Chemo-Enzymatic Synthesis of Oligoglycerol Derivatives.

A cleaner and greener method has been developed and used to synthesize 14 different functionalized oligomer derivatives of glycerol in moderate 29%-39% yields over three steps. After successive regioselective enzymatic acylation of the primary hydroxyl groups, etherification or esterification of the secondary hydroxyl groups and chemoselective enzymatic saponification, the target compounds can efficiently be used as versatile building blocks in organic and supramolecular chemistry.

1. Introduction

Glycerol is a considered green feedstock due to its bioavailability. An enormous quantity of glycerol is being produced by the oleochemical and biodiesel industry and is used as a base chemical for the production of value added products [1,2,3,4,5,6,7,8,9,10,11,12,13]. It has enormous applications in the food industry, pharmaceutical and personal care preparations. Oligomerization of glycerol and the physical and chemical properties of its oligomers have also been well studied. In particular the low molecular weight oligomers such as di-, tri-, and tetraglycerol are more hydrophilic than higher ones and thus have better solubility in polar solvents. These oligomers are used in personal care formulations for their mild humectant properties and ability to enhance fragrance/flavor impact and longevity. Glycerol oligomers also act as plasticizers in PVA films and starch-based biodegradable thermoplastic compositions. The oligomer derivatives have been explored for various applications, e.g., thickners, emulsifiers, antifogging agents, etc. [14,15,16,17,18,19]. Glycerol oligomers may also be considered as superior building blocks for polymerization or polycondensation reactions in comparison to the monomer as the latter leads to low molecular weight reaction products which have a considerable effect on the properties of the macromolecular compounds. The preparation of polymers based on the conversion of oligomerss into macromolecular compounds is emerging as an interesting line of development in the synthesis of polymers [20,21]. Such a method is associated with the synthesis of oligomers with reactive groups at the ends of the molecules.

In order to provide more extensive data for a wider structure-activity relationship (SAR), analogues having 2-O-alkyl and 2-O-acyl groups have to be synthesized selectively via full or partial green chemistry. Unfortunately, glycerol and its oligomers have two types of hydroxyl groups having similar pKa and consequently, regioselective differentiation of the primary and secondary hydroxyl groups is difficult. In this regard, a protection-functionalization-deprotection strategy has to be realized. Our preliminary work reported that an enzymatic method using immobilized Candida antarctica lipase (Novozym 435) [22,23,24] can distinguish the primary and secondary hydroxyl groups in glycerol and polyglycerol moieties to synthesize a wide variety of polymeric and dendritic architectures for biomedical applications [25,26,27,28,29]. Starting from dimers and trimers of glycerol, regio-selective enzymatic protection of the primary hydroxyl groups by acetylation could be envisaged followed by 2-O-alkylation or 2-O-acylation of the secondary hydroxyl groups, then followed by deprotection of primary hydroxyls. Herein, we report the synthesis of 14 new building blocks based on diglycerol and triglycerol wherein aromatic/azido groups have been incorporated in the secondary carbon via an ether or an ester so as to provide versatility besides facilitating the monitoring of their reactions and product purification. The presence of aromatic moiety may provide the possibility of additional π-π interactions in the macromolecules and thus controlling the aggregation phenomenon and encapsulation behavior. The incorporation of the azido group and an alkynyl group on the other hand provide a site for the 1,3-dipolar cycloaddition reaction under click conditions [30,31,32].

2. Results

Commercially available diglycerol and triglycerol were subjected to Novozym 435-catalyzed acylation by following the literature procedure [26]. Since the commercially available oligomers are not in pure form and rather are mixtures of glycerol, diglycerol and triglycerol, consequently, a mixture of products is obtained after acylation using Novozym 435 (20 wt %) and vinyl acetate in THF (Scheme 1).

Scheme 1

Synthesis of diglycerol/triglycerol based building blocks. Reagents and Conditions: (i) Novozym 435, vinyl acetate, THF, 6 h, 30 °C; (ii) DIAD, TPP, THF, ethyl,4-hydroxybenzoate, 15 h, 40 °C (iii) (a) MsCl, DCM, TEA, 2.5 h, 0 to −5 °C. (b) DMF, NaN3, 90 °C, 15 h. (iv) EDC.HCl, DCM:DMF (4:1), 4-(prop-2-yn-1-yloxy)benzoic acid, 15 h, 30 °C. (v) Novozym 435, n-butanol, THF, 72 h, 60 °C.

