Multiple hydride reduction pathways in isoflavonoids
BackgroundIsoflavonoids are of interest owing to their appearance in metabolic pathways of isoflavones, and their estrogenic and other physiological properties, making them promising lead compounds for drug design. ResultsThe reduction of isoflavones by various hydride reagents occurs by a 1,4-pathway in contrast to ordinary β-alkoxy-α,β-unsaturated ketones. Isoflavan-4-ones, cisand trans-isoflavan-4-ols, α-methyldeoxybenzoins or 1,2-diphenylprop-2-en-1-ols are obtained depending on the hydride reagent, mostly in good yields. The stereoselective reduction of isoflavan-4-ones is also discussed. ConclusionThe work described in this paper shows that most structural types of reduced isoflavonoids are now reliably available in satisfactory or good yields by hydride reductions to be used as authentic reference compounds in analytical and biological studies.
Background<!>Results and Discussion<!>2-Isoflaven-4-ols (11)<!>Supporting Information
<p>The reduction of isoflavones 1 has been actively studied during the last twenty years, owing to the range of interesting biological effects [1][2][3] -estrogenic activity, promise in cancer, osteoporosis, and coronary heart disease prevention -shown by the isoflavones themselves and their reduced metabolites. There are some reports [4][5][6][7][8] of total syntheses of reduced isoflavonoid structures from commercially available starting materials but overall yields in these multistep procedures tend to be low, and free hydroxy groups are not compatible. Another strategy involves the hydrogenation of isoflavones using a palladium or platinum catalyst but mixtures of reduction products are often formed [9,10]. Certain hydride reagents have been tested, as discussed below, for the reduction of simple or protected isoflavones but many of the early results are contradictory or rely on indeterminate product characterization.</p><p>The reduction of simple (nonflavonoid) β-alkoxy-α,β-unsaturated ketones by hydride reagents (NaBH 4 , LiAlH 4 , DIBAH) occurs normally by 1,2-attack, this being a key step in the well known "carbonyl transposition" of 1,3-diketone enol ethers into enones (R 3 O-CR=CR 1 -COR 2 → R-CO-CR 1 =CHR 2 ). [11][12][13][14][15] Prior to our work there were no reports on the reduction of isoflavones containing free hydroxy groups which nevertheless are common, usually at one or several of the C-5, C-7 and C-4' sites, in the naturally occurring isoflavonoids. It is to be expected that the presence of the phenolic hydroxyls, or the derived phenolate anions, will alter the reactivity pattern of the parent isoflavone system, besides perhaps decreasing the overall reactivity due to solubility reasons. For example, any tendency of hydride attack at the C-2 will be opposed by electron feeding from the 4'-OH group while an OH group at C-5 or C-7 will discourage attack at C-2 and C-4. Thus there was ample room for the development of reliable methods for the synthesis of hydroxy-substituted isoflavone metabolites, and for clarification of the course of reduction of isoflavones with various hydride reducing agents. We present here experimental details of our own results in this field together with a thorough survey of the literature. The discussion is based on the types of reduced isoflavonoid structures formed (Figure 1), i.e., isoflavanones 2, cis-isoflavan-4-ols 3, trans-isoflavan-4-ols 4, the ring opened α-methyldeoxybenzoins 5 and 1,2-diaryl-2-propen-1-ols 6, and isoflavenes 7 and 8 (hydride reductions do not lead to isoflavans 9, which however are obtained by catalytic hydrogenation [10,16,20]). Incidentally, it is appropriate to point out that flavones do not undergo similar reductive metabolism in mammals as described above for isoflavones. We will nevertheless present at a later date certain findings on the hydride reduction pathways in flavones.</p><!><p>Isoflavanones (2) Isoflavanones result from the 1,4-reduction of isoflavones. The resonance contributor 10 will encourage this mode of attack. DIBAH, normally a preferential 1,2-reducer, reacts with methoxy-, benzyloxy-, MOMO-and MEMO-substituted isoflavones to give the isoflavanones in 40-93% yield [16,[21][22][23] (Table 1). Our own work has shown that even unprotected hydroxy-substituted isoflavones are reduced by a large excess of DIBAH in 50-70% yield (Table 1). The simple borohydride reagents reduce isoflavones to the isoflavanols (see below) but the Selectrides ® give 60-88% yields of isoflavanones in the absence of hydroxy substituents according to our results (Table 1). There is no significant difference between the K-and L-Selectride ® . In the literature, there is an isolated report of the reduction of a MOM substituted isoflavone by L-Selectride ® in 40% yield (Table 2) [16]. Methoxy substituted isoflavones have also been reduced by sodium hydrogen telluride to give isoflavan-4-ones in 61-71% yields [24].    trans-isoflavan-4-ols 4 (Table 3). [16,29,34] Isoflavanones are reduced by electrophilic hydrides (borane-tetrahydrofuran, bistert-butylthioethane borane) diastereoselectively to cisisoflavan-4-ols, but a large excess of the reducing agent is usually needed [29].</p><p>Although it was realized by the early workers that diastereomeric mixtures of isoflavanols would presumably be formed, there were no reliable methods to determine their structures. In some papers, the products are summarily assigned the cis [25,26] or trans [35][36][37][38] structures. However, rigorous NMR analysis has recently made it possible to establish cisand trans-structures for the isoflavanol products and to study their conformational equilibria [32,34]. Borohydride reductions generally give a small preference for the cis products as suggested by Cram's rule [39]. We are not aware of any examples in the literature of single enantiomers of isoflavanone or isoflavanol metabolites, nor have such compounds been reported as hydride reduction products of isoflavones. In view of the significant biological properties of the reduced metabolites it will be interesting to examine the behaviour of the pure enantiomers.