A New Generation of Glycoconjugated Azo Dyes Based on Aminosugars

The third generation of glycoconjugated azo dyes (GADs) was prepared linking monoazo dyes to 6-amino-6-deoxy-d-galactose or 6amino-6-deoxylactose through mixed amido-ester connections. The complementary conjugation reactions were studied using the succinyl derivative of either the acetal protected aminosugar or the azo dye. Target “naturalized” GADs were obtained after acid hydrolysis of the acetal protecting groups present on the sugar moiety.


Introduction
The most commonly employed class of industrial textile dyes are the so-called "disperse dyes" family, characterised by their extremely low solubility in water.These, in a finely dispersed state, are used in dyeing processes of synthetic fibres, such as polyester, polyacetate, and polyamide.Owing to the absence of ionic groups in these synthetic fibres, only apolar dyes can be utilized [1][2][3].Among the various troublesome aspects related to the use of disperse dyes and their dyeing processes, the highest concerns are raised by (a) the use of substantial amounts of dispersing agents, which are needed to bring the insoluble dyes in a stable colloidal dispersion throughout the application process, and (b) the use of high temperatures, typically around 130 ∘ C, which demand large amounts of energy and require appropriate dyeing machines able to operate under pressure [1][2][3].Azo and anthraquinone dyes represent the two most relevant types of disperse dyes.Although disperse dyes can differ considerably in molecular weight, only those with a low mass (typically between 200 and 500 Da) [1] are effectively used in the dyeing of textile apolar synthetic fibres.
Recently, an innovative class of azo dyes [4][5][6] has been proposed.These have been obtained through the glycoconjugation of common mono-and disaccharides, such as d-glucose, d-galactose, and lactose, with some model synthetic monoazo dyes.Glycoconjugated azo dyes (GADs) have been prepared by the so-called "naturalisation" procedure, which is to link the azo dye component and the sugar component through difunctional spacers by means of either two consecutive esterification reactions or two consecutive alkylation reactions.Glycoconjugated azo dyes (GADs, Figure 1) type 1 [4,5] and type 2 [6] characterised, respectively, by diester and diether bonds have been evaluated for their tinctorial properties, giving unexpected results.In fact, GADs dye synthetic fibres quickly in boiling water, at ambient pressure, and without the need of dispersant, thanks to the increased hydrophilic characteristics.Uniquely, GADs can also be used effectively to dye natural fibres, such as wool and silk, allowing the study of solutions to novel dyeing technologies.Moreover, biodegradation and detoxification of a selected diestereal GAD by Fusarium oxysporum gave promising results [7] for the treatment of the aqueous waste, representing a step further towards environmentally friendly tinctorial processes.
In a more recent investigation [8] high molecular weight anthraquinone or bis-azo disperse dyes only become watersoluble after conjugation with two lactose units.The doubleconjugation procedure required a preliminary insertion of a malonate spacer followed by condensation with two 6  -2 International Journal of Carbohydrate Chemistry amino-6  -deoxylactose units [8].Also, glutamic acid was used as the additional spacer in order to allow the doubleconjugation [9].Presented here is an extension of our previous results, namely, the synthesis of the third generation of naturalized GADs by conjugation of low molecular weight monoazo dyes with either 6-amino-6-deoxy-d-galactose or 6  -amino-6  -deoxylactose through mixed amido-ester connections (3).

Results and Discussion
In the search of new derivatives to be tested as new GADs, we followed the same strategy used for the synthesis of the di-estereal [4,5] (1) and diethereal [6] (2) compounds.We were interested in preparing structures with different functional groups at the attachment point with the linker in order to evaluate the synthetic accessibility and, at a later stage, the effect of these changes both on the stability during the dyeing process and on the tinctorial properties.To construct the amido-estereal mixed GADs, we used appropriately protected 6-amino-galactose derivatives in either the condensation reaction with monosuccinyl-azo dye or after their conversion into monosuccinyl derivatives.This would allow us to study the two complementary strategies and to identify the best pathway.
In particular, the known yellow azo dye (8) [12] and the commercial Disperse Red 13 (9) were used in the preparation of GADs following route A (Scheme 2) while derivative 10, prepared by succinylation of commercial alcoholic azo dye Disperse Red 1, [4] was employed in the condensation reaction with the aminosugar derivatives 4 [10] and 5 [8] (route B).
It is noteworthy that the succinyl derivatives 7 and 10 could be used as crudes without affecting the outcome of the condensation reaction.In the case of protected compounds 11-13 the final deprotected GADs 16-18 (Figure 3) were obtained by simply removing the acetal protecting groups by means of acid hydrolysis as we previously reported [4][5][6] for type 1 and type 2 derivatives (90% aqueous CF 3 COOH, room temperature).
Isomeric forms of the 16-18 were identified by comparison of their NMR data with those reported for analogous compounds (types 1-2) [4][5][6].The UV-Vis absorption spectra for a deprotected red GAD (17) and for the deprotected yellow GAD (18) were recorded in MeOH.As for the previous two generations of GADs, [4,5] the glycoconjugation process did not affect the absorption properties of the dyes and 17 and 18 were characterised by almost the same  max values of the parent not conjugated dyes ( max around 495 nm for the red compounds 9, 12, and 17 and around 410 nm for the yelloworange compounds 8, 13, and 18).

Conclusions
In conclusions, we presented the access to the third generation of GADs exploring two complementary routes.The obtained compounds are characterized by different attachment points between the chromophore or the sugar and the linker.The presence of the amido bond could permit a novel synthetic pathway to GADs, for example, by using enzymes [14] for the condensation reaction.Detailed analysis of the stability and tinctorial properties of the third generation of GADs are currently under investigation, the results of which will be published in due course.