Showing papers in "Journal of Organic Chemistry in 1992"

The Osmium-Catalyzed Asymmetric Dihydroxylation: A New Ligand Class and a Process Improvement

Abstract: Two key improvements in the osmiuun-catalyzed asymmetric dihydroxylation have led to a simple procedure which is applicable to a wide range of olefins.

Lactobacillus kefir alcohol dehydrogenase: a useful catalyst for synthesis

Abstract: The alcohol dehydrogenase from Lactobacillus kefir simultaneously catalyzes carbonyl reductions and NADPH regeneration in the presence of 2-propanol. Representative synthesis of a number of chiral alcohols was accomplished in good yield and high enantiomeric excess (94-99%). This NADPH-requiring enzyme tranfers the pro-R hydride from the cofactor to the si face of carbonyls to give (r) alcohols

Design, synthesis, and evaluation of an improved enantioselective naproxen selector

Abstract: The design, synthesis, and evaluation of an improved selector for the enantiomers of the nonsteroidal anti- inflammatory drug, naproxen, are described. So as to utilize the principle of reciprocity, two chiral stationary phase (CSPs) derived from (S)-naproxen were produced and HPLC techniques were used to screen candidate naproxen selectors. By determining how the structural features of the candidates influence enantioselective recognition by naproxen, a hypothetical chiral recognition mechanism for enantioselective recognition of naproxen was developed and used to design a selector which was incorporated into a new CSP.

Synthesis and ion selectivity of all conformational isomers of tetrakis[(ethoxycarbonyl)methoxy]calix[4]arene

Abstract: We have found that the conformer distribution in tetra-O-alkylation of 5,11,17,23-tetra-tert-butylcalix[4]arene-25,26,27,28-tetrol by ethyl bromoacetate is remarkably affected by the metal cation present in the base. In general, the cone conformer predominantly results when the base contains template metal cations whereas the partial-cone and 1,3-alternate conformers result when the base contains nontemplate metal cations; in acetone solvent one can realize the change from the 100% cone selectivity to the 100% partial-cone selectivity. By combining the metal template effect with a protection-deprotection method with a benzyl group, we developed synthetic routes to all of the four conformers. Two-phase solvent extraction established that the cone conformer shows Na + selectivity whereas the remaininig three conformers show K + selectivity

Cobalt(II) chloride catalyzed acylation of alcohols with acetic anhydride: scope and mechanism

Abstract: Cobalt(II) chloride catalyzes the acetylation of a variety of alcohols with acetic anhydride in excellent yield. Primary hydroxyl groups can be selectively acylated in the presence of secondary and tertiary ones while the secondary hydroxyl groups can be preferentially acetylated in the presence of tertiary ones. Tertiary alcohols have been found to give ketones, acetoacetates, olefins, and diketene in addition to the acetate

Optimization of solid-phase synthesis of [Ala8]-dynorphin A

Abstract: [Ala 8 ]-dynorphin A, a potent k-selective opioid heptadecapeptide with numerous sensitive side-chain residues, has been prepared by solid-phase synthesis using base-labile N α -9-fluorenylmethyloxycarbonyl (Fmoc) protection and side-chain anchoring to a tris(alkoxy)benzylamide PAL-resin

Highly diastereoselective alkylations of chiral amide enolates: new routes to hydroxyethylene dipeptide isostere inhibitors of HIV-1 protease

Abstract: The nonchelate enforced chiral amide enolates derived from 4-7 react with alkyl iodide and protected α-amino epoxide electrophiles to produce the HIV protease inhibitors 10 and 16-19 with high diastereoselectivity

Synthesis of enantiomerically pure (2'R,5'S)-(-)-1-(2-hydroxymethyloxathiolan-5-yl)cytosine as a potent antiviral agent against hepatitis B virus (HBV) and human immunodeficiency virus (HIV)

Abstract: The synthesis of the ennatiomerically pure 2'R,5'S-(-) form of BCH-189 from L-gulose has been accomplished, and this isomer has been found to exhibit the most potent anti-HIV and anti-HBV activities among the four possible isomers. This is the first example of an L-like nucleoside which is more potent than its D-like isomer

Substitution reactions of a nucleofugal group in heptamethine cyanine dyes. Synthesis of an isothiocyanato derivative for labeling of proteins with a near-infrared chromophore

Abstract: The reactions of dye 1 with MeONa, MeNH 2 , PhONa, PhSNa, PhSH, and 4-H 2 NPhSH to yield the corresponding derivatives 2a-e, hydrodechlorination of 1 to 3 in the presence of EtSNa or PhSNa/Ph 2 PH, and synthesis of the SCN-substituted dye 4, a new reagent for ultratrace detection of proteins, are described

Catalytic asymmetric synthesis of quarternary carbon centers. Palladium-catalyzed formation of either enantiomer of spirooxindoles and related spirocyclics using a single enantiomer of a chiral diphosphine ligand

Abstract: The asymmetric synthesis of eight spirooxindoles and related spirocyclics from the corresponding aryl iodide by palladium catalyzed cyclizations in the presence of the diphosphine ligand (R)-(+)-BINAP is reported

Enhanced coupling efficiency in solid-phase peptide synthesis by microwave irradiation

