The kinetics of ligand substitution in bis(N-alkylsalicylaldiminato)nickel(II) Complexes: a study on the reactivity of square-planar versus octahedral coordination

The bis(N-alkylsalicylaldiminato)nickel(II) complexes Ni(R-sal)₂ with R = CH(CH₂OH)CH(OH)Ph (I), R = CH(CH₃)CH(OH)Ph (II) and R = CH₂CH₂Ph (III; Ph = phenyl) were prepared and characterized. In the solid state I and II are paramagnetic (μ = 3.2 and 3.3 BM at 20 °C, respectively), whereas III is diamagnetic. It follows from the UV-Vis spectra that in acetone solution I is six-coordinate octahedral and III is four-coordinate planar, the spectrum of II showing characteristics of both modes of coordination. Vis spectrophotometry and stopped-flow spectrophotometry were applied to study the kinetics of ligand substitution in I–III by H₂salen (= N,N′-disalicylidene-ethylenediamine) in the solvent acetone at different temperatures. The kinetics follow a second-order rate law, rate = k[H₂-salen] [complex]. At 20 °C the sequence of rate constants is k(III):k(II):k(I) = 11 850:40.6:1. The activation parameters are ΔH^≠(I) = 112, ΔH^≠(II) = 40.7, ΔH^≠(III) = 35.7 kJ mol⁻¹ and ΔS^≠(I) = 92, ΔS^≠(II) = −103, ΔS^≠(III) = −89 J K⁻¹ mol⁻¹. The enormous difference in rate between complexes I, II and III, which is less pronounced in methanol, is attributed to the existence of a fast equilibrium planar ⇌ octahedral, which is established in the case of I and II by intramolecular octahedral coordination through the hydroxyl groups present in the organic group R. An A-mechanism is suggested to control the substitution in the sense that the entering ligand attacks the four-coordinate planar complex, the octahedral complex being kinetically inert.     

Isolation and characterization of substituted 4-hydroxy-cyclohex-2-enones as metabolites of 3,4-dimethoxybenzyl alcohol and its methyl ether in ligninolytic cultures of Phanerochaete chrysosporium

The metabolism of quinones formed in the enzymatic oxidation of veratryl alcohol (3,4-dimethoxybenzyl alcohol) (Ia) and its methyl ether Ib in ligninolytic cultures of Phanerochaete chrysosporium was studied. A metabolite of 2-hydroxymethyl-5-methoxy-2,5-cyclohexadiene-1,4-dione (IIa, formed from Ia by oxidation) was isolated and identified as cis-4-hydroxy-6-hydroxymethyl-3-methoxy-cyclohex-2-en-one (IVa), formally the reduced hydroquinone IIIa. The formation of IVa was also observed when both veratryl alcohol Ia or 2,5-dihydroxy-4-methoxybenzyl alcohol (IIIa), the hydroquinone of IIa, were used as substrates. Analogously, cis-4-hydroxy-3-methoxy-6-methoxymethyl-cyclohex-2-en-one (IVc) was isolated and identified as a metabolite from either 3,4-dimethoxybenzyl methyl ether (Ib) or from its oxidation product 5-methoxy-2-methoxymethyl-2,5-cyclohexadiene-1,4-dione (IIb) as well as from the corresponding hydroquinone 2,5-dihydroxy-4-methoxybenzyl methyl ether (IIIc). The physiological role of these unprecedented conversions is discussed. Correspondence to: H. E. Schoemaker     

Interactions of five- and six-membered cyclic esters with α-chymotrypsin: Kinetic evidence for covalent enzyme intermediates

Interactions of α-chymotrypsin with 2-coumaranone (I), 3,4-dihydrocoumarin (II), o-hydroxy-α-toluenesulfonic acid sultone (III), and β-o-hydroxyphenylethanesulfonic acid sultone (IV) were studied in the presence of 14% acetonitrile at pH 7.0 by means of the proflavin displacement technique and by inhibition of N-acetyl-l-tryptophan ethyl ester (ATrEE) hydrolysis. Under saturating conditions of either I, II, or III, an enzyme intermediate was shown to accumulate using either the proflavin displacement technique or the ATrEE activity assay. The intermediates have characteristics of covalent enzyme-substrate compounds and are believed to decompose simultaneously by two pathways, one to give free enzyme and hydrolyzed cyclic ester, and the other to give the original cyclic ester and free enzyme. With α-chymotrypsin and III the observed first-order rate constant for decomposition of the intermediate by the two pathways was 0.19 ± 0.04 min^?1, while the rate constant for the hydrolytic pathway alone was 0.013 ± 0.0009 min^?1. These results indicate that the covalent-like intermediate with this sultone is not only capable of reverting to starting cyclic ester but prefers this pathway over hydrolysis. Sultone IV was found to bind to enzyme; but in contrast to the behavior of esters I–III, the binding did not result in accumulation of a covalent-like intermediate.     

