Individually, heterocycles and fluorinated compounds have attracted much attention of chemists in pharmaceutical and agrochemical industries over the past few decades. Thus, a search for novel drug candidates based on fluorinated heterocyclic frameworks has become a new dependable strategy in modern medicinal chemistry. In 2004, 3,5-diaryl-5-(trifluoromethyl)-2-isoxazoline derivatives were disclosed as promising agrochemicals of pest control agents by Nissan chemical industries. This discovery promptly induced many academic and industrial chemists to focus on these small trifluoromethylated heterocycles as important leads of agrochemicals and veterinary medicines. A large number of trifluoromethylated heterocyclic variants have been designed, including isoxazolines, pyrrolines and pyrazolines with a quaternary carbon bearing a CF3 group at the 5-position as a common structural feature (Figure 1).[4-6] In this context, we were interested in 4-functionalized 3,5-diaryl-5-(trifluoromethyl)-2-isoxazoline derivatives as novel lead candidates of drug discovery in a future market. The potential of 1-, 2-, 3- and 5-positions at this small ring are well investigated; however, research on the functionalization at the 4-position of this scaffold is rather immature.
The trifluoromethanesulfonyl (triflyl, SO2CF3, Tf) group is the strongest electron-withdrawing group (EWG), equivalent to the nitro group (Hammett substituent constants: SO2CF3, σp=0.83, σm=0.96; NO2, σp=0.71, σm=0.78), while the lipophilicity of the two is exactly opposite (SO2CF3, πp=0.55; NO2, πp=−0.28)[1a],  Thus, introduction of the triflyl group into organic molecules is an effective method to dramatically alter the chemical properties of parent molecules without changing molecular complexities. Indeed, the triflyl group has been successfully used in the fields of chiral catalysts, pharmaceuticals, and advanced functional materials. As part of our research programs directed at the design and synthesis of biologically attractive heteroaryl triflones and diaryl-5-(trifluoromethyl)-2-isoxazoline derivatives, we were fascinated by unknown 4-functionalized triflones derivatives 1. However, functionalization at the 4-position using a well-studied building block strategy is rare.[2-6] In 2011, we reported trifluoromethylation at the 5-position of isoxazoles by nucleophilic addition using the Ruppert–Prakash reagent, (trifluoromethyl)trimethylsilane (Me3SiCF3).[12b] The key for this transformation is the activation of nonreactive aromatic isoxazoles with a nitro group at the 4-position, which achieved the first trifluoromethylation of aromatic isoxazoles. In contrast to the nitro group, we envisioned that the triflyl group should (1) activate aromatic isoxazoles, (2) promote additional functionalization at the 4-position, and (3) have a potential biological function. We disclose herein the synthesis of 5-trifluoromethyl-4-triflyl-2-isoxazolines 1 (X=H, Figure 1, Scheme 1) by direct additive trifluoromethylation of isoxazole triflones 2 with Me3SiCF3 in high yields and with high diastereoselectivities. The resulting isoxazoline triflones were efficiently halogenated under natural conditions to provide all-carbon functionalized isoxazoline triflones 1 (X=F, Cl, Br) in excellent diastereoselectivities individually, or in a one-pot sequential procedure from isoxazole triflones 2. The triflyl analogue of antiparasiticide 1 p was prepared by this method. Cinnamyl-substituted isoxazoline triflone 1 q was also synthesized regioselectively under the same reaction condition (Scheme 1).
