Zinc Carbenoid-Promoted Methylene Insertion in Saturated Heterocycles: Mechanistic Insights and Reactivity Profiles

Dedicated to Professor Hiriyakkanavar Ila on her 80th birthday

Abstract

The ring expansion of saturated heterocycles through methylene insertion into N–O bonds using a zinc carbenoid is described. This transformation is applied to 1,2-oxazetidines and 1,2-oxazolidines, while N-tosylated 1,2-oxazinane affords a ring-opened product. Density functional theory calculations suggest a stepwise reaction mechanism of the ring expansion and elucidate the origins of the different reactivities observed.

Skeletal editing involves the direct modification of molecular skeletons, especially heterocycles.[1] The strategy can be categorized into three types: ring expansion, ring contraction, and atom exchange. These approaches have attracted much attention in medicinal chemistry for enabling late-stage transformations of molecular frameworks.[2] Among the ring expansions, insertion reactions with free carbenes or transition-metal carbene complexes have been reported.[3] For example, ring expansions of nitrogen-containing heterocycles[4] and 1,2-azoles[5] through insertion into carbon–heteroatom or nitrogen–heteroatom bonds are well documented.

While ring expansion of heterocycles containing unsaturated bonds has been extensively developed, investigations into saturated heterocycles with N–O bonds remain limited. Gleason et al. reported an example of an insertion reaction into the N–O bond of 1,2-oxazolidine using a rhodium carbene complex (Scheme [1a]).[6] The proposed mechanism involves the N-alkylation of the nitrogen atom within the ring by the rhodium carbene complexes, followed by a rearrangement known as Stevens rearrangement.[7] Additionally, reactions involving the 1,2-shift of a methyl group on the nitrogen atoms have been used to insert carbon units into N–O bonds (Scheme [1b]). Kang et al. developed the rearrangement of N-methyl 1,2-oxazolidine to N–H-1,3-oxazinane using a ruthenium catalyst,[8] and Kürti et al. reported the conversion of N-methyl isoxazolidinium salt to the corresponding 1,3-oxazinane under basic conditions.[9] The base-mediated rearrangement of a methyl group at nitrogen was also applied to methylene moiety insertion into N–O bonds in linear compounds.[10]

On the other hand, we have developed methods for the functionalization of isoxazole[11] and recently reported the insertion of zinc carbenoids[12] into N–X (X = N, O, S) bonds, enabling ring-expansion reactions of 1,2-azoles and cyclic oximes, such as isoxazolines (Scheme [1c]).[11c] [d] Zinc carbenoids have been primarily studied as reagents for the Simmons–Smith reaction and homologation reactions.[13] A zinc carbenoid-mediated skeletal rearrangement[14] was reported as a side reaction of the Simmons–Smith reaction, but the reactivity of zinc carbenoids to other types of bonds, such as N–O bonds, has not been clear. In this study, we report the ring expansion of saturated heterocycles containing N–O bonds using zinc carbenoids (Scheme [1d]). We investigate the mechanism of zinc carbenoid reactions through both experimental approaches and computational studies using density functional theory (DFT) calculations.

We first examined the ring expansion of saturated heterocycles having N–O bonds by conducting conditions screening, employing N-tosylated 1,2-oxazetidine 1aa [15] as a model substrate (Table [1]). Initially, 1aa was treated with CH₂I₂ (6 equiv) and Et₂Zn (3 equiv) in DCE, resulting in 84% yield of the corresponding 1,3-oxazolidine 2aa (entry 1). Treatment of 1aa with reduced amounts of both CH₂I₂ (2 equiv) and Et₂Zn (1 equiv) resulted in a low conversion rate of 1aa, giving the 2aa in 76% yield (entry 2). The use of CH₂I₂ (4 equiv) and Et₂Zn (2 equiv) afforded 2aa at the same level as that obtained under the conditions given in entry 1 (entry 3). Next, we investigated the effect of solvent on the ring expansion (entries 4–8). The conversion rate of 1aa decreased in solvents other than DCE, even when the temperature was increased to room temperature after mixing for 1 h at 10 °C. Although 1aa was completely consumed in CH₂Cl₂, the yield of 2aa decreased to 53% (entry 4). Other solvents also did not improve the yield of 2aa (entries 5–8). In our previous work,[11d] the order of addition of the reagents affected the reaction outcome significantly. Therefore, we altered the addition timing of CH₂I₂, mixing it into the solution of 1aa in DCE before adding Et₂Zn. In contrast to the previous results, this modification increased the yield of 2aa to 88% (entry 9). We then adjusted the reaction temperature to –20 °C and room temperature (entries 10 and 11), but the yield of 2aa was not improved. Finally, we examined the reaction time (entries 12 and 13). When the reaction was quenched after 5 min, 22% of 1aa remained, but the yield of 2aa reached 75% (entry 12). Encouraged by this result, we extended the reaction time to 30 min, giving 2aa in 93% yield (entry 13).

With the optimized reaction conditions in hand, we investigated the substrate scope of the ring expansion (Scheme [2]). First, we examined 1,2-oxazetidines 1a with various types of aryl sulfonyl groups at the nitrogen atom. The substrate 1ab containing an electron-withdrawing group such as chlorine at the para-position of the aryl group afforded the product 2ab in 87% yield. However, 1ac, having trifluoromethyl group at the same position, resulted in a reduced yield of product 2ac (58%). The substrate 1ad, containing a methoxy group, afforded 2ad in moderate yield (65%). In contrast, 1ae, having a naphthyl group, gave the product 2ae in a high yield (90%). For comparison, N-tert-butoxycarbonyl 1,2-oxazetidine 1af did not afford the corresponding product 2af, resulting in a complex mixture. We also attempted the ring expansion of azetidine 1ag [16] under the same conditions, but the reaction did not proceed at all.

Table 1 Conditions Optimization for the Ring Expansion of 1,2-Oxa­zetidine 1aa ^a

Entry	Equiv		Solvent	Temp (°C)	Time (min)	Yield (%)b
	X	Y				2aa	1aa
1	6	3	DCE	10	60	84	0
2	2	1	DCE	10	60	76	24
3	4	2	DCE	10	60	84	0
4c	4	2	CH2Cl2	10 to r.t.	14 h	53	0
5c	4	2	THF	10 to r.t.	14 h	8	91
6c	4	2	EtOAc	10 to r.t.	14 h	38	60
7c	4	2	n-Hexane	10 to r.t.	14 h	43	18
8c	4	2	toluene	10 to r.t.	14 h	54	0
9d	4	2	DCE	10	60	88	0
10d	4	2	DCE	–20	60	64	9
11d	4	2	DCE	r.t.	60	76	0
12d	4	2	DCE	10	5	75	22
13d	4	2	DCE	10	30	93 (81)e	0

^a Reaction conditions: 1aa (0.10 mmol), CH₂I₂ (X equiv), Et₂Zn (Y equiv), solvent (2 mL). Et₂Zn was added first into the reaction mixture, followed by the addition of CH₂I₂ after 10 min.

