Synthesis of O-benzyl hydroxamates employing the sulfonate esters of N-hydroxybenzotriazole

Abstract

The direct conversion of various carboxylic acids, that include sterically hindered amino acids and di-peptides, to O-benzyl hydroxamates is demonstrated using sulfonate esters of benzotriazoles under ambient and milder conditions without significant racemization. This simple and efficient protocol is extended to the synthesis of O-benzyl hydroxamates, using in situ generated solid supported TsOBt to facilitate the recovery and re-usability of HOBt and to render the isolation of the products easier. Such in situ generation and further application of a coupling reagent is novel and industrially important.

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Introduction

The medicinal and biological importance of hydroxamic acids (R-CONHOH) is overwhelming, as they are the key structural constituents of a wide spectrum of bioactive agents including various antibacterial, antifungal, and anticancer agents.¹ As they are very effective metal-ion chelators, they have led to the rational design and discovery of potent inhibitors of metalloenzymes such as thermolysin.² The internal hydroxamic acid motifs are found in siderophores.³ Biologically active peptides are often structurally manipulated in order to improve the properties of these peptides for research in drug development. An important class of these compounds is the backbone-modified peptides or “pseudopeptides”, such as aza-peptides and oxa-peptides.⁴ However, a thorough review of synthetic strategies that are available for the synthesis of hydroxamic acids directly from acids, has revealed that the methods available so far are either associated with routine peptide coupling reagents or with expensive reagents.⁵ Furthermore, the transformation has been carried out from various carboxylate esters and either O or N-protected hydroxylamines.⁶ Since such methods involve amino acids and peptides, it is important to retain the stereochemistry as well.

Additionally, one of the major difficulties encountered during the synthesis of large chemical combinatorial libraries is resolving the need for highly diverse arrays of compounds with heterogeneous behaviour. Organic synthesis by solid phase methods is therefore emerging as a better tool for the clean synthesis of compounds. Tethering starting materials or reagents to an insoluble polymer allows remarkable simplifications in handling steps, rendering the automation of the process feasible. Alternatively, unlike classical solid phase synthesis, the reagents remain attached to the insoluble matrix, while the desired product is easily recovered by merely washing with the respective solvent(s), and the supported reagent is recycled. For example, a similar application of the solid supported N-hydroxybenzotriazole (HOBt) and differently substituted carbodiimides is reported.⁷ However, synthesis of such solid supported reagents involves cumbersome multi-step procedures. In a few cases, such reagents are commercially available but expensive and sensitive to prolonged storage.

Herein we report a novel synthetic route to synthesize the hydroxamates of various carboxylic acids under milder and ambient conditions (Scheme 1), applying the sulfonate esters of N-hydroxybenzotriazole that can be prepared easily from cheap synthetic sources. In addition, we have introduced the novel concept of in situ generation of the solid supported reagent, the sulfonate ester of N-hydroxybenzotrizole, and demonstrated its utility for coupling carboxylic acid with O-benzylhydroxylamine hydrochloride to obtain the mentioned product. Such reagents can also be easily regenerated and recycled.

Scheme 1 Synthesis of O-benzyl hydroxamates employing the sulfonate esters of N-hydroxybenzotriazole (VI).

Results and discussion

Initially, we stirred phenyl acetic acid (1 mmol), TsOBt (¹H benzo[d][1,2,3]triazol-1-yl 4-methanebenzenesulfonate, 1 mmol), DIPEA (Diisopropyl ethyl amine, 2 mmol) and O-benzylhydroxylamine hydrochloride (1 mmol pre-neutralized with 1 mmol DIPEA in 1 mL acetonitrile) at room temperature. Interestingly, the coupling took place within 2 h and gave the desired product (IIIA) in good yield, along with a small amount of the sulfonamide of O-benzylhydroxylamine of p-toulene sulfonic acid (IIIB). In order to explore the chemistry of this particular transformation, we went on to synthesize various sulfonate esters as shown in Scheme 2 and Table 1 using a previously reported procedure.^8,9

Scheme 2 Synthesis of the sulfonate esters of HOBt.

Table 1 Synthesis of various sulfonate esters of HOBt

Entry	Vl, R^2, R^3	Yield^a (%)	Time^b (h)
a Yields refer to isolated yields after column chromatography.b TLC is checked every 30 min. All the products were completely characterized with ^1H-NMR, ^13C-NMR and ESI-MS.
a	R^2=CH3, R^3=H	84	1.5
b	R^2=NO2, R^3=H	81	1.5
c	R^2=H, R^3=NO2	80	1.5

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Next, we examined the ability of each of these coupling reagents (VIa–c) to undergo this transformation. Although the insertion of the electron withdrawing group accelerated the reaction, generation of the sulfonamide corresponding to the O-benzylhydroxylamine side product (IIIB) was greater. Therefore, we continued to perform this reaction with TsOBt. The reactivity of TsOBt was moderate and produced the desired product (IIIA) in good to excellent yield. Formation of the side product was also reduced. The base and solvent screening experiments revealed that DIPEA and acetonitrile were the best.