Synthesis and insecticidal activity of new oxime derivatives of podophyllotoxin-based phenazines against Mythimna separata Walker

To discover new natural-product-based insecticidal agents, a series of novel oxime derivatives of podophyllotoxin-based phenazines modified in the C, D and E rings of podophyllotoxin were prepared and tested as insecticidal agents against the pre-third-instar larvae of oriental armyworm, Mythimna separata (Walker) in vivo at 1 mg/mL. The steric configuration of IIIc was unambiguously confirmed by single-crystal X-ray diffraction analysis. Compounds IIIa–d, and IIIi exhibited an equal or higher insecticidal activity than toosendanin.     

Novel tricyclic Δ2-isoxazoline and 3-oxo-2-methyl-isoxazolidine derivatives: Synthesis and binding affinity at neuronal nicotinic acetylcholine receptor subtypes

A group of novel tricyclic Δ²-isoxazolines (4b, 5b, 7a–b, and 8a–b) and 3-oxo-isoxazolidines (6a–b and 9a–b), structurally related to cytisine or norferruginine, was prepared through 1,3-dipolar cycloadditions involving suitable olefins and bromonitrile oxide. The target compounds were assayed at α4β2 and α7 neuronal acetylcholine receptors (nAChRs). The results of competition binding experiments indicated for the new derivatives a reduction of the affinity at the α4β2 subtype in comparison with the reference molecules, coupled with an overall negligible affinity at the α7 subtype. The binding mode of the bromo-Δ²-isoxazolines 4b and 7b, which were the highest affinity ligands in the series (K_i = 0.92 and 0.75 μM, respectively), was analyzed by applying a recently developed model of the α4β2 nAChRs.     

Synthesis of 4β-acyloxypodophyllotoxin analogs modified in the C and E rings as insecticidal agents against Mythimna separata Walker

To discover the new natural-product-based insecticidal agents, four series of sixty novel 4β-acyloxypodophyllotoxin analogs modified in the C and E rings were prepared, and their insecticidal activity was tested against the pre-third-instar larvae of oriental armyworm, Mythimna separata (Walker) in vivo at the concentration of 1 mg/mL. It demonstrated that the position of the dioxo group on the E-ring of 4′-demethylepipodophyllotoxin was regioselectively controlled by the chlorine atom at its C-2′ position when 2′-chloro-4′-demethylepipodophyllotoxin was oxidized by sodium periodate. Among all the derivatives, IIIi exhibited the best potent insecticidal activity with the final mortality rate of 63.3%. To alkylacyloxy series, the proper length of the side chain at the C-4 position of Ia–g, IIa–g and IIIa–g was important for their insecticidal activity.     

Comparative Secretome Analysis of Magnaporthe oryzae Identified Proteins Involved in Virulence and Cell Wall Integrity

Plant fungal pathogens secrete numerous proteins into the apoplast at the plant–fungus contact sites to facilitate colonization. However, only a few secretory proteins were functionally characterized in Magnaporthe oryzae, the fungal pathogen causing rice blast disease worldwide. Asparagine-linked glycosylation 3 (Alg3) is an α-1,3-mannosyltransferase functioning in the N-glycan synthesis of N-glycosylated secretory proteins. Fungal pathogenicity and cell wall integrity are impaired in Δalg3 mutants, but the secreted proteins affected in Δalg3 mutants are largely unknown. In this study, we compared the secretomes of the wild-type strain and the Δalg3 mutant and identified 51 proteins that require Alg3 for proper secretion. These proteins were predicted to be involved in metabolic processes, interspecies interactions, cell wall organization, and response to chemicals. Nine proteins were selected for further validation. We found that these proteins were localized at the apoplastic region surrounding the fungal infection hyphae. Moreover, the N-glycosylation of these proteins was significantly changed in the Δalg3 mutant, leading to the decreased protein secretion and abnormal protein localization. Furthermore, we tested the biological functions of two genes, INV1 (encoding invertase 1, a secreted invertase) and AMCase (encoding acid mammalian chinitase, a secreted chitinase). The fungal virulence was significantly reduced, and the cell wall integrity was altered in the Δinv1 and Δamcase mutant strains. Moreover, the N-glycosylation was essential for the function and secretion of AMCase. Taken together, our study provides new insight into the role of N-glycosylated secretory proteins in fungal virulence and cell wall integrity.     