The reaction progress was monitored by TLC (methanol/chloroform, 1:9, v/v) and on completion of the reaction the desired diacylated product was purified through column chromatography over silica gel using CHCl3/MeOH as eluent It should be noted that compounds 2 and 10 contain two and three stereocenters, respectively. In our hands, both of these compounds were obtained as a mixture of three and seven stereoisomers, respectively and this was confirmed by the observance of the peak multiplicity in the 13C-NMR spectrum of compounds 2 and 10 (see Supplementary Materials Figures S1 and S8). This suggests that the enzymatic conditions produced the protected glycerol derivatives regioselectively but not stereoselectively. In the 1H-NMR spectrum of compounds 2 and 10, the methyl protons of the acetyl group appeared at 2.08 ppm, whereas the methylene and methine protons appeared in the range of 3.43 to 4.24 ppm (Figure 1 and Figure 2).

Figure 1

1H-NMR spectrum of compound 2, 4, and 8.

Figure 2

1H-NMR spectrum of compound 10, 12 and 16.

The resulting diacyl oligomers 2 and 10 were further functionalized at the secondary hydroxyl via ether/ester linkage (Scheme 1). The 2-O-alkylation of compounds 2 and 10 was realized via Mitsunobu reaction using 4-hydroxybenzoic acid ethyl ester and furnished the two ethers 3 and 11 in 70% and 65% yield, respectively [33]. The 2-O-acylation of compounds 2 and 10 using benzoic acid derivative in the presence of EDC gave the corresponding esters 7 and 15 in 75% and 70% yield, respectively [25,34].

Furthermore, the azido group was also incorporated in a stepwise manner i.e., first carrying out mesylation of the secondary hydroxyl group followed by treatment with sodium azide in DMF. The formation of products 5 and 13 was monitored by the appearance of an azide peak in the IR spectra. Our conditions permitted us to produce regioselectively, in the secondary position, the ethers 3 and 11, esters 7 and 15 and azido derivatives 5 and 13. Saponification and transesterification were not observed, meaning that the regioselective protection of the primary hydroxyl groups supported the described nucleophilic conditions.

In the next step the resulting diacyl oligomer derivatives 3, 5, 7, 11, 13, and 15 were subjected to Novozym 435-catalyzed deacetylation in the presence of excess n-butanol in THF [35]. After column chromatography, the target compounds 4, 6, 8, 12, 14 and 16 were obtained in good yields. A high chemo-selectivity was observed for deacylation of compounds 3, 7, 11 and 15, the aromatic acid ester remained intact and only the selective hydrolysis of aliphatic acid ester was observed. Moreover, in our conditions, no transesterification of the aromatic esters was observed from the secondary hydroxyl group to the primary hydroxyl group. All the compounds were characterized by 1H-, 13C-NMR and HRMS analysis. The methine proton in all of these compounds undergoes a characteristic chemical shift on functionalization i.e., O-alkylation of secondary hydroxyl groups in compounds 4, 8, 12 and 16 led to a shift from 3.90–4.02 to 4.50–4.55 ppm (Figure 1 and Figure 2). However, on acylation of the secondary hydroxyl group, the corresponding methine proton undergoes a more significant shift to 5.20–5.44 ppm (Figure 1 and Figure 2). The azido compounds 6 and 14 exhibited a characteristic peak for azide group at about 2100 cm−1 in the IR spectrum. While in the compounds 8 and 16 the characteristic acetylinic proton was observed at 2.55–2.56 ppm (Figure 1 and Figure 2) in the 1H-NMR spectrum and the acetylinc (-C≡C-) moiety led to the observance of a peak in the rang 2100–2200 cm−1 in the IR spectrum.

3. Experimental Section

3.1. General Information

All the compounds were characterized by their physical and spectral data. Infrared spectra were recorded on a Perkin-Elmer FT-IR Model 9 Spectrophotometer (Perkin-Elmer, Singapore). The 1H- and 13C-NMR spectra (400 MHz/100.5 MHz) were recorded on Jeol-400 NMR spectrometer (Jeol, Tokyo, Japan) using TMS as an internal standard. The chemical shift values are measured on δ scale and the coupling constant values (J) are reported in Hz. The HRMS data were recorded on Agilent-6530, Q-TOF LCMS (Agilent, Singapore).

