Total synthesis of bassiatin and its stereoisomers: novel divergent behavior of substrates in Mitsunobu cyclizations

Total syntheses of the morpholine-2,5-dione, Bassiatin, and its stereoisomers have been completed. A key step in the syntheses was the Mitsunobu cyclization of hydroxyacid acyclic precursors. The hydroxyacid precursors are hindered alcohols and two substrates underwent Mitsunobu cyclization with retention of configuration. The other two substrates underwent Mitsunobu cyclization with either retention or inversion of configuration depending on reaction conditions. This divergence in outcome of the Mitsunobu reaction for the same substrate depending on effective concentration is novel.     

The coordination chemistry of simple phospholes: Complexes of nickel(II), palladium(II) and platinum(II)

The reactions between four very simply substituted phospholes and the chlorides of Ni(II), Pd(II) and Pt(II) are described. The phospholes 1-phenylphosphole, 3-methyl-1-phenyl-phosphole and 3,4dimethyl-1-phenylphosphole all readily form bis-complexes of formula L₂MCl₂ [L = phosphole ligand and M = Ni(II), Pd(II) or Pt(II)] or tris-complexes of formula L₃MCI₂. 1-n-Butyl-3,4-dimethylphosphole appears to form stable complexes only with Ni(II). Evidence is put forward which indicates that the L₂MCl₂ complexes exist in a four-coordinate, square-planar monomeric/five coordinate equilibrium while the L₃MCl₂ complexes are primarily the ionic species [L₃MCl]⁺ Cl^? in solution. Comparisons are made with the behaviour of other simple phospholes which do not form Ni(II) complexes and the results are discussed briefly in terms of both aromatic and non-aromatic phosphole models.     

New pyrimidine- and fluorene-containing oligo(arylene)s: synthesis, crystal structures, optoelectronic properties and a theoretical study

New pyrimidine containing oligo(arylene)s, notably the pyrimidine-fluorene hybrid systems 13-16, have been synthesised by Suzuki cross-coupling methodology. An efficient synthesis of the key reagent 9,9-dihexylfluorene-2,7-diboronic acid 10 from 2,7-dibromo-9,9-dihexylfluorene 9 is reported. Cross-coupling of 10 with two equivalents of 2-bromopyrimidine, 5-bromopyrimidine and 2,5-dibromopyrimidine gave 2,7-bis(2-pyrimidyl)-9,9-dihexylfluorene 13. 2,7-bis(5-pyrimidyl)-9,9-dihexylfluorene 14 and 2,7-bis(5-bromo-2-pyrimidyl)-9,9-dihexylfluorene 15 in 23-34% yields. A further two-fold Suzuki reaction of benzeneboronic acid with compound 15 gave 2,7-bis(5-phenyl-2-pyrimidyl)-9,9-dihexylfluorene 16 (35% yield). Ab initio calculations of the geometries and electronic structures at the Hartree Fock (HF) and density functional theory (DFT) levels of theory are reported for compounds 13, 14 and 16 (with ethyl substituents replacing hexyl) and for their dipyrazinyl and bistetraazenyl analogues, 17, 18, 20 and 21. The heterocyclic nitrogen atoms of 13 and 16 facilitate planarisation of the system, compared to 14, which is in agreement with X-ray structural data obtained for 5-bromo-2-phenylpyrimidine 6, 2,5-diphenylpyrimidine 7 and compound 15. Bistetrazenyl derivative 21 is calculated to be a fully planar system. The cyclic voltammogram (CV) of compound 16 in dichloromethane solution shows a quasi-reversible oxidation wave at E(1/2)0 = +1.36 V (vs. Ag/Ag+). Compound 13 is a poorer donor with an oxidation observed at Epa = +1.50 V which is in good agreement with the difference in the energies of their HOMO orbitals calculated at both HF and DFT levels of theory (0.11-0.12 eV). For compound 14 we were not able to measure an Eox potential which should lie at much more positive potentials. Compounds 15 and 16 are blue emitters in solution, with photoluminescence quantum yields (PLQY) of 25% and 85%, respectively. For thin films of 16 the PLQY is reduced to 21%. An OLED using compound 16 as the emissive layer has been fabricated in the configuration ITO/PEDOT/16/Ca/Al: blue-green light (lambda max 500 nm) most likely emanating primarily from excimer states is emitted at a high turn-on voltage.     

A general preparation of pyridines and pyridones via the annulation of ketones and esters

A general preparation of pyridines 4a-f from stabilized ketones 3a-c and aryl ketones 3d-f is described. The annulation of stabilized esters 3g,h gives access to the corresponding 2-pyridones 4g,h. The annulation reactions proceed in fair to excellent yields (46-87%) with vinamidinium hexafluorophosphate salts 2a-d containing electron-withdrawing groups at the beta-position. The mechanism of the reaction was investigated by NMR and proceeds through the formation of a dienaminone intermediate.     