The current methodology was compatible with aliphatic carboxylic acids (entry 1 and 6, Table 2) and aromatic acids (entries 2–5, Table 2). The yields were good to excellent in all the cases and no di- and tri-acylated products were observed in the reaction mixture. The method was also compatible with common N-protecting groups (entries 7–17, Table 2 and entries 1 and 2, Table 3). Various side chain protecting groups such as, tert-butyl ether (entry 16, Table 2) and benzyl ester (entry 17, Table 2) were also tolerated. Apart from the functional group tolerance, the important advantage of this method is the applicability to the sterically hindered amino acids such as alanine (entries 8, 9 and 13, Table 2), phenyl alanine (entries 10 and 14, Table 2) and leucine (entry 15, Table 2). In all these cases, reaction yields were found to be good but lower than their less sterically hindered analogues. Furthermore, sulfonate esters in condensation chemistry were found to cause no/less racemization.⁹ In our observations, when compared with the optical rotation of the entries (8 and 10, Table 2), the values were found to be close to the reported values^6a,10 and it can be concluded that this transformation does not cause significant racemization.

Table 2 Synthesis of hydroxamic acids using sulfonate esters of HOBt

a Yields refer to the isolated yields after column chromatography. TLC is checked every 30 min. All the unreported products were characterized completely. Reported compounds were characterized with ¹H-NMR, IR and ESI-MS, and matched with the reported data.

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Table 3 Synthesis of hydroxymic acids of peptides

Entry	Product	Yield^a (%)
a Yields refer to the isolated yields after column chromatography. TLC is checked every 30 min. All the products were completely characterized with ^1H-NMR, ^13C-NMR, IR and ESI-MS.
1		69
2		61

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The present methodology was also extended to the synthesis of hydroxamates of di-peptides. Two di-peptides were synthesized using the pentafluorophenol activation strategy and then converted to the corresponding O-benzyl hydroxamates using the current methodology (Table 3). An investigation into the extent of racemization was performed using LCMS. The diastereomeric ratio that was noticed in the case of the C-terminus free di-peptides, was found to be retained even after the application of the present protocol (Fig. 1 and ESI†), which confirmed that this protocol does not cause much racemization.

Fig. 1 Racemization study using HPLC, before (A) and after (B) hydroxamate formation.

In order to facilitate the recovery and re-usability of HOBt, we further developed the solid supported reagent, the sulfonate ester of N-hydroxybenzotriazole (VIII), and applied it to hydroxamate synthesis employing the same conditions as that of the solution phase version of the reaction.

Such in situ generated solid supported HOBt esters of p-toulene sulfonic acid (VII, in Scheme 3) enabled the recovery of HOBt which can be recycled effectively. The recyclability of this reagent was initially tested for 3 cycles with phenyl acetic acid and O-benzylhydroxylamine hydrochloride, and each time the isolated yields were very good (entry 1, Table 4). We further applied the same strategy to the synthesis of the O-benzyl hydroxamates of Fmoc-Gly-OH (entry 2, Table 4) and Fmoc-Phe-OH (entry 3, Table 4). Each time, the yield with the solid supported TsOBt was found to be as good as with the TsOBt in solution.

Scheme 3 In situ generation of the coupling reagent and its application for hydroxamic acid synthesis.

Table 4 Synthesis of hydroxymates using solid supported TsOBt^a

Entry	Acid	Cycle	Yield (%)
a The yields refer to the isolated yields after work up (experimental details are provided in the Experimental section). Characterization data of the products were matched with those of the previously synthesized products, Table 2.
1	Phenyl acetic acid	I	81
		II	80
		III	80
2	Fmoc-Gly-OH	I	82
		II	82
		III	80
		IV	76
		V	75
		VI	71
3	Fmoc- Phe-OH	I	58
		II	58
		III	57
		IV	58
		V	54
		VI	52

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The reaction mechanism can be depicted as shown in Scheme 4 based on the possible products. The desired O-benzyl hydroxamates (IIIA) can be generated following paths a and b. On the other hand, direct attachment of the O-benzylhydroxylamine (II) on the sulfur center of the reagent VI (path c) and the intermediate IX can lead to the formation of the by-product IIIB. As shown in Scheme 4, the formation of both the intermediates (IX and X) was identified by IR and ¹H-NMR spectroscopy. In our model reaction, the peaks at δ 4.04–4.03 were assigned as the glycinic CH₂ protons of Fmoc glycine (Fig. 2A). A set of two peaks emerged at δ 3.72–3.71 and δ 3.56–3.49 (B) 2 min after the addition of TsOBt to the Fmoc-Gly-OH, along with the DIPEA in acetonitrile solvent, which persisted as long as the amine component was not added (C). These peaks were probably for the formation of the intermediates IX and X, which is in agreement with the observations of Carpino et al.^9a Furthermore, in the IR spectra, the C=O stretching frequencies corresponding to the sulfonate ester and the HOBt ester were observed at 1362 cm⁻¹ and 1756 cm⁻¹ respectively (see supporting information†).¹¹

Scheme 4 Proposed mechanism.