^b NMR yield using dibromomethane as an internal standard.

^c Reaction temperature was increased to room temperature after mixing for 1 h at 10 °C.

^d CH₂I₂ was added first into the reaction mixture, followed by the addition of Et₂Zn after 10 min.

^e Isolated yield.

We next examined 1,2-oxazolidines 1b, five membered rings, as substrates for this ring expansion. Similar to the results with 1,2-oxazetidines 1a, substrates having a tosyl group and an aryl group with a chlorine substituent afforded the 1,3-oxazinanes 2ba and 2bb in good yields (71% and 81%, respectively). Substrate 1bc, having a trifluoromethyl group, gave the corresponding product 2bc in a lower yield (43%). Substrates 1bd and 1be, containing a methoxy moiety and a naphthyl ring, respectively, afforded the corresponding products 2bd and 2be in good yields (86% and 72%, respectively). Additionally, a substrate with an amide group on the aromatic ring was tolerated in this reaction, producing 2bf in 33% yield. However, 1bg, having two phenyl groups on the saturated ring, did not afford the corresponding product 2bg at all.

We also explored alternative sources of the zinc carbenoid by employing iodoform instead of CH₂I₂ (Scheme [3]). Under these conditions, 1aa did not afford the corresponding ring-expanded product but instead yielded the O-formylated acyclic product 3. This product would be formed through ring expansion by the zinc carbenoid and subsequent hydrolysis.

We also examined the ring expansion of other sizes of saturated heterocycles containing N–O bonds (Scheme [4]). First, we employed oxaziridine 4 [17] as a substrate, but the corresponding 1,3-oxazitidine 5 was not obtained. Instead, imine 6 was obtained, which is the same type of byproduct generated through Davis oxidation (Scheme [4a]). This result indicated that 4 reacted as an electrophile toward nucleophilic Et₂Zn. Second, the six-membered ring, 1,2-oxazinane 7, also did not give the corresponding seven-membered product 8. Instead, it afforded N-methylated aldehyde 9, indicating that the introduced methylene moiety at the nitrogen atom absorbed the hydrogen atom from the adjacent position of the oxygen atom (Scheme [4b]). On the other hand, the seven-membered ring, 1,2-oxazepane 10, did not yield either the ring-expanded product 11 or the ring-opened aldehyde 12, resulting in the full recovery of 1,2-oxazepane 10 (Scheme [4c]).

To elucidate the mechanism of the reaction using a zinc carbenoid, we conducted DFT calculations on the ring expansion of 1,2-oxazetidine 1aa (Figure [1a]). All calculations were performed using the Gaussian 16 program at the B3LYP-D3/LANL2DZ level for I and Zn, and 6-31G(d,p) for other elements in 1,2-dichloroethane using the polarizable continuum model (PCM). To reduce computational load, MeZnCH₂I[11d] was used as a model of zinc carbenoid. Based on our previous calculations, we examined the stepwise reaction pathway for the ring expansion (Figure [1a]). 1,2-Oxazetidine 1aa and MeZnCH₂I form the intermediate Int1 through S_N2-like transition-state TS1 with an energy barrier of 23.8 kcal/mol (ΔG ^‡ _1aa→TS1). After the N-alkylation, the methylene moiety of the intermediate Int1 shifts into the N–O bond of 1,2-oxazetidine ring via transition-state TS2 to afford the product 2aa. The calculations indicated that this ring-expansion step proceeded in an asynchronous concerted manner.[18] The C–Zn bond cleavage, the N–O bond cleavage, and the formation of the C–O bond occurred stepwise without any intermediates (Figure S1, the Supporting Information). The energy barrier for this step (ΔG ^‡ _Int1→TS2) is 2.7 kcal/mol. The calculated free-energy profile indicates that the rate-determining step of the ring expansion is the N-alkylation step.

We then calculated the energy profile for O-alkylation by MeZnCH₂I and identified the transition-state TS1′ for O-alkylation. The energy barrier for this path (ΔG ^‡ _1aa→TS1′) is 29.4 kcal/mol, and the resulting intermediate Int1′ has higher energy than the corresponding N-alkylated intermediate Int1. This result indicates that the alkylation with the zinc carbenoid occurs at the nitrogen atom, despite the steric hindrance of the tosyl group. The reaction profile of 1,2-oxazolidine 1ba showed a similar pattern with successive N-alkylation and 1,2-shift reaction pathway (Figure S3, the Supporting Information). In addition, we calculated the activation energies for N-alkylation of substrates that did not undergo the reaction. 1,2-Azetidine 1ag, diphenyl 1,2-isoxazolidine 1bg, and 1,2-oxazepane 10 required higher activation energies (1ag: 33.7 kcal/mol, 1bg: 31.8 kcal/mol, and 10: 28.5 kcal/mol, see Figure S4–6, the Supporting Information), which explains their lack of reactivity. Especially, a comparison between 1,2-isoxazolidine 1ba and diphenyl 1,2-isoxazolidine 1bg indicated the interaction between the IZnCH₃ and the oxygen atom in the rings. The orbital interaction between the lone pair of the oxygen atom and the empty p-orbital of the Zn would play an important role in the stabilization of the transition state of the alkylation step (Figure S7, the Supporting Information).

We investigated the energy profile of the reaction between 1,2-oxazepane 7 and MeZnCH₂I (Figure [1b]). The first step, N-alkylation, proceeds through the transition-state TS3 to afford the N-alkylated intermediate Int2. The energy barrier for this step (ΔG ^‡ _7→TS3) is 24.1 kcal/mol. After N-alkylation, however, the cleavage of the N–O bond accompanied by the proton transfer affords the ring-opened product 9 via transition–state TS4. This step also involves a concerted asynchronous process: the cleavage of the C–Zn bond, the N–O bond, and the C–H bond, and the formation of the C–H bond and C=O bond in this order (Figure S2, the Supporting Information). The energy barrier for this step (ΔG ^‡ _int2→TS4) is 6.6 kcal/mol.