Photoreactive 111In-cyclodextrin inclusion complex: a new heterobifunctional reagent for antibody labeling

The compound of interest, N-5-azido-2-nitrobenzoylaminomethyl-¹¹¹In-acetylacetone-α-cyclodextrin (CD) (V) was synthesized by the selective tosylation of α-CD to form 6-tosyl-6-deoxy-CD, which was then reacted with NaN₃ to form 6-azido-6-deoxy-CD (II). This was followed by catalytic hydrogenation to yield III. Compound III and ¹¹¹In-acetylacetone were mixed to form an inclusion complex, which was then reacted with N-5-azido-2-nitrobenzoyloxysuccinimide to yield compound V. Anti-melanoma MAbTP41.2 was added to compound V, followed by immediate photoreactivation labeling by u.v. light at 320 nm. The final product VI was purified from a Sephadex G-50 column. ¹¹¹In-DTPA-MAbTP41.2 was also prepared as a control.Immunoreactivity via the cell-binding assay of VI was 87%, compared with 57% by the BADTPA method. Biodistribution in non-tumor rats yielded a liver concentration in %ID/g of 3.5, 1.7 and 1.0 for compound VI, compared to the 5.5, 5.2 and 3.1 for the BADTPA compound, at 4, 24 and 48 h post-injection, respectively.     

Reactions of N-cyanomethyl groups attached to a tetraaza macrocyclic copper(II) complex leading to the formation of various hetero-functionalized macrocyclic complexes

New hetero-functionalized macrocyclic complexes [CuL²](ClO₄)₂ (I) and [CuL³](ClO₄)₂ (II) bearing one N-CH₂CONH₂ or one N-CH₂C(NH)NH(CH₂)₂CH₃ pendant arm as well as one N-CH₂CN group have been prepared by the selective reaction of water or n-propylamine with one of the two N-CH₂CN groups in [CuL¹](ClO₄)₂ (L¹ = 2,13-bis(cyanomethyl)-5,16-dimethyl-2,6,13,17-tetraazatricyclo[16.4.0.^1.180^7.12]docosane). The complex [CuL⁴](ClO₄)₂ (III) bearing both N-CH₂CONH₂ and N-CH₂C(NH)NH(CH₂)₂CH₃ pendant arms can be prepared by either the reaction of I with n-propylamine or the hydrolysis of II. The N-CH₂CONH₂ and/or N-CH₂C(NH)NH(CH₂)₂CH₃ groups of I, II, and III are coordinated to the metal ion. The crystal structure of II shows that the complex has distorted square-pyramidal coordination polyhedron with a considerably strong apical Cu-N (N-CH₂C(NH)NH(CH₂)₂CH₃) bond (2.101(6) Å). The addition of HClO₄ (?0.01 M) to an acetonitrile (or DMSO) solution of II or III produces [Cu(HL³)](ClO₄)₃ (IIa) or [Cu(HL⁴)](ClO₄)₃ (IIIa), showing that the N-CH₂C(NH)NH(CH₂)₂CH₃ pendant arm of each complex is readily protonated in the non-aqueous solvent; the resulting N-CH₂C()NH(CH₂)₂CH₃ group of IIa or IIIa is not involved in coordination. However, the N-CH₂C(NH)NH(CH₂)₂CH₃ group of II is not protonated even in ?1.0 M HClO₄ aqueous solution. In the case of III, most of the complex exists as the protonated form [Cu(HL⁴)]³⁺ in ?0.1 M HClO₄ aqueous solutions.     

Cu2+-catalyzed peptide bond formation in the reaction of 5-deoxypyridoxal and α-phenyl-α-aminomalonic acid

The reaction of 5-deoxypyridoxal with α-phenyl-α-aminomalonic acid in the presence of excess Cu²⁺ ions is shown to lead to the formation of N-5-deoxypyridoxoyl-α-phenylglycine, III, as the only 5-deoxypyridoxal-derived product. This reaction occurs anaerobically under very mild conditions of temperature and pH and involves the oxidative formation of a peptide bond. It represents a hitherto undescribed reaction type for vitamin B₆ and its analogs; a mechanism for the reaction is proposed.     

