A selective and mild glycosylation method of natural phenolic alcohols

Several bioactive natural p-hydroxyphenylalkyl β-D-glucopyranosides, such as vanillyl β-D-glucopyranoside, salidroside and isoconiferin, and their glycosyl analogues were prepared by a simple reaction sequence. The highly efficient synthetic approach was achieved by utilizing acetylated glycosyl bromides as well as aromatic moieties and mild glycosylation promoters. The aglycones, p-O-acetylated arylalkyl alcohols, were prepared by the reduction of the corresponding acetylated aldehydes or acids. Various stereoselective 1,2-trans-O-glycosylation methods were studied, including the DDQ–iodine or ZnO–ZnCl2 catalyst combination. Among them, ZnO–iodine has been identified as a new glycosylation promoter and successfully applied to the stereoselective glycoside synthesis. The final products were obtained by conventional Zemplén deacetylation.

The activities of the above mentioned glycosides are primarily related to the structure of the aglycone. The glycosylation of the poorly soluble hydroxyalkylphenols, such as 4-hydroxybenzyl, vanillyl, 4-hydroxyphenethyl and coniferyl alcohols, significantly increases their water solubility. Further it influences the physicochemical and pharmacological properties of these phenols and often reduces their potential toxicity. Moreover, p-hydroxyarylalkyl glycosides are also key starting building blocks for the synthesis of more complex bioactive natural compounds with promising therapeutic potential (e.g., phenylpropanoid glycosides) [15,16]. Therefore, the development of relatively simple and safe procedures is needed for a rapid multigram-scale synthesis of arylalkyl glycosides in good yields.
Enzymatic glycosylations of arylalkyl alcohols are easily accomplished, however, the glycosides have often been obtained in low to moderate yields (usually below 30%). Glucosi-dases from fruit seed meals are the most commonly used biocatalysts for reverse hydrolysis reactions carried out in organic media or ionic liquids as co-solvents [24][25][26][27]. The enzymatic transglucosylation using 4-nitrophenyl β-D-glucopyranoside as the donor and almond β-glucosidase as biocatalyst gave salidroside (2) in moderate yield [28].
Recently, we have published the enzymatic glycosylation of tyrosol (2-(4-hydroxyphenyl)ethanol) with cellobiose, lactose and melibiose as donors for the preparation of salidroside and its α-and β-galactoside analogues [31]. However, all transglycosylation reactions required a distinct pair of the disaccharide donor and the glycosidase for which the reaction conditions had to be optimized. The current paper deals with an efficient, safe and uniform chemical synthesis of various p-hydroxyarylalkyl glycosides, including compounds 1-3.

Preparation of aglycones
The synthesis of the appropriate aglycones 6a-c was commenced from readily available commercial p-hydroxyphenylcarbaldehydes 4a-c which are less expensive than the corresponding p-hydroxybenzyl alcohols (Scheme 1). The conventional acetylation of 4a-c with acetic anhydride in pyridine gave p-acetoxyphenylcarbaldehydes 5a-c in more than 98% yields. Subsequently the aldehyde function was reduced by NaBH 4 at pH 7-8, which was kept constant by the continuous addition of 85% H 3 PO 4 to avoid phenolic acetyl-group cleavage. The p-acetoxybenzyl alcohols 6a-c were isolated in 85-95% yields.  p-Hydroxyphenylacetic acid (7) and ferulic acid (10) are also more readily available on the market than the corresponding alcohols, tyrosol and coniferyl alcohol. Therefore p-O-acetylated tyrosol (9) and p-O-acetylated coniferyl alcohol (12) were prepared from acids 7 and 10 by a two-step sequence: acid-catalysed acetylation of the phenolic hydroxy group (isolated yields >94%) followed by the reduction of the carboxylic function with NaBH 4 -I 2 in THF (Scheme 2). Due to the higher lability of the phenolic acetate under basic conditions, the method published by Kanth and Periasamy [32] was slightly modified. For this, the reagents were added at lower temperature, the reaction time was prolonged and a solution of NaHCO 3 instead of NaOH was used for washing. Under these conditions, the deacetylated product was formed only in traces (<5%). Regarding the reduction of acetylated ferulic acid 11, no formation of the 1,4-reduction product was observed and the double bond remained untouched.

