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Thermal decompositions of S‐2‐benzothiazole thiocarbonates have been investigated. The results are best rationalized by an Sni ion‐pair return mechanism from which S‐substituted benzothiazoles are the major products obtained. The ready ionization of the carbon‐oxygen bond of thiocarbonates at elevated temperature renders a concerted Sni mechanism (internal substitution with allylic rearrangement) inoperative. Of particular interest is the mixed benzo‐thiazole‐2‐thiol and 2‐thione compound isolated from the decomposition of 2 (or 3)‐halogen‐substituted ethyl (or propyl) thiocarbonates. Evidence is presented that the thione 14 results from the nucleophilic opening of the cyclic intermediate, 2,3‐dihydrothiazolo[2,3‐b]benzothi‐azolium chloride (20). The latter is readily formed under the experimental condition by intramolecular cyclization of the corresponding 2‐(2‐chloroethyl)benzothiazole (23), a primary product from the decomposition of S‐2‐benzothiazole 2‐chloroethylthiocarbonate (7).

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ChemInform Abstract The salicylaldehydes (I) and (V) react with triethyl phosphonoacetate (II) to give the cyclization products (III) and (IV) or (VI) and (VII). The absorption and fluorescence properties of the derivatives prepared are compared to those of the known coumarin (VIII).

627 4,4'~ButS-ene-l,4-dienylidene>bi-4H-pyran and thiopyran derivatives were prepared by allowing the anion prepared from diethyl 4-(2,6-diphenyI-4H-pyranyl)phosphonate to react with 4-pyranylidene butenaldehydes or the corresponding thiopyranylidene aldehydes. The oxidation potentials are compared of some bi-4Hpyrans and thiopyrans with zero, two, and four methenyl groups separating the heterocyclic nuclei.J. Heferocycfic Chem., 18, 627 (1981). W e have investigated new methods for synthesizing A4.4'-bi-4H-pyrans and thiopyrans (1, X = 0 or S) (1-4), ~~ . .compounds which can form conducting charge-transfer salts (5). For studying the effect of changes in the structure of 1 on their physical properties, we prepared derivatives of 1 with a n ethanediylidene group separating the heterocyclic nuclei (2) (6). ( x = O o r S)