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Porcine pancreatic lipase mediated selective acylation of primary alcohols in organic solvents
Tomhedron hers. Vol.3 I. No.24, pi 34OM408.1~ Primed in GreatBritah
PORCINE
PANCREATIC
LIPASE
w40-4039/90 53.00 + .oo Pexpmon Ress pk
MEDIATED
ALCOHOLS
SELECTIVE
ACYLATION
OF PRIMARY
IN ORGANIC’ SOLVENTS
Sowmianarayanan Ramaswamy,l Brian Morgan8 and Allan. C. Oehtschlagerl’ 1 Derttmgnt of Chemistry, Simon Fra8er University, Bumaby, B.C.,Canada V5A 1S6 Phero Tech. Inc., 7672 Progress Way, R.R.#S, Delta, B.C., Canada V4C 1E9
Abstract:
Porcine pancreatic lipase mediated acylation of l,n-diols
and triols affords primary
acytated products in high yields.
The regio- and stereospecificity of enzyme catalyzed reactions are the envy of synthetic chemists.1 The use of enzymes in organic synthesis has become increasingly popular since it was realized that many enzymes can function in organic sofvents without lo88 of specificity.8 While investigating the kinetic resolution of racemic 1&butanediol using porcine pancreatic lipase (PPL) in Et80 with trifluoroethyi butyrate as the acylating agent, we noted that acylation of the primary hydroxyl was essentially complete before reaction occurred at the secondary hydroxyt.3 This enzyme-catalyzed primary hydroxyls selective
reaction offered promise as a convenient
in the presence of secondary
protection
using acetic anhydride
hydroxyls.
- pytidine
method for the acylation of
Conventional
in dichloromethane
procedures for such afford substantial
amounts of diacylated product even at temperatures below 0% (Scheme 1).
Scheme 1
Biphasic system8 employing acetic anhydride and solld K8C08’yield
somewhat less diacylated
product but substantial amounts of starting material remain. Attempt8 to drive these reactions to completion by addition of excess anhydrfde result in increased yield8 of the diacylated product. 3465
Lipase medtated acytattons in organic solvents have been demonstrated to be regiospecfflc for the primary hydroxyts of carbohydrate&~ We have now applied this technique to aliphatic dials, using porcine pancreatic lipase (PPL) and wetic or butyric anhydride as the acylating agent.6 These reactbns produce high yields of the primary acylated product which contain only traces (l-2%) of diester. SystemattcInv(egtlgattonof this method using 1,ndlols l-4 and trlol S (Rgure l), reveal the method to be general. P
2
a1
+ 1
J.4
OR1
s
a.R,b.R,=C
C.R1=l+r OH&o d. R1= C2H$0, R2. H
Lo% 4 Figure
=H
PI2W,R2-H
o.R,-R,=C2H,C0
1
Reactions were carrled out by vigorously stirrtng a mixture of the diol with 1.5 eq of the anhydrlde and PPL in ether. Because of the poor solubility of trio15 In ether, THF was used as a solvent for this substrate. There was a remarkable selectivity for primary acylation in the case of both vi&al and distal diols (entries 1-4) and butyrylatton pmceeded faster than acetylatlon (see entries 1 and 5). This might be expected since the natural substrates for lipases are the triglycerides of long-chain fatty acids. Selectlvtt was poor between sterlcally differentiated primary alcohols (entry 5).7 Llpase Catalyzed Acylatio.n of 1-S. The following general procedure was followed: PPL* (50 mg of solid per mmol of the substrate) was added to a stltred solution of 1eq of diol or trio1and 1.5 eq of anhydrlde In Et20 or THF (1M solution). The reaction was stirred at room temperature for the indicated time (Table l), after which the PPL was removed by flltration. The organic phase was diluted with ether and excess anhydrlde was removed by washing with aqueous 2M NaOH In the cass of acetic anhydtide, or by distillation In the case of butyric anhydrtde (50°C at 0.1 ton). The product was then either distilled (Kugelrohr) or isolated by column chromatography. Analytical samples were obtained by column chromatography.s In conclusion, the method reported here Illustrates the potential of PPL In selective hydroxyl protection.
Tabte 1. Porcine Pancreatic Llpase Mediated Acylatlon on Mols and Trlols
24
87
3d, 38
948
(C3H7co)zC
7
89
4d,4e
97:3
(CH3CC)20
48 15
92 94
5b, 5c 5d, 5e
98:2 99:l
3
3a
K3wQ20
4
4a
5
5a
G3bw20
Isolated yields * Ratios determined by gas chromatographicanalysis l
l
ACKNOWLEDGEMENT. This work was supported by a Strategic (Biotechnology) Grant by the Natural Sciences and Engineering Research Council of Canada.
