Chalcone dibromides

Ceroplastol synthesis, 1, 428 Cetyl alcohol synthesis, 1, 478 Chaetoglobasins structures, 4, 376 Chalcone, o -azido-2 -oxy-synthesis, 3, 823 Chalcone, 2-hydroxy-reduction, 3, 751 Chalcone, 2 -hydroxy-mass spectra, 3, 618 Chalcone dibromides flavone synthesis from, 3, 823 Chalcones polymers, 1, 304 Chanoclavine synthesis, 6, 423 Charge density waves in stacks of ions, 1, 351-352 Chartreusin... 

Chalcone dibromides are advantageous intermediates for the preparation of various nitrogen-containing heterocycles (refs. 1-4). In the case of exocyclic a,P-unsaturated ketones, however, only few examples are known concerning the utilization of their dibromides for such purposes (ref. 5). Our aim was, therefore, the synthesis of the dibromides of various exocyclic a,P-unsaturated ketones (ref. 6) and to study their chemical transformations. In our present paper the reaction of such dibromides with azide nucleophile is reported. 

The imine 217, formed from acetophenone and the 1-aminoimidazole, has been transformed into the enamine 218 which is then cyclized to 219 in trifluoroacetic acid (TFA) (Equation 40) <1995SC3271 >. The diaminoimidazole 220 reacts with ynone 221 to form 222 (Equation 41) <1998CHE1189>. Bromochalcones and chalcone dibromides can also be used in place of the ynone. Similar reactions have been used to prepare the dihydro analogues of 222 <1999CHE1207>. The reaction between 1-aminoimidazoles and 1,3-diketones has been extended to prepare bis-heterocyclic compounds, for example, 223 as ligands for transition metals (Equation 42) <2005EJI4382>. 

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). 

Reaction of chalcone dibromides with sodium azide and subsequent treatment with base gives access to 3-aminoflavones (79LA174). The a-azido-2 -oxychalcones are also available from phenacyl bromides by reaction with sodium azide and then benzaldehyde. 

Synthesis of annelated diazepines based on unsaturated aromatic diazepines may involve the preliminary transformation of ketones into the corresponding l,3-diaryl-2,3-dibromopropane-3-ones (chalcone dibromides). The interaction between o-PDA or some of its substituted analogues with chalcone dibromides leads to the formation of aziridine derivatives [64] (see Chap. 1). However, in the case of 4-nitro-o-PDA, either azirenoquinoxalynes 53 or benzodiazepine derivatives 54 may be obtained depending on the reaction conditions [65] (Scheme 4.15). Diazepine derivatives 56 are obtained by the condensation of chalcone dibromides 51 with 5,6-diamino-1,3-dimethyluracil 55 [66], but aziridine derivatives are not isolated in this reaction. It should be noted that compounds 54 and 56 are formed owing to cyclization of the intermediate (3-enaminoketones [65, 66, 67] and are easily isolated from the reaction mixture. 

Methoxy-l-benzopyrans are prepared at or below room temperature from 2-bromophenols and 3-trimethylsilyloxyacroleins. Protection of the phenolic group and lithiation precede treatment with the acrolein when the protecting group is removed with cold mineral acid, cyclization occurs in good yield. Chalcone dibromides cyclize in a basic medium but yields of pyrans are often better under weakly basic conditions, as is demonstrated in this synthesis of a flavone in dilute alkali at room temperature. 

In agreement with earlier studies on the chalcone dibromides , the dehydro-chlorinations of 4,4 -dichlorochalcone dichlorides are not stereospecific . The threo isomer yields the trans olefin chiefly (see (30) below), as expected for elimination of HX from an " anti conformation (see Section 2.3), but the erythro isomer gives a mixture of olefins in which the trans form predominates over the cis by a factor of 2 1 (31). 

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