Fig. 2 Evidence of the mechanism from ¹H NMR. Selected regions of the NMR spectra of (A) Fmoc-Gly-OH, (B) after mixing Fmoc-Gly-OH and TsOBt for 2 min, (C) after mixing Fmoc-Gly-OH and TsOBt for 10 min, and (D) of Fmoc-Gly-NHOBn.

Conclusions

In conclusion, we have demonstrated the synthesis of O-benzyl hydroxamate using the sulfonate ester of N-hydroxybenzotriazole, which was successfully applied to a wide variety of acids that include amino acids with various side chain protecting groups, and common N-protecting groups such as Boc, Cbz and Fmoc, under milder and ambient conditions in a green solvent, acetonitrile. This strategy was also successfully applied to the synthesis of the O-benzyl hydroxamates of di-peptides. Gratifyingly, the method enjoys success with in situ generated solid supported TsOBt, which facilitates the recovery of N-hydroxybenzotriazole and was successfully reused for up to six cycles without losing its efficiency. This process even facilitates the isolation of the product without purification using column chromatography. Yields of the isolated products were also found to be good.

Experimental

All reagents were purchased from commercial sources and were used without any further purification unless mentioned otherwise. Melting points were uncorrected. Thin layer chromatograms were run on glass plates coated with silica gel G for TLC, using an EtOAc–hexane solvent system. All the compounds were purified by column chromatography using 60–120 mesh silica gel. ¹H-NMR (400 MHz for ¹H) was recorded using a 400 MHz spectrometer using CDCl₃ as the solvent unless mentioned otherwise. Chemical shifts were reported in parts per million (ppm), using tetra methyl silane (0.05% to 1%) as the internal reference. Coupling constants (J) were reported in Hz singlet (s), doublet (d), triplet (t), doublet of doublet (dd), multiplet (m), or broad (br). Low resolution mass spectra were recorded on a Q-TOF ESI-MS instrument, HRMS was recorded on a Q-TOF LC/MS system, IR was recorded on a FT-IR spectrometer and LC-MS was performed on a Q-TOF instrument. Specific rotation was measured on a Perkin Elmer (model 343) polarimeter at 589 nm, 27.1 °C, with the temperature correction off. Data for previously reported compounds (cited therein) matched well with our observed data.

General procedure for the synthesis of sulfonate esters of N-hydroxy benzotriazole from sulfonyl chlorides and HOBt

The representative procedure for the synthesis of TsOBt is common to all entries in Table 1 and is as follows. An oven dried 25 mL R.B. flask equipped with a magnetic stirrer bar, was loaded with HOBt (1 mmol, 1 equiv.) which was dissolved in 1 mL of dry CH₂Cl₂. To this, DIPEA (1 mmol, 1 equiv.) was added under nitrogen. Then, it was cooled to 0 °C, followed by the slow addition (over 30 min) of tosyl chloride using a syringe. Next, the reaction mixture was stirred at room temperature. The progress of the reaction was monitored by TLC. After completion of the reaction, the reaction mixture was washed with saturated NaCl solution (2 × 10 mL) and dried over anhydrous CaCl₂. It was then dried in vacuo and purified by recrystallization using hexane and CH₂Cl₂.

General procedure for the synthesis of O-benzyl hydroxamates

The representative procedure for the synthesis of N-benzyloxy-benzamide is common to all entries in Table 2 and is as follows. An oven dried 25 mL R.B. flask equipped with a magnetic stirrer bar, was loaded with benzoic acid (1 mmol, 1 equiv.), TsOBt (1 mmol, 1 equiv.) and DIPEA (2 mmol, 2 equiv.), and was dissolved in 1 mL of acetonitrile at 0 °C, followed by the slow addition of preneutralized NH₂OBn with DIPEA (1 equiv. 1 mmol). Then, the reaction mixture was stirred at room temperature. The progress of the reaction was monitored by TLC. After completion of the reaction, the reaction mixture was diluted with EtOAc, then concentrated under reduced pressure, after which it was washed with 5% NaHCO₃ (2 × 10 mL), 5% HCl (2 × 10 mL) (for Boc protected acids this acid wash was excluded) and saturated NaCl solution (2 × 10 mL), and dried over anhydrous CaCl₂. Next, it was dried in vacuo and purified by column chromatography using hexane and EtOAc.

