Chromones 4-hydroxy

The initial compound (9) was prepared by stirring 1,3,5 trimethoxybenzene with 1 -methyl-4-piperidone in glacial acetic acid. The tram alcohol (10) was obtained by hydration of the double bond and the corresponding cis compound (11) then made via the ketone by oxidation then reduction. Treatment with acetic anhydride and an acid catalyst such as boron trifluoride etherate gave (12). Treatment of this compound at room temperature with ethyl benzoate in the presence of sodium metal produced the diketone (13) which was cyclised to (14) by the addition of mineral acid. Final demethoxylation of the chromone hydroxy groups with pyridine hydrochloride was carried out for several hours at 180°. 

Chromone, 5-hydroxy-2-methyl-biosynthesis, 3, 876 Chromone, 6-hydroxy-2-methyl-reactions... 

Chromone, 7-hydroxy-2-methyl-bromination, 3, 707 Chromone, 3-hydroxymethylene-synthesis, 3, 821... 

Chromone-2-carboxylic acid, 6,8-dibromo-7- hydroxy-ethyl ester... 

Chromone-5-carboxylic acid, 7-hydroxy-2-methyl-decarboxylation, 3, 711 Chromonecarboxylic acids decarboxylation, 3, 710 Chromone-2-carboxylic acids esters... 

Kojic acid — see also Pyran-4-one, 5-hydroxy-2-hydroxymethyl-, 3, 611 acylation, 3, 697 application, 3, 880 occurrence, 3, 692 reactions, 3, 714, 715 with amines, 3, 700 with phenylhydrazine, 3, 700 synthesis, 3, 810 Kokusagine occurrence, 4, 989 Kokusaginine occurrence, 4, 989 synthesis, 4, 990 Koopmans theorem, 2, 135 Kostanecki-Robinson reaction chromone and coumarin formation in, 3, 819-821 mechanism, 3, 820 flavones, 3, 819... 

In order to study any chemoselectivity influences of microwave irradiation on the domino Knoevenagel/hetero-Diels-Alder process (the so-called Tietze reaction), Raghunathan and coworkers [31a] investigated the transformation of 4-hydroxy coumarins (10-85) with benzaldehydes 10-86 in EtOH to afford pyrano[2,3-c]cou-marin 10-87 and pyrano[2,3-b]chromone derivatives 10-88. Normal heating of 10-85a and 10-86a at reflux for 4h gave a 68 32 mixture of 10-87a and 10-88 in 57% yield, whereas under microwave irradiation a 97 7 mixture in 82% yield was obtained. Similar results were found using the benzo-annulated substrates 10-85b and 10-86b. 

Chromones are also Michael acceptors, and Scheme 18 shows how 3-bromochromone reacts with 1,3-diketones in basic media. The reaction is fairly general and the yields can be as high as 90%, moreover, phenolic furans are not common and the approach provides an effective way of protecting the phenolic hydroxy group during furan ring formation.100... 

Dihydro- 7 -hydroxy-2S, 3 R -dimethyl-2-[4-methyl-5-(4-methyl-2-furyl) -3 (E), 7 -pentenyl]-furo[2,3-B]chromone ... 

Bei der analogen Nitrierung von 7-Hydroxy-4-methyl-2H-chromon erhalt man zu 73% 7-Hydroxy-4-methyl-3,6,8-trinitro-2H-chromon [Schmp. 216 217 (Aceton)]1 ... 

However, chromones react differently, because the phenolic hydroxy group in the ring-opened intermediate is unreactive. Thus, isoxazoles (264) result from the reaction with hydroxylamine, and a pyrazole is formed with hydrazine. y-Pyrones also give pyrazoles with hydrazine. 

Chromones with hydroxy or methoxy groups in the aromatic ring show high intensity bands at 240-260 and 290-360 nm. Both bands are red-shifted relative to chromone and in sodium methoxide solution these shifts are enhanced as a result of ionization of the phenolic groups (76JHC211). 

Chromones are readily oxidized by permanganate or dichromate with opening of the pyran ring and the formation of a salicylic acid (402). Flavones and isoflavones are also degradatively oxidized for example oxidation of munetone (403) with alkaline hydrogen peroxide yields 2-methoxybenzoic acid and 4-hydroxy-2-isopropylbenzofuran-5-carboxylic (isotubaic) acid. 

When a homoisoflavanone, for example 3-benzylidenechromanone (649), is heated with a base in DMF, two products are obtained. The main reaction is the migration of the exocyclic double bond to form 3-(3 -hydroxy-4 -methoxybenzyl)chromone (650) a skeletal rearrangement through a ring opening reaction accounts for the formation of the other minor product, 3 -hydroxy-4 -methoxy-3-methylflavone (651). Labelling with 14C showed... 

Salicylaldehyde also reacts with enamines to form chromans, although these have not usually been isolated but rather have been oxidized directly to a chromone (66JOC1232). The synthesis has been applied to both cyclic and acyclic enamines and to 2-hydroxy-1-naphthaldehyde. The initial intermediate undergoes an intramolecular cyclization involving participation of the neighbouring hydroxyl group. 

Coumarins and isocoumarins appear to be of varied origins. Simple coumarins, such as umbelliferone, are formed by the shikimic acid pathway in which hydroxylation of p-hydroxycinnamic acid occurs. Other coumarins, for example alternariol (690), are derived from a polyketide unit, as are a number of chromanones, chromones, pyranones and isocoumarins (B-78MI22400). The biosynthesis of 5-hydroxy-2-methylchromone has been shown to involve the chromanone (60JCS654). However, isocoumarins are also derived from the mixed acetate-shikimate route, through initial cyclization of the polyketide and subsequent lactonization. 

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