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Selective Acylation of Sugar Derivatives Catalyzed by Immobilized Lipase
M. Guisnet et al. (Editors),HCrcrogeneous Catalysis and Fine Chemicals 111 0 1993 Elsevier Science Publishers B.V. All rights reserved.
513
Selective Acylation of Sugar Derivatives Catalyzed by Immobilized Lipase A.T.J.W. de Goede, M. van Oosterom, M.P.J. van Deurzen, R.A. Sheldon, H. van Bekkum and F. van Rantwijk Laboratory of Organic Chemistry and Catalysis, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands Abstract Alkyl derivatives of glucose, galactose and fructose were acylated by lipase-catalyzed transesterification with alkanoic esters. The best results were obtained with immobilized lipases of the Candidu untarcticu type. Primary alcohol functions were acylated first; followed by secondary ones depending on the structure of the glycoside. The water activity in the reaction medium had a striking effect on both the rate and the selectivity of the process. The size and orientation of the alkyl substituent and the structure of the acyl acceptor were also found to exert a profound influence on the course of the reaction. 1. INTRODUCTION Interest in the chemistry and applications of renewable raw materials and of products derived from these is rapidly growing. In this respect alkylation and acylation of monoand disaccharides combine the essential features of two major renewable classes, viz. triglycerides and carbohydrates, while leading to bio-friendly surfactants and emulsifiers. Introduction of acyl groups by chemical means usually results in mixtures of compounds, although some exceptions are known'. Lipase-catalyzed transesterification, in which the sugar acts as an acyl acceptor, offers an elegant way for selective conversion of these multifuctional systems without resorting to protection-deprotection schemes2>? Under the usual conditions for enzyme-catalyzed (trans)esterification, the composition at equilibrium is often far from optimal. Complete conversion can be achieved by removal of alcohol and/or water by vacuum3' or chemical means, i.e. by the use of enolk or oxime4 esters. We endeavoured to fix the water activity at a low level by adding zeolite to the reaction mixture. Immobilized enzymes which are stable in these very dry media have recently become available5. In this paper we present the results obtained with lipase-catalyzed transesterification of alkyl derivatives of glucose, galactose and fructose.
514
2. EXPERIMENTAL
Rhizomucor miehei lipase immobilized on a macroporous anion exchange resin (lipozym IM 20) and Candida antarctica Ii ase SP 435 (immobilized on an unspecified carrier) were kindly donated b Novo NorJsk A/S. The a- and 8-methyl glucosides and galactosides were supplied by sigma. The allotted amounts of reagents were mixed and shaken for the time indicated in the text. Sam les were withdrawn at regular intervals and analyzed by reversed-phase HPLC, and by after trimeth Isilylation. Pure com ounds were oitamed by chromatography and identified by 'H and 13C NMR at 400 MEz. Full spectral data will be reported elsewhere.
&
3. RESULTS AND DISCUSSION 3.1. a-Alkyl glucosides
1-0-Alkylglucopyranosides (alkyl is C, to C,2) can be prepared by direct acetalization of glucose with the appropriate alcohol under proton catalysis. Thus, in the case of 1-0octyl glucoside the a-isomer can be easily and almost quantitatively obtained as its monohydrate by crystallization followed by recycling of the other product components6. Various immobilized 0 lipases were tested in the +3AO/C2H5 transesterification of I-0-octyl a-D-glucopyranoside ethyl acrylate, using the ( 1) latter with HO compound both as reactant \\*H\& HO and solvent. By far the best Ho O C a H l 7 Lipose HoOCaH, results were obtained with lipase preparations of the 1 C2H50H 2 Candida antarctica type7 (see Table 1).Acylation occurred mainly at the 6-0 position, in line with the usual preference of lipases for primary alcohol functions3. The resulting 6-0-acryl ester may serve as a starting material for specialty polymers. Acylation at the 2-0-position was the main sidereaction. The selectivity and rate of the C. antarctica lipase catalyzed reaction could be improved substantially by adding zeolite CaA which selectively adsorbs water and The role of water in lipase-catalyzed reactions has been studied extensively, but still remains enigmatic. A minute amount of water is essential for establishing the proper tertiary structure, although a low water concentration enhances the stability of the enzyme. Water also engages in unwanted side-reactions such as hydrolysis and it inhibits the desired reaction by acting as a competitor for the enzyme domain". At low water activity", the rate enhancing factors apparently prevail over the rate decreasing factors, as far as the lipases from C. antarctica are concerned (see Table 1). The amount of enzyme could even be reduced by 90 % (i.e. to 4 mg) when zeolite CaA was present, for the same conversion. Surprisingly, the parallel and consecutive esterification at the 2-position was slowed down in the presence of the zeolite. The opposite is true for RIz. miehei lipase: this enzyme is partially deactivated and becomes less selective at low water activity.
