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. 2020 Aug 13;5(33):21007–21014. doi: 10.1021/acsomega.0c02516

Yb(OTf)3-Catalyzed and Di-tert-butyl Dicarbonate-Mediated Decarboxylative Etherification and Esterification Reactions

Arshpreet Kaur †, Arshdeep Pannu †, Deshkanwar S Brar †, Surinder K Mehta †, Deepak B Salunke †,‡,*
PMCID: PMC7450632  PMID: 32875237

Abstract

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Protecting group chemistry has invariably captured the fascination of chemists because of its extensive viability in chemical synthesis. The present report describes our pioneer work of applying ytterbium triflate as a catalyst, for the reaction of alcohols with di-tert-butyl dicarbonate (Boc2O) leading to the formation of tert-butyl ethers. There exists no recorded evidence for the use of Yb(OTf)3 as a catalyst for the protection of alcohols to tert-butyl ethers, despite its excellent utility in various reactions. Yb(OTf)3 has been used predominantly in the catalytic deprotection studies such as selective deprotection of tert-butyl esters to carboxylic acids as well as prenyl ethers to alcohols. This study involved the critical evaluation of solvent, time, and temperature that finally led to an efficient protocol for the formation of tert-butyl ethers. Yb(OTf)3 catalyzed the formation of tert-butyl ethers, notably reducing the reaction time, which is exemplified by the achievement of up to 92% conversion of alcohols to tert-butyl ethers within an hour. Additionally, the report demonstrates the utility of this synthetic protocol for the protection of carboxylic acids.

1. Introduction

Various protecting groups like esters, silyl ethers, alkoxy alkyl ethers, and so forth have been reported for the protection of alcohols (Figure 1).1 However, tert-butyl ether is one of the most versatile protecting group because of its extreme stability in basic conditions.2 It can be successfully employed to aliphatic alcohols as well as phenols and is compatible with other protecting groups and functionalities such as carbonyls, nitriles, hydroxyl moieties, and esters. Bartoli’s mild and chemoselective deprotection strategy using anhydrous CeCl3 and NaI in acetonitrile contributed to the popularity of the tert-butyl ether as a desirable protecting group for alcohols.3

Figure 1.

Figure 1

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Protecting groups for hydroxy functionality.

Protection of alcohols to tert-butyl ethers using isobutene in the presence of strong acids came into the limelight during the 1980s.4 However, this conventional route did not work well in the case of aromatic alcohols as it preferentially resulted in Friedel–Craft ring alkylation. Bartoli et al. evaluated various Lewis acid catalysts for the protection of alcohols using tert-butyl dicarbonate (Boc2O) as the reagent to replace harsh conditions for the formation of tert-butyl ethers.5 During their investigation, it was observed that the catalytic amount (10 mol %) of perchlorates [Ce(ClO4)3, Zn(ClO4)2, Mg(ClO4)2, Ca(ClO4)2, and HClO4] and triflates [In(OTf)3, Sc(OTf)3, Al(OTf)3, Ce(OTf)3, Zn(OTf)2, and Mg(OTf)2] resulted in tert-butyl ethers with more than 94% yield in each case. Mg(ClO4)2 stood out to be the best in terms of cost and efficiency of the reaction, while in the case of aromatic alcohols, Sc(OTf)3 was found to be the best choice as tert-butyl ethers are not much stable in the case of perchlorates.

Later, Procopio et al. reported the Er(OTf)3-catalyzed protection of alcohols to tert-butyl ethers.6 The protection step was carried out in solvent-free conditions at room temperature using a catalytic amount (5 mol %) of Er(OTf)3. The broad range of substrates such as aliphatic (primary, secondary, and tertiary) alcohols, unsaturated aliphatic alcohols, phenols bearing electron-releasing as well as electron-withdrawing substituents, tert-butyloxycarbonyl (Boc)-protected amino acids, and so forth were tested which resulted in the demonstration of high yield and selectivity in the presence of other interfering functional groups. Additionally, deprotection studies were also carried out using microwave irradiation.

Overall, the rare-earth metal triflates served as a key that unlocked various catalytic reactions in the protection chemistry. However, no evidence for the use of Yb(OTf)3 as a catalyst for the protection of alcohols to tert-butyl ethers was found despite its excellent utility in various reactions such as selective activation of aldimines over aldehyde for nucleophilic addition, diastereoselective radical cyclization, polymerization of methacrylate, cycloaddition and cyclization reactions, and so forth.7,8 Yb(OTf)3 has been used mostly in the catalytic deprotection studies such as selective deprotection of tert-butyl esters to carboxylic acids,9 selective deprotection of methoxyacetates10 and acetates to alcohols,11 and chemoselective deprotection of acetonide.12

Gooßen and Döhring employed Yb(OTf)3 for the protection of carboxylic acids as tert-butyl esters using nitromethane as solvent at room temperature.13 On the other hand, Yb(OTf)3 was used for deprotection of tert-butyl esters in nitromethane at 50 °C.9 Similarly in the case of ethers, Sharma and Mahalingam14 demonstrated the application of Yb(OTf)3 for the deprotection of prenyl ethers to alcohols at room temperature in nitromethane (CH3NO2), whereas the conversion of alcohols into p-methoxybenzyl ethers was reported in dichloromethane (DCM) at room temperature by Sharma and Mahalingam.15 These studies showed that the critical evaluation of time, solvent, and temperature might prove useful to identify the appropriate condition required for the formation of tert-butyl ethers from alcohols as in the case of Er(OTf)3.6,16

Thus, intrigued by these observations and the relative low cost of Yb(OTf)3, we investigated the application of Yb(OTf)3 as a catalyst for the conversion of alcohols to tert-butyl ethers. The study involved the critical evaluation of solvent, time, and temperature that finally led to an efficient protocol for the formation of tert-butyl ethers. The synthetic utility of the optimized reaction protocol was also successfully employed for the protection of carboxylic acids for the synthesis of tert-butyl esters.

