Rh(II)-Catalyzed Highly Diastereoselective Cascade Transannulation

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

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Rh(II)-Catalyzed Highly Diastereoselective Cascade Transannulation of N‑Sulfonyl-1,2,3-triazoles and Vinyl Benzoxazinanones Kuntal Pal, Geetanjali S. Sontakke, and Chandra M. R. Volla* Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai-400076, India

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S Supporting Information *

ABSTRACT: An efficient, Rh(II)-catalyzed, denitrogenative reaction of 4-vinyl benzoxazinanones with N-sulfonyl-1,2,3triazoles has been developed for the synthesis of structurally diverse tricyclic 2-imidazolones in moderate to good yields with excellent diastereoselectivities. The reaction consists of the sequential formation of four new bonds: two C−N and two C−O bonds in a cascade fashion. The reaction works under operationally simple conditions and also represents the first catalytic nondecarboxylative cyclization of vinyl benzoxazinanones with triazoles.

T

Scheme 1. Overview of the Work

ransition metal-catalyzed synthesis of heterocyclic skeletons has been the attention of broad investigation because of their fundamental significance in organic, bioorganic, medicinal, and materials chemistry.1 Imidazolidin-2-ones are one such class of heterocycles, widely present in the core structure of natural products, pharmaceuticals, and biologically active compounds (Figure 1).2 They are also valuable synthetic

Figure 1. Selected 2-imidazolidinones.

intermediates for accessing a wide variety of complex molecular scaffolds.3 In addition, they have been extensively utilized as chiral ligands and auxiliaries in asymmetric synthesis.4 Given the significance of imidazolidinones, many excellent synthetic strategies have been developed over the past few decades for the construction of these structural frameworks.5 In contrast to the rich assortment of synthetic methods available for the formation of imidazolidinones, protocols for the effective construction of fused imidazolidinones in a one-pot operation are rather limited. Since the influential work of Tunge in 2008, vinyl benzoxazinanones were extensively employed in palladiumcatalyzed interceptive decarboxylative allylic cycloaddition (IDAC) reactions.6 The zwitterionic π-allyl palladium complex formed from vinyl benzoxazinanones after decarboxylation displays unique reactivity and can be trapped efficiently by various reagents to produce biologically important heterocyclic compounds (Scheme 1a).7 Recently, Xiao and co-workers reported a [4 + 2] cycloaddition reaction of vinyl benzox© XXXX American Chemical Society

azinanones and α-diazoketones via sequential visible-light photo activation and palladium catalysis.8 The reaction proceeds through the interception of π-allyl palladium complex by ketenes derived from α-diazoketones via visible-light-induced Wolff rearrangement (Scheme 1b). While these palladium-catalyzed decarboxylations of vinyl benzoxazinanones are well established, the nondecarboxylative reaction pathways are still underdeveloped and highly desirable. For the past few years, the in situ generated Rh-azavinyl carbenes (Rh-AVC) from N-sulfonyl-1,2,3-triazoles have also gained considerable interest due to their versatile reactivity toward a large number of nucleophiles.9 Particularly, these highly reactive carbenes can undergo annulation10 and cycloaddition11 reactions to produce a broad range of nitrogenous Received: April 3, 2019

A

DOI: 10.1021/acs.orglett.9b01174 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

catalyzed transannulation reaction was explored with vinyl benzoxazinanone 1a (Scheme 2). Gratifyingly, under the

heterocycles. Insertion reactions of Rh-AVC followed by rearrangement are another important class of reactions in triazole chemistry.12,13 The groups of Murakami14 and Fokin15 have extensively explored the utility of Rh-azavinyl carbenes for formal 1,3-insertion of O−H, N−H, and B−H bonds. Recently, our group established Rh(II)-catalyzed O−H insertions of isatins and isatoic anhydrides with Rh-AVC for accessing indigoids and 2-amino-benzoxazinones, respectively.16 The selective O−H insertion over the N−H insertion is the key feature in these reactions. With inspiration from the previous reports on vinyl benzoxazinanones and our continuous interest in Rh-catalyzed annulation reaction of triazoles, we tested vinyl benzoxazinanones with Rh-AVC, and herein, we report a highly diastereoselective cascade transannulation of N-sulfonly-1,2,3triazoles and vinyl benzoxazinanones for accessing fused tricyclic imidazolidinones in a one-pot operation (Scheme 1c). Our initial investigation was commenced by treating a mixture of easily preparable vinyl benzoxazinanone 1a (0.1 mmol) and N-tosyl-4-phenyl-triazole 2a (0.12 mmol) with 2 mol % of Rh2(Oct)4 in 1,2-DCE. Interestingly, after 2 h at room temperature, tricyclic 2-imidazolone, 3a-phenyl-5-vinyl-3,3adihydro-5H-benzo[d]imidazo[5,1-b][1,3]oxazin-1(2H)-one 3a was isolated, although in lower yields (Table 1, entry 1).

