Some chemistry of bromobenzopyrans

Rahman, Md. Moshfiqur (1995) Some chemistry of bromobenzopyrans. Doctoral thesis, University of Central Lancashire.

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A series of 2,2-disubstituted chroman-4-ones has been prepared by the base catalysed condensation of a carbonyl compound with 2'-hydroxyacetophenone. Chroman-4-ones substituted in the aromatic ring were more conveniently prepared by the cyclocondensation of a phenol with an a,j3-unsaturated carboxylic acid.
2,4-Dimethylphenyl-3-methylbut-2-enoate was obtained as a by-product during the preparation of 2,2,6,8 tetramethylchroman-4-one from 2,4-dimethyiphenol and 3,3-dimethylacrylic acid in polyphosphoric acid. The ester in polyphosphoric acid underwent a Fries rearrangement to give the chroman-4-one, but in methanesulfonic acid it gave 3,4-dihydro-4,4,6,8-tetramethylcoumarin.
Reduction and dehydration of the resulting chroman-4-ols gave the 2H-chromenes in excellent yield.
Addition of N-bromosuccinimide in moist dimethyl sulfoxide to the 2H-chromenes gave the trans-3-bromochroman-4-ols in good yield. Subsequent dehydration using a catalytic amount of 4-toluenesulfonic acid in refluxing toluene afforded the 3-bromo-2H-chromenes. A novel bis-4-(3-bromochromanyl)ether was isolated from the dehydration of 3-bromo-2,2-dimethylchroman-4-ol. Its formation is thought to arise from intermolecular capture of the benzylic cation by a second molecule of the bromohydrin.
4-Bromo-2H-chromenes have been obtained in a single step and in excellent yield from the chroman-4-ones by reaction with phosphorus tribromide. A novel 2H-chromene-4-phosphonic acid was characterised from the reaction of 2,2-dimethylchroman-4-one with PBr3.
Lithiation of the 4-bromo-2H-chromenes gave the 4-chromenyl lithium which reacted with a range of electrophiles to afford novel 4-substituted 2H-chromenes. However, lithiation of the isomeric 3-bromo-2H-chromenes did not lead to the corresponding 3-substituted 2H-chromenes but gave instead substituted 2'-hydroxyphenyl allenes in good yields. This method constitutes a new route to the allene system.
Reactivity of the allenes towards bromination, hydroboration and cycloaddition reactions have been investigated.
3,4-Dibromo-2,2,6,8-tetramethyl-2H-chromene has been prepared by the bromination of 4.bromo-2,2,6,8-tetramethyl-2H-chromene. 3,4,7-Tribromo-2,2,6,8-tetramethyl- 2H-chromene was obtained as a by-product
The reaction of 3,4-dibromo-2,2,6,8-tetramethyl-2H-chromene with alkyllithium reagents gave mainly the 4-ailcyl substituted 211-chromenes with trace amounts of the 4-unsubstituted 2H-chromenes.
Reaction of 3,4-dibromo-2,2,6,8-tetramethyl-2H-chrOmene with magnesium in THF in the presence of furans gave the Diels-Alder adducts, as a result of the trapping of the heteroaryne, 3,4-didehydro-2H[ 1 ]ben zopyran. This reaction constitutes the
generation of a novel aryne.
Diels-Alder adducts obtained by the use of 2,5-dimethylfuran and 1,3-diphenylisobenzofuran have been aromatised providing facile access to dibenzo[bdjpyrans.

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