Nicolas Birlirakis

Dissecting the mechanisms of a class of chemical glycosylation using primary 13C kinetic isotope effects

Although arguably the most important reaction in glycoscience, chemical glycosylations are among the least well understood of organic chemical reactions, resulting in an unnecessarily high degree of empiricism and a brake on rational development in this critical area. To address this problem, primary 13C kinetic isotope effects have now been determined for the formation of β- and α-manno- and glucopyranosides using a natural abundance NMR method. In contrast to the common current assumption, for three of the four cases studied the experimental and computed values are indicative of associative displacement of the intermediate covalent glycosyl trifluoromethanesulfonates. For the formation of the α-mannopyranosides, the experimentally determined KIE differs significantly from that computed for an associative displacement, which is strongly suggestive of a dissociative mechanism that approaches the intermediacy of a glycosyl oxocarbenium ion. The application of analogous experiments to other glycosylation systems should shed further light on their mechanisms and thus assist in the design of better reactions conditions with improved stereoselectivity. Chemical glycosylations are perhaps the most important reactions in glycoscience, but the mechanisms are not well understood. Here, quantum chemical calculations combined with natural-abundance NMR measurements of 13C kinetic isotope effects reveal both associative and dissociative mechanisms at the extremes of a continuum that depends on the relative stereochemistry of the substrate and the anomeric configuration of the product.

Siderophore peptide, a new type of post-translationally modified antibacterial peptide with potent activity

Microcin E492 (MccE492, 7886 Da), the 84-amino acid antimicrobial peptide from Klebsiella pneumoniae, was purified in a post-translationally modified form, MccE492m (8717 Da), from culture supernatants of either the recombinant Escherichia coli VCS257 strain harboring the pJAM229 plasmid or the K. pneumoniae RYC492 strain. Chymotrypsin digestion of MccE492m led to the MccE492m-(74–84) C-terminal fragment that carries the modification and that was analyzed by mass spectrometry and nuclear magnetic resonance at natural abundance. The 831-Da post-translational modification consists of a trimer of N-(2,3-dihydroxybenzoyl)-l-serine linked via a C-glycosidic linkage to a β-d-glucose moiety, itself linked to the MccE492m Ser-84-carboxyl through an O-glycosidic bond. This modification, which mimics a catechol-type siderophore, was shown to bind ferric ions by analysis of the collision-induced dissociation pattern obtained for MccE492m-(74–84) by electrospray ion trap mass spectrometry experiments in the presence of FeCl3. By using a series of wild-type and mutant isogenic strains, the three catechol-type siderophore receptors Fiu, Cir, and FepA were shown to be responsible for the recognition of MccE492m at the outer membrane of sensitive bacteria. Because MccE492m shows a broader spectrum of antibacterial activity and is more potent than MccE492, we propose that by increasing the microcin/receptor affinity, the modification leads to a better recognition and subsequently to a higher antimicrobial activity of the microcin. Therefore, MccE492m is the first member of a new class of antimicrobial peptides carrying a siderophore-like post-translational modification and showing potent activity, which we term siderophore-peptides.

Oxidative cleavage of unsaturated 1,2-diols using chiral lead-tetracarboxylates obtained by in situ metathesis

Abstract A combination of an acetate metathesis/cascade transformations process, providing ring enlarged systems decorated with a chiral auxiliary, is presented. The use of a chiral carboxylic acid, such as ( S )-2-acetoxypropionic acid, gives diastereomeric mixtures when performed in the racemic series, offering the possibility of a chemical resolution.

INFLUENCE OF THE SUBSTITUTION PATTERN ON THE PB(OAC)4 MEDIATED OXIDATIVE CLEAVAGE OF STEROIDAL 1,2-DIOLS

Abstract A rapid increase in molecular complexity, modulated by the substitution pattern, upon treatment of steroidal unsaturated diols with lead tetraacetate is presented. The steric and electronic factors involved in these cascade type transformations are investigated, the products serving as useful mechanistic probes.

DIPOLAR SPECTRAL DENSITIES FROM OFF-RESONANCE 1H NMR RELAXATION MEASUREMENTS

Abstract The use of an off-resonance rf magnetic field in cross-relaxation measurements opens new ways for the study of dipolar proton-proton spectral densities, basic indicators of local motions. The effective relaxation rates in these experiments can be expressed as weighted averages of longitudinal and transverse terms. By varying the value of the spin-lock frequency in ROESY-type pulse sequences one determines these terms accurately and thus has access to the spectral density function at three frequencies, without any assumption on the type of motion. Experimental illustrations are given.

Iodobenzene diacetate-mediated hetero-domino transformations

Abstract Treatment of a series of unsaturated diols with iodobenzene diacetate (PhI(OAc) 2 ), in various solvents gave cyclic ene-acetals by a sequential oxidative cleavage-intramolecular [4+2] cycloaddition. The reaction is easy to perform, can be scaled up safely and occurs efficiently irrespective of the diol stereochemistry.

Studies towards the taxoid diterpene ABC-ring system: practical access to highly functionalized enantiomerically pure analogues of major group representatives†

Abstract Short routes for practical syntheses of enantiopure taxoid subunits which possess oxygenation at sites appropriate for further elaboration into various members of the major taxoid families are described along with detailed structure elucidation.

Pb(OAc)4 mediated hetero-domino transformations: can any unsaturated 1,2-diol be regarded as a substrate?

We report the oxidative cleavage of unsaturated diols 6 and 7 derived from monocyclic precursors, used as substrates to determine the limits of the olefin as a source of diversity.

Lead tetraacetate mediated domino reactions on (R)-(−)-carvone-derived bicyclic unsaturated 1,2-diols and further rearrangements

Abstract Carvone derived octaline diols 1 and 2 were subjected to Pb(OAc)4-mediated glycol fission conditions. The isolable ‘half-cascade’ intermediate 3 was subjected to ozonolysis in methylene chloride, and subsequent basic treatment, providing bis-angularly substituted bicyclic lactone 5 via an intramolecular Cannizzaro type oxidoreduction. The ring expanded products 4 and 6 subjected to basic treatment afforded bicyclic aldols 8 and 7, respectively, via a fused-to-bridged ring system interchange. The methods described here represent efficient approaches to a variety of conveniently functionalized chiral backbones offering chemoselectivity. The mechanistic pathways involved in both the oxidative cleavage induced domino transformations and in the base-induced ring-system interchange reactions are discussed.

PB(OAC)4 MEDIATED OXIDATIVE CLEAVAGE OF STEROIDAL UNSATURATED 1,2-DIOLS: INFLUENCE OF THE ANGULAR SUBSTITUTION

Abstract To evaluate the effect of angular substitution at C-10, the required precursor 2b was prepared by the methods described earlier and subjected to oxidative cleavage with lead tetraacetate.