The diglycerol and triglycerol were obtained from Solvay Bruxelles (Solvay, Bruxelles, Belgium) and Sigma Aldrich (Saint Louis, MO, USA) respectively. All other chemicals and solvents used were purchased from the Spectrochem Pvt. Ltd. (Mumbay, India) and SD Fine Chemicals Pvt. Ltd. (Mumbay, India). All the solvents were distilled prior to their use. Novozym 435 (immobilized Candida antarctica lipase) was purchased from Novo Nordisk A/S, Bagsvaerd, Denmark. Reactions were monitored by pre-coated TLC plates (Merck silica gel 60 F254, Darmstadt, Germany), by visualizing the spot in ceric solution stain and iodine. All the compounds were purified by column chromatography using silica gel (100–200 mesh).

3.2. Synthesis and Characterization

3.2.1. Synthesis of Oxybis(2-hydroxypropan-3,1-diyl) Diacetate (2)

In a 250 mL RB flask diglycerol (1, 30.1 mmol) was dissolved in THF (150 mL) followed by the addition of Novozym 435 (20 wt % of monomers). After stirring for 10 min vinyl acetate (69.3 mmol, 5.96 g) was added, the reaction mixture was placed in an incubator shaker at 230 rpm for 6 h at 30 °C. The progress of the reaction was monitored by TLC (methanol–chloroform, 1:9, v/v). On completion of the reaction, the enzyme was filtered off and washed with methanol. The organic solvent was evaporated under reduced pressure. The obtained crude product was purified by column chromatography using CHCl3–MeOH to give the desired compound 2 as a viscous liquid (75%); IR (neat) νmax: 3778, 3698, 2983, 1743, 1712 cm−1; 1H-NMR (CDCl3): δ 4.24–3.97 (m, 6H, H-3, H-4, H-7, H-8), 3.67–3.53 (4H, m, H-5, H-6), 3.22 (br s, 1H, OH), 3.00 (br s, 1H, OH), 2.08 (s, 6H, H-1, H-10) ppm; 13C-NMR (CDCl3): δ 171 (C-2, C-9), 72 (C-5, C-6), 68 (C-4, C-7), 65 (C-3, C-8), 20 (C-1, C-10) ppm; HRMS (positive, acetonitrile): m/z calcd. for C10H18O7: 250.1053; found [M + Na]+: 273.0946.

3.2.2. Synthesis of ((2-Hydroxypropane-1,3-diyl)bis(oxy))bis(2-hydroxypropane-3,1-diyl) Diacetate (10)

In a 250 mL RB flask triglycerol (9, 30.1 mmol) was dissolved in THF (150 mL) followed by the addition of Novozym 435 (20 wt % of monomers). After stirring for 10 min vinyl acetate (69.3 mmol, 5.96 g) was added, the reaction mixture was placed in an incubator shaker at 230 rpm for 6 h at 30 °C. The progress of the reaction was monitored by TLC (methanol–chloroform, 1:9, v/v). On completion of the reaction, the enzyme was filtered off and washed with methanol. The organic solvent was evaporated under reduced pressure. The obtained crude product was purified by column chromatography using CHCl3-MeOH to give the desired compound 10 as a viscous liquid (70%); IR (neat) νmax: 3399, 2880, 1724, 1694 1443 cm−1; 1H-NMR (CDCl3): δ 4.14–4.08 (m, 4H, H-3, H-11), 4.02–3.93 (m, 3H, H-4, H-7, H-10), 3.58–3.43 (m, 8H, H-5, H-6, H-8, H-9), 2.08 (s, 6H, H-1, H-13) ppm; 13C-NMR (CDCl3): δ 171 (C-2, C-12), 72 (C-5, C-6, C-8, C-9), 69, 68 (C-4, C-7, C-10), 65 (C-3, C-11), 21 (C-1, C-13) ppm; HRMS (positive, acetonitrile): m/z calcd. for C13H24O9: 324.1420; found [M + Na]+: 347.1909.