We examined the different reaction pathways among four- to six-membered ring cycles (Figure [2]). First, we compared the structures[19] of the transition states at the cleavage of N–O bonds (Figure [2a]). For 1,2-oxazetidine and 1,2-oxazolidine, the distance between the carbon atom and the oxygen atom is shorter than the distance between the carbon atom and the hydrogen atom. In the case of 1,2-oxazepane, however, this relationship is reversed (C–O: 2.65 Å vs. C–H: 2.59 Å), as the corresponding hydrogen atom is positioned at the axial position of the six-membered ring. Next, we calculated the ring-strain energy of the corresponding N-H rings using homodesmotic models[20] at the MP2/6-311G(d,p) level of theory and compared the stability of the original rings with those of the one-carbon expanded rings (Figure [2b]). For 1,2-oxazetidine and 1,2-oxazolidine, the ring strain is reduced after one-carbon insertion, indicating that the ring expansion is thermodynamically favorable. On the other hand, the ring-strain energy is increased in the case of 1,2-oxazepane. These results indicate that the ring expansion of 1,2-oxazepane via methylene insertion is kinetically and thermodynamically unfavorable.

In conclusion, we have developed a method for the ring expansion of saturated heterocycles having N–O bonds using a zinc carbenoid (EtZnCH₂I). This transformation can be applied to strained four-membered rings, enabling the insertion of a single carbon unit into N–O bonds. DFT calculations revealed that the ring expansion proceeds through a stepwise mechanism involving N-alkylation followed by successive 1,2-shift of the methylene moiety. Additionally, the calculation provides insights into the differences in reactivity among four- to seven-membered rings. Further investigations of ring expansion of heterocycles using zinc carbenoids are in progress in our laboratory.

¹H and ¹³C NMR spectra were recorded with a Bruker Biospin AVANCE II (400 MHz for ¹H, 100 MHz for ¹³C) or a Bruker Biospin AVANCE III (500 MHz for ¹H, 125 MHz for ¹³C) instrument in the indicated solvent. Chemical shifts are reported in parts per million (ppm) relative to the signal for internal tetramethylsilane (0.00 ppm) for solutions in CDCl₃ (7.26 ppm for ¹H, 77.16 ppm for ¹³C). IR spectra were recorded with a JASCO FT/IR-4200 spectrophotometer. Only the strongest and/or structurally important peaks are reported as IR data (units of cm^–1). Mass spectra were measured with a JMS-700 Mstation and Bruker micrOTOF II. All reactions were monitored by thin-layer chromatography carried out on E. Merck silica gel plates (60F-254, 0.2 mm) with UV light (254 nm), and were visualized using an aqueous alkaline KMnO₄ solution. Column chromatography was performed on Wakogel® 60 N, purchased from FUJIFILM Wako Pure Chemical Corporation. Preparative thin-layer chromatography (PTLC) was performed using Wakogel B5-F silica-coated plates (1.0 mm) prepared in our laboratory. DFT calculations were performed with the Gaussian 16 program package (revision C.01). See the Supporting Information for details.

Starting materials 1,2-oxazetidines 1aa–af,[15] 1,2-diazetidine 1ag,[16] and oxaziridine 4 [17] are known compounds and were synthesized according to the reported procedures; their spectroscopic data are in agreement with the reports.

2-Tosyl-1,2-oxazetidine; Typical Procedure A[15]

(1) To a solution of N-hydroxyphtalimide (9.79 g, 1 equiv, 60 mmol) in N,N-dimethyl formamide (72 mL) was added 1,2-dibromoethane (7.8 mL, 1.5 equiv, 90 mmol) and triethylamine (16.7 mL, 2.0 equiv, 120 mmol). The solution was stirred at r.t. for 12 h then the precipitate of triethylammonium bromide was filtered under suction. The filtrate was diluted with ice-cold water and the solid precipitate was filtered off. The precipitate was purified by column chromatography on silica gel (hexane/EtOAc, 6:4) to afford 2-(2-bromoethoxy)isoindoline-1,3-dione S [1] (9.37 g, 34.7 mmol, 58%) as a white solid.

(2) A suspension of S [1] (1.35 g, 1.0 equiv, 5.0 mmol) in glacial acetic acid (7.5 mL) and 48% hydrobromic acid (5.0 mL) was stirred at 130 °C for 30 min. After cooling, 1,2-phtalic acid was filtered off with a Celite pad. The filtrate was concentrated in vacuo, and the resulting residue S [2] was used for the next reaction without further purification.

(3) The crude product S [2] was dissolved in pyridine at 0 °C under argon atmosphere and stirred for 10 min. Then p-toluenesulfonyl chloride (1.91 g, 2.0 equiv, 10.0 mmol) was added in one portion. After stirring at r.t. for 4 h, aq. HCl (1.0 M) was added, the mixture was poured into CH₂Cl₂, and the aqueous layer was extracted with two portions of CH₂Cl₂. The combined extract was washed with brine, dried over MgSO₄­, and concentrated in vacuo. The residue was purified by column chromatography on silica gel (hexane/EtOAc, 7:3) to afford N-(2-bromoethoxy)-4-methylbenzenesulfonamide S [3] (960 mg, 3.3 mmol, 66% in two steps).

(4) To a solution of S [3] in anhydrous THF (10 mL) under argon atmosphere was added NaH (60%, dispersion in paraffin, 264 mg, 2.0 equiv, 6.6 mmol). After stirring for 1 h, sat. aq. NH₄Cl was added, the mixture was poured into EtOAc, and the aqueous layer was extracted with two portions of EtOAc. The combined extract was washed with brine, dried over MgSO₄, and concentrated in vacuo. The residue was purified by column chromatography on silica gel (hexane/EtOAc, 7:3) to afford 1,2-oxazetidine 1aa (539 mg, 2.5 mmol, 77%).

1,2-Oxazetidines 1aa–af,[15] 1,2-oxazolidines 1ba–bf, 1,2-oxazinane 7, and 1,2-oxazepane 10 were synthesized according to Typical Procedure A using 1,2-dibromoehane, 1,3-dibromopropane, 1,4-dibromobutane, and 1,5-dibromopentane as alkyl reagents, respectively.

Note : When 1,3-dibromopropane was used as the starting material, the intermediate S [2] afforded the cyclized products 1ba–bf directly in this step.

2-Tosylisoxazolidine (1ba)

According to Typical Procedure A using 2-(3-bromopropoxy)isoindoline-1,3-dione[21] (1.14 g, 1.0 equiv, 4.0 mmol), 1ba was obtained.

Yield: 1.55 g (6.80 mmol, 97% in 2 steps); white solid; mp 93–94 °C.

IR (neat): 2966, 2893, 1596, 1354, 1168, 675 cm^–1.

