Bicyclic C-glycosides

In order to cleave the bicyclic C-glycoside to the desired functionalized dihydropyran, 284 was exposed to sodium amalgam. Syn-dihydroxylation of 285 (OSO4-NMO) gave a mixture of diastereomeric tetrads, which after separation, protection-deprotection procedure and oxidation of the product C-l alcohol, using generated in situ ruthenium tetraoxide, gave the acid 254. 

Elongation of unprotected carbohydrates can be achieved by Knoevenagel condensation employing (3-ketoesters promoted by L-Pro. The subsequent oxa-Michael step gives rise to bicyclic c-glycoside fused tetrahydrofuranes. 

Craig, D, Munasinghem, V R N, Stereoselective template-directed C-glycosidation. Synthesis of bicyclic keto oxetanes via intramolecular cyclization reactions of (2-pyridylthio)glycosidic silyl enol ethers, J. Chem. Soc., Chem. Commun., 901-903, 1993. 

A new general approach to the synthesis of C-glycosidic compounds and C-nucleosides was elaborated by Noyori. In his approach the starting material, unsaturated bicyclic ketone 236, was prepared in a cyclocoupling reaction between 1,1,3,3-tetrabromoacetone and furan, followed by a zinc-copper couple... 

Spiro- and 1,2-fused Bicyclic Systems. Spiroketals involving C-glycosidic structures can more correctly be considered to be chain-extended sugars (Chapter 2, Section 2.3). Sharpless asymmetric dihydroxylation of 5-aryl-2-vinylfuran... 

The six-membered cyclic sugar nitrone 67 was generated in situ from D-ribose derivative 66 and directly trapped with suitable dipolarophiles. Bicyclic isoxazolidines such as 68 were then converted into 1,5-dideoxy-1,5-iminoribitol iminosugar C-glycosides that were tested as glycosidase inhibitors (14JOC4398). 

Addition of an aryl Grignard reagent to the y-lactone led tt> 160. Subsequent treatment with triethylsilane and SnCU led stereoselecdvely to the -C-glycoside 161, convertible to the analogue 162 of nicotinamide tiboside.188 Maeba s group have extended their previous work (see Vol. 25, p. 260, and earlier) to the synthesis of the bicyclic structures 163 (both cis-fiised isomers), the 1,4-thiazine 164, and related compounds. 1 ... 

Only very recently [77], Sinay et al. presented a series of Ferrier-type rearrangements [78] of 5,6-unsaturated sugars to cyclohexenones. In this study, the same catalyst, applied to the reaction of furanyl C-glycoside 216, yielded exclusively, after reduction of the intermediate ketone, cyclohexanol 217 (Scheme 45). A conceivable Claisen rearrangement to a bicyclic product 218 was apparently not observed. 

The bases are either monocyclic pyrimidines or bicyclic purines (see Section 14.1). Three pyrimidine bases are encountered in DNA and RNA, cytosine (C), thymine (T) and uracil (U). Cytosine is common to both DNA and RNA, but uracil is found only in RNA and thymine is found only in DNA. In the nucleic acid, the bases are linked through an A-glycoside bond to a sugar, either ribose or deoxyribose the combination base plus sugar is termed a nucleoside. The nitrogen bonded to the sugar is that shown. 

Reaction of the unsaturated bromoethyl glycoside 216 in the foregoing manner gives the bicyclic product 217,218 and similar treatment of the propargyl ether 219 with a tributyltin radical results in carbon-radical generation and cyclization to afford the tin-containing adduct 220 in 90% yield. On oxidation with osmium tetraoxide and periodate ion, the Sn-C bond is cleaved, and the corresponding ketone 221 is produced in excellent yield.219... 

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