Highly Efficient a -Sialylation by Virtue of Fixed Dipole Effects of N -Phthalyl Group:Application to Continuous Flow Synthesis of a (2-3)-and a (2-6)-Neu5Ac-Gal Motifs by Microreactor
Shin-ichi Tanaka,Takashi Goi,Katsunori Tanaka,
and Koichi Fuka
Department of Chemistry,Graduate School of Science,Osaka University,Toyonaka,Osaka,Japan
power stripHighly a -lective sialylation of sialic acid N -phenyltrifluoroacetimidate with various galacto and lacto acceptors has been achieved by introducing the C-5N -phthalyl group on the donor.The “fixed dipole effect”of the N -phthalyl group was propod to explain the high reactivity and a -lectivity .The microfluidic system was applied to the prent a -sialylation,which is amenable to large-scale synthesis.The N -phthalyl group was removed by treatment with methylhydrazine acetate,for which protocol can be readily applied to the synthesis of a variety of sialic acid-containing
oligosaccharides.Received February 6,2007;Accepted July 24,2007
Address correspondence to Koichi Fuka,Department of Chemistry,Graduate School of Science,Osaka University,Machikaneyama 1-1,Toyonaka,Osaka 560-0043,Japan.E-mail:koichi@chem.sci.osaka-u.ac.jp
Journal of Carbohydrate Chemistry,26:369–394,2007
Copyright #Taylor &Francis Group,LLC
ISSN:0732-8303print 1532-2327online
DOI:
10.1080/07328300701634796
369
Keywords a -Sialylation,N -Phthalyl group,Fixed dipole effects,Microreactor,
gonna什么意思Oligosaccharides
N -Acetylneuramic acid (Neu5Ac),the most abundant sialic acid congener in nature,is found at the termini of glycoproteins and glycolipids on mammalian cell surfaces,usually linked with galacto or N -acetylgalactosamine through a (2-3)or a (2-6)sialoglycoside bonds.Since Neu5Ac on the cell surfaces plays diver and important roles in cell /cell interaction process,[1]such as pathogen /host recognition,tumor metastasis,and cell differentiation /prolifer-ation,much effort has been devoted to the development of efficient and stereo-lective synthesis of a (2-3)and a (2-6)-Neu5Ac-Gal units in order to further investigate their biological functions.[2,3]However,an efficient and general sia-lylation with high a -lectivity still has not been fully realized,becau (1)the prence of the electron-withdrawing carboxylic acid groups at C-2of the sialyl acid donors deactivates the oxocarbenium ions,thus electronically and steri-cally interfering with efficient sialylation.This leads to the formation of a sig-nificant amount of the dehydrated byproduct,2,3-glycal.In addition,(2)the abnce of the neighboring participation group at C-3cannot ensure the stereo-chemical outcome for the a -lective sialylation.This inherent reactivity of the sialyl donor renders the a -lective sialylation as one of the most difficult [4]and challenging topics in the field of oligosaccharide synthes
is.[5]
Recently,Cai and Yu have successfully enhanced the reactivity of sialic acid donors by utilizing phenyltrifluoroacetimidate as a leaving group,and achieved efficient sialylation with a variety of acceptors in 59%to 90%yields:while (2-3)-Neu5Ac-Gal synthesis has been realized in 81%yield (a :b ¼3:1),the corre-sponding a -lective (2-6)-sialylation was achieved in 61%yield.[6]Kiso and coworkers [7]and Takahashi and coworkers [8]have ud the C-5-N -Troc-protected thiophenyl derivatives as the sialic acid donors and achieved good a -lectivity,namely ,a :b ¼54:10for the (2-3)-sialylation ca.[7]Apparently,one of the most exciting achievements in this field is the a -(2-8)-linkage formation between the sialosides,quite recently realized by Kiso’s [9a]and Takahashi’s [9b]groups.Especially,Takahashi’s group has utilized the 4-O ,5-N -oxazolidinone-protecting group both on the sialoside donor and the acceptor and realized both a -(2-9)-and a -(2-8)-oligosialoside synthesis with excellent lectivity.[9b]
In our program directed toward the establishment of a general and practical synthesis of N -linked oligosaccharides and other neuramic acid-containing natural products,highly yielding and a -lective sialylation is esntial.The recent successful precedents mentioned above led us to utilize the phenyl-trifluoroacetimidate donor and to pursue the possibility of increasing the a -lectivity by
modifying the C-5-N -protecting groups of the sialic acid donors (i.e.,the amide groups of which dipole moments direct the stereochemical cour of glycosylation and /or the carbonyls take part in the neighboring group
S.-i.Tanaka et al.
370ace of ba
participation).In this paper,we disclo in detail[10]highly efficient sialylation with excellent a-lectivity by using the N-phthalyl group,which led to a general synthesis of the a(2-3)-and a(2-6)-Neu5Ac-Gal moieties.The method is applied to microfluidic a-sialylation in pursuit of a scaled-up synthesis of neuramic acid-containing compounds.The possible mechanisms for the enhanced a-lectivity bad on the conformational analysis and electronic property calculations are also described.