Scheme 2. Scope of the Reaction with Different Triazoles

Table 1. Optimization of Reaction Conditionsa

entry

Rh(II)-cat.

temp (°C)

solvent

yield (%)b

1 2 3 4 5 6 7 8 9 10

Rh2(Oct)4 Rh2(Oct)4 Rh2(Oct)4 Rh2(Oct)4 Rh2(Oct)4 Rh2(Oct)4

rt 60 100 80 60 100 100 100 100 100

DCE DCE DCE CHCl3 DCM toluene DCE DCE DCE DCE

12 (8) 53 94(90)c 45 51 39 0 79 92 0

Rh2(OAc)4 Rh2(esp)2 Rh2(TFA)4

a

Reaction conditions: 0.1 mmol of vinyl benzoxazinanone 1a, 0.12 mmol of triazole 2a, 2 mol % Rh-catalyst in 1 mL of solvent. bNMR yield was taken by using 0.1 mmol of 1,3,5-trimethoxybenzene as an internal standard. cYield of the isolated product in parentheses for 0.23 mmol of vinyl benzoxazinanone 1a.

standard conditions, different alkyl groups on the aromatic ring of triazoles 2b−2e were compatible and provided the corresponding tricyclic 2-imidazolone derivatives 3b−3e in good to excellent yields (77−93%). While 4-methoxyphenyl and 6-methoxynaphthyl derived triazoles led to the formation of the corresponding products 3f and 3g in 82% and 75%, 2methoxyphenyl triazole gave only trace amounts of product probably because of the increased steric hindrance. After prolonged heating, the triazole was decomposed. Different halogen (F, Cl, and Br) substituted triazoles were also reacted smoothly under the reaction conditions to furnish 3i−3k in 78− 87% yields. Additionally, heteroaryl- and alkyl-substituted triazoles were found to be amenable substrates for the formation of desired tricyclic 2-imidazolones in moderate to good yields. Furthermore, triazoles with other sulfonyl protected groups 2o− 2t were also well-suited and delivered the corresponding products 3o−3t in good to excellent yields (82−90%). The structure and the stereochemistry of the tricyclic 2-imidazolones were confirmed unambiguously by the single-crystal X-ray diffraction analysis of 3i, 3n, and 3p. In all cases, excellent

Remarkably, only one diastereomer of the desired product was observed, and the whole process involves the sequential formation of four new bonds in an efficient manner. These scaffolds were tested as lifespan-altering compounds or as cardiotonic drugs.17 After increasing the temperature to 60 °C (entry 2), the yield of 3a was increased to 53%. To further improve the yield, the temperature is increased to 100 °C, which led to the clean formation of 3a in 94% (entry 3, 90% isolated yield). Other solvents like DCM, CHCl3, or toluene were found to be less effective for the transformation (entries 4−6). While Rh2(OAc)4 was relatively less efficient, Rh2(esp)2 displayed similar reactivity to that of Rh2(Oct)4 and provided the product 3a in 92% NMR yield (entries 8−9). However, Rh2(TFA)4 failed to produce any product at 100 °C (entry 10). With the optimal reaction conditions in hand (Table 1, entry 3), the substrate scope of various triazoles 2 in the Rh(II)B


Letter

Organic Letters

To test the synthetic utility of this transannulation reaction, a gram-scale reaction with 1.0 g of vinyl benzoxazinanone 1a and triazole 2a was performed. Pleasingly, the reaction proceeded smoothly with only 0.5 mol % Rh2(Oct)4 and produced 1.8 g of 3a (70% yield) (Scheme 4a). Additionally, to simplify the

diastereoselectivities were observed, indicating the formation of a single diastereomer as confirmed by 1H NMR of the crude reaction mixture. After establishing the broad scope of N-sulfonyl-1,2,3triazoles, we turned our attention toward exploring the scope with differently substituted vinyl benzoxazinanones 1b−1h to delineate the generality of the Rh(II)-catalyzed denitrogenative transannulation reaction (Scheme 3). Evidently, 6-methyl

