Annulations of N-Acyl Sulfilimines with Ynamides for the Synthesis of 4

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Gold(III)-Catalyzed Formal [3 + 2] Annulations of N‑Acyl Sulfilimines with Ynamides for the Synthesis of 4‑Aminooxazoles Xianhai Tian,† Lina Song,† Chunyu Han,† Cheng Zhang,† Yufeng Wu,† Matthias Rudolph,† Frank Rominger,† and A. Stephen K. Hashmi*,†,‡ †

Institut für Organische Chemie, Heidelberg University, Im Neuenheimer Feld 270, Heidelberg 69120, Germany Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia

‡

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S Supporting Information *

ABSTRACT: A gold-catalyzed formal 1,3-dipolar [3 + 2] annulation using readily accessible N-acyl sulfilimines and ynamides is reported. This reaction includes the cleavage of a N−S bond and subsequent C−O bond formation. In total, 30 oxazole derivatives bearing diverse functionalities could be prepared in 43−98% yield from the corresponding sulfilimines and ynamides.

O

xazoles1 are significant structural motifs in the research fields of natural products, synthetic bioactive compounds, coordination chemistry, and material science. Thus, the synthesis of functionalized oxazoles has been the subject of intense research in recent decades. Transition-metal-catalyzed cyclizations of amide derivatives with alkynes are commonly used methods.2 For example, our group developed a goldcatalyzed cyclization of N-propargyl carboxamides affording oxazoles through oxazoline intermediates.2a Zhang et al.3 disclosed a catalytic [2 + 2 + 1]-type reaction of terminal alkynes, amides, and N-oxides via an oxygen-transfer process. Although great progress has been made in preparing oxazoles, methods for the synthesis of more useful 4aminooxazoles4 (Figure 1) are still rare. A traditional synthesis of 4-aminooxazoles from α-oxonitriles and aldehydes was reported by Lakhan et al.5 Recently, gold-promoted nitrene transfer processes from nitrene equivalents to C−C triple bonds of ynamides6 have provided facile, efficient, and highly regioselective synthetic routes to this important compound class. A [3 + 2] annulation between N-acylaminides and ynamides via the generation of α-imino gold carbenes by using a cleavage of the N−N bond was disclosed by Davies et al. (Figure 1).7 Then dioxazoles8 were employed for the synthesis of 4-aminooxazoles through a gold-catalyzed N−O bond cleavage. A Tf2NH-promoted formal [3 + 2] cyclization was also discovered.9 Beyond these advances, a novel approach using readily available nitrene transfer reagents for the synthesis of 4-aminooxazoles is still desirable. Gold-catalyzed transformations covering important and challenging ring formation processes have rapidly been considered as key steps for the total synthesis of natural products and complex bioactive compounds because of the distinctive catalytic activity of gold complexes.10 Especially functionalized gold carbenes generated from alkynes and carbene precursors have served as versatile and crucial intermediates for constructing heterocyclic diversity. By introduction of a conjugated trapping functionality, gold© XXXX American Chemical Society

Figure 1. Bioactive 4-aminooxazoles and synthetic routes involving nitrene transfer pathways.

promoted dipolar [3 + 2] annulations for the synthesis of fivemembered rings occur. Using this principle, several carbene precursors such as sulfonium ylides,11 pyridinium ylides,7,12 azides,13 2H-azirines,14 and isoxazole derivatives8,9,15 have been utilized in [3 + 2] annulations for the syntheses of important valuable heterocycles. Recently, we successfully demonstrated the fruitful partnership between N-arylsulfilimines and ynamides by applying [3 + 2] annulations for the syntheses of indoles and N-fused imidazoles.16 Inspired by elegant previous reports17 and in line with our long-standing interest in oxazole synthesis, we speculated that, by goldReceived: March 22, 2019

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DOI: 10.1021/acs.orglett.9b01011 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Scheme 1. Scope with Respect to Ynamidesa,b

catalyzed reactions with ynamides, N-acyl sulfilimines could be ultilized as α-imino gold carbene precursors. The introduced acyl group on the anion, whose oxygen atom could supposedly trap the gold carbene to form an oxazole, tunes the stability and reactivity of the N−S bond. Herein, we report our new results on the intermolecular synthesis of 4-aminooxazoles from N-acyl sulfilimines and ynamides. The starting materials N-acyl sulfilimines can be readily prepared from commercially available amides, DMS and NCS. With S,S-dimethyl-N-acylsulfilimine 1a and ynamide 2a, we commenced our study by employing IPrAuCl/AgNTf2 as the catalyst. 4-Aminooxazole 3aa was given in 11% yield (Table 1, Table 1. Optimization of Reaction Conditionsa

entry

R

catalyst

solvent

yieldb

1 2 3 4 5 6 7 8 9

CH3 CH3 CH3 CH3 CH3 CH3 CH3 Ph CH3

IPrAuCl/AgNTf2 PPh3AuCl/AgNTf2 PPh3AuCl/AgOTf NaAuCl4·2H2O PicAuCl2 PicAuCl2 PicAuCl2 PicAuCl2 −

toluene toluene toluene toluene toluene DCE THF toluene toluene

11% 15% 13% 75% 92% 55% 26% 63% n.d.c

a

General reaction conditions: 1 (0.3 mmol, 1.5 equiv), 2a (0.2 mmol, 1.0 equiv), catalyst (5 mol %), solvent (2.0 mL, 0.1 M). bIsolated yield of product 3aa. cn.d.: not detected.