2. Results and Discussion

Di-tert-butyl dicarbonate (Boc2O) is universally used in the protection of amines as their N-Boc derivatives. Bartoli et al. reported the reaction of Boc2O (1.5 equiv) with octan-1-ol in the presence of anhydrous Mg(ClO4)2 that resulted in the formation of tert-butyl octyl ether (2, 66%) without formation of any Boc-derivative (3). About 30% of the starting material remained unreacted in this reaction, and the yield was further enhanced by increasing the equivalents of Boc2O (2.3 equiv).17 These preliminary findings served as a potential solution to the protection of alcohol and was also successfully employed to explore the catalytic activity of Er(OTf)3.6 Interestingly, our first attempt utilizing Yb(OTf)3 as the catalyst in the reaction of octan-1-ol (1) (Scheme 1, entry 1) with Boc2O (2.3 equiv) at room temperature in nitromethane resulted in the mixture of tert-butyl octyl ether (2, 40%) and a small amount of the Boc-derivative (3, 28%) along with unreacted octan-1-ol (1, 20%) in 8 h.

Scheme 1. Reaction of Octan-1-ol with Boc2O (2.3 equiv) Using Yb(OTf)3 (5 mol %) as the Catalyst.

Scheme 1

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On raising the reaction temperature to 60 °C for 1 h, Yb(OTf)3 directed the reaction selectively toward the desired product, wherein tert-butyl octyl ether 2 was obtained with 70% yield. Even with Er(OTf)3, it was reported that the increase in temperature improved the transformation of tert-butyl carbonate to tert-butyl ether.6 Likewise, we explored this conversion of octan-1-ol (1) to tert-butyl octyl ether 2 in different anhydrous solvents such as DCM (CH2Cl2), acetonitrile (CH3CN), 1,4-dioxane, and dimethylformamide (DMF). The reaction in CH3CN showed an abrupt change in the product selectively and yield with variation in the temperature. At room temperature, the reaction in CH3CN afforded only Boc-derivative (3, undesired product) in 38% yield (Scheme 1, entry 2), whereas at 60 °C, the reaction resulted in the desired product 2 (54%) along with the formation of Boc-derivative 3. Moreover, the use of CH2Cl2, 1,4-dioxane and DMF at 60 °C in sealed vessels also resulted in the formation of the desired product 2 with 34, 15, and 28% yield, respectively in 1 h. During the screening of solvents, other reaction conditions such as equivalents of Boc2O (2.3 equiv) and catalyst loading (5 mol %) were kept constant. Out of five polar aprotic solvents, CH3CN and CH3NO2 showed good results and were selected for further optimization.

After selecting a couple of solvents, systematic reaction optimization was planned (Table 1). The temperature of the reaction was fixed at 60 °C, and the catalyst loading of 1 and 10 mol % was explored to compare with the earlier optimized results of 5 mol % catalyst loading (Table 1, entry 1–3). The 5 mol % catalyst loading was found to be optimum in terms of the yield as well as selectivity of the reaction. No intermediate carbonate formation was observed under this optimized condition. The higher catalyst loading resulted in the lowering of the yield, suggesting a possible deprotection of the product formed (entry 3). Lowering the temperature of the reaction (from 60 to 40 °C) at 10 mol % catalyst loading (Table 1, entry 4) improved the yield from 16 to 29%. We also noticed that the reaction with 5 mol % Yb(OTf)3 when carried out for 12 h (Table 1, entry 5) resulted in a lower yield which confirmed the possible deprotection of tert-butyl ether functionality. It was clear from these observations that a window does exist for the protection over deprotection in Yb(OTf)3-catalyzed ether formation. Boc2O is known to get decomposed at a higher temperature; therefore, higher equivalents of Boc2O were used in one of the reactions (Table 1, entry 6) which resulted in a slightly improved yield (77%). Further improvement in the yield (82%) was observed when the reaction was carried out at 80 °C (entry 7). It was surprising to observe that the change of solvent from nitromethane to acetonitrile at 60 °C (Table 1, entry 8) resulted in the mixture of ether as well as carbonate suggesting a need of appropriate temperature and solvent combination for better results. An improved yield (92%) was obtained in the catalytic reaction carried out in acetonitrile at 80 °C (Table 1, entry 9). We also noticed that the deprotection of tert-butyl ether was more feasible in acetonitrile at 80 °C than in nitromethane at 60 °C (Table 1, entry 5 and 10).

Table 1. Optimization of Reaction Conditions for tert-Butyl Octyl Ether (2) Formation.

entry Yb(OTf)3 (mol %) Boc2O (equiv) solventa temp. (°C) time (h) % yield (2)/(3)
1 5 2.3 CH3NO2 60 1 70:00
2 1 2.3 CH3NO2 60 1 15:10
3 10 2.3 CH3NO2 60 1 16:00
4 10 2.3 CH3NO2 40 1 29:00
5 5 2.3 CH3NO2 60 12 40:00
6 5 5.0 CH3NO2 60 1 77:00
7 5 5.0 CH3NO2 80 1 82:00
8 5 2.3 CH3CN 60 1 54:38
9 5 2.3 CH3CN 80 1 92:00
10 5 2.3 CH3CN 80 12 15:00
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a

The reactions were carried out at 1 M concentration.