¹H NMR (CDCl₃, 400 MHz): δ = 7.85 (d, J = 8.0 Hz, 2 H), 7.34 (d, J = 8.0 Hz, 2 H), 3.97 (t, J = 7.2 Hz, 2 H), 3.70 (t, J = 7.4 Hz, 2 H), 2.44 (s, 3 H), 2.20 (quint., J = 7.2 Hz, 2 H).

¹³C NMR (CDCl₃, 100 MHz): δ = 145.1, 133.0, 129.8, 129.3, 69.7, 47.4, 28.7, 21.8.

HRMS (ESI-TOF): m/z [M + Na] calcd for C₁₀H₁₃NO₃S: 250.0508; found: 250.0506.

2-((4-Chlorophenyl)sulfonyl)isoxazolidine (1bb)

According to Typical Procedure A using 2-(3-bromopropoxy)isoindoline-1,3-dione[21] (1.14 g, 1.0 equiv, 4.0 mmol), 1bb was obtained.

Yield: 693 mg (2.64 mmol, 66% in 2 steps); white solid; mp 84–85 °C.

IR (neat): 2965, 2891, 1558, 1362, 1167, 913, 745 cm^–1.

¹H NMR (CDCl₃, 400 MHz): δ = 7.91 (d, J = 8.8 Hz, 2 H), 7.53 (d, J = 8.8 Hz, 2 H), 3.97 (t, J = 7.2 Hz, 2 H), 3.73 (t, J = 7.2 Hz, 2 H), 2.29 (quint., J = 7.3 Hz, 2 H).

¹³C NMR (CDCl₃, 100 MHz): δ = 140.8, 134.7, 130.7, 129.5, 69.9, 47.2, 28.8.

HRMS (ESI-TOF): m/z [M + Na] calcd for C₉H₁₀ClNO₃S: 269.9962; found: 269.9963.

2-((4-(Trifluoromethyl)phenyl)sulfonyl)isoxazolidine (1bc)

According to Typical Procedure A using 2-(3-bromopropoxy)isoindoline-1,3-dione[21] (1.14 g, 1.0 equiv, 4.0 mmol), 1bc was obtained.

Yield: 716 mg (2.54 mmol, 64% in 2 steps); white solid; mp 99–100 °C.

IR (neat): 3012, 2896, 1735, 1357, 1167, 839, 745 cm^–1.

¹H NMR (CDCl₃, 500 MHz): δ = 8.12 (d, J = 8.5 Hz, 2 H), 7.82 (d, J = 8.0 Hz, 2 H), 4.09 (t, J = 7.3 Hz, 2 H), 3.77 (t, J = 7.3 Hz, 2 H), 2.33 (quint., J = 7.3 Hz, 2 H).

¹³C NMR (CDCl₃, 125 MHz): δ = 139.9, 135.6 (q, J _C–F = 33.0 Hz), 129.8, 128.2 (q, J _C–F = 3.8 Hz), 123.3 (q, J _C–F = 271 Hz), 70.0, 47.0, 28.7.

HRMS (ESI-TOF): m/z [M + Na] calcd for C₁₀H₁₀F₃NO₃S: 304.0226; found: 304.0229.

2-((4-Methoxyphenyl)sulfonyl)isoxazolidine (1bd)

According to Typical Procedure A using 2-(3-bromopropoxy)isoindoline-1,3-dione[21] (1.14 g, 1.0 equiv, 4.0 mmol), 1bd was obtained.

Yield: 668 mg (2.74 mmol, 69% in 2 steps); white solid; mp 96–97 °C.

IR (neat): 2970, 2843, 1736, 1595, 1354, 1263, 1160, 743, 681 cm^–1.

¹H NMR (CDCl₃, 400 MHz): δ = 7.90 (d, J = 8.8 Hz, 2 H), 7.01 (d, J = 8.8 Hz, 2 H), 3.98 (t, J = 7.2 Hz, 2 H), 3.88 (s, 3 H), 3.70 (t, J = 7.4 Hz, 2 H), 2.22 (quint., J = 7.2 Hz, 2 H).

¹³C NMR (CDCl₃, 100 MHz): δ = 164.1, 131.5, 127.2, 114.4, 69.7, 55.8, 47.4, 28.7.

HRMS (ESI-TOF): m/z [M + Na] calcd for C₁₀H₁₃NO₄S: 266.0457; found: 266.0462.

2-(Naphthalen-2-ylsulfonyl)isoxazolidine (1be)

According to Typical Procedure A using 2-(3-bromopropoxy)isoindoline-1,3-dione[21] (1.14 g, 1.0 equiv, 4.0 mmol), 1be was obtained.

Yield: 655 mg (2.49 mmol, 62% in 2 steps); white solid; mp 84–85 °C.

IR (neat): 3057, 2969, 2894, 1737, 1328, 1130, 749, 670 cm^–1.

¹H NMR (CDCl₃, 400 MHz): δ = 8.56 (s, 1 H), 8.00–7.91 (m, 4 H), 7.69–7.60 (m, 2 H), 4.01 (t, J = 7.2 Hz, 2 H), 3.78 (t, J = 7.4 Hz, 2 H), 2.23 (quint., J = 7.2 Hz, 2 H).

¹³C NMR (CDCl₃, 100 MHz): δ = 135.6, 133.0, 132.2, 131.2, 129.6, 129.5, 129.4, 128.1, 127.7, 123.9, 69.8, 47.4, 28.7.

HRMS (ESI-TOF): m/z [M + Na] calcd for C₁₃H₁₃NO₃S: 286.0508; found: 286.0508.

N-(4-(Isoxazolidin-2-ylsulfonyl)phenyl)acetamide (1bf)

According to Typical Procedure A using 2-(3-bromopropoxy)isoindoline-1,3-dione[21] (1.14 g, 1.0 equiv, 4.0 mmol), 1bf was obtained.

Yield: 550 mg (2.03 mmol, 51% in 2 steps); white solid; mp 170–171 °C.

IR (neat): 3006, 2968, 1735, 1590, 1523, 1354, 1160, 913, 743, 640 cm^–1.

¹H NMR (CDCl₃, 400 MHz): δ = 7.91 (d, J = 8.4 Hz, 2 H), 7.71 (d, J = 8.4 Hz, 2 H), 3.98 (t, J = 7.0 Hz, 2 H), 3.71 (t, J = 7.2 Hz, 2 H), 2.26–2.18 (m, 5 H).

¹³C NMR (CDCl₃, 100 MHz): δ = 168.8, 143.3, 130.7, 130.2, 119.2, 69.7, 47.5, 28.7, 24.9.

