Dehalogenation, ring-closure

Another benz-analog of 289, namely 296, has been prepared by ring closure of bisacetylene 295 with subsequent dehalogenation in 10% yield. 296 forms orange-red stable crystals with indefinite melting point a solution in benzene is photochemically unstable and is destroyed in a few minutes." ... 

Photochemical reactions of PCDEs have been studied in different solvents, but no difference in the amount of PCDFs has been observed [72,73], In the case of PCBs, the rate of dechlorination is faster in polar solvents than in hydrocarbon solvents [3]. Dehalogenated PCDEs have dominated in photolysis in -hexane [72], but photolysis of PCDEs in acetone solution inhibits dechlorination and favors photocyclization [73]. Therefore, it has been suggested that reactions in the environment in conditions near to acetone solution could have significance in the formation of PCDFs from PCDEs [73]. Photolysis in acetone solution can be used to synthesize PCDF congeners [74]. Carbonium ion is a possible intermediate in ring closure of PCDEs to PCDFs [75]. PCDFs can be synthesized from PCDEs... 

A similar reaction is noted on treating the 3-(a-haloacylamino)-rhodamine (285) with a secondary alkylamine. Nucleophilic attack at the ring-carbonyl, with concomitant opening of the thiazolidine ring, rather than amino-dehalogenation at the acyl-side-chain, is followed by ring closure to the 4//-l,3,4-thiadiazin-5(6//)-one (286) (Equation (19)) <93AP(326)249>. 

Dicyanopropenes provide a valuable source for ring-closure reactions and have been used in a variety of syntheses of N-heterocycles. When a,j -unsaturated esters 424 reacted with 10, pyridones 425 were obtained and were in turn converted into dinitrile 426 with cyanamide. 426 was cyclized by hydrogen halides and yielded products 427 and 428 (X = Cl, Br, I). Dehalogenation afforded 429 and 430 Ionic species like 431 or... 

Alkylation of salicylamide with bromoacetaldehyde diethylacetal and subsequent thermal ring closure yielded 1,4-benzoxazepinone, which was converted to the 1,4-benzoxazepine (26) via simple chlorination and dehalogenation (Scheme 9) <90LA917>. 

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