Efficient Synthesis of O-tert-Propargylic Oximes via Nicholas Reaction

Abstract

A synthetic protocol to access O-tert-propargylic oximes derived from tertiary propargylic alcohols was established via Nicholas reaction. Thus, BF₃·OEt₂-mediated reaction between the dicobalt hexacarbonyl complex of tert-propargylic alcohols and p-nitrobenzaldoxime followed by decomplexation with cerium(IV) ammonium nitrate afforded the corresponding O-tert-propargylic oximes in good to high yields. The obtained O-tert-propargylic oximes were effectively converted into heterocycles, such as four-membered cyclic nitrones, oxazepines, and isoxazolines, by using π-Lewis acidic catalysts.

N-Propargyloxyamine derivatives have been frequently utilized as a synthetic intermediate of isoxazoline derivatives[1] [2] via various transformations, such π-Lewis acidic metal-catalyzed reactions[3] and iodocyclization reactions (Scheme [1]).[4] Moreover, we have recently disclosed that O-propargylic oximes serve as an intriguing platform for unique heterocycles, such as azete-N-oxides[5] and 1,4-oxazepines,[6] by the action of π-Lewis acidic metal catalysts such as Cu, Rh, and Au.[7] In general, N-propargyloxyamines derived from primary and secondary propargylic alcohols have been prepared through Mitsunobu reaction between the corresponding propargylic alcohols and N-hydroxyphthalimide (NHPI, Scheme [1]).[8] However, the protocol is practically inapplicable to propargyloxyamines derived from tert-propargylic alcohols 1 due to severe steric repulsion in the S_N2 substitution (Scheme [2a]). Thus, it is important to develop efficient and robust approaches to the O-tert-propargylic hydroxylamine derivatives for heterocyclic synthesis.[9] Although acid-mediated S_N1-type reactions (Scheme [2b])[10] as well as electrophilic amination reactions (Scheme [2c])[11] have been typically utilized for the synthesis of tertiary alkoxyamine derivatives, our preliminary attempts to synthesize the O-tert-propargylic hydroxylamines by these methods from tert-propargylic alcohols 1 were unsuccessful, presumably due to instability of the alkyne moiety. Thus, we envisioned that application of the Nicholas reaction[12] would be an effective way to substitute the hydroxy group of the tert-propargylic alcohols with the aminooxy group through protection of the alkyne moiety by ligation with two cobalt atoms. Herein, we report an efficient protocol to access O-tert-propargylic oximes 2 from the corresponding tertiary-propargyl alcohols 1 via Nicholas reaction (Scheme [2d]) and we summarize their reactivity in π-Lewis acidic metal-catalyzed reactions for heterocyclic synthesis.

Initially, alkyne 1 were efficiently reacted with Co₂(CO)₈ under standard conditions. As represented by the results of 1a–d, the corresponding dicobalt hexacarbonyl complex 3a–d were obtained in good to excellent yields (Scheme [3] and Table S1).

Next, Nicholas reactions between the alkyne-dicobalt complex 3 and hydroxylamine derivatives 4 were examined, as summarized in Table [1]. The reaction between 3a and 3 equivalents of the oxime 4a, derived from p-nitrobenzaldehyde in the presence of 2.5 equivalents of BF₃·OEt₂ at –10 °C, afforded the corresponding O-propargylic oxime-dicobalt complex 5aa in good yield (entry 1). The reaction using either p-anisaldoxime (4b) or cyclohexanecarbaldoxime (4c) resulted in lower chemical yields due to the formation of the elimination byproduct 6a (entries 2 and 3). The use of NHPI 4d as a nucleophile was totally inefficient (entry 4). The Nicholas reaction was applicable to various tert-propargylic alcohols 3a–j. For example, substrate 3c, having geminal diphenyl groups at the propargylic position, was efficiently converted into the corresponding product 5ca (entry 6), while the use of 5 equivalents of 4a was effective for the reaction of 3b, having phenyl and methyl groups (entry 5). Substrates 3d–f, having a cycloalkyl moiety, reacted with 4a to afford the desired products 5da–fa, respectively, in good to acceptable yields (entries 7–9). Substrates 3g and 3h, having an aryl group at the alkyne terminus, reacted with 4a to afford the desired product in excellent yields, irrespective of its electronic character (entries 10 and 11). An alkyl substituent was tolerated at the alkyne terminus (entry 12). In addition, a trimethylsilyl group was tolerated at R¹, affording the desired product 5ja in good yield (entry 13).