The observed low yield of the desired product (tert-butyl ether 2) after prolong reaction hours (Table 1, entry 5) and the utility of Yb(OTf)3 in the catalytic deprotection reactions prompted us to carry out stability studies.9,14 We evaluated the compatibility of this catalyst on tert-butyl ethers in acetonitrile. Slight deprotection of tert-butyl ether 2 to octan-1-ol 1 started after an hour, but the complete deprotection was not achieved even after stirring the reaction mixture at room temperature for 24 h (45% deprotection occurred, Table 2, entry 1). No further deprotection was observed even when the reaction was kept at room temperature for 120 h. Subsequently, the effect of catalyst (5 mol %) on tert-butyl ether functionality was studied at 80 °C. After 4 h of initiating the reaction, 30% deprotection of tert-butyl ether 2 to octan-1-ol 1 was observed (Table 2, entry 2), and up to 90%, deprotection was achieved after 18 h (Table 2, entry 3).

Table 2. Stability of tert-Butyl Ether 2 in the Presence of Yb(OTf)3 (5 mol %) in CH3CN.

2.

sr. no. temp. (°C) time (h) conversion (%)
1 25 24 45
2 80 4 30
3 80 18 90
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Based on the detailed study performed on octan-1-ol 1, it emerged that the best experimental condition for achieving the catalytic tert-butyl ether protection of alcohols is by using 5 mol % of Yb(OTf)3 as the catalyst at 80 °C in acetonitrile using 2.3 equiv of Boc2O. The applicability of the developed protocol was further explored by performing the reaction on wide series of primary and secondary alcohols as well as phenols (Table 3).

Table 3. Substrate Scope for Yb(OTf)3 (5 mol %)-Catalyzed tert-Butyl Ether Formation in CH3CN at 80 °C.

2.

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a

The reactions were carried out using 2.3 equiv of Boc2O, and the products formed were characterized by 1H NMR and 13C NMR spectroscopy.

b

Amount of catalyst used: 10 mol %.

c

Solvent used: nitromethane, heating temperature: 60 °C.

d

Solvent used: DCM, heating temperature: 40 °C (no reaction was observed in acetonitrile).

e

Solvent used: DCM, heating temperature: 60 °C.

Overall, the aliphatic primary alcohol gave significantly higher yield compared to the aliphatic secondary alcohols (Table 3, entry 1 and 2) confirming the effect of stearic hindrance. The optimized reaction conditions resulted in the chemoselective formation of tert-butyl ethers, in the presence of alkene, alkynes, and bromo functionality (Table 3, entry 3–5). The protocol also worked well in case of Boc-2-aminoethanol (entry 6), wherein the desired product 6 was formed in good yield in 1 h using CH3NO2 at 60 °C. No reaction was observed in the case of cyclohexanol and cholesterol when the reactions were performed in acetonitrile, whereas the desired products were obtained in DCM (Table 3, entry 7, 8). Notably, the etherification reaction worked well for benzylic alcohol in acetonitrile (Table 3, entry 9), whereas lower yield was observed for the protection of β-naphthol (Table 3, entry 10).

Moreover, we attempted selective derivatization of symmetrical and unsymmetrical diols by employing milder conditions (Table 4). Several methods have been reported for the mono-derivatization of symmetrical diols, such as selective mono-tosylation,18 mono-tetrahydropranylation,19 mono-dithiocarbonylation,20 and mono-acylation.21 Very few reports22,23 for the mono-etherification of diols to tert-butylether are reported with a limited substrate scope. We have successfully demonstrated the decarboxylative etherification protocol using Yb(OTf)3 as a catalyst for the mono-protection of octan-1,8-diol (Table 4, entry 1), wherein a moderate yield of the desired product (13) was obtained particularly after lowering the concentration of the reaction (0.5 M). In addition, selective protection of primary alcohol over secondary alcohol functionality of octan-1,2-diol (Table 4, entry 2) was achieved, but only up to 15% of the desired product was obtained in this reaction.

Table 4. Mono tert-Butyl Ether Derivatives of Diols Catalyzed by Yb(OTf)3 (5 mol %) in CH3NO2 at 60 °C.

2.

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a

The reactions were carried out using 2.3 equiv of Boc2O, and the products formed were characterized by 1H NMR and 13C NMR spectroscopies.

b

Amount of catalyst used: 10 mol %.

Interestingly, the reaction of salicylic acid with Boc2O (2.3 equiv) and Yb(OTf)3 (5 mol %) in CH3CN at 80 °C (Table 5, entry 1) selectively resulted in the formation of tert-butyl ester, and no tert-butyl ether formation was observed.

Table 5. tert-Butyl Ester Derivatives of Carboxylic Acids Catalyzed by Yb(OTf)3 (5 mol %) in CH3CN at 80 °C.

2.

2.

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a

All the products were characterized by 1H NMR and 13C NMR spectroscopy; 5 equiv of Boc2O was used for all the reactions except for the entries 1 and 3, wherein 2.3 equiv of Boc2O was used.

b

Amount of catalyst used: 10 mol %.

c

Solvent used: nitromethane, heating temp.: 60 °C.

