Chalcones, rearrangement with

The treatment of various chalcones 210 with HTIB or DAIB-TSOH in a methanolic medium leads to the / -keto dimethylacetals 211 (Scheme 59) (02S2490). This rearrangement can also be effected with poly[4-(diacetoxy)-iodo]styrene and has been developed into a one-pot synthesis of isoflavones (e.g., 213) from benzoyl-protected 2/-hydroxychalcones (e.g., 212), via base-catalyzed cyclization of the acetal intermediates. 

Farkas et al., [123] synthesised different natural isoflavanones (255) and isoflavans (256) by oxidative rearrangement of 2-hydroxy chalcones (254) with thallium nitrate in methanol followed by acid-catalyzed cyclization as shown in Fig. (11). 

Chalcones can be easily converted to isoflavones rather than flavones by rearrangement with TTN (thallium trinitrate) followed by cyclisation. 

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%... 

Thus, there takes place the formation of only compound 166, but not the regioisomer-5-(4-bromophenyl) -octyl-2-phenyl-rH-spiro[pyrrol-3,2 -qui-noxalin]-3 (4 fl)-one 166 (Scheme 6.74). In an attempt to perform the rearrangement in boiling AcOH, compound 166 was converted to chalcone 167 with the extmsion of the nitrogen atom from the pyrrole ring. The analysis of crude products obtained from the reaction mixture after the evaporation of solvents by H NMR spectroscopy reveals the presence of a spiro-compound 166 and a trace amount of the unreacted starting compounds (37v and 146b). 

Scheme 20), these acetals could readily be converted into isofiavones. Unfortunately, very low yields of rearrangement products were obtained using thallium(III) acetate, and separation and purification of acetals such as (XXXIV) was extremely tedious. Reaction of chalcones with TTN, on the other hand, is generally complete within a few hours at room temperature 95), and Farkas et al. (J75) have developed the Ollis procedure into a simple method for the preparation of isofiavones (Scheme 21). 

A carbon labelling study has elucidated the rearrangement mechanism for formation of chalcone (97) which accompanies formation of (91) by the expected vicinyl elimination of trimethylsilyl and benzotriazolyl groups from 2-benzotriazolyl-2-aryl-3-ketopropylsilanes, on reaction with fluoride ion in DMF. ° Thus, it has been possible to distinguish between the two alternative mechanisms depicted in Scheme 11 (via intermediates (93) or (95), respectively, by determining the fate of the labelled quaternary carbon of substrate (89). The results are consistent with the formation of a cyclopropane intermediate (95) which subsequently ring opens, with relief of strain, to form delocalized carbanion (96), from which the chalcone (97) is obtained (labelled... 

The oxidative rearrangement of chalcones is a valuable route to isoflavones which has been thoroughly investigated. Initially, the conversion was achieved in two distinct steps. Epoxidation of a 2 -benzyloxychalcone, carried out by conventional techniques, is followed by treatment with a Lewis acid, such as boron trifluoride etherate, which brings about the rearrangement. 

As noted already, the reactions of oc,(3-unsaturated ketones with or /zo-diamines are often accompanied by various rearrangements and side processes. For instance, the o-PDA condensation with chalcone in the presence of an acid catalyst or during overheating of the reaction mixture produces a final product of 2-phenylbenzimidazole [2, 7] a similar phenomenon is also observed for 2,3-diaminopyridine [54]. Owing to the existence of much more suitable approaches to the synthesis of 2-aryl derivatives of benzimidazole and their heteroannelated analogues (e.g., proceeding from diamines and acids or aldehydes [121]), such a trend in the interaction of chalcones with diamines is unlikely to have any application for this purpose. 

The formation of pyrazoline derivatives 175 (but not dihydrotetrazolotria-zepines 173, as assumed earlier [53]) is brought about by the condensation of chalcones with diaminotetrazole 172 [128,129]. The structure of the compounds 175 is convincingly verified using X-rays. The mechanism of their formation implies a Dimrothe rearrangement for either the initial diamine or for one of the cyclocondensation intermediates (Scheme 4.51). 

Chalcones under acidic conditions are known to undergo 1,2-aryl migrations with concomitant reductive elimination of iodobenzene yielding rearranged acetals [Eq. (61)] [106]. 

Homopterocarpin (92), a phytoalexin isolated from Pterocarpus santali-nus, was synthesized from chalcone 177 via a rearrangement product (178) by treatment with PIFA in the presence of TFA by Miki and co-workers [135] (Scheme 44). 

Polymer-supported (diacetoxyiodo)benzene (PSDIB) can promote an oxidative rearrangement of 2 -benzyloxy-chalcones to form the intermediate acetal 748, which can be isolated or directly treated with aqueous base to afford... 

Enol acetates seem to be more reactive than ketones, since a steroidal substrate gave the corresponding a-trifluoroacetoxy-ketone at room temperature [5], Chalcones with DIB in trimethyl orthoformate and acid give normally rearranged methoxy-propanoates (Section 3.1). However, a substituted chalcone with BTI yielded a different product which was an intermediate in the synthesis of homopterocarpin [6] ... 

In this case the carboxonium ion captures an external nucleophile which gains electrophilic properties due to this addition. The reaction of carbonyl compounds with nitriles stimulating by acylium ions142 (equation 103) as well as the known synthesis of chalcones 363 from aldehydes and acetylenes154 (equation 104) belong to this type of rearrangement. 

Oxidation of chalcones with Tl(III) salts gives the aryl group rearranged ketals via... 

Molecular oxygen inserts into substituted cyclopentadienes under acidic conditions producing pyrylium salts a hydroperoxide rearrangement is proposed <05JOC5768>. In a further development of the use of benzotriazole (Bt) in synthesis, P-lithiation of the vinyl ether 16 and quenching of the anion with chalcone affords an enol ether. Cyclisation with PCI5 produces 2,4,6-triarylpyrylium salts <05S245>. 

Oxidative rearrangement of chalcones. 4-Methoxychalcone (1) on treatment with thallium triacetate in boiling methanol is converted into the 1,2-diaryl-3,3-dimethoxy-... 

Phenyl-substituted propargyl alcohol couples with 3-iodopyridine to furnish (49) (Scheme 10). On reaction of the 2-iodopyridine (50), however, it was found that the initial alkynylation product (51) rearranged to form the corresponding chalcone (52). The same rearrangement occurs in pyrimidines when the iodine is located in an electrophilic position. In reactions with the corresponding methylpropargyl alcohol, the reaction stops after... 

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