HRMS (ESI-TOF): m/z [M + Na] calcd for C₁₁H₁₄N₂O₄S: 293.0566; found: 293.0571.

3,5-Diphenyl-2-tosylisoxazolidine (1bg)[22]

To a mixture of oxaziridine 4 [17] (413 mg, 1.0 equiv, 1.5 mmol) and 4 Å molecular sieves (650 mg) in anhydrous toluene, styrene (259 μL, 1.5 equiv, 2.25 mmol) was added. Boron trifluoride ethyl ether complex (19 μL, 0.10 equiv, 0.15 mmol) was then added slowly by syringe. After stirring for 4 h, the reaction mixture was filtered through a Celite pad, and the residue was concentrated in vacuo. The resulting mixture was purified by column chromatography on silica gel (hexane/EtOAc, 9:1) to afford 1bg.

Yield: 285 mg (0.75 mmol, 50%); white solid; mp 155–156 °C.

IR (neat): 3033, 2971, 2925, 1734, 1457, 1362, 913, 749, 698 cm^–1.

¹H NMR (CDCl₃, 400 MHz): δ = 7.94 (d, J = 8.0 Hz, 2 H), 7.51 (d, J = 7.6 Hz, 2 H), 7.40–7.28 (m, 10 H), 5.51 (t, J = 8.2 Hz, 1 H), 5.35 (dd, J = 6.0 Hz, 10.4 Hz, 1 H), 3.18–3.13 (m, 1 H), 2.47–2.42 (m, 4 H).

¹³C NMR (CDCl₃, 100 MHz): δ = 145.1, 140.9, 136.3, 133.2, 129.8, 129.4, 128.9, 128.8, 128.7, 127.8, 127.2, 126.6, 83.9, 63.4, 46.7, 21.8.

HRMS (ESI-TOF): m/z [M + Na] calcd for C₂₂H₂₁NO₃S: 402.1134; found: 402.1134.

2-Tosyl-1,2-oxazinane (7)

According to Typical Procedure A using 1,4-dibromobutane, 7 was obtained.

Yield: 666 mg (2.76 mmol, 30% in 4 steps); white solid; mp 99–100 °C.

IR (neat): 2951, 2860, 1734, 1362, 1169, 913, 749 cm^–1.

¹H NMR (CDCl₃, 400 MHz): δ = 7.75 (d, J = 8.0 Hz, 2 H), 7.34 (d, J = 8.0 Hz, 2 H), 4.08 (t, J = 5.4 Hz, 2 H), 3.14 (t, J = 5.6 Hz, 2 H), 2.45 (s, 3 H), 1.86 (quint, J = 5.8 Hz, 2 H), 1.55 (quint., J = 5.8 Hz, 2 H).

¹³C NMR (CDCl₃, 100 MHz): δ = 144.7, 130.8, 129.6, 129.5, 72.8, 49.4, 24.1, 23.1, 21.8.

HRMS (ESI-TOF): m/z [M + Na] calcd for C₁₁H₁₅NO₃S: 264.0665; found: 264.0665.

2-Tosyl-1,2-oxazepane (10)

According to Typical Procedure A using 1,5-dibromopropane, 10 was obtained.

Yield: 154 mg (0.60 mmol, 6% in 4 steps); white solid; mp 99–100 °C.

IR (neat): 2934, 2866, 1735, 1362, 1231, 913, 772 cm^–1.

¹H NMR (CDCl₃, 400 MHz): δ = 7.77 (d, J = 8.4 Hz, 2 H), 7.34 (d, J = 8.0 Hz, 2 H), 3.97 (t, J = 6.4 Hz, 2 H), 3.10 (t, J = 6.0 Hz, 2 H), 2.44 (s, 3 H), 1.89–1.83 (m, 2 H), 1.74 (quint., J = 5.7 Hz, 2 H), 1.69–1.64 (m, 2 H).

¹³C NMR (CDCl₃, 100 MHz): δ = 144.6, 131.2, 129.6, 129.4, 74.4, 52.2, 29.1, 28.9, 23.4, 21.8.

HRMS (ESI-TOF): m/z [M + Na] calcd for C₁₂H₁₇NO₃S: 278.0821; found: 278.0821.

3-Tosyloxazolidine (2aa); Typical Procedure B

To a mixture of 1aa (42.7 mg, 0.20 mmol, 1.0 equiv) in 1,2-dichloroethane (4.0 mL), diiodomethane (46.5 μL, 4.0 equiv) was added at 10 °C under an argon atmosphere. After stirring at the same temperature for 10 min, diethylzinc (ca. 1 M in hexane, 0.40 mL, 2.0 equiv) was added to the mixture. The reaction mixture was stirred at the same temperature for 30 min and the reaction was followed by TLC analysis. After completion of the reaction, sat. aq. NH₄Cl was added to the mixture. After stirring for 10 min, the residue was poured into water and the aqueous layer was extracted with two portions of CH₂Cl_2­. The combined extract was dried over Na₂SO₄ and concentrated in vacuo. The residue was purified by PTLC (hexane/EtOAc, 8:2) to afford 2aa.

Yield: 33.8 mg (0.149 mmol, 74%); white solid.

Spectral properties were identical to those previously reported.[23]

3-((4-Chlorophenyl)sulfonyl)oxazolidine (2ab)

According to Typical Procedure B using 2-((4-chlorophenyl)sulfonyl)-1,2-oxazetidine (1ab) (23.4 mg, 0.10 mmol), 2ab was obtained.

Yield: 21.6 mg (0.087 mmol, 87%); pale-yellow solid; mp 84–85 °C.

IR (neat): 2969, 2880, 1735, 1351, 1169, 753, 627 cm^–1.

¹H NMR (CDCl₃, 500 MHz): δ = 7.80 (d, J = 9.0 Hz, 2 H), 7.52 (d, J = 9.0 Hz, 2 H), 4.85 (s, 2 H), 3.72 (t, J = 6.8 Hz, 2 H), 3.43 (t, J = 6.5 Hz, 2 H).

¹³C NMR (CDCl₃, 125 MHz): δ = 140.1, 135.8, 129.8, 129.3, 80.9, 66.3, 46.2.

HRMS (ESI-TOF): m/z [M + Na] calcd for C₉H₁₀ClNO₃S: 269.9962; found: 269.9969.

3-((4-(Trifluoromethyl)phenyl)sulfonyl)oxazolidine (2ac)

According to Typical Procedure B using 2-((4-(trifluoromethyl)phenyl)sulfonyl)-1,2-oxazetidine (1ac) (26.7 mg, 0.10 mmol), 2ac was obtained.