The carboxylic acid functionalities in aliphatic substrates such as lauric acid (Table 5, entry 2), oleic acid bearing cis-double bond (Table 5, entry 3), and unsaturated carboxylic acid with trans-double bond such as cinnamic acid (Table 5, entry 4) were also protected to corresponding tert-butyl esters in moderate to good yield. The carboxylic acid functionality of Nα-Fmoc-Nε-Boc-l-lysine was also protected to the respective ester functionality (19) in excellent yield (Table 5, entry 5).

3. Reaction Mechanism

The reaction mechanism for the formation of tert-butyl ether from alcohol is hypothesized and well supported by Bartoli and co-worker’s.5 The catalytic reaction initiates through the formation of a chelate complex 20 (Figure 2) on reaction of Boc2O with Yb(OTf)3. The alcohol undergoes nucleophilic addition to electron-deficient carboxyl functionality to form an intermediate 21. The activated intermediate 21 via path A result in the formation of carbonate, so as to restore the carbon-oxygen double bond. The carbonate formation occurs in an irreversible manner with the elimination of one molecule of carbon dioxide and tert-butanol. The intermediate 21 via path B can equilibrate to intermediate 23 which on elimination of tert-butanol leads to the formation of mixed anhydride 22. The mixed anhydride 22 on further complexation with metal ion via synchronous “six-membered transition state” (TS) decomposes to form tert-butyl ether with elimination of two molecules of carbon dioxide (Figure 2).

Figure 2.

Figure 2

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Reaction mechanism for the formation of tert-butyl ether.

During this investigation, the carbonate formation (Scheme 1, entry 1) was only observed at room temperature and the reaction was directed toward tert-butyl ether formation with increase in temperature. A similar change in product selectivity was also observed by Procopio et al. while working on Er(OTf)3.6 The carbonate formation was usually observed with the Lewis acid catalyst that contain highly nucleophilic anions which do not dissociate from counter metal ion such as metal acetates.5 The increment in temperature favors the dissociation of the ytterbium cation which can be attributed to increased activation energy and improved solubility of the catalyst. This dissociation allows the ytterbium metal ion to tightly hold the oxygen atom and shift the equilibrium via path B to form mixed anhydride 22 which via the six-membered TS leads to the formation of tert-butyl ether, tert-butanol, and carbon dioxide. Additionally, the increase in equivalents of Boc2O shifted the equilibrium in the forward direction resulting in the improvement of reaction yield (Table 1, entry 1, 6). The poor yield in the case of phenol (Table 3, entry 10) is attributed to lower nucleophilicity of the phenoxide anion, whereas the lower yield in the case of secondary alcohol as compared to primary alcohol (Table 3, entry 1, 2) is attributed to the steric hindrance for nucleophilic addition on carbonyl carbon. Moreover, the formation of tert-butyl ester also occurs through the same mechanism and is supported by the already known synthesis of tert-butyl ester through the mixed anhydride approach.9

4. Conclusions

Yb(OTf)3 can be effectively employed for the protection of primary alcohols to the corresponding tert-butyl ethers in a short period with up to 92% conversion in an hour. The reaction can be used for the selective protection of alcohols in the presence of alkenes, alkynes, N-Boc, and alkyl halide functionalities. The temperature and reaction time played a significant role in the protection as well as deprotection studies. At room temperature, the substrate was stable for an hour, and the deprotection gradually begins when the temperature of the reaction was raised to 80 °C. The applicability of the optimized protocol for the preparation of tert-butyl ester derivatives of salicylic acid, lauric acid, oleic acid, cinnamic acid, and Fmoc-l-Lys(Boc)-OH was successfully demonstrated. Overall, the decarboxylative etherification and esterification with di-tert-butyl dicarbonate using Yb(OTf)3 as a catalyst can be effortlessly performed provided solvent, temperature, and reaction time are carefully optimized.

5. General Methods

1H and 13C NMR spectra were recorded on FT NMR AL 300 MHz (Jeol) and FT NMR Avance-II 400 MHz and 500 MHz (Bruker) using CDCl3 as solvent and tetramethylsilane as the internal standard. Chemical shifts are reported in ppm downfield from the standard (δ = 0.00), and coupling constants are reported in Hz. Thin-layer chromatography (TLC) was performed using silica gel 60-F254 precoated aluminium sheets, commercially available from Merck. Column chromatography was performed using 60–120 or 230–400 mesh silica gel. All the solvents were distilled before use by employing standard methods. The commercially available chemicals/reagents were used as obtained.

5.1. Experimental Section

5.1.1. Typical Experimental Procedure for the Protection of Alcohols with Yb(OTf)3

Octan-1-ol (1) (2.0 mmol) was dissolved in acetonitrile (2 mL) and allowed to stand at 80 °C for 10 min. The reaction mixture was further treated with di-tert-butyl dicarbonate (4.6 mmol) and Yb(OTf)3 (62 mg, 5 mol %) and stirred at 80 °C for 1 h. After completion of the reaction (TLC analysis), the reaction mixture was diluted with water (25 mL) and extracted with ethyl acetate (3 × 25 mL). Combined organic layers were washed with water (2 × 25 mL), dried over sodium sulphate, and evaporated under reduced pressure. Purification of the crude residue by column chromatography (silica gel, 1% ethyl acetate/hexane system) afforded tert-butyl octyl ether 2 (92% yield). The workup process was avoided for the reactions carried out on a small scale. The chromatographic purification was carried out by directly loading the reaction mixture on silica gel columns. A similar reaction protocol was used for the protection of phenols as well as carboxylic acids. The different reagents used and reaction conditions maintained are mentioned in Tables 2–5.

