Biomimetic Approach to the Catalytic Enantioselective Synthesis of Flavonoids
Herein is reported the direct asymmetric addition of phenol nucleophiles to benzopyrylium salts as a means to produce enantioenriched flavonoid-like compounds. This enantioselective C\xe2\x80\x93C bond construction was achieved through a chiral anion phase-transfer strategy that mimics the proposed biosynthesis of this structurally diverse set of natural products. The utility of this methodology was demonstrated in enantioselective synthesis of a 2,8-dioxabicyclo[3.3.1]nonane and a 2,4-diarylbenzopyran.
<p>Flavonoid compounds containing 2,4-diarylbenzopyran or 2,8-dioxabicyclo[3.3.1]nonane skeletons comprise a large class of natural products with diverse activity (Figure 1A).1,2 The proposed biosynthetic pathway proceeds through a benzopyrylium salt (A) that engages a phenolic compound (B) in either a simple addition reaction or a formal cycloaddition to form structures of the type C or D, respectively (Figure 1B).2a,3 While progress has been made in the development of methods for the stereoselective construction of these skeletons, to date they have generally relied on the de novo formation of the pyran ring4 and, therefore, have not provided access to the diversity enabled by a biomimetic route.</p><p>The development of an enantioselective biomimetic flavonoid synthesis requires a catalyst capable of promoting stereoselective addition to a planar cationic electrophile. These species generally fall outside of the purview of typical Lewis and Brønsted acid catalysis. On the other hand, chiral anion phase-transfer (CAPT) catalysis has been used to engage a variety of cationic electrophiles as a means to construct asymmetric C–F, C–Br, C–I, and C–N bonds (Figure 2A).5,6 Inspired by these works, we envisioned that this method might be used for the desired enantioselective C–C bond formation. In this scenario, CAPT of an insoluble benzopyrylium cation generates a soluble chiral benzopyrylium ion pair, which can react with phenols to generate enantioenriched flavonoid-type products and release the chiral phosphate (Figure 2B).</p><p>The reaction between 6-Br, 4′-MeO-benzopyrylium hexafluorophosphate (1a) and 1-naphthol was chosen as model reaction to investigate this hypothesis (Table 1).7 In the presence of 2 equiv of Na3PO4 and 10 mol % of (S)-C8-MES (2a), 1a reacted smoothly with 1-naphthol in toluene to give the desired product (3a) with good conversion and low enantioselectivity (–28% ee; Table 1, entry 1). Investigation of other catalysts revealed that chiral phosphoric acids (R)-C8-TRIP (2b) and (R)-C8-TCYP (2c) provided the highest levels of selectivity (61% ee, Table 1, entries 2 and 3). Variation in the reaction solvents or the stoichiometric bases did not improve the enantioselectivity of 3a using (R)-C8-TRIP (2b) as the catalyst (Table 1, entries 7–12). We hypothesized that the product might be sensitive to oxygen (air).8,9 Accordingly, performing the reaction under nitrogen resulted in an improvement in the enantioselectivity to 83% ee (Table 1, entry 13). Furthermore, we found that the reaction reached full conversion even if in the absence of the catalyst (Table 1, entry 15). Finally, lowering the reaction temperature to 0 °C and adding 4 Å molecular sieves furnished 3a in 83% yield and 93% ee (Table 1, entry 17).</p><p>The scope of the chiral anion-catalyzed C–C bond forming reaction was explored using the optimized conditions; however, changing from 1-naphthol to less nucleophilic phenols resulted in lower conversion. Increasing the temperature from 0 °C to room temperature improved conversion, allowing neutral and electron-rich phenols to afford the corresponding products in good yields and excellent enantioselectivities (Table 2, 3a–3e). Substrates in which the C-ring of the benzopyrylium salt was substituted with an electron-rich group at the para or meta position furnished the corresponding product in excellent enantioselectivities (Table 2, 3g–3j, 3l). On the other hand, substitution at the ortho-positions of C-ring decreased the enantioselectivity, although good yield was maintained (Table 2, 3k). With respect to the substitution at A-ring of benzopyrylium salt, substrates bearing electron-poor and electron-rich groups delivered the corresponding product in moderate yields with excellent enantioselectivities (Table 2, 3m–3o).</p><p>On the basis that many flavonoid natural products possess 7-hydroxy and 4′-hydroxysubstituents, we sought to extend the method to a 7 and 4′-substituted benzopyrylium hexafluorophosphate salt. After many attempts, 7-MeO, 4′-Br-benzopyrylium hexafluorophosphate (1j) was identified as optimal for this substitution pattern. For example, 3o was formed in 50% yield and 93% ee.9 Moreover, chiral anion-catalyzed reaction of benzopyrylium salt 1j with 3,5-dimethoxyphenol was followed by acid-catalyzed cyclization to furnish 2,8-dioxabicyclo[3.3.1]nonane 5 in 56% yield and 94% ee (Scheme 1). This skeleton is found in a variety of natural products that demonstrate a wide range of biological activity.10 Derivatization of the enantioenriched product in Table 1 was also investigated. Reaction 1j with 2 (on a 1.0 mmol scale) furnished compound 3o in 44% yield and 92% ee (no loss of ee).11 Palladium-catalyzed conversion of the bromosubstituent of 3o to a methoxy group,12 followed by reducing with H2 produced phenol 6 without loss of enantiomeric purity (71% in 2 steps, 91% ee).13 Notably, phenol 6 contains the core of myristinin B and C (Figure 1A).</p><p>In conclusion, the first example of CAPT catalysis for the enantioselective delivery of carbon-based electrophiles is reported. The use of benzopyrylium-type cations as electrophiles for phenols mimics the pathway proposed for the biosynthesis of flavonoid natural products and, therefore, allows for an expedient enantioselective entry into this class of compounds.</p>