3.2.3. Synthesis of Diethyl 4,4′-((Oxybis(1-acetoxypropane-3,2-diyl))bis(oxy))dibenzoate (3)

To a stirred solution of compound 2 (4.0 mmol), ethyl,4-hydroxybenzoate (8.4 mmol, 1. 39 g) and triphenylphosphine (12.0 mmol, 3.15 g) in THF (20 mL), DIAD (10 mmol, 2.02 g) in THF (5 mL) was added dropwise. The reaction mixture was stirred for 15 h at 40 °C. The progress of the reaction was monitored by TLC (ethyl acetate–petroleum ether, 1:1, v/v). On completion of the reaction, the reaction mixture was concentrated under reduced pressure and the desired compound was extracted with ethyl acetate (3 × 30 mL). The combined organic layer was dried over anhydrous Na2SO4 and concentrated in vacuuo. The obtained crude product was purified through column chromatography using petroleum ether-ethyl acetate to give the desired compound 3 as a viscous liquid (70%); IR (neat) νmax: 2981, 1740, 1708, 1603, 1506 cm−1; 1H-NMR (CDCl3): δ 7.97–7.94 (m, 4H, H-3′, H-5′), 6.95–6.93 (m, 4H, H-2′, H-6′), 4.69–4.64 (m, 2H, H-4, H-7), 4.34–4.28 (m, 8H, H-3, H-8, H-8′), 3.77–3.70 (m, 4H, H-5, H-6), 2.02 (s, 6H, H-1, H-10), 1.36 (t, 6H, J = 8.0, H-9′) ppm; 13C-NMR (CDCl3): δ 170 (C-2, C-9), 166 (C-7′), 161 (C-1′), 131 (C-3′, C-5′), 123 (C-4′), 115 (C-2′, C-6′), 74 (C-4, C-7), 70 (C-5, C-6), 63 (C-3, C-8), 60 (C-8′), 20 (C-1, C-10), 14 (C-9′) ppm; HRMS (positive, acetonitrile): m/z calcd. for C28H34O11: 546.2101; found [M + H]+: 547.2162.

3.2.4. Synthesis of Diethyl 4,4′-((9-(4-(Ethoxycarbonyl)phenoxy)-2,16-dioxo-3,7,11,15-tetraoxahepta-decane-5,13-diyl)bis(oxy))dibenzoate (11)

To a stirred solution of compound 10 (4.0 mmol), ethyl,4-hydroxybenzoate (8.4 mmol, 1.39 g) and triphenylphosphine (12.0 mmol, 3.15 g) in THF (20 mL), DIAD (10 mmol, 2.02 g) in THF (5 mL) was added drop wise. The reaction mixture was stirred for 15 h at 40 °C. The progress of the reaction was monitored by TLC (ethyl acetate–petroleum ether, 1:1, v/v). On completion of the reaction, the reaction mixture was concentrated under reduced pressure and the desired compound was extracted with ethyl acetate (3 × 30 mL). The combined organic layer was dried over anhydrous Na2SO4 and concentrated in vacuuo. The obtained crude product was purified through column chromatography using petroleum ether–ethyl acetate to give the desired compound 11 as a viscous liquid (65%); IR (neat) νmax: 2992, 1741, 1707, 1603, 1507 cm−1; 1H-NMR (CDCl3): δ 7.97–7.90 (m, 6H, H-3′, H-5′), 6.95-6.85 (m, 6H, H-2′, H-6′), 4.67–4.54 (m, 3H, H-4, H-7, H-10), 4.36–4.31 (m, 6H, H-8′), 4.28–4.25 (m, 4H, H-3, H-11), 3.76–3.66 (m, 8H, H-5, H-6, H-8, H-9), 2.06, 2.02 (s, 6H, H-1, H-13), 1.39–1.35 (m, 9H, H-9′) ppm; 13C-NMR (CDCl3): δ 170 (C-2, C-12), 166 (C-7′), 161 (C-1′), 131 (C-3′, C-5′), 123 (C-4′), 115 (C-2′, C-6′), 76, 74 (C-4, C-7, C-10), 70 (C-5, C-6, C-8, C-9), 63 (C-3, C-11), 60 (C-8′), 20 (C-1, C-13), 14 (C-9′) ppm; HRMS (positive, acetonitrile): m/z calcd for C40H48O15: 768.2993; found [M + NH4]+: 786.3316.

3.2.5. Synthesis of Oxybis(2-azidopropane-3,1-diyl) Diacetate (5)

A solution of compound 2 (4.0 mmol) in DCM (30 mL) was cooled under nitrogen atmosphere over ice bath, triethylamine (16.0 mmol, 1.62 g) and methanesulfonyl chloride (12 mmol, 1.37 g) were then added with maintaining the temperature of the reaction mixture at 0 °C. The solution was then stirred at 30 °C for 2.5 h, the progress of the reaction was monitored by TLC (methanol–chloroform, 1:19, v/v). On completion of the reaction, the salt was filtered and the solvent evaporated under reduced pressure. To the mesylated product (4 mmol) so obtained, sodium azide (24 mmol, 1.56 g) and DMF (30 mL) were added, the reaction mixture was heated at 90 °C for 15 h. The progress of the reaction was monitored by TLC (methanol–chloroform, 1:19, v/v). On completion of the reaction, DMF was removed under reduced pressure, the residue obtained was extracted with ethyl acetate (3 × 30 mL). The combined organic layer was dried over anhydrous sodium sulphate followed by evaporation of solvent. The crude product was purified by column chromatography using petroleum ether-ethyl acetate to give the desired compound 5 as a viscous liquid (75%); IR (neat) νmax: 2922, 2875, 2094, 1739, 1449 cm−1; 1H-NMR (CDCl3): δ 4.24–4.06 (m, 4H, H-3, H-8), 3.78-3.67 (m, 6H, H-4, H-5, H-6, H-7), 2.09, 2.08 (s, 6H, H-1, H-10) ppm; 13C-NMR (CDCl3): δ 170 (C-2, C-9), 71 (C-5, C-6), 63 (C-3, C-8), 59 (C-4, C-7), 20 (C-1, C-10) ppm; HRMS (positive, acetonitrile): m/z calcd. for C10H16N6O5: 300.1182; found [M + H]+: 301.1409.

