Synthesis of three 3-C-hydroxymethylpentoses with the D-ribo-, D-xylo- and L-lyxo-configurations. Identification of the latter with a monosaccharide isolated from phase I Coxiella burnetii lipopolysaccharide

1,2-O-Isopropylidene-β-D-lyxo-furanose

In the title compound C8H14O5, the pentofuranose five-membered ring has a twisted conformation on two carbon atoms while the five-membered ring of the isopropylidene group has an envelope conformation on an oxygen atom. Hydroxy groups are involved an infinite network of O—H...O hydrogen bonds that leads to the formation of a layer parallel to the (001) plane. Only weak C—H...O contacts exist between neighboring layers.

Total Synthesis, Configuration Assignment, and Cytotoxic Activity Evaluation of Protulactone A

The first total synthesis and absolute configuration assignment of protulactone A (1) has been achieved. Four stereoisomers, 1a, ent-1a, 1b, and ent-1b, of this natural polyketide were prepared by chiral pool synthesis starting from l- and d-arabinose, respectively. The absolute and relative configurations of all isomers were assigned by single-crystal X-ray analysis. Target compounds were screened for their in vitro cytotoxicity toward certain human tumor cells (NCI₆₀ cancer cell line panel).

Construction of quaternary stereocentres on carbohydrate scaffolds

This review describes a glimpse of the various strategies for constructing stereo-defined quaternary centres in densely functionalised carbohydrates moiety of structurally intriguing and biologically potent natural products and building blocks.

Influence of Steric Crowding on Diastereoselective Arabinofuranosylations

The occurrence of arabinofuranosides on the cell surface of Mycobacterium tuberculosis (Mtb) and their significance in controlling disease spurred interest in developing strategies for their diastereoselective synthesis. Mtb uses enzymes to achieve diastereoselectivity through noncovalent interactions. Of the two possible glycosidic linkages, chemically, 1,2-trans linkage is relatively easy to synthesize by taking advantage of neighboring group participation, whereas synthesis of the 1,2-cis linkage is notoriously difficult. In this article, stereochemical effects on the diastereoselectivity of arabinofuranosidation are investigated with thiopyridyl, imidate, and thiotolyl donors as well as differently crowded glycosyl acceptors; subtle differences in the stereochemical environment of the acceptors were observed to alter the diastereoselectivity of the furanoside formation. Results from this endeavor suggest that 1,2-cis arabinofuranosides can be synthesized conveniently by conducting the reaction at lower temperature on sterically demanding and less reactive substrates.

Development of 2-Deoxy-2-[(18)F]fluororibose for Positron Emission Tomography Imaging Liver Function in Vivo

Life-threatening acute liver failure can be triggered by a variety of factors, including common drugs such as acetaminophen. Positron emission tomography (PET) is rarely used to monitor liver function, in part because of a lack of specific imaging agents for liver function. Here we report a new PET probe, 2-deoxy-2-[(18)F]fluororibose ([(18)F]-2-DFR), for use in imaging liver function. [(18)F]-2-DFR was synthesized and validated as a competitive substrate for the ribose salvage pathway. [(18)F]-2-DFR was prepared through an efficient late stage radiofluorination. The desired selectivity of fluorination was achieved using an unorthodox protecting group on the precursor, which could withstand harsh SN2 reaction conditions with no side reactions. [(18)F]-2-DFR accumulated preferentially in the liver and was metabolized by the same enzymes as ribose. [(18)F]-2-DFR could distinguish between healthy liver and liver damaged by acetaminophen. [(18)F]-2-DFR is expected to be a useful PET probe for imaging and quantifying liver functions in vivo, with likely significant clinical utility.

3-De-oxy-1,2-di-O-isopropyl-idene-5-O-tosyl-d-threo-pentofuran-ose

In the crystal structure of the title compound, C(15)H(20)O(6)S, the two independent mol-ecules crystalllize in a chiral setting with two different conformations, twisted (4)T(3) and envelope (4)E, for the furan-ose rings. Weak C-H⋯O contacts strengthen the crystal structure.