515
Table 1 Transesterificationa of l-O-octyl a-D-glucopyranoside (1)catalyzed by immobilized lipases. Zeolite
Without zeolite Time Conv. (h) (%I
Lipase
a
Select.
Time Conv. Select. (h) (%I (% 2)
(%'.I
73 89 99
43 94 95
45 Rhizomucor Miehei Candida antarctica A t B 24 Candida antarctica B (SP 435) 4
With zeolite CaA
100 6 4
12 99 99
66 99 99
Compound 1, 40 mg; ethyl acrylate, 4 mi; immobilized lipase, 40 mg; zeolite CaA, 0.4 g; 40".
This pattern changed when the transesterification of 1-O-methyl a-D-glucopyranoside (3) with ethyl butanoate in the presence of C. antarctica lipase SP 435 was investigated12. Without drying agent the 6-O-acylated compound 4 was the main product. In the presence of zeolite the reaction continued, although more sluggishly, until complete conversion to diesters was attained. The 2,6-diester 5 was mainly formed, together with a small amount of the 3,6-isomer (6). It would seem that the size of the axial substituent at the l-position profoundly influences the rate of reaction at the neighbouring equatorial 2-position. We tentatively ascribe this effect to steric interactions with the apolar residues of the aminoacids which surround the active site13.
HoOCH3
0
-
HO
5
4
3
Table 2 Transesterificationa of l-O-methyl a-D-glucopyranoside (3) catalyzed by lipase SP 435 Drying agent
Cosolvent
4 (%)
98 Zeolite CaA Zeolite C ~ A ~ - B U O H ~ 95 a
5
6
(96)
(%)
95
5
Compound 3,40 mg; ethyl butanoate, 4 ml; lipase SP 435, 4 mg; zeolite CaA,0.4 g where appropriate; 5 days at 40'. Ethyl butanoate, 2 ml; t-butyl alcohol, 2 ml.
\
C3H7
C H 3 7
6
516 The solvent also has an effect on the selectivity, as became clear when 50 % t-butyl alcohol14 was added with zeolite present, and the reaction stopped at the mono-ester stage. It would seem that t-butyl alcohol competes with 4 for the active site of the catalyst. 33. 8-Alkyl glucosides
With 8-alkyl glucosides, the picture became more complex1*. Transesterification of 1-O-methyl- and -0ctylB-D-glucopyranoside(7a and 7b, respectively) with ethyl butanoate without drying gave mainly the 6-0-acylated products 8, as would be expected, but with zeolite present complete conversion to the diesters took place. The 3,6-diesters 9 were mainly formed, together with the 2,6-isomers (lo), depending on the size of the anorneric substituent. 0
HO
C3H7COOEt
Ho&Ok C
SP 435
HO
OH
7
H
0
OH
4
OH
8
R
=
b: R
=
a:
'
CH3 C8HI5
HO
Table 3 TransesterificatiotP of 1-0-alkyl 8-D-glucopyranosides (7) with lipase SP 435 Drying agent
Cosolvent
Zeolite CaA Zeolite CaA
-
a
t-BuOHb
8a (%)
98 95
9a (%) 88
10a (%)
12
8b
(%I 98 95
9b
10b (%)
60
40
Compound 7, 40 mg; ethyl butanoatc, 4 ml; lipase SP 435, 4 mg; zeolite CaA, 0.4 g; 5 days at 40" Ethyl butanoate, 2 ml;1-butyl alcohol, 2 ml.
3.3. Methyl galactosides
It has already been shown that the equatorial/axial configuration of the secondary alcohol groups in sugar derivatives profoundly influences the rate and selectivity of their
517
acylation, although the effect is rather dependent on which lipase is used". We found that, in the presence of lipase SP 435, the 1-0-methyl a-D-6-0-butanoylgalactopyranoside (12) was converted into the 2,6-diester considerably faster than 4. When the reaction was run in the presence of zeolite CaA, 14 was formed in a nearly quantitative yield, even in 50 % t-butyl alcohol solution (see Scheme 1). 0
C3H7COOEt H
HO
SP 435
50
11
Z t-BuOH
0
I-
50
7
99
Zeolite CoA
11 ,
Compound
36
I
40 mg; ethyl butonoote, 2 ml; lipose
sp 435, 4 0 mg; t-BuOH, 2 ml; zeolite CoA. 0 . 4 g; 50 h at 40'
Transesterification of I-@Methyl a-D-galactopyranoside (11).
Scheme 1.
'
1
e
O
C
H
o\
-.
