Trifluoromethyl Vinyl Sulfide: A Building Block for the Synthesis of CF3S-Containing Isoxazolidines.

Trifluoromethyl vinyl sulfide, a potential building block for pharmaceutically and agrochemically relevant products, is prepared and used for the first time in high-pressure-mediated 1,3-dipolar cycloaddition reactions with nitrones to synthesize (trifluoromethyl)sulfanyl isoxazolidines.

Introduction

The (trifluoromethyl)sulfanyl group (SCF3) represents a privileged substituent in agrochemicals and pharmaceuticals because of the strong electron-withdrawing effect and the large Hansch lipophilicity parameter (π = 1.44).[1] Cefazaflur,[2] a first-generation cephalosporin antibiotic, toltrazuril,[3] an antiprotozoal agent, and a losartan analogue,[4] developed as a potential hypotensive agent, are prominent examples of biologically active compounds bearing the SCF3 group (Figure ).

Figure 1

Bioactive heterocycles containing the SCF3 group.

Hence, CF3S-containing compounds are considered appealing targets in the agrochemical and pharmaceutical fields, and consequently, modern research has focused on efficient (trifluoromethyl)sulfanylation methods. The main strategies that have been developed to synthesize CF3S-containing compounds have focused on direct C–S bond formation[5] and trifluoromethylation of sulfur-containing compounds.[6] However, incorporation via simple CF3S-containing building blocks is still unprecedented so that the use of a CF3S-containing reagent could be an alternative strategy to construct CF3S-substituted heterocycles. More specifically, trifluoromethyl vinyl sulfide (2) could be an attractive building block to provide straightforward access to a wide variety of hetero- and carbocycles via cycloaddition reactions. Previously reported examples of cycloaddition reactions of alkene 2 are rather scarce: there is a single example of a Diels–Alder reaction with 2,3-dimethylbutadiene to yield the corresponding CF3S-cyclohexene,[7] and two cyclopropanation examples with organomercury reagents to yield CF3S-substituted cyclopropanes.[8]

The potential of this unexplored chemistry and the inherent biological attractiveness of these molecules motivated us to further study the reactivity of alkene 2 in cycloaddition reactions. Thus, we herewith present the study of the first 1,3-dipolar cycloaddition reactions of 2 with several nitrones[9] to synthesize a novel group of isoxazolidines (Scheme ).

Scheme 1

Synthesis of CF3S-Containing Isoxazolidines

Results and Discussion

We commenced our investigations by forming alkene 2 in situ. Because of its high volatility,[7] we first performed the elimination reactions in deuterated solvents in order to instantly measure conversions from commercially available chloro alkane 1 into alkene 2, with no need for further treatment or isolation of the alkene. We tested several bases (Et3N, DBU, KOBu, KOH and KOTMS) in the presence of various deuterated solvents (CD2Cl2, THF-d8, CD3OD and DMF-d7; see the Supporting Information (SI) for experimental details) to eventually choose a solution of KOBu in THF-d8 at 21 °C for 90 min as the final method for the in situ synthesis of alkene 2 with full conversion.

Initially, we focused on the synthesis of 2,5-disubstituted isoxazolidines 3 using N-substituted hydroxylamines 5 and paraformaldehyde as precursors for the in situ synthesis of the dipoles (Table ). The nitrone intermediates were used without isolation in a one-pot process with a preformed solution of alkene 2 (3 equiv) and In(OTf)3 (0.06 equiv). In this manner, we obtained isoxazolidine 3a in 22% yield after column chromatography. Increasing the concentration of 2 (6 equiv) resulted in a yield of 38% (entry 1). Isoxazolidines 3b–e were synthesized in yields between 20 and 30% under the same reaction conditions with different substrates (entries 2–5). However, yields were rather low after column chromatography in all cases (see the SI for experimental details).

Table 1

Synthesis of Isoxazolidines 3a–e

entry	compd	R	yield (%)
1	3a	C6H5	38
2	3b	3-CF3C6H4	20
3	3c	3,5-Cl2C6H3	22
4a	3d	C6H5CH2	30
5a	3e	Cy	23

Et3N (1.0 equiv) was used.

With these results in hand and because of the low yields in the synthesis of 3a–e, we changed our strategy to the synthesis of 2,3,5-trisubstituted isoxazolidines 4, exhibiting a second (hetero)aromatic group at the 3-position and introducing consequently a second chiral center in the molecule (Scheme , Table ).

Scheme 2

Formation of Compounds 4, 7, and 8

Table 2

Optimization Process for the 1,3-Dipolar Cycloaddition Reaction of Nitrones 6 with Alkene 2

entry	nitrone	R	2 (equiv)	T (°C)	scavenger	cis-4/trans-4	8a (%)	conva (yield)b (%)
1	6c	4-MeC6H4	1.3	21		76:22	5	83 (41)c
2	6c	4-MeC6H4	3	21		79:21	22	93 (−)
3	6c	4-MeC6H4	3	50		76:24	6	96 (85)c
4	6b	4-FC6H4	1.3	21		79:21	7	56 (35)c
5	6b	4-FC6H4	3	21		79:21	38	75 (−)
6	6b	4-FC6H4	3	50		77:23	17	91 (79)c
7	6b	4-FC6H4	3	50	galvinoxyl (3%)	76:24	0	100 (78)
8	6b	4-FC6H4	3	50	galvinoxyl (1%)	76:24	1	100 (−)

Calculated by integration of the 1H NMR signals of the crude mixtures.

Isolated after column chromatography.

trans-Isomer contaminated with 7.

We first performed the reaction with nitrone 6a (R = H), a catalytic amount of In(OTf)3, and alkene 2 at 21 °C in THF, but no conversion into the product was observed. When the same conditions were applied at 80 °C with nitrone 6c (R = Me), less than 7% conversion into the final product was observed by 1H NMR, presumably because of the low reactivity of this alkene and perhaps due to its volatility. Therefore, we decided to perform these reactions under high-pressure conditions (Table ), which is an established and powerful tool to increase the rate of cycloaddition reactions.[10] Thus, when 1.3 equiv of 2 at 21 °C under a 15 kbar pressure was used (entry 1) we gratifyingly observed an 83% conversion into isoxazolidines 4c and 7c (regioisomeric ratio 4c/7c 13:1, diastereomeric ratio [dr] cis-4c/trans-4c 7:2). Nitrone 6b gave only 56% conversion under the same reaction conditions (entry 4), with similar regio- and diastereoisomeric ratios. When increasing the concentration of alkene 2, 93 and 75% conversions into compounds 4c and 4b were obtained, respectively (entries 2 and 5). Finally, an increase of the temperature to 50 °C gave nearly full conversion into the desired cis- and trans-products 4c and 4b (entries 3 and 6).

Azoxybenzene[11]8 was formed in all reactions as a side product (Scheme ) and could not be separated in any case from trans-4 by column chromatography. Thus, we used galvinoxyl to prevent the formation of compound 8.[12] We were pleased to see that when using 3 equiv of alkene 2 at 50 °C, in the presence of 1–3 mol % of galvinoxyl as radical scavenger (entries 7 and 8), not only full conversion into the desired product 4b was obtained, but also the formation of the azoxy compound 8 was fully suppressed.

Furthermore, our reactions gave the exo-products as the major diastereoisomers (the cis-isomers), showing a 3:1 diastereomeric ratio (entries 7 and 8). Having these results in hand, the scope of nitrones in this 1,3-dipolar cycloaddition was examined by employing the conditions shown in entry 7. We first synthesized a total of 18 nitrones (6a–r) with phenyl (containing both electron-donating and electron-withdrawing groups at the 2-, 3-, and 4- positions), heterocyclic and carbamoyl substituents in excellent yields (see the SI). We then studied the scope of the 1,3-dipolar cycloaddition reactions between alkene 2 and nitrones 6a–r (Table ).

Table 3

Scope of the High-Pressure-Promoted 1,3-Dipolar Cycloaddition Reaction

Combined yield.

Calculated by 1H NMR of the crude.

Galvinoxyl was not used in the reaction.

cis-4o,p/trans-4o,p/(mixture of isomers 7).

As shown in Table , high and excellent yields (total yields) were observed for compounds 4a–c, 4f–i, 4l, and 4n. In addition, in most of the cases the major product (cis-4) was separated from the other isomers by column chromatography in good yields (cis-4a–c, cis-4f–j, cis-4m, and cis-4q). Unfortunately, in a few cases the isomers 4 were obtained as a mixture that could not be separated (4d,e, 4k,l, and 4n). For compounds 4o and 4p, the trans-4 isomer and the corresponding regioisomers 7 were present as a mixture.[13] The assignment of the cis- and trans-isomers was performed by 2D NMR studies (NOESY).

The diastereoselectivity was lower for compounds 4o and 4p, which contained an indole and a pyrrole substituent, respectively. In these cases, a mixture of cis- and trans-diastereoisomers was obtained approximately in a 3:2 ratio. In addition, for compounds 4o and 4p, a 7:3 ratio of a mixture cis-7 and trans-7 from the regioisomer 7 was observed. For these two compounds (4o and 4p), the total yields were significantly lower because of incomplete conversion into the products and purification problems. Finally, when using a nonaromatic nitrone, the final benzyl amide isoxazolidine 4r was synthesized in 60% yield.

To evaluate the apparent regioselectivity of the cycloaddition, high-level DFT calculations were carried out at the BP86/TZ2P[14] level, using the ADF program.[15] Tetrahydrofuran was simulated using the COSMO solvation model.[16] Thermochemical corrections were computed using the temperature and pressure under which the experiments were performed (see the SI for computational details).

We focused on understanding the regioselectivity of the cycloaddition between a simple model nitrone 9 with alkene 2 (Scheme ). This reaction proceeds in a concerted and asynchronous manner with a Gibbs free energy barrier of 21.3 and 24.0 kcal mol–1 for 10 and 11, respectively (Table ). The ΔΔG⧧ of 2.7 kcal mol–1 facilitates a high degree of regioselectivity, resulting in a calculated product ratio of 98:2 for 10/11. Furthermore, the experimentally observed regioisomer 10 is more stable than 11 (ΔΔGrxn = 3.9 kcal mol–1). The calculated product ratio for the model nitrone cycloaddition is in line with experimentally observed regioselectivity of reactions involving the majority of the nitrones 6.

Scheme 3

Formation of Compounds 10 and 11

Table 4

Computed Activation Barriers, Reaction Energies (kcal mol–1), and Product Distribution Computed at the COSMO(THF)-BP86/TZ2P Level of Theory

compd	ΔE⧧ (ΔG⧧)	ΔErxn (ΔGrxn)	product ratioa
10	12.8 (21.3)	–17.6 (−6.1)	98
11	15.0 (24.0)	–14.3 (−2.2)	2

Calculated at 50 °C and 15 kbar based on the ΔΔG⧧ between 10 and 11 (see the SI for details).

Insight into why regioisomer 10 is favored over the other, 11, is provided by the activation strain model (ASM)[17] (also known as the distortion-interaction model).[18] In this framework, the potential energy surface ΔE(ζ) is decomposed along the reaction coordinate ζ into the strain ΔEstrain(ζ) associated with deforming the individual reactants plus the actual interaction ΔEint(ζ) between the deformed reactants.

The ΔEint(ζ) between the reactants is further analyzed by an energy decomposition analysis (EDA) in the conceptual framework provided by the Kohn–Sham molecular orbital (KS-MO) model[19] and is decomposed into three physically meaningful terms:The ΔVelstat(ζ) term corresponds to the classical electrostatic interaction between unperturbed charge distributions, ΔEPauli(ζ) is responsible for any steric repulsion, and the ΔEoi(ζ) accounts for charge transfer (HOMO–LUMO interactions) and polarization.

Applying the ASM along the reaction coordinate defined by the bending of the 1,3-dipole, it is revealed that reactivity differences in the reaction between 9 and 2 leading to 10 (blue curve) and 11 (red curve) are a result of ΔEint (Figure a). ΔEstrain remains nearly constant for both reactions along the reaction coordinate. Next, the EDA terms were analyzed, and the ΔEPauli dominates and is chiefly responsible for the difference in the interaction energies (Figure b). The more favorable ΔVelstat and ΔEoi curves associated with the reaction leading to 11 are unable to overcome for increased steric repulsion associated with this approach. These results highlight the fact that the reaction results from the least hindered approach, leaving the SCF3 group far away from the CH2 group. See the SI for an ASA and EDA analysis on 6m and 6p.

Figure 2

(a) Activation strain and (b) energy decomposition analyses of the cycloaddition reactions of nitrone 9 with alkene 2 up to their respective TS (indicated by the dot) computed at the BP86/TZ2P level.

In summary, we have developed the synthesis of a new class of 5-[(trifluoromethyl)sulfanyl]isoxazolidines. The reactions utilized for their synthesis were high-pressure-promoted 1,3-dipolar cycloaddition reactions between trifluoromethyl vinyl sulfide 2 and various easily synthesized nitrones. The results of our DFT computations are in harmony with experimental results and were leveraged to show that the high regioselectivity of these cycloaddition reactions originates from minimizing steric repulsion.

