Furanoid glycals

Reactions of some furanoid glycals having free 3-hydroxyl (518 and 522) and 3-0-benzyl groups (520 and 524) with AcOF gave different products in the former and the latter groups of compounds, respectively, addition reaction occurred mainly from the same (to give 519 and 523, respectively) and from the opposite sides [to give 521 and 525 (with 526), respectively] of the substituents at C-3 (see Table V). 

The latter elimination route to D-ribal is illustrative of the approach necessary for the preparation of the sensitive furanoid glycals, which are not accessible via the... 

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. 

Stereocontrolled glycosylation of furanoid glycals with pyrimidine or purine bases has been accomplished via a Lewis acid-mediated sulphenylation279. 

Dihydrooxadiazines can be made in related fashion and they too offer novel access to important products. For example, the (9-silylated D-glucal 70, irradiated at 350 nm in cyclohexane with dibenzyl azodicarboxylate, affords adduct 74 in 71% yield,74 and this with an acid catalyst can be used as a glycosylating agent to make /J-linked 2-amino-2-deoxy-D-glucosides.75 This chemistry also is applicable to furanoid glycals.74 Scheme 6 outlines the regio- and stereoselectivities of these cycloaddition reactions and their applications in synthetically useful processes. 

F. Bravo, M. Kassou, and S. Castillon, Synthesis of erythro and threo furanoid glycals using 5-endo trig selenoetherification as key step, Tetrahedron Lett., 40 (1999) 1187-1190. See also 35 (1994) 5513-5516. 

H.-C. Zhang, M. Brakta, and G. D. Daves, Preparation of l-(tri-n-butylstannyl)furanoid glycals and their use in palladium-mediated coupling reactions, Tetrahedron Lett., 34 (1993) 1571-1574. 

The fluorination and stereochemistry of the reaction of pyranoid and furanoid glycals with acetyl hypofluorite are described41. In related work42 the synthesis of 2-deoxy-2-fluoro-D-galactapyranose by treatment of 2-deoxy-D-galactapyranose with acetyl hypofluorite is described and the 18F-labelled product was similarly prepared through the use of Ac018F. 

The synthesis of the nucleoside 20 was eventually performed by addition of a silylated base to the double bond of the furanoid glycal 19 after activation with iodine. The reaction proceeds with total stereoselectivity and the configuration of the newly introduced stereogenic centers was assigned by H NMR214. 

By the cycloaddition of dibenzyl diazenedicarboxylate and furanoid and pyranoid glycals it is possible to diastereoselectively introduce an amino function at the C-2 position of a carbohydrate29-34. A single diastereomer is produced from furanoid glycals, where the cycloaddition takes place trans to the C-3 hydroxy substituent protected as the silyl ether, e.g., formation of 14-1630. The more reactive bis(2,2,2-trichloroethyl) diazenedicarboxylate can be employed even with less reactive 3-acetyl glycals31. 

One equiv of the furanoid glycal was dissolved in Et O and the solution was cooled to — 78 C. After addition of 2,4,6-triisopropylphenylsulfenyl chloride (1 equiv), the reaction was kept at — 78 °C for 30 min. Silyiated 6-chloropurine (1 equiv) and TMSOTf (1.15 equiv) as Lewis acid were then introduced and the reaction was allowed to warm to rt. Isolation of the products was accomplished by column chromatography and the a,/J-ratio of anomers formed was determined by H NMR spectroscopy of the purified products. 

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]. 

Lithiated glycals can also be transformed into more stable and more s)mthetically useful organozinc reagents [158]. Nevertheless, as has already been shown, even the primary lithiation products of the furanoid glycals 59 can be efficiently trapped by appropriate electrophiles to 60 (O Scheme 19) [159]. 

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%. 

An elimination reaction was also successfully used in the synthesis of furanoid glycals from 2-deoxyribose. The substrates used were 2-deoxy-l-seleno-furanosides 75, which were transformed to their respective glycals 76 via selenoxide elimination in yields ranging from 71 to... 

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]. 

Furanoid glycals are not stable to the acidic conditions used to prepare the pyranoid congeners. Scheme 4 illustrates a very mild method for preparing furanosyl chlorides, which undergo efficient reductive elimination on treatment with lithium in liquid ammonia. Alternatively, a Zn-Ag couple adsorbed on graphite will reduce furanosyl bromides and the corresponding chlorides react with the radical anion prepared from naphthalene and sodium. ... 

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. ... 

This approach was adopted by Daves for the coupling of iodo derivatives of nitrogen heterocycles with cyclic enol ethers and furanoid glycals in a water/ethanol mixture, using tetrabutylammonium chloride as a promoter (Eq. 5) [10]. Surprisingly, the use of absolute ethanol as reaction solvent was ineffective. 

The rearrangement of /i-methoxyallyl propanoates has been investigated in order to study the stereoelectronic effect of the oxygen in acyclic systems in comparison to the rearrangement of pyranoid and furanoid glycal systems (cf. Section 1.6.3.1.1.4.2.4. p 3457)515. 

Ireland and Vevret developed a route for the synthesis of both (+)- and (-)-nonactic acids, with the stereochemistry at C-6 derived from C-4 of D-gulonolactone and D-mannose, respectively (70). For the synthesis of (+)-nonactic acid 301 (Scheme 40), the furanoid glycal 295 was prepared in 10 steps from D-gulonolactone 292 by fairly straightforward functional group manipulations. The... 

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