Y
C3H7COOEt
3
"&0CH3
lipase SP 435
HO
HO
50 % 1-BuOH
15
: (
:(
-
:( 27
Zeolite CaA
15 , 40
47
HH&OCH3
52
'&
mg; ethyl butonoote. 2 ml; lipose
SP 435. 40 mg; I-&OH.
Scheme 2.
1
-
16
Drying agent
Compound
C3H7
2 rnl, zeolite CaA, 0 4 g, 50 h at 4 0'
Transesterification of 1-0-Methyl 8-D-galactopyranoside (15).
CH3
518
1-0-Methyl 8-D-galactopyranoside (IS)was rapidly acylated by ethyl butanoate in the presence of SP 435 to a mixture of the 6-mono-ester 16 and the 2,6- and 3,6- diesters 17 and 18, respectively (see Scheme 2). In the presence of zeolite CaA the di-esters were the sole products. 3.4. Alkyl fructosides
Alkyl fructosides have remained rather inaccessible and have, consequently, received only scant attention. We have found that the reaction of fructose and alkyl alcohols can be achieved by taking special precautions16. Initially, furanoid compounds are formed which are in the course of the reaction partly transformed into the B-fructopyranoside. Separation of the products by preparative HPLC allowed us to study the esterification of structurally homogeneous compounds16. 2-0-Dodecyl F D OC 2'25 CgH gCOOEt OC1ZH25 fructopyranoside (19) HOe 0 y c 9 H 19 was converted into the OH OH Lipase HQ 1-0-decanoyl ester 20 by 50 'Z a c e t o n e lipase catalyzed 19 20 transesterification with ethyl decanoate (Table 4). It should be noted that here Rhizotwcor miellei lipase is activated by zeolite, whereas the reverse was true in the case of 1 and ethyl acrylate; 20 is formed less selectively under these conditions, however. Without cosolvent, di-esters whose structure still has to be elucidated, were mainly formed.
*,
Table 4 Transesterificationa of 2-0-dodecyl fl-D-fructopyranosidecatalyzed by immobilized li pases. Zeolite Lipase Rhizomucor Miehei Candida antarctica SP 435 a
Without zeolite Time
(h)
168 24
Conv. (%)
22 74
Select. (%20) 94 100
With zeolite CaA Time (h)
Conv.
168 24
60 96
(%)
Select. (%20) 82 100
Compound 19, 40 mg; ethyl decanoatc 2 ml; immobilized lipase, 40 mg; acetone, 2nd; zeolite CaA, 0.4 g where appropriate; 40".
Transesterification of 2-0-dodecyl cr-D-fructofuranoside (21) with ethyl decanoate and SP 435 lipase gave initially the 6-mono-ester 22 which was slowly converted into di-esters, mainly the 1.6-di-ester 23. Addition of zeolite CaA allowed selective formation of either 22 or 23, depending on the reaction time.
519
) g o ,
21
1””
040+p Y
CSHISCOOEt
6 C l2H25 C. ontarctico SP 435
IY
23
22
170 h
Table 5 Transesterificationa of 2-0-dodecyl a-D-fructofuranoside (21) catalyzed by lipase SP 435
Drying agent
Zeolite CaA a
time (h)
mono-esters 22 div (%) (‘31)
25 168
75 59
4
25 75
93
0 10
di-esters 23 div (%) (%)
8
3 3
0 80
0 4
Compound 21,40 mg; ethyl decanoate 4 ml; lipase SP 435, 40 mg; zeolite CaA, 0.4 g where appropriate; 170 h at 40’.
4. Concluding remarks Summarizing, our results on the lipase-catalyzed transesterification of 1-0-alkyl glycosides first of all underline the potential of C. antarctica type lipases. Generally, the primary 6-hydroxyl function of glucosides and galactosides is esterified first. The rate and selectivity of the consecutive esterification of the secondary hydroxyl groups is strongly influenced by the water activity in the medium, the anomeric configuration of the glycoside and the configuration of the 4-hydroxyl group. The size of the anomeric substituent exerts considerable influence on the esterification of the secondary hydroxyl groups in the 6-0-acylated a-D-glucopyranosides 2 and 4; with an octyl group as the anomeric substituent (2), the reaction is quite sluggish whilst in the case of a methyl group (4) it proceeds readily. Regioselective acylation of the 2-hydroxyl group was found for 4 as well as for the 6-0-acyl-a-D-galactoside 12. The influence of the size of the anomeric substituent in the I-0-alkyl 6-0-acyl-8-Dglucopyranosides 8a and 8b was much less pronounced. A tendency towards acylation of the 3-position was observed for the glucosides 8 as well as for the galactoside 16. The alkyl fructosides 19 and 21 represent a novel class of reactants and the investigations are far from complete. It is already clear, however, that in this case also the primary hydroxyl functions are acylated first.