Abstract: Procedures have been developed for increasing coupling efficiency in solid-phase peptide synthesis by microwave irradiation using a kitchen microwave oven

Preparation of functionalized dialkylzinc reagents via an iodine-zinc exchange reaction. Highly enantioselective synthesis of functionalized secondary alcohols

Abstract: Functionalized dialkylzincs are obtained by the reaction of polyfunctional alkyl iodides with Et 2 Zn in excellent yield. Their treatment with aldehydes, in the presence of the titanium catalyst 6, affords functionalized secondary alcohols with high enantioselectivity

Acyl radicals: intermolecular and intramolecular alkene addition reactions

Abstract: A full study of the use of phenyl selenoesters as precursors to acyl radicals and their subsequent participation in intermolecular and intramolecular alkene addition reactions is detailed. Primary alkyl-, vinyl-, and aryl- substituted acyl radicals generated by Bu 3 SnH treatment of the corresponding phenyl selenoesters participate cleanly un intermolecular addition reactions with alkenes bearing electron-withdrawing or radical-stabilizing substituents at rates that exceed those of the potentially competitive decarbonylation or reduction

Preparation of enantiomerically pure protected 4-oxo .alpha.-amino acids and 3-aryl .alpha.-amino acids from serine

Abstract: The organozinc reagent 13, prepared from the protected β-iodo alanine derivative 3c using ultrasonic activation, is efficiently acylated using acid chlorides in the presence of bis(triphenylphosphine)palladium dichloride to give enantiomerically pure protected 4-oxo-α-amino acids 17 in 39-90% yield (13 examples). Zinc reagent 13 can also be coupled with aryl iodides in the presence of bis(triphenylphosphine)palladium dichloride to give enantiomerically pure protected phenylalanine analogues 26, 29, and 30 in 10-67% yield (11 examples)

Nickel(0)-catalyzed cross coupling of aryl O-carbamates and aryl triflates with Grignard reagents. Directed ortho metalation-aligned synthetic methods for polysubstituted aromatics via a 1,2-dipole equivalent

Abstract: The first Ni(0)-catalyed cross-coupling reactions of aryl O-carbamates and aryl triflates with Grignard reagents (Scheme I) to give diversely polysubstituted aromatics 2d and 2e (table I) which feature regiospecficity based on directed ortho metalation (carbamate), minimal β-hybride elimination (triflate), and dependence on steric and electronic effects are described

Gambieric acids, new potent antifungal substances with unprecedented polyether structures from a marine dinoflagellate Gambierdiscus toxicus

Abstract: From the dinoflagellate Gambierdiscus toxicus, we isolated gambieric acids A, B, C, and D which possess highly potent antifungal properties. The structures of gambieric acids A and B were elucidated from two-dimensional NMR data; they have novel brevetowin-type structures consisting of nine contiguous ether rings (7/6/6/7/9/6/6/6/6) and one isolated tetrahydrofuran

Tris(trimethylsilyl)silane: an efficient hydrosilylating agent of alkenes and alkynes

Abstract: tris(trimethylsilyl)silane adds across the double bond of a variety of mono-, di-, and trisubstituted under free-radical conditions in good yields. The reaction, which proceeds via a free-radical chain mechanim, is highly regioselective (anti-Markovnikov). Addition to prochiral olefins bearing an ester group is highly stereoselective. The factors that control the stereochemistry have been discussed in term of preferred conformations of the intermediate carbon-centered radicals and are thought to be of steric origin

Reaction of fullerenes and benzyne

Abstract: Benzyne reacts with C 60 to form a series of compounds [C 60 +(C 6 H 4 ) n ], n=1,2,3,4, identified by mass spectrometry; the monoaddition product was isolated by chromatography and structurally characterized by 1 H- and 13 C-NMR spectroscopies

Samarium(II) iodide-induced reductive cyclization of unactivated olefinic ketones. Sequential radical cyclization/intermolecular nucleophilic addition and substitution reactions

Abstract: Samarium (II) iodide in the presence of HMPA effectively promotes the intramolecular coupling of unactivated olefinic ketones by a reductive ketyl-olefin radical-cyclization process. The reaction is quite for the formation of 5- and 6-membered carbocycles and even provides modest yields in less facile cyclization processes as evidenced by the generation of methylcyclooctanol via an 8-endo cyclization. Sequential radical cyclization-intermolecular nucleophilic addition/substitution process set the SmI 2 reaction apart from its radical-chain,photochemical, and electrochemical counterparts

Palladium-catalyzed arylation of unsymmetrical olefins. Bidentate phosphine ligand controlled regioselectivity

Abstract: The palladium-catalyzed arylation of several unsymmetrical olefins by aryl triflates in the presence of bidentate phosphine ligands is described. The use of these ligands increases the influence that electronic factors have in determining the regioselectivity of the reaction. The catalyst performances allow a revisiting of the scheme that describes the regioselectivity outcome in Heck-type reactions. Furthermore, a general mechanism for the palladium-catalyzed arylation of olefins is proposed

Affinity capillary electrophoresis can simultaneously measure binding constants of multiple peptides to vancomycin

Abstract: Affnity capillary electrophoresis is a sensitive and convenient technique for studying molecular recognition involving low molecular weight receptors