Chemical synthesis of aspidosperma alkaloids inspired by the reverse of the biosynthesis of the rhazinilam family of natural products

Pyrrole reduction: Iterative metal-catalyzed C?H functionalization reactions facilitated the preparation of a highly substituted pyrrole derivative. This derivative could be transformed into the pyrrole-containing secondary metabolite, rhazinilam, which could in turn be transformed through a reductive transannular cascade process into the structurally complex pyrrolidine-containing alkaloid natural product, aspidospermidine.     

Bacterial dioxygenase- and monooxygenase-catalysed sulfoxidation of benzo[b]thiophenes

Asymmetric heteroatom oxidation of benzo[b]thiophenes to yield the corresponding sulfoxides was catalysed by toluene dioxygenase (TDO), naphthalene dioxygenase (NDO) and styrene monooxygenase (SMO) enzymes present in P. putida mutant and E. coli recombinant whole cells. TDO-catalysed oxidation yielded the relatively unstable benzo[b]thiophene sulfoxide; its dimerization, followed by dehydrogenation, resulted in the isolation of stable tetracyclic sulfoxides as minor products with cis-dihydrodiols being the dominant metabolites. SMO mainly catalysed the formation of enantioenriched benzo[b]thiophene sulfoxide and 2-methyl benzo[b]thiophene sulfoxides which racemized at ambient temperature. The barriers to pyramidal sulfur inversion of 2- and 3-methyl benzo[b]thiophene sulfoxide metabolites, obtained using TDO and NDO as biocatalysts, were found to be ca.: 25-27 kcal mol(-1). The absolute configurations of the benzo[b]thiophene sulfoxides were determined by ECD spectroscopy, X-ray crystallography and stereochemical correlation. A site-directed mutant E. coli strain containing an engineered form of NDO, was found to change the regioselectivity toward preferential oxidation of the thiophene ring rather than the benzene ring.     

Solid phase synthesis of a cyclic peptide using fmoc chemistry

Using the Fmoc protection strategy of solid phase peptide synthesis, a 10 amino acid peptide was prepared and cyclized in a “head-to-tail” fashion while it was attached to the solid support. Cyclization was accomplished with either BOP or carbodiimide and the peptide was cleaved from the resin and purified using standard protocols.     

Characterization of high purity GaAs far-infrared photoconductors

In this paper we report the results of an extensive study on the far-infrared photoconductivity of high purityn-type GaAs. The crystal, which was grown at Max-Plank-Institute for Solid State Physics using liquid-phase epitaxy, exhibited the fine structures of the excited state transitions of the residual shallow level impurities. The major peak in the spectral response belongs to the 1s-2p transition, with its responsivity about thirty five times higher than the continuum. At 3.4K detector temperature, 625 mV bias, and 100 Hz chopping frequency the detector responsivity at 35.4 cm^–1 (279 µm) was measured to be 0.017 A/W. Under these same conditions, the NEP was 5.9×10^–14 W/Hz. The (DC) dark current at 25 mV bias was 5.6×10^–14 A.     

The27Al(n,d)26Mg reaction at 22 MeV

Deuteron spectra from the reaction²⁷Al(n, d)²⁶Mg has been measured at reaction angles 0°→60°, with a spectrometer comprising three multiwire proportional counters followed by a curved plastic scintillator. Spectra were obtained with an energy resolution of ～0.7 MeV and an angle resolution of ～5°. Angular distributions for transitions to the ground state and excited states at 1.81, 2.93 and 4.3 MeV in²⁶Mg were obtained. Distorted waves analysis of the angular distributions yield spectroscopic factors of 0.29(8), 0.9(3), 0.18(7) and 1.9(6) for the ground and excited states, respectively. The results are compared with previous (d,³He) and (n, d) measurements, and with theoretical predictions.     

On deletions of largest bonds in graphs

A well-known conjecture of Scott Smith is that any two distinct longest cycles of a k-connected graph must meet in at least k vertices when k≥2. We provide a dual version of this conjecture for two distinct largest bonds in a graph. This dual conjecture is established for k?6.     

Copper-catalyzed alkene arylation with diaryliodonium salts

Alkenes and arenes represent two classes of feedstock compounds whose union has fundamental importance to synthetic organic chemistry. We report a new approach to alkene arylation using diaryliodonium salts and Cu catalysis. Using a range of simple alkenes, we have shown that the product outcomes differ significantly from those commonly obtained by the Heck reaction. We have used these insights to develop a number of new tandem and cascade reactions that transform readily available alkenes into complex arylated products that may have broad applications in chemical synthesis.     

Toluene dioxygenase-catalyzed cis-dihydroxylation of benzo[b]thiophenes and benzo[b]furans: synthesis of benzo[b]thiophene 2,3-oxide

Enzymatic cis-dihydroxylation of benzo[b]thiophene, benzo[b]furan and several methyl substituted derivatives was found to occur in both the carbocyclic and heterocyclic rings. Relative and absolute configurations and enantiopurities of the resulting dihydrodiols were determined. Hydrogenation of the alkene bond in carbocyclic cis-dihydrodiols and ring-opening epimerization/reduction reactions of heterocyclic cis/trans-dihydrodiols were also studied. The relatively stable heterocyclic dihydrodiols of benzo[b]thiophene and benzo[b]furan showed a strong preference for the trans configuration in aqueous solutions. The 2,3-dihydrodiol metabolite of benzo[b]thiophene was utilized as a precursor in the chemoenzymatic synthesis of the unstable arene oxide, benzo[b]thiophene 2,3-oxide.