</p><!><p>There are very few reports of this class of compounds, either from reductive processes or otherwise. In 1965, the reduction of the parent isoflavone by NaBH 4 in EtOH or diborane in THF was claimed [28] to furnish 2-isoflaven-4-ol in 75-76% yield, but unfortunately the characterization of this product relied on elemental analysis only. More recently, Japanese workers [40] reported that the reduction of 12 (Figure 2) by NaBH 4 in the presence of PdCl 2 in THF-H 2 O gave a 1:1 mixture of the ketone 13a and the diol 13b (Figure 2). A 1 H NMR spectrum was given for the latter isoflavenol but there appear to be certain discrepancies, notably in the δ value (8.69) reported for the H-8 which is some 2 δ units in excess of what would be expected for such a vinyl ether proton. Thus more work is required to fully confirm the nature of this class of reduction products. In the event, in our studies the reduction of isoflavone 1a by NaBH 4 in the presence of PdCl 2 in THF-H 2 O gave a 82:18 mixture of cisand trans-isoflavan-4-ol 3a, 4a while no 2-isoflaven-4-ols were observed. The absence of such 1,2-reduction products, or structures conceivably derivable thereof such as 2-or 3-isoflavenes (7,8), even in the CeCl 3 -complexed NaBH 4 reductions, must reflect the good stabilization obtainable via resonance heteroring stabilization (10) in the isoflavones. As already mentioned, this is in contrast to the behaviour of simple nonflavonoid β-alkoxy-α,β-unsaturated ketones which prefer 1,2attack by hydride.</p><p>Isoflavenes (7,8) and isoflavans (9) In the early work, [41] there is a mention of 7,4'-dimethoxy-2methyl-3-isoflavene being obtained in 15% yield from the reduction of the corresponding isoflavone by LiAlH 4 in Et 2 Obenzene but there is no structural data on the product other than elemental analysis, itself quite accurate. Similarly 2',4'dimethoxy-3',6,7-trihydroxyisoflav-3-ene was reported from the reduction of 2',4'-dimethoxy-3',6,7-trihydroxyisoflavan-4-one with LiAlH 4 in low yield [42]. More recent work by us [43] and others [16,44] would indicate that the normal course of LiAlH 4 reduction of isoflavones leads to deoxybenzoins and propenols (see below and Table 4).</p><p>Published syntheses of 2-and 3-isoflavenes involve the reduction of 3-arylcoumarins, [45][46][47] the corresponding aldehyde hemiacetals, [7] isoflavylium salts [48][49][50] or of isoflavones by the Clemmensen reaction [51]. Low-yielding non-reductive routes to 2-and/or 3-isoflavenes have also been reported [52,53]. 2-Isoflavenes 7 however remain mostly poorly character-ized, and some of the NMR spectral details reported [7,51,52] appear inconsistent with the 2-isoflavenoid structure. As far as the NMR spectra of 2-isoflavenes are concerned, a recent study clears this issue by 2D NMR work on a natural 2-isoflavene, [54] establishing that the H-2 and H-4 protons appear at δ 6.87 and 3.61, respectively, much as expected by correlation data shift calculations. Isoflavans (9) have not been prepared by hydride reductions, but by catalytic hydrogenation of isoflavones [10,[16][17][18][19][20].</p><p>Deoxybenzoins ( 5) and propenols (6) Isoflavanones undergo a facile retro-Michael-type ring opening [21,55] under basic conditions to give the propenone intermediate 14 (Figure 2), sometimes considered [56] to be an independent isoflavone metabolite but presumably just an artefact in reality. As regards the synthesis of isoflavanones by DIBAH reduction of isoflavones (see above), we found that unless the workup is done with cold methanolic HCl, some amount of the propenone 14 will be formed and reduced further to the deoxy-benzoin 5. If on the other hand the deoxybenzoins are the actual synthetic targets, the reducing agent of choice is LiAlH 4 in THF. This works very well for isoflavones bearing a hydroxy group at C-7 such as genistein, but in isoflavones lacking a 7-OH group another reaction pathway competes leading to the propenols 6 [43] as byproducts (see below). We have discussed a possible mechanism to explain these hydroxyl-dependent divergent pathways [43].</p><p>To summarize, all hydride addition reactions with isoflavones appear to involve an initial 1,4-addition to give the isoflavanone enolate. In a hydroxylic solvent, or even on workup under basic conditions, the ketone is generated, and reduced further to the saturated alcohol (NaBH 4 ). In a nonprotic solvent, the β-aryloxyenolate will undergo a retro-Michael addition, giving the phenolate anion of the ring opened 2-propen- The dietary isoflavonoids are mainly metabolized in man via reductive pathways, leading to the reduced structural types discussed above (Figure 3). These compounds are often more estrogenic than the starting isoflavones. Research interest in many fields including medicine, nutrition and biosynthesis and metabolism thus converge on the reduced isoflavonoids. The work described in this paper shows that most structural types of reduced isoflavonoids are now reliably available in satisfactory or good yields by hydride reductions. Although not discussed here, it is clear that D atoms may be introduced in the same way which is very useful in the quantitation of the naturally occurring compounds by GC-MS selected ion monitoring techniques [61].</p><!><p>Supporting Information File 1</p><p>Experimental details and characterisation data.</p><p>[http://www.beilstein-journals.org/bjoc/content/ supplementary/1860-5397-2-16-S1.doc]</p>