Flavones, 3-aroyl

Flavanols are similarly accessed from 2-(2-(benzoyloxy)acetyl)phenyl benzoates 752 via a Baker-Venkataraman rearrangement to form 3-benzoyloxy flavones followed by deprotection of the hydroxyl group (Scheme 194) <2000JOC583>. 3-Aroyl flavones are prepared form or/ o-hydroxyacetophenones and aroyl chlorides in modest yield, using a Kostanecki-Robinson approach (Equation 305) <2005SC315>. 

The value of l-(o-hydroxyphenyl)-3-arylpropane-l,3-diones in the synthesis of O-heterocycles by the Baker-Venkataraman reaction with aroyl chlorides has been exemplified by their conversion into 3-acylflavones, flavones, flavanones and... 

Banerji Goomer [116] introduced a general method for the synthesis of flavones in which the key step of the synthesis was the aroylation of O-hydroxy-acetophenones (236) to / -ketones (237) using very mild reaction conditions [Fig. (5)]. Subsequent cyclization of 237 yielded flavones (238) and the authors succeeded in preparing a series of 7 flavones in yields of between 84-91 %. More recently Bois et al., [117] using a similar approach but with improved reaction conditions synthesised 5-hydroxyflavones in a single step reaction from 2,6-dihydroxyacetophenone by treating with aroyl chloride in the presence of excess base. 

Chromones.- Enamines have previously been converted into chromones and in another approach, the enamine (104) reacted with thionyl chloride to give the chromone (105 n=2) in moderate yield. The sulphinic acid was readily oxidized to the sulphonic acid (105 n=3) by 3-chloroperoxybenzoic acid.110 A convenient synthesis has been described of 2-methylchromene-4-thione in 57% yield from 4-(2-hydroxyphenyl)butane-1,3-dione.111 The t-amino-methylchromones (106) have been synthesized and shown to be central nervous system depressants. A new synthesis of 2-substituted chromones (including flavones) uses 2-acetoxyphenacyl bromide (107) which is converted into its phosphorane (108). Heating this with an alkanoyl or aroyl chloride or anhydride in pyridine gave good yields of the chromone (109).113... 

Also related is the oxidative cyclization of 2-aroyl-3-aryl-acrylophenones (166) to 3-acyl-chromones (167) with selenium dioxide in isoamyl alcohol/ Oxidation of 2-phenyl-2H-l-benzopyrans with potassium permanganate provides a new synthesis of flavones/ A radical mechanism has been suggested. 

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