3.2.6. Synthesis of ((2-Azidopropane-1,3-diyl)bis(oxy))bis(2-azidopropane-3,1-diyl) Diacetate (13)

A solution of compound 10 (4.0 mmol) in DCM (30 mL) was cooled under nitrogen atmosphere over ice bath, triethylamine (16.0 mmol, 1.62 g) and methanesulfonyl chloride (12 mmol, 1.37 g) were then added with maintaining the temperature of the reaction mixture at 0 °C. The solution was then stirred at 30 °C for 2.5 h, the progress of the reaction was monitored by TLC (methanol–chloroform, 1:19, v/v). On completion of the reaction, the salt was filtered and the solvent evaporated under reduced pressure. To the mesylated product (4 mmol) so obtained, sodium azide (24 mmol, 1.56 g) and DMF (30 mL) were added, the reaction mixture was heated at 90 °C for 15 h. The progress of the reaction was monitored by TLC (methanol–chloroform, 1:19, v/v). On completion of the reaction, DMF was removed under reduced pressure, the residue obtained was extracted with ethyl acetate (3 × 30 mL). The combined organic layer was dried over anhydrous sodium sulphate followed by evaporation of solvent. The crude product was purified by column chromatography using petroleum ether-ethyl acetate to give the desired compound 13 as a viscous liquid (70 %); IR (neat) νmax: 2919, 2101, 1737, 1450 cm−1; 1H-NMR (CDCl3): δ 4.24–4.09 (m, 4H, H-3, H-11), 3.82–3.49 (m, 11H, H-4, H-5, H-6, H-7, H-8, H-9, H-10), 2.08 (s, 6H, H-1, H-13) ppm; 13C-NMR (CDCl3): δ 170 (C-1, C-13), 71 (C-5, C-6, C-8, C-9,), 70 (C-3, C-11), 63 (C-7), 59 (C-4, C-10), 20 (C-1, C-13) ppm; HRMS (positive, acetonitrile): m/z calcd. for C10H18O7: 399.1615; found [M + NH4]+: 417.1971.

3.2.7. Synthesis of Oxybis(1-acetoxypropane-3,2-diyl) bis(4-(prop-2-yn-1-yloxy)benzoate) (7)

Compound 2 (4.0 mmol) and 4-(prop-2-yn-1-yloxy)benzoic acid (8.4 mmol) were dissolved in a mixture of DCM and DMF in 4:1 ratio (30 mL). The reaction mixture was stirred at 0 °C, then EDC.HCl (10 mmol, 1.91 g) and DMAP (6 mmol, 0.73 g) were added, the reaction mixture was stirred for 15 h at 40 °C. The progress of the reaction was monitored by TLC (petroleum ether–ethyl acetate, 1:1, v/v). On completion of the reaction, the solvent was removed under reduced pressure and the residue obtained was extracted with ethyl acetate (3 × 30 mL), the organic layer was dried over anhydrous Na2SO4 and solvent evaporated under reduced pressure. The resulting crude product was purified by column chromatography using petroleum ether–zethyl acetate to give the desired compound 7 as a viscous liquid (75%); IR (neat) νmax: 3284, 2932, 2878, 2123, 1712, 1603, 1508 cm−1; 1H-NMR (CDCl3): δ 7.98–7.96 (m, 4H, H-3′, H-7′), 6.98–6.95 (m, 4H, H-4′, H-6′), 5.44–5.38 (m, 2H, H-4, H-7), 4.75–4.74 (m, 4H, H-8′), 4.41–4.29 (m, 4H, H-3, H-8), 3.82–3.70 (m, 4H, H-5, H-6), 2.56 (m, H-10′), 2.03 (s, 6H, H-1, H-10) ppm; 13C-NMR (CDCl3): δ 171 (C-2, C-9), 165 (C-1′), 161 (C-5′), 132 (C-3′, C-7′), 122 (C-2′), 114 (C-4′, C-6′), 77 (C-9′), 76 (C-10′), 70 (C-4, C-7), 69 (C-5, C-6), 62 (C-3, C-8), 55 (C-8′), 21 (C-1, C-10) ppm; HRMS (positive, acetonitrile): m/z calcd. for C30H30O11: 566.1788; found [M + NH4]+: 584.2123.