General procedure for the synthesis of O-benzyl hydroxamate over solid phase resin

The solid supported HOBt was swelled for 20 min using dichloromethane as the solvent, with a rotator in a 5 mL sintered syringe. To this, 2 equivalents of DIPEA were added and rotated for 5 min. Then 1.5 equivalents of tosyl chloride were added with continuous rotation using a blood tube rotator. Then the reaction mixture was washed 5 times with CH₂Cl₂. To this, 2.5 equivalents of the acid were added along with 2 equivalents of DIPEA and rotated for a further 20 min. Finally, the preneutralized mixture of NH₂OBn and DIPEA in DCM (1 mL) was added and rotated for 3 h. Then it was washed 5 times with DCM and the washings were collected together. The combined DCM solution was finally washed with 5% NaHCO₃ (2 × 3 mL), 5% HCl (2 × 3 mL) and brine (2 × 10 mL), which afforded the desired product for the number of cycles mentioned, while the same solid supported HOBt was repeatedly used in the above mentioned cycle.

N-Benzyloxy-acetamide (entry 1, Table 2). R_f product 0.56 (EtOAc–hexane, 1:4) mp 103 °C, yield (182 mg, 80%), colorless crystals. IR (KBr, ν cm⁻¹) 3093, 2977, 1754, 1591, 942, 764. ¹H NMR (400 MHz, CDCl₃) δ ppm 7.56–7.43 (m, 5H, 5 × ArH), 4.99 (s, 2H, CH₂), 2.18 (s, 3H, CH₃); LRMS (ESI) m/z calcd for [M + H]⁺: 166.08, found: 166.08.

N-Benzyloxy-benzamide (entry 2, Table 2). R_f product 0.61 (EtOAc–hexane, 1:4) mp 102–103 °C (Lit. 102–104 °C), yield (173 mg, 76%), colorless crystals. IR (KBr, ν cm⁻¹) 3093, 2977, 1754, 1591, 942, 764. ¹H NMR (400 MHz, CDCl₃) δ ppm 7.56–7.43 (m, 10H, 10 × ArH), 4.99 (s, 2H, CH₂); LRMS (ESI) m/z calcd for [M + H]⁺: 228.10, found: 228.10.

N-Benzyloxy-2-nitro-benzamide (entry 3, Table 2). R_f product 0.57 (EtOAc–hexane, 1 : 4) mp 103 °C (Lit. 104 °C), yield (207 mg, 81%), yellow colorless crystals. IR (KBr, ν cm⁻¹) 3099, 2797, 1741, 1561, 769. ¹H NMR (400 MHz, CDCl₃) δ ppm 8.44 (s, 1H, 1 × ArH), 8.26–8.28 (d, 1H, J = 8.4 Hz, 1 × ArH), 7.98–7.96 (d, 1H, J = 7.6 Hz, 1 × ArH), 7.56–7.52 (t, 1H, J = 8.0 Hz, 1 × ArH), 7.35–7.30 (m, 4H, 4 × ArH), 7.18 (s, 1H, 1 × ArH), 4.94 (s, 2H, CH₂); LRMS (ESI) m/z calcd for [M + H]⁺: 261.05, found: 261.05.

N-Benzyloxy-3-chlorobenzamide (entry 4, Table 2). R_f product 0.53 (EtOAc–hexane, 1 : 4) mp 165–166 °C (Lit. 166–167 °C), yield (215 mg, 79%), colorless crystals. IR (KBr, ν cm⁻¹) 2993, 1733, 1574, 971, 758. ¹H NMR (400 MHz, CDCl₃) δ ppm 8.58 (br s, 1H, NH), 8.42–8.40 (s, 1H, 1 × ArH), 8.36–8.38 (t, 1H, J = 8.0 Hz, 1 × ArH), 7.34–7.33 (m, 4H, 4 × ArH), 7.19 (s, 1H, 1 × ArH), 4.98 (s, 2H, CH₂); LRMS (ESI) m/z calcd for [M + H]⁺: 273.08, found: 273.08.

N-Benzyloxy-4-nitro-benzamide (entry 5, Table 2). R_f product 0.53 (EtOAc–hexane, 1:4) mp 167–169 °C (Lit. 166–167 °C), Yield (215 mg, 79%), colorless crystals. IR (KBr, ν cm⁻¹) 2993, 1733, 1574, 971, 758. ¹H NMR (400 MHz, CDCl₃) δ ppm 8.58 (br s, 1H, NH), 8.20–8.15 (m, 2H, 2 × ArH), 7.77–7.75 (d, 1H, J = 8.8 Hz, 1 × ArH), 7.34–7.33 (m, 5H, 5 × ArH), 7.19 (s, 1H, 1 × ArH), 4.98 (s, 2H, CH₂); LRMS (ESI) m/z calcd for [M + H]⁺: 273.08, found: 273.08.