Experimental Section

General Information

Reagents were obtained from commercial suppliers and were used without purification. Standard syringe techniques were applied for the transfer of dry solvents and air- or moisture-sensitive reagents. Tetrahydrofuran was used as a solvent after distillation. Potassium tert-butoxide was used as a 1.0 M solution in tetrahydrofuran. The high-pressure experiments were run in a high-pressure apparatus equipped with a one-wall-piston cylinder for pressures up to 15 kbar (1.5 GPa). Reactions were performed in 1–1.5 mL PTFE ampules closed by screwed stainless steel stoppers. These ampules were inserted into the high-pressure vessel filled with 80–100 petroleum ether as transmission medium. Reactions were followed, and R values were obtained, using thin layer chromatography (TLC) on silica gel-coated plates (Merck 60 F254) with the indicated solvent mixture. Detection was performed with UV light, and/or by charring at ca. 150 °C after dipping into a solution of KMnO4. Infrared spectra were recorded on an IR-ATR Bruker TENSOR 27 spectrometer. High-resolution mass spectra were recorded on a JEOL AccuTOF (ESI) or a MAT900 (EI, CI, and ESI) GC-TOF. NMR spectra were recorded at 298 K on a Varian Inova 400 (400 MHz), Bruker Avance III 400 MHz or Bruker Avance III 500 MHz spectrometer in the solvent indicated. Chemical shifts are given in parts per million (ppm) with respect to tetramethylsilane (0.00 ppm) as internal standard for 1H NMR; and CDCl3 (77.16 ppm) as internal standard for 13C NMR. Coupling constants are reported as J values in hertz (Hz). 1H NMR data are reported as follows: chemical shift (ppm), multiplicity (s = singlet, d = doublet, dd = doublet of doublets, dt = doublet of triplets, dtdq = doublet of triplet of doublet of quartets, dq = doublet of quartets, ddd = doublet of doublet of doublets, ddt = doublet of doublets of triplets, dddd= doublet of doublets of doublets of doublets, ddquint = doublet of doublet of quintets, dddquint = doublet of doublet of doublet of quintets, quint = quintet, t = triplet, td = triplet of doublets, tt = triplet of triplets m = multiplet, b = broad), coupling constants (Hz), integration and assignment. Compounds were fully characterized by 1H, 13C, 2D gDQCOSY, gHSQC, gHMBC, NOESY, and HOESY spectra. The characterization and assignment of isoxazolidines 4a–r were done either from isolated compounds or mixtures of isomers. Column or flash chromatography was carried out using ACROS silica gel (0.035–0.070 mm, and 60 Å pore diameter).

General Procedure for the Synthesis of Trifluoromethyl Vinyl Sulfide 2

2-Chloroethyl trifluoromethyl sulfide (1.0 equiv) was dissolved in distilled THF (0.40 M) under inert atmosphere and cooled to 0 °C. Then, KOBu (1.0 equiv, 1.0 M solution in THF) was added slowly, and the mixture was warmed to 21 °C for 90 min. This solution was used for the cycloaddition reactions of alkene 2. 1H NMR [400 MHz, δ (ppm), THF-d8]: 6.54 (dd, J = 16.5, 9.4 Hz, 1 H), 5.75–5.66 (m, 2 H). 13C NMR [101 MHz, δ (ppm), THF-d8]: 129.8 (q, J = 306.5 Hz), 124.4 (q, J = 1.0 Hz), 121.3 (q, J = 3.2 Hz). 19F NMR [377 MHz, δ (ppm), THF-d8]: −43.6.[7]

General Procedure for the Synthesis of Hydroxylamines 5b and 5c

The corresponding nitrobenzene (12b or 12c; 1 equiv) was added to a flame-dried Schlenk tube containing a suspension of Pd/C (0.25 equiv) in dry THF (10 mL). The mixture was cooled to 0 °C, and N2H4·H2O (2.0 equiv) was added dropwise. The reaction was carefully monitored by TLC until the starting material disappeared. The crude mixture was diluted with THF (10 mL) and filtered over a pad of diatomaceous earth. The solvent was evaporated in vacuo, and the crude mixture was purified by column chromatography using CH2Cl2 under a nitrogen atmosphere.

N-[3-(Trifluoromethyl)phenyl]hydroxylamine (5b)

According to general procedure, the reaction of 1-nitro-3-(trifluoromethyl)benzene 12b (1.0 g, 5.23 mmol) afforded hydroxylamine 5b (885 mg, 5.00 mmol) as a pale yellow oil. 1H NMR [400 MHz, δ (ppm), CDCl3]: 7.41–7.34 (m, 1 H), 7.28–7.26 (m, 1 H), 7.23–7.19 (m, 1 H), 7.15–7.10 (m, 1 H), 6.74–4.98 (bs, 2 H). R: 0.31 (CH2Cl2). Yield: 95%. NMR spectral data are in accordance with previously reported data.[20]

N-(3,5-Dichlorophenyl)hydroxylamine (5c)

According to general procedure, the reaction of 1,3-dichloro-5-nitrobenzene 12c (1.0 g, 5.22 mmol) afforded hydroxylamine 5c (827 mg, 4.65 mmol) as a yellow solid. 1H NMR [400 MHz, δ (ppm), CDCl3]: 6.93 (t, J = 1.8 Hz, 1 H), 6.88 (d, J = 1.8 Hz, 2 H), 6.79 (bs, 1 H), 5.31 (bs, 1 H). R: 0.33 (CH2Cl2). Yield: 89%. NMR spectral data are in accordance with previously reported data.[20]

General Procedure for the Synthesis of Isoxazolidines 3a–c

A solution of the corresponding hydroxylamine 5 (1 equiv) in THF (0.22 M) was added to a solution of paraformaldehyde (3 equiv) in THF (0.22 M) at 0 °C. Then a cooled solution (0 °C) of alkene 2 (6.0 equiv) was subsequently added to the solution containing paraformaldehyde and the hydroxylamine. Finally, a solution of In(OTf)3 (0.06 equiv) in dry THF (44 mM) was added to the reaction mixture. The reaction mixture was warmed to 21 °C and stirred for 16 h. Then, the reaction mixture was quenched with a saturated aqueous solution of NaHCO3 (10 mL), and the aqueous layer was extracted with AcOEt (2 × 10 mL). The combined organic layers were dried over MgSO4, filtered off and concentrated in vacuo. The crude mixture was purified by column chromatography using the indicated eluent to afford isoxazolines 3a–c.

2-Phenyl-5-[(trifluoromethyl)sufanyl]isoxazolidine (3a)

According to the general procedure, the reaction of N-phenylhydroxylamine 5a (17.8 mg, 0.163 mmol) afforded isoxazolidine 3a (15.3 mg, 0.163 mmol) as a brown solid after column chromatography (heptane/AcOEt, 19:1 → 4:1). 1H NMR [500 MHz, δ (ppm), CDCl3]: 7.34–7.27 (m, 2 H), 7.08–7.05 (m, 2 H), 7.03 (tt, J = 7.3, 1.1 Hz, 1 H), 5.98 (dd, J = 8.1, 4.3 Hz, 1 H), 3.85–3.71 (m, 1 H), 3.24 (ddd, J = 9.3, 8.2, 7.6 Hz, 1 H), 2.91 (dtdq, J = 13.2, 8.2, 4.0, 0.8 Hz, 1 H), 2.48–2.28 (m, 1 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 149.8, 130.1 (q, J = 307.7 Hz), 129.0, 123.0, 116.1, 81.1 (q, J = 2.2 Hz), 52.0, 36.0 (q, J = 1.1 Hz). 19F NMR [471 MHz, δ (ppm), CDCl3]: −39.8. FTIR [ν̅ (cm–1)]: 2860, 1599, 1491, 1295, 1109, 752, 692. HRMS (EI) m/z: [M+•] calcd for C10H10F3NOS 249.0435, found 249.0458. R: 0.56 (heptane/AcOEt, 4:1). Yield: 38%.

2-[3-(Trifluoromethyl)phenyl]-5-[(trifluoromethyl)sulfanyl]isoxazolidine (3b)

According to the general procedure, the reaction of hydroxylamine 5b (56.2 mg, 0.318 mmol) afforded isoxazolidine 3b (23.5 mg, 0.071 mmol) as a colorless oil, after column chromatography (heptane/AcOEt/Et3N, 12.2:1:0.13). 1H NMR [500 MHz, δ (ppm), CDCl3]: 7.41 (ddquint, J = 8.2, 7.2, 0.8 Hz, 1 H), 7.29–7.25 (m, 2 H), 7.23–7.19 (m, 1 H), 6.01 (dd, J = 8.0, 4.2 Hz, 1 H), 3.83 (ddd, J = 9.1, 8.2, 4.3 Hz, 1 H), 3.30 (ddd, J = 9.1, 8.5, 7.4 Hz, 1 H), 3.02–2.90 (m, 1 H), 2.48–2.38 (m, 1 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 150.1, 131.3 (q, J = 32.3 Hz), 129.8 (q, J = 307.9 Hz), 129.4, 124.0 (q, J = 272.4 Hz), 119.2 (q, J = 3.8 Hz), 118.9, 112.3 (q, J = 4.0 Hz), 81.1 (q, J = 2.4 Hz), 51.7, 35.8 (q, J = 1.2 Hz). 19F NMR [471 MHz, δ (ppm), CDCl3]: −39.9, −62.8. FTIR [ν̅ (cm–1)]: 2956, 2925, 2855, 1593, 1328, 1120, 795, 698. HRMS (EI) m/z: [M+•] calcd for C11H9F6NOS 317.0309, found 317.0325. R: 0.44 (heptane/AcOEt, 4:1). Yield: 22%.

2-(3,5-Dichlorophenyl)-5-[(trifluoromethyl)sulfanyl]isoxazolidine (3c)

According to the general procedure, the reaction of hydroxylamine 5c (29.3 mg, 0.166 mmol) afforded isoxazolidine 3c (10.33 mg, 0.033 mmol) as a green oil after column chromatography (heptane/AcOEt/Et3N, 12.2:1:0.13). 1H NMR [500 MHz, δ (ppm), CDCl3]: 6.99 (t, J = 1.8 Hz, 1 H), 6.91 (d, J = 1.8 Hz, 2 H), 5.98 (dd, J = 8.0, 4.1 Hz, 1 H), 3.75 (ddd, J = 8.9, 8.1, 4.4 Hz, 1 H), 3.27 (ddd, J = 9.2, 8.6, 7.1 Hz, 1 H), 3.01–2.87 (m, 1 H), 2.47–2.36 (m, 1 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 151.6, 135.4, 129.7 (q, J = 307.9 Hz), 122.5, 114.2, 81.2 (q, J = 2.6 Hz), 51.6, 35.8 (q, J = 1.1 Hz). FTIR [ν̅ (cm–1)]: 2924, 2850, 1587, 1260, 1115, 1051, 796. HRMS (EI) m/z: [M+•] calcd for C10H8Cl2F3NOS 316.9656, found 316.9657. R: 0.45 (heptane/AcOEt, 4:1). Yield: 20%.

General Procedure for the Synthesis of Isoxazolidines 3d and 3e

A solution of hydroxylamine hydrochloride 5d or 5e (1 equiv) in THF (0.22 M) and Et3N (1 equiv) was added to a solution of paraformaldehyde (3 equiv) in THF (0.22 M) at 0 °C. Then a cooled solution (0 °C) of alkene 2 (6.0 equiv) was subsequently added to the solution containing paraformaldehyde and the hydroxylamine. Finally, a solution of In(OTf)3 (0.06 equiv) in dry THF (22 mM) was added to the reaction mixture. The reaction mixture was warmed to 21 °C and stirred for 16 h. Then, the reaction mixture was quenched with a saturated aqueous solution of NaHCO3 (10 mL), and the aqueous layer was extracted with AcOEt (2 × 10 mL). The combined organic layers were dried over MgSO4, filtered off, and concentrated in vacuo. The crude mixture was purified by column chromatography using the indicated eluent to afford isoxazolines 3d and 3e.

2-Benzyl-5-[(trifluoromethyl)sulfanyl]isoxazolidine (3d)

According to the general procedure, the reaction of hydroxylamine hydrochloride 5d (26.8 mg, 0.169 mmol) afforded isoxazolidine 3d (13.2 mg, 0.050 mmol) as a colorless oil after column chromatography (heptane/AcOEt/Et3N, 9:1:0.1). 1H NMR [500 MHz, δ (ppm), CDCl3]: 7.40–7.36 (m, 2 H), 7.35–7.30 (m, 2 H), 7.30–7.27 (m, 1 H), 5.86 (dd, J = 8.3, 4.4 Hz, 1 H), 4.35–3.82 (m, 2 H), 3.34–3.11 (m, 1 H), 3.11–2.40 (m, 1 H), 2.83 (dtd, J = 11.9, 8.0, 3.3 Hz, 1 H), 2.40–2.18 (m, 1 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 136.2, 130.3 (q, J = 307.9 Hz), 128.9, 128.4, 127.5, 81.3 (q, J = 2.4 Hz), 61.7, 53.3, 36.5. 19F NMR [471 MHz, δ (ppm), CDCl3]: −39.9. FTIR [ν̅ (cm–1)]: 2849, 1606, 1497, 1455, 1117, 755, 698. HRMS (EI) m/z: [M+•] calcd for C11H12F3NOS 263.0592, found 263.0604. R: 0.41 (heptane/AcOEt, 4:1). Yield: 30%.

2-Cyclohexyl-5-[(trifluoromethyl)sulfanyl]isoxazolidine (3e)

According to the general procedure, the reaction of hydroxylamine hydrochloride 5e (23.1 mg, 0.152 mmol) afforded isoxazolidine 3e (20.4 mg, 0.080 mmol) as a colorless oil, after column chromatography (heptane/AcOEt, 19:1 → 5:1). 1H NMR [400 MHz, δ (ppm), CDCl3]: 5.82 (dd, J = 8.1, 4.2 Hz, 1 H), 3.43–3.13 (m, 1 H), 2.85–2.73 (m, 1 H), 2.68–2.50 (m, 1 H), 2.68–2.18 (m, 1 H), 2.31–2.18 (m, 1 H), 1.83–1.69 (m, 2 H), 1.66–1.52 (m, 2 H), 1.39–1.13 (m, 6 H). 13C NMR [101 MHz, δ (ppm), CDCl3]: 130.4 (q, J = 308.1 Hz), 80.8 (q, J = 2.2 Hz), 65.8, 51.5, 36.6, 26.0, 24.8, 24.4. 19F NMR [377 MHz, δ (ppm), CDCl3]: −40.0. FTIR [ν̅ (cm–1)]: 2924, 2854, 1728, 1455, 1258, 1082, 1009, 788. R: 0.44 (heptane/AcOEt, 9:1). Yield: 53%.