1.(a) Chen, C.S., Sib, C.J. Angew. Chem., Inf. Ed. Engl. 1989,28, 895. (b) Jones, J.B. Tetrahedron 1988,42, 3351-3403. (c) Whltesides, G. M., Wong, C. -H. Angew. Chem. Inr. Ed. Eng/. 1985,24, 817-838. 2. Zaks, A., lUibanov,A.M.Pruc. Natl. Acad. Sci. USA. 1995,82,3192-3198. 3. When the reaction proceeded beyond monoacylation, only minor degree of kinetic resolution was observed. A mixture of 1,3_butanediol(3.0 g, 33.7 mmol), trifluoroethyl butyrate (15.2 g, 89.1 mmol) and PPL (5 g) was stlrred In Et20 (50 ml) for 119 h. After removal of the enzyme, the mono- and dlbutyrate were separated by column chromatography. Hydrolysis of the dlbutyrate, followed by dertvatlzation with (S)-acetyl Iactyl chloride4 yielded an eep value of 0.378. A similar derlvatization of the monobutyrate yielded ees of 0.132. From these values the calculated conversion was 28.00/o,
and the selectivhy (E) was 2.5.
4. Slessor, K. N.; King, G. G. S.; Miller, D. Ft.; Winston, M. L.; Cutforth, T. L. J. ,Ckvn. Eco/.,
1985,
17,
1659. 5. The&rod, M., Klibanov, A. M., J. Am. C/tern. Sot. 1966, 108,5636. 6. Bianchi, D., Cesti, P., Battistel, E., J. Org. Chem., 1966,53,5531. 7. A similar lack of selectivity was observed in the acylation of 1,3 propanediol, 1,Cbutanediol and P-methyl-l ,4-butanediol.
In the case of 1,2-ethanediol,
acylation with 1.0 equivalent
of butyric
anhydride in the presence of PPL yielded a 5 : 1 ratio of mono- to dibutyrate, as determined by GC analysis. However this ratio fell to 3 : 1 as the water-soluble
monoester was extracted during
aqueous work-up. 6. PPL was purchased from Sigma Chemical Company (Type II); the listed activity was 16 units per mg solid using olive oil at pH 7.7, with a 30 min incubation. 9. All compounds were analyzed by a combination of 1H and 1% NMR, CIMS, IR, and elemental analysis. (Received in USA 28 March 1990)
PORCINE
PANCREATIC
LIPASE
w40-4039/90 53.00 + .oo Pexpmon Ress pk
MEDIATED
ALCOHOLS
SELECTIVE
ACYLATION
OF PRIMARY
IN ORGANIC’ SOLVENTS
Sowmianarayanan Ramaswamy,l Brian Morgan8 and Allan. C. Oehtschlagerl’ 1 Derttmgnt of Chemistry, Simon Fra8er University, Bumaby, B.C.,Canada V5A 1S6 Phero Tech. Inc., 7672 Progress Way, R.R.#S, Delta, B.C., Canada V4C 1E9
Abstract:
Porcine pancreatic lipase mediated acylation of l,n-diols
and triols affords primary
acytated products in high yields.
The regio- and stereospecificity of enzyme catalyzed reactions are the envy of synthetic chemists.1 The use of enzymes in organic synthesis has become increasingly popular since it was realized that many enzymes can function in organic sofvents without lo88 of specificity.8 While investigating the kinetic resolution of racemic 1&butanediol using porcine pancreatic lipase (PPL) in Et80 with trifluoroethyi butyrate as the acylating agent, we noted that acylation of the primary hydroxyl was essentially complete before reaction occurred at the secondary hydroxyt.3 This enzyme-catalyzed primary hydroxyls selective
reaction offered promise as a convenient
in the presence of secondary
protection
using acetic anhydride
hydroxyls.
- pytidine
method for the acylation of
Conventional
in dichloromethane
procedures for such afford substantial
amounts of diacylated product even at temperatures below 0% (Scheme 1).