5.1.1.1. 1-(tert-Butoxy)octane (2)

It was obtained as colorless liquid; yield (92%, 0.343 g); Rf (4% ethylacetate/hexane): 0.70.

1H NMR (300 MHz, CDCl3): δ 0.87 (t, J = 7.2 Hz, 3H, C8H, −CH3), 1.18 (s, 9H, C2′H, −C(CH3)3), 1.48–1.51 (m, 12H, C2H, C3H, C4H, C5H, C6H, C7H, CH2), 3.41 (t, J = 6.6, 2H, C1H, OCH2). The 1H NMR was consistent with that reported in the literature.17

5.1.1.2. tert-Butyl Octyl Carbonate (3)

It was obtained as colorless liquid; yield (38%, 0.142 g); Rf (4% ethylacetate/hexane): 0.68.

1H NMR (300 MHz, CDCl3): δ 0.88 (t, J = 6.6 Hz, 3H, C8H, CH3), 1.27–1.29 (m, 8H, CH2), 1.51 (s, 9H, C2′H, −C(CH3)3), 1.58–1.71 (m, 4H, CH2), 4.05 (t, J = 6.9, 2H, C1H, OCH2). The 1H NMR was consistent with that reported in the literature.16

5.1.1.3. 2-(tert-Butoxy)octane (4)

It was obtained as colorless liquid; yield (22%, 0.031 g); Rf (6% ethylacetate/hexane): 0.68.

1H NMR (400 MHz, CDCl3): δ 0.89 (t, J = 6.5 Hz, 3H, CH3), 1.09 (d, J = 6.1 Hz, 3H, CH3), 1.18 (s, 9H, −C(CH3)3), 1.22–1.45 (m, 10H, 5CH2), 3.52–3.57 (m, 1H, O–CH−). 13C NMR (100 MHz, CDCl3): δ 14.1 (C8, CH3), 22.6 (CH2), 23.1 (C1, CH3), 26.1 (CH2), 28.6 (C2′, −C(CH3)3), 29.5 (CH2), 31.9 (CH2), 39.1 (C3, CH2), 67.4 (C2, CH–O), 73.0 (C1′, −C(CH3)3). The 1H NMR and 13C NMR were consistent with that reported in the literature.17

5.1.1.4. 1-Bromo-11-(tert-butoxy)undecane (5)

It was obtained as pale yellow oil; yield (40%, 0.051 g); Rf (6% ethylacetate/hexane): 0.48.

1H NMR (400 MHz, CDCl3): δ 1.11 (s, 9H, C2′H, −C(CH3)3), 1.18–1.26 (m, 12H), 1.31–1.37 (m, 2H), 1.40–1.46 (m, 2H) (C2H, C3H, C4H, C5H, C6H, C7H, C8H, C9H, CH2), 1.74–1.82 (m, 2H, C10H, CH2), 3.25 (t, J = 6.8 Hz, 2H, C1H, CH2–Br), 3.33 (t, J = 6.9 Hz, 2H, C11H, CH2–O). 13C NMR (100 MHz, CDCl3): δ 27.6 (C2′, −C(CH3)3), 26.3, 28.2, 28.8, 29.4, 29.50, 29.55, 29.6, 30.7, 32.9 (C2, C3, C4, C5, C6, C7, C8, C9, C10, CH2), 34.0 (C1, CH2–Br), 61.7 (C11, CH2–O), 72.4 (C1′, O–C(CH3)3). The 1H NMR and 13C NMR were consistent with that reported in the literature.24

5.1.1.5. 10-(tert-Butoxy)dec-1-yne (6)

It was obtained as colorless oil; yield (85%, 0.116 g); Rf (5% ethylacetate/hexane): 0.45.

1H NMR (500 MHz, CDCl3): δ 1.18 (s, 9H, C2′H, −C(CH3)3), 1.27–1.35 (dd, J = 9.6, 4.8 Hz, 6H), 1.36–1.44 (dd, J = 9.6, 4.9 Hz, 2H), 1.47–1.57 (m, 4H) (C4H, C5H, C6H, C7H, C8H, C9H, CH2), 1.93 (t, J = 2.6 Hz, 1H, C1H, HC≡), 2.18 (td, J = 7.1, 2.6 Hz, 2H, C3H, CH2–C≡), 3.32 (t, J = 6.8 Hz, 2H, C10H, CH2–O). 13C NMR (125 MHz, CDCl3): δ 18.4 (C3, CH2–C≡), 27.6 (C2′, −C(CH3)3), 26.2, 28.5, 28.7, 29.1, 29.4, 30.7 (C4, C5, C6, C7, C8, C9, CH2), 61.6 (C10, CH2–O), 68.0 (C1, HC≡C−), 72.4 (C1′, −C(CH3)3), 84.7 (C2, ≡C−). The 1H NMR and 13C NMR were consistent with that reported in the literature.25

5.1.1.6. (S)-8-(tert-Butoxy)-2,6-dimethyloct-2-ene (7)

It was obtained as colorless oil; yield (63, 0.086 g); Rf (0.5% ethylacetate/hexane): 0.40.

