Studies in Benzopyran chemistry

Abstract

6-Bromo-, 7-bromo-, 8-bromo- and 6,8-dibromo- 2,2-dimethylchromenes have been synthesised. The 6-bromoand 8-bromo-derivatives were prepared by Claisen rearrangement of aryl propargyl ethers, derived from phenols and 3-chloro-3-methylbut-1-yne. 7-Bromo- and 6,8-dibromo-2,2-dimethylchromenes were obtained by dehydration of the corresponding chroman-4-ols, available from the chroman-4-ones. Addition of bromine to the chromenes gave the corresponding 3,4-dibromochromans which were hydrolysed with aqueous acetone to yield the 3-bromochroman-4-ols. These chromanols were also obtained directly from the chromenes by reaction with N-bromosuccinimide in moist dimethyl sulphoxide.
The bromohydrins were oxidised to 3-bromochroman-4-ones and conversion to the chroman-4-ones was achieved by
subsequent reaction with benzo±n. The same chroman-4-ones have been synthesised by the condensation of the appropriate brominated 2-hydroxyacetophenories with acetone in the presence of pyrrolidine. Róduction of the chroman-4-ones using sodium borohydride afforded the chroman-4-ols.
The bromohydrins have been converted to the corresponding 3,4-epoxychromans and ring opening reactions with lithium aluminium hydride, hydrobromic acid and methanol were studied. Dehydration of the 3-bromochroman- 4-ols yielded the 3-bromochromenes. The isomeric 4-bromochromenes resulted from treatment of the -- - 3,4-dibromochromans with sodium methoxide.
Routes to 2,2-dimethylchroman-4-ones based on the Fries rearrangement of phenyl 3-methylbut-2-enoates and the reaction between a phenol and 3-methylbut-2-enoic acid have been examined. In many instances, the isomeric 3,4-dihydro-4,4-dimethylcoumarins were formed either as sole products or admixed with chromanones.
Treatment of 4-bromo- 1 6-bromo-, 7-bromo- and 8-bromo-2 ,2-dimethylchromenes with n-butyl-lithium followed by Michler's ketone has given a series of dyes related to Malachite Green, h'.it incorporating an oxygen heterocyclic function. The 6- and 7-bromochromenes gave longitudinally conjugated dyes, whereas transversely conjugated dyes resulted from the 4- and 8-bromochromenes.