Yield: 14.9 mg (0.058 mmol, 58%); pale-yellow solid; mp 56–57 °C.

IR (neat): 2971, 2892, 1734, 1355, 1170, 1062, 713, 669 cm^–1.

¹H NMR (CDCl₃, 500 MHz): δ = 7.99 (d, J = 8.0 Hz, 2 H), 7.82 (d, J = 8.5 Hz, 2 H), 4.87 (s, 2 H), 3.75 (t, J = 6.8 Hz, 2 H), 3.45 (t, J = 6.8 Hz, 2 H).

¹³C NMR (CDCl₃, 125 MHz): δ = 141.0, 135.1 (q, J _C–F = 32.0 Hz), 128.4, 126.6 (q, J _C–F = 3.8 Hz), 123.3 (q, J _C–F = 271 Hz), 80.8, 66.5, 46.1.

HRMS (ESI-TOF): m/z [M + Na] calcd for C₁₀H₁₀F₃NO₃S: 304.0226; found: 304.2623.

3-((4-Methoxyphenyl)sulfonyl)oxazolidine (2ad)

According to Typical Procedure B using 2-((4-methoxyphenyl)sulfonyl)-1,2-oxazetidine (1ad) (22.9 mg, 0.10 mmol), 2ad was obtained.

Yield: 18.7 mg (0.0768 mmol, 77%); pale-yellow solid; mp 81–82 °C.

IR (neat): 3009, 2970, 1735, 1349, 1260, 750 cm^–1.

¹H NMR (CDCl₃, 500 MHz): δ = 7.79 (d, J = 9.0 Hz, 2 H), 6.99 (d, J = 9.0 Hz, 2 H), 4.84 (s, 2 H), 3.87 (s, 2 H), 3.66 (t, J = 6.8 Hz, 2 H), 3.42 (t, J = 6.8 Hz, 2 H).

¹³C NMR (CDCl₃, 125 MHz): δ = 163.6, 130.1, 128.8, 114.6, 80.9, 66.2, 55.8, 46.3.

HRMS (ESI-TOF): m/z [M + Na] calcd for C₁₀H₁₃NO₄S: 266.0457; found: 266.0458.

3-(Naphthalen-2-ylsulfonyl)oxazolidine (2ae)

According to Typical Procedure B using 2-(naphthalen-2-ylsulfonyl)-1,2-oxazetidine (1ae) (24.9 mg, 0.10 mmol), 2ae was obtained.

Yield: 23.7 mg (0.090 mmol, 90%); white solid; mp 119–120 °C.

IR (neat): 2983, 2889, 1342, 1161, 929, 832, 756, 643 cm^–1.

¹H NMR (CDCl₃, 500 MHz): δ = 8.44 (d, J = 1.0 Hz, 1 H), 7.99 (d, J = 8.5 Hz, 2 H), 7.92 (d, J = 7.5 Hz, 1 H), 7.83 (dd, J = 2.0 Hz, 9.0 Hz, 1 H), 7.68–7.61 (m, 2 H), 4.93 (s, 2 H), 3.66 (t, J = 6.8 Hz, 2 H), 3.49 (t, J = 6.8 Hz, 2 H).

¹³C NMR (CDCl₃, 125 MHz): δ = 135.2, 134.2, 132.3, 129.8, 129.6, 129.4, 129.2, 128.1, 127.8, 122.9, 80.9, 66.3, 46.3.

HRMS (ESI-TOF): m/z [M + Na] calcd for C₁₃H₁₃NO₃S: 286.0508; found: 286.0509.

3-Tosyl-1,3-oxazinane (2ba)

According to Typical Procedure B using 1ba (45.5 mg, 0.20 mmol), 2ba was obtained.

Yield: 34.2 mg (0.14 mmol, 71%); pale-yellow solid.

Spectral properties identical to those previously reported.[24]

3-((4-Chlorophenyl)sulfonyl)-1,3-oxazinane (2bb)

According to Typical Procedure B using 1bb (52.3 mg, 0.20 mmol), 2bb was obtained.

Yield: 44.8 mg (0.162 mmol, 81%); pale-yellow solid; mp 96–97 °C.

IR (neat): 2948, 2854, 1585, 1157, 972, 808, 637 cm^–1.

¹H NMR (CDCl₃, 500 MHz): δ = 7.83 (d, J = 8.5 Hz, 2 H), 7.49 (d, J = 8.5 Hz, 2 H), 4.96 (s, 2 H), 3.72 (t, J = 5.3 Hz, 2 H), 3.55 (t, J = 6.0 Hz, 2 H), 1.35 (quint., J = 5.5 Hz, 2 H).

¹³C NMR (CDCl₃, 125 MHz): δ = 139.4, 139.1, 78.5, 67.6, 44.6, 23.6.

HRMS (ESI-TOF): m/z [M + Na] calcd for C₁₀H₁₂ClNO₃S: 284.0119; found: 284.0117.

3-((4-(Trifluoromethyl)phenyl)sulfonyl)-1,3-oxazinane (2bc)

According to Typical Procedure B using 1bc (56.3 mg, 0.20 mmol), 2bc was obtained.

Yield: 25.5 mg (0.0863 mmol, 43%); pale-yellow solid; mp 76–77 °C.

IR (neat): 2857, 1464, 1330, 1160, 1064, 748, 629 cm^–1.

¹H NMR (CDCl₃, 500 MHz): δ = 8.04 (d, J = 8.0 Hz, 2 H), 7.79 (d, J = 8.0 Hz, 2 H), 4.99 (s, 2 H), 3.73 (t, J = 5.3 Hz, 2 H), 3.59 (t, J = 5.8 Hz, 2 H), 1.35 (quint., J = 5.6 Hz, 2 H).

¹³C NMR (CDCl₃, 125 MHz): δ = 140.1, 135.8, 129.8, 129.3, 80.9, 66.3, 46.2.

HRMS (ESI-TOF): m/z [M + Na] calcd for C₁₁H₁₂F₃NO₃S: 318.0382; found: 318.0382.

3-((4-Methoxyphenyl)sulfonyl)-1,3-oxazinane (2bd)

According to Typical Procedure B using 1bd (48.7 mg, 0.20 mmol), 2bd was obtained.

Yield: 44.4 mg (0.172 mmol, 86%); pale-yellow solid; mp 83–84 °C.

IR (neat): 2959, 2844, 1596, 1260, 1152, 1020, 669 cm^–1.