N-Benzyloxy-2-phenyl-acetamide (entry 6, Table 2). R_f product 0.52 (EtOAc–hexane, 1:4), mp 75–77 °C (Lit. 76.5–77.5 °C), Yield (203 mg, 84%), yellow color solid. IR (KBr, ν cm⁻¹) 3113, 2986, 1748, 1591, 958. ¹H NMR (400 MHz, CDCl₃) δ ppm 8.50 (br s, 1H, NH), 7.38–7.17 (m, 10H, 10 × ArH), 4.84 (s, 2H, CH₂), 3.44 (s, 2H, CH₂); LRMS (ESI) m/z calcd for [M + H]⁺: 242.11, found: 242.11.

(Benzyloxycarbamoyl-methyl)-carbamic acid tert-butyl ester (entry 7, Table 2). R_f product 0.48 (EtOAc–hexane, 1:4) Yield (241 mg, 86%), semi solid, IR (KBr, ν cm⁻¹) 3093, 2897, 1754, 1620, 983, 886. ¹H NMR (400 MHz, CDCl₃) δ ppm 9.68 (br s, 1H, NH), 7.41–7.27 (m, 5H, 5 × ArH), 5.50 (br s, 1H, NH), 4.88 (s, 2H, CH₂), 3.67 (s, 2H, CH₂), 1.44 (s, 9H, C(CH₃)₃); LRMS (ESI) m/z Calcd for [M + H]⁺: 281.15, found: 281.15.

(1-Benzyloxycarbamoyl-ethyl)-carbamic acid tert-butyl ester (entry 8, Table 2). R_f product 0.48 (EtOAc–hexane, 1:4) Yield (233 mg, 79%), colorless semi solid, mp 96 °C. IR (KBr, ν cm⁻¹) 3164, 2974, 1751, 15941, 764. ¹H NMR (400 MHz, CDCl₃) δ ppm 9.41 (br s, 1H, NH), 7.38–7.34 (m, 5H, 5 × ArH), 5.12 (br s, 1H, NH), 4.88 (s, 2H, CH₂), 4.05 (q, 1H, CH), 1.41 (s, 9H, C(CH₃)₃), 1.33–1.31 (d, 3H, J = 6.8 Hz, CH₃); [α]_D = −55.1°, 25 °C, c = 1.1, CHCl₃ LRMS (ESI) m/z Calcd. for [M + H]⁺: 295.16, found: 295.16.

(1-Benzyloxycarbamoyl-ethyl)-carbamic acid benzyl ester (entry 9, Table 2). R_f product 0.49 (EtOAc–hexane, 1:4) Yield (282 mg, 86%), colorless solid, mp 121–124 °C (Lit. 121–122 °C). IR (KBr, ν cm⁻¹) 3102, 2973, 1752, 1568, 760. ¹H NMR (400 MHz, CDCl₃) δ ppm 8.79 (br s, 1H, NH), 7.37–7.26 (m, 10H, 10 × ArH), 5.10–5.04 (t, J = 11.6 Hz, 2H, CH₂), 4.89 (s, 2H, CH₂), 4.07 (m, 1H, α-CH), 1.36–1.34 (d, J = 6.8 Hz, 3H, CH₃); LRMS (ESI) m/z calcd for [M + H]⁺: 329.15, found: 329.15.

(1-Benzyloxycarbamoyl-2-phenyl-ethyl)-carbamic acid tert-butyl ester: (entry 10, Table 2). R_f product 0.51 (EtOAc–hexane, 1:4), Yield (263 mg, 71%), Colorless solid. Mp. 129–130 °C (Lit. 129–131 °C). IR (KBr, ν cm⁻¹) 3328, 1754, 1664. ¹H NMR (400 MHz, CDCl₃) δ ppm 8.50 (br s, 1H, NH), 7.29–7.17 (m, 10H, 10 × ArH), 5.01 (br s, 1H, NH), 4.77 (s, 2H, CH₂), 4.67 (m, 1H, α-CH), 4.13–4.11 (d, J = 6.8 Hz, 1H, CH_aH_b), 3.07–2.96 (m, 1H, CH_aH_b) 1.36 (s, 9H, –C(CH₃)₃); [α]_D = −26.4 [25 °C, c 1.1, CHCl₃]; LRMS (ESI) m/z calcd for [M + H]⁺: 371.19, found: 371.19.

tert-Butyl 2-(benzyloxycarbamoyl)pyrrolidine-1-carboxylate (entry 11, Table 2). R_f product 0.43 (EtOAc–hexane, 1 : 4) Yield (243 mg, 76%), mp 182–185 °C (Lit. 182–185 °C), colorless solid, IR (KBr, ν cm⁻¹) 3093, 2977, 1754, 1591, 942, 764. ¹H NMR (400 MHz, CDCl₃) δ ppm 9.80 (br s, 1H, NH), 7.29–7.17 (m, 5H, 5 × ArH), 4.89 (s, 2H, CH₂), 4.18 (m, 1H, CH), 3.32–3.30 (m, 3H, CH_2, CH), 2.04–1.80 (m, 4H, 2 × CH₂), 1.41 (s, 9H, C(CH₃)₃); LRMS (ESI) m/z calcd for [M + H]⁺:321.18, found: 321.18.

