Glycals pyranose-derived

Coupled with his achievements in carrying out highly regio and stereo selective additions to glycals (1,2-unsaturated pyranose derivatives) to give glycosides of 2-amino-2-deoxy sugars, NMR spectroscopy enabled Lemieux to lead the subject into the next phase of its development. 

Pyranose-derived glycals are also useful as substrates in C-glycosidations with pyrimidinyl mercuric acetates. As shown in Scheme 4.1.8, Farr, eta/.,7 carried out such reactions utilizing 3-O-silyl protected glycals. Palladium acetate was used as the catalyst and no salt additives were used. Compared to the results presented in the previous four schemes, the illustrated coupling proceeded cleanly with the formation of an 84% yield of a p-C-glycoside. 

Parker has outlined an elegant, enantioselective synthesis of L-vancosamine derivatives commencing from noncarbohydrate precursors (Scheme 17.38) [116]. This approach features a diastereoselective allenylstannane addition and W(CO)5-catalyzed cycloisomerization to construct the pyranose core. Oxidative cyclization of the C4-carba-mate 128 is performed with 10 mol% Rh2(OAc)4 and proceeds stereospecifically to give the crystalline oxazolidinone 129 (86%). All told, synthesis of this useful L-vancosa-mine glycal equivalent covers seven steps from (S)-(-)-ethyl lactate 127 and is accomplished in 44% overall yield. 

To obtain this type of D-glucosamine donor in a stereocontrolled manner, another glycal derivative, namely 3-0-benzyl-2-deoxy-5,6-O-isopropyli-dene-D-araW o-l,4-anhydro-hex-l-enitol (151), was chosen as starting material because it is easily prepared from D-mannose and because the j8-face of the double bond is very hindered.79 As anticipated, azido-phenylselenyla-tion of 151 afforded an a//3 mixture of phenyl 2-azido-2-deoxy-l-seleno-D-glucofuranosides 152 in high yield. In order to obtain the glucosamine equivalent in pyranose form, compound 152 was treated under acidic conditions in the presence of mercury acetate to effect simultaneous cleavage of... 

The use of DMD in carbohydrate epoxidations has also been explored. Pyranose glycals (117)-(119), which with peroxy acids do not lead to isolable oxirane derivatives due to reaction with solvent or acid, can now be epoxidized with DMD to the corresponding oxiranes (120)-(122) without side reactions in excellent yields and stereoselectivity (Scheme 23) <89JA6661>. 

The synthesis and use of glycal derivatives in the preparation of staurosporine and en/-staurosporine is mentioned in Chapter 24 and an investigation into the isomerization of 2-oxo-pyranosides in pyridine affording a hydroxyenone product is covered in Chapter 15. The synthesis of a 1,2-unsaturated C-2 branched, 3,4-annulated sugar from a 2,3-unsaturated pyranose bearing a 4-0-allyl substituent is covered in Chapter 14. 

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