1H NMR (500 MHz, CDCl3): δ 0.82 (d, J = 6.5 Hz, 3H, C2′H, CH3), 1.11 (s, 9H, C2″H, −C(CH3)3), 1.05–1.09 (m, 1H, C6H), 1.22–1.30 (m, 2H, C5H), 1.45–1.51 (m, 2H, C7H), 1.53 (s, 3H), 1.61 (s, 3H) (C1H, C1′H, CH3), 1.88–1.93 (m, 2H, C4H, −H2C–C=), 3.24–3.31 (m, 2H, C8H, −H2C–O), 5.03 (t, J = 7.1 Hz, 1H, C3H, C=CH). 13C NMR (125 MHz, CDCl3): δ 17.6 (C1′, CH3), 19.7 (C6, CH), 25.5 (CH2), 25.7 (C1′, CH3), 27.6 (C2″, −C(CH3)3), 29.7 (C6, CH), 37.3 (C5, CH2), 37.81 (C7, CH2), 59.8 (C8, CH2–O), 72.4 (C1″, OC(CH3)3), 125.0 (C3, −HC=C), 131.0 (C2, −HC=C−). DEPT-135, HSQC and COSY are provided in Supporting Information.

5.1.1.7. tert-Butyl (2-(tert-Butoxy)ethyl)carbamate (8)

It was obtained as colorless viscous liquid; yield (73%, 0.076 g); Rf (20% ethylacetate/hexane): 0.70.

1H NMR (400 MHz, CDCl3): δ 1.17 (s, 9H, C2′H, tBuO), 1.43 (s, 9H, C2″H, Boc), 3.15–3.25 (m, 2H, C1H, CH2), 3.40 (t, J = 5.1 Hz, 2H, C2H, CH2), 4.90 (br s, 1H, NH). 13C NMR (100 MHz, CDCl3): δ 27.5 (C2′, CH3), 28.4 (C2″, CH3), 41.2 (C1, CH2), 60.6 (C2, CH2), 73.0 (C1″), 79.1 (C1′), 156.1 (C=O). The 1H NMR and 13C NMR were consistent with that reported in the literature.17

5.1.1.8. tert-Butoxycyclohexane (9)

It was obtained as colorless liquid; yield (51%, 0.160 g); Rf (5% ethylacetate/hexane): 0.65.

1H NMR (500 MHz, CDCl3): δ 1.18 (s, 9H, C2′H), 1.09 (m, 1H), 1.20–1.30 (m, 4H) (H-2ax, H-3ax, H-4ax, H-5ax, H-6ax), 1.50–1.59 (m, 1H, H-4eq), 1.67–1.80 (m, 4H, H-2eq, H-3eq, H-5eq, H-6eq), 3.45–3.21 (m, 1H, C1H). 13C NMR (125 MHz, CDCl3): δ 25.1 (C3, C5, CH2), 25.5 (C6, CH2), 28.4 (C2′, CH3), 35.5 (C2, C6, CH2), 70.1 (C1′), 73.0 (C1, CH). The 1H NMR and 13C NMR were consistent with that reported in the literature.26

5.1.1.9. tert-Butyl Cholesteryl Ether (10)

It was obtained as a white solid; yield (73%, 0.084 g); Rf (5% ethylacetate/hexane): 0.75.

IR (neat, cm–1) ν: 2935 (s), 2885 (s), 1464 (m), 1363 (m), 1198 (s), 1068 (s).

1H NMR (500 MHz, CDCl3): δ 0.67 (s, 3H, CH3), 0.86 (dd, J = 6.6, 2.2 Hz, 6H), 0.91 (d, J = 6.5 Hz, 3H, CH3), 0.99 (s, 3H), 1.02–1.17 (m, 8H), 1.19 (s, 9H, CH3), 1.22–1.72 (m, 14H), 1.79–1.89 (m, 2H), 1.91–2.05 (m, 2H), 2.10–2.15 (m, 1H), 2.21–2.34 (m, 1H), 3.25–3.40 (m, 1H), 5.21–5.42 (m, 1H). 13C NMR (125 MHz, CDCl3): δ 11.9 (CH3), 18.7 (CH3), 19.4 (CH3), 21.1 (CH2), 22.6 (CH3), 22.8 (CH3), 23.8 (CH2), 24.3 (CH2), 28.0 (CH), 28.2 (CH2), 28.5 (−C(CH3)3), 31.3 (CH2), 31.9 (CH), 32.0 (CH2), 35.8 (CH), 36.2 (CH2), 36.6 (C), 37.8 (CH2), 39.5 (CH2), 39.8 (CH2), 42.1 (CH2), 42.3 (C), 50.3 (CH), 56.2 (CH), 56.8 (CH), 71.5 (CH), 73.3 (C), 120.9 (C=CH), 142.0 (−C=CH). The 1H NMR and 13C NMR were consistent with that reported in the literature.17

5.1.1.10. (tert-Butoxymethyl)benzene (11)

It was obtained as colorless oil; yield (78%, 0.118 g); Rf (4% ethylacetate/hexane): 0.64.