Table 1 Nicholas Reaction of 3 with Hydroxylamine Derivatives 4 ^a

Entry	3	R1	R2, R3	4	Time (h)	5	Yield (%)b
1	3a	Ph	Me, Me,	4a	2.5	5aa	91
2	3a	Ph	Me, Me,	4b	1.5	5ab	53
3	3a	Ph	Me, Me,	4c	0.75	5ac	47
4	3a	Ph	Me, Me,	4d	16.5	5ad	<1c
5	3b	Ph	Ph, Me	4a d	9	5ba	95
6	3c	Ph	Ph, Ph	4a	1.75	5ca	94
7	3d	Ph	-(CH2)3-	4a	2	5da	75
8	3e	Ph	-(CH2)4-	4a	0.5	5ea	36
9	3f	Ph	-(CH2)5-	4a	2	5fa	58
10	3g	p-MeOC6H4	Me, Me	4a	12.5	5ga	>99
11	3h	p-F3CC6H4	Me, Me	4a	11.5	5ha	>99
12	3i	n-Hex	Me, Me	4a	11.5	5ia	94
13	3j	Me3Si	Me, Me	4a	1	5ja	64

^a The reaction of 3 (0.4 mmol) and 4 (0.8 mmol) was carried out in the presence of BF₃·OEt₂ in CH₂Cl₂ at –10 °C for 0.5–16.5 h.

^b Isolated yield.

^c Compound 6a was obtained in 70% yield.

^d Compound 4a (2.0 mmol, 5 equiv) was used.

Decomplexation of 5aa, bearing a p-nitrophenyl group at the oxime moiety (R⁴), was efficiently promoted by using 4 equivalents of cerium ammonium nitrate (CAN) to quantitatively afford the O-tert-propargylic oxime 2aa (Table [2], entry 1; see also the Supporting Information). The propargylic oxime 2ga, having an electron-rich p-anisyl group at the alkyne terminus, was obtained by reducing the loading amount of CAN (2 equiv), due to instability of the substrate 5ga under the oxidative conditions (entry 2). However, it should be noted that O-tert-propargylic oximes with a variety of aryl groups at the alkyne terminus are potentially accessible from 2ja via desilylation followed by Sonogashira reaction (entry 5). Substrate 5ia, having an alkyl group at the alkyne terminus, was readily decomplexed to afford the desired product 2ia in excellent yield (entry 4), whereas 5ac, bearing an alkyl group at the oxime moiety, was not converted into the desired product, but suffered decomposition under the reaction conditions (entry 6).

Table 2 Decomplexation of 5 ^a

Entry	5	R1	R4	2	Yield (%)b
1	5aa	Ph	p-O2NC6H4	2aa	quant
2c	5ga	p-MeOC6H4	p-O2NC6H4	2ga	25
3	5ha	p-F3CC6H4	p-O2NC6H4	2ha	58
4	5ia	n-Hex	p-O2NC6H4	2ia	91
5	5ja	Me3Si	p-O2NC6H4	2ja	66
6	5ac	Ph	Cy	–	<1

^a The reaction of 5 (0.1 mmol) was carried out in the presence of CAN (0.4 mmol) in acetone at 0 °C for 1 hour.

^b Isolated yield.

^c CAN (0.2 mmol) was used.

Obtained O-tert-propargylic oximes 2 were employed for π-Lewis acidic metal-catalyzed reactions to synthesize heterocycles. For example, the reaction of the propargylic oximes 2ca in the presence of a catalytic amount of [CuCl(cod)]₂ at 80 °C afforded the corresponding four-membered cyclic nitrone (azete-N-oxide) 7ca in good yield (Table [3], entry 1). The reaction proceeds via [2,3]-rearrangement from propargylic oxime 2 to N-allenylnitrone 9 via the vinylmetal intermediate 8 followed by 4π-electrocyclization. The substrate 2ba, having phenyl and methyl groups at the propargylic position, afforded a ca. 1:1 mixture of E/Z stereoisomers at the exo-olefin moiety (entry 2). The chemical yield of 7aa, derived from 2aa having two methyl groups at the propargylic position, was improved by using (PPh₃)AuNTf₂ instead of [CuCl(cod)]₂ (entries 4 vs. 3). We previously reported that Au catalysts did not promote [2,3]-rearrangement of O-propargylic oximes derived from secondary alcohols, presumably because the ring-opening process from 8 to 9 involves cleavage of the C–Au bond, which is much stronger than the C–Cu bond due to the relativistic nature of the gold atom.[5] Accordingly, the present results indicate that substitution by two alkyl groups at the propargylic position facilitates the ring-opening process even in the gold-catalyzed reaction.