For the sialic acid donors,we planned six differently protected derivatives on the C-5nitrogen,namely,acetyl(2a),Troc(2b),bis-acetyl(2c),dimethylma-leoyl(DMM,2d),tetrachlorophthalyl(TCP,2e),and phthalyl(Pht,2f)(Sch.1). The donors were easily prepared from thioglycoside1,[11]according to Higuchi’s procedure:[3e](1)deacetylation,(2)protection
of the C-5nitrogen, (3)peracetylation of the remaining hydroxyls,(4)hydrolysis of the thioglyco-side,and(5)phenyltrifluoroacetimidate formation.
a-Selective(2-6)-sialylation trials using donors2a–2f thus prepared and 1-O-allyl-2,3-O-benzoylgalacto3,appropriately protected for further func-tional group manipulation,are shown in Table1.First,the reactivity of the sialy donors2a–2f was screened using50mg of each sample(Table1).All reac-tions were performed at2788C in the prence of TMSOTf as the Lewis acid activator,using1.5equivalents of acceptor3with respect to the donors.Propio-nitrile was ud as the optimal solvent,by taking advantage of“nitrile solvent effect.”[13]The reaction of N-mono-acetyl donor2a with acceptor3provided the corresponding disacchalide4a in good yield(entry1,93%)and with moderate lectivity(a:b¼77:23).It is noteworthy that,when N-Troc derivative2b was
ud(entry2),the reaction was significantly accelerated(6h for N-Ac
电灯泡英文and
Scheme1:Preparation of sialic acid donors2a–f bearing a variety of N-protected groups.
Highly Efficient a-Sialylation371bostoncollege
30min for N -Troc),and a -lectivity also incread up to a :b ¼92:8.This obrvation is in accordance with that of Kiso and coworkers.[7]
Encouraged by the promising results,we then examined the N -bis-acylated donors 2c –2f .Bis-N -acetyl derivative 2c gave the corresponding dis-acchalide 4c in moderate yield and with moderate a -lectivity (75%and a :b ¼72:28,entry 3),similar to that obrved for monoacylate 2a (entry 1).However,we were glad to find that,when two acyl groups on the C-5nitrogen were fixed within the five-membered rings,the a -lectivity incread dramatically.Thus,N -DMM-and N -TCP-protected donors 2d and 2e successfully provided disacchalides 4d and 4e in 94%and 87%yields,respectively,and both with excellent a -lectivity (a :b ¼96:4,entries 4and
5).Furthermore,the utilization of the N -Pht-protected donor 2f resulted in perfect a -lectivity and excellent yield (92%on 50-mg scale,entry 6).It is worthwhile mentioning that,when the solvent was exchanged from propioni-trile to the noncoordinating dichloromethane,the stereolectivity was reverd (entry 7,a :b ¼9:91),indicating the importance of the “nitrile solvent effect”in order to obtain good a -lectivity.However,when the scale of the reaction was incread by only two,the product yield markedly decread,although the a -lectivity remained high.Thus,the 100-mg
Table 1:a -Selective (2-6)-sialylation Using 2a–f .
a
Entry
Donor Solvent Temp Time Product Yield (%)b a :b c 1
2a EtCN 2788C 6h 4a 9377:232
2b EtCN 2788C 30min 4b 8492:83
2c EtCN 2788C ,5min 4c 7572:284
2d EtCN 2788C ,5min 4d 9496:45
tick tock2e EtCN 2788C ,5min 4e 8796:46
2f EtCN 2788C 30min 4f 92d a only 7
2f CH 2CI 2rt ,5min 4f 879:918
2f EtCN 2788C 30min 4f 60e a only a All sialylations were performed using 1.5equiv of acceptor 3relative to donors 2a –2f .The mixture of anomeric stereoisomers for imidates 2a –2f was ud.b Isolated yields.c The a /b -ratio was determined by NMR analysis:a -and b -isomers were identified bad on the empirical rule for their characteristic proton chemical shifts.[12]
d Yield at 50mg-scal
研究生成绩查询2017e o
f 3.e Yield at 100mg-scale of 3.
S.-i.Tanaka et al.
372
scale reaction of 3gave only 60%of a -sialoside accompanied by a significant amount of a glycal byproduct (entry 8).
The decrea in sialylation efficiency obrved in entry 8might be due to the high reactivity of 2f .For such a ca,preci reaction control is very diffi-cult under the conventional batch process conditions,especially when the reaction is scaled up.Thus,the disorder of the reaction factors in the scaled-up batch reaction,that is,(1)preci temperature control;(2)mixing efficiency between acceptor,donor,and Lewis acid;and (3)reaction time might lead to the glycal production.In order to circumvent the problems,we ud a continuous flow microreactor,which is reported to realize efficient mixing and fast heat transfer and,therefore,is recognized as innovative technology in recent organic synthesis.[14]Once the reaction conditions are optimized for the small-scale reaction,the same conditions are directly applied to a large-scale synthesis,since the reaction is conducted under the flow process conditions.[15]An application of the microfluidic system to the glycosylation reaction was first reported by Seeberger and coworkers on a -mannosylation.[16a]We also have established an efficient microfluidic glycosylation in combination with the affinity paration method.[16b]
For the prent microfluidic sialylation,a propionitrile solution of sialyl donor 2f and acceptor 3with various concentrations was mixed with TMSOTf solution in dichloromethane at 2788C using an IMM micromixer [17]at the flow rate of 1.0mL /min (Table 2).After the reaction mixture was allowed to flow at 2788C for additional 47c through a reactor tube (F ¼1.0mm,Table 2:Optimization of a -(2-6)-sialylation between donor 2f and acceptor 3using
microreactor.huke
Entry
taste怎么读Donor 2f (M)Acceptor 3(M)TMSOTf (M)Yield of 4f (%)a :b a 1
0.150.10.0814a only 2
0.150.10.1588a only 3
0.20.10.15.99a only a Bad on 1H NMR analysis.Highly Efficient a -Sialylation 373
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