1H NMR (400 MHz, CDCl3): δ 1.29 (s, 9H, C2″H, CH3), 4.45 (s, 2H, C1H, CH2–O), 7.23–7.35 (m, 5H, ArCH). 13C NMR (100 MHz, CDCl3): δ 27.7 (C2″, CH3), 64.1 (C1, CH2), 73.4 (C1″), 127.1 (ArCH), 127.3 (ArCH), 127.4 (ArCH), 128.3 (ArCH), 139.9 (C1′, ArC−). The 1H NMR and 13C NMR were consistent with that reported in the literature.17

5.1.1.11. 2-(tert-Butoxy)naphthalene (12)

It was obtained as colorless oil; yield (16%, 0.222 g); Rf (8% ethylacetate/hexane): 0.72.

1H NMR (400 MHz, CDCl3): δ 1.41 (s, 9H, −C(CH3)3), 7.18 (dd, J = 2.3, J = 8.8 Hz, 1H, ArH), 7.35–7.45 (m, 3H, ArH), 7.72–7.80 (m, 3H, ArH). 13C NMR (100 MHz, CDCl3): δ 29.0 (−C(CH3)3), 78.9 (O–C(CH3)3), 119.8 (ArCH), 124.5 (ArCH), 125.0 (ArCH), 126.0 (ArCH), 127.2 (ArCH), 127.6 (ArCH), 128.6 (ArCH), 130.3 (ArC=), 134.2 (ArC=), 153 (ArC=). The 1H NMR and 13C NMR were consistent with that reported in the literature.17

5.1.1.12. 8-(tert-Butoxy)octan-1-ol (13)

It was obtained as colorless oil; yield (46%, 0.063 g); Rf (15% ethylacetate/hexane): 0.40.

1H NMR (400 MHz, CDCl3): δ 1.18 (s, 9H, C2′H, CH3), 1.32 (s, 8H), 1.45–1.65 (m, 4H) (C2H, C3H, C4H, C5H, C6H, C7H), 3.32 (t, J = 6.8 Hz, 2H, C8H, CH2-OtBu), 3.64 (t, J = 6.6 Hz, 2H, C1H, CH2–OH). 13C NMR (125 MHz, CDCl3): δ 27.5 (C2′, −C(CH3)3), 25.7, 26.2, 29.37, 29.45, 30.6, 32.7 (C2, C3. C4, C5, C6, C7, CH2), 61.6 (C8, CH2), 62.9 (C1, CH2), 72.4 (C1′, −C(CH3)3). ESI-MS: calcd for C12H26NaO2+, 225.18; found, 225.1 [M + Na]+.

5.1.1.13. 1-(tert-Butoxy)octan-2-ol (14)27

It was obtained as colorless oil; yield (15%, 0.021 g); Rf (10% ethylacetate/hexane): 0.65.

1H NMR (400 MHz, CDCl3): δ 0.88 (t, J = 6.8 Hz, 3H, C8H), 1.20 (s, 9H), 1.25–1.35 (m, 6H), 1.37–1.52 (m, 2H) (C3H, C4H, C5H, C6H, C7H, CH2), 3.15 (t, J = 8.0 Hz, 1H, C1H), 3.38 (dd, J = 8.8, 3.1 Hz, 1H, C1H), 3.57–3.80 (m, 1H, C2H). 13C NMR (125 MHz, CDCl3): δ 14.1 (C8, CH3), 22.6 (C2′, −C(CH3)3), 25.6, 27.6, 29.4, 31.8, 33.2 (C3, C4, C5, C6, CH2), 66.0 (C1, CH2), 70.7 (C2, CH), 73.1 (C1′, −C(CH3)3). ESI-MS: calcd for C12H26O2Na+, 225.18; found, 225.2 [M + Na]+.

5.1.1.14. tert-Butyl 2-Hydroxybenzoate (15)

It was obtained as colorless oil; yield (28%, 0.079 g); Rf (5% ethylacetate/hexane): 0.75.

IR (neat, cm–1) ν: 3150 (br), 2979 (m), 1667 (s), 1613 (m), 1326 (s), 1146 (s).

1H NMR (500 MHz, CDCl3): δ 1.54 (s, 9H, C2′H, CH3), 6.77 (t, J = 7.5 Hz, 1H, ArH), 6.87 (d, J = 8.3 Hz, 1H, ArH), 7.31–7.37 (m, 1H, ArH), 7.70 (dd, J = 7.9, 1.5 Hz, 1H, ArH), 10.96 (s, 1H, OH). 13C NMR (125 MHz, CDCl3): δ 28.2 (C2′, −C(CH3)3), 82.7 (C1′, −C(CH3)3), 113.9 (C1, ArC−), 117.5 (C3, ArCH), 118.8 (C5, ArCH), 130.1 (C6, ArCH), 135.1 (C4, ArCH), 161.8 (C2, ArC−), 169.8 (C=O). The 1H NMR and 13C NMR were consistent with that reported in the literature.28

5.1.1.15. tert-Butyl Dodecanoate (16)29

It was obtained as colorless oil; yield (47%, 0.061 g); Rf (5% ethylacetate/hexane): 0.70.

1H NMR (400 MHz, CDCl3): δ 0.88 (t, J = 6.9 Hz, 3H, C12H, CH3), 1.26 (br s, 16H, C4H, C5H, C6H, C7H, C8H, C9H, C10H, C11H, CH2), 1.50 (s, 9H, C2′H, CH3), 1.52–1.63 (m, 2H, C3H, CH2), 2.20 (t, J = 7.5 Hz, 2H, C2H, CH2). 13C NMR (100 MHz, CDCl3): δ 14.1 (C12, CH3), 22.7 (C3, CH2), 25.1 (C10, CH2), 28.1 (C2′, −C(CH3)3), 29.1, 29.3, 29.3, 29.5, 29.6, 31.9 (C4, C5, C6, C7, C8, C10, C11, CH2), 35.6 (C2, CH2), 79.8 (C1′, −C(CH3)3), 173.3 (C=O).