520
5. ACKNOWLEDGEMENT The authors wish to thank Novo-Nordisk A/& Denmark for supplying the sample of lipase SP 435. This work has benefited from numerous discussions with Dr. L. Maat, for which the authors wish to express their gratitude. Thanks are due to Miss W. Benckhuijsen, who performed part of the experimental work. Financial support by Unichema BV, Royal Gist-brocades BV, Suiker-Unie BV and the Innovation-oriented Research Program on Carbohydrates (10P-k) is gratefully acknowledged. 6. REFERENCES
10 11 12 13
14
15
16
K.S. Mufti, R.A. Khan, GB Appl. 80/22320 (1980) (Chem. Abstr., 96 (1982) 1631121). For a review see: D.G. Drueckhammcr, W.J. Hcnnen, R.L. Pedcrson, C.F. Barbas and C.M. Gautheron, T. Krach, C.-H. Wong, Synthesis, (1991) 499. a. M. Therisod and A.M. Klibanov, J. Am. Chem. Soc., 108 (1986) 5638; b. M. Therisod and A.M. Klibanov, J . Am. Chem. SOC.,109 (1087) 3977; c. S. Riva, J. Chopineau, A.P.G. Kieboom and A.M. Klibanov, J. Am. Chem. Soc., 110 (1988) 584; d. W.J. Hennen, H.M. Swcers, Y.-F. Wang and C.-H. Wong, J. Org. Chem., 53 (1988) 4939; e. Y.-F. Wang, J.J. Lalonde, M. Momongan, D.E. Bergbreiter and C.-H. Wong, J. Am. Chem. SOC.,110 (19%) 7200;f. 0. Kirk, F. Bjorkling and S.E. Godtfredsen, PCT WO 89/01480 (Chcm. Abstr., 114 (1991) 183876c);g. M.P. de Nijs, L. Maat and A.P.G. Kieboom, Rccl. Trav. Chim. Pays-Bas, 109 (1990) 429. V. Gotor and R. Pulido, J. Chem. Soc., Perkin Trans., 491 (1991). S. Pedersen and P. Eigtved (Novo-Nordisk A/S), PCT Int. Appl. WO 90/15868 (Chem Abstr., 114 (1991). 224573r). A.J.J. Straathof, H. van Bekkum and A.P.G. Kieboom, Starch, 40 (1988) 229. M. Ishii (Novo Industri A/S), PCT Int. Appl. WO 8802,775 (Chem. Abstr., 110 (1989) 20529t). C’ W.F. Holderich and H. van Bekkum, Stud. Surf. Sci. Catal., 58 (1991) 642. Full paper: A.T.J.W. de Goedc, W. Benckhuijsen, F. van Rantwijk, L. Maat and H. van Bekkum, submitted to Recl. Trav. Chim. Pays-Bas. M.J.S. Dewar, Enzymc, 36 (1986) X; M.J.S. Dewar and K.M. Dieter, Biochemistry, 27 (1988) 3302. The water content in water-saturated and CaA-dried cthyl acrylatc was determined at 1.68 and 0.0075 ‘K, respectively, by Karl-Fischcr titration, corresponding with a water activity of 0.004. Full paper: A.T.J.W. de Gocdc, F. van Rantwijk and H. van Bckkum, in prcparation. See: L. Brady, A.M. Brzozowski, Z.S. Dcrcwcnda, E. Dodson, G.Dodson, S. Tolley, J.P. Turkenburg, L. Christiansen, B. Huge-.lensen, L. Norskov, L. Thim, U. Menge, Nature, 343 (1990) 767; A.M. Brzozowski, U. Derewenda, Z.S. Derewenda, G.C. Dodson, D.M. Dodson, J.P. Turkenburg, F. Bjorkling, B. Huge-Jensen, S.A. Patkar, L. Thim, Nature, 351 (1991) 491. The combination of the slightly acidic CaA zcolite and 1-butyl alcohol would seem to raise the problem of Ca-catalyzed dehydratation of thc cosolvent. Experiments with methyl butanoate as acyl donor and zeolite N a A as drying agent gave results which were very similar to those obtained with the combination ethyl butanoate/CaA. The same was true when zeolite powder not containing a binding agent was used instead of pellets. a. P. Ciuffreda, F. Ronchetti and L. Toma, J. Carbohydr. Chem., 9 (1990) 125; b. P. Ciuffreda, D. Colombo, F. Ronchetti and L. Toma, J. Org. Chem., 55 (1990)4187; c. D. Colombo, F. Ronchetti and L. Toma, Tetrahedron, 47 (1991) 103; d. D. Colombo, F. Ronchetti, A. Scala and L. Toma, J. Carbohydr. Chem., 11 (1992) 89. Full paper: A.TJ.W. de Goede, M.PJ. van Deurzen, F. van Rantwijk and H. van Bekkum, in preparation.
