J . Org. Chem. 1994,59, 7027-7032
7027
Regioselective Acetylations of Alkyl f?-D-Xylopyranosidesby Use of Lipase PS in Organic Solvents and Application to the Chemoenzymatic Synthesis of Oligosaccharides Rosa Lbpez, Esther Montero, Felicitas Sdnchez, Javier Caiiada, and Alfonso Ferndndez-Mayoralas* Grupo de Carbohidratos, Znstituto de Quimica Organica General, CSZC, CalLe Juan de la Cierva 3, 28006 Madrid, Spain Received March 28, 1994@
Aglycon structure and solvent can change the regioselectivity of the acetylation of alkyl p-Dxylopyranosides, catalyzed by lipase PS. The acetylation of methyl 8-D-xylopyranoside (3)in acetonitrile gave the 3,4-diacetate 4 exclusively, whereas the reaction of octyl B-D-xylopyranoside (5) gave a mixture of 2,4-and 3,4-diacetates (6 and 7)in 1.0:1.3 and 3.6:l.O ratios, in acetonitrile and hexane, respectively. The effect of several solvents on the selectivity in the monoacetylation of 5 was studied. The 2-monoacetate 8 was preferentially formed over the 3- and 4-monoacetates (9 and 10) in hydrophobic solvents. High yields of partially acetylated xylose derivatives were obtained, which were used in the syntheses of a disaccharide showing liquid crystal properties, a n intermediate for the synthesis of proteoglycan fragments, and a trisaccharide potential inhibitor of plant growth.
Scheme 1
Introduction One of the attractive features of conducting enzymecatalyzed reactions in organic solvents is the potential to alter the selectivity by simply changing the solvent. This effect has been reported in enantio-,l chemo-,2 and regio~elective~ acylations and deacylations. A recent study3 of solvent effect on enzyme regioselectivity in the lipase PS-catalyzed deacylation of a diester substrate containing a n octyl group showed that the regioselectivity correlated with the hydrophobicity of the organic solvent. This correlation could be accounted for by a n interaction of the octyl group of the substrate and a hydrophobic cleft in the enzyme. The results of this study could mitigate the limitations of conventional acylatioddeacylation methodologies with carbohydrate substrates to provide partially acylated sugars-useful precursors for the synthesis of oligosaccharides and chiral intermediates for the synthesis of natural products. We have recently reported4 results on the lipase PScatalyzed regioselective deacetylation of tri-0-acyl-p-Dxylopyranosides, which bear a methyl or octyl group a s aglycon, in organic solvents. With all of the substrates and in all of the solvents used, deacylation at only the 4 position was observed. For example, the deacetylation of methyl 2,3,4-tri-O-acetyl-~-~-xylopyranoside (1)in tertamyl alcohol furnishes only the HO-4 free derivative 2 in high yield (Scheme 1). This persistent regioselectivity was attributed to steric effects. Due to this observation, we planned to study the regioselectivity of the acylation, in organic solvents, of the unprotected and less sterically hindered, methyl and octyl P-D-xylopyranosides (3 and Abstract published in Advance ACS Abstracts, October 15, 1994. (1)Sakurai, T.; Margolin, A. L.; Russell, A. J.; Klibanov, A. M. J. Am. Chem. SOC.1988,110, 7236. Fitzpatrick, P. A.; Klibanov, A. M. J.Am. Chem. SOC.1991,113, 3166. Parida, S.; Dordick, J. S. J.Am. Chem. SOC. 1991, 113, 2253. Secundo, F.; Riva, S.; Carrea, G. Tetrahedron: Asymmetry 1992, 3, 267. (2) Chinsky, N.; Margolin, A. L.; Klibanov, A. M. J.Am. Chem. SOC. 1989. - - - , -I -1 -1., 386. - - -. (3) Rubio, E.; FemAndez-Mayoralas, A.; Klibanov, A. M. J. Am. L991, 113, 695. @
0022-326319411959-7027$04.50/0
-
Lipase PSI nPentOH W-oMe
1 OAc
tAmyl-OH (93%)
o H aM -fe 2 OAC
Lipase PS
HO-OMe
Ho
OH
3
5
AcOCH=CH2/Acetonitrile (85Yo)
-"
4
OH
A AcO C O ~ O C a H , , Acetonitrile 6/7 =1.0/1.3 Hexane 617= 3.6/1 .O
7
OH
5, Scheme 1). We planned to evaluate the effect of different solvents on the regioselectivity of the acetylation of 5, owing to its solubility in a wide range of solvents and its potential for hydrophobic-type interactions. The results described herein show that hydrophobic solvents favor the acetylation a t the C-2 position of 5 over the C-3 and C-4positions. Thus, high yields of partially acetylated xylopyranosides have been obtained and their utility has been demonstrated in the synthesis of oligosaccharides.
Results and Discussion The acetylations of 3 and 5, solubilized in organic solvent, were carried out by treatment with vinyl acetate (5 equiv) in the presence of lipase PS (25 mg1mL of organic solvent) at 30 "C.The formation of products was followed by GLC and HPLC. The course of the acetylations always followed the same pattern: monoacetates were formed first, they reached a maximum, and they subsequently gave diacetates. When nearly all monoacetates were consumed and diacetates prevailed, the reactions stopped. No appreciable amounts of triacetates were formed.
0 1994 American Chemical Society
7028
L6pez et al.
