Synthesis of multiply fluorinated N-acetyl-D-glucosamine and D-galactosamine analogs via the corresponding deoxyfluorinated glucosazide and galactosazide thiodonors

Multiple fluorination of glycostructures has emerged as an attractive way of modulating their protein affinity, metabolic stability, and lipophilicity. Here we described the synthesis of a series of mono-, diand trifluorinated N-acetyl-D-glucosamine and D-galactosamine analogs. The key intermediates were the corresponding multiply fluorinated glucosazide and galactosazide thiodonors prepared from deoxyfluorinated 1,6-anhydro-2-azido-β-D-hexopyranose precursors by ring-opening reaction with phenyl trimethylsilyl sulfide. Nucleophilic deoxyfluorination at C4 and C6 by reaction with DAST, thioglycoside hydrolysis and azide/acetamide transformation completed the synthesis.


Results and discussion
Our approach to the synthesis is summarized in Scheme 1. Challenging regio-and stereoselective introduction of fluorine at C3 and C4 of the pyranose ring, together with azide installation at C2, can be accomplished by nucleophilic fluorination and azidolysis starting from dianhydro derivatives 1 and 2 as we described previously [26]. The resulting intermediates 3 can be transformed into 2-azido-hexopyranosides  [38]. This instability of amino sugar hemiacetals underscores the requirement to both protect the anomeric position with a robust protecting group and to conduct final deprotection under neutral conditions. After initial experimentation with benzyl glycosides (Scheme 1, Pg = OBn), phenyl thioglycosides (Scheme 1, Pg = SPh), readily available from 1,6anhydropyranoses [39] as we described earlier [40] were found to fulfill this requirement satisfactorily.

Scheme 1. Retrosynthetic analysis of the target fluoro analogs
Accordingly, the synthesis started from known fluorinated 1,6-anhydro-2-azido-hexopyranoses 7-13 (Scheme 2) [26,40]. Reaction of compounds 7-10 with phenyl trimethylsilyl sulfide (PhSTMS) and ZnI2 delivered phenyl thioglycosides 14-17 [40]. 1,6-Anhydropyranoses 11 and 12 under these conditions produced the expected thioglycosides 18 and 19, respectively. Difluorinated derivative 13 decomposed on reaction with PhSTMS/ZnI2 system. Separation of anomers of the products 14-19 was attempted because of the risk of thiophenyl migration in the subsequent C6 deoxyfluorination, which would likely occur with the β-anomers of 14-19 [41]. Complete separation of the α-anomer by conventional silica gel column chromatography was possible for thioglycosides 14, 16, 17, and 19, while the products 15, and 18 were obtainable as enriched α-anomers (α/β ≥ 3.3:1). Cleavage of the internal acetal with PhSTMS was accompanied by the formation of low quantities of side-products detectable by TLC and separable by careful chromatography except for the cleavage of 12 where the side products co-eluted with the fraction containing the β-anomer of the product. In the case of the cleavage of 1,6-anhydroderivative 10, we were able to isolate one of the side-products in sufficient purity and quantity to be assigned the structure of Cfuranoside 20 (Scheme 2). This compound resulted from pyranose ring contraction probably caused by intramolecular displacement of the C2 azide aided by coordination of ZnI2. When the α-anomer of thioglycoside 17 was separately subjected to the reaction conditions, the by-product 20 started to form in trace amounts in accordance with the suggested mechanism. The ring contraction may involve formation of 5 a transient oxiranium cation as suggested in Scheme 2 [42][43][44][45]. Analogous ring-contraction reactions have been described for substrates possessing a good C2 leaving group [42,[46][47][48][49][50].

Scheme 2.
Conversion of 1,6-anhydro derivatives into thioglycosides, and a possible mechanism for the formation of C-furanosides by ring contraction.

Scheme 3. Deoxyfluorination and O-benzylation of thioglycosides, hydrolysis to hemiacetals and thioaglycone migration
As the C3/C4 difluorinated thiogalactoside could not be accessed from compound 13 by reaction with PhSTMS/ZnI2 (Scheme 2), it was necessary to obtain 3,4-difluoro and 3,4,6-trifluoro analogs of GalNAc from 3-fluoro-4,6-diol 18. According to precedents in the literature [53], deoxyfluorination of the C4hydroxyl group in compound 18 was expected to occur with inversion of configuration to give the desired galacto-configured 4-fluoro products. Accordingly, treatment of diol 18 with DAST resulted in deoxyfluorination of both hydroxyl groups to yield trifluoro galactosazide 36 after thioglycoside hydrolysis To obtain the target fluoro analogs, the hemiacetals 22-31, 36 and 38 were debenzylated and their azide group converted to an acetamide. Although palladium-catalyzed hydrogenolysis in ethanol/acetic anhydride appeared to be a logical deprotection step [26], the desired fluoro sugars were contaminated with varying quantities of unidentified by-products. However, clean debenzylation was achieved by first converting the azide to an acetamide on reaction with thioacetic acid [54][55]. Hence, the hemiacetals were reacted with thioacetic acid in pyridine to give acetamides 39-48 (Scheme 4) and the target trifluoro analogs 49 and 50. Reversing the order of hemiacetal and azide/acetamide formation was not an option because NBS-promoted hydrolysis of 2-acetamido thioglycosides was sluggish and incomplete. Protecting the primary hydroxyl at C6 by O-benzylation (Scheme 3) was essential before treatment with thioacetic acid; otherwise, an O6-acetylated by-product was formed. Acetylation of the anomeric hydroxyl occurred only to a very limited degree upon reaction with AcSH in pyridine and traces of O1 acetates were removed by chromatography or recrystallization. Palladium-catalyzed hydrogenolytic debenzylation of 39-48 then yielded the target fluoro analogs 51-60. To complete the series of fluorinated analogs for the purpose of comparing their NMR spectra, the known C6 monodeoxyfluorinated compounds 61 [27,28] and 62 [29] were prepared by published procedures [27,29].  The magnitudes of the vicinal 3 JH-H, 3  the previous reports by Giguѐre [13,15,46].
In summary, we have demonstrated that multiply deoxyfluorinated GlcNAc and GalNAc are accessible via the corresponding multifluorinated 1-thiophenyl gluco-and galactosazides. Installation of the thiophenyl aglycone permits C6 deoxyfluorination and circumvents the problems resulting from the low stability of amino sugar hemiacetals. The prepared polyfluorinated thiodonors and hemiacetals are valuable intermediates in oligosaccharide synthesis and their utility in glycosylation is currently being studied in our group.

Supporting Information
Supporting Information File 1: Experimental. Experimental procedures and spectral data.
Supporting Information File 2: Copies of NMR spectra. Copies of 1 H, 13 C, 19 F, and 2D NMR spectra for new compounds.