General Procedure A for the Syntheses of Nitrones 6d–g, 6i, 6j, 6l–r

The corresponding aldehyde (1.0 equiv, dissolved in 1.5 mL of dry EtOH for solid compounds) was added to a solution of N-phenylhydroxylamine (1.05 equiv) in dry EtOH (1.0 mL) at 21 °C under a nitrogen atmosphere. The reaction mixture was stirred for 16–21 h, and then the solvent was removed under vacuo. The crude mixture was purified by column chromatography using the indicated eluent to afford nitrones 6d–g, 6i, 6j, 6l–r.

General Procedure B for the Syntheses of Nitrones 6a–c, 6h, and 6k

The corresponding aldehyde (1.0 equiv) was added to a solution of N-phenylhydroxylamine (1.0 equiv) in dry EtOH (1.0 mL), and the flask was wrapped with aluminum foil. The reaction mixture was stirred at 21 °C for 16–21 h, and then the solvent was removed under vacuo. The crude mixture was purified by column chromatography using the indicated eluent to afford nitrones 6a–c, 6h, and 6k.

(Z)-N,1-Diphenylmethanimine Oxide (6a)[21]

According to general procedure B, the reaction of benzaldehyde (47.0 μL, 0.458 mmol) with N-phenylhydroxylamine (50 mg, 0.458 mmol) afforded nitrone 6a (71 mg, 0.360 mmol) as a white solid, after column chromatography (heptane/AcOEt, 5:1 → 1:1). 1H NMR [400 MHz, δ (ppm), CDCl3]: 8.44–8.36 (m, 2 H), 7.92 (s, 1 H), 7.81–7.74 (m, 2 H), 7.53–7.43 (m, 6 H). Yield: 79%.

(Z)-1-(4-Fluorophenyl)-N-phenylmethanimine Oxide (6b)[22]

According to general procedure B, the reaction of 4-fluorobenzaldehyde (98.0 μL, 0.916 mmol) with N-phenylhydroxylamine (100 mg, 0.916 mmol) afforded nitrone 6b (178.5 mg, 0.829 mmol) as a white solid. 1H NMR [400 MHz, δ (ppm), CDCl3]: 8.49–8.42 (m, 2 H), 7.91 (s, 1 H), 7.80–7.75 (m, 2 H), 7.55–7.44 (m, 3 H), 7.23–7.14 (m, 2 H). Yield: 91%.

(Z)-N-Phenyl-1-(4-tolyl)methanimine Oxide (6c)[22]

According to general procedure B, the reaction of 4-methylbenzaldehyde (163.0 μL, 1.375 mmol) with N-phenylhydroxylamine (127 mg, 1.375 mmol) afforded nitrone 6c (267.2 mg, 1.26 mmol) as yellow crystals, after column chromatography (heptane/AcOEt, 5:1 → 1:1). 1H NMR [400 MHz, δ (ppm), CDCl3]: 8.34–8.27 (m, 2 H), 7.89 (s, 1 H), 7.81–7.75 (m, 2 H), 7.51–7.43 (m, 3 H), 7.33–7.28 (m, 2 H), 2.42 (s, 3 H). Yield: 92%.

(Z)-N-Phenyl-1-[4-(trifluoromethyl)phenyl]methanimine oxide (6d)[23]

According to general procedure A, the reaction of 4-(trifluoromethyl)benzaldehyde (91 mg, 0.523 mmol) with N-phenylhydroxylamine (60 mg, 0.549 mmol) afforded nitrone 6d (139 mg, 0.523 mmol) as a yellow solid. 1H NMR [400 MHz, δ (ppm), CDCl3]: 8.54–8.48 (m, 2 H), 8.00 (s, 1 H), 7.83–7.76 (m, 2 H), 7.76–7.70 (m, 2 H), 7.55–7.47 (m, 3 H). Yield: 99%.

(Z)-1-[4-(Pentafluoro-λ6-sulfanyl)phenyl]-N-phenylmethanimine Oxide (6e)

According to general procedure A, the reaction of 4-(pentafluoro-λ6-sulfanyl)benzaldehyde (120 mg, 0.517 mmol) with N-phenylhydroxylamine (59 mg, 0.543 mmol) afforded nitrone 6e (163.3 mg, 0.505 mmol) as a white-yellow solid. 1H NMR [400 MHz, δ (ppm), CDCl3]: 8.52–8.45 (m, 2 H), 8.00 (s, 1 H), 7.89–7.81 (m, 2 H), 7.81–7.74 (m, 2 H), 7.58–7.46 (m, 3 H). 13C NMR [101 MHz, δ (ppm), CDCl3]: 154.5, 149.1, 133.6, 132.5, 130.7, 129.5, 128.9, 126.5 (quint, J = 4.4 Hz), 121.9. FTIR [ν̅ (cm–1)]: 3057, 1594, 1573, 1074, 821, 810, 765. HRMS (ESI-TOF) m/z: [M + H]+ calcd for C13H11F5NOS 324.0476, found 324.0482. R: 0.45 (CH2Cl2). Yield: 98%.

(Z)-1-(4-Methoxyphenyl)-N-phenylmethanimine oxide (6f)[24]

According to general procedure A, the reaction of 4-methoxybenzaldehyde (0.063 mL, 0.514 mmol) with N-phenylhydroxylamine (59 mg, 0.540 mmol) afforded nitrone 6f (110 mg, 0.484 mmol) as a yellow-brown solid, after column chromatography (CH2Cl2 → CH2Cl2/MeOH, 10:0.2). 1H NMR [400 MHz, δ (ppm), CDCl3]: 8.44–8.36 (m, 2 H), 7.84 (s, 1 H), 7.79–7.72 (m, 2 H), 7.50–7.37 (m, 3 H), 7.01–6.93 (m, 2 H), 3.86 (s, 3 H). Yield: 94%.

(Z)-1-(3-Fluorophenyl)-N-phenylmethanimine Oxide (6g)[25]

According to general procedure A, the reaction of 3-fluorobenzaldehyde (0.060 mL, 0.564 mmol) with N-phenylhydroxylamine (62 mg, 0.564 mmol) afforded nitrone 6g (118 mg, 0.548 mmol) as a light brown solid, after column chromatography (heptane/CH2Cl2, 1:4 → CH2Cl2/MeOH, 10:1). 1H NMR [400 MHz, δ (ppm), CDCl3]: 8.44 (ddd, J = 10.6, 2.6, 1.6 Hz, 1 H), 7.94 (s, 1 H), 7.90 (ddt, J = 8.1, 1.6, 0.7 Hz, 1 H), 7.80–7.73 (m, 2 H), 7.54–7.47 (m, 3 H), 7.44 (td, J = 8.1, 5.9 Hz, 1 H), 7.17 (tdd, J = 8.1, 2.6, 1.0 Hz, 1 H). 13C NMR [101 MHz, δ (ppm), CDCl3]: 162.8 (d, J = 245.5 Hz), 149.1, 133.5, 132.6 (d, J = 9.1 Hz), 130.4, 130.1 (d, J = 8.4 Hz), 129.4, 125.1 (d, J = 3.0 Hz), 121.8, 118.0 (d, J = 21.7 Hz), 115.3 (d, J = 24.7 Hz). Yield: 97%.

(Z)-N-Phenyl-1-(3-tolyl)methanimine Oxide (6h)[25]

According to general procedure B, the reaction of 3-methylbenzaldehyde (42 μL, 0.458 mmol) with N-phenylhydroxylamine (50 mg, 0.458 mmol) afforded nitrone 6h (95.3 mg, 0.451 mmol) as a yellow oil, after column chromatography (heptane/AcOEt, 5:1 → 1:1). 1H NMR [400 MHz, δ (ppm), CDCl3]: 8.32 (tq, J = 1.8, 0.7 Hz, 1 H), 8.12 (dddquint, J = 7.8, 1.8, 1.2, 0.7 Hz, 1 H), 7.89 (s, 1 H), 7.81–7.75 (m, 2 H), 7.53–7.44 (m, 3 H), 7.38 (t, J = 7.8 Hz, 1 H), 7.32–7.27 (m, 1 H), 2.43 (s, 3 H). 13C NMR [101 MHz, δ (ppm), CDCl3]: 149.3, 138.5, 134.9, 132.0, 130.8, 130.0, 129.4, 129.3, 128.7, 126.6, 121.9, 21.6. FTIR [ν̅ (cm–1)]: 3092, 2963, 2920, 1652, 1590, 1459, 1088, 781, 692. HRMS (ESI-TOF) m/z: [M + H]+ calcd for C14H14NO 212.1070, found 212.1085. Yield: 98%.

(Z)-1-[3-(Methoxycarbonyl)phenyl]-N-phenylmethanimine Oxide (6i)

According to general procedure A, the reaction of methyl 3-formylbenzoate (90 mg, 0.548 mmol) with N-phenylhydroxylamine (63 mg, 0.576 mmol) afforded nitrone 6i (112 mg, 0.439 mmol) as a yellow oil, after column chromatography (CH2Cl2 → CH2Cl2/MeOH, 10:1). 1H NMR [400 MHz, δ (ppm), CDCl3]: 8.88 (dddd, J = 7.9, 1.7, 1.2, 0.5 Hz, 1 H), 8.80 (tt, J = 1.7, 0.5 Hz, 1 H), 8.13 (ddd, J = 7.9, 1.7, 1.2 Hz, 1 H), 8.01 (d, J = 0.5 Hz, 1 H), 7.83–7.75 (m, 2 H), 7.59 (tt, J = 7.9, 0.5 Hz, 1 H), 7.54–7.46 (m, 3 H), 3.95 (s, 3 H). 13C NMR [101 MHz, δ (ppm), CDCl3]: 166.5, 149.0, 133.6, 132.6, 131.6, 131.0, 130.6, 130.4, 130.2, 129.3, 129.0, 121.7, 52.4. FTIR [ν̅ (cm–1)]: 3064, 2952, 1718, 1434, 1278, 1187, 1072, 912, 729, 684. HRMS (ESI-TOF) m/z: [M + H]+ calcd for C15H14NO3 256.0968, found 256.0984. R: 0.07 (CH2Cl2). Yield: 80%.

(Z)-1-(3-Methoxyphenyl)-N-phenylmethanimine Oxide (6j)

According to general procedure A, the reaction of 3-methoxybenzaldehyde (0.063 mL, 0.514 mmol) with N-phenylhydroxylamine (59 mg, 0.540 mmol) afforded nitrone 6j (115 mg, 0.506 mmol) as a yellow oil, after column chromatography (CH2Cl2 → CH2Cl2/MeOH, 10:0.2). 1H NMR [400 MHz, δ (ppm), CDCl3]: 8.38 (dd, J = 2.7, 1.5 Hz, 1 H), 7.92 (s, 1 H), 7.81–7.75 (m, 2 H), 7.66 (dddd, J = 7.7, 1.5, 1.0, 0.5 Hz, 1 H), 7.52–7.44 (m, 3 H), 7.38 (dd, J = 8.3, 7.7 Hz, 1 H), 7.05 (ddd, J = 8.3, 2.7, 1.0 Hz, 1 H), 3.89 (s, 3 H). 13C NMR [101 MHz, δ (ppm), CDCl3]: 159.7, 149.1, 134.7, 131.9, 130.0, 129.5, 129.2, 122.3, 121.8, 118.1, 112.7, 55.4. FTIR [ν̅ (cm–1)]: 3060, 2834, 1553, 1270, 1025, 769, 684. HRMS (ESI-TOF) m/z: [M + Na]+ calcd for C14H13NO2Na 250.0839, found 250.0857. R: 0.06 (CH2Cl2). Yield: 98%.

(Z)-N-Phenyl-1-[3-(trifluoromethyl)phenyl]methanimine Oxide (6k)[25]

According to general procedure B, the reaction of 3-(trifluoromethyl)benzaldehyde (61 μL, 0.458 mmol) with N-phenylhydroxylamine (50 mg, 0.458 mmol) afforded nitrone 6k (116 mg, 0.437 mmol) as a white solid, after column chromatography (heptane/AcOEt, 5:1 → 1:1). 1H NMR [400 MHz, δ (ppm), CDCl3]: 8.70 (dq, J = 2.3, 1.0 Hz, 1 H), 8.59 (dddd, J = 7.9, 1.7, 1.2, 0.6 Hz, 1 H), 8.01 (s, 1 H), 7.84–7.75 (m, 2 H), 7.72 (dq, J = 8.5, 1.1 Hz, 1 H), 7.61 (t, J = 7.9 Hz, 1 H), 7.55–7.48 (m, 3 H). 13C NMR [101 MHz, δ (ppm), CDCl3]: 149.1, 133.1, 131.8 (q, J = 1.0 Hz), 131.5, 131.5, 130.5, 129.5, 129.3, 127.3 (q, J = 3.7 Hz), 125.7 (q, J = 3.9 Hz), 122.6, 121.9. FTIR [ν̅ (cm–1)]: 3124, 1604, 1330, 1120, 1068, 770, 692. HRMS (ESI-TOF) m/z: [M + H]+ calcd for C14H11F3NO 266.0787, found 266.0799. R: 0.35 (CH2Cl2). Yield: 95%.

