Conformational Plasticity in Glycomimetics: Fluorocarbamethyl-L-idopyranosides Mimic the Intrinsic Dynamic Behaviour of Natural Idose Rings

Sugar function, structure and dynamics are intricately correlated. Ring flexibility is intrinsically related to biological activity; actually plasticity in L-iduronic rings modulates their interactions with biological receptors. However, the access to the experimental values of the energy barriers and free-energy difference for conformer interconversion in water solution has been elusive. Here, a new generation of fluorine-containing glycomimetics is presented. We have applied a combination of organic synthesis, NMR spectroscopy and computational methods to investigate the conformational behaviour of idose- and glucose-like rings. We have used low-temperature NMR spectroscopic experiments to slow down the conformational exchange of the idose-like rings. Under these conditions, the exchange rate becomes slow in the (19) F NMR spectroscopic chemical shift timescale and allows shedding light on the thermodynamic and kinetic features of the equilibrium. Despite the minimal structural differences between these compounds, a remarkable difference in their dynamic behaviour indeed occurs. The importance of introducing fluorine atoms in these sugars mimics is also highlighted. Only the use of (19) F NMR spectroscopic experiments has permitted the unveiling of key features of the conformational equilibrium that would have otherwise remained unobserved.

gem-Difluoro-carbasugars, the cases of mannopyranose and galactopyranose

5a-Difluoro-5a-carbamannopyranose (gem-difluoro-carbamannopyranose) and 5a-difluoro-5a-carbagalactopyranose (gem-difluoro-carbagalactopyranose), close congeners of their respective natural sugars, in which the endocyclic oxygen atom has been replaced by a gem-difluoromethylene group, were synthesized from D-mannose and D-galactose, using a rearrangement strategy.

Structural characterisation of flavonoids and flavonoid-O-glycosides extracted from Genista tenera by fast-atom bombardment tandem mass spectrometry

Genista tenera is a plant native to the Madeira Island (Portugal). From the ethanol extract of its powdered aerial parts, two flavones, three isoflavones and one 7-O-glucosyl isoflavone were isolated. A mass spectrometric study of these compounds was performed using liquid secondary ion mass spectrometry (LSIMS) in combination with high-energy collision-induced dissociation (CID) and tandem mass spectrometry (MS/MS). Characteristic fragmentation patterns were observed in all the investigated compounds; the loss of small neutral species from the protonated molecules was useful for identifying the presence of specific functional groups in the A- and B-rings. In order to help to establish the proposed structures, NMR and UV studies were also performed.

Bioactive humulene derivatives from Asteriscus vogelii

The bioactive fractions obtained from the extract of Asteriscus vogelii afforded a new humulene derivative, the 8-oxo-6,7,9,10-tetrahydrohumulen-1,12-olide (1), in addition to the known asteriscunolides A (2), C (3) and D (4), and the acids 8-oxo-alpha-humula-6Z,9Z-dien-12-oic acid (5), 8-oxo-alpha-humula-6E,9Z-dien- 12-oic acid (6) and 8-oxo-alpha-humula-6E,9E-dien-12-oic acid (7), characterized as their methyl esters. Evaluation of their phytotoxic and cytotoxic activities was accomplished. Compounds 3 and 4 gave the highest inhibition of plant cell cultures and of the plant Lemna paucicostata. Compound 4 was also the most active against P-338 lymphoma in mice, A-549 carcinoma of human lung, HT-29 carcinoma of human colon and MEL-28 human melanoma.

Synthesis of pseudo-C-nucleosides

Whereas the reaction of 1,2-O-isopropylidene-alpha-D-xylo-5-hexulofuranuronamide (1) with the Wittig reagent ethoxycarbonylmethylenetriphenylphosphorane gave 3-(1,2-O-isopropylidene-beta-L-threofuranos-4-yl)maleimide (2, 15%) and ethyl 5-carbamoyl-5,6-dideoxy-1,2-O-isopropylidene-alpha-D-xylo-hept-5-enof uranuronate (3, 76%), a similar reaction of 3-O-benzyl-1,2-O-isopropylidene-alpha-D-ribo-5-hexulofuranuronamide++ + (4) gave only 3-(3-O-benzyl-1,2-O-isopropylidene-alpha-D-erythrofuranos-4-yl)mal eimide (5), 80%), and that of 3-O-benzyl-1,2-O-isopropylidene-alpha-D-xylo-5-hexulofuranuronamide++ + (6) gave only ethyl 3-O-benzyl-5-carbamoyl-5,6-dideoxy-1,2-O-isopropylidene- alpha-D-xylo-hept-5-enofuranuronate (7, 85%). The formation of the maleimide ring depended on the orientation and substitution of HO-3'. Compounds 2 and 5 are analogous of showdomycin.