Dehydrobromination preferential

Strained cydic allenes, in situ generated by dehydrobromination of 1-bromocycloalk-enes with tBuOK, react with diphenylisobenzofuran (DPIBF) to give the [4 + 2]-adducts. Preferential formation of endo adducts was observed [141]. 

The bromination-dehydrobromination route is in many cases preferable [Scheme 2 80JCS(P1)2081], using bromine, N-bromosuceinimide (NBS), or phenyltrimethylammonium tribromide (PTAB). The resulting mono-bromo, geminal, or vicinal dibromo and tribromo derivatives (e.g., 5a,b) are often preferentially dehydrobrominated by Jones method [73JCS(P1)968] using lithium salts in dimethylformamide (DMF). Bases such as triethyla-mine, sodium methoxide, and l,8-diazabicyclo[5.4.0]undec-7-ene (DBU) were also used. 

The decalone 166 undergoes preferential enolization to C-3, and it was proposed to first block the C-3 position by introducing a double bond. Bromi-nation-dehydrobromination proceeded readily to give 167, which proved intractable in all attempts to functionalize C-1 (167 — 168). Direct functionalization... 

The E2 reaction of most compounds is known to proceed preferentially by removal of a proton anti-periplanar to the leaving group. Based upon this generalization, predict the geometry of the l-bromo-l,2-diphenylethylene that is produced as the intermediate in the double dehydrobromination of meso-stilbene dibromide. 

Bu4NBrCl2 80a took place preferentially at the electronically favored site to afford a mixture of compound 78 (42%), compound 79 (21%), and other isomeric products. Unfortunately, both compounds 78 and 79 turned out to be 1 1 diastereomeric mixtures, respectively, indicating that essentially no diastereoinduction occurred. Whereas an originally planned conversion of major 78 into halomon 67 proved unfruitful, the dehydrobromination of tetrahalide 79 followed by bromochlorination with Bu4NBrCl2 80a successfully furnished the target compound 67. 

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