(Z)-N-Phenyl-1-(2-tolyl)methanimine Oxide (6l)[26]

According to general procedure A, the reaction of 2-methylbenzaldehyde (0.061 mL, 0.524 mmol) with N-phenylhydroxylamine (60 mg, 0.551 mmol) afforded nitrone 6l (107 mg, 0.508 mmol) as a yellow-brown solid, after column chromatography (CH2Cl2 → CH2Cl2/MeOH, 10:0.2). 1H NMR [400 MHz, δ (ppm), CDCl3]: 9.42–9.32 (m, 1 H), 8.07 (s, 1 H), 7.81–7.73 (m, 2 H), 7.53–7.44 (m, 3 H), 7.39–7.33 (m, 2 H), 7.28–7.23 (m, 1 H), 2.46 (s, 3 H). Yield: 97%.

(Z)-1-(2-Methoxyphenyl)-N-phenylmethanimine Oxide (6m)[25]

According to general procedure A, the reaction of 2-methoxybenzaldehyde (70 mg, 0.514 mmol) with N-phenylhydroxylamine (59 mg, 0.540 mmol) afforded nitrone 6m (106 mg, 0.464 mmol) as an orange-brown solid, after column chromatography (CH2Cl2 → CH2Cl2/MeOH, 10:0.2). 1H NMR [400 MHz, δ (ppm), CDCl3]: 9.48 (dd, J = 7.9, 1.7 Hz, 1 H), 8.40 (s, 1 H), 7.85–7.73 (m, 2 H), 7.53–7.39 (m, 4 H), 7.13–7.06 (m, 1 H), 6.93 (dd, J = 8.4, 1.1 Hz, 1 H), 3.89 (s, 3 H). 13C NMR [101 MHz, δ (ppm), CDCl3]: 157.6, 149.8, 132.3, 129.8, 129.4, 129.2, 128.9, 122.0, 121.0, 120.1, 110.0, 55.8. FTIR [ν̅ (cm–1)]: 3058, 1577, 1288, 1046, 746, 691, 611. R: 0.07 (CH2Cl2). Yield: 90%.

(Z)-N-Phenyl-1-(thiophen-3-yl)methanimine Oxide (6n)[27]

According to general procedure A, the reaction of thiophene-3-carbaldehyde (0.047 mL, 0.535 mmol) with N-phenylhydroxylamine (61 mg, 0.562 mmol) afforded nitrone 6n (109 mg, 0.535 mmol) as a white-yellow solid, after column chromatography (heptane/CH2Cl2, 1:1 → heptane/CH2Cl2, 0:1). 1H NMR [400 MHz, δ (ppm), CDCl3]: 9.16 (ddd, J = 3.1, 1.2, 0.7 Hz, 1 H), 8.06 (d, J = 0.6 Hz, 1 H), 7.82–7.76 (m, 2 H), 7.52–7.46 (m, 3 H), 7.45 (dd, J = 5.2, 1.2 Hz, 1 H), 7.39 (dd, J = 5.1, 3.0 Hz, 1 H). Yield: 99%.

(Z)-1-(1H-Indol-2-yl)-N-phenylmethanimine Oxide (6o)

According to general procedure A, the reaction of 1H-indole-2-carbaldehyde (80 mg, 0.551 mmol) with N-phenylhydroxylamine (63 mg, 0.579 mmol) afforded nitrone 6o (130 mg, 0.551 mmol) as a yellow-brown solid. 1H NMR [500 MHz, δ (ppm), CDCl3]: 11.69 (s, 1 H), 8.14 (s, 1 H), 7.87–7.80 (m, 2 H), 7.67 (dd, J = 8.1, 1.0 Hz, 1 H), 7.55–7.45 (m, 4 H), 7.32 (ddd, J = 8.2, 6.9, 1.1 Hz, 1 H), 7.16 (ddd, J = 8.0, 6.9, 1.0 Hz, 1 H), 6.94 (s, 1 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 147.4, 136.0, 130.2, 129.9, 129.4, 127.6, 126.8, 125.2, 121.3, 120.8, 112.3, 108.6. FTIR [ν̅ (cm–1)]: 3315, 3067, 1593, 1502, 1345, 1234, 818, 733. HRMS (ESI-TOF) m/z: [M + Na]+ calcd for C15H12N2ONa 259.0842, found 259.0858. R: 0.13 (CH2Cl2). Yield: 99%.

(Z)-N-Phenyl-1-(1H-pyrrol-2-yl)methanimine Oxide (6p)

According to general procedure A, the reaction of 1H-pyrrole-2-carbaldehyde (52 mg, 0.547 mmol) with N-phenylhydroxylamine (60 mg, 0.550 mmol) afforded nitrone 6p (62 mg, 0.333 mmol) as a dark brown solid, after column chromatography (CH2Cl2 → CH2Cl2/MeOH, 10:0.1). 1H NMR [500 MHz, δ (ppm), CDCl3]: 12.21 (s, 1 H), 7.95 (s, 1 H), 7.81–7.73 (m, 2 H), 7.52–7.38 (m, 3 H), 7.07 (td, J = 2.7, 1.3 Hz, 1 H), 6.70 (dt, J = 3.6, 1.6 Hz, 1 H), 6.40 (dt, J = 3.9, 2.5 Hz, 1 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 147.1, 129.5, 129.3, 126.1, 125.3, 121.3, 121.0, 115.9, 111.3. FTIR [ν̅ (cm–1)]: 3317, 3062, 1580, 1403, 1294, 1074, 1045, 740, 686. HRMS (ESI-TOF) m/z: [M + H]+ calcd for C11H11N2O 187.0866, found 187.0885. R: 0.45 (heptane/AcOEt, 1:1). Yield: 61%.

(Z)-1-(Furan-2-yl)-N-phenylmethanimine Oxide (6q).[28]

According to general procedure A, the reaction of furan-2-carbaldehyde (0.043 mL, 0.520 mmol) with N-phenylhydroxylamine (60 mg, 0.546 mmol) afforded nitrone 6q (97 mg, 0.520 mmol) as a light brown solid, after column chromatography (CH2Cl2 → CH2Cl2/MeOH, 10:0.2). 1H NMR [400 MHz, δ (ppm), CDCl3]: 8.16 (s, 1 H), 8.01 (dt, J = 3.5, 0.7 Hz, 1 H), 7.83–7.77 (m, 2 H), 7.58 (dd, J = 1.8, 0.7 Hz, 1 H), 7.52–7.44 (m, 3 H), 6.65 (dd, J = 3.6, 1.8 Hz, 1 H). Yield: 99%.

(Z)-2-(Benzylamino)-2-oxo-N-phenylethan-1-imine Oxide (6r)

According to general procedure A, the reaction of N-benzyl-2-oxoacetamide (56 mg, 0.343 mmol) with N-phenylhydroxylamine (26 mg, 0.240 mmol) afforded nitrone 6r (34.5 mg, 0.135 mmol) as a white solid. 1H NMR [400 MHz, δ (ppm), CDCl3]: 10.39 (s, 1 H), 7.72–7.67 (m, 2 H), 7.66 (s, 1 H), 7.58–7.46 (m, 3 H), 7.40–7.27 (m, 5 H), 4.64 (d, J = 5.9 Hz, 2 H). 13C NMR [101 MHz, δ (ppm), CDCl3]: 160.8, 147.5, 137.8, 131.7, 130.3, 129.7, 128.9, 128.0, 127.7, 121.6, 43.3. FTIR [ν̅ (cm–1)]: 3255, 3121, 2945, 1634, 1516, 1059, 768, 751, 683. HRMS (ESI-TOF) m/z: [M + Na]+ calcd for C15H14N2O2Na 277.0948, found 277.0956. R: 0.68 (CH2Cl2/MeOH, 10:0.2). Yield: 39%.

General Procedure for the Synthesis of Isoxazolidines 4a, 4b, 4d–r

The solution containing alkene 2 in distilled THF (3 equiv, 1.48 M) at 0 °C was added to a PTFE tube containing the corresponding nitrone 6 (1 equiv) and galvinoxyl (0.03 equiv). The tube was filled up with distilled THF, closed and brought to 15 kbar at 50 °C for 16 h. Then, the reaction mixture was filtered off, and the solvent was removed under vacuo. The crude mixture was purified by column chromatography using the indicated eluent to afford isoxazolidines 4a, 4b, and 4d–r.

2,3-Diphenyl-5-[(trifluoromethyl)sulfanyl]isoxazolidine (4a)

According to the general procedure, the reaction of nitrone 6a (26 mg, 0.119 mmol) and alkene 2 in a 1 mL PTFE high-pressure tube afforded cis-4a (24.9 mg, 0.77 mmol, 65%) as a white solid and a mixture of cis- and trans-4a and regioisomers 7a (12.6 mg, 0.039 mmol, 33%) as a yellow oil after column chromatography (heptane/CH2Cl2, 9:1). Total yield: 98%.

rac-(3R,5R)-2,3-Diphenyl-5-[(trifluoromethyl)sulfanyl]isoxazolidine (cis-4a)

1H NMR [500 MHz, δ (ppm), CDCl3]: 7.52–7.47 (m, 2 H), 7.43–7.37 (m, 2 H), 7.35–7.30 (m, 1 H), 7.24–7.18 (m, 2 H), 7.04–6.99, (m, 1 H), 6.99–6.95 (m, 2 H), 6.01 (dd, J = 8.1, 4.4 Hz, 1 H), 4.41 (dd, J = 9.0, 7.1 Hz, 1 H), 3.41 (dddq, J = 13.6, 9.0, 8.1, 0.9 Hz, 1 H), 2.44–2.31 (m, 1 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 148.8, 139.8, 130.2 (q, J = 307.7 Hz), 129.3, 128.8, 128.3, 127.2, 123.8, 117.6, 80.5 (q, J = 2.5 Hz), 68.3, 47.2. FTIR [ν ® (cm–1)]: 3070, 2963, 1598, 1489, 1258, 1114, 1031, 966, 928, 752, 700. HRMS (EI) m/z: [M+•] calcd for C16H14F3NOS 325.0748, found 325.0765. R: 0.20 (heptane/CH2Cl2, 9:1).

rac-(3R,5S)-2,3-Diphenyl-5-[(trifluoromethyl)sulfanyl]isoxazolidine (trans-4a)

1H NMR [500 MHz, δ (ppm), CDCl3]: 7.50–7.45 (m, 2 H), 7.41–7.36 (m, 2 H), 7.35–7.30 (1 H), 7.25–7.18 (m, 2 H), 7.04–6.98 (m, 2 H), 6.98–6.93 (m, 1 H), 6.07 (dd, J = 6.8, 3.8 Hz, 1 H), 4.90 (t, J = 7.3 Hz, 1 H), 3.02–2.92 (m, 1 H), 2.88 (ddd, J = 13.3, 7.3, 3.8 Hz, 1 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 151.0, 139.9, 129.9 (q, J = 307.8 Hz), 129.2, 128.8, 128.2, 126.7, 122.4, 115.2, 82.3 (q, J = 2.0 Hz), 68.1, 46.3. FTIR [ν̅ (cm–1)]: 3065, 1598, 1489, 1206, 1108, 1049, 842, 796, 665. HRMS (EI) m/z: [M+•] calcd for C16H14F3NOS 325.0748, found 325.0750. R: 0.11 (heptane/CH2Cl2, 9:1).

3-(4-Fluorophenyl)-2-phenyl-5-[(trifluoromethyl)sulfanyl]isoxazolidine (4b)

According to the general procedure, the reaction of nitrone 6b (27 mg, 0.125 mmol) and alkene 2 in a 1 mL PTFE high-pressure tube afforded cis-4b (27.6 mg, 0.080 mmol, 64%) as a white solid, a mixture of cis- and trans-4b and regioisomers 7b (13.1 mg, 0.038 mmol, 27%) as a yellow oil and trans-4b (3.5 mg, 0.010 mmol, 8%) as a pale yellow oil, after column chromatography (heptane/CH2Cl2, 9:1). Total yield: 99%.

rac-(3R,5R)-3-(4-Fluorophenyl)-2-phenyl-5-[(trifluoromethyl)sulfanyl]isoxazolidine (cis-4b)

1H NMR [500 MHz, δ (ppm), CDCl3]: 7.49–7.43 (m, 2 H), 7.25–7.19 (m, 2 H), 7.11–7.05 (m, 2 H), 7.05–7.01 (m, 1 H), 6.99–6.93 (m, 2 H), 6.01 (dd, J = 8.1, 4.3 Hz, 1 H), 4.40 (dd, J = 9.0, 7.0 Hz, 1 H), 3.39 (dddq, J = 13.7, 9.0, 8.1, 0.9 Hz, 1 H), 2.39–2.28 (m, 1 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 162.6 (d, J = 247.0 Hz), 148.6, 135.4 (d, J = 3.1 Hz), 130.1 (q, J = 307.8 Hz), 128.92, 128.91 (d, J = 7.2 Hz), 124.0, 117.7, 116.2 (d, J = 21.6 Hz), 80.5 (q, J = 2.3 Hz), 67.7, 47.1. 19F NMR [471 MHz, δ (ppm), CDCl3]: −39.9. FTIR [ν̅ (cm–1)]: 3071, 1597, 1455, 1256, 1226, 1110, 1029, 905, 804, 754. HRMS (EI) m/z: [M+•] calcd for C16H13F4NOS 343.0654, found 343.0650. R: 0.26 (heptane/CH2Cl2, 17:3).

rac-(3R,5S)-3-(4-Fluorophenyl)-2-phenyl-5-[(trifluoromethyl]sulfanyl]isoxazolidine (trans-4b)

1H NMR [500 MHz, δ (ppm), CDCl3]: 7.44–7.40 (m, 2 H), 7.25–7.20 (m, 2 H), 7.10–7.04 (m, 2 H), 6.99–6.92 (m, 3 H), 6.05 (dd, J = 6.8, 3.9 Hz, 1 H), 4.88 (t, J = 7.2 Hz, 1 H), 2.98–2.90 (m, 1 H), 2.87 (ddd, J = 13.3, 7.3, 3.9 Hz, 1 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 162.3 (d, J = 242.3 Hz), 150.7, 135.6, 128.9, 128.4 (d, J = 8.3 Hz), 122.6, 116.2 (d, J = 21.7 Hz), 115.3, 81.2, 67.4, 46.2. The carbon signal of SCF3 was not observed. 19F NMR [471 MHz, δ (ppm), CDCl3]: −39.4. FTIR [ν̅ (cm–1)]: 3071, 1686, 1598, 1154, 1052, 937, 693. HRMS (EI) m/z: [M+•] calcd for C16H13F4NOS 343.0654, found 343.0676. R: 0.21 (heptane/CH2Cl2, 17:3).