3.2.8. Synthesis of 2,16-Dioxo-9-((4-(prop-2-yn-1-yloxy)benzoyl)oxy)-3,7,11,15-tetraoxaheptadecane-5,13-diyl bis(4-(prop-2-yn-1-yloxy)benzoate) (15)

Compound 10 (4.0 mmol) and 4-(prop-2-yn-1-yloxy)benzoic acid (8.4 mmol) were dissolved in a mixture of DCM and DMF in 4:1 ratio (30 mL). The reaction mixture was stirred at 0 °C, then EDC.HCl (10 mmol, 1.91 g) and DMAP (6 mmol, 0.73 g) were added, the reaction mixture was stirred for 15 h at 40 °C. The progress of the reaction was monitored by TLC (petroleum ether–ethyl acetate, 1:1, v/v). On completion of the reaction, the solvent was removed under reduced pressure and the residue obtained was extracted with ethyl acetate (3 × 30 mL), the organic layer was dried over anhydrous Na2SO4 and solvent evaporated under reduced pressure. The resulting crude product was purified by column chromatography using petroleum ether–ethyl acetate to give the desired compound 15 as a viscous liquid (70%); IR (neat) νmax: 3286, 2923, 2122, 1714, 1604, 1507 cm−1; 1H-NMR (CDCl3): δ 8.18–7.95 (m, 6H, H-3′, H-7′), 7.26–6.94 (m, 6H, H-4′, H-6′), 5.39–5.24 (m, 3H, H-4, H-7, H-10), 4.80–4.73 (m, 6H, H-8′), 4.37–4.27 (m, 4H, H-3, H-11), 3.90–3.62 (m, 8H, H-5, H-6, H-8, H-9), 2.58–2.50 (m, 3H, H-10′), 2.03 (s, 6H, H-1, H-13) ppm; 13C-NMR (CDCl3): δ 170 (C-2, C-12), 165 (C-1′), 161 (C-5′), 131 (C-3′, C-7′), 123 (C-2′), 114 (C-4′, C-6′), 77 (C-9′), 76 (C-10′), 70 (C-4, C-7, C-10), 70 (C-5, C-6, C-8, C-9), 62 (C-3, C-11), 55 (C-8′) 20 (C-1, C-13) ppm; HRMS (positive, acetonitrile): m/z calcd. for C43H42O15: 798.2524; found [M + NH4]+: 816.2855.

3.2.9. Synthesis of 4,4′-((Oxybis(1-hydroxypropane-3,2-diyl))bis(oxy))dibenzoate (4)

Compound 3 (0.91 mmol) was dissolved in THF (1 mL), then n-butanol (1 mL) and Novozym 435 (50 wt % of monomer) were added. The reaction mixture was kept in an incubator shaker at 230 rpm for 72 h at 60 °C. The progress of the reaction was monitored by TLC (methanol–chloroform, 1:9, v/v). On completion of the reaction, the enzyme was filtered off and washed with methanol. The filtrate was concentrated under reduced pressure. The crude product so obtained was purified by column chromatography using CHCl3–MeOH to give the desired compound 4 as a viscous liquid (75%); IR (neat) νmax: 3440, 2935, 17033, 1603, 1507 cm−1; 1H-NMR (CDCl3): δ 7.95 (d, 4H, J = 8.0, H-3′, H-5′), 6.93 (d, 4H, J = 8.0, H-2′, H-6′), 4.56–4.54 (m, 2H, H-2, H-5), 4.36–4.30 (m, 4H, H-8′), 4.00–4.72 (m, 8H, H-1, H-3, H-4, H-6), 2.39 (br s, 2H, OH), 1.37 (t, 6H, H-9′) ppm; 13C-NMR (CDCl3): δ 166 (C-7′), 161 (C-1′), 131 (C-3′, C-5′), 123 (C-4′), 115 (C-2′, C-6′), 77 (C-2, C-5), 70 (C-3, C-4), 62 (C-1, C-6), 61 (C-8′), 14 (C-9′) ppm; HRMS (positive, acetonitrile): m/z calcd. for C24H30O9: 462.1890; found [M + H]+: 463.1964.