(Benzyloxycarbamoyl-methyl)-carbamic acid 9H-fluoren-9-ylmethyl ester (entry 12, Table 2). R_f product 0.42 (EtOAc–hexane, 1:4) Yield (354 mg, 84%), colorless liquid, IR (KBr, ν cm⁻¹) 3093, 2977, 1754, 1591, 942, 764. ¹H NMR (400 MHz, CDCl₃) δ ppm 7.75–7.73 (d, J = 7.6 Hz, 2H, 2 × ArH), 7.56 (br s, 3H, 3 × ArH), 7.40–7.27 (m, 8H, 8 × ArH), 6.79 (br s, 1H, NH), 4.89 (s, 2H, CH₂), 4.38–4.36 (d, J = 7.6 Hz, 4H, CH₂), 4.21–4.19 (t, J = 7.2 Hz, 1H, CH), 4.23 (d, 2H, CH₂); LRMS (ESI) m/z calcd for [M + H]⁺: 425.14, found: 425.14.

(1-Benzyloxycarbamoyl-ethyl)-carbamic acid 9H-fluoren-9-ylmethyl ester (entry 13, Table 2). R_f product 0.42 (EtOAc–hexane, 1 : 4) Yield (296 mg, 75%), Colorless solid, mp 145–147 °C (Lit. 146–148 °C). IR (KBr, ν cm⁻¹) 3093, 2977, 1754, 1591, 942, 764. ¹H NMR (400 MHz, CDCl₃) δ ppm 7.76–7.75 (d, 2H, J = 7.6 Hz, 2 × ArH), 7.57 (br s, 3H, 3 × ArH), 7.41–7.39 (m, 3H, 3 × ArH), 7.28–7.26 (m, 5H, 5 × ArH), 5.35 (br s, 1H, NH), 4.89 (d, 2H, CH₂), 4.36–4.34 (t, J = 7.2 Hz, 1H, 1H, CH), 4.28–4.26(m, 1H, CH), 1.48–1.46 (d, J = 6.8 Hz, 3H, CH₃); LRMS (ESI) m/z Calcd. for [M + Na]⁺: 417.18, found: 417.18.

(1-Benzyloxycarbamoyl-2-phenyl-ethyl)-carbamic acid 9H-fluoren-9-ylmethyl ester (entry 14, Table 2). R_f product 0.46 (EtOAc–hexane, 1 : 4) Yield (315 mg, 64%), white solid, IR (KBr, ν cm⁻¹) 3411, 2928, 1714, 1599, 1396, 1047, 738. ¹H NMR (400 MHz, CDCl₃) δ ppm 7.73–7.71 (d, J = 7.6 Hz, 3H, 3 × ArH), 7.50–7.7 (t, J = 7.2 Hz, 3H, 3 × ArH), 7.37–7.33 (m, 4H, 4 × ArH), 7.24–7.21 (m, 8H, 8 × ArH), 5.41 (br s, 1H, NH), 4.64 (br s, 1H, NH), 4.39–4.37 (t, J = 6.4 Hz, 2H, CH₂), 4.25 (s, 2H, CH₂), 4.14–4.08 (m, 2 × 1H, 2 × CH), 3.18–3.17 (m, 1H, CH_aH_b), 3.06–3.05 (m, 1H, CH_aH_b); ¹³C NMR (100 MHz, CDCl₃) δ ppm 175.2, 170.8, 143.7, 141.1, 129.4, 128.3, 127.5, 126.9, 126.8, 125.0, 119.8, 66.7, 47.1, 37.8; HRMS (ESI) m/z Cald. for [M + H]⁺: 493.2127, found: 493.2159.

(9H-Fluoren-9-yl)methyl 1-(benzyloxyamino)-4-methyl-1-oxopentan-2-ylcarbamate (entry 15, Table 2). R_f product 0.51 (EtOAc–hexane, 2:4) Yield (257 mg, 56%), white solid, IR (KBr, ν cm⁻¹) 3432, 2923, 1670, 1539, 1289, 741. ¹H NMR (400 MHz, CDCl₃) δ ppm 9.50 (br s, 1H, NH), 7.74–7.72 (d, J = 7.6 Hz, 2H, 2 × ArH), 7.52–7.50 (d, J = 7.2 Hz, 2H, 2 × ArH), 7.37–7.24 (m, 9H, 9 × ArH), 5.5–5.51 (d, J = 8.8 Hz, 1H, NH), 4.86 (s, 4H, 2 × CH₂), 4.27–4.23 (m, 2H, CH_aH_b and CH), 4.13–4.07 (m, 2H, CH_aH_b and α-CH), 1.59–1.52 (m, 1H, CH), 0.88 (br s, 6H, 2 × CH₃); ¹³C NMR (100 MHz, CDCl₃) δ ppm 175.4, 166.3, 143.8, 141.8, 136.0, 129.5, 128.7, 127.9, 127.2, 125.2, 120.1, 84.2, 75.7, 67.2, 47.2, 42.3; HRMS (ESI) m/z Calcd. for [M + H]⁺: 459.2284, found: 459.2298.

