-thiapentalene were unsuccessful. 1,6-Dioxa-6aλ⁴-thiapentalene couples with p-nitrobenzenediazonium tetrafluoroborate with rearrangement to give 1-p-nitrophenyl-6-oxa-6aλ⁴-thia-1,2-diazapentalene-3-carbaldehyde, the first derivative of the 6-oxa-6aλ⁴-thia-1,2-diazapentalene system to be reported. 1,6-Dioxa-6aλ⁴-thia-pentalenes show aldehyde reactivity. 3,4-Dibromo- and 3,4-di-iodo-1,6-dioxa-6aλ⁴-thiapentalene reacted with methylamine in aqueous acetonitrile to give 3,4-dibromo-6-methyl- and 3,4-di-iodo-6-methyl-1-oxa-6aλ⁴-thia-6-azapentalene, respectively, the first derivatives of the 1-oxa-6aλ⁴-thia-6-azapentalene system to be reported. 3-Iodo-1,6-dioxa-6aλ⁴-thiapentalene reacted with methylamine to give a mixture of 3-iodo-6-methyl- and 4-iodo-6-methyl-1-oxa-6aλ⁴-thia-6-azapentalene, which were not separated. 3-lodo-1,6-dioxa-6aλ⁴-thiapentalene underwent halogen–metal exchange with n-butyl-lithium at –70 °C. The resulting 3-lithio-derivative reacted in situ with dimethylformamide or with the Vilsmeier reagent (Me₂[graphic omitted]CHCl·PO₂Cl₂^–) to give 1,6-dioxa-6aλ⁴-thiapentalene-3-carbaldehyde in low yield.