Table 3 Synthesis of Azete-N-oxides 7 by Cu- and Au-Catalyzed Reactions of 2 ^a

Entry	2	R2, R3	Catalyst (mol%)	7	Yield (%)b
1	2ca	Ph, Ph	[CuCl(cod)]2 (5)	7ca	85
2	2ba	Ph, Me	[CuCl(cod)]2 (5)	7ba	86c
3	2aa	Me, Me	[CuCl(cod)]2 (5)	7aa	30
4	2aa	Me, Me	(PPh3)AuNTf2 (5)	7aa	56
5d	2ea	-(CH2)4-	(PPh3)AuNTf2 (5)	7ea	52

^a Reaction conditions for Cu catalysis: [CuCl(cod)]₂ (0.01 mmol) in CH₃CN (0.2 mL) at 80 °C for 44 h. For Au catalysis: (PPh₃)AuNTf₂ (0.01 mmol) in 1,2-dichloroethane (0.2 mL) at 70 °C for 9 h.

^b Isolated yield.

^c A 44:56 mixture of E/Z isomers was obtained.

^d At 80 °C.

We have previously reported that the Cu-catalyzed reaction between O-secondary propargylic oxime and electron-deficient olefins, such as N-methylmaleimide 10, proceeded through [2,3]-rearrangement, [3+2] cycloaddition between the N-allenylnitrone 9 that was generated in situ and the maleimide, and [1,3]-oxygen rearrangement from the nitrogen atom to the allene center carbon, affording the corresponding 1,4-oxazepines (Scheme [4]). Thus, the copper-catalyzed cascade reaction was applied to the O-tert-propargylic oxime 2aa. The reaction with N-methylmaleimide 10 proceeded at 80 °C in dioxane, affording the corresponding oxazepine 11aa in good yield with excellent anti-selectivity.[6]

Moreover, the gold-catalyzed reaction of 2aa in methanol gave isoxazoline 12 in good yield (Scheme [5]). In particular, the reaction of 2da, bearing a cyclobutyl moiety at the propargylic position, was efficiently converted into the corresponding spirocyclic isooxazoline 12da.[13] The reaction proceeds via alcoholysis of the cyclized vinylgold intermediate 8′ with rapid protodeauration in methanol. In fact, the reaction involved formation of the acetal 13, supporting the proposed mechanism.

In conclusion, we have developed an efficient approach to O-tert-propargylic oximes derived from tertiary propargylic alcohols by using the Nicholas reaction. Given that the oximes serve as a platform for catalytic rearrangement reactions and also as an equivalent of propargyloxyamines in π-acidic metal catalysis, the present protocol is useful for the synthesis of highly functionalized heterocycles.

¹H and ¹³C NMR spectra were recorded on a JEOL JNM-ECS400 (400 MHz for ¹H and 100 MHz for ¹³C) spectrometer. Chemical shifts are reported in ppm relative to CHCl₃ (for ¹H, δ 7.26), and CDCl₃ (for ¹³C, δ 77.00). Infrared (IR) spectra were recorded on a JASCO FT/IR- 4100 spectrophotometer. High-resolution mass spectra analysis was performed on a Bruker Daltonics APEX III FT-ICR-MS spectrometer and Bruker Daltonics solariX FT-ICR-MS spectrometer at Research and Analytical Center for Giant Molecules, Graduate School of Science, Tohoku University. Flash column chromatography was performed on silica gel 60N (Merck 40-63 µm or Kanto 40-50 µm). Analytical thin layer chromatography (TLC) was performed on Merck pre-coated TLC plates (silica gel 60 F254).

Acetone, hexane, CH₃CN, 1,4-dioxane, 1,2-dichloroethane, and methanol were purchased form Fujifilm Wako Pure Chemical Corporation. CH₂Cl₂ was purchased from Kanto Chemical Co., Inc. Co₂(CO)₈, BF₃·OEt₂, and CAN were purchased from TCI. These reagents were used as received. [Cu(cod)]₂ and (PPh₃)AuNTf₂ were prepared according to reported procedures.[14] [15] CDCl₃ was purchased from Merck. All air- and moisture-sensitive manipulations were performed under argon atmosphere using oven-dried glassware, including glovebox techniques.