2-Phenyl-3-(4-tolyl)-5-[(trifluoromethyl)sulfanyl]isoxazolidine (4c)

The solution containing alkene 2 (3.0 equiv) in distilled THF (500 μL) at 0 °C was added to a PTFE tube containing nitrone 6c (1.0 equiv). The tube was filled up with distilled THF, closed and brought to 15 kbar at 50 °C for 16 h. Then, the reaction mixture was filtered off and the solvent was removed under vacuo. The crude mixture was purified by column chromatography (heptane/CH2Cl2, 4:1) to afford isoxazolidine cis-4c (17.1 mg, 0.050 mmol, 50%) as white solid and trans-4c (12.1 mg, 0.36 mmol, 35%) as a brown oil, contaminated with regioisomers 7c and azoxybenzene 8. Total yield: 85%.

rac-(3R,5R)-2-Phenyl-3-(4-tolyl)-5-[(trifluoromethyl)sulfanyl]isoxazolidine (cis-4c)

1H NMR [500 MHz, δ (ppm), CDCl3]: 7.40–7.34 (m, 2 H), 7.23–7.16 (m, 4 H), 7.03–6.95 (m, 3 H), 6.00 (dd, J = 8.1, 4.3 Hz, 1 H), 4.36 (dd, J = 9.0, 7.2 Hz, 1 H), 3.38 (dddq, J = 13.7, 9.0, 8.1, 0.9 Hz, 1 H), 2.39–2.32 (m, 1 H), 2.36 (s, 3 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 148.9, 138.1, 136.6, 130.2 (q, J = 307.7 Hz), 130.0, 128.8, 127.1, 123.7, 117.6, 80.5 (q, J = 2.2 Hz), 68.1, 47.3, 21.3. 19F NMR [471 MHz, δ (ppm), CDCl3]: −39.9. FTIR [ν̅ (cm–1)]: 3056, 2962, 1597, 1489, 1257, 1107, 752, 691. HRMS (EI) m/z: [M+•] calcd for C17H16F3NOS 339.0905, found 339.0890. R: 0.21 (heptane/CH2Cl2, 4:1).

rac-(3R,5S)-2-Phenyl-3-(4-tolyl)-5-[(trifluoromethyl)sulfanyl]isoxazolidine (trans-4c)

1H NMR [500 MHz, δ (ppm), CDCl3]: 7.36–7.33 (m, 2 H), 7.25–7.16 (m, 4 H), 7.02–6.98 (m, 2 H), 6.97–6.92 (m, 1 H), 6.06 (dd, J = 6.8, 3.8 Hz, 1 H), 4.85 (t, J = 7.2 Hz, 1 H), 2.95 (dddq, J = 13.2, 7.6, 6.8, 0.9 Hz, 1 H), 2.85 (ddd, J = 13.2, 7.2, 3.8 Hz, 1 H), 2.36 (s, 3 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 151.0, 137.9, 136.9, 130.1 (q, J = 307.7 Hz), 129.8, 128.8, 126.6, 122.2, 115.2, 82.3 (q, J = 2.2 Hz), 67.9, 46.4, 21.3. 19F NMR [471 MHz, δ (ppm), CDCl3]: −39.3. FTIR [ν̅ (cm–1)]: 3063, 2925, 1599, 1487, 1152, 1053, 756, 686. HRMS (EI) m/z: [M+•] calcd for C17H16F3NOS 339.0905, found 339.0924. R: 0.13 (heptane/CH 2Cl2, 4:1).

2-Phenyl-3-[4-(trifluoromethyl)phenyl]-5-[(trifluoromethyl)sulfanyl]isoxazolidine (4d)

According to the general procedure, the reaction of nitrone 6d (50 mg, 0.190 mmol) and alkene 2 in a 1.5 mL PTFE high-pressure tube, containing approximately 300 μL of glass beads, afforded a mixture of isoxazolidines cis- and trans-4d and regioisomers 7d (53.1 mg, 0.137 mmol, 73%) as a pale yellow solid oil, after column chromatography (heptane/CH2Cl2, 17:3). FTIR [ν̅ (cm–1)]: 1597, 1490, 1325, 1104, 1051, 850, 754, 692. HRMS (EI) m/z: [M+•] calcd for C17H13F6NOS 393.0622, found 393.0624. R: 0.20 (heptane/CH2Cl2, 17:3). Total yield: 73%.

rac-(3R,5R)-2-Phenyl-3-[4-(trifluoromethyl)phenyl]-5-[(trifluoromethyl)sulfanyl]isoxazolidine (cis-4d)

1H NMR [500 MHz, δ (ppm), CDCl3]: 7.72–7.55 (m, 4 H), 7.25–7.18 (m, 2 H), 7.05–7.00 (m, 1 H), 6.97–6.93 (m, 2 H), 6.00 (dd, J = 8.0, 4.3 Hz, 1 H), 4.50 (dd, J = 9.1, 6.7, 1 H), 3.43 (dddq, J = 13.7, 9.1, 8.0, 0.9, 1 H), 2.33 (ddd, J = 13.7, 6.7, 4.3 Hz, 1 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 148.4, 144.1, 130.6 (q, J = 32.6 Hz), 130.0 (q, J = 307.8 Hz), 129.0, 127.4, 126.3 (q, J = 3.8 Hz), 124.09 (q, J = 272.1 Hz), 124.05, 117.4, 80.4 (q, J = 2.2 Hz), 67.5, 46.9.

rac-(3R,5S)-2-Phenyl-3-[4-(trifluoromethyl)phenyl]-5-[(trifluoromethyl)sulfanyl]isoxazolidine (trans-4d)

1H NMR [500 MHz, δ (ppm), CDCl3]: 7.70–7.56 (m, 4 H), 7.27–7.18 (m, 2 H), 7.00–6.96 (m, 3 H), 6.04 (dd, J = 6.3, 4.6 Hz, 1 H), 4.97 (t, J = 7.1 Hz, 1 H), 3.01–2.83 (m, 2 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 150.5, 144.1, 130.5 (q, J = 32.5 Hz), 130.0 (q, J = 307.8 Hz), 129.0, 127.0, 126.2 (q, J = 3.9 Hz), 122.7, 115.1, 82.2 (q, J = 2.1 Hz), 67.5, 45.9.

3-[4-(Pentafluoro-λ6-sulfanyl)phenyl]-2-phenyl-5-[(trifluoromethyl)sulfanyl]isoxazolidine (4e)

According to the general procedure, the reaction of nitrone 6e (37.8 mg, 0.143 mmol) and alkene 2 in a 1 mL PTFE high-pressure tube afforded a mixture of isoxazolidines cis- and trans-4e and regioisomers 7e (41 mg, 0.091 mmol, 72%) as a white solid, after column chromatography (heptane/CH2Cl2, 17:3). FTIR [ν̅ (cm–1)]: 2963, 1670, 1585, 1261, 1120, 1104, 817, 753, 584. HRMS (EI) m/z: [M+•] calcd for C16H13F8NOS2 451.0311, found 451.0318. R: 0.21 (heptane/CH2Cl2, 17:3). Total yield: 72%.

rac-(3R,5R)-3-[4-(Pentafluoro-λ6-sulfanyl)phenyl]-2-phenyl-5-[(trifluoromethyl)sulfanyl]isoxazolidine (cis-4e)

1H NMR [500 MHz, δ (ppm), CDCl3]: 7.85–7.72 (m, 2 H), 7.64–7.55 (m, 2 H), 7.30–7.21 (m, 2 H), 7.09–7.02 (m, 1 H), 7.02–6.92 (m, 2 H), 6.00 (dd, J = 8.0, 4.3 Hz, 1 H), 4.52 (dd, J = 9.2, 6.6, 1 H), 3.43 (dt, J = 13.7, 8.6, 1 H), 2.32 (ddd, J = 13.7, 6.6, 4.3 Hz, 1 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 153.7–153.4 (m), 148.3, 144.0, 130.0 (q, J = 307.9 Hz), 129.1, 127.3, 127.0, 124.2, 117.3, 80.4 (q, J = 2.3 Hz), 67.1, 46.7.

rac-(3R,5S)-3-[4-(Pentafluoro-λ6-sulfanyl)phenyl]-2-phenyl-5-[(trifluoromethyl)sulfanyl]isoxazolidine (trans-4e)

1H NMR [500 MHz, δ (ppm), CDCl3]: 7.80–7.74 (m, 2 H), 7.60–7.56 (m, 2 H), 7.28–7.21 (m, 2 H), 7.01–6.97 (m, 3 H), 6.03 (dd, J = 5.9, 5.0, 1 H), 4.97 (t, J = 7.1 Hz, 1 H), 3.01–2.80 (m, 2 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 153.7–153.4 (m), 150.4, 143.9, 130.0 (q, J = 307.9 Hz), 129.0, 127.1, 127.0, 122.8, 115.1, 82.2 (q, J = 2.2 Hz), 67.1, 45.8.

3-(4-Methoxyphenyl)-2-phenyl-5-[(trifluoromethyl)sulfanyl]isoxazolidine (4f)

According to the general procedure, the reaction of nitrone 6f (54 mg, 0.238 mmol) and alkene 2 in a 1.5 mL PTFE high-pressure tube afforded isoxazolidine cis-4f (44.3 mg, 0.125 mmol, 53%), a mixture of isoxazolidines cis- and trans-4f (27.3 mg, 0.077 mmol, 32%) and isoxazolidine trans-4f mixed with cis-4f and regioisomers 7f (12.4 mg, 0.035 mmol, 15%) as yellow oils, after column chromatography (heptane/CH2Cl2, 7:3). Total yield: 99%.

rac-(3R,5R)-3-(4-Methoxyphenyl)-2-phenyl-5-[(trifluoromethyl)sulfanyl]isoxazolidine (cis-4f)

1H NMR [500 MHz, δ (ppm), CDCl3]: 7.43–7.35 (m, 2 H), 7.23–7.18 (m, 2 H), 7.04–6.99 (m, 1 H), 6.99–6.95 (m, 2 H), 6.94–6.90 (m, 2 H), 6.01 (dd, J = 8.1, 4.3 Hz, 1 H), 4.34 (dd, J = 8.9, 7.3 Hz, 1 H), 3.82 (s, 3 H), 3.37 (dddq, J = 13.8, 8.9, 8.2, 0.9 Hz, 1 H), 2.46–2.25 (m, 1 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 159.6, 148.8, 131.4, 130.2 (q, J = 307.8 Hz), 128.8, 128.4, 123.8, 117.8, 114.6, 80.5 (q, J = 2.2 Hz), 68.0, 55.4, 47.2. 19F NMR [471 MHz, δ (ppm), CDCl3]: −39.9. FTIR [ν̅ (cm–1)]: 3003, 1614, 1515, 1298, 1103, 1031, 816, 773. HRMS (EI) m/z: [M+•] calcd for C17H16F3NO2S 355.0854, found 355.0861. R: 0.15 (heptane/CH2Cl2, 4:1).

rac-(3R,5S)-3-(4-Methoxyphenyl)-2-phenyl-5-[(trifluoromethyl)sulfanyl]isoxazolidine (trans-4f)

1H NMR [500 MHz, δ (ppm), CDCl3]: 7.38–7.35 (m, 2 H), 7.22–7.18 (m, 2 H), 7.01–6.98 (m, 2 H), 6.97–6.93 (m, 1 H), 6.92–6.89 (m, 2 H), 6.06 (dd, J = 6.8, 3.7 Hz, 1 H), 4.87–4.77 (m, 1 H), 3.81 (s, 3 H), 3.00–2.90 (m, 1 H), 2.84 (ddd, J = 13.2, 7.2, 3.7 Hz, 1 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 159.5, 151.0, 131.8, 130.0 (q, J = 307.8 Hz), 128.8, 127.9, 122.4, 115.4, 114.5, 82.3 (q, J = 2.3 Hz), 67.7, 55.5, 46.3. FTIR [ν̅ (cm–1)]: 3002, 1512, 1489, 1248, 1109, 1030, 794, 693. HRMS (EI) m/z: [M+•] calcd for C17H16F3NO2S 355.0854, found 355.0855. R: 0.10 (heptane/CH2Cl2, 4:1).

