Synthesis of glycoconjugates containing a 1 , 2 , 3-triazole unit

The preparation of several alkynyl esters, derived from amino acids, coumarins and an alkynyl derivative of acetylated D-glucose is described. Eight new glycoconjugates containing the 1,2,3-triazole unit were obtained, by a click approach from the above referred alkynyl derivatives with tetracetyl--D-glucosylazide, prepared in situ from acetobromoglucose.


Introduction
The glycoconjugates have an enormous potential in drug design 1 .Between them, glycopeptides are particularly important as they combine the structural features of amino acids and carbohydrates in the same molecule.Glycoconjugates containing the 1,2,3-triazole unit find application in medicinal chemistry, particularly in those cases where this unit acts as a bridge between an amino acid/peptide and the sugar moiety. 2 In this work the synthesis of several glycoconjugates containing the 1,2,3-triazole unit as a bridge between a sugar (D-glucose) moiety and an amino acid or heteroaromatic unit is described.The 1,2,3-triazole unit was formed by an azide-alkyne 1,3-dipolar cycloaddition, catalysed by a Cu(I) species, a chemical process usually known as click chemistry. 3,4The azido component was prepared in situ from -acetobromoglucose. 4,5

Results and Discussion
The starting alkynyl esters 1-5 (figure 1) were prepared by reaction between Nprotected glycine, tyrosine and phenylalanine, 7-hydroxycoumarin and 7-hydroxy-4methylcoumarin and propargyl bromide with high yields.All these compounds showed [a026]     in the proton NMR spectra the coupling patterns for the alkynyl function, for instance for compound 1 a triplet (J=2.4Hz) at 3.58 and a doublet (J=2.4Hz) at 4.73 ppm for ≡CH and CH 2 C≡, respectively.
All the final compounds showed the NMR spectra consistent with the proposed structure, namely the signal for the proton of the triazole ring (a singlet 8.13-8.59ppm).
Compounds 13 and 14 were prepared using as starting material the acetylenic derivative of glucose and but-3-yn-1-ol and propargyl alcohol, respectively (scheme 3).These compounds were isolated in 60-69% yields and were fully characterized.The NMR confirms their structures, it can be observed besides the typical glucosyl moiety Compound 15 was synthesised, under the conditions of click reaction, using glycosylazide and compound 13 as the acetylenic component in 80% yield.The NMR showed the triazole proton at 8.09 ppm, along with the signals expected for the two glucosyl moieties.

Conclusions
Glyconjugates containing the 1,2,3-triazole unit were obtained by an azide-alkyne 1,3dipolar cycloaddition, catalysed by Cu(I).The azide component, glycosylazide, was obtained in situ from -acetobromoglucose and the alkyne components were prepared by reaction of propargyl bromide with N-protected glycine, tyrosine and phenylalanine, 7-hydroxycoumarin and 7-hydroxy-4-methylcoumarin with high yields.The final glyconjugates were isolated with a wide range of yields, varying from low, 14% to as high as 80%.

Experimental
Melting points were determined on a Gallenkamp melting point apparatus and are uncorrected. 1

General method for the preparation of alkynyl esters-method A
To a solution of appropriated substrate in DMSO (5 mL) anhydrous K 2 CO 3 (1.01 equiv.) and propargyl bromide (1 equiv.)were added and the reaction mixture was stirred at room temperature for 4 hours.Water was added, and the mixture extracted with ethyl acetate and the organic extracts were combined, dried (MgSO 4 ) and evaporated to dryness.

General method for the cycloaddition reaction by click chemistry-method B
To a solution of -acetobromoglucose in dry DMSO dry NaN 3 (1.2equiv.) was added and the mixture stirred at room temperature for 20 mins, forming the glycosylazide in situ.The acetylenic substrate (1.5 equiv.for the but-3-yn-1-ol, but-3-yn-2-ol and propargyl alcohol; 1.05 equiv.for the others), sodium L-ascorbate (1M aqueous solution, 2.5 mL/mmol azide) and CuSO 4 .5H 2 O (1M aqueous solution, 2.5 mL/mmol azide) were then added to the reaction mixture, that which was stirred at room temperature for the time indicated.After filtration water was added to the filtrate, and the mixture extracted with ethyl acetate and the organic extracts were combined, dried (MgSO 4 ) and evaporated to dryness.

Synthesis of Z-Phe-OCH 2 C≡CH (2)
Following the general method A, and starting from Z-PheOH, compound 2 was isolated as a pure brown solid (88%), m.p. 64.

7-propargyloxycoumarin (4)
The titled compound was prepared by the general method A, starting from the 7hydroxycoumarin, and was isolated as a pure brownish solid, 99%, m.p. 118.Following the general method B and using but-

Scheme 2 .
Scheme 2. General method for click reactions.

Figure 1 .
Figure 1.Structures and numbering of the compounds prepared.
Scheme 3. Synthesis of acetylenic derivatives of D-Glucose.