3.2.10. Synthesis of Diethyl 4,4′-((((2-(4-(ethoxycarbonyl)phenoxy)propane-1,3-diyl)bis(oxy))bis(1-hydroxypropane-3,2-diyl))bis(oxy))dibenzoate (12)

Compound 11 (0.91 mmol) was dissolved in THF (1 mL), then n-butanol (1 mL) and Novozym 435 (50 wt % of monomer) were added. The reaction mixture was kept in an incubator shaker at 230 rpm for 72 h at 60 °C. The progress of the reaction was monitored by TLC (methanol–chloroform, 1:9, v/v). On completion of the reaction, the enzyme was filtered off and washed with methanol. The filtrate was concentrated under reduced pressure. The crude product so obtained was purified by column chromatography using CHCl3–MeOH to give the desired compound 12 as a viscous liquid (65%); IR (neat) νmax: 3465, 2980, 2928, 1702, 1603, 1507 cm−1; 1H-NMR (CDCl3, 25 °C): δ 7.97–7.94 (m, 6H, H-3′, H-5′), 6.98–6.89 (m, 6H, H-2′, H-6′), 4.58–4.53 (m, 3H, H-2, H-5, H-8), 4.37–4.31 (m, 6H, H-8′), 3.92–3.70 (m, 12H, H-1, H-3, H-4, H-6, H-7, H-9), 2.27 (br s, 2H, OH), 1.39 (m, 9H, H-9′) ppm; 13C-NMR (CDCl3): δ 166 (C-7′), 161 (C-1′), 131 (C-3′, C-5′), 124 (C-4′), 115 (C-2′, C-6′), 75, 70 (C-2, C-5, C-8), 62 (C-3, C-4, C-6, C-7), 62 (C-1, C-9), 60 (C-8′), 14 (C-9′) ppm; HRMS (positive, acetonitrile): m/z calcd. for C36H44O13: 684.2773; found [M + NH4]+: 702.3113.

3.2.11. Synthesis of 3,3′-Oxybis(2-azidopropan-1-ol) (6)

Compound 5 (1.6 mmol) was dissolved in THF (1 mL), then n-butanol (1 mL) and Novozym 435 (50 wt % of monomer) were added. The reaction mixture was kept in an incubator shaker at 230 rpm for 72 h at 60 °C. The progress of the reaction was monitored by TLC (methanol–chloroform, 1:9, v/v). On completion of the reaction, the enzyme was filtered off and washed with methanol. The filtrate was concentrated under reduced pressure. The crude product obtained was purified by column chromatography using CHCl3–MeOH as an eluent to give the desired compound 6 as a viscous liquid (70%); IR (neat) νmax: 3357, 2926, 2978, 2088, 1638, 1464 cm−1; 1H-NMR (CDCl3): δ 3.78–3.65 (m, 10H, H-1, H-2, H-3, H-4, H-5, H-6), 2.45 (br s, 2H, OH) ppm; 13C-NMR (CDCl3): δ 71 (C-3, C-4), 62 (C-2, C-5), 62 (C-1, C-6) ppm. HRMS (positive, acetonitrile): m/z calcd. for C6H12N6O3: 216.0971; found [M + H]+: 217.1044.

3.2.12. Synthesis of 3,3′-((2-Azidopropane-1,3-diyl)bis(oxy))bis(2-azidopropan-1-ol) (14)

Compound 13 (1.6 mmol) was dissolved in THF (1 mL), then n-butanol (1 mL) and Novozym 435 (50 wt % of monomer) were added. The reaction mixture was kept in an incubator shaker at 230 rpm for 72 h at 60 °C. The progress of the reaction was monitored by TLC (methanol–chloroform, 1:9, v/v). On completion of the reaction, the enzyme was filtered off and washed with methanol. The filtrate was concentrated under reduced pressure. The crude product obtained was purified by column chromatography using CHCl3–MeOH as an eluent to give the desired compound 14 as a viscous liquid (60%); IR (neat) νmax: 3390, 2915, 1988, 2105, 1748, 1480 cm−1; 1H-NMR (CDCl3): δ 3.78–3.55 (m, 15H, H-1–H-9) ppm; 13C-NMR (CDCl3, 25 °C): δ 71 (C-4, C-6), 70 (C-3, C-7), 62 (C-1, C-9), 62, 60 (C-2, C-5, C-8) ppm; HRMS (positive, acetonitrile): m/z calcd. for C9H17N9O4: 315.14.04; found [M + H]+: 338.1298.