¹H NMR (CDCl₃, 500 MHz): δ = 7.81 (d, J = 9.0 Hz, 2 H), 6.97 (d, J = 9.0 Hz, 2 H), 4.92 (s, 2 H), 3.70 (t, J = 5.3 Hz, 2 H), 3.50 (t, J = 5.8 Hz, 2 H), 1.34 (quint., J = 5.5 Hz, 2 H).

¹³C NMR (CDCl₃, 125 MHz): δ = 163.1, 131.9, 129.8, 114.3, 78.5, 67.5, 55.7, 44.4, 23.6.

HRMS (ESI-TOF): m/z [M + Na] calcd for C₁₁H₁₅ClNO₄S: 280.0614; found: 280.0613.

3-(Naphthalen-2-ylsulfonyl)-1,3-oxazinane (2be)

According to Typical Procedure B using 1be (52.7 mg, 0.20 mmol), 2be was obtained.

Yield: 39.9 mg (0.143 mmol, 72%); white solid; mp 116–117 °C.

¹H NMR (CDCl₃, 500 MHz): δ = 8.46 (d, J = 1.5 Hz, 1 H), 7.97 (d, J = 8.0 Hz, 2 H), 7.91 (d, J = 7.5 Hz, 1 H), 7.87 (dd, J = 1.5 Hz, 8.5 Hz, 2 H), 5.02 (s, 2 H), 3.69 (t, J = 5.3 Hz, 2 H), 3.59 (t, J = 5.8 Hz, 2 H), 1.33 (quint., J = 5.5 Hz, 2 H).

¹³C NMR (CDCl₃, 125 MHz): δ = 140.1, 135.8, 129.8, 129.3, 80.9, 66.3, 46.2.

HRMS (ESI-TOF): m/z [M + Na] calcd for C₄H₁₅NO₃S: 300.0665; found: 300.0663.

N-(4-((1,3-Oxazinan-3-yl)sulfonyl)phenyl)acetamide (2bf)

According to Typical Procedure B using 1bf (54.1 mg, 0.20 mmol), 2bf was obtained.

Yield: 19.1 mg (0.067 mmol, 33%); pale-yellow solid; mp 156–157 °C.

IR (neat): 2980, 2854, 1676, 1591, 1260, 1152, 972, 750 cm^–1.

¹H NMR (CDCl₃, 500 MHz): δ = 7.81 (d, J = 8.5 Hz, 2 H), 7.77 (brs, 1 H), 7.68 (d, J = 9.0 Hz, 2 H), 4.94 (s, 2 H), 3.71 (t, J = 5.5 Hz, 2 H), 3.52 (t, J = 5.8 Hz, 2 H), 2.21 (s, 3 H), 1.36 (quint., J = 5.5 Hz, 2 H).

¹³C NMR (CDCl₃, 125 MHz): δ = 169.0, 142.3, 134.9, 129.0, 119.4, 78.5, 67.6, 44.5, 24.9, 23.7.

HRMS (ESI-TOF): m/z [M + Na] calcd for C₁₂H₁₆N₂O₄S: 307.0723; found: 307.0727.

2-((4-Methylphenyl)sulfonamido)ethyl Formate (3)

According to Typical Procedure B using iodoform (157 mg, 4.0 equiv), 3 was obtained.

Yield: 12.4 mg (0.051 mmol, 51%); pale-yellow oil.

IR (neat): 3279, 2923, 1724, 1327, 1158, 1092, 663 cm^–1.

¹H NMR (CDCl₃, 500 MHz): δ = 7.96 (s, 1 H), 7.75 (d, J = 8.5 Hz, 2 H), 7.32 (d, J = 8.0 Hz, 2 H), 4.98 (t, J = 6.0 Hz, 1 H), 4.19 (t, J = 5.3 Hz, 2 H), 3.26 (t, J = 5.7 Hz, 2 H), 2.43 (s, 3 H).

¹³C NMR (CDCl₃, 125 MHz): δ = 160.6, 143.9, 136.9, 130.0, 127.2, 62.6, 42.1, 21.7.

HRMS (ESI-TOF): m/z [M + Na] calcd for C₁₀H₁₃NO₄S: 266.0457; found: 266.0457.

N,4-Dimethyl-N-(4-oxobutyl)benzenesulfonamide (9)

According to Typical Procedure B using 7 (48.3 mg, 0.20 mmol), 9 was obtained.

Yield: 36.5 mg (0.162 mmol, 81%); transparent oil.

IR (neat): 2922, 2871, 2728, 1720, 1350, 1150, 913, 775 cm^–1.

¹H NMR (CDCl₃, 500 MHz): δ = 9.81 (s, 1 H), 7.63 (d, J = 8.5 Hz, 2 H), 7.30 (d, J = 8.0 Hz, 2 H), 2.99 (t, J = 6.5 H, 2 H), 2.98 (s, 3 H), 2.69–2.59 (m, 2 H), 2.41 (s, 3 H), 1.84 (quint., J = 6.9 Hz, 2 H).

¹³C NMR (CDCl₃, 125 MHz): δ = 201.5, 143.6, 134.4, 129.8, 127.5, 49.4, 40.7, 34.9, 21.6, 20.0.

HRMS (ESI-TOF): m/z [M + Na] calcd for C₁₂H₁₇NO₃S: 278.0821; found: 278.0823.

Conflict of Interest

The authors declare no conflict of interest.

Acknowledgment

The DFT calculations were carried out on the TSUBAME3.0 supercomputer at Tokyo Institute of Technology supported by the MEXT Project of the Tokyo Tech Academy for Convergence of Materials and Informatics (TAC-MI).