Compound 3a

To a suspension of Co₂(CO)₈ (5.6 g, 16.5 mmol) in CH₂Cl₂ (150 mL) in a 300 mL three-neck flask was added 1a (15 mmol) at room temperature. The mixture was stirred at room temperature for 3 h, then the solvents were removed in vacuo, the crude product was purified by silica gel column chromatography using hexane/CH₂Cl₂ (1:2) as eluent to afford 3a.

Yield: 6.18 g (13.9 mmol, 93%).

IR (neat): 3606, 3535, 3082, 2988, 2933, 2475, 2089, 2041, 2001, 1606, 1573, 1482, 1459, 1443, 1377, 1362, 1312, 1231, 1153, 1106, 1073, 1030, 998, 954, 918, 828, 761, 736, 691, 672, 621 cm^–1.

¹H NMR (CDCl₃, 400 MHz): δ = 7.61 (m, 2 H), 7.35 (m, 3 H), 1.72 (s, 6 H).

¹³C NMR (CDCl₃, 100 MHz): δ = 199.59, 164.68, 137.71, 129.73, 128.83, 127.76, 106.99, 96.10, 91.69, 73.40, 32.44.

HRMS (FD): m/z [M + Na]⁺ calcd for C₁₇H₁₂Co₂O₇: 468.9139; found: 468.9139.

Compound 5aa

To a mixture of 3a (6.18 g, 13.9 mmol) and oxime 4a (6.94 g, 41.8 mmol) in CH₂Cl₂ (140 mL) was added BF₃·OEt₂ (4.42 mL, 35.2 mmol) dropwise at –10 °C. After stirring at –10 °C for 2.5 h, the reaction was quenched with aqueous NaHCO₃ solution and the mixture was extracted with CH₂Cl₂. The organic layer was washed with water and brine and dried over anhydrous sodium sulfate. After solvents were removed in vacuo, the residue was purified by silica gel column chromatography using hexane/EtOAc (5:1) as eluent to obtain 5aa.

Yield: 7.57 g (12.7 mmol, 91%).

IR (neat): 3428, 3413, 3370, 3359, 2989, 2937, 2376, 2362, 2347, 2334, 2323, 2090, 2051, 2021, 1601, 1587, 1523, 1482, 1442, 1377, 1345, 1140, 1109, 962, 852, 795, 762, 691 cm^–1.

¹H NMR (CDCl₃, 400 MHz): δ = 8.19–8.15 (m, 2 H), 8.11–8.10 (m, 1 H), 7.58–7.54 (m, 4 H), 7.36–7.34 (m, 3 H), 1.88 (s, 1 H).

¹³C NMR (CDCl₃, 100 MHz): δ = 199.58, 148.01, 138.64, 138.25, 129.73, 128.75, 127.50, 123.85, 102.45, 92.61, 83.65, 46.24, 29.15.

HRMS (FD): m/z [M + Na]⁺ calcd for C₂₄H₁₆Co₂N₂O₉: 616.9412; found: 616.9412.

Compound 2aa

To 5aa (4.87 g, 8.16 mmol) in acetone (81.6 mL) in a 200 mL round-bottom flask was added CAN (17.9 g, 32.6 mmol) at 0 °C. After stirring at 0 °C for 5 h, the reaction was quenched with water and the mixture was extracted with ether. The organic layer was washed with water and brine and dried over Na₂SO₄. After removing solvents in vacuo, the residue was purified by silica gel column chromatography using hexane/EtOAc (10:1) as eluent to obtain 2aa in analytically pure form.

Yield: 2.52 g (8.16 mmol, quant).

IR (neat): 2997, 2949, 1598, 1589, 1519, 1491, 1443, 1421, 1409, 1341, 1305, 1245, 1215, 1174, 1154, 1102, 1070, 1028, 1013, 947, 851, 795, 757, 690, 642, 622 cm^–1.

¹H NMR (CDCl₃, 400 MHz): δ = 8.25–8.23 (d, J = 8.7 Hz, 2 H), 8.17 (s, 1 H), 7.82–7.80 (d, J = 8.7 Hz, 2 H), 7.45–7.43 (m, 2 H), 7.31–7.29 (m, 3 H), 1.74 (s, 6 H).

¹³C NMR (CDCl₃, 100 MHz): δ = 148.29, 147.56, 138.40, 131.78, 128.35, 128.20, 127.81, 123.92, 122.62, 90.45, 84.87, 83.79, 77.89, 35.06, 14.04.

HRMS (FD): m/z [M + Na]⁺ calcd for C₁₈H₁₆N₂O₃: 331.1053; found: 331.1053.

