Furanoid intermediates

A novel approach to the preparation of hex-2-enos-4-uloses from glycals which proceeds via furanoid intermediates has been described (Scheme 13). ... 

O-isopropylidene derivative (57) must exist in pyridine solution in a conformation which favors anhydro-ring formation rather than elimination. Considerable degradation occurred when the 5-iodo derivative (63) was treated with silver fluoride in pyridine (36). The products, which were isolated in small yield, were identified as thymine and l-[2-(5-methylfuryl)]-thymine (65). This same compound (65) was formed in high yield when the 5 -mesylate 64 was treated with potassium tert-hx Xy -ate in dimethyl sulfoxide (16). The formation of 65 from 63 or 64 clearly involves the rearrangement of an intermediate 2, 4 -diene. In a different approach to the problem of introducing terminal unsaturation into pento-furanoid nucleosides, Robins and co-workers (32,37) have employed mild base catalyzed E2 elimination reactions. Thus, treatment of the 5 -tosylate (59) with potassium tert-butylate in tert-butyl alcohol afforded a high yield of the 4 -ene (60) (37). This reaction may proceed via the 2,5 ... 

Heterogeneous catalysts, particularly zeolites, have been found suitable for performing transformations of biomass carbohydrates for the production of fine and specialty chemicals.123 From these catalytic routes, the hydrolysis of abundant biomass saccharides, such as cellulose or sucrose, is of particular interest. The latter disaccharide constitutes one of the main renewable raw materials employed for the production of biobased products, notably food additives and pharmaceuticals.124 Hydrolysis of sucrose leads to a 1 1 mixture of glucose and fructose, termed invert sugar and, depending on the reaction conditions, the subsequent formation of 5-hydroxymethylfurfural (HMF) as a by-product resulting from dehydration of fructose. HMF is a versatile intermediate used in industry, and can be derivatized to yield a number of polymerizable furanoid monomers. In particular, HMF has been used in the manufacture of special phenolic resins.125... 

Carbene-mediated methylenation of aldonolactones provides a direct route to 1-methylene sugars, which may be used as intermediates for the preparation of furanoid or pyranoid C-glycosyl compounds, or chiral precursors for the synthesis of natural products. 

The biotransformation of (/f,5)-Iinalool by fungi is a useful method for the preparation of natural linalool oxides. The stereospecific conversion of (J ,5)-linalool by Corynespora cassiicola DSM 62475 led to 5/f-configured furanoid linalool oxides and 55-configured pyranoid linalool oxides, both via bS -configured epoxylinalool as postulated intermediate (Figure 12.6). The biotransformation protocol affords an almost total conversion of the substrate with high enantioselectivities and a molar conversion yield close to 100% (Table 12.4). Pure linalool oxides are of interest for lavender notes in perfumery. ... 

When applied to N-acylglycofuranosylamines, the periodate oxidation showed an abnormal uptake of oxidant ( overoxidation ). For example, when oxidized with lead tetraacetate12 and with periodate,1065 N-acetyl-a-D-glucofuranosylamine (15) afforded formaldehyde (indicating a furanose structure), and it consumed more than 5 moles of oxidant per mole. This result can be attributed to subsequent oxidation of the formic acid produced,69 or to the formation,10 by hydrolysis, of the intermediate 2-hydroxypropanedial (tartron-aldehyde) (77) that would then be oxidized. This tendency to undergo overoxidation has been found common for the furanoid N-acyl-gly cos y lam ines.24,25... 

The synthesis of natural products by chirality transfer from carbohydrates has been used for a total synthesis of (-)-(7S)-nonactic acid (199). The furanoid glycal (197) was prepared from D-mannose, which is the appropriate chiral precursor (Scheme 46) (80JOC4259). A [3,3]-sigmatropic rearrangement of the silylated ketene-acetal (198) led to the control of the C-2 configuration. The intermediate furanoid glycal was prepared in ten steps from the carbohydrate precursor. 

An entropy value for the acidic hydrolysis of ethyl 0-D-galactofuranoside was reported by Overend and coworkers.136 The negative value (—7.1 e.u.) was interpreted as being diagnostic of an A-2 mechanism, in contrast to the positive values (+13.7 e.u., mean value) and A-l mechanism found for a large number of pyranosides. The proposed intermediate is shown in formula (34). It was considered that the planar, furanoid ring is capable of... 

Alkoxy radicals produced by homolysis of hypohalites can attack non-activated C-H bonds, and lead to tetrahydro-furanoid structures after basic treatment of the intermediate halo-alcohols. Hypochlorites of [2y,2g] and C(20)-tertiary alcohols [28,2gJ gave 6 8,19- and 18,20-oxido derivatives respectively on irradiation and hydrolysis, but the more reactive hypobromites and hypoiodites are now preferred. Akhtar ef al. [30] distinguish between two modes of reaction of a 6jS-hypobromite (2), generated in situ by treating the alcohol with bromine and silver acetate. Experiments with... 

Besides the conventional methods, the metallo-carbene route to access cyclic compounds has become a versatile tool in sugar chemistry. Synthesis of stavudine 112, an antiviral nucleoside, from an allyl alcohol [101] is realized by a Mo(CO)5-mediated cyclization reaction (O Scheme 26). Molybdenum hexacarbonyl smoothly reacts with the triple bond of 113 to generate the intermediate Mo-carbene, which undergoes a clean cyclorearrangement to yield the furanoid glycal 114. Alkynol isomerization is effected by group-6 transition metal carbonyl complexes [102]. 

Furanoid glycals utilizing elimination of selenoxides were prepared from unusual substrates, the 4-phenylselenyltetrahydrofurans. These intermediates were obtained with good yields from D-glyceraldehyde (O Scheme 21). Their oxidations have led to selenoxides, however, the elimination reaction required heating. The best yields were noted for reactions refluxed in 1,2-dichloroethane and yields ranged from 62 to 95%. 

A brand-new methodology for synthesizing glycals from noncarbohydrate precursors, one based on cyclization of acetylenic alcohols, has emerged from the field of metalorganics. Molybdenum pentacarbonyl-trialkylamine complexes have been found to efficiently catalyze cyclization of l-alkyn-4-ols to substituted dihydrofurans [233,234]. This same transformation has been successfully carried out on asymmetrically substituted alcohols the furanoid glycals 132, 134, and 135 (O Scheme 45) so obtained have in turn been used as intermediates in the synthesis of nucleosides [235]. 

The reactions of organopalladium compounds with glycals are formally like Ferrier rearrangements, but the intermediates appear to be a- and 7i-bonded palladium organometallics rather than stabilised oxocarbenium ions, so the reactions can be carried out on furanoid glycals without yielding furans. ... 

Conformational factors in furanoid sugars can dictate the stability of a frans-benzylthio-sulfonate system toward formation of episulfonium ion. Christensen and Goodman noted that the 3-)S-benzyl-3-thio-D-arabinoside ester (311) requires vigorous conditions for conversion into the mixture of azides (313) that arises by opening of the intermediate ion... 

An illustration of the potential value of 2,3-unsaturated furanoid compounds, particularly in nucleoside synthesis, is the preparation, by way of an epoxy intermediate, of a crystalline compound claimed to be ethyl 3-amino-3-deoxy-DL-arabinofuranoside from racemic 2-eth-oxy-2,5-dihydro-5-(tetrahydropyranyloxymethyl)furan (41, R = OEt,... 

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