• References

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Corresponding Author

Publication History

Received: 19 May 2024

Accepted after revision: 19 June 2024

Article published online:
16 July 2024

© 2024. Thieme. All rights reserved

Georg Thieme Verlag KG
Rüdigerstraße 14, 70469 Stuttgart, Germany

• References

• 1a Joyson BW, Ball LT. Helv. Chim. Acta 2023; 106: e202200182
  CrossrefSearch in Google Scholar
• 1b Jurczyk J, Woo J, Kim SF, Dherange BD, Sarpong R, Levin MD. Nat. Synth. 2022; 1: 352
  CrossrefPubMedSearch in Google Scholar
• 2a Campos KR, Coleman PJ, Alvarez JC, Dreher SD, Garbaccio RM, Terrett NK, Tillyer RD, Truppo MD, Parmee ER. Science 2019; 363: eaat0805
  CrossrefPubMedSearch in Google Scholar
• 2b Blakemore DC, Castro L, Churcher I, Rees DC, Thomas AW, Wilson DM, Wood A. Nat. Chem. 2018; 10: 383
  CrossrefPubMedSearch in Google Scholar
• 3 Liu Z, Sivaguru P, Ning Y, Wu Y, Bi X. Chem. Eur. J. 2023; 29: e202301227
  CrossrefPubMedSearch in Google Scholar
• 4a Dherange BD, Kelly PQ, Liles JP, Sigman MS, Levin MD. J. Am. Chem. Soc. 2021; 143: 11337
  CrossrefPubMedSearch in Google Scholar
• 4b Reisenbauer JC, Green O, Franchino A, Finkelstein P, Morandi B. Science 2022; 377: 1104
  CrossrefPubMedSearch in Google Scholar
• 5a Manning JR, Davies HM. L. Tetrahedron 2008; 64: 6901
  CrossrefPubMedSearch in Google Scholar
• 5b Koronatov AN, Rostovskii NV, Khlebnikov AF, Novikov MS. J. Org. Chem. 2018; 83: 9210
  CrossrefPubMedSearch in Google Scholar
• 5c Hyland EE, Kelly PQ, McKillop AM, Dherange BD. I, Levin MD. J. Am. Chem. Soc. 2022; 144: 19258
  CrossrefPubMedSearch in Google Scholar
• 5d Li L, Ning Y, Chen H, Ning Y, Sivaguru P, Liao P, Zhu Q, Ji Y, de Ruiter G, Bi X. Angew. Chem. Int. Ed. 2023; e202313807
• 6 Hughes JM. E, Gleason JL. Tetrahedron 2018; 74: 759
  CrossrefSearch in Google Scholar
• 7 Stevens TS, Creighton EM, Gordon AB, MacNicol M. J. Chem. Soc. 1928; 3193
  Crossref
• 8 Yao C.-Z, Xiao Z.-F, Ning X.-S, Liu J, Zhang X.-W, Kang Y.-B. Org. Lett. 2014; 16: 5824
  CrossrefPubMedSearch in Google Scholar
• 9 Siitonen JH, Kattamuri PV, Yousufuddin M, Kürti L. Org. Lett. 2020; 22: 2486
  CrossrefPubMedSearch in Google Scholar
• 10 Malik M, Senatore R, Langer T, Holzer W, Pace V. Chem. Sci. 2023; 14: 10140
  CrossrefPubMedSearch in Google Scholar
• 11a Morita T, Fuse S, Nakamura H. Angew. Chem. Int. Ed. 2016; 55: 13580
  CrossrefPubMedSearch in Google Scholar
• 11b Morita T, Fukuhara S, Fuse S, Nakamura H. Org. Lett. 2018; 20: 433
  CrossrefPubMedSearch in Google Scholar
• 11c Tsuda M, Morita T, Nakamura H. Chem. Commun. 2022; 58: 1942
  CrossrefPubMedSearch in Google Scholar
• 11d Tsuda M, Morita T, Morita Y, Takaya J, Nakamura H. Adv. Sci. 2024; 11: 2307563
  CrossrefPubMedSearch in Google Scholar
• 12a Furukawa J, Kawabata N, Nishimura J. Tetrahedron Lett. 1966; 3353
  Crossref
• 12b Furukawa J, Kawabata N, Nishimura J. Tetrahedron 1968; 24: 53
  CrossrefSearch in Google Scholar
• 13 Pirovano V, Vincente R. Homologation Reactions Based on Zinc Carbenoids and Related Reagents. In Homologation Reactions: Reagents, Applications, and Mechanisms. Wiley-VCH; Weinheim: 2023: 217-264
  CrossrefSearch in Google Scholar
• 14 Ryu I, Murai S, Otani S, Sonoda N. Tetrahedron Lett. 1977; 1995
  Crossref
• 15a Javorskis T, Sriubaitė S, Bagdžiūnas G, Orentas E. Chem. Eur. J. 2015; 21: 9157
  CrossrefPubMedSearch in Google Scholar
• 15b Yang J, Wu B, Hu L. Asian J. Org. Chem. 2020; 9: 197
  CrossrefSearch in Google Scholar
• 15c Ganie MA, Bhat MS, Rizvi MA, Raheem S, Shah BA. Chem. Commun. 2022; 58: 8508
  CrossrefPubMedSearch in Google Scholar
• 16a Lakmal HH. C, Xu JX, Xu X, Ahmed B, Fong C, Szalda DJ, Ramig K, Sygula A, Webster CE, Zhang D, Cui X. J. Org. Chem. 2018; 83: 9497
  CrossrefPubMedSearch in Google Scholar
• 16b Higuchi D, Matsubara S, Kadowaki H, Tanaka D, Murakami K. Chem. Eur. J. 2023; 29: e202301071
  CrossrefPubMedSearch in Google Scholar
• 17a Williamson KS, Michaelis DJ, Yoon TP. Chem. Rev. 2014; 114: 8016
  CrossrefPubMedSearch in Google Scholar
• 17b Ravindra S, Jesin CP. I, Shabashini A, Nandi GC. Adv. Synth. Catal. 2021; 363: 1756
  CrossrefSearch in Google Scholar
• 18a Dewar MJ. S, Pierini AB. J. Am. Chem. Soc. 1984; 106: 203
  CrossrefSearch in Google Scholar
• 18b Beno BR, Houk KN, Singleton DS. J. Am. Chem. Soc. 1996; 118: 9984
  CrossrefSearch in Google Scholar
• 19 All 3D structures were described by the CYLview visualization program, see: Legault CY. CYLview 20 . Université de Sherbrooke; Québec: 2020. http://www.CYLview.org
  Search in Google Scholar
• 20a Magers DH, Davis SR. J. Mol. Struct.: THEOCHEM 1999; 487: 205
  CrossrefSearch in Google Scholar
• 20b Wheeler SE. Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2012; 2: 204
  Search in Google Scholar
• 21 Molla R, Joshi PN, Reddy NC, Biswas D, Rai V. Org. Lett. 2023; 25: 6385
  CrossrefPubMedSearch in Google Scholar
• 22 Partridge KM, Anzovino ME, Yoon TP. J. Am. Chem. Soc. 2008; 130: 2920
  CrossrefPubMedSearch in Google Scholar
• 23 Wang F, Stahl SS. Angew. Chem. Int. Ed. 2019; 58: 6385
  CrossrefPubMedSearch in Google Scholar
• 24 Bates RW, Lu Y, Cai MP. Tetrahedron 2009; 65: 7852
  CrossrefSearch in Google Scholar

• Supplementary Material