Issue in Honor of Prof Richard R. Schmidt ARKIVOC 2013 (ii) 201-212
requirement of expensive catalysts,8,9 and involvement of expensive equipment such as microwave. A few years ago, Ye et al.11 first demonstrated O-glycosidation reaction using a combination of bromodimethylsulfonium bromide and silver triflate. From this successful result, we conceived that bromodimethylsulfonium bromide (BDMS) can be further exploited for Oglycosylation reaction since our research group is actively engaged to devise newer synthetic methodologies by employing the same reagent in carbohydrate chemistry.12 The unique property and usefulness of bromodimethylsulfonium bromide (BDMS) have been established by our laboratory as well as by others in last few years, which was compiled by us.13 Very recently we have shown the same catalyst is also effective for multicomponent reaction for the synthesis of heterocyclic compounds14a as well as other organic molecules.14b,c In this paper, we would like to report the synthesis of 2,3-unsaturated-O-glycosides from 3,4,6-tri-O-acetyl-D-glucal and various alcohols in presence of BDMS as a pre-catalyst as shown in Scheme 1.
Scheme 1. Synthesis of 2,3-unsaturated-O-glycosides.
Results and Discussion
For the present study, we have prepared the catalyst BDMS by following the literature procedure.13 To find out the optimal reaction conditions, we have chosen as 3,4,6-tri-O-acetyl-Dglucal (1) and p-bromobenzylalchol (2a) as model substrates. As per our assumption, various reactions were carried out in presence of 5 mol%, 10 mol% and 15 mol% BDMS in dry DCM, respectively (Table 1, entries 1-3). The desired product 3a was obtained in good yield, which was characterized from 1H NMR, 13C NMR spectra and elemental analysis. To verify the suitability of other solvents, the similar reactions were conducted in CH3CN, dry ether and mixed solvent system such as DCM/CH3CN (1:1) successively. The successful results are summarized in Table 1 (entries 4-6). To compare the efficacy with other catalysts, similar reactions were performed again with different metal triflates namely Co(OTf)2 and Mn(OTf)2 in acetonitrile (Table 1, entries 7 and 8), which also provided the product 3a in low yield and poor selectivity. The best result was obtained by using 10 mol% BDMS in dichloromethane (Table 1, entry 2) at room temperature in terms of reaction time, yield and selectivity. It is noteworthy to mention that on increasing the amount of catalyst, the reaction time reduced, but the yield did not increase significantly. From these results, it is quite clear that BDMS plays a major role in terms of .selectivity and yield.
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