513
Selective Acylation of Sugar Derivatives Catalyzed by Immobilized Lipase A.T.J.W. de Goede, M. van Oosterom, M.P.J. van Deurzen, R.A. Sheldon, H. van Bekkum and F. van Rantwijk Laboratory of Organic Chemistry and Catalysis, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands Abstract Alkyl derivatives of glucose, galactose and fructose were acylated by lipase-catalyzed transesterification with alkanoic esters. The best results were obtained with immobilized lipases of the Candidu untarcticu type. Primary alcohol functions were acylated first; followed by secondary ones depending on the structure of the glycoside. The water activity in the reaction medium had a striking effect on both the rate and the selectivity of the process. The size and orientation of the alkyl substituent and the structure of the acyl acceptor were also found to exert a profound influence on the course of the reaction. 1. INTRODUCTION Interest in the chemistry and applications of renewable raw materials and of products derived from these is rapidly growing. In this respect alkylation and acylation of monoand disaccharides combine the essential features of two major renewable classes, viz. triglycerides and carbohydrates, while leading to bio-friendly surfactants and emulsifiers. Introduction of acyl groups by chemical means usually results in mixtures of compounds, although some exceptions are known'. Lipase-catalyzed transesterification, in which the sugar acts as an acyl acceptor, offers an elegant way for selective conversion of these multifuctional systems without resorting to protection-deprotection schemes2>? Under the usual conditions for enzyme-catalyzed (trans)esterification, the composition at equilibrium is often far from optimal. Complete conversion can be achieved by removal of alcohol and/or water by vacuum3' or chemical means, i.e. by the use of enolk or oxime4 esters. We endeavoured to fix the water activity at a low level by adding zeolite to the reaction mixture. Immobilized enzymes which are stable in these very dry media have recently become available5. In this paper we present the results obtained with lipase-catalyzed transesterification of alkyl derivatives of glucose, galactose and fructose.
514
2. EXPERIMENTAL
Rhizomucor miehei lipase immobilized on a macroporous anion exchange resin (lipozym IM 20) and Candida antarctica Ii ase SP 435 (immobilized on an unspecified carrier) were kindly donated b Novo NorJsk A/S. The a- and 8-methyl glucosides and galactosides were supplied by sigma. The allotted amounts of reagents were mixed and shaken for the time indicated in the text. Sam les were withdrawn at regular intervals and analyzed by reversed-phase HPLC, and by after trimeth Isilylation. Pure com ounds were oitamed by chromatography and identified by 'H and 13C NMR at 400 MEz. Full spectral data will be reported elsewhere.
&
3. RESULTS AND DISCUSSION 3.1. a-Alkyl glucosides
1-0-Alkylglucopyranosides (alkyl is C, to C,2) can be prepared by direct acetalization of glucose with the appropriate alcohol under proton catalysis. Thus, in the case of 1-0octyl glucoside the a-isomer can be easily and almost quantitatively obtained as its monohydrate by crystallization followed by recycling of the other product components6. Various immobilized 0 lipases were tested in the +3AO/C2H5 transesterification of I-0-octyl a-D-glucopyranoside ethyl acrylate, using the ( 1) latter with HO compound both as reactant \\*H\& HO and solvent. By far the best Ho O C a H l 7 Lipose HoOCaH, results were obtained with lipase preparations of the 1 C2H50H 2 Candida antarctica type7 (see Table 1).Acylation occurred mainly at the 6-0 position, in line with the usual preference of lipases for primary alcohol functions3. The resulting 6-0-acryl ester may serve as a starting material for specialty polymers. Acylation at the 2-0-position was the main sidereaction. The selectivity and rate of the C. antarctica lipase catalyzed reaction could be improved substantially by adding zeolite CaA which selectively adsorbs water and The role of water in lipase-catalyzed reactions has been studied extensively, but still remains enigmatic. A minute amount of water is essential for establishing the proper tertiary structure, although a low water concentration enhances the stability of the enzyme. Water also engages in unwanted side-reactions such as hydrolysis and it inhibits the desired reaction by acting as a competitor for the enzyme domain". At low water activity", the rate enhancing factors apparently prevail over the rate decreasing factors, as far as the lipases from C. antarctica are concerned (see Table 1). The amount of enzyme could even be reduced by 90 % (i.e. to 4 mg) when zeolite CaA was present, for the same conversion. Surprisingly, the parallel and consecutive esterification at the 2-position was slowed down in the presence of the zeolite. The opposite is true for RIz. miehei lipase: this enzyme is partially deactivated and becomes less selective at low water activity.
