Chalcone, bromination

The A-bromosuccinimidc-hydrogen fluoride/pyridine system was employed to add bromine monofluoride to chalcones in dimethylformamide with the formation of. mt-2-bromo-3-fluoro-... 

The cyclization of a number of chalcone dibromides to flavones has been achieved by treatment with pyridine (81JOC638). It is thought that the reaction may proceed through debromination and dehydrobromination to the chalcone and bromochalcone, respectively (Scheme 166). The formation of pyridine perbromide would account for the nuclear brominated flavones which sometimes accompany the flavone (63CB913). 

Chalcone 1 (2.08 g, 0.01 mol Scheme A.l) was dissolved in 10 ml of chloroform and a solution of 1.6 g (0.01 mol) bromine in 10 ml of chloroform was added dropwise with vigorous stirring (each new portion after decoloration of the reaction mixture). The mixture was allowed to stand overnight and the precipitate of 2,3-dibromo-l,3-diphenylpropan-l-one 2 formed was filtered. Yield 80%. Melting point 160-162°C. 

Chiral crystals generated from non-chiral molecules have served as reactants for the performance of so-called absolute asymmetric synthesis. The chiral environments of such crystals exert asymmetric induction in photochemical, thermal and heterogeneous reactions [41]. Early reports on successful absolute asymmetric synthesis include the y-ray-induced isotactic polymerization of frans-frans-l,3-pentadiene in an all-frans perhydropheny-lene crystal by Farina et al. [42] and the gas-solid asymmetric bromination ofpjp -chmethyl chalcone, yielding the chiral dibromo compound, by Penzien and Schmidt [43]. These studies were followed by the 2n + 2n photodimerization reactions of non-chiral dienes, resulting in the formation of chiral cyclobutanes [44-48]. In recent years more than a dozen such syntheses have been reported. They include unimolecular di- r-methane rearrangements and the Nourish Type II photoreactions [49] of an achiral oxo- [50] and athio-amide [51] into optically active /Mactams, photo-isomerization of alkyl-cobalt complexes [52], asymmetric synthesis of two-component molecular crystals composed from achiral molecules [53] and, more recently, the conversion of non-chiral aldehydes into homochiral alcohols [54,55]. 

BrF is an effective reagent in aromatic brominations of deactivated systems 567,568. IC1 is a selective reagent for the nuclear iodination of hydroxy chalcones without affecting the double bond. This reagent is the active species in the a-iodination of carboxylic acids with I2/SOCl2570. 

If the alkyne in the second step is a 1-aryl propargyl alcohol the product is a bischalcone as a consequence of a CIR-CIR sequence. Furthermore, it is also possible to perform CIR-Heck and CIR-Suzuki sequences if alkenes or boronates and potassium carbonate are added after completion of the initial CIR step. Here again, the gradual differences in reactivity in the oxidative addition between an electron-deficient and an electro-neutral carbon-bromine bond can be readily exploited for selective cross-coupling, first to furnish an aryl propargyl alcohol that is slowly isomerized upon base catalysis to give the bromo chalcone for further cross-coupling. 

Nitroflavone has been obtained by conversion of o-benzyloxy-2-nitro-acetophenone to the chalcone, debenzylation and cyclization to the 3-nitro-flavanone,102 and aromatization by bromination-dehydrobromination (Scheme 21).103... 

Just as bromine chloride is a more reactive electrophile than iodine chloride so one would expect bromine azide acting as an electrophile to be more reactive than iodine azide. In fact in solvents suitable for heterolytic reaction, bromine azide adds to chalcone less readily than iodine azide ° and this further indicates an Adj mechanism. The evidence is strengthened by the occurrence of acid catalysis of the bromine azide addition. 

At a lower oxidation level, orffto-hydroxy-acyl-arenes undergo base-catalysed aldol condensations with aromatic aldehydes to give chalcones, °° which can be cyclised to 2,3-dihydro-chromones via an intramolecular Michael process the dihydro-chromones can in turn be dehydrogenated to produce chromones by a variety of methods, for example by bromination then dehydrobromination or by oxidation with the trityl cation, iodine, dimethyldioxirane or iodobenzene diacetate. ° ... 

Achiral molecules can also crystallize into enantiomorphic 3-D crystals such that, when the conformational deracemization is faster than the processes of crystallization, unequal amounts of the two enantiomorphous crystals will be formed. Such a crystallization event will, therefore, lead to a neat process of mirror-symmetry breaking. A topochemical asymmetric transformation in appropriate motifs of such single crystals should result in the formation of polymers of a single handedness. The pioneering study of such class of absolute asymmetric transformations, by Penzien and Schmidt [35], was the gas-sohd asymmetric bromination of the nonchiral p, pf -dimethyl-chalcone, which crystaUizes in an enantiomorphous crystal. Following this study, the details of over 20 such photoinitiated and thermally initiated reactions have been reported [36-42]. 

Since elemental bromine (Br2) is volatile and highly corrosive, special systems, for example, pyridinium tribromide and related compound, are commonly used to generate Br2 in situ [2]. A method for stereoselective bromination of stilbene and chalcone in a water suspension medium using pyridinium tribromide has been reported [2]. Bromination reactions of ( )-stilbene and ( )-chalcone in a water suspension medium by salts... 

The chalcone is brominated in the second step (Experiment [F2]) to )4eld eryflzro-2,3-dibromo-3-(4-nitrophenyl)propiophenone ... 

As the first example of asymmetric synthesis using chiral crystals involving solid-gas reaction, ) some other interesting examples of solid-gas reaction using chiral crystals were reported. Reaction of chiral crystals of chalcone derivative with bromine in connection with rearrangement gave optically active dibromide in 8%... 

One such reaction is the heterogeneous addition of gaseous Br2 to 4,4 -dimethylchal-cone. This chalcone molecule is achiral in solution, but crystallizes into a chiral crystal. When bromine gas is added to a single crystal of this material one obtains a chiral dibromide, with one enantiomer in excess [7] (Scheme 4). 

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