“Green” oxidation reactions—application to carbohydrate chemistry

Abstract

Oxidation of glycals by either methyltrioxorhenium (MTO)-hydrogen peroxide or hydrogen peroxide-acetonitrile in methanol provides direct access to methyl glycosides. The two different reagents proceed with a complementary stereochemical outcome enabling the synthesis of b-D-gluco-pyranosides or a-D-manno-pyranosides from the same carbohydrate precursor. Green ContextDevelopment of greener methods for carrying out synthetic transformations often is limited to a few simple test reactions, with more complex systems being less thoroughly investigated. Here, novel clean oxidation approaches based on hydrogen peroxide are described which work well for several polyfunctional molecules. High yields and interesting stereochemical outcomes are recorded for a range of protected carbohydrates.

Tandem Epoxidation‐Alcoholysis or Epoxidation‐Hydrolysis of Glycals Catalyzed by Titanium(IV) Isopropoxide or Venturello’s Phosphotungstate Complex

“…Another general feature of these methods is the high yield obtained, in the range of 85 to 90%. Similar yields have been achieved with MTO for a few substrates, [10][11] and in the work of Danishefsky and others using DMDO [5] but most other methods are associated with yields under 80%.…”

“…Whereas the glucal derivatives all exhibit almost exclusive selectivity towards formation of 1,2-trans glycosides, the galactal derivatives give significant proportions of 1,2-cis glycosides, suggesting that the substituent at C-4 may assist in stabilizing a carbenium ion at C-1 formed by opening of the epoxide. The results of reactions with TiA C H T U N G T R E N N U N G (O-i-Pr) 4 in methanol can be compared with similar tandem catalytic oxidations in methanol using methyltrioxorhenium (MTO) as catalyst and either H 2 O 2 or urea-hydrogen peroxide (UHP) as oxidant, as reported independently by Quayle and co-workers [11] and Goti and coworkers [10a] and summarized in Table 2. While reaction times for the TiA C H T U N G T R E N N U N G (O-i-Pr) 4 -catalyzed reactions were significantly longer than for the other reactions, the selectivities towards a-epoxides were markedly superior in most cases.…”

“…Catalysts for epoxidation of glycals have only recently been identified, including Ru-porphyrin [9] and CH 3 ReO 3 (MTO). [10,11] . The latter has also been applied in a heterogeneous version.…”

Methyltrioxorhenium‐Catalyzed Epoxidation‐Methanolysis of Glycals under Homogeneous and Heterogeneous Conditions

“…A comparison of these results with those reported by Quayle and co-workers with the use of aqueous H 2 O 2 , [32] where only glucal and galactal derivatives have been used as substrates, allows us to establish that our procedure with UHP in MeOH affords generally higher product yields. From the point of view of stereoselectivity of epoxidation, while the glucals 8 and 9 gave similar ratios, a considerably higher selectivity was observed by us in the reaction of galactal 10 (7.5 : 1) with respect to that reported in aqueous H 2 O 2 (3 : 1).…”

“…[31] Shortly after our preliminary communication, Quayle and coworkers reported full details of a similar procedure, employing aqueous H 2 O 2 instead of UHP. [32] Lewis base adducts of MTO with nitrogen-containing ligands, such as pyridine, [33] pyridine derivatives, [34] pyrazole [35] and others are known to influence significantly the oxidation processes, for example, decreasing the formation of diols in epoxidation reactions, especially in the case of sensitive substrates, and increasing the catalytic efficiency. MTO reacts with monodentate and bidentate nitrogen ligands to give trigonal bipyramidal and distorted octahedral adducts, respectively.…”