515
Table 1 Transesterificationa of l-O-octyl a-D-glucopyranoside (1)catalyzed by immobilized lipases. Zeolite
Without zeolite Time Conv. (h) (%I
Lipase
a
Select.
Time Conv. Select. (h) (%I (% 2)
(%'.I
73 89 99
43 94 95
45 Rhizomucor Miehei Candida antarctica A t B 24 Candida antarctica B (SP 435) 4
With zeolite CaA
100 6 4
12 99 99
66 99 99
Compound 1, 40 mg; ethyl acrylate, 4 mi; immobilized lipase, 40 mg; zeolite CaA, 0.4 g; 40".
This pattern changed when the transesterification of 1-O-methyl a-D-glucopyranoside (3) with ethyl butanoate in the presence of C. antarctica lipase SP 435 was investigated12. Without drying agent the 6-O-acylated compound 4 was the main product. In the presence of zeolite the reaction continued, although more sluggishly, until complete conversion to diesters was attained. The 2,6-diester 5 was mainly formed, together with a small amount of the 3,6-isomer (6). It would seem that the size of the axial substituent at the l-position profoundly influences the rate of reaction at the neighbouring equatorial 2-position. We tentatively ascribe this effect to steric interactions with the apolar residues of the aminoacids which surround the active site13.
HoOCH3
0
-
HO
5
4
3
Table 2 Transesterificationa of l-O-methyl a-D-glucopyranoside (3) catalyzed by lipase SP 435 Drying agent
Cosolvent
4 (%)
98 Zeolite CaA Zeolite C ~ A ~ - B U O H ~ 95 a
5
6
(96)
(%)
95
5
Compound 3,40 mg; ethyl butanoate, 4 ml; lipase SP 435, 4 mg; zeolite CaA,0.4 g where appropriate; 5 days at 40'. Ethyl butanoate, 2 ml; t-butyl alcohol, 2 ml.
\
C3H7
C H 3 7
6
516 The solvent also has an effect on the selectivity, as became clear when 50 % t-butyl alcohol14 was added with zeolite present, and the reaction stopped at the mono-ester stage. It would seem that t-butyl alcohol competes with 4 for the active site of the catalyst. 33. 8-Alkyl glucosides
With 8-alkyl glucosides, the picture became more complex1*. Transesterification of 1-O-methyl- and -0ctylB-D-glucopyranoside(7a and 7b, respectively) with ethyl butanoate without drying gave mainly the 6-0-acylated products 8, as would be expected, but with zeolite present complete conversion to the diesters took place. The 3,6-diesters 9 were mainly formed, together with the 2,6-isomers (lo), depending on the size of the anorneric substituent. 0
HO
C3H7COOEt
Ho&Ok C
SP 435
HO
OH
7
H
0
OH
4
OH
8
R
=
b: R
=
a:
'
CH3 C8HI5
HO
Table 3 TransesterificatiotP of 1-0-alkyl 8-D-glucopyranosides (7) with lipase SP 435 Drying agent
Cosolvent
Zeolite CaA Zeolite CaA
-
a
t-BuOHb
8a (%)
98 95
9a (%) 88
10a (%)
12
8b
(%I 98 95
9b
10b (%)
60
40
Compound 7, 40 mg; ethyl butanoatc, 4 ml; lipase SP 435, 4 mg; zeolite CaA, 0.4 g; 5 days at 40" Ethyl butanoate, 2 ml;1-butyl alcohol, 2 ml.
3.3. Methyl galactosides
It has already been shown that the equatorial/axial configuration of the secondary alcohol groups in sugar derivatives profoundly influences the rate and selectivity of their
517
acylation, although the effect is rather dependent on which lipase is used". We found that, in the presence of lipase SP 435, the 1-0-methyl a-D-6-0-butanoylgalactopyranoside (12) was converted into the 2,6-diester considerably faster than 4. When the reaction was run in the presence of zeolite CaA, 14 was formed in a nearly quantitative yield, even in 50 % t-butyl alcohol solution (see Scheme 1). 0
C3H7COOEt H
HO
SP 435
50
11
Z t-BuOH
0
I-
50
7
99
Zeolite CoA
11 ,
Compound
36
I
40 mg; ethyl butonoote, 2 ml; lipose
sp 435, 4 0 mg; t-BuOH, 2 ml; zeolite CoA. 0 . 4 g; 50 h at 40'
Transesterification of I-@Methyl a-D-galactopyranoside (11).
Scheme 1.
'
1
e
O
C
H
o\
-.
Y
C3H7COOEt
3
"&0CH3
lipase SP 435
HO
HO
50 % 1-BuOH
15
: (
:(
-
:( 27
Zeolite CaA
15 , 40
47
HH&OCH3
52
'&
mg; ethyl butonoote. 2 ml; lipose
SP 435. 40 mg; I-&OH.