(1-Benzyloxycarbamoyl-2-tert-butoxy-ethyl)-carbamic acid 9H-fluoren-9-ylmethyl ester (entry 16, Table 2). R_f product 0.48 (EtOAc–hexane, 1 : 4) Yield (299 mg, 61%), white solid, mp 154–157 °C. IR (KBr, ν cm⁻¹) 3309, 2974, 1672, 1537, 1298, 738. ¹H NMR (400 MHz, DCl₃) δ ppm 9.20 (br s, 1H, NH), 7.74–7.73 (d, J = 7.6 Hz, 2H, 2 × ArH), 7.57–7.55 (d, J = 7.6 Hz, 2H, 2 × ArH), 7.39–7.25 (m, 9H, 9 × ArH), 5.74–7.53 (d, J = 5.6 Hz, 1H, NH), 4.94–4.85 (s, 2H, CH₂), 4.37–4.35 (d, J = 6.2 Hz, 2H, CH₂), 4.20–4.16 (t, J = 6.8 Hz, 1H, CH), 4.10–4.09 (m, 2H, 2 × CH), 3.70–3.69 (m, 1H, CH_aH_b), 3.30–3.29 (m, 1H, CH_aH_b), 1.10 (s, 9H, C(CH₃)₃); ¹³C NMR (100 MHz, CDCl₃) δ ppm 175.2, 166.2, 143.7, 141.3, 129.1, 128.6, 127.7, 125.1, 120.0, 74.5, 67.2, 61.2, 47.1, 29.7, 27.2 HRMS (ESI) m/z Calcd. for [M + H]⁺: 489.2389, found: 489.2294.

(1-Benzyloxycarbamoyl-3-oxo-4-phenyl-butyl)-carbamic acid 9H-fluoren-9-ylmethyl ester (entry 17, Table 2). R_f product 0.39 (EtOAc–hexane, 2:3) Yield (299 mg, 56%), white solid, mp 108–111 °C. IR (KBr, ν cm⁻¹) 3093, 2977, 1754, 1591, 942, 764. ¹H NMR (400 MHz, CDCl₃) δ ppm 8.95 (br s, 1H, NH), 7.76–7.75 (d, J = 7.6 Hz, 2H, 2 × ArH), 7.57–7.55 (d, J = 7.6 Hz, 2H, 2 × ArH) 7.41–7.23 (m, 14H, 14 × ArH), 5.74–5.73 (d, J = 5.6 Hz, 1H, NH), 4.89 (s, 2H, CH₂), 4.51 (m, 1H, α-CH), 4.47 (s, 2H, CH₂), 4.35 (t, 1H, CH), 3.02–2.97 (m, 2H, CH_aH_b); ¹³C NMR (100 MHz, CDCl₃) δ ppm 155.9, 135.8, 133.1, 131.5, 129.7, 129.4, 106.1, 64.7, 13.8; LRMS (ESI) m/z Calcd for [M + H]⁺: 551.2182, found: 551.2374.

tert-Butyl 1-(1-(benzyloxyamino)-1-oxo-3-phenylpropan-2-ylamino)-1-oxopropan-2-ylcarbamate (entry 1, Table 3). R_f product 0.36 (EtOAc–hexane, 2 : 3) Yield (349 mg, 79%), white solid, 125–128 °C. IR (KBr, ν cm⁻¹) 3340, 2978, 1725, 1577, 1210, 978. ¹H NMR (400 MHz, CDCl₃) δ ppm 7.28–7.26 (m, 4H, 4 × ArH), 7.23–7.19 (m, 4H, 4 × ArH), 6.85–6.83 (d, 2H, J = 7.2 Hz, 2 × ArH), 6.92 (br s, 1H, 1 × NH), 5.24 (br s, 1H, NH), 4.50 (s, 2H, CH₂), 3.77 (m, 1H, CH), 3.05 (br s, 2H, CH_aH_b), 1.37 (s, 9H, –C(CH₃)₃, 1.16–1.15 (d, 3H, J = 6.0 Hz, CH₃). ¹³C NMR (100 MHz, CDCl₃) δ ppm 175.2, 170.8, 156.1, 143.7, 143.6, 141.2, 135.1, 128.6, 128.4, 128.3, 127.7, 127.0, 125.0, 119.9, 77.2, 67.4, 67.1, 50.1, 47.0, 36.4; HRMS (ESI) m/z Calcd for [M + K]⁺: 479.1826 found: 479.1823.

