SYNTHESIS OF 7-HYDROXY-2,8-DIMETHYL-4-OXO-3-PHENOXY-4H-6-CHROMENECARBALDEHYDE
DOI: https://doi.org/10.17721/1728-2209.2018.1(55).13
Keywords:
6-dialkylaminomethyl-7-hydroxy-4H-4-chromenones, 7-hydroxy-4-oxo-4H-6-chromenecarbaldehydes, Duff reaction.Abstract
Ortho-hydroxyformylchromones are convenient synthones for the construction of linear and angular hetarenochromones. Usually, 7-hydroxy-6-formylchromones were synthesized by oxidation of natural linear furochromones: visnagin and kellin and their synthetic analogues. The Duff reaction, which is the formylation of phenols in the ortho-position by heating with hexamethylenetetramine followed by acidic hydrolysis of intermediate imine, was also used to convert 7-hydroxychromones into 7-hydroxy-6-formylchromones, but in this case there were some difficulties because of the passivity of position 6 in 7-hydroxychromones compared to position 8 to the electrophilic attack. Thus, for the preparation of 7-hydroxy-6-formylchromones, it is necessary to use 8-substituted derivatives and to provide formylation for a long time. A method for the synthesis of 7-hydroxy-6-formylchromones based on 8-substituted 7-hydroxy-6-dialkylaminomethylchromones and hexamethylenetetramine was developed using the Duff reaction conditions. This method was demonstrated on the synthesis of 7-hydroxy-2,8-dimethyl-4-oxo-3-phenoxy-4H-6-chromenecarbaldehyde from 6-dimethylaminomethyl-7-hydroxy-2,8-dimethyl-3-phenoxy-4H-4-chromenone and hexamethylenetetramine in glacial acetic acid at reflux. It should be noted that when carrying out this reaction under heating on a water bath with subsequent hydrochloric acid hydrolysis only Mannich base hydrochloride was isolated from the reaction mixture.
The starting 6-dimethylaminomethyl-7-hydroxy-2,8-dimethyl-3-phenoxy-4H-4-chromenone was synthesized from 1-(2,4-dihydroxy-3-methylphenyl)-2-phenoxyethanone in three steps. Acylation of the latter with acetic anhydride in the presence of trimethylamine followed by condensation afforded 2,8-dimethyl-4-оxо-3-phenoxy-4Н-7-chromenylаcetate. Subsequent removal of acetyl protection resulted in 7-hydroxy-2,8-dimethyl-3-phenoxy-4H-4-chromenone, which on introduction into the Mannich reaction with bisdimethylaminomethane in dioxane gave rise to the desired 6-dimethylaminomethyl derivative.
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