Convenient conversion of wheat hemicelluloses pentoses (D-xylose and L-arabinose) into a common intermediate

The transformation of D-xylose and L-arabinose, the two major components of wheat straw and bran, into a unique multifunctional, optically pure, five-carbon synthon has been achieved. The synthetic sequence requires three steps: suitable protection of the hydroxyl groups of the pentoses, introduction of an iodide at the C-5 position and zinc-mediated opening of the furanose ring leading to the formation of a common substituted pent-4-enal.

Synthesis of putative chain terminators of mycobacterial arabinan biosynthesis

The synthesis of a variety of arabinose derivatives that have been modified at C-5 was achieved from d-arabinose. The 5-fluoro and 5-methoxy compounds were converted into the corresponding farnesyl phosphodiesters as putative chain terminators of mycobacterial arabinan biosynthesis. Biological testing of these materials revealed no effective anti-mycobacterial activity.

From Common Carbohydrates to Enantiopure Cyclooctane Polyols and Glycomimetics via Deoxygenative Zirconocene Ring Contraction

D-Arabinose and D-glucose are transformed into the identical vinyl furanoside, whose role is to serve as the precursor to enantiopure cyclooctadienone 6. The key steps of this relay involve a zirconocene-promoted ring contraction and [3,3] sigmatropic rearrangement of an enynol. Subsequently defined are convenient synthetic routes to several cyclooctane-1,2,3-triols, 1,2,3,4,5-pentaols, and structurally related glycomimetics.

Zirconocene-mediated route to enantiopure 9-oxabicyclononanes functionalized on both carbon bridges

[reaction: see text] A zirconocene-mediated ring contraction of 4-vinylfuranosides generated either from d-arabinose or d-glucose is followed by sequential oxidation to the ketone and alkynyl Grignard addition. The resulting cis-cyclobutanediols are subjected in turn to thermal rearrangement and intramolecular oxymercuration-demercuration. The regiochemistry of the final ring closure is controlled by the nature of R.

Carbohydrate-based synthesis of naturally occurring marine metabolites slagenins B and C

[reaction: see text] The first enantioselective syntheses of slagenins B and C, marine metabolites from Agelas nakamurai, starting from L-arabinose have been described.

Diastereoselectivity control in the zirconocene-mediated ring contraction of 4-vinylfuranosides to enantiopure multiply functionalized cyclobutanes

[reaction: see text] The ring contraction induced by treatment of 4-vinylfuranosides with zirconocene in the presence of boron trifluoride etherate can be rendered highly diastereoselective by proper adjustment of substituent stereochemistry. The two competing transition states that are seemingly involved appear to be usefully sensitive to nonbonded steric interactions.

Alpha-LNA (locked nucleic acid with alpha-D-configuration): synthesis and selective parallel recognition of RNA

Alpha-LNA is presented as a stereoisomer of LNA (locked nucleic acid) with alpha-D-configuration. Three different approaches towards the thymine alpha-LNA monomer as well as the 5-methylcytosine alpha-LNA monomer are presented. Different alpha-LNA sequences have been synthesised and their hybridisation with complementary DNA and RNA has been evaluated by means of thermal stability experiments and circular dichroism spectroscopy. In a mixed pyrimidine sequence, alpha-LNA displays unprecedented parallel-stranded and selective RNA binding. Furthermore, a remarkable selectivity for hybridisation with RNA over DNA is indicated.

Synthesis of a bicyclic analogue of AZT restricted in an unusual O4'-endo conformation

The [3.2.0]bicyclic beta-nucleoside analogue 5 has been designed as a conformationally restricted analogue of the anti-HIV drug AZT. The synthesis of 5 as well as its alpha-anomer 29 is hereby described. The synthesis was accomplished from D-arabinose via a modified Corey-Link procedure stereoselectively incorporating the azide moiety as well as a methyl ester function. When the tert-butyldiphenylsilyl group was used as a permanent protecting group, a selective formation of an oxetane ring failed. When using the p-methoxyphenyl group as a permanent protecting group, 5 and 29 were efficiently obtained via a selective reduction of the ester, a nucleobase coupling followed by separation of the anomers and ring-closing procedures. The nucleoside 5 is conformationally restricted in an unusual O4'-endo (East) conformation, which is an intermediate between the North- and South-type conformations. Nevertheless, neither 5 nor 29 displayed any anti-HIV activity.