Scheme 2.
1
-
16
Drying agent
Compound
C3H7
2 rnl, zeolite CaA, 0 4 g, 50 h at 4 0'
Transesterification of 1-0-Methyl 8-D-galactopyranoside (15).
CH3
518
1-0-Methyl 8-D-galactopyranoside (IS)was rapidly acylated by ethyl butanoate in the presence of SP 435 to a mixture of the 6-mono-ester 16 and the 2,6- and 3,6- diesters 17 and 18, respectively (see Scheme 2). In the presence of zeolite CaA the di-esters were the sole products. 3.4. Alkyl fructosides
Alkyl fructosides have remained rather inaccessible and have, consequently, received only scant attention. We have found that the reaction of fructose and alkyl alcohols can be achieved by taking special precautions16. Initially, furanoid compounds are formed which are in the course of the reaction partly transformed into the B-fructopyranoside. Separation of the products by preparative HPLC allowed us to study the esterification of structurally homogeneous compounds16. 2-0-Dodecyl F D OC 2'25 CgH gCOOEt OC1ZH25 fructopyranoside (19) HOe 0 y c 9 H 19 was converted into the OH OH Lipase HQ 1-0-decanoyl ester 20 by 50 'Z a c e t o n e lipase catalyzed 19 20 transesterification with ethyl decanoate (Table 4). It should be noted that here Rhizotwcor miellei lipase is activated by zeolite, whereas the reverse was true in the case of 1 and ethyl acrylate; 20 is formed less selectively under these conditions, however. Without cosolvent, di-esters whose structure still has to be elucidated, were mainly formed.
*,
Table 4 Transesterificationa of 2-0-dodecyl fl-D-fructopyranosidecatalyzed by immobilized li pases. Zeolite Lipase Rhizomucor Miehei Candida antarctica SP 435 a
Without zeolite Time
(h)
168 24
Conv. (%)
22 74
Select. (%20) 94 100
With zeolite CaA Time (h)
Conv.
168 24
60 96
(%)
Select. (%20) 82 100
Compound 19, 40 mg; ethyl decanoatc 2 ml; immobilized lipase, 40 mg; acetone, 2nd; zeolite CaA, 0.4 g where appropriate; 40".
Transesterification of 2-0-dodecyl cr-D-fructofuranoside (21) with ethyl decanoate and SP 435 lipase gave initially the 6-mono-ester 22 which was slowly converted into di-esters, mainly the 1.6-di-ester 23. Addition of zeolite CaA allowed selective formation of either 22 or 23, depending on the reaction time.
519
) g o ,
21
1””
040+p Y
CSHISCOOEt
6 C l2H25 C. ontarctico SP 435
IY
23
22
170 h
Table 5 Transesterificationa of 2-0-dodecyl a-D-fructofuranoside (21) catalyzed by lipase SP 435
Drying agent
Zeolite CaA a
time (h)
mono-esters 22 div (%) (‘31)
25 168
75 59
4
25 75
93
0 10
di-esters 23 div (%) (%)
8
3 3
0 80
0 4
Compound 21,40 mg; ethyl decanoate 4 ml; lipase SP 435, 40 mg; zeolite CaA, 0.4 g where appropriate; 170 h at 40’.
4. Concluding remarks Summarizing, our results on the lipase-catalyzed transesterification of 1-0-alkyl glycosides first of all underline the potential of C. antarctica type lipases. Generally, the primary 6-hydroxyl function of glucosides and galactosides is esterified first. The rate and selectivity of the consecutive esterification of the secondary hydroxyl groups is strongly influenced by the water activity in the medium, the anomeric configuration of the glycoside and the configuration of the 4-hydroxyl group. The size of the anomeric substituent exerts considerable influence on the esterification of the secondary hydroxyl groups in the 6-0-acylated a-D-glucopyranosides 2 and 4; with an octyl group as the anomeric substituent (2), the reaction is quite sluggish whilst in the case of a methyl group (4) it proceeds readily. Regioselective acylation of the 2-hydroxyl group was found for 4 as well as for the 6-0-acyl-a-D-galactoside 12. The influence of the size of the anomeric substituent in the I-0-alkyl 6-0-acyl-8-Dglucopyranosides 8a and 8b was much less pronounced. A tendency towards acylation of the 3-position was observed for the glucosides 8 as well as for the galactoside 16. The alkyl fructosides 19 and 21 represent a novel class of reactants and the investigations are far from complete. It is already clear, however, that in this case also the primary hydroxyl functions are acylated first.