Alkali Metal- and Ammonium Picrate Extraction and Complex Forming Capabilities of d-Glucose and d-Mannose-based Lariat Ethers

The alkali metal- and ammonium picrate extracting ability of d-glucose- and d-mannose-based 15-crown-5 ethers and related lariat ethers was investigated in dichloromethane – water system. A heteroatom was waried in the crown ether containing a 4,6-O-benzylidene-α-d-glucopyranoside unit 6, (X=O), 2 (X=S) and 8a (X=NH). Extracting ability of the latter species (8a) was excellent (97–99%) in regard of all cations (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺ and NH₄⁺) examined, it was not, howewer, selective. Introduction of a side arm on the nitrogen atom of 8a decreased the extracting ability, but increased the selectivity. In this series of compounds (8b–f, 4), 4 with a pyridylethyl substituent allowed the extraction of sodium picrate in 72%. The glucose-based macrocycles 8a, 8e and 8f formed a stronger complex with the cations examined than the mannose-based analogues 9a, 9e and 9f, that can be explained by the all-gauche conformation of the former ones. It was pointed out that in the case of crowns with tertiary amine moieties, the basicity increases the quantity of the picrates extracted. According to complex forming measurements by FAB-MS, the best sodium ion selectivity was achieved by the γ-hydroxypropyl substituted lariat ether (8e). Possible structures of the complexes formed by the two types of monosacharides with sodium cation were evaluated by molecule modelling calculations.     

Strain-release 2-azaallyl anion addition/borylation of [1.1.1]propellane: synthesis and functionalization of benzylamine bicyclo[1.1.1]pentyl boronates

We report a 3-component reaction between N-benzyl ketimines, [1.1.1]propellane, and pinacol boronates to generate benzylamine bicyclo[1.1.1]pentane (BCP) pinacol boronates. These structures are analogous to highly sought diarylmethanamine cores, which are common motifs in bioactive molecules. We demonstrate the versatility of the boronate ester handle via downstream functionalization through a variety of reactions, including a challenging Pd-catalyzed (hetero)arylation that exhibits a broad substrate scope. Together, these methods enable the synthesis of high-value BCP benzylamines which are inaccessible by existing methods. Furthermore, we demonstrate the successful application of these newly developed (hetero)arylation conditions to a variety of challenging tertiary pinacol boronates, including nitrogen-containing heterocycles, 1,1-disubstituted cyclopropanes, and other BCP cores.

We report a 3-component reaction between N-benzyl ketimines, [1.1.1]propellane, and pinacol boronates to generate benzylamine bicyclo[1.1.1]pentane (BCP) pinacol boronates.     

Recombinant phytochrome A in yeast differs by its spectroscopic and photochemical properties from the major phyA' and is close to the minor phyA": evidence for posttranslational modification of the pigment in plants.

Previously, two pools of phytochrome A (phyA' and phyA") have been detected by in situ low-temperature fluorescence spectroscopy and photochemistry; it was suggested that they might differ in the nature of their posttranslational modification. In order to verify this possibility Arabidopsis and rice (Oryza) phyA were expressed in yeast and the pigments were assembled in vivo with phycocyanobilin (PCB) and phytochromobilin (P phi B). The resulting recombinant phytochromes in the red-light-absorbing form (Pr) were characterized in the yeast cell by (1) the fluorescence emission spectra; (2) the temperature dependence of Pr fluorescence intensity and activation energy of fluorescence decay; and (3) the extent of photoconversion of Pr into photoproduct lumi-R (gamma 1) or far-red-light absorbing form (Pfr) (gamma 2). Both Arabidopsis phyA/PCB and Oryza phyA/P phi B had low gamma 1 of ca 0.05, allowing their attribution to the Pr" phenomenological type of phytochrome comprising phyA", phyB and cryptogam phytochromes. The spectroscopic properties of Oryza phyA/P phi B were also very close to phyA". However, both investigated holoproteins differed from phyA", both with respect to the character of temperature dependence of the fluorescence yield and activation energy. Thus, recombinant Oryza phyA/P phi B is similar but not identical to phyA". The data demonstrate that the low-abundance-fraction plant phyA (phyA") comes from the same gene as the major (phyA') fraction. Because both endogenous phyA fractions differ from the phytochrome expressed in yeast, they appear to be posttranslationally modified and/or bound to partner proteins or cellular substructures. However, the character of the presumed chemical modification is different in phyA' and phyA" and its extent is more profound in the case of the former.     

The Effect of Acid Strength on the Mitsunobu Esterification Reaction: Carboxyl vs Hydroxyl Reactivity

In the presence of carboxylic acids, the adduct formed between triphenylphosphine and diisopropyl azodicarboxylate reacts to form mono- and bis-acylated hydrazides and the carboxylic acid anhydrides. These products are formed via attack of the carboxylate on the triphenylphosphonium group of the adduct, with weaker acids reacting much faster than stronger acids. This provides an explanation for the observation in the literature that acids stronger than acetic acid, such as 4-nitrobenzoic acid and chloroacetic acid, provide better yields in esterification reactions, since reaction of the alcohol with the phosphonium group of the adduct is more rapid than the competing reaction of the carboxylate for the phosphonium group.