NMR study of virenose and dihydrohydroxystreptose isolated from Coxiella burnetii phase I lipopolysaccharide

A lipopolysaccharide (LPS) isolated from Coxiella burnettii in virulent phase I contains in its O-polysaccharide chain two unusual sugars, virenose (6-deoxy-3-C-methylgulose) and dihydrohydroxystreptose [3-C-(hydroxymethyl)lyxose]. The sugars were isolated from LPS I, after acid hydrolysis and removal of lipid A, by a combination of HPLC and preparative paper chromatography. Their enantiomeric forms and ring conformations were established from optical rotation and NMR data. Two-dimensional COSY, HSQC, and HMBC as well as one- and two-dimensional NOEs were used to assign all proton and carbon signals in both monosaccharides. Virenose was found to be the D-gulo enantiomer with the 4C1 ring conformation and dihydrohydroxystreptose was shown to be the L-lyxo enantiomer also with the 4C1 conformation. The latter sugar was reported [S. Schramek, J. Radziejewska-Lebrecht, and H. Mayer, Eur. J. Biochem., 148 (1985) 455-461] to be present in LPS I in a furanose form, and it appears that a furanose to pyranose tautomerization took place in the course of the isolation procedure.

Chemical and immunochemical studies on lipopolysaccharides of Coxiella burnetii phase I and phase II

Lipopolysaccharides of Coxiella burnetii phase I and II were comparatively investigated by chemical and immunochemical methods. LPS of phase I (LPS I) and phase II cells (LPS II) show no serological cross reaction, indicating that the serological determinants of LPS II are masked in LPS I. Chemical analysis of LPS I and II show that phase I and II cells can be considered as S and R forms of Coxiella burnetii. The structure of LPS II has recently been elucidated and shows a dimannosylated core of an alpha(1,3)-linked heptose-disaccharide which is attached to a "KDO-like" substance. In enterobacterial core-types, alpha(1,3)-linked heptose-disaccharide is also part of the inner core structure, although the heptose occurring in enterobacterial R cores is the L-glycero-D-manno-heptose. In Coxiella burnetii we have only the rare D-glycero-D-manno-heptose which is the biosynthetic precursor of the former and is in many enteric LPS, present only in addition to L-glycero-D-mannoheptose. In these R-cores, it is occupying mostly terminal positions (Radziejewska-Lebrecht et al., 1981) and is absent from the main chain. The complete structure of LPS I is not yet available, but some important points could recently be clarified. The immunodominant sugars in LPS I are C-3-branched sugars, 6-deoxy-3-C-methyl-L-gulose (L-virenose) and 3-C-(hydroxymethyl)-L-lyxose (dihydro-hydroxy-L-streptose). These two sugars have not been found so far in other lipopolysaccharides and the latter one not previously in any other natural product. Their identification is based on GLC-MS comparison with authentic and synthetic compounds. Both branched sugars (and in addition part of the mannose) are the terminal sugars in LPS I. Sites of attachment of phase I-specific sugars to the LPS II-core are: the 3-position of a branched heptose and, presumably, the 4-position of a terminal D-mannose. The extreme acid-lability of the linkages of both branched sugars was investigated in detail and is caused by the nature of the branched sugars (deoxyhexose with bulky axial substituents; pentofuranose with axial OH-groups). No information is so far available on the (penultimate) sugars to which the branched sugars are linked, but methylation analyses with LPS I, and with the recently described I/CR mutant, which is selectively lacking the virenopyranose, are presently performed.