520
5. ACKNOWLEDGEMENT The authors wish to thank Novo-Nordisk A/& Denmark for supplying the sample of lipase SP 435. This work has benefited from numerous discussions with Dr. L. Maat, for which the authors wish to express their gratitude. Thanks are due to Miss W. Benckhuijsen, who performed part of the experimental work. Financial support by Unichema BV, Royal Gist-brocades BV, Suiker-Unie BV and the Innovation-oriented Research Program on Carbohydrates (10P-k) is gratefully acknowledged. 6. REFERENCES
10 11 12 13
14
15
16
K.S. Mufti, R.A. Khan, GB Appl. 80/22320 (1980) (Chem. Abstr., 96 (1982) 1631121). For a review see: D.G. Drueckhammcr, W.J. Hcnnen, R.L. Pedcrson, C.F. Barbas and C.M. Gautheron, T. Krach, C.-H. Wong, Synthesis, (1991) 499. a. M. Therisod and A.M. Klibanov, J. Am. Chem. Soc., 108 (1986) 5638; b. M. Therisod and A.M. Klibanov, J . Am. Chem. SOC.,109 (1087) 3977; c. S. Riva, J. Chopineau, A.P.G. Kieboom and A.M. Klibanov, J. Am. Chem. Soc., 110 (1988) 584; d. W.J. Hennen, H.M. Swcers, Y.-F. Wang and C.-H. Wong, J. Org. Chem., 53 (1988) 4939; e. Y.-F. Wang, J.J. Lalonde, M. Momongan, D.E. Bergbreiter and C.-H. Wong, J. Am. Chem. SOC.,110 (19%) 7200;f. 0. Kirk, F. Bjorkling and S.E. Godtfredsen, PCT WO 89/01480 (Chcm. Abstr., 114 (1991) 183876c);g. M.P. de Nijs, L. Maat and A.P.G. Kieboom, Rccl. Trav. Chim. Pays-Bas, 109 (1990) 429. V. Gotor and R. Pulido, J. Chem. Soc., Perkin Trans., 491 (1991). S. Pedersen and P. Eigtved (Novo-Nordisk A/S), PCT Int. Appl. WO 90/15868 (Chem Abstr., 114 (1991). 224573r). A.J.J. Straathof, H. van Bekkum and A.P.G. Kieboom, Starch, 40 (1988) 229. M. Ishii (Novo Industri A/S), PCT Int. Appl. WO 8802,775 (Chem. Abstr., 110 (1989) 20529t). C’ W.F. Holderich and H. van Bekkum, Stud. Surf. Sci. Catal., 58 (1991) 642. Full paper: A.T.J.W. de Goedc, W. Benckhuijsen, F. van Rantwijk, L. Maat and H. van Bekkum, submitted to Recl. Trav. Chim. Pays-Bas. M.J.S. Dewar, Enzymc, 36 (1986) X; M.J.S. Dewar and K.M. Dieter, Biochemistry, 27 (1988) 3302. The water content in water-saturated and CaA-dried cthyl acrylatc was determined at 1.68 and 0.0075 ‘K, respectively, by Karl-Fischcr titration, corresponding with a water activity of 0.004. Full paper: A.T.J.W. de Gocdc, F. van Rantwijk and H. van Bckkum, in prcparation. See: L. Brady, A.M. Brzozowski, Z.S. Dcrcwcnda, E. Dodson, G.Dodson, S. Tolley, J.P. Turkenburg, L. Christiansen, B. Huge-.lensen, L. Norskov, L. Thim, U. Menge, Nature, 343 (1990) 767; A.M. Brzozowski, U. Derewenda, Z.S. Derewenda, G.C. Dodson, D.M. Dodson, J.P. Turkenburg, F. Bjorkling, B. Huge-Jensen, S.A. Patkar, L. Thim, Nature, 351 (1991) 491. The combination of the slightly acidic CaA zcolite and 1-butyl alcohol would seem to raise the problem of Ca-catalyzed dehydratation of thc cosolvent. Experiments with methyl butanoate as acyl donor and zeolite N a A as drying agent gave results which were very similar to those obtained with the combination ethyl butanoate/CaA. The same was true when zeolite powder not containing a binding agent was used instead of pellets. a. P. Ciuffreda, F. Ronchetti and L. Toma, J. Carbohydr. Chem., 9 (1990) 125; b. P. Ciuffreda, D. Colombo, F. Ronchetti and L. Toma, J. Org. Chem., 55 (1990)4187; c. D. Colombo, F. Ronchetti and L. Toma, Tetrahedron, 47 (1991) 103; d. D. Colombo, F. Ronchetti, A. Scala and L. Toma, J. Carbohydr. Chem., 11 (1992) 89. Full paper: A.TJ.W. de Goede, M.PJ. van Deurzen, F. van Rantwijk and H. van Bekkum, in preparation.