Glycals, azidonitration

For the synthesis of glycans containing 2-amino-2-deoxysugars, the regioselective azidophenylselenation of glycals is popular [524—528]. This methodology follows on from the classic azidonitration work of Lemieux and Ratdiffe, first reported in the... 

Scheme 16 Azidonitration of Glycals and Conversion of the Nitro Intermediate into Various Glycosyl Donors1276-2861...

Of great use is the azidonitration of glycals by a similar procedure85,86. It is probable, but not certain, that the reaction is initiated by attack of an azide radical at the double bond rather than by a ceric azide species, although the steric bulk of the latter can explain the stereochemical course of the reaction. 

The azidonitration route to 2-amino-2-deoxy sugars from D-glycal should be compared with the alternative route exploiting the addition of nitrosyl chloride (Section 7.2.1.1). Both methods have been tested on the synthesis of intermediates of D-lactosamine, and the azidonitration method was considered more efficient95. 

An alternative to azidonitration has recently been reported (Czemecki and Ayadi 1995). Glycal 10.43 reacted with (diacetoxydiodo)benzene PhI(OAc)2, sodium azide, and diphenyldiselenide PhSeSePh, in dichloromethane solution at room temperature gives the cyrstalline azido phenylselenide 10.46 as the only product in 92% yield. Hydrolysis with N-iodosuccinimide then gives hemiketal 10.47 (87%). A few modifications allow the preparation of analogues with a benzyl protection. These hemiketals may be activated by conversion to imidates. 

Azidonitration of the 5-thio-D-xylal 290 with sodium azide in acetonitrile and, subsequently with ceric ammonium nitrate, afforded a separable 1 1 mixture ofthe D-xylo (291) and the D-lyxo derivatives (292). " " They were converted into the corresponding 1-0-acetyl derivatives 293 and 294, on treatment with sodium acetate in acetic acid. In a similar way, azidonitration of 3,4-di-0-bcnzoyl-5-thio-D-ribal (295) afforded a mixture of the corresponding 2-azido-l-O-nilro derivatives that were converted, without separation, into the corresponding 1-O-acetyl derivatives, from which a 3 7 mixture of the a and anomers of 296 could be isolated in 72% y ield.- The steric arrangement of the axially oriented 3-0-benzoyl substituent in the erythro compoimd 295, which in compound 290 occupies an equatorial position, explains the differences found in the addition of the azido group at C-2 of both glycals. 

Amino-2-deoxy glycopyranosides may also be obtained by azidonitration of glycals using ceric ammonium nitrate 24... 

Acetamido-4-amino-2,4,6-trideoxy-D-galactopyranosyl precursors such as trichltHoacetimidate 76, were prepared from ethyl 1-thio-a-D-mannopyranoside. Key steps were stereoselective reduction of oxine 74 (available in 5 steps), and azidonitration of glycal 75 (Scheme 23). Their application in the synthesis of disaccharides is covered in Chapter 3. Benzyl 2,4-diacetamido-3-6>-benzyl-2,4,6-trideoxy-P-D-galactopyranoside has been synthesized in 5 steps from benzyl 2-acetamido-3-6>-benzyl-2-deoxy-P-D-galactopyranoside. ... 

Azidonitration of D-glycal triacetates was the first step in a synthesis of C-glycosyl derivatives of 2-amino-2-deoxy sugars which is covered in Chapter 9. yJ-o-Glucopyranoside tetranitrate and -cellobiose octanitrate are referred to In Chapter 21. 

Glycals have been extensively used for the synthesis of a wide range of carbohydrate derivatives such as C-, S-/ and N-glycosides, cyclopropanated carbohydrates, and natural products. In particular, the Perrier and the azidonitration reactions, together with Danishefsky s glycal methodology (through epoxides), represent the most widely exploited applications. 

A convenient method for the removal of nitrate groups from the anomeric position, e.g., those introduced by azidonitration of glycals, involving treatment with benzenethiol and ethyldiisopropylamine, has been developed. A study on denitration with hydioxylamine in pyridine established that, whereas trinitrocellulose reacts at the 2-position with high selectivity, the denitration of monomeric model compounds (tetranitrates of methyl a- and P-D-glucopyranoside, methyl a-D-mannopyranoside, methyl p-D-galactoside) takes place... 

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