Recent developments in the enantioselective synthesis of polyfunctionalized pyran and chromene derivatives

Recent developments in the synthesis of 4H- and 2H-pyrans as well as structurally related chromene derivatives that have enabled the enantioselective synthesis of these scaffolds have been surveyed. The role of chiral catalysts in orienting initial reactions of active methylenes, methines and methyl ketones, to unsaturated ketones and nitriles in multi-component reactions or Friedel–Craft alkylations of phenols is discussed to show their involvement in transition states leading to end products. Chromene synthesis via [4+2] cycloadditions, [3+3] and [4+2] annulations as well as ring opening and recyclization leading to high enantio- and diasteroselectivity is also demonstrated. The enantioselectivity in such catalytic asymmetric reactions despite starting with non-chiral starting materials is discussed. On the other hand, in surveying ring opening and recyclization, the starting materials are chiral and the chiral center was not part of the reaction leading to the final product.     

Amphiphilic allylation of arylidene-1,3-oxazol-5(4H)-one using bis-π-allylpalladium complexes: an approach to synthesis of cyclohexyl and cyclohexenyl α-amino acids

An efficient method for synthesis of cyclohexyl and cyclohexenyl α-amino acids via palladium-catalyzed three-component assemblies followed by ring-closing metathesis (RCM) is described. The present catalytic reaction is successfully extended to substituted benzylidene azlactones 2a-j RCH=(1,3-oxazole): R = alkyl or aryl. The amphiphilic bis-allylation of these substrates has been achieved by replacing toxic allylstannanes with allyltrifluoroborate and the reaction proceeded smoothly to afford the corresponding 1,7-diene derivatives 3a-j in acceptable to good yields. RCM of the resulting octadienes using the first generation Grubbs catalyst gave easy access to stereodefined substituted cyclohexene derivatives 7-11 in high yields. Acid hydrolysis of the oxazolone ring of 7-10 gave protected amino acids 12-16. Debenzoylation of 13 and 15 afforded 1-amino-6-aryl-cyclohex-3-enecarboxylic acids 17 and 18 in excellent yields, respectively. Moreover, catalytic reduction of 13 gave the corresponding cyclohexane derivative 19 which could be debenzoylated to give 1-amino-2-phenylcyclohexene-1-carboxylic acid (20). The structures of compounds 9, 12 and 13 were confirmed by X-ray structural analysis. It is an excellent method for creating a wide range of cyclic α,α-disubstituted α-amino acids.     

Spectroscopic studies of charge transfer complexes of meso‐tetra‐p‐tolylporphyrin and its zinc complex with some aromatic nitro acceptors in different organic solvents

The charge transfer complex (CTC) formation of 5,10,15,20‐tetra(p‐tolyl)porphyrin (TTP) and zinc 5,10,15,20‐tetra(p‐tolyl)porphyrin with some aromatic nitro acceptors such as 2,4,6‐trinitrophenol (picric acid), 3,5‐dinitrosalicylic acid, 3,5‐dinitrobenzoic acid (DNB) and 2,4‐dinitrophenol (DNP) was studied spectrophotometrically in different organic solvents at different temperatures. The spectrophotometric titration, Job's and straight line methods indicated the formation of 1:1 CTCs. The values of the equilibrium constant (K_CT) and molar extinction coefficient (ε_CT) were calculated for each complex. The ionization potential of the donors and the dissociation energy of the charge transfer excited state for the CTC in different solvents was also determined and was found to be constant. The spectroscopic and thermodynamic properties were observed to be sensitive to the electron affinity of the acceptors and the nature of the solvent. No CT band was observed between Zn‐TTP as donor and DNP or DNB as acceptors in various organic solvents at different temperature. Bimolecular reactions between singlet excited TTP (¹TTP*) and the acceptors were investigated in solvents with various polarities. A new emission band was observed. The fluorescence intensity of the donor band decreased with increasing the concentration of the acceptor accompanied by an increase in the intensity of the new emission. The new emission of the CTCs can be interpreted as a CT excited complex (exciplex). Copyright © 2007 John Wiley & Sons, Ltd.     

Observation of bubble domains at the cholesteric to homeotropic-nematic transition in a confined chiral nematic liquid crystal

We report observations on a chiral nematic liquid crystal confined in a plane cell treated for homeotropic alignment. The characteristics of the specimen investigated are such that at room temperature a uniform homeotropic alignment is observed and at higher temperatures the material displays the helical structure. It is observed that the changeover between homeotropic and helical textures does not occur at one single threshold. There are two thresholds, between which there is an intermediate texture where small circular domains are observed. These domains may be regarded as circular analogues of the plane solitons predicted by previous authors.     