Glycosylation reactions
Acetyl-protecting groups are the simplest choice also for the protection of the glycone part since the deprotection of both, sugar and aromatic moieties, can be accomplished in one step. Naturally occurring O-glycosides possess mostly 1,2-transglycosidic linkages. Therefore, neighbouring group participation is usually exploited in the trans-O-glycosylation of appropriate aglycones.
In the course of our synthetic studies, 1,2-trans-glycosylation reactions utilizing per-O-acetyl-D-glucopyranose as a donor were initially investigated. However, the reaction of 6b with per-O-acetylated-D-glucopyranose promoted by a Lewis acid (SnCl 4 ) in DCM failed. The deacetylated aglycone -vanillyl alcohol along with some amounts of 2,3,4,6-tetra-O-acetyl-Dglucopyranose were isolated. It is evident that these frequently used reaction conditions require more acid-stable derivatives. Therefore, it was reasonable to look for milder conditions for an efficient and inexpensive method of glycosylation while excluding the use of toxic mercury salts as promoter (Helferich reaction) or silver salts. The latter are often rather expensive, moisture and light sensitive, and uncomfortable to handle.
Accordingly, various acetylated glycosyl bromides 13, 15-20 derived from pyranoses, furanoses and a disaccharide ( Figure 2) were prepared as glycosyl donors in one step and high yields starting from the peracetylated sugars.
The glycosyl bromides depicted in Figure 2 were subsequently examined in the glycosylation of acceptors 6a-c, 9 and 12 (Scheme 3, Table 1). The choice of the glycosylation promoter was strongly limited by the instability of the phenolic acetyl group under basic as well as strongly acidic conditions. Only mild, neutral promoters were therefore selected and investigated. Thus, the reactions were performed in the presence of Scheme 3: General reaction scheme for the synthesis of p-hydroxyphenylalkyl glycosides.  [35] (method C). In addition ZnO-I 2 (method D) was successfully applied as a new promoter in the stereoselective 1,2-trans-glycoside synthesis (Table 1, entries 4, 9, 13, 17, 19 and 25). The selection of methods C and D was based on the common knowledge that iodine, either alone or in combination with other promoters such as salts of various metals (other than the traditional Koenigs-Knorr heavy metals), serves as an effective activator of disarmed glycosyl halides in the 1,2-trans-glycoside synthesis [35][36][37][38]. Despite the fact that the precise mechanism is not clear, it is assumed that the reaction of the glycosyl bromide promoted by iodine (through the formation of an iodobromonium ion) results in a carbohydrate-derived oxocarbonium ion that functions as the reactive intermediate [35].
In the first step, acceptor 6b and glucopyranosyl bromide 13 as the donor were selected and tested in the presence of the above mentioned promoters (see Table 1, entries 1-4) in order to identify the optimal glycosylation conditions in terms of yield and selectivity. In all cases, only the 1,2-trans-glycosylation product, β-glucoside 21b, was obtained. While method B (ZnO-ZnCl 2 ) performed in DCM instead of ACN afforded only a moderate yield (Table 1, entry 2, 46%) of 21b, the reactions in DCM promoted by Ag 2 O (Table 1, entry 1, 57%) and ZnO-I 2 ( Table 1, entry 4, 56%) gave comparably good yields. DDQ-I 2 in ACN (Table 1, entry 3, 68%) gave 21b in the highest yield, in addition to the exclusive selectivity and the shortest reaction time (Table 1, entry 5). Therefore this promoter was selected for the glucosylation reactions of acceptors 6a and 6c with bromide 13, affording compounds 21a and 21c with full β-selectivity.
On the other side, the glucosylation of p-O-acetylated coniferyl alcohol 12 with bromide 13 failed under these conditions. Coniferyl aldehyde 24 was detected and isolated as a major product. For example, the DDQ-I 2 -promoted reaction provided aldehyde 24 in 58% yield along with less than 5% of the desired product 23. This may be caused by the oxidative nature of the promoter and by the existence of a conjugated electronic push-pull system of coniferyl alcohol that is enhanced by the electron-withdrawing acetoxy group. On the contrary, the TMSOTf-promoted glycosylation [39] (method E) of coniferyl alcohol 12 with trichloroacetimidate 14 at low temperature was found to be more efficient and glycoside 23 was obtained in high yield with full β-selectivity as proved by NMR spectroscopy. The phenolic acetyl group remained intact under these conditions. In contrast to the above mentioned results, the reaction of D-xylosyl bromide 20 with 6b did not proceed stereoselectively. An anomeric mixture of vanillyl xylosides 30α/β in a ratio varying from 1:2.3 to 1:4 was obtained when the glycosylation was promoted with DDQ-I 2 in either DCM or ACN at room temperature ( In the final step, the removal of the acetyl groups under Zemplén conditions proceeded smoothly and the desired target glycosides 1-3, 31a,b and 32-37 were isolated in high yields (Figure 3).

Conclusion
The glycosylation methods studied in this work represent a simple and convenient approach to bioactive natural p-hydroxyphenylalkyl glycosides and their analogues. The mild reaction conditions with exclusive stereoselectivity can be used as an alternative to the common Koenigs-Knorr or Helferich glycosyla- tion. In many cases, the DDQ-I 2 -promoted reaction provided products in a stereoselective way and in the highest yields. It is noteworthy that ZnO-I 2 is a new glycosylation promoter, which was found to well activate also less reactive disarmed tetra-Oacetyl-α-D-glycopyranosyl bromides, to give stereoselectively only the 1,2-trans glycosides in good to high yields. These conditions were efficiently used in the stereoselective xyloside synthesis that is not trivial. All used glycosylation conditions were compatible with acetyl protective groups of the phenolic function. The coupling reaction and deprotection were achieved in two steps, thus providing the rapid access to the targeted glycosides.

Supporting Information
Supporting Information File 1 Experimental procedures and analytical data.