Experimental and theoretical evidence for cyclic selenurane formation during selenomethionine oxidation

The oxidation products of selenomethionine (SeMet) have been studied via experimental (77)Se NMR and theoretical (77)Se chemical shifts. Four signals are observed: a diastereomeric pair of selenoxides at 840 ppm and two unidentified resonances at 703 and 716 ppm. Theoretical DeltaG and chemical shifts suggest the 703 and 716 ppm resonances correspond to hypervalent selenium heterocycles, called selenuranes, formed by reaction with the amine or acid group of the amino acid and the selenoxide. To identify which of these selenuranes is formed, the amine and acid groups were individually protected. The N-formyl SeMet formed only the selenoxide pair at 840 ppm. The oxidized SeMet methyl ester produced signals at 703 and 716 ppm which are assigned as the Se-N selenurane.     

Synthesis of 1,2 difluoro-1,2-bis(pentafluorophenyl)dichlorane

1,2-Difluoro-1,2-bis(pentafluorophenyl)dichlorane is a new class of organic polyvalent chlorine compound. The closeness of the retention time of this compound and that of chloropentafluoro- benzene made its purification difficult. All attempts to obtain this compound in high yield have failed. 1,2-Difluoro-1,2-bis- (pentafluorophenyl)dichlorane is prepared by fluorination of chloropentafluorobenzene at 128°C with elemental fluorine. It has been characterized by ¹⁹F n.m.r., i.r., mass spectroscopy and elemental analysis.     

Wannier states of molecular impurity in a liquid rare gas

We have measured the vacuum ultraviolet absorption spectra of CH₃I in solid and in liquid krypton in the spectral region 2000–1450 Å. In both phases we have observed two Wannier series n(²E_{$\text{3}\text{2}$}) and n(²E_{$\text{1}\text{2}$}) up to n = 3. Information has been obtained concerning the features of the conduction band in a liquid rare gas.     

Synthesis of lipoxins: Total synthesis of conjugated trihydroxy eicosatetraenoic acids

The first synthesis of a conjugated trihydroxy eicosatetraenoic acid, a possible structure for Lipoxin A is described. A biomimetic approach was utilized.     

Arabinofuranose disaccharide analogs as inhibitors of Mycobacterium tuberculosis

Several octyl 5-O-(α-d-arabinofuranosyl)-α-d-arabinofuranoside disaccharide analogs substituted at the 5-position of the non-reducing end sugar were synthesized and tested in vitro against Mycobacterium tuberculosis (M.tb.), Mycobacterium avium complex (MAC) as well as in a cell free assay system for arabinosyltransferase acceptor/inhibitor activity. A few compounds showed interesting inhibitory activity in the cell free assay as well as against the whole microorganism in vitro.     

The reactions of pyranylidenemethylpyrylium salts with sodium sulfide

Symmetrical pyranylidenemethylpyrylium salts react with sodium sulfide to give, after acidification, pyranylidenemethylthiapyrylium salts. In the case of unsymmetrical salts in which the methylene group is joined at the 2-position of one ring and the 4-position of the other, the ortho oxygen atom is displaced by sulfur. An unsymmetrical dye which is substituted in the 2,6-positions of one ring with phenyl groups and with alkyl groups in the other ring gave a product which contained the sulfur atom in the aryl-substituted ring.     

Iterative chain elongation by a pikromycin monomodular polyketide synthase

The unique ability of the pikromycin polyketide synthase (Pik PKS) to generate 12- and 14-membered ring macrolactones presents an opportunity to explore the fundamental processes of polyketide synthesis, specifically, the mechanistic details of the chain extension process. We have overexpressed and purified PikAIII and PikAIV and demonstrated the ability of these proteins to generate triketide lactone products using (14)C-methylmalonyl-CoA as the sole substrate. Monomodular PikAIII generates TKL (1) when reacted alone, and synthesizes TKL (2) upon reaction in combination with PikAIV. Product formation remains dependent on the enzymatic decarboxylation of methylmalonyl-CoA and transfer of the acyl chain within the enzyme rather than acylation by propionyl-CoA from spontaneous decarboxylation. We propose that synthesis of TKL (1) by PikAIII involves iterative assembly of the triketide chain within a PikAIII homodimer analogous to the nonmodular type I PKS systems.     

2+2] Cycloaddition of ketenes with ynamides. A general method for the synthesis of 3-aminocyclobutenone derivatives

Ynamides react with ketenes in [2+2] cycloadditions leading to a variety of substituted 3-aminocyclobut-2-en-1-ones. The ynamides employed in these reactions are readily available via the copper-promoted N-alkynylation of carbamates and sulfonamides with alkynyl bromides and iodides. The scope of the [2+2] cycloaddition with regard to both the ketene and ynamide component is described.     

Chemoselective arylamidine cyclizations: mild formation of 2-arylimidazole-4-carboxylic acids

A versatile, one-pot synthesis of 2-arylimidazole-4-carboxylic acids from arylamidines and methyl-2-chloroacetoacetate is described. The transformation is chemoselective, and reaction conditions are mild. Moreover, the flexibility of the strategy offers rapid access to two important classes of biaryl compounds, both 2-arylimidazoles and 2-arylpyrimidines, depending simply upon solvent and base selection. [reaction: see text]     

A simple and efficient route to β-substituted pyrroles

$\text{N}$-Phenylsulfonylpyrrole undergoes Friedel-Crafts acylation exclusively at the β-position of the pyrrole ring, thus allowing a simple and efficient synthesis of β-acylated pyrroles.