3-(3-Fluorophenyl)-2-phenyl-5-[(trifluoromethyl)sulfanyl]isoxazolidine (4g)

According to the general procedure, the reaction of nitrone 6g (52 mg, 0.242 mmol) and alkene 2 in a 1.5 mL PTFE high-pressure tube afforded isoxazolidine cis-4g (46.6 mg, 0.136 mmol, 56%) as a white solid, a mixture of cis- and trans-4g (27.8 mg, 0.081, 34%) as a yellow oil, and isoxazolidine trans-4g mixed with cis-4g and regioisomers 7g (8.5 mg, 0.025 mmol, 10%) as a yellow solid, after column chromatography (heptane/CH2Cl2, 9:1). Total yield: 99%.

rac-(3R,5R)-3-(3-Fluorophenyl)-2-phenyl-5-[(trifluoromethyl)sulfanyl]isoxazolidine (cis-4g)

1H NMR [500 MHz, δ (ppm), CDCl3]: 7.34 (td, J = 8.1, 5.9 Hz, 1 H), 7.28–7.18 (m, 4 H), 7.06–6.98 (m, 2 H), 6.98–6.93 (m, 2 H), 5.98 (dd, J = 8.1, 4.3 Hz, 1 H), 4.41 (dd, J = 9.1, 6.8 Hz, 1 H), 3.39 (dddq, J = 13.7, 9.1, 8.1, 0.9 Hz, 1 H), 2.33 (ddd, J = 13.7, 6.8, 4.3 Hz, 1 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 163.4 (d, J = 247.0 Hz), 148.6, 142.5 (d, J = 7.2 Hz), 130.9 (d, J = 8.3 Hz), 130.1 (q, J = 307.9 Hz), 128.9, 123.9, 122.7 (d, J = 2.9 Hz), 117.4, 115.3 (d, J = 21.3 Hz), 114.1 (d, J = 22.6 Hz), 80.4 (q, J = 2.4 Hz), 67.5 (d, J = 1.9 Hz), 46.9. 19F NMR [471 MHz, δ (ppm), CDCl3]: −39.9. FTIR [ν̅ (cm–1)]: 3062, 1594, 1489, 1448, 1258, 1111, 1044, 915, 833, 752, 690. HRMS (EI) m/z: [M+•] calcd for C16H13F4NOS 343.0654, found 343.0675. R: 0.22 (heptane/CH2Cl2, 17:3).

rac-(3R,5S)-3-(3-Fluorophenyl)-2-phenyl-5-[(trifluoromethyl)sulfanyl]isoxazolidine (trans-4g)

1H NMR [500 MHz, δ (ppm), CDCl3]: 7.35 (td, J = 8.1, 5.9 Hz, 1 H), 7.25–7.20 (m, 4 H), 7.04–6.94 (m, 4 H), 6.05 (dd, J = 6.8, 4.0 Hz, 1 H), 4.90 (t, J = 7.4 Hz, 1 H), 2.99–2.91 (m, 1 H), 2.33 (ddd, J = 13.2, 7.4, 4.0 Hz, 1 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 163.4 (d, J = 247.1 Hz), 150.6, 142.6 (d, J = 7.2 Hz), 130.7 (d, J = 8.3 Hz), 128.9, 122.6, 122.2 (d, J = 2.8 Hz), 115.2 (d, J = 21.6 Hz), 115.1, 113.7 (d, J = 22.7 Hz), 80.2 (q, J = 2.5 Hz), 67.4 (d, J = 2.1 Hz), 46.0. 19F NMR [471 MHz, δ (ppm), CDCl3]: −39.4. FTIR [ν̅ (cm–1)]: 3070, 1593, 1488, 1451, 1256, 1109, 947, 787, 691. HRMS (EI) m/z: [M+•] calcd for C16H13F4NOS 343.0654, found 343.0667. R: 0.15 (heptane/CH2Cl2, 17:3).

2-Phenyl-3-(3-tolyl)-5-[(trifluoromethyl)sulfanyl]isoxazolidine (4h)

According to the general procedure, the reaction of nitrone 6h (49 mg, 0.234 mmol) and alkene 2 in a 1.5 mL PTFE high-pressure tube afforded isoxazolidine cis-4h (46 mg, 0.136 mmol, 57%) as an off white solid and isoxazolidine trans-4h and regioisomers 7h (26 mg, 0.077 mmol, 31%) as a yellow solid, after column chromatography (heptane/CH2Cl2, 17:3 → 7:3). Total yield: 88%.

rac-(3R,5R)-2-Phenyl-3-(3-tolyl)-5-[(trifluoromethyl)sulfanyl]isoxazolidine (cis-4h)

1H NMR [500 MHz, δ (ppm), CDCl3]: 7.31–7.28 (m, 1 H), 7.27–7.24 (m, 2 H), 7.22–7.17 (m, 2 H), 7.14–7.10 (m, 1 H), 7.02–6.94 (m, 3 H), 5.98 (dd, J = 8.1, 4.4 Hz, 1 H), 4.35 (dd, J = 9.0, 7.2 Hz, 1 H), 3.37 (dddq, J = 13.6, 9.0, 8.1, 0.9 Hz, 1 H), 2.38–2.32 (m, 1 H), 2.36 (s, 3 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 148.9, 139.7, 139.1, 130.2 (q, J = 307.7 Hz), 129.1, 129.1, 128.8, 127.6, 124.2, 123.6, 117.5, 80.5 (q, J = 2.2 Hz), 68.3, 47.2, 21.6. FTIR [ν̅ (cm–1)]: 3028, 1599, 1489, 1040, 1001, 839, 737, 724, 692. HRMS (EI) m/z: [M+•] calcd for C17H16F3NOS 339.0905, found 339.0925. R: 0.41 (heptane/CH2Cl2, 4:1).

rac-(3R,5S)-2-Phenyl-3-(3-tolyl)-5-[(trifluoromethyl)sulfanyl]isoxazolidine (trans-4h)

1H NMR [500 MHz, δ (ppm), CDCl3]: 7.31–7.18 (m, 5 H), 7.14–7.10 (m, 1 H), 7.03–6.98 (m, 2 H), 6.98–6.91 (m, 1 H), 6.06 (dd, J = 6.8, 3.7 Hz, 1 H), 4.85 (t, J = 7.3 Hz, 1 H), 2.95 (dddq, J = 13.2, 7.6, 6.8, 0.9 Hz, 1 H), 2.86 (ddd, J = 13.2, 7.3, 3.7 Hz, 1 H), 2.36 (s, 3 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 151.2, 139.9, 139.0, 130.0 (q, J = 307.8 Hz), 129.0, 128.9, 128.8, 127.1, 123.7, 122.3, 115.1, 82.4 (q, J = 2.1 Hz), 68.1, 46.4, 21.6. 19F NMR [471 MHz, δ (ppm), CDCl3]: −39.3. FTIR [ν̅ (cm–1)]: 3029, 1598, 1489, 1150, 1111, 1045, 785, 692. HRMS (EI) m/z: [M+•] calcd for C17H16F3NOS 339.0905, found 339.0931. R: 0.34 (heptane/CH2Cl2, 4:1).

Methyl 3-{2-Phenyl-5-[(trifluoromethyl)sulfanyl]isoxazolidin-3-yl}benzoate (4i)

According to the general procedure, the reaction of nitrone 6i (26 mg, 0.102 mmol) and alkene 2 in a 1 mL PTFE high-pressure tube, containing approximately 150 μL of glass beads, afforded isoxazolidine cis-4i (17.3 mg, 0.045 mmol, 44%) as an off-white solid and a mixture of isoxazolidines cis- and trans-4i and regioisomers 7i (18.2 mg, 0.047 mmol, 47%) as a brown-yellow solid, after column chromatography (heptane/CH2Cl2, 3:2). Total yield: 91%.

Methyl 3-{rac-(3R,5R)-2-Phenyl-5-[(trifluoromethyl)sulfanyl]isoxazolidin-3-yl}benzoate (cis-4i)

1H NMR [500 MHz, δ (ppm), CDCl3]: 8.11 (t, J = 1.8 Hz, 1 H), 8.01 (dt, J = 7.7, 1.4 Hz, 1 H), 7.76 (dddd, J = 7.7, 1.8, 1.2, 0.5 Hz, 1 H), 7.48 (t, J = 7.7 Hz, 1 H), 7.24–7.19 (m, 2 H), 7.07–7.00 (m, 1 H), 6.98–6.94 (m, 2 H), 6.01 (dd, J = 8.0, 4.5 Hz, 1 H), 4.50 (dd, J = 8.9, 7.0 Hz, 1 H), 3.93 (s, 3 H), 3.43 (dddq, J = 13.6, 8.9, 8.0, 0.9 Hz, 1 H), 2.42–2.30 (m, 1 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 166.8, 148. 6, 140.4, 131.6, 131.1, 130.0 (q, J = 307.7 Hz), 129.6, 129.0, 128.3, 124.0, 117.6, 80.4 (q, J = 2.5 Hz), 67.9, 52.4, 46.9. 19F NMR [471 MHz, δ (ppm), CDCl3]: −39.9. FTIR [ν̅ (cm–1)]: 3015, 1729, 1598, 1489, 1301, 1247, 1114, 1025, 786, 692, 674. HRMS (EI) m/z: [M+•] calcd for C18H16F3NO3S 383.0803, found 383.0824. R: 0.27 (heptane/CH2Cl2, 3:2).

Methyl-3-{rac-(3R,5S)-2-phenyl-5-[(trifluoromethyl)sulfanyl]isoxazolidin-3-yl}benzoate (trans-4i)

1H NMR [500 MHz, δ (ppm), CDCl3]: 8.12 (t, J = 1.8 Hz, 1 H), 8.02–7.98 (m, 1 H), 7.69 (dt, J = 7.8, 1.6 Hz, 1 H), 7.47 (t, J = 7.7, 1 H), 7.26–7.18 (m, 2 H), 7.00–6.98 (m, 2 H), 6.97–6.94 (m, 1 H), 6.06 (dd, J = 6.7, 4.0 Hz, 1 H), 4.96 (t, J = 7.3 Hz, 1 H), 3.93 (s, 3 H), 3.01–2.94 (m, 1 H), 2.91 (ddd, J = 13.3, 7.4, 4.0 Hz, 1 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 166.8, 150.7, 140.5, 131.19, 131.14, 130.0 (q, J = 307.9 Hz), 129.57, 129.55, 128.9, 127.8, 122.6, 115.2, 82.3 (q, J = 2.2 Hz), 67.6, 52.4, 46.1. 19F NMR [471 MHz, δ (ppm), CDCl3]: −39.4. FTIR [ν̅ (cm–1)]: 3064, 1720, 1598, 1489, 1285, 1107, 1020, 799, 751, 692. HRMS (EI) m/z: [M+•] calcd for C18H16F3NO3S 383.0803, found 383.0810. R: 0.22 (heptane/CH2Cl 2, 3:2).

3-(3-Methoxyphenyl)-2-phenyl-5-[(trifluoromethyl)sulfanyl]isoxazolidine (4j)

According to the general procedure, the reaction of nitrone 6j (50 mg, 0.190 mmol) and alkene 2 in a 1.5 mL PTFE high-pressure tube, containing approximately 300 μL of glass beads, afforded isoxazolidine cis-4j (33.8 mg, 0.095 mmol, 53%) as a white solid, a mixture of isoxazolidines cis- and trans-4j (3 mg, 0.0084 mmol, 5%) as a yellow oil and isoxazolidines trans-4j and 7j (18.2 mg, 0.051 mmol, 22%) as a yellow oil, after column chromatography (heptane/CH2Cl2, 7:3). Total yield: 80%.

rac-(3R,5R)-3-(3-Methoxyphenyl)-2-phenyl-5-[(trifluoromethyl)sulfanyl]isoxazolidine (cis-4j)

1H NMR [400 MHz, δ (ppm), CDCl3]: 7.29 (t, J = 7.9 Hz, 1 H), 7.24–7.17 (m, 2 H), 7.09–6.95 (m, 5 H), 6.85 (ddd, J = 8.3, 2.7, 1.1 Hz, 1 H), 5.99 (dd, J = 8.1, 4.3 Hz, 1 H), 4.37 (dd, J = 9.1, 7.0 Hz, 1 H), 3.81 (s, 3 H), 3.43–3.32 (m, 1 H), 2.36 (ddd, J = 13.7, 7.0, 4.3 Hz, 1 H). 13C NMR [101 MHz, δ (ppm), CDCl3]: 160.4, 148.9, 141.6, 130.3, 130.2 (q, J = 307.7 Hz), 128.8, 123.6, 119.3, 117.3, 113.8, 112.4, 80.5 (q, J = 2.5 Hz), 68.0, 55.4, 47.2 (q, J = 1.2 Hz). 19F NMR [377 MHz, δ (ppm), CDCl3]: −39.9. FTIR [ν̅ (cm–1)]: 3006, 2838, 1599, 1490, 1117, 757, 694. HRMS (EI) m/z: [M+•] calcd for C17H16F3NO 2S 355.0854, found 355.0868. R: 0.16 (heptane/CH2Cl2, 7:3).

rac-(3R,5S)-3-(3-Methoxyphenyl)-2-phenyl-5-[(trifluoromethyl)sulfanyl]isoxazolidine (trans-4j)

1H NMR [400 MHz, δ (ppm), CDCl3]: 7.29 (t, J = 8.1 Hz, 1 H), 7.25–7.19 (m, 2 H), 7.07–6.92 (m, 5 H), 6.89–6.81 (m, 1 H), 6.06 (dd, J = 6.8, 3.8, 1 H), 4.86 (t, J = 7.4 Hz, 1 H), 3.81 (s, 3 H), 2.96 (dddq, J = 13.3, 7.6, 6.8, 0.9 Hz, 1 H), 2.87 (ddd, J = 13.3, 7.3, 3.8 Hz, 1 H). 13C NMR [101 MHz, δ (ppm), CDCl3]: 160.3, 151.0, 141.6, 130.2, 130.0 (q, J = 307.8 Hz), 128.8, 122.3, 118.8, 115.1, 113.7, 112.0, 82.3 (q, J = 2.3 Hz), 68.0, 55.5, 46.3. 19F NMR [377 MHz, δ (ppm), CDCl3]: −39.3. FTIR [ν̅ (cm–1)]: 3064, 1720, 1598, 1489, 1347, 1107, 1020, 799, 750, 692. HRMS (ESI-TOF) m/z: [M + H]+ calcd for C17H17F3NO2S 356.0932, found 356.0939. R: 0.13 (heptane/CH2Cl2, 7:3).

