Chemistry of 4-fluoroglutamic acid. Part 1. A critical survey of its syntheses: an attempt to optimize reaction conditions for large-scale preparation

Synthetic methods for the preparation of 4-fluoroglutamic acid (1) are reviewed. Theutility of every method for routine laboratory work is evaluated with respect to theaccessibility of starting materials and reagents, the overall yields and also from theviewpoint of safety. Optimum reaction conditions are given for the selected pathway,suitable for large-scale synthesis.     

Composite alignment media for the measurement of independent sets of NMR residual dipolar couplings

The measurement of independent sets of NMR residual dipolar couplings (RDCs) in multiple alignment media can provide a detailed view of biomolecular structure and dynamics, yet remains experimentally challenging. It is demonstrated here that independent sets of RDCs can be measured for ubiquitin using just a single alignment medium composed of aligned bacteriophage Pf1 particles embedded in a strained polyacrylamide gel matrix. Using this composite medium, molecular alignment can be modulated by varying the angle between the directors of ordering for the Pf1 and strained gel matrix, or by varying the ionic strength or concentration of the Pf1 particles. This approach offers significant advantages in that greater experimental control can be exercised over the acquisition of multi-alignment RDC data while a homogeneous chemical environment is maintained across all of the measured RDC data.     

Synthesis and biological evaluation of nucleosides containing 8-amino-imidazo[1,2-α]pyrazine as an isosteric replacement for adenine

A number of novel C-nucleosides related to purine derivatives are described in which the purine moiety has been replaced by the isosteric heterocycle, 8-aminoimidazo[1,2-α]pyrazine. The nucleosides prepared include the ribo, 3′-deoxy, 2′,3′-dideoxy, and 2′,3′-unsaturated derivatives. These C-nucleosides represent derivatives containing acid stable glycosyl bonds and they can be considered as analogs of adenine- or 3-deazaade-nine-containing nucleosides. Preparation of the parent ribonucleoside was accomplished by reaction of the C-l functionalized sugar, (2ξ)-1-amino-3,6-anhydro-l-deoxy-4,5-O-isopropylidene-7-O-trityl-D-allo-heptitol with 2,3-dichloropyrazine, followed by ring closure to the 8-chloroimidazo[1,2-α]pyrazine nucleoside, conversion to the 8-amino derivative and deblocking. A single crystal X-ray structure of the parent 8-amino-3-(β-D-ribofuranosyl)imidazo[1,2-α]pyrazine is described and the conformation compared to that of formycin. The sugar-modified analogs were prepared by subsequent functional group manipulations on the sugar moiety. Biological evaluation against HIV in H9 T-lymphoid cell culture showed the nucleosides to be devoid of significant antiviral activity compared to DDA. The 3-deazaadenosine analog also demonstrated weak suppression of mouse splenic NK activity toward YAC cells (mouse lymphoma cell targets). The imidazo[1,2-α]pyrazine analog of 3-deazaadenosine showed antiinflammatory activity in vivo in the rat pleurisy carrageenan model in the same range with 3-deazaadenosine.     

Stereoelective polymerization of D,L-lactide using N-heterocyclic carbene based compounds

A new Zn alkoxide catalyst supported by an N-heterocyclic carbene rapidly polymerizes D,L-lactide (D,L-LA) to heterotactic enriched poly(lactide)(PLA), while the free carbene and analogs instead yield highly isotactic enriched PLA.     

Snapshots of dioxygen activation by copper: the structure of a 1:1 Cu/O(2) adduct and its use in syntheses of asymmetric Bis(mu-oxo) complexes

The X-ray structure of a 1:1 Cu/O(2) adduct revealed side-on (eta(2)) O(2) coordination. Density functional calculations corroborated the structure, indicated a significant contribution of a Cu(III)-(O(2)(2-)) resonance form, and provided insights into the key bonding interactions. Reaction of a 1:1 adduct supported by a slightly different beta-diketiminate ligand with Cu(I) reagents resulted in the formation of novel asymmetric bis(mu-oxo) complexes that were identified by EPR, UV-vis, and Raman spectroscopy, as well as by an X-ray structure in one instance.     

Pyrrolopyrimidine nucleosides VII. A study on electrophilic and nucleophilic substitution at position six of certain pyrrolo[2,3-d] pyrimidine nucleosides

A study involving the reactivity of the pyrrolo[2,3-d] pyrimidine ring system at position 6 with another exocyclic group (CN or -NH₂) already residing at C5 has established that hydrogen and bromine are susceptible to electrophilic and acid-catalyzed nucleophilic substitution, respect-tively. In one instance a strong nucleophile (hydrazine) gave nucleophilic substitution at position 6 which was followed by a reaction with the o-nitrile group to afford the tricyclic nucleoside 4,5-diamino-8-(β-D-ribofuranosyl)pyrazolo[3′, 4′ :5,4] pyrrolo[2,3-d] pyrimidine (4).     

New routes to 1-deoxy-(3,4-dihydro-7,8-dimethyl-2,4-dioxopyrimido[4,5-b]-quinolin-10(2H)yl)-D-ribitols (5-deazariboflavins)

The acid-catalyzed reaction of 6-(N-D-ribityl-3,4-xylidino)uracil ( 1 ) with trimethyl ortho-formate yields a bis(methoxymethylene) derivative ( 2 ), which is readily deprotected to give 5-deazariboflavin ( 3 ). Correspondingly, 5-methyl-5-deazariboflavin ( 6 ) is produced by cyclization of the tetraacetate of 1 with acetyl chloride in the presence of stannic chloride followed by deacetylation.     

Mechanistic studies on the formation and reactivity of dioxygen adducts of diiron complexes supported by sterically hindered carboxylates

Dioxygen activation by enzymes such as methane monooxygenase, ribonucleotide reductase, and fatty acid desaturases occurs at a nonheme diiron active site supported by two histidines and four carboxylates, typically involving a (peroxo)diiron(III,III) intermediate in an early step of the catalytic cycle. Biomimetic tetracarboxylatodiiron(II,II) complexes with the familiar "paddlewheel" topology comprising sterically bulky o-dixylylbenzoate ligands with pyridine, 1-methylimidazole, or THF at apical sites readily react with O(2) to afford thermally labile peroxo intermediates that can be trapped and characterized spectroscopically at low temperatures (193 K). Cryogenic stopped-flow kinetic analysis of O(2) adduct formation carried out for the three complexes reveals that dioxygen binds to the diiron(II,II) center with concentration dependences and activation parameters indicative of a direct associative pathway. The pyridine and 1-methylimidazole intermediates decay by self-decomposition. However, the THF intermediate decays much faster by oxygen transfer to added PPh(3), the kinetics of which has been studied with double mixing experiments in a cryogenic stopped-flow apparatus. The results show that the decay of the THF intermediate is kinetically controlled by the dissociation of a THF ligand, a conclusion supported by the observation of saturation kinetic behavior with respect to PPh(3), inhibition by added THF, and invariant saturation rate constants for the oxidation of various phosphines. It is proposed that the proximity of the reducing substrate to the peroxide ligand on the diiron coordination sphere facilitates the oxygen-atom transfer. This unique investigation of the reaction of an O(2) adduct of a biomimetic tetracarboxylatodiiron(II,II) complex provides a synthetic precedent for understanding the electrophilic reactivity of like adducts in the active sties of nonheme diiron enzymes.