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Synlett 2008(14): 2178-2182  
DOI: 10.1055/s-2008-1077981
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Chemoenzymatic Synthesis of Enantiopure Structured Triacylglycerols

Björn Kristinsson, Gudmundur G. Haraldsson
Science Institute, University of Iceland, Dunhaga 3, 107 Reykjavik, Iceland
e-Mail: gghar@raunvis.hi.is;
Further Information

Publication History

Received 2 June 2008
Publication Date:
31 July 2008 (online)

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Abstract

A highly efficient chemoenzymatic method for the synthesis of enantiopure ABC-type asymmetrically structured triacyl­glycerols has been developed starting from enantiopure (S)-solketal and involving two lipase steps.

Key words

asymmetrically structured triacylglycerols - regioselectivity - lipase - eicosapentaenoic acid - acyl migration

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7

Novozym 435, CAL-B; a gift from Novozyme A/S (Bagsvaerd, Denmark).

14

Sigma-Aldrich (Steinheim, Germany): (S)-(+)-1,2-O-isopropylidene-sn-glycerol of 98% purity and 99% ee.

17

[α]D ²0 +5.5 (c 20, CHCl3), identical to that in ref. 18.

18

Aldrich Chemical Catalog, (R)-(+)-benzyloxy-1,2-propanediol, 99%: [α]D ²0 +5.5 (c 20, CHCl3).

20

Procedure for the Preparation of 1- O -Octadecanoyl- sn -glycerol [( S )-4]
To a solution of 3-O-benzyl-sn-glycerol [(R)-3, 1.497 g, 6.73 mmol] and vinyl stearate (3.827 mg, 12.3 mmol) in CH2Cl2 (8.2 mL) was added immobilized CAL (99 mg). The resulting suspension was stirred at r.t. for approx. 70 min when TLC monitoring (EtOAc-PE, 1:1) indicated a complete reaction. The lipase preparation was separated by filtration and the solvent removed in vacuo on a rotary evaporator. The resulting residue was dissolved in THF (30 mL) without further purification followed by addition of n-hexane (70 mL) and Pd/C catalyst (370 mg). The reaction was performed in a PARR reactor under hydrogen pressure (5 bar) during which the product precipitated. When the reaction had proceeded to completion (about 2 h), THF was added until all the product had been dissolved. The catalyst was separated off by filtration by the aid of Celite and the solvent removed in vacuo on a rotary evaporator. The residue was redissolved in minimum amount of THF and a fourfold volume of n-hexane added. The resulting mixture was allowed to stand at r.t. overnight to afford the product as white crystals (2.057 g, 5.73 mmol) in 85% overall yield; mp 59.5-60.7 ˚C. [α]D ²0 +2.43 (c 6, THF). ¹H NMR (400 MHz, CDCl3): δ = 4.21 (dd, J = 11.6, 4.8 Hz, 1 H, OCOCH2), 4.15 (dd, J = 11.6, 6.0 Hz, 1 H, OCOCH2), 3.96-3.90 (m, 1 H, CH2CHCH2), 3.72-3.67 (m, 1 H, CH 2OH), 3.63-3.57 (m, 1 H, CH 2OH), 2.52 (d, J = 5.2 Hz, 1 H, CHOH), 2.35 (t, J = 7.6 Hz, 2 H, CH2COO), 2.08 (t, J = 6.0 Hz, 1 H, CH2OH), 1.65-1.59 (m, 2 H, CH 2CH2COO), 1.36-1.18 (m, 28 H, CH2), 0.88 (t, J = 6.5 Hz, 3 H, CH2CH 3). ¹³C NMR (100 MHz, CDCl3): δ = 174.3, 70.3, 65.2, 63.3, 34.1, 31.9, 29.7 (5 C), 29.6 (2 C), 29.4 (2 C), 29.3, 29.2, 29.1, 24.9, 22.7, 14.1. FT-IR: 3150-3600 (br, OH), 2918 (vs, CH), 2849 (vs, CH), 1735 (vs, C=O) cm-¹. HRMS (APCI): m/z calcd for C21H42O4 - OH: 341.3050; found: 341.3042 amu.

