Addition Reactions of Glycal Esters: Access to Glycosyl Donors of Kdo,d-glycero-d-talo- and d-glycero-d-galacto-2-Octulosonic Acid Residues

ABSTRACT

Addition reactions of O-acetylated glycal esters of Kdo mono-, α-(2→8)- and α-(2→4)- linked Kdo disaccharide derivatives 1a - c with NIS in acetic acid afforded good yields of trans-diaxial as well as minor amounts of trans-diequatorial and cis-configured 2-O-acetyl-3-deoxy-3-iodo derivatives, which were efficiently reduced with Bu₃SnH/AIBN to give the corresponding per-O-acetylated Kdo methyl ester derivatives. Similar reactions of 1a with NBS or NCS furnished the trans-diaxial 2-O-acetyl-3-bromo-3-deoxy- as well as 3-chloro-3-deoxy derivatives as the main products. Reaction of 1a with NBS in aqueous MeCN provided the 2,3-trans-bromohydrin derivative 11c, which upon treatment with DBU in MeCN gave the elimination product 11 and the α-2,3-anhydro derivative 12 as a suitable donor of glycosides with D-glycero-D-talo- or D-glycero-D-galacto configuration, respectively.     

Wavelength-independent ultrafast dynamics and coherent oscillation of a metal–carbon stretch vibration in photodissociation of Cr(CO)6 in the region of 270–345 nm

In the group-6 metal hexacarbonyls a number of metal-to-ligand charge-transfer (MLCT) and ligand-field (LF or d → d) states can be excited in the near UV. The latter are repulsive. In equilibrium geometry, most of them are higher than the MLCT states. We probed the dynamics of photodissociation of M(CO)₆ → M(CO)₅ + CO (M = Cr; some data also for M = Mo) with improved time resolution (10–40 fs), pumping at different wavelengths (mainly 270–345 nm) and probing by nonresonant photoionization. The initial relaxation (e.g. within 12.5 fs from T_1u excited at 270 nm) is assigned to direct crossing over to the repulsive surface, from where the subsequent dissociation is also remarkably fast (18 fs in this example). That is, there is no detour via the lowest excited singlet state, in contrast to the usual assumption. Also with 318 and 345 nm excitation a direct MLCT → LF relaxation seems to occur before dissociation. The product M(CO)₅ is generated in the S₁ state, also at pump wavelengths (345 nm) with barely sufficient energy. It relaxes to S₀ through a Jahn–Teller induced conical intersection along pseudorotation coordinates, which stimulates a coherent oscillation in S₀ in this vibration. A higher-frequency oscillation, assigned to totally symmetric MC stretch vibrations, is already found in the Franck–Condon region; it persists (with different wavenumbers) also during dissociation and over the subsequent product states. This vibration is transverse to the valley of dissociation, which is barrierless. The wavelength-independent mechanism also implies that there is no triplet contribution (which was previously supposed at long wavelengths) to photochemical dissociation of the hexacarbonyls.     

Studies on DMSO-containing carbanilation mixtures: chemistry,oxidations and cellulose integrity

The oxidative effect of carbanilation mixtures containing dimethylsulfoxide (DMSO) was demonstrated by means of alcohol model substances in which competitive carbanilation was prevented due to steric hindrance of the hydroxyl function, rendering those compounds specific probes for oxidation effects. Dimethylsulfonium ions and derived ylide species were shown to be the actually oxidizing species according to trapping methodology using lipophilic olefins which were converted into the corresponding cyclopropane and epoxide derivatives. The experimental data were in good agreement with DFT computations carried out on the B3LYP/6-311+G(d,p) level of theory. The direct interaction of cellulose and sulfoxide solvent was proven by means of methyl-(2-naphthyl)sulfoxide (MNSO) as a model for DMSO, which caused introduction of UV-detectable methylthionaphthyl ether moieties into the cellulose, formed in Pummerer-type side reaction paralleling the chemical behavior of DMSO. A facile color test—responding to sulfoxide-derived oxidizing species—was developed to assess the suitability of carbanilation conditions with regard to cellulose oxidation and degradation. DMSO-based carbanilation systems have to be used with great caution for determination of molecular weight parameters and for similar purposes which require complete maintenance of the cellulose integrity. Cellulose oxidation/degradation by DMSO-derived intermediates upon carbanilation can be minimized but cannot be avoided completely. Thus, if cellulose integrity is an issue as it is in cellulose analytics, it is recommended to replace DMSO by solvent components of similar solution behavior but without the inherent danger of generating oxidants, such as pyridine or DMAc, whenever possible.