J. Org. Chem., Vol. 59,No. 23, 1994
Table 1. Monoacetylationof 5 Catalyzed by Lipase PS in Organic Solvents 5
+
AcOCH-CH2
Lipase PS
solvent
R ' BRq-OCIJHl,
R'O
8 : R' -Ac; $- R3=H 9:R2=Ac;R'-R3-H 10: R3=Ac; R ' = R 2 - H
solvent hexane
toluene benzene
triethylamine ethyl acetate tetrahydrofurane acetonitrile
time (h) 3 4 5 12 16 68 22
yield of re1 % monoacetates(%) 8 9 10 53 52 47 44 44 51 42
83
77 59 25 42 33 35
6 13 30 39 14 20
5
11 10 11 36 44 47 60
The acetylation of 3 in acetonitrile gave the 3,4-di-0acetyl derivative 4 as the only diacetylated product (Scheme 1). The reaction of the octyl derivative 6 under the same conditions gave a mixture of 2,4- and 3,4-di-Oacyl derivatives (6 and 7, respectively) in a 1.0:1.3 ratio (determined by gas chromatography). However, the latter reaction performed in hexane led to a mixture of 6 and 7 in a 3.6:l.O ratio. These three results alone show the significant influence of both aglycon and solvent on the regioselectivity of the reaction. To study the origin of the regioselectivity change in the acetylation of 6, the effect of several solvents on the formation of the initial products, i.e., the monoacetates, was determined. The acetylations of 6 in the various solvents were carried out on a preparative scale and were interrupted when the maximum formation of monoacetates was reached. In Table 1, the solvents used (from more to less hydrophobic), the reaction times, the yields, and the relative percentages of isolated monoacetates are shown. From the relative percentage of monoacetates, a clear tendency of the 2-monoacetate 8 to be formed in the more hydrophobic solvents could be seen, whereas the 4-monoacetate 10 seems to be formed in the most polar solvents. Behavior leading to the 3-monoacetate 9 is not so clear. In the structure of 6, HO-3 lies between the other two hydroxyls. The values in Table 1must be considered in a qualitative manner since two factors can affect the regioselectivity observed at the time of maximum monoacetates: as the monoacetates are produced, each regioisomer can be further acetylated a t a different rate and intramolecular migration could occur during the reaction and p ~ r i f i c a t i o n . ~ In order to circumvent these problems, the rates of formation of the monoacetates 8-10 were measured a t the initial stages of the reaction, before diacetates formed. Samples were analyzed by HPLC after making sure t h a t no intramolecular migrations of acetates took place during the analysis time. In Table 2, the results are summarized. In general, the catalytic efficiency of the lipase increased as the solvent hydrophobicity increased. Examination of the individual rate values corroborates the observation a t the time of maximum formation of monoacetates: acetylation at HO-2 is preferred in hydrophobic solvents while acetylation a t HO-4 is favored (5) The monoacetates are stable in the selected solvents for the time of the reaction in the absence or in the presence of enzyme; however, intramolecular migrations during the separation by silica gel column cannot be precluded.
Table 2. Initial Rate@ of Formation of 8 We), 9 We), and 10 (VIO)in the Monoacetylationof 5 (10 mM) Catalyzed by Lipase PS in Organic Solvents
solvent hexane toluene benzene triethylamine ethyl acetate tetrahydrofurane acetonitrile a pM.h-l
-
1
0
2
4
Log P
B
P 'ap >
-1
0
1
2
3
4
5
6
7
Polarity Index Figure 1. Regioselectivity 244-acetate (Vfl10)in the monoacetylation of 5 catalyzed by lipase PS in anhydrous solvents as function of solvent log P (A) and solvent polarity index7 (B):a, hexane; b, toluene; c, benzene; d, triethylamine; e, ethyl acetate; f, tetrahydrofuran; g, acetonitrile.
as the solvent becomes more hydrophilic. In the last column of Table 2, the ratios of Vfllo clearly show this behavior. In changing from acetonitrile to hexane, the ratio increases 11-fold. In Figure 1A-B, the Vfl10 ratios are plotted versus log p6 (solvent hydrophobicity) and the polarity index' of the solvents. Except for the basic triethylamine, a correlation was found with these solvent parameters. To determine if monoacetates disappear at different velocities under the reaction conditions, we measured the initial rates of formation of the diacetates from the three monoacetyl regioisomers separately, in two distinct solvents, acetonitrile and toluene.8 From the results shown in Table 3, it is noteworthy that the enzyme in toluene (6)Laane, C.; Boeren, S.; Vos, K.; Veeger, C. Biotechnol. Bioeng. 1987,30,81. (7) Synder, L. R. J. Chromatogr. Sci. 1978,16,223. (8)We chose toluene as the hydrophobic solvent since monoacetate 9 was not completely soluble in hexane at 20 mM.
Acetylations of Alkyl P-D-Xylopyranosides
J. Org. Chem., Vol. 59, No. 23, 1994 7029
Table 3. Initial Ratesa for the Acetylation of Monoacetates 8,9, and 10 (20 mM) Catalysed by Lipase PS, in Toluene and Acetonitrile at 30 "C monoacetates
solvent
8 9 10 8
toluene toluene toluene acetonitrile acetonitrile acetonitrile
9
10 a
rateb 1.66 5.57 18.02 0.99 1.44 1.30
Scheme 2
diacetate products (ratio)
6 7 7 6 7 6
11
2) NaOMe
+ 7 (1:3)
Determined by GC. mM*h-l