Scheme 1
Biphasic system8 employing acetic anhydride and solld K8C08’yield
somewhat less diacylated
product but substantial amounts of starting material remain. Attempt8 to drive these reactions to completion by addition of excess anhydrfde result in increased yield8 of the diacylated product. 3465
Lipase medtated acytattons in organic solvents have been demonstrated to be regiospecfflc for the primary hydroxyts of carbohydrate&~ We have now applied this technique to aliphatic dials, using porcine pancreatic lipase (PPL) and wetic or butyric anhydride as the acylating agent.6 These reactbns produce high yields of the primary acylated product which contain only traces (l-2%) of diester. SystemattcInv(egtlgattonof this method using 1,ndlols l-4 and trlol S (Rgure l), reveal the method to be general. P
2
a1
+ 1
J.4
OR1
s
a.R,b.R,=C
C.R1=l+r OH&o d. R1= C2H$0, R2. H
Lo% 4 Figure
=H
PI2W,R2-H
o.R,-R,=C2H,C0
1
Reactions were carrled out by vigorously stirrtng a mixture of the diol with 1.5 eq of the anhydrlde and PPL in ether. Because of the poor solubility of trio15 In ether, THF was used as a solvent for this substrate. There was a remarkable selectivity for primary acylation in the case of both vi&al and distal diols (entries 1-4) and butyrylatton pmceeded faster than acetylatlon (see entries 1 and 5). This might be expected since the natural substrates for lipases are the triglycerides of long-chain fatty acids. Selectlvtt was poor between sterlcally differentiated primary alcohols (entry 5).7 Llpase Catalyzed Acylatio.n of 1-S. The following general procedure was followed: PPL* (50 mg of solid per mmol of the substrate) was added to a stltred solution of 1eq of diol or trio1and 1.5 eq of anhydrlde In Et20 or THF (1M solution). The reaction was stirred at room temperature for the indicated time (Table l), after which the PPL was removed by flltration. The organic phase was diluted with ether and excess anhydrlde was removed by washing with aqueous 2M NaOH In the cass of acetic anhydtide, or by distillation In the case of butyric anhydrtde (50°C at 0.1 ton). The product was then either distilled (Kugelrohr) or isolated by column chromatography. Analytical samples were obtained by column chromatography.s In conclusion, the method reported here Illustrates the potential of PPL In selective hydroxyl protection.
Tabte 1. Porcine Pancreatic Llpase Mediated Acylatlon on Mols and Trlols
24
87
3d, 38
948
(C3H7co)zC
7
89
4d,4e
97:3
(CH3CC)20
48 15
92 94
5b, 5c 5d, 5e
98:2 99:l
3
3a
K3wQ20
4
4a
5
5a
G3bw20
Isolated yields * Ratios determined by gas chromatographicanalysis l
l
ACKNOWLEDGEMENT. This work was supported by a Strategic (Biotechnology) Grant by the Natural Sciences and Engineering Research Council of Canada.
1.(a) Chen, C.S., Sib, C.J. Angew. Chem., Inf. Ed. Engl. 1989,28, 895. (b) Jones, J.B. Tetrahedron 1988,42, 3351-3403. (c) Whltesides, G. M., Wong, C. -H. Angew. Chem. Inr. Ed. Eng/. 1985,24, 817-838. 2. Zaks, A., lUibanov,A.M.Pruc. Natl. Acad. Sci. USA. 1995,82,3192-3198. 3. When the reaction proceeded beyond monoacylation, only minor degree of kinetic resolution was observed. A mixture of 1,3_butanediol(3.0 g, 33.7 mmol), trifluoroethyl butyrate (15.2 g, 89.1 mmol) and PPL (5 g) was stlrred In Et20 (50 ml) for 119 h. After removal of the enzyme, the mono- and dlbutyrate were separated by column chromatography. Hydrolysis of the dlbutyrate, followed by dertvatlzation with (S)-acetyl Iactyl chloride4 yielded an eep value of 0.378. A similar derlvatization of the monobutyrate yielded ees of 0.132. From these values the calculated conversion was 28.00/o,
and the selectivhy (E) was 2.5.
4. Slessor, K. N.; King, G. G. S.; Miller, D. Ft.; Winston, M. L.; Cutforth, T. L. J. ,Ckvn. Eco/.,
1985,
17,
1659. 5. The&rod, M., Klibanov, A. M., J. Am. C/tern. Sot. 1966, 108,5636. 6. Bianchi, D., Cesti, P., Battistel, E., J. Org. Chem., 1966,53,5531. 7. A similar lack of selectivity was observed in the acylation of 1,3 propanediol, 1,Cbutanediol and P-methyl-l ,4-butanediol.
In the case of 1,2-ethanediol,
acylation with 1.0 equivalent
of butyric
anhydride in the presence of PPL yielded a 5 : 1 ratio of mono- to dibutyrate, as determined by GC analysis. However this ratio fell to 3 : 1 as the water-soluble
monoester was extracted during
aqueous work-up. 6. PPL was purchased from Sigma Chemical Company (Type II); the listed activity was 16 units per mg solid using olive oil at pH 7.7, with a 30 min incubation. 9. All compounds were analyzed by a combination of 1H and 1% NMR, CIMS, IR, and elemental analysis. (Received in USA 28 March 1990)