2-Phenyl-3-[3-(trifluoromethyl)phenyl]-5-[(trifluoromethyl)sulfanyl]isoxazolidine (4k)

According to the general procedure, the reaction of nitrone 6k (49 mg, 0.234 mmol) and alkene 2 in a 1.5 mL PTFE high-pressure tube afforded a mixture of isoxazolidines cis- and trans-4k and 7k (87.4 mg, 0.222 mmol, 95%) as a white-yellow solid, after column chromatography (heptane/CH2Cl2, 17:3 → 7:3). FTIR [ν̅ (cm–1)]: 2927, 1598, 1490, 1263, 1144, 1045. HRMS (EI) m/z: [M+•] calcd for C17H13F6NOS 393.0622, found 393.0639. R: 0.38 (heptane/CH2Cl2, 4:1). Total yield: 95%.

rac-(3R,5R)-2-Phenyl-3-[3-(trifluoromethyl)phenyl]-5-[(trifluoromethyl)sulfanyl]isoxazolidine (cis-4k)

1H NMR [500 MHz, δ (ppm), CDCl3]: 7.73–7.69 (m, 2 H), 7.60–7.56 (m, 1 H), 7.54–7.48 (m, 1 H), 7.26–7.20 (m, 2 H), 7.05–7.00 (m, 1 H), 6.98–6.93 (m, 2 H), 5.99 (dd, J = 8.0, 4.5 Hz, 1 H), 4.51 (dd, J = 9.0, 6.7 Hz, 1 H), 3.41 (dddq, J = 13.6, 9.0, 8.0, 0.8 Hz, 1 H), 2.37–2.28 (m, 1 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 148.6, 141.1, 131.6 (q, J = 32.4 Hz), 130.4 (q, J = 1.2 Hz), 129.9 (q, J = 308.0 Hz), 129.9, 129.0, 125.2 (q, J = 3.8 Hz), 124.0 (q, J = 272.4 Hz), 124.1, 123.9 (q, J = 3.8 Hz), 117.4, 80.4 (q, J = 2.5 Hz), 67.8, 46.8.

rac-(3R,5S)-2-Phenyl-3-[3-(trifluoromethyl)phenyl]-5-[(trifluoromethyl)sulfanyl]isoxazolidine (trans-4k)

1H NMR [500 MHz, δ (ppm), CDCl3]: 7.73–7.70 (m, 1 H), 7.68–7.64 (m, 1 H), 7.60–7.56 (m, 1 H), 7.53–7.48 (m, 1 H), 7.26–7.20 (m, 2 H), 6.92–6.88 (m, 3 H), 6.06–6.02 (m, 1 H), 4.96 (t, J = 7.2 Hz, 1 H), 2.98–2.86 (m, 2 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 150.6, 141.1, 131.6 (q, J = 32.4 Hz), 130.4 (q, J = 1.2 Hz), 129.7, 128.9, 125.1 (q, J = 4.0 Hz), 123.5 (q, J = 3.8 Hz), 122.7, 115.2, 82.3 (q, J = 2.4 Hz), 67.5, 46.0.

2-Phenyl-3-(2-tolyl)-5-[(trifluoromethyl)sulfanyl]isoxazolidine (4l)

According to the general procedure, the reaction of nitrone 6l (49 mg, 0.234 mmol) and alkene 2 in a 1.5 mL PTFE high-pressure tube afforded isoxazolidine cis-4l (1.9 mg, 0.0056 mmol; used for characterization) as a colorless oil and a mixture of isoxazolidines cis- and trans-4l (79.5 mg, 0.234 mmol, 99%) as a yellow solid, after column chromatography (heptane/CH2Cl2, 4:1). FTIR [ν̅ (cm–1)]: 3025, 1596, 1489, 1258, 1109, 1041, 972, 754, 695. HRMS (EI) m/z: [M+•] calcd for C17H16F3NOS 339.0905, found 339.0923. R: 0.23 (heptane/CH2Cl2, 4:1). Total yield: 99%.

rac-(3R,5R)-2-Phenyl-3-(2-tolyl)-5-[(trifluoromethyl)sulfanyl]isoxazolidine (cis-4l)

1H NMR [500 MHz, δ (ppm), CDCl3]: 7.72 (dd, J = 7.2, 1.7 Hz, 1 H), 7.26–7.17 (m, 5 H), 7.00 (tt, J = 7.4, 1.2 Hz, 1 H), 6.96–6.88 (m, 2 H), 6.01 (dd, J = 8.1, 4.2 Hz, 1 H), 4.61 (dd, J = 9.1, 7.1 Hz, 1 H), 3.44 (dddq, J = 13.5, 9.1, 8.1, 0.9 Hz, 1 H), 2.35 (s, 3 H), 2.25 (ddd, J = 13.5, 7.1, 4.2 Hz, 1 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 148.9, 138.0, 134.6, 130.8, 130.1 (q, J = 307.4 Hz), 128.9, 127.8, 127.3, 126.6, 123.5, 117.2, 80.5 (q, J = 2.2 Hz), 64.5, 45.6, 19.7. 19F NMR [471 MHz, δ (ppm), CDCl3]: −40.0.

rac-(3R,5S)-2-Phenyl-3-(2-tolyl)-5-[(trifluoromethyl)sulfanyl]isoxazolidine (trans-4l)

1H NMR [500 MHz, δ (ppm), CDCl3]: 7.65–7.62 (m, 1 H), 7.27–7.15 (m, 5 H), 6.97–6.93 (m, 3 H), 6.06 (dd, J = 6.7, 3.9 Hz, 1 H), 5.04 (t, J = 7.5 Hz, 1 H), 2.89 (ddd, J = 13.1, 7.5, 3.9 Hz, 1 H), 2.80 (dddq, J = 13.1, 7.5, 6.7, 0.9 Hz, 1 H), 2.39 (s, 3 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 151.1, 137.8, 134.3, 130.8, 128.7, 127.8, 127.0, 126.2, 122.1, 114.7, 82.2 (q, J = 2.3 Hz), 65.3, 44.6, 19.4.

3-(2-Methoxyphenyl)-2-phenyl-5-[(trifluoromethyl)sulfanyl]isoxazolidine (4m)

According to the general procedure, the reaction of nitrone 6m (53 mg, 0.234 mmol) and alkene 2 in a 1.5 mL PTFE high-pressure tube afforded isoxazolidine cis-4m (35.6 mg, 0.100 mmol, 43%) as an off white solid and a mixture of isoxazolidines cis- and trans-4m (29.6 mg, 0.083 mmol, 28%) as a yellow solid, after column chromatography (heptane/CH2Cl2, 7:3). Total yield: 71%.

rac-(3R,5R)-3-(2-Methoxyphenyl)-2-phenyl-5-[(trifluoromethyl)sulfanyl]isoxazolidine (cis-4m)

1H NMR [500 MHz, δ (ppm), CDCl3]: 7.65 (dd, J = 7.6, 1.7 Hz, 1 H), 7.28 (ddd, J = 8.2, 7.5, 1.8 Hz, 1 H), 7.25–7.19 (m, 2 H), 7.01–6.94 (m, 4 H), 6.91 (dd, J = 8.3, 1.0 Hz, 1 H), 5.94 (dd, J = 8.0, 4.5 Hz, 1 H), 4.87 (dd, J = 9.1, 6.1 Hz, 1 H), 3.86 (s, 3 H), 3.40 (dddq, J = 13.6, 9.1, 8.0, 0.9 Hz, 1 H), 2.30–2.20 (m, 1 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 156.3, 149.4, 130.3 (q, J = 307.7 Hz), 128.9, 128.4, 127.5, 123.1, 121.3, 116.7, 110.5, 80.6 (q, J = 2.4 Hz), 62.1, 55.5, 45.0. FTIR [ν̅ (cm–1)]: 3075, 1599, 1490, 1243, 1106, 1027, 897, 751, 664. HRMS (EI) m/z: [M+•] calcd for C17H16F3NO2S 355.0854, found 355.0875. R: 0.19 (heptane/CH2Cl2, 7:3).

rac-(3R,5S)-3-(2-Methoxyphenyl)-2-phenyl-5-[(trifluoromethyl)sulfanyl]isoxazolidine (trans-4m)

1H NMR [500 MHz, δ (ppm), CDCl3]: 7.59 (dd, J = 7.6, 1.5 Hz, 1 H), 7.31–7.26 (m, 2 H), 7.06–7.02 (m, 2 H), 7.00–6.88 (m, 4 H), 5.99–5.96 (m, 1 H), 5.26 (dd, J = 7.5, 5.8 Hz, 1 H), 3.88 (s, 3 H), 2.90 (ddd, J = 12.7, 7.5, 5.0 Hz, 1 H), 2.84–2.76 (m, 1 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 156.3, 151.2, 128.9, 128.8, 128.1, 127.2, 122.0, 121.1, 114.9, 110.4, 82.4 (q, J = 2.2 Hz), 62.9, 55.5, 43.7. The carbon signal of SCF3 was not observed. FTIR [ν̅ (cm–1)]: 2839, 1599, 1490, 1256, 1106, 1026, 751, 692. HRMS (EI) m/z: [M+•] calcd for C17H16F3NO2S 355.0854, found 355.0862. R: 0.15 (heptane/CH2Cl2, 7:3).

2-Phenyl-3-(thiophen-3-yl)-5-[(trifluoromethyl)sulfanyl]isoxazolidine (4n)

According to the general procedure, the reaction of nitrone 6n (48 mg, 0.234 mmol) and alkene 2 in a 1.5 mL PTFE high-pressure tube afforded a mixture of isoxazolidines cis- and trans-4n (68.4 mg, 0.206 mmol, 88%) as a yellow solid, after column chromatography (heptane/CH2Cl2, 3:2). FTIR [ν̅ (cm–1)]: 3095, 1598, 1489, 1257, 1163, 1110, 1031, 837, 792, 754, 692. HRMS (EI) m/z: [M+•] calcd for C14H12F3NOS2 331.0312, found 331.0330. R: 0.19 (heptane/CH2Cl2, 17:3). Total yield: 88%.

rac-(3R,5R)-2-Phenyl-3-(thiophen-3-yl)-5-[(trifluoromethyl)sulfanyl]isoxazolidine (cis-4n)

1H NMR [500 MHz, δ (ppm), CDCl3]: 7.36 (dd, J = 5.0, 3.0 Hz, 1 H), 7.30–7.27 (m, 1 H), 7.25–7.20 (m, 2 H), 7.19 (dd, J = 5.0, 1.3 Hz, 1 H), 7.02 (tt, J = 7.4, 1.2 Hz, 1 H), 7.00–6.96 (m, 2 H), 6.00 (dd, J = 8.2, 4.2 Hz, 1 H), 4.51 (dd, J = 8.8, 6.7 Hz, 1 H), 3.36–3.28 (m, 1 H), 2.38 (ddd, J = 13.6, 6.7, 4.2 Hz, 1 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 148.9, 140.4, 130.2 (q, J = 307.7 Hz), 128.8, 127.3, 126.1, 123.9, 122.7, 117.5, 80.5 (q, J = 2.5 Hz), 64.6, 45.9.

rac-(3R,5S)-2-Phenyl-3-(thiophen-3-yl)-5-[(trifluoromethyl)sulfanyl]isoxazolidine (trans-4n)

1H NMR [500 MHz, δ (ppm), CDCl3]: 7.32 (dd, J = 5.0, 3.0 Hz, 1 H), 7.30–7.27 (m, 1 H), 7.26–7.20 (m, 2 H), 7.09 (dd, J = 5.0, 1.4 Hz, 1 H), 7.06–6.95 (m, 3 H), 6.03 (dd, J = 7.2, 4.9 Hz, 1 H), 5.01 (t, J = 6.5 Hz, 1 H), 2.96 (dt, J = 13.0, 6.4 Hz, 1 H), 2.78 (ddd, J = 12.7, 7.3, 4.7 Hz, 1 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 150.2, 140.6, 130.2 (q, J = 307.7 Hz), 128.8, 127.0, 125.8, 122.7, 122.4, 115.7, 81.9 (q, J = 2.2 Hz), 64.2, 44.6.