We first examined the reaction of 3,5-diphenyl-4-(trifluoromethanesulfonyl)isoxazole (2 a) as a model substrate under the same reaction condition employed for the previously reported additive trifluoromethylation of 4-nitroisoxazoles,[12b] namely, with Me3SiCF3 using NaOAc in N,N-dimethylformamide (DMF) in the presence of a catalytic amount of cetyltrimethylammonium bromide ([CH3(CH2)15N(CH3)3]Br) at ambient temperature. The desired trifluoromethylated product was obtained with moderate yield and excellent diastereoselectivity (43 %, d.r.=94:6; Entry 1, Table 1). The yield improved to 65 % in the absence of cetyltrimethylammonium bromide (65 %, Entry 2). After screening several bases (Entries 1–9), the yield of 1 a was increased to 76 % when the reaction was carried out using KOAc (Entry 9). The choice of solvent is crucially important in conversion, and the best result was obtained by treating 2 a with Me3SiCF3 (2.0 equiv) at room temperature in dimethyl sulfoxide (DMSO) in the presence of KOAc (1.5 equiv). Substrate 2 a was completely consumed in 3 h, and the desired product 1 a was obtained in 91 % yield (Entry 13).
|Entry[a]||Base||Solvent||t [h]||Yield [%][b]|
With optimal conditions in hand, the scope of the trifluoromethylation of isoxazole triflones 2 was explored with a variety of substrates selected in order to establish the generality of the process using this strategy (see Table 2). A series of 3,5-diary-4-triflyl-isoxazole 2 b–g with a variety of substituents at their aromatic rings (Ar) such as methyl, methoxy, bromo, chloro, nitro as well as sterically demanding naphthyl moiety, were nicely converted to the corresponding trifluoromethylated adducts 1 b–g in high to excellent yield (80–96 %) with excellent diastereoselectivities (d.r.=93:7–97:3; Table 2, Entries 2–7). Heteroaryl-substituted isoxazole triflone 2 h was also a suitable substrate for this transformation, affording trifluoromethylated adduct 1 h in 85 % yield with d.r.=99:1 (Entry 8). Interestingly, for isoxazole triflone 2 i, which has an enolizable proton, the trifluoromethylation reaction proceeded nicely to provide the corresponding CF3 adduct 1 i in good yield as a single diastereomer (Entry 9). We next examined the substrate scope differing in the nature of the aryl substituents of Ar1. A series of isoxazole triflones 2 j–o were nicely converted to SO2CF3-substituted 5-trifluoromethyl-2-isoxazolines 1 j–o in 87–99 % yield with high diastereoselectivities (d.r.=93:7–95:5), these being almost independent of the functional groups on the aromatic ring Ar1, such as, methyl, methoxy, chloro, bromo and nitro as well as sterically demanding naphthyl moiety (Entries 10–15). With facile access to this range of 5-trifluoromethyl-4-triflyl-2-isoxazolines 1, we next considered the synthesis of multiply substituted triflyl-isoxazoline 1 p. 5-Trifluoromethyl-2-isoxazoline 3 is a compound possessing high antiparasitic activity (Scheme 1); therefore, its SO2CF3 derivative 1 p would be an attractive biologically active candidate (Figure 1). As expected, trifluoromethylation of 2 p smoothly proceeded to give 1 p in 98 % yield. The relative stereochemistry of (4S*,5R*)-1 h was clearly determined by X-ray analysis (Figure 3), and the stereochemistry of all the other products 1 were tentatively assigned by 1H and 19F NMR spectral comparison with 1 h.
|Entry[a]||2||Ar (or Me)||Ar1||1||d.r.[b]||Yield [%][c]|
|1||2 a||Ph||Ph||1 a||94:6||91|
|2[d]||2 b||4-MeC6H4||Ph||1 b||93:7||85|
|3||2 c||4-MeOC6H4||Ph||1 c||95:5||96|
|4[d]||2 d||4-ClC6H4||Ph||1 d||97:3||88|
|5||2 e||4-BrC6H4||Ph||1 e||96:4||90|
|6||2 f||4-NO2C6H4||Ph||1 f||97:3||80|
|7[d]||2 g||2-naphthyl||Ph||1 g||96:4||93|
|8||2 h||2-furanyl||Ph||1 h||99:1||85|
|9||2 i||Me||Ph||1 i||100:0||64|
|10||2 j||Ph||4-MeC6H4||1 j||93:7||87|
|11||2 k||Ph||4-MeOC6H4||1 k||93:7||89|
|12[d]||2 l||Ph||4-ClC6H4||1 l||94:6||89|
|13[d]||2 m||Ph||4-BrC6H4||1 m||94:6||89|
|14||2 n||Ph||4-NO2C6H4||1 n||95:5||92|
|15[d]||2 o||Ph||2-naphthyl||1 o||96:4||99|
|16||2 p||3,5-Cl2C6H3||4-MeOC6H4||1 p||95:5||98|
Trifluoromethylation of cinnamyl-substituted isoxazole triflone 2 q was next investigated (Scheme 2). Under the same reaction condition, trifluoromethylated adduct 1 q at the 5-position of isoxazoline was regioselectively obtained in 80 % yield as single diastereomer, accompanied with small amounts of 1,6-conjugated adduct 4 (9 %).