A Novel Method for Analysis of Xanthate Group Distribution in Viscoses

Analytical monitoring of xanthation in the viscose process along with xanthate group analysis in the viscose material is a long-debated problem in cellulose chemistry. The task is rendered extremely intricate by the lability of the starting material and the harshness of the reaction medium, which adds to a lack of suitable analytical approaches. In a four-years' endeavor in our lab, a method is being developed which allows to analyze the distribution of xanthate groups in viscoses relative to the anhydroglucose units and along the cellulose chain. In a first step the xanthate groups are stabilized by alkylation, which was optimized towards quantitative conversion. In a second step, the remaining free hydroxyl groups are protected by carbanilation, followed by selective removal of the stabilized xanthate groups. Steps two and three thus generate an inverse image of the initial xanthate pattern. In the forth and fifth step, the liberated hydroxyl groups are methylated, and the carbanilates are removed, so that in the overall process the xanthates were replaced by methyl groups. All reaction steps have been comprehensively tested with regard to completeness of conversion and orthogonality of the protecting groups.     

A facile synthesis of 2,7‐diazapyrene

2,7‐Diazapyrene is synthesized in three high‐yield steps from commercially available 1,4,5,8‐naphthalene tetracarboxylic dianhydride, which first reacts with concentrated ammonium hydroxide solution at room temperature to give 1,4,5,8‐naphthalenetetracarboxylic diimide (96%). The latter compound is subsequently reduced with borane in refluxing tetrahydrofuran to give 1,2,3,6,7,8‐hexahydro‐2,7‐diazapyrene (77%), which in turn is oxidized with manganese dioxide in refluxing benzene giving 2,7‐diazapyrene (71%).     

Synthesis and purity assessment of tetra- and pentaacyl lipid A of Chlamydia containing (R)-3-hydroxyicosanoic acid

Based on structural data of lipid A from Chlamydia trachomatis strains, chemically pure tetra- and pentaacyl 1,4′-bisphosphoryl as well as the related 4′-monophosphoryl derivatives of lipid A were synthesized. (R)-3-Hydroxyicosanoic acid as a chiral constituent was prepared via Noyori-reduction of methyl-3-oxoicosanoic acid. Synthetic intermediates were O-acylated with myristoic acid residues at positions 3 and 3′ and N-acylated with (R)-3-hydroxyicosanoic acid at both glucosamine units. Efficient purification methods for highly hydrophobic long-chain tri-, tetra- and pentaacyl progenitors of lipid A have been developed. Purity and homogeneity of the synthetic target compounds were confirmed by NMR and MS-data as well as a sensitive immunostaining approach. The tetra- and pentaacyl species serve as biomedical probes to investigate the endotoxic potential of chlamydial lipid A and to clarify its role in Chlamydia associated infections.     

Investigation of the unusual behavior of projectile fragments using nuclear chemistry techniques

In recent years the question of the unusual behavior of projectile fragments formed in the interaction of relativistic heavy ions with copper nuclei was studied using nuclear chemistry techniques. As targets we used various copper disk arrangements. It is not the intention of this conference contribution to give a complete account of the entire procedure and its results. However, we want to present a few more recent experimental findings, some of them even in a preliminary form. We want to stimulate the discussion about the entire problem which might be part of the so-called anomalon phenomenon, as observed in the field of relativistic heavy ion interactions with a few other techniques such as nuclear emulsions or bubble chambers.     

Novel tocopheryl compounds. Part 15: One-pot formation of furotocopheryl derivatives

Reaction of tocopheryl bromide 2a or chromanyl bromide 2b with triphenyl phosphine produced phosphomium salt intermediates (3a-b), which further reacted with acyl chlorides to novel furotocopherol compounds 4-11 in good yields. The cyclization proceeded according to a two step esterification-Wittig mechanism. Similarly, furotocopheryl dimer 12 was prepared starting from oxalyl chloride. The coupling of tocopheryl phosphonium salt 3a onto modified polystyrene provided a new, vitamin E-loaded resin.