3.2.13. Synthesis of Oxybis(1-hydroxypropane-3,2-diyl) bis(4-(prop-2-yn-1-yloxy)benzoate) (8)

Compound 7 (0.88 mmol) was dissolved in THF (1 mL), then n-butanol (1 mL) and Novozym 435 (50 wt % of monomer) were added. The reaction mixture was kept in an incubator shaker at 230 rpm for 72 h at 60 °C. The progress of the reaction was monitored by TLC (methanol–chloroform, 1:9, v/v). On completion of the reaction, the enzyme was filtered and washed with methanol. The filtrate was concentrated under reduced pressure. The obtained crude product was purified by column chromatography using CHCl3–MeOH to give the desired compound 8 as a viscous liquid (70%); IR (neat) νmax: 3459, 3289, 2932, 2880, 2123, 1701, 1603, 1508 cm−1; 1H-NMR (CDCl3): δ 8.01–7.98 (m, 4H, H-3′, H-7′), 6.98–6.95 (m, 4H, H-4′, H-6′), 5.26–5.22 (m, 2H, H-2, H-5), 4.74 (m, 4H, H-8′), 3.92–3.73 (m, 8H, H-1, H-3, H-4, H-6), 2.56 (m, 2H, H-10′), 2.43 (br s, 2H, OH) ppm; 13C-NMR (CDCl3): δ 166 (C-1′), 161 (C-5′), 132 (C-3′, C-7′), 123 (C-2′), 114 (C-4′, C-6′), 77 (C-9′), 76 (C-10′), 73 (C-2, C-5), 70 (C-3, C-4), 62 (C-1, C-6), 55 (C-8′) ppm; HRMS (positive, acetonitrile): m/z calcd. for C26H26O9: 482.1577; found [M + Na]+: 505.1459.

3.2.14. Synthesis of ((2-((4-(Prop-2-yn-1-yloxy)benzoyl)oxy)propane-1,3-diyl)bis(oxy))bis(1-hydroxypropane-3,2-diyl) bis(4-(Prop-2-yn-1-yloxy)benzoate) (16)

Compound 15 (0.88 mmol) was dissolved in THF (1 mL), then n-butanol (1 mL) and Novozym 435 (50 wt % of monomer) were added. The reaction mixture was kept in an incubator shaker at 230 rpm for 72 h at 60 °C. The progress of the reaction was monitored by TLC (methanol–chloroform, 1:9, v/v). On completion of the reaction, the enzyme was filtered and washed with methanol. The filtrate was concentrated under reduced pressure. The obtained crude product was purified by column chromatography using CHCl3–MeOH to give the desired compound 16 as a viscous liquid (65%); IR (neat) νmax: 3286, 2955, 2878, 2123, 1713, 1603, 1508 cm−1; 1H-NMR (CDCl3): δ 8.17–7.94 (m, 6H, H-3′, H-7′), 7.08–6.94 (m, 6H, H-4′, H-6′), 5.39–5.18 (m, 3H, H-2, H-5, H-8), 4.79–4.73 (m, 6H, H-8′), 3.96–3.67 (m, 12H, H-1, H-3, H-4, H-6, H-7, H-9), 2.66 (br s, 2H, OH), 2.59–2.55 (m, 3H, H-10′) ppm; 13C-NMR (CDCl3): δ 165 (C-1′), 161 (C-5′), 131 (C-3′, C-7′), 123 (C-2′), 114 (C-4′, C-6′), 77 (C-9′), 76 (C-10′), 73 (C-2, C-5, C-8), 70 (C-3, C-4, C-6, C-7), 62 (C-1, C-9), 55 (C-8′) ppm; HRMS (positive, acetonitrile): m/z calcd. for C39H38O13: 714.2303; found [M + Na]+: 737.2198.

4. Conclusions

A series of 13 novel glycerol oligomer derivatives 3–8 and 10–16 have been synthesized using chemo-enzymatic approach that can be used further for the synthesis of supramolecular architectures. The enzyme catalyzed approach exhibits high chemo- and regioselectivity as the primary hydroxyl group can be selectively acylated in the presence of secondary hydroxyl group. The acetyl groups of the hydroxymethyl of polymers 2 and 10 were stable during the acylation, etherification and azidation. Moreover selective enzymatic deprotection of the primary hydroxyl groups was efficient without saponification and trans-esterification of the ester in the secondary hydroxyl group.