5.1.1.16. tert-Butyl Oleate (17)

It was obtained as colorless oil; yield (35%, 0.042 g); Rf (2% ethylacetate/hexane): 0.50.

1H NMR (400 MHz, CDCl3): δ 0.88 (t, J = 6.8 Hz, 3H, C18H), 1.22–1.36 (m, 20H, C4H, C5H, C6H, C7H, C12H, C13H, C14H, C15H, C16H, C17H), 1.44 (s, 9H, C2′H, CH3), 1.52–1.62 (m, 2H, C3H, CH2), 2.07–1.89 (m, 4H, C8H, C11H), 2.20 (t, J = 7.5 Hz, 2H, C2H, CH2), 5.28–5.40 (m, 2H, C10H, HC=CH). 13C NMR (100 MHz, CDCl3): δ 14.1 (C18, CH3), 22.7 (C17, CH2), 25.1 (C3, CH2), 27.18, 27.22 (C8, C11, CH2), 28.1 (C2′, CH3), 29.1, 29.1, 29.2, 29.3, 29.5, 29.7, 29.7, 31.9 (C4, C5, C6, C7, C12, C13, C14, C15, C16, CH2), 35.6 (C2, CH2–CO), 79.9, 129.8 (C1′, CH3), 129.9 (C9, C10), 173.3 (C=O). The 1H NMR and 13C NMR were consistent with that reported in the literature.30

5.1.1.17. tert-Butyl Cinnamate (18)

It was obtained as colorless oil; yield (58%, 0.04 g); Rf (5% ethylacetate/hexane): 0.50.

1H NMR (400 MHz, CDCl3): δ 1.54 (s, 9H, C2″H, CH3), 6.37 (d, J = 16.0 Hz, 1H, C2H, =CH), 7.33–7.43 (m, 3H, ArH, C3′H, C4′H, C5′H), 7.51 (dd, J = 6.5, 3.1 Hz, 2H, ArH, C2′H, C6′H), 7.59 (d, J = 16.0 Hz, 1H, C3H, HC=). 13C NMR (100 MHz, CDCl3): δ 28.2 (C2″, CH3), 80.5 (C1″), 120.2 (C2, =CH–CO), 128.0 (C1′, ArC), 128.8 (C2′, C6′, ArCH), 130.0 (C3′, C5′, ArCH), 134.7 (C4′, ArCH), 143.6 (C3, =CH–C), 166.4 (CO). The 1H NMR and 13C NMR were consistent with that reported in the literature.31

5.1.1.18. tert-Butyl N2-(((9H-Fluoren-9-yl)methoxy)carbonyl)-N6-(tert-butoxycarbonyl)-l-lysinate (19)

It was obtained as white solid; yield (89%, 0.1 g); Rf (20% ethylacetate/hexane): 0.4.

IR (neat, cm–1) ν: 3311 (s), 3042 (s), 2979 (s), 1686 (s), 1534 (m), 1451 (m), 1271.9 (m), 1154 (m), 744 (s).

1H NMR (400 MHz, CDCl3): δ 1.43 (s, 9H, –C(CH3)3, Boc), 1.47 (s, 9H, −C(CH3)3, COOtBu), 1.52–1.94 (m, 4H, CH2), 3.11 (d, J = 5.8 Hz, 2H, CH2), 4.15–4.30 (m, 2H, CH), 4.34–4.45 (m, 2H, CH2O), 4.56 (s, 1H, NH), 5.38 (d, J = 7.6 Hz, 1H, NH), 7.32 (td, J = 7.4, 1.0 Hz, 2H, ArH), 7.40 (t, J = 7.5 Hz, 2H, ArH), 7.61 (d, J = 7.4 Hz, 2H, ArH), 7.76 (d, J = 7.5 Hz, 2H, ArH). 13C NMR (100 MHz, CDCl3): δ 22.3 (CH2), 28.0 (CH3), 28.4 (CH3), 29.6 (CH2), 32.4 (CH2), 40.2 (CH2), 47.2 (CH), 54.2 (CH), 66.9 (CH2), 79.1 (−C(CH3)3), 82.1 (−C(CH3)3), 120.0 (ArCH), 125.1 (ArCH), 127.0 (ArCH), 128.0 (ArCH), 141.3 (ArC=), 143.8 (ArC=), 144.0 (ArC=), 156.0 (C=O), 156.0 (C=O), 171.6 (COOtBu). ESI-MS: calcd for C30H41N2O6+, 525.29; found, 525.30 [M + H]+. The 1H NMR was consistent with that reported in the literature.32

Acknowledgments

D.B.S. is thankful to DBT New Delhi for the award of Ramalingaswami Fellowship (BT/RLF/Re-entry/16/2013). The support from UGC-CAS and Panjab University Development fund is gratefully acknowledged. A.K. is thankful to UGC, New Delhi, for the award of Junior and Senior Research Fellowships.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c02516.

  • Spectral data for compounds 2–19 including IR, 1H NMR, 13C NMR, DEPT-135, MS (ESI), HSQC, COSY, and comparison list for spectral data with the literature (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao0c02516_si_001.pdf (4.3MB, pdf)

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