22

Procedure for 3- O -Decanoyl-1-octadecanoyl- sn -glycerol [( S )-5]
Immobilized Candida antarctica lipase (45 mg) was added to a 10 mL round-bottom flask containing a mixture of 1-octadecanoyl-sn-glycerol [(S)-4, 203 mg, 0.566 mmol] and vinyl decanoate (168 mg, 0.849 mmol) dissolved in THF (2 mL). The resulting suspension was stirred at r.t. for 2-3 h or until TLC monitoring (EtOAc-PE, 1:1) indicated a complete reaction. The lipase preparation was removed by filtration and the solvent removed in vacuo on a rotary evaporator to afford pure 3-O-decanoyl-1-octadecanoyl-sn-glycerol [(S)-5, 246 mg, 0.480 mmol] as a white crystalline material in 85% yield after recrystallization from MeOH; mp 53.4-54.0 ˚C; [α]D ²0 +0.02 (c 10, CH2Cl2). ¹H NMR (400 MHz, CDCl3): δ = 4.20-4.06 (m, 5 H, CH2CHCH2), 2.47 (d, J = 3.6 Hz, 1 H, OH), 2.34 (t, J = 7.6 Hz, 4 H, CH2COO), 1.68-1.59 (m, 4 H, CH 2CH2COO), 1.36-1.18 (m, 40 H, CH2), 0.87 (t, J = 6.8 Hz, 6 H, CH3). ¹³C NMR (100 MHz, CDCl3): δ = 173.9 (2 C), 68.4, 65.1 (2 C), 34.1 (2 C), 32.0, 31.9, 29.7 (5 C), 29.6 (2 C), 29.5 (2 C), 29.4 (3 C), 29.3 (2 C), 29.1 (2 C), 24.9 (2 C), 22.7 (2 C), 14.1 (2 C). FT-IR: 3300-3600 (br, OH), 2914 (vs, CH), 2849 (vs, CH), 1732 (vs, C=O), 1708 (vs, C=O) cm-¹. HRMS (APCI): m/z calcd for C31H60O5 - OH 495.4408; found: 495.4412 amu.

23

Procedure for 3- O -Decanoyl-2-eicosapentaenoyl-1-octadecanoyl- sn -glycerol [( S )-2]
To a solution of 3-O-decanoyl-1-octadecanoyl-sn-glycerol [(S)-5, 100 mg, 0.195 mmol] and EPA as free acid (66.0 mg, 0.218 mmol) in CH2Cl2 (2 mL) were added DMAP (20 mg, 0.16 mmol) and EDAC (50 mg, 0.26 mmol). The resulting solution was stirred on a magnetic stirrer hotplate at r.t. for 12 h. The reaction was disconnected by passing the reaction mixture through a short column packed with silica gel by use of Et2O-CH2Cl2 (10:90). Solvent removal in vacuo afforded the pure product as a yellowish oil (141 mg, 0.177 mmol, 91% yield). [α]D ²0 +0.16 (c 10, CH2Cl2). ¹H NMR (400 MHz, CDCl3): δ = 5.42-5.33 (m, 10 H, =CH), 5.32-5.23 (m, 1 H, CH2CHCH2), 4.29 (dd, J = 12.0, 4.4 Hz, 2 H, CH 2CHCH 2), 4.14 (dd, J = 12.0, 6.0 Hz, 2 H, CH 2CHCH 2), 2.86-2.78 (m, 8 H, =CCH2C=), 2.34 (t, J = 7.6 Hz, 2 H, CH2COO in EPA), 2.31 (t, J = 7.6 Hz, 4 H, CH2COO), 2.14-2.04 (m, 4 H, =CCH 2CH2 and =CCH 2CH3), 1.70 (quint, J = 7.6 Hz, 2 H, CH 2CH2COO in EPA), 1.64-1.55 (m, 4 H, CH 2CH2COO), 1.34-1.20 (m, 40 H, CH2), 0.97 (t, J = 7.6 Hz, 3 H, CH3 in EPA), 0.88 (t, J = 6.8 Hz, 6 H, CH3 in C18). ¹³C NMR (100 MHz, CDCl3): δ = 173.3 (2 C), 172.6, 132.0, 128.9, 128.8, 128.6, 128.3, 128.2, 128.1, 128.0, 127.8, 127.0, 69.0, 62.1 (2 C), 34.0 (2 C), 33.6, 31.9, 31.8, 29.7 (5 C), 29.6 (2 C), 29.5, 29.4 (2 C), 29.3 (2 C), 29.2 (3 C), 29.1, 26.5, 25.6 (3 C), 25.5, 24.8 (2 C), 24.7, 22.7, 22.6, 20.5, 14.3, 14.1 (2 C). FT-IR: 3013 (s, CH), 2925 (vs, CH), 2854 (vs, CH), 1745 (vs, C=O) cm-¹. HRMS (APCI): m/z calcd for C51H88O6 + NH4: 814.6919; found: 814.6913 amu.