Template-directed oligonucleotide strand ligation, covalent intramolecular DNA circularization and catenation using click chemistry

The copper-catalyzed azide-alkyne cycloaddition reaction has been used for the template-mediated chemical ligation of two oligonucleotide strands, one with a 5'-alkyne and the other with a 3'-azide, to produce a DNA strand with an unnatural backbone at the ligation point. A template-free click-ligation reaction has been used for the intramolecular circularization of a single stranded oligonucleotide which was used as a template for the synthesis of a covalently closed DNA catenane.     

Comparison of reversed-phase liquid chromatography-mass spectrometry with electrospray and atmospheric pressure chemical ionization for analysis of dietary tocopherols

ESI and APCI ionization techniques in both negative and positive ion modes were evaluated for simultaneous LC-MS analysis of the four tocopherol homologues (alpha, beta, gamma and delta). The ESI and APCI ionization of tocopherols in positive ion mode was not efficient and proceeded via two competitive mechanisms, with the formation of protonated pseudo-molecular ions and molecular ions, which adversely influenced the repeatability of MS signal. Ionization in negative ion mode in both ESI and APCI was more efficient as it only produced target deprotonated pseudo-molecular ions. The APCI in negative ion mode showed larger linearity range, lower detection limits and was less sensitive to the differences in chemical structure of analytes and nature of applied solvents than negative ion ESI. Negative ion APCI was, therefore, chosen for the development of LC-MS method for simultaneous determination of the four tocopherols in foods. A baseline separation of the tocopherols was achieved on novel pentafluorophenyl silica-based column Fluophase PFP. The use of methanol-water (95:5, v/v) as a mobile phase was preferable to the use of acetonitrile-water due to considerable gain in MS signal. The limits of quantifications were 9 ng/mL for alpha-tocopherol, 8 ng/mL for beta- and gamma- and 7.5 ng/mL for delta-tocopherol when 2 microL was injected. This method was successfully applied to determination of tocopherols in sunflower oil and milk.     

Novel glycosylated carboranylquinazolines for boron neutron capture therapy of tumors: synthesis,characterization, and in vitro toxicity studies

The synthesis of a series of N‐glycosyl caboranylquinazolines is described. The condensation reaction of nitro‐acetylanthranilic acid with aminophenylcarborane gave 3‐[(o‐carboran‐1‐yl)phenyl]‐2‐methyl‐6‐nitroquinazolin‐4(3H)‐one 1 followed by reduction with Na₂S to the corresponding 6‐amino‐3‐[(o‐carboran‐1‐yl)phenyl]‐2‐methylquinazolin‐4(3H)‐one 2 . Reaction of compound 2 with D‐glucose or D‐ribose in methanol in the presence of a catalytic amount of acetic acid affords boronated N‐glycosylaminoquinazolines namely: 2‐methyl‐3‐[4‐(o‐carboran‐1‐yl)phenyl]‐6‐[N‐β‐D‐glucopyranosyl)]aminoquinazolin‐4(3H)‐one 3 or 2‐methyl‐3‐[4‐(o‐carboran‐1‐yl)phenyl]‐6‐[N‐β‐D‐ribofuranosyl)]aminoquinazolin‐4(3H)‐one 4 , respectively. Degradation of the o‐caborane cage of compounds 3 and 4 yielded highly water‐soluble compounds of sodium 2‐methyl‐3‐[4‐( nido ‐undecarborate‐1‐yl)phenyl]‐6‐[N‐β‐D‐glucopyranosyl]aminoquinazolin‐4(3H)‐one 5 and sodium 2‐methyl‐3‐[4‐( nido ‐undecarborate‐1‐yl)phenyl]‐6‐[N‐β‐D‐ribofuranosyl)]aminoquinazolin‐4(3H)‐one 6 , respectively. The structures were established on the basis of elemental analysis, NMR, IR and mass spectrometry. The in vitro toxicity test using B16 melanoma cells showed that N‐glycosyl of nido ‐undecaboranylquinazolines ( 5 and 6 ), with higher water solubility, is not toxic at boron concentration of 3000 µg boron ml⁻¹, whereas, N‐glycosyl of closo ‐carboranylquinazolines ( 3 and 4 ) has LD₅₀ > 200 µg boron ml⁻¹. The compounds described here may be considered as potential agents for BNCT. Copyright © 2008 John Wiley & Sons, Ltd.