Chalcones reductions

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... 

The Robinson annulation of ethyl acetoacetate and trans-chalcone proceeded smoothly to give 6-ethoxycarbonyl-3,5-diphenyl-2-cyclohexenone in 48 % yield. The product was separated from the ionic liquid by solvent extraction with toluene. In both these reactions, the ionic liquid [HMIM][PF6] was recycled and reused with no reduction in the product yield. 

Table 3 summarizes the scope and limitation of substrates for this hydrogenation. Complex 5 acts as a highly effective catalyst for functionalized olefins with unprotected amines (the order of activity tertiary > secondary primary), ethers, esters, fluorinated aryl groups, and others [27, 30]. However, in contrast to the reduction of a,p-unsaturated esters decomposition of 5 was observed when a,p-unsaturated ketones (e.g., trans-chalcone, trans-4-hexen-3-one, tra s-4-phenyl-3-buten-2-one, 2-cyclohexanone, carvone) were used (Fig. 3) [30],... 

Phenyllactic acid is converted to the (S)-( - )-hydroxyketone via its ester and amide. Reduction of the (- )-epoxychalcone yields the ( + )-rotating a-hydroxyketone. Therefore the configuration of the a-carbon of the epoxy-chalcone must be R. Since epoxychalcones are derived from /rans-chalcones, the configuration of the (3-carbon in the epoxychalcone must be 5, as shown in Figure 12. 

Hydride ion transfer from formic acid and its salts finds widespread application in the reduction of organic substrates, but limited use has been made of the procedure under phase-transfer catalytic conditions. However in the presence of a ruthenium complex catalyst, it is possible to selectively reduce the C=C bonds of conjugated ketones with sodium formate [11], The rate of reduction is fastest with tetrahexyl-ammonium hydrogensulphate and Aliquat the complete reduction of chalcone being effected within one hour, whereas with benzyltriethylammonium chloride only ca. 15% reduction is observed after two hours under similar conditions. 

The stereoselective epoxidation of chalcones, followed by acid-catalysed ring closure and concomitant cleavage of the epoxide ring, provides a very efficient route to chiral flavon-3-ols and, subsequently, by borohydride reduction to produce flavan-3,4-diols [13, 14], It has been shown that diastereoselective reduction of the chiral flavon-3-ols by sodium borohydride in methanol yields the trans-2,3-dihydroxy compounds, whereas borohydride reduction in dioxan produces the cis-isomers [14] the synthetic procedure confirms the cis configuration of the 2,3-hydroxy groups of naturally occurring leucodelphinidins [14]. 

In effect, antimalarial trojan horse drugs of this type should deliver a double blow to the parasite by exploiting the presence of high concentrations of ferrous ion present in the parasite food vacuole as the trigger for protease inhibitor release. In model studies with prototype 81d, in the presence of ferrous ions, these systems readily degrade to produce the desired chalcone (82b, R = H, in 45% yield from 81d), in tandem with secondary carbon-centred radical 82a (Scheme 29). Furthermore, analogues 81d-f have superior in vitro antimalarial activity to that of arteflene (<25 nM in vitro versus Plasmodium falciparum, arteflene >50 nM). The other product obtained is the diol (82c), a product of two-electron reduction of the endoperoxide bridge. 

FIGURE 16.3 Overview of the biosynthesis of (I) chalcones and (II) 6 -deoxychalcones. The sequential condensation of three molecules of malonyl-CoA (acetate pathway) and p-coumaroyl-CoA (shikimate pathway) is catalyzed by the enzyme chalcone synthase.The production of 6 -deoxychalcones is thought to involve an additional reduction step at the tri- or tetraketide level, catalyzed by polyketide reductase.The origin of the A-ring carbons derived from the acetate pathway is indicated in bold. CoA, coenzyme A. 

Bokadia, M. M, B, R. Brown, D. Cobern, A. Roberts, and G. A. Somer-field Polymerisation of flavans. Part VI. Reduction of flavanoids and chalcones with lithium aluminium hydride in the presence of aluminium chloride. J. chem. Soc. [London] 1962, 1658. 

A study of the influence of the nature of the solid support showed that silica, celite, cellulose or magnesium sulphate in combination with borohydride can also be used successfully in the microwave-assisted reduction of carbonyl compounds. The choice of the solid support has been reported to influence the chemoselectivity of the reduction of chalcone. Under optimised conditions the reduction of the alkene can be suppressed using borohydride on silica, whereas the use of cellulose as solid support seems to favour C=C reduction (Scheme 4.11 )27. 

The retrosynthetic analysis of 2,4,6-triphenylpyrylium tetrafluoroborate (86), involving an initial reduction followed by a disconnection of one carbon-oxygen bond (cf. disconnection of 2,5-dimethylfuran, Section 8.3.1, p. 1146), reveals the substituted 1,5-dicarbonyl compound (89). Further rational disconnection then reveals acetophenone and l,3-diphenylprop-2-en-l-one (chalcone) clearly the latter may originate from acetophenone and benzaldehyde (cf. Section 6.12.2, p. 1032). 

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

When dienones such as 55 are subjected to the epoxidation conditions the electron-poorer C=C double bond is selectively epoxidized. The other C=C bond can be functionalized further, for example, it can be dihydroxylated, as shown in the synthesis of the lactone 56 (Scheme 10.11) [82]. Stannyl epoxides such as 57 (Scheme 10.11, see also Table 10.8, R1 = n-Bu3Sn) can be coupled with several electrophiles [72], reduction of chalcone epoxide 58 and ring opening with alkyl aluminum compounds provides access to, e.g., the diol 59 and to phenylpropionic acids (for example 60). Tertiary epoxy alcohols such as 61 can be obtained with excellent diastereoselectivity by addition of Grignard reagents to epoxy ketones [88, 89]. 

Optically pure (+)-(i )-3-phenylsulfanyl-l,3-diarylpropan-l-ones are readily available by the enantioselective Michael addition of thiophenols to chalcones. After reduction, acidic dehydration of the racemic alcohol affords a mixture of the racemic cis- and trans- 2,4-disubstituted thiochromans (Scheme 164). A detailed consideration of the stereochemical outcome of the reaction with unsymmetrically substituted diaryl derivatives suggests the involvement of a [1,3] PhS shift via a four-membered sulfonium intermediate and this is backed up by theoretical calculations <2003T3621>. 

---