We finally examined the halogenation reactions of trifluoromethylated adduct 1 a to all-carbon functionalized isoxazolines (Scheme 3), since 4-halogenated 3,5-diaryl-5-(trifluoromethyl)-2-isoxazoline derivatives have emerged as targets in 2009. Fluorination of 1 a with Selectfluor® in acetonitrile gave a highly functionalized 4-fluoro-5-(trifluoromethyl)-4-(trifluoromethanesulfonyl)-2-isoxazoline 1 a-F in high yield with good diastereoselectivity (89 %, d.r.=80:20). It should be noted that the fluorination of nitro-analogue 5 under the same reaction condition failed to provide any fluorination product. This phenomenon apparently resulted from strong lipophilic and electron-withdrawing features of the triflyl group. Other halogenation reactions were also smoothly achieved, namely chlorination and bromination, under simple conditions using N-chlorosuccinimide (NCS) and N-bromosuccinimide (NBS) to afford 1 a-Cl and 1 a-Br in excellent yields with 94 % and 96 %, respectively.
The trifluoromethylation/halogenation reactions proceeded in a one-pot sequential protocol without any loss of yield and selectivity. Namely, after completion of the first trifluoromethylation of 2 a with Me3SiCF3, monitored by TLC analysis, a solution of halogenating reagents (Selectfluor®, NCS or NBS) in acetonitrile was added into the reaction mixture. It should be noted that both yields and diastereoselectivities were comparable to the results achieved for the two-step reactions (Scheme 4).
It is known that nucleophilic trifluoromethylation to conjugated alkenes essentially proceeds by a 1,2-addition, not a 1,4-addition except for several specific cases. We thus propose that this trifluoromethylation is explained by a 1,2-type addition of a CF3 anion to a species A, which is a resonance structure of 2 a, to provide intermediates B. The electrophilic approach of H+ or X+ to B from the face opposite to the existing phenyl group furnish 1 a or 1 a-X with high diastereoselectivities (Scheme 5).
In summary, activation of aromatic isoxazoles with a triflyl group at the 4-position realized the regio- and diastereoselective trifluoromethylation of isoxazole triflones 2 by Me3SiCF3 at the 5-position, which directly provided biologically potent, highly functionalized 3,5-disubstituted-5-(trifluoromethyl)-2-isoxazoline featuring the SO2CF3 group at the 4-position. Hence, the triflyl group not only activates the 5-position of aromatics and 4-position of resulting isoxazolines but also provides a new class of highly functionalized 5-trifluoromethyl-2-isoxazoline derivatives 1 attractive for agrochemicals. Biological activities of selected 5-(trifluoromethyl)-2-isoxazoline derivatives 1 and asymmetric variants of this method are now under consideration.
This study was financially supported in part by the Platform for Drug Discovery, Informatics, and Structural Life Science, Grant-in-Aids for Scientific Research on Innovative Areas (“Advanced Molecular Transformations by Organocatalysts”) (24105513), and Scientific Research (B) from MEXT Japan (25288045), ACT-C from JST, Exploratory Research (25670055). We also thank TOSOH F-TECH INC. for a gift of Me3SiCF3.