Lithium carbonate dehydrobromination with

Benzofuranyl)butanoic acid readily forms the acid chloride, and this undergoes intramolecular Friedel-Crafts acylation on treatment with tin(IV) chloride in carbon disulfide at room temperature, providing 1,2,3,4-tetra-hydro-l-dibenzofuranone (54%). " This intermediate has been converted to dibenzofuran by lithium aluminum hydride reduction and subsequent dehydrogenation, to 1-methyldibenzofuran by Grignard reaction and dehydrogenation, and to 1-dibenzofuranol by reaction with iV-bromosuccinimide and subsequent dehydrobromination with pyridine. 

The enone 445 was then converted to ( )-oxocrinine (415) by a sequence that commenced with the bromination of 445 using excess 5,5-dibromo-2,2-di-methyl-4,6-dioxo-l,3-dioxane to provide a mixture of bromo ketones 446. Removal of the jV-carbobenzyloxy protecting group according to the protocol previously detailed gave 448 as a mixture (a-Br (3-Br = 3 1) of diastereomers, but only the a-bromo isomer underwent dehydrobromination on heating with lithium bromide and lithium carbonate in dry DMF to furnish 415. Interestingly, treatment of the (3-bromo derivative of 448 under similar conditions afforded the debrominated product 447 (200). 

Cyclization of enone (9) in hexane with boron trifluorideetherate in presence of 1,2-ethanedithiol, followed by hydrolysis with mercury (II) chloride in acetonitrile, yielded the cis-isomer (10) (16%) and transisomer (11) (28%). Reduction of (10) with lithium aluminium hydride in tetrahydrofuran followed by acetylation with acetic anhydride and pyridine gave two epimeric acetates (12) (32%) and (13) (52%) whose configuration was determined by NMR spectroscopy. Oxidation of (12) with Jones reagent afforded ketone (14) which was converted to the a, 3-unsaturated ketone (15) by bromination with pyridinium tribromide in dichloromethane followed by dehydrobromination with lithium carbonate and lithium bromide in dimethylformamide. Ketone (15), on catalytic hydrogenation with Pd-C in the presence of perchloric acid, produced compound (16) (72%) and (14) (17%). The compound (16) was converted to alcohol (17) by reduction with lithium aluminium hydride. 

Dehydrobromination (8, 244). Dehydrobromination of 1,2-dibromocyclohexanc with lithium chloride and lithium carbonate in HMPT at 160° provides 1,3-cyclohcxadicnc of >95% purity in yields of 75-90%. Even higher yields can be obtained by dehydrobromination of 3-bromocyclohexene. ... 

Bromination of an olefin and double dehydrobromination of the resulting 1,2-dibromide is a classical method for the generation of 1,3-dienes (Table 1). Bromination of a double bond can be done with molecular bromine or, more conveniently, with pyridinium bromide perbromide . A variety of bases has been employed for dehydrobromination. While potassium hydroxide and sodium methoxide have been used for a long time, lithium carbonate-lithium chloride in DMF or hexamethylphosphoric triamide (HMPA) works well in many cases . Double dehydrobromination with hindered bases such as potassium r-butoxide or diazabicyclononene (DBN) and diazabicycloundecene (DBU) give good results. 

Extension of the chain of double bonds in androstenes provides yet another site for modifying the basic strucmre. Bromination of the intermediate 14-2 that features the l,4-dien-3-one system proceed on the allylic position at Cg (20-1) (Scheme 5.20). Dehydrobromination by means of lithium carbonate in DMF leads to formation of the 1,4,6-trien-3-one system (20-2). Treatment of that intermediate with thioacetic acid proceeds to add one thiol group to each end of the conjugated systems to afford the bisthiolated product thiomestrone (20-3). 

The paper includes a method for cw-dehydrobromination of a-bromo ketones. Thus dehydrobromination of a 80 20 mixture of (1) and (2) with lithium fluoride and lithium carbonate in DMF at 120° requires a reaction period of... 

Dehydrobromination. King and Paquette have reported that HMPT is an excellent solvent for dehydrobrominations with lithium carbonate and lithium fluoride. Powdered glass is added to facilitate CO2 evolution. These conditions were used for preparation of 1,3-cycloalkadienes by a bromination-dehydro-bromi nation sequence. 

Addition of bromine to the dienol acetate (49) gives the 6j5-bromo-A -3-ketone (50). Dehydrobromination of the crude bromo compound in DMF with lithium or calcium carbonate gives the title compound (51). ... 

Dehydrobromination of the 16a-bromo-17-ketone (330) with dimethylaceta-mide containing lithium bromide and carbonate gave a mixture of the 14-en-17-one (331) and the 14j3-15-en-17-one (332). " Although it was possible to separate these products by fractional crystallization, a more satisfactory use for the mixed products involved enol acetylation, which gave the A " -dien-17-ol acetate (333) in overall yield of 65%. Perchloryl fluoride treatment of the dienol... 

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