Pyrano chromone

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. 

Chloroquinolines are reactive groupings due to electron-deficient carbon to which the halogen is attached. This carbon is electron-deficient due to the combined electron-withdrawing effects of the chlorine substituent and the quinoline nitrogen. The electrophilic carbon is thus able to react readily with nucleophiles present in the body. The impact of this grouping on a molecule is illustrated by 6-chloro-4-oxo-10-propyl-4H-pyrano[3,2-g]quinoline-2,8-dicarboxylate (Figure 8.28). In contrast to many related compounds (chromone-carboxylates) lacking the chloroquinoline, 6-chloro-4-oxo-10-propyl-4H-pyrano[3,2-g]quinoline-2,8-dicarboxylate is excreted as a... 

Fig. 8.28 Structure of 6-chloro-4-0X0-10-propyl-4 H-pyrano[3,2-g]quino-line-2,8-dicarboxylate which, in contrast to many related compounds (chromone-carboxylates) lacking the chloroquinoline, is excreted as a glutathione conjugate.

The cycloaddition of ketene acetals to 3-formylchromone exhibits good diastereoselectivity and meihanolysis of the pyrano[4,3- ] -pyran affords the chromanone ester (21) without racemisation. The chiral auxiliary, the diol (22), can be recyclised and the overall process represents an asymmetric conjugate addition to the chromone (95 JCS(P1 )2293). 

The reactivity of 47/-benzopyran-4-ones in Diels-Alder reactions is well documented <1987T3075>, and recently high asymmetric induction has been achieved in the reaction of 3-alkoxycarbonyl-substituted chromones with chiral auxiliaries and Danishefsky s diene <1991JOC2058>. It should be noted that 3-formylchromones can react as heterodienes in the stereoselective inverse electron Diels-Alder reaction with enol ethers <1994T11755> to provide a route to pyrano[4,3-A][l]benzopyrans a heterocyclic nucleus which occurs naturally in the fungal metabolite fulvic acid <1984CC1565>. The thermal Diels-Alder reaction of 477-pyran-4-one 405 in the presence of an excess of Danishefsky s diene 404 provided cycloadduct 406 (Equation 32) <1996H(43)745>. 

The Pd-catalysed Miyaura-Suzuki coupling of aryltrifluoroborates with 4-tosyloxycoumarins in aqueous conditions offers an attractive route to 4-arylcoumarins <06TL1525> and 4-hydroxycoumarins have been converted into a mixture of predominantly thiopyrano[5, 4 3,4]- pyrano[5,6-c]coumarins and [6,5-c]chromones in a one-pot tandem Knoevenagel - HDA sequence with an S-prenylated 1-phenyl-lff-pyrazole-4-carbaldehyde <06TL2265>. The benzopyrano[4,3-c]benzopyranone system can be obtained from 3-aryl-4-methylcoumarins by deprotonation of the methyl function and subsequent elaboration <06TL5909>. 

The fruits of Ammi visnapya contain the pharmacologically active chromone (khellin) and pyrano coumarins (visnadin). 

The pyrano[3,2-c]xanthen-7-one exhibits the two absorption bands associated with a heteroatom at a peri position of the benzopyran nucleus. Pyrano[2,3-a]xanthen-12-one 44, in which the fusion of the chromone ring is reversed, shows an intense single band blue-shifted some 14 nm relative to 2,2-diphenyl-2//-naphtho[l,2-6]pyran [Xmax 403, 482 nm (PhMe)]. Both isomers are readily degraded <01HCA117>. It is noteworthy that 44 could not be... 

A variety of 2-substituted 2//-chromenes can be obtained from the facile reaction of 2-hydroxybenzaldehydes with vinylboronic acids in ionic liquid solvents <04SL2194>. In a one-pot sequence also in an ionic liquid, a Knoevenagel condensation between O-prenylated salicylaldehydes and 4-hydroxycoumarins is followed by an intramolecular hDA reaction to yield ci5-fused chromano[4, 3 4,5]pyrano[3,2-c]coumarins e.g. 20 small amounts of the corresponding chromone are also formed <04S1783>. In like manner, cii-fused furopyranopyran derivatives have been obtained from sugar aldehydes <04TL3493>. 

Chromene ring formation can be effected by oxidative cyclisation of 2-prenylphenols or by reaction of pyrano compounds with DDQ (dichlorodicyanobenzoquinone), by reduction of a chromone with a hydride followed by dehydration with a toluenesulphonic acid, and directly by the use of 1,1-dimethoxy-3,3-dimethyl-3-hydroxypropane. 

Suitably 3-substituted chromones take part in cycloaddition reactions. 3-Acyl derivatives react with 1-alkoxyalkenes in an eiu/o-selective fashion to form pyrano[4,3-/ ][l]benzopyran-10-one derivatives from which a range of chromones and chromanones can obtained (94T11755). 

Sulfur analogs of fluorinated chromones and coumarins 13JSC432. Supramolecular photodimerization of coumarins 12MOL1408. Synthesis and chemical reactivity of pyrano[3,2-c]quinolinones 12JHC 1269. 

Stephanidou-Stephanatou and co-workers reported a 4-picoline-catalyzed het-ero-Diels-Alder type cycloadditions of chromone-3-carboxaldehydes 124 with acetylenedicarboxylates 125, providing an one-pot synthesis of pyrano[4,3-c] chrotnenesl27, Scheme 3.42 [57],... 

Hydroxy-8,8-dimethyl-4 l,8li-benzo[l,2-h 3,4-h ]dipyran-4-one, 9CI. 5 Hydroxy-8,8-dimethyl-8H-pyrano[2,3-h] chromone [99365-32-9]... 

Hydroxy-7,7-dimethylpubeschin, see T-30038 5-Hydroxy-8,8-dimethyl-8//-pyrano[2,3-/ ] chromone, see E-30191... 

Pyrano[3,2-g]benzothiazoles.—[CsNS-CsO-Cel- Five isomeric thiazolo-chromones (320)—(324) have been synthesized from the appropriately substituted acetylhydroxybenzothiazoles by their base-catalysed condensation with diethyl oxalate, followed by acid-catalysed cyclization and hydrolysis of the resulting diketone esters [e.g. (319) (320)]. The mass... 

An efficient synthesis of pyrrlo[2,3-d]pyrimidine annulated pyrano[5,6-c]cou-marin/[6,5-c]chromone derivatives was achieved by intramolecular hetero-Diels-Alder reaction by microwave irradiation under solvent-free and solid supported conditions (Ramesh and Raghunathan, 2008). 

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