3-(1H-Indol-2-yl)-2-phenyl-5-[(trifluoromethyl)sulfanyl]isoxazolidine (4o)

According to the general procedure, the reaction of nitrone 6o (56 mg, 0.237 mmol) and alkene 2 in a 1.5 mL PTFE high-pressure tube afforded isoxazolidines cis- and trans-4o (20.3 mg, 0.056 mmol, 24%; contaminated with 8) as a brown-yellow oil and a mixture of isoxazolidine trans-4o and regioisomers 7o (35.6 mg, 0.098 mmol, 41%) as a brown oil, after column chromatography (heptane/CH2Cl2, 11:9). FTIR [ν̅ (cm–1)] of isoxazolidines cis- and trans-4o: 3413, 3060, 2962, 1599, 1436, 1258, 1110, 1022, 788, 756, 684, 660. FTIR [ν̅ (cm–1)] of isoxazolidine trans-4o and regioisomers 7o: 3406, 3059, 2963, 1597, 1455, 1231, 1107, 1014, 793, 693. Total yield: 65%.

rac-(3R,5R)-3-(1H-Indol-2-yl)-2-phenyl-5-[(trifluoromethyl)sulfanyl]isoxazolidine (cis-4o)

1H NMR [500 MHz, δ (ppm), CDCl3]: 8.67 (bs, 1 H), 7.61–7.58 (m, 1 H), 7.40–7.37 (m, 1 H), 7.25–7.18 (m, 3 H), 7.15–7.12 (m, 1 H), 7.05–7.02 (m, 3 H), 6.50 (d, J = 2.0 Hz, 1 H), 6.07 (dd, J = 8.3, 3.5 Hz, 1 H), 4.77 (dd, J = 9.4, 5.6 Hz, 1 H), 3.44–3.32 (m, 1 H), 2.50 (ddd, J = 13.8, 5.6, 3.5 Hz, 1 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 148.6, 136.2, 135.6, 130.0 (q, J = 306.8 Hz), 129.1, 128.6, 123.9, 122.6, 120.6, 120.4, 116.7, 111.5, 100.9, 80.7 (q, J = 2.4 Hz), 62.2, 44.9. R: 0.51 (heptane/CH2Cl2, 1:1).

rac-(3R,5S)-3-(1H-Indol-2-yl)-2-phenyl-5-[(trifluoromethyl)sulfanyl]isoxazolidine (trans-4o)

1H NMR [500 MHz, δ (ppm), CDCl3]: 8.50 (bs, 1 H), 7.57–7.47 (m, 3 H), 7.32 (dt, J = 8.2, 0.9 Hz, 1 H), 7.06–7.03 (m, 1 H), 6.98–6.90 (m, 2 H), 6.86–6.82 (m, 2 H), 6.40 (d, J = 2.1 Hz, 1 H), 5.84 (dd, J = 8.3, 4.0 Hz, 1 H), 4.28 (dd, J = 9.4, 5.7 Hz, 1 H), 2.82 (dt, J = 13.7, 8.9 Hz, 1 H), 2.16–2.09 (m, 1 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 148.5, 134.4, 132.7, 129.0, 128.6, 122.6, 122.5, 120.6, 119.5, 116.2, 111.3, 100.5, 82.1 (q, J = 2.4 Hz), 59.8, 44.6. R: 0.42 (heptane/CH2Cl2, 1:1).

3-(1H-Indol-2-yl)-2-phenyl-4-[(trifluoromethyl)sulfanyl]isoxazolidine (7o)

1H NMR [500 MHz, δ (ppm), CDCl3]: 7.64–7.59 (m, 2 H), 7.38–6.97 (m, 16 H), 6.59–6.55 (m, 2 H), 5.25 (d, J = 8.2 Hz, 1 H), 4.94 (d, J = 4.4 Hz, 1 H), 4.67–4.61 (m, 1 H), 4.61–4.56 (m, 1 H), 4.31 (q, J = 7.6 Hz, 1 H), 4.25–4.18 (m, 2 H), 4.11 (dd, J = 9.3, 6.2 Hz, 1 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 149.3, 148.9, 135.9, 135.7, 135.5, 132.8, 129.2, 129.2, 128.5, 127.9, 123.4, 123.2, 122.7, 122.5, 120.8, 120.6, 120.3, 115.1, 115.0, 111.3, 111.2, 103.1, 100.6, 73.2, 72.7, 70.1, 65.6, 52.4, 48.1.

2-Phenyl-3-(1H-pyrrol-2-yl)-5-[(trifluoromethyl)sulfanyl]isoxazolidine (4p)

According to the general procedure, the reaction of nitrone 6p (44 mg, 0.234 mmol) and alkene 2 in a 1.5 mL PTFE high-pressure tube afforded a mixture of isoxazolidines cis- and trans-4p and 7p (33 mg, 0.105 mmol, 45%) as a brown oil, after column chromatography (heptane/CH2Cl2, 4:1 → CH2Cl2/MeOH, 10:1). FTIR [ν̅ (cm–1)]: 3421, 2960, 1597, 1489, 1154, 1109, 1082, 1029, 756, 722. HRMS (EI) m/z: [M+•] calcd for C14H13F3N2OS 314.0701, found 314.0706. R (unassigned mixture): 0.46, 0.30, 0.24, 0.20 (heptane/CH2Cl2, 1:1). Total yield: 45%.

rac-(3R,5R)-2-Phenyl-3-(1H-pyrrol-2-yl)-5-[(trifluoromethyl)sulfanyl]isoxazolidine (cis-4p)

1H NMR [400 MHz, δ (ppm), CDCl3]: 8.65 (bs, 1 H), 7.28–7.22 (m, 2 H), 7.08–6.98 (m, 3 H), 6.81–6.77 (m, 1 H), 6.21–6.18 (m, 1 H), 6.15–6.12 (m, 1 H), 6.03 (dd, J = 8.3, 3.6 Hz, 1 H), 4.62 (dd, J = 8.9, 5.6 Hz, 1 H), 3.34–3.17 (m, 1 H), 2.48–2.36 (m, 1 H). 13C NMR [101 MHz, δ (ppm), CDCl3]: δ 148.9, 130.0 (q, J = 307.9 Hz), 129.0, 128.8, 123.8, 116.8, 115.2, 109.1, 106.7, 80.7 (q, J = 2.3 Hz), 62.5, 44.6.

rac-(3R,5S)-2-Phenyl-3-(1H-pyrrol-2-yl)-5-[(trifluoromethyl)sulfanyl]isoxazolidine (trans-4p)

1H NMR [500 MHz, δ (ppm), CDCl3]: 8.57 (bs, 1 H), 7.31–6.66 (m, 7 H), 6.27–6.15 (m, 1 H), 5.97 (t, J = 6.4 Hz, 1 H), 5.07 (dd, J = 7.0, 4.4 Hz, 1 H), 3.00 (ddd, J = 12.2, 7.1, 4.4 Hz, 1 H), 2.68 (dt, J = 13.1, 6.4 Hz, 1 H). 13C NMR [101 MHz, δ (ppm), CDCl3]: 150.0, 129.9, 128.9, 123.8, 118.0, 115.8, 107.2, 106.3, 82.1, 62.7, 43.2. The carbon signal of SCF3 was not observed.

2-Phenyl-3-(1H-pyrrol-2-yl)-4-[(trifluoromethyl)sulfanyl]isoxazolidine (7p)

1H NMR [400 MHz, δ (ppm), CDCl3]: 8.65 (bs, 1 H), 7.30–7.23 (m, 2 H), 7.06–6.99 (m, 3 H), 6.81–6.77 (m, 1 H), 6.27–6.19 (m, 1 H), 6.19 (m, 1 H), 5.11 (d, J = 7.8 Hz, 1 H), 4.66–4.55 (m, 1 H), 4.22 (q, J = 7.5 Hz, 1 H), 4.15 (dd, J = 8.6, 6.8 Hz, 1 H). 13C NMR [101 MHz, δ (ppm), CDCl3]: 149.2, 129.3, 126.0, 123.4, 118.8, 115.2, 109.0, 108.8, 73.5, 65.3, 48.7 (q, J = 1.3 Hz). The carbon signal of SCF3 was not observed.

1H NMR [500 MHz, δ (ppm), CDCl3]: 8.57 (bs, 1 H), 7.31–6.66 (m, 7 H), 6.27–6.15 (m, 1 H), 4.82 (d, J = 4.2 Hz, 1 H), 4.65–4.50 (m, 2 H), 4.26–4.10 (m, 1 H). 13C NMR [101 MHz, δ (ppm), CDCl3]: 149.6, 129.4, 126.0, 124.1, 118.1, 115.2, 109.1, 108.7, 70.3, 65.3, 48.7. The carbon signal of SCF3 was not observed.

3-(Furan-2-yl)-2-phenyl-5-[(trifluoromethyl)sulfanyl]isoxazolidine (4q)

According to the general procedure, the reaction of nitrone 6q (44 mg, 0.234 mmol) in a 1.5 mL PTFE high-pressure tube afforded isoxazolidine cis-4q (33.2 mg, 0.105 mmol, 45%) as an off white solid and isoxazolidines trans-4q and 7q (24.7 mg, 0.078 mmol, 34%) as a yellow oil, after column chromatography (heptane/CH2Cl2, 23:2 → heptane/CH2Cl2, 0:1). Total yield: 79%.

rac-(3R,5R)-3-(Furan-2-yl)-2-phenyl-5-[(trifluoromethyl)sulfanyl]isoxazolidine (cis-4q)

1H NMR [400 MHz, δ (ppm), CDCl3]: 7.45 (dd, J = 1.8, 0.9 Hz, 1 H), 7.31–7.22 (m, 2 H), 7.10–6.98 (m, 3 H), 6.37 (dd, J = 3.2, 1.8 Hz, 1 H), 6.35 (d, J = 3.2 Hz, 1 H), 6.00 (dd, J = 8.1, 4.5 Hz, 1 H), 4.50 (dd, J = 8.8, 6.5 Hz, 1 H), 3.23 (dddq, J = 13.5, 8.8, 8.1, 0.9 Hz, 1 H), 2.63 (ddd, J = 13.5, 6.5, 4.5 Hz, 1 H). 13C NMR [101 MHz, δ (ppm), CDCl3]: 151.0, 148.7, 143.0, 130.2 (q, J = 307.7 Hz), 128.9, 124.2, 117.5, 110.8, 109.0, 80.5 (q, J = 2.6 Hz), 63.0, 42.4 (q, J = 1.2 Hz). FTIR [ν̅ (cm–1)]: 2922, 1489, 1453, 1254, 1106, 1037, 936, 743, 680. HRMS (EI) m/z: [M+•] calcd for C14H12F3NO2S 315.0541, found 315.0538. R: 0.48 (heptane/CH2Cl2, 7:3).

rac-(3R,5S)-3-(Furan-2-yl)-2-phenyl-5-[(trifluoromethyl)sulfanyl]isoxazolidine (trans-4q)

1H NMR [400 MHz, δ (ppm), CDCl3]: 7.37 (dd, J = 1.8, 0.9 Hz, 1 H), 7.27–7.22 (m, 2 H), 7.09–7.05 (m, 2 H), 6.98 (tt, J = 7.3, 1.2 Hz, 1 H), 6.30 (dd, J = 3.3, 1.8 Hz, 1 H), 6.26 (dt, J = 3.3, 0.8, 1 H), 6.11 (dd, J = 7.4, 5.2 Hz, 1 H), 5.00 (dd, J = 7.7, 4.3, 1 H), 3.17 (dddq, J = 13.7, 7.4, 4.3, 0.8, 1 H), 2.77–2.67 (m, 1 H). 13C NMR [101 MHz, δ (ppm), CDCl3]: 151.6, 149.2, 142.8, 130.0 (q, J = 307.7 Hz), 128.8, 123.0, 116.2, 110.6, 108.4, 81.8 (q, J = 2.5 Hz), 61.6, 41.6 (q, J = 1.2 Hz). FTIR [ν̅ (cm–1)]: 2872, 1599, 1490, 1262, 1118, 1046, 757, 694. HRMS (EI) m/z: [M+•] calcd for C14H12F3NO2S 315.0541, found 315.0545. R: 0.39 (heptane/CH2Cl2, 7:3).

N-Benzyl-2-phenyl-5-[(trifluoromethyl)sulfanyl]isoxazolidine-3-carboxamide (4r)

According to the general procedure, the reaction of nitrone 6r (25 mg, 0.098 mmol) and alkene 2 in a 1 mL PTFE high-pressure tube afforded isoxazolidine cis-4r (3.2 mg, 0.0084 mmol, 9%; used for NMR characterization) as a white solid and a mixture of isoxazolidines cis- and trans-4r (16.1 mg, 0.042 mmol, 51%) as a yellow-white solid, after column chromatography (heptane/CH2Cl2, 17:3 → CH2Cl2/MeOH, 24:1). FTIR [ν̅ (cm–1)]: 3385, 3309 2930, 1668, 1597, 1454, 1161, 1029, 697. HRMS (EI) m/z: [M+•] calcd for C18H17F3N2O2S 382.0963, found 382.0964. Total yield: 60%.

rac-(3R,5R)-N-Benzyl-2-phenyl-5-[(trifluoromethyl)sulfanyl]isoxazolidine-3-carboxamide (cis-4r)

1H NMR [400 MHz, δ (ppm), CDCl3]: 7.59–7.52 (m, 1 H), 7.37–7.27 (m, 7 H), 7.11–7.03 (m, 1 H), 7.02–6.97 (m, 2 H), 5.88 (dd, J = 8.1, 4.7 Hz, 1 H), 4.56 (dd, J = 14.8, 6.1 Hz, 1 H), 4.49 (dd, J = 14.8, 5.9 Hz, 1 H), 4.31 (dd, J = 9.6, 3.5 Hz, 1 H), 3.19–3.07 (m, 1 H), 2.75–2.65 (m, 1 H). 13C NMR [101 MHz, δ (ppm), CDCl3]: 169.7, 148.8, 137.6, 129.6 (q, J = 308.1 Hz), 129.5, 128.9, 127.9, 127.8, 123.9, 115.1, 80.3 (q, J = 2.5 Hz), 67.6, 43.7, 39.4.

rac-(3R,5S)-N-Benzyl-2-phenyl-5-[(trifluoromethyl)sulfanyl]isoxazolidine-3-carboxamide (trans-4r)

1H NMR [500 MHz, δ (ppm), CDCl3]: 7.52–7.43 (m, 1 H), 7.36–7.23 (m, 7 H), 7.11–7.02 (m, 3 H), 5.91 (t, J = 6.8 Hz, 1 H), 4.54–4.49 (m, 2 H), 4.46 (dd, J = 15.0, 5.6 Hz, 1 H), 3.29 (ddd, J = 13.5, 7.3, 3.9 Hz, 1 H), 2.62 (ddd, J = 13.9, 8.1, 6.2 Hz, 1 H). 13C NMR [126 MHz, δ (ppm), CDCl3]: 169.1, 149.4, 137.6, 129.6 (q, J = 308.1 Hz), 129.0, 128.8, 127.7, 127.6, 123.6, 115.1, 82.3 (q, J = 2.5 Hz), 67.1, 43.5, 39.0.