entry 1). Although another two gold(I) catalysts (entries 2−3) did not improve the reaction dramatically, a gold(III) catalyst, NaAuCl4·2H2O, afforded a significantly improved yield of 3aa (entry 4). Further testing of gold(III) catalysts showed that PicAuCl2 furnished product 3aa in excellent yield (entry 5).18 Other solvents like DCE and THF also worked well, but the yields are lower (entries 6−7). S,S-Diphenylsulfilimine 1l also underwent the desired transformation with ynamide 2a to deliver 3aa in 63% yield (entry 8). Furthermore, a control experiment without any gold catalyst provided no trace of the desired product 3aa (entry 9). Subsequently, we investigated the reaction scope with respect to ynamides under the optimized reaction conditions (Table 1, entry 5) by using sulfilimine 1a as a model substrate. As described in Scheme 1, a series of N-alkyl-substituted ynamides (2a−d) bearing methyl, n-butyl, cyclopropyl, and isobutyl were converted into the corresponding oxazoles 3aa− ad in 87−95% yields. Allyl- and phenyl-substituted ynamides 2e and 2f are also suitable substrates for this transformation. With a 3,5-dimethoxyphenyl group on the nitrogen atom, ynamide 2g is very well tolerated, giving product 3ag in 84% yield. Ynamides 2h−j with other protecting groups also underwent the transformation smoothly. This reaction is also adaptable with ynamides bearing diverse functionalities, including alkyl, halogen, and electron-withdrawing groups, at ortho, meta, and para positions on the benzene rings (2k−p, R2 = Ar), and the corresponding oxazoles 3ak−ap were prepared in 77−87% yields. The structure of 3ao was confirmed by an X-ray structure analysis (Figure 2). In the case of thiophen-3-yl

a

Reaction conditions: 1 (0.3 mmol, 1.5 equiv), 2 (0.2 mmol, 1.0 equiv), PicAuCl2 (3.8 mg, 5 mol %), toluene (2.0 mL, 0.1 M). b Isolated yields. c3.0 equiv 1a were used.

Figure 2. Solid state molecular structure of 3ao.

ynamide 2q, the product 3aq bearing two heterocyclic units could be synthesized in 90% yield. An alkyl ynamide 2r (R2 = phenylpropyl) delivered 3hr in 96% yield, and no side product of a 1,2-H-shift was obtained. Furthermore, both bis-ynamides B

DOI: 10.1021/acs.orglett.9b01011 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Scheme 4. Gram-Scale Synthesisa

2s and 2t can react well with 3 equiv of ylide 1a, yielding 3as and 3at in 43% and 50% yields. The scope was further explored by using diverse sulfilimines (Scheme 2). We initially focused on different aryl-substituted Scheme 2. Scope with Respect to Sulfiliminesa,b

a

Isolated yield.

In conclusion, N-acyl sulfilimines were successfully applied for the generation of α-imino gold carbenes, a subsequent oxygen nucleophilic attack of which offers a facile and promising access to 4-aminooxazoles. The broad scope, high efficiency, DMS as easily removable waste, and the easy synthesis of the N-acyl sulfilimines from inexpensive amides, NCS and DMS, are the main advantages of this protocol.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b01011. Experimental procedures and compound characterization (PDF) Accession Codes

a

Reaction conditions: 1 (0.3 mmol, 1.5 equiv), 2a (0.2 mmol, 1.0 equiv), PicAuCl2 (3.8 mg, 5 mol %), toluene (2.0 mL, 0.1 M). b Isolated yields. cNo reaction.

CCDC 1868317 contains 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.

ylides. Sulfilimines 1b−d bearing a methyl group at different positions were compatible with the reaction under the standard conditions, with good to excellent yields. We found that substrate S,S-dimethylsulfilimine, bearing a methoxy group, appeared to be sensitive to decomposition under the reaction conditions, and we therefore chose S,S-diphenyl sulfilimine 1m (R = Ph) as the substrate for the synthesis of 2-(3methoxylphenyl)oxazole 3ma, since the phenyl groups could improve the stability of the N−S double bond (Scheme 3). As

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

A. Stephen K. Hashmi: 0000-0002-6720-8602 Notes

Scheme 3. Synthesis of 2-(3-Methoxylphenyl)oxazole 3ma from S,S-Diphenylsulfilimine 1ma

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS X.T., L.S., C.H., C.Z., and Y.W. are grateful to the CSC (China Scholarship Council) for a PhD fellowship. a

Isolated yield.

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a result, product 3ma was obtained in 64% yield. This method was also applicable to ylides 1e−g bearing diverse halogen substituents (F, Cl, Br) on the benzene rings. A thiophenebased ylide 1h was incorporated, with annulation with ynamide 2a to afford product 3ha in 98% yield. In addition to alkylsubstituted substrates, n-butyl- and cyclohexyl-containing sulfilimines led to the corresponding products in 90% and 95% yields. Notably, tert-butyl substituted 1k showed low reactivity, probably because of the strong steric hindrance. Furthermore, this reaction is efficiently scalable, which was exemplified by the 3.0 mmol scale reaction of 1a and 2a. As shown in Scheme 4, the desired oxazole 3aa was obtained in 85% yield. C

DOI: 10.1021/acs.orglett.9b01011 Org. Lett. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.orglett.9b01011 Org. Lett. XXXX, XXX, XXX−XXX