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.

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A route to 2-phenyl-4,5-dihydrooxazole-4-carbonitrile N-oxide 7 has been developed starting from (S) -serine methyl ester and involving dehydration of the 4-nitromethyl derivative 13 using mtolylene diisocyanate and triethylamine. Cycloaddition of the nitrile oxide to styrene afforded a 5 5 : 4 5 mixture of diastereoisomeric dihydroisoxazoles 21 and 22; furazan N-oxides 19 and 20 were also formed in a competing dimerisation. The analogous (R) -0xazolidine-4-carbonitrile oxide 8, also prepared from (S)-serine methyl ester, reacted similarly with styrene, oct-I -ene and diethyl fumarate to afford ca. 1 : 1 mixtures of adducts. The structures of the dihydrooxazole-dihydroisoxazole adduct 21 and furazan N-oxide 20 were determined by X-ray crystallography.The formation of 2-isoxazolines (4,5-dihydroisoxazoles) by 1,3dipolar cycloaddition of nitrile oxides to alkenes has been known for many years,' and the scope and mechanism of the reaction have been studied in detai1.2,3 More recently, the topic has received renewed attention in view of the now widespread use of nitrile oxide-isoxazoline chemistry for the synthesis of natural products and analogues. Of particular importance for the successful application of this methodology is an understanding of the factors affecting the stereochemistry of the cycloaddition step. It has been established '7 8 56, 5729. 40, 177.

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