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• 2b Baraldi PG, Barco A, Benetti S, Pollini GP, Simori D. Synthesis 1987; 857
• 3a Knight DW, Proctor AP, Clough JM. Synlett 2010; 628
• 3b Yeom H.-S, Lee E.-S, Shin S. Synlett 2007; 2292
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• 4a Foot OF, Knight DW, Low AC. L, Li YF. Tetrahedron Lett. 2007; 48: 647
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• 4b Okitsu T, Sato K, Potewar TM, Wada A. J. Org. Chem. 2011; 76: 3438
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• 5a Nakamura I, Araki T, Zhang D, Kudo Y, Kwon E, Terada M. Org. Lett. 2011; 13: 3616
  Search in Google Scholar
• 5b Nakamura I, Kudo Y, Araki T, Zhang D, Kwon E, Terada M. Synthesis 2012; 44: 1542
  Thieme ConnectSearch in Google Scholar
• 6 Nakamura I, Kudo Y, Terada M. Angew. Chem. Int. Ed. 2013; 52: 7536
  Search in Google Scholar
• 7a Nakamura I, Zhang D, Terada M. J. Am. Chem. Soc. 2010; 132: 7884
  CrossrefPubMedSearch in Google Scholar
• 7b Nakamura I, Okamoto M, Sato Y, Terada M. Angew. Chem. Int. Ed. 2012; 51: 10816
  CrossrefPubMedSearch in Google Scholar
• 7c Nakamura I, Sato Y, Takeda K, Terada M. Chem. Eur. J. 2014; 20: 10214
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• 7d Nakamura I, Gima S, Kudo Y, Terada M. Angew. Chem. Int. Ed. 2015; 54: 7154
  CrossrefPubMedSearch in Google Scholar
• 7e Gima S, Nakamura I, Terada M. Eur. J. Org. Chem. 2017; 4375
• 7f Gima S, Shiga K, Terada M, Nakamura I. Synlett 2019; 30: 393
  Thieme ConnectSearch in Google Scholar
• 7g Shiga K, Terada M, Nakamura I. Chem. Sci. 2019; 10: 5283
  CrossrefPubMedSearch in Google Scholar
• 8a Somanadhan B, Loke W.-K, Sim M.-K, Go M.-L. Bioorg. Med. Chem. 2002; 10: 207
  CrossrefPubMedSearch in Google Scholar
• 8b Proctor AJ, Beautement K, Clough JM, Knight DW, Li Y. Tetrahedron Lett. 2006; 47: 5151
  CrossrefSearch in Google Scholar
• 9a Gaudemer F, Gaudemer A. Tetrahedron Lett. 1980; 21: 1445
  CrossrefSearch in Google Scholar
• 9b Sabbasani V, Lee D. Org. Lett. 2015; 17: 4878
  CrossrefPubMedSearch in Google Scholar
• 10a Reddy CR, Radhika L, Kumar TP, Chandrasekhar S. Eur. J. Org. Chem. 2011; 5967
• 10b Ren Z, Mo F, Dong G. J. Am. Chem. Soc. 2012; 134: 16991
  CrossrefPubMedSearch in Google Scholar
• 10c Kang T, Kim H, Kim JG, Chang S. Chem. Commun. 2014; 50: 12073
  CrossrefPubMedSearch in Google Scholar
• 11a Choong IC, Ellman JA. J. Org. Chem. 1999; 64: 6528
  CrossrefPubMedSearch in Google Scholar
• 11b Sun R, Li Y, Lü M, Xiong L, Wang Q. Bioorg. Med. Chem. Lett. 2010; 20: 4693
  CrossrefPubMedSearch in Google Scholar
• 12a Lockwood RF, Nicholas KM. Tetrahedron Lett. 1977; 18: 4163
  CrossrefSearch in Google Scholar
• 12b Teobald BJ. Tetrahedron 2002; 58: 4133
  CrossrefSearch in Google Scholar
• 12c Diaz DD, Betancort JM, Martin VS. Synlett 2007; 343
• 13 Kotha S, Deb AC, Lahiri K, Manivannan E. Synthesis 2009; 165
• 14 Chi KM, Shin H.-K, Hampden-Smith MJ, Duesler EN, Kodas TT. Polyhedron 1991; 10: 2293
  CrossrefSearch in Google Scholar
• 15 Mézailles N, Ricard L, Gagosz F. Org. Lett. 2005; 7: 4133
  CrossrefPubMedSearch in Google Scholar

• Supplementary Material