(9H-Fluoren-9-yl)methyl 1-(1-(benzyloxyamino)-1-oxo-3-phenylpropan-2-ylamino)-1-oxopropan-2-ylcarbamate (entry 2, Table 3). R_f product 0.48 (EtOAc–hexane, 2 : 3) Yield (445 mg, 79%), white solid, mp 115–117 °C. IR (KBr, ν cm⁻¹) 3233, 2929, 1693, 1535, 1082, 738. ¹H NMR (400 MHz, CDCl₃) δ ppm 7.28–7.26 (m, 4H, 4 × ArH), 7.23–7.19 (m, 6H, 6 × ArH), 6.85–6.83 (d, 1H, J = 7.2 Hz, 1 × NH), 5.24 (br s, 1H, 1 × NH), 4.50(s, 2H, CH₂), 3.77– 3.74 (m, 1H, α-CH), 3.05 (br s, 2H, CH_aH_b), 1.37 (s, 9H, –C(CH₃)₃, 1.16–1.15 (d, 3H, J = 6.0 Hz, CH₃); ¹³C NMR (100 MHz, CDCl₃) δ ppm 172.6, 172.3, 156.7, 155.5, 143.8, 143.6, 140.6, 137.2, 135.0, 133.2, 129.1, 129.1, 128.6, 127.9, 127.6, 127.4, 126.9, 126.0, 125.1, 123.7, 120.9, 119.8, 77.2, 74.6, 74.4, 65.6, 60.3, 53.3, 53.2, 49.9, 47.5, 46.6, 36.7, 34.0, 33.3, 31.6, 29.6, 25.3, 25.0, 25.2, 21.0; HRMS (ESI) m/z Calcd. for [M + K]⁺: 564.2498, found: 564.2456.

Acknowledgements

PNB and DD thank IIT Guwahati for the fellowship. The authors sincerely thank the Central Instrumentation Facility, IIT Guwahati for the ¹H-NMR, ¹³C-NMR and LRMS facilities. Our funding agency DST (FAST TRACK scheme, sanction no. SR/FT/CS-011/2008) is also greatly acknowledged.

Notes and references

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4. D. Sabatino, C. Proulx, S. Klocek, C. B. Bourguet, D. Boeglin, H. Ong and W. D. Lubell, Org. Lett., 2009, 11, 3650.
5. (a) G. Giacomelli, A. Porcheddu and M. Salaris, Org. Lett., 2003, 5, 2715; (b) M. A. Bailén, R. Chinchilla, D. J. Dodsworth and C. Na`jera, Tetrahedron Lett., 2001, 42, 5013; (c) M. C. Pirrung and J. H. L. Chau, J. Org. Chem., 1995, 60, 8084; (d) E. Thouin and W. D. Lubell, Tetrahedron Lett., 2000, 41, 457; (e) A. R. Tunoori, J. M. White and G. I. George, Org. Lett., 2000, 2, 4091; (f) J. G. Kim and D. O. Jang, Bull. Korean Chem. Soc., 2010, 31, 171.
6. (a) A. Gissot, A. Volonterio and M. Zanda, J. Org. Chem., 2005, 70, 6925; (b) C. Y. Ho, E. Strobel, J. Ralbovsky, Jr and R. A. Galemmo, J. Org. Chem., 2005, 70, 4873; (c) J. C. S. Woo, E. Fenster and G. R. Dake, J. Org. Chem., 2004, 69, 8984; (d) K. Ramasamy, R. K. Olsen and T. Emery, J. Org. Chem., 1981, 46, 5438.
7. (a) J. E. Pop, B. P. Déprez and A. L. Tartar, J. Org. Chem., 1997, 62, 2594; (b) M. C. Desai and L. M. S. Stramiello, Tetrahedron Lett., 1993, 34, 7685; (c) C. F. Sturino and M. Labelle, Tetrahedron Lett., 1998, 39, 5891.
8. N. B. Palakurthy and B. Mandal, Tetrahedron Lett., 2011, 52, 7132.
9. (a) L. A. Carpino, J. Xia and A. J. El-Faham, J. Org. Chem., 2004, 69, 62; (b) B. Devadas, B. Kundu, A. Srivastava and K. B. Mathur, Tetrahedron Lett., 1993, 34, 6455; (c) S. N. Khattab, Chem. Pharm. Bull., 2010, 58, 501.
10. (a) [α]_D = −55.1° at 25 °C, c =1.1, CHCl₃ for entry 8, and [α]_D = −6.4° at 25 °C, c = 1.1, CHCl₃ for entry 10; (b) M. P. Sibi, H. Hasegawa and S. R. Ghorpade, Org. Lett., 2002, 4, 3343.
11. F. X. Webster and R. M. Silverstein, in Spectrometric identification of Organic Compounds, John Wiley & Sons, 6th Edn., 1997.

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Footnote

† Electronic supplementary information (ESI) available: copies of spectra. See DOI: 10.1039/c3ra44294b

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