Scheme 4. (a) Gram-Scale Synthesis; (B) One-Pot Protocol

Scheme 3. Scope of the Reaction with Different Vinyl Benzoxazinanones

reaction pathway, we sought to synthesize the tricyclic 2imidazolone derivatives 3 by combining the Cu(I)-catalyzed azide−alkyne cycloaddition and Rh(II)-catalyzed denitrogenative transannulation reaction in a one-pot fashion (Scheme 4b). Satisfyingly, the yield of one-pot protocol was comparable with the stepwise pathway. In order to gain more insights on the reaction mechanism, the reaction of N-tosyl protected vinyl benzoxazinanone 6 with triazole 2a was investigated under the standard reaction conditions. As expected, no product formation was detected in 1 H NMR of the crude reaction mixture, signifying the importance of the free N−H group of vinyl benzoxazinanone (Scheme 5a). In addition, when 4-ethyl benzoxazinanone 7 was reacted with triazole 2a, we observed the exclusive formation of 2-amino-benzoxazinone 8, suggesting that a vinyl group is crucial for the ring-opening and intramolecular cyclization (Scheme 5b). Furthermore, vinyl benzoxazinanone 9 with an addition methyl group at the benzylic position did not undergo Scheme 5. Control Studies and Further Functionalization

substituted vinyl benzoxazinanone 1b followed a similar pathway and afforded the desired products 3u and 3v in 81% and 73% yield, respectively. Also, we examined different 6- and 7-halo substituted vinyl benzoxazinanones, and pleasingly, all of them underwent this transformation smoothly giving the tricyclic imidazolidinones in synthetically useful yields (79− 86%). However, when vinyl benzoxazinanone 1g having electron-deficient CF3 group on the C7-position was tested in the reaction with 2a, an inseparable mixture of desired product 3ad and the 2-amino-benzoxazinone 3ad′ was isolated in 67% yield in the ratio of 0.6:1.0. Although the reaction led to a mixture of products, it shed some light on the mechanism of the reaction and the intermediates present. A strong electronwithdrawing group like CF3 in the para-position of the leaving group disfavors allyl etherification (see proposed mechanism, Scheme 6). Similarly, the reaction with 8-methyl substituted vinyl benzoxazinanone 1h also provided the open chain 2amino-benzoxazinone 3af′ as major product (10:1 ratio), indicating that the intramolecular cyclization is sensitive to both electronic and steric effects. C


Organic Letters

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cyclization and delivered 2-amino-benzoxazinone 10 in 87%, suggesting the reaction was sensitive to steric hindrance (Scheme 5c). Selected transformations of 3a were carried out to exhibit the synthetic potential of this approach. The imidazolidinone 3a was reduced with 10 mol % Pd/C using H2 at room temperature to isolate 11 in 96% yield. In another transformation, 3a was converted to α,β-unsaturated ester 12 in good yield by using cross-metathesis reaction in the presence of 10 mol % of Ru catalyst (Scheme 5d,e). Finally the plausible mechanism based on the control experiments and previous reports15 is proposed in Scheme 6.

Letter

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b01174. Additional experimental procedures, X-ray crystallographic analysis, and spectroscopic data for synthesized compounds (PDF) Accession Codes

CCDC 1906603−1906605 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

Scheme 6. Proposed Reaction Mechanism

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kuntal Pal: 0000-0002-9005-0874 Geetanjali S. Sontakke: 0000-0003-1253-3640 Chandra M. R. Volla: 0000-0002-8497-1538 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This activity is supported by SERB, India (funding to CMRV, EMR/2015/002047). K.P. and G.S.S. thank Council of Scientific & Industrial Research (C.S.I.R., India) for the fellowship.

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The reaction is initiated by the trapping of electrophilic α-imino carbenoid 2′ by the O−H tautomer of vinyl benzoxazinanone 1′ to form the Rh(II)-zwitterionic intermediate A. Intramolecular nucleophilic addition leads to spiro-oxazole intermediate C, which upon ring-opening followed by a keto−enol tautomerization forms the key 2-amino-benzoxazinone intermediate E. Rh(II)-salt catalyzes the ring-opening to form intermediate F.18 Intramolecular nucleophilic addition of amide on to the carbonyl group leads to the formation of intermediate G. Diastereoselective allylic etherification provides the desired tricyclic 2-imidazolones 3 as a single stereoisomer. The steric repulsion between the allyl group and the imidazolidinone moiety in the nucleophilic addition of alkoxide onto the allyl system18 controls the direction of the allyl group, leading to excellent diastereoselectivity. In conclusion, we have successfully developed a highly diastereoselective Rh(II)-catalyzed nondecarboxylative O−H insertion reaction of vinyl benzoxazinanones with easily accessible N-sulfonyl triazoles. The reaction proceeds through a highly selective O−H insertion onto Rh-azavinyl carbenes followed by an intramolecular rearrangement and involves the formation of four new bonds in an efficient manner. A variety of structurally diverse tricyclic 2-imidazolones were synthesized under operationally simple conditions. The efficiency of this methodology was demonstrated by a gram-scale and one-pot synthesis of tricyclic-fused imidazolidinones and postfunctionalization of the vinyl group.

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Rh(II)-Catalyzed Highly Diastereoselective Cascade Transannulation

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