Three coordination modes of the pentadentate ligand 2,6-diacetylpyridinedisemicarbazone

The pentadentate ligand 2,6-diacetylpyridinedisemicarbazone, DAPSC, reacts with Cr(NO₃)₃·9H₂O and forms two kinds of complexes. At pH=3, the ligand is singly-deprotonated and crystals of [Cr- (DAPSCH)(H₂O)₂](NO₃)₂·H₂O (Ia) are obtained. Evaporation of a solution at pH=0, yields crystals of [Cr(DAPSC)(H₂O)₂](NO₃)₃·2H₂O (II) in which the ligand is fully protonated. The reaction of DAPSC with UO₂(O₂CCH₃)₂ in methanol, followed by crystallization of the product from DMSO yields crystals of [UO₂(DAPSC2H)(H₂O)]·2DMSO (III) in which the ligand is fully deprotonated. Compound Ia is monoclinic, space group P2₁/n with a=11.746(1), b=14.752(2), c=11.866(1) Å,β=105.53(2)°, V= 1981(1) ų and Z=4. Compound II is monoclinic, space group, P2₁/n with a=38.000(3), b= 14.939(2), c=8.233(1) Å, β=96.12(2)°, V= 4647(1) Å and Z=8. Compound III is monoclinic, space group P2₁/n with a=18.048(2), b=15.207(2), c=8.842(1) Å,β=97.72(2)°, V=2405(1) ų and Z=4. The structures were refined using 2084, 4169 and 2516 reflections to R values of 4.4%, 7.8% and 4.8% respectively.     

Synthesis and investigations into the anticancer and antibacterial activity studies of β-carboline chalcones and their bromide salts

A series of sixteen β-carbolines, bearing chalcone moiety at C-1 position, were prepared from easily accessible 1-acetyl-β-carboline and various aldehydes under basic conditions followed by N²-alkylation using different alkyl bromides. The prepared compounds were evaluated for in vitro cytotoxicity against a panel of human tumor cell lines. N²-Alkylated-β-carboline chalcones 13a-i represented the interesting anticancer activities compared to N²-unsubstituted β-carboline chalcones 12a-g. Off the prepared β-carbolines, 13g exhibited broad spectrum of activity with IC₅₀ values lower than 22.5?µM against all the tested cancer cell lines. Further, the N²-alkylated-β-carboline chalcone 13g markedly induced cell death in MDA-MB-231 cells by AO/EB staining assay. The most cytotoxic compound 13g possessed a relatively high drug score of 0.48. Additionally, the prepared β-carboline chalcones displayed moderate antibacterial activities against tested bacterial strains.     

Synthesis and structures of dimeric iron(III)-Oxo and -imido complexes containing intramolecular hydrogen bonds

Hydrogen bonding networks proximal to metal centers are emerging as a viable means for controlling secondary coordination spheres. This has led to the regulation of reactivity and isolation of complexes with new structural motifs. We have used the tridenate ligand bis[(N′-tert-butylureido)-N-ethyl]-N-methylaminato ([H₂1]²⁻) that contains two hydrogen bond donors to examine the oxidation of the Fe^II-acetate complex, [Fe^IIH₂1(η²-OAc)]⁻ with dioxygen, amine N-oxides, and xylyl azide. A complex with Fe^III-O-Fe^III core results from the oxidation with dioxygen and amine N-oxides, in which the oxo ligand is involved in hydrogen bonding to the [H₂1]²⁻ ligand. A distinctly different hydrogen bonding network was found in Fe^III dimer isolated from the reaction with the xylyl azide: a rare Fe^III-N(R)-Fe^III core was observed that does not have hydrogen bonds to the bridging nitrogen atom. The intramolecular H-bond networks within these dimers appear to adjust to the presence of the bridging species and rearrange to its size and electron density.     

Synthesis of alkyl glyco-pyranosides and -furanosides of 2-amino-2-deoxy-d-glucose. Crystal structure of 2-deoxy-2-[(4,4-dimethyl-2,6-dioxocyclohexylidenemethyl)amino]-α-d-glucopyranose

The reaction of 2-amino-2-deoxy-d-glucose hydrochloride with 5,5-dimethyl-2-phenylaminomethylene-1,3-cyclohexanedione in MeOH in the presence of Et₃N afforded 2-deoxy-2-[(4,4-dimethyl-2,6-dioxocyclohexylidenemethyl)amino]-d-glucose (6) in yields > 75%. Glycosidation of 6 with different alcohols (MeOH, CH₂CHCH₂OH, BnOH) under the Fischer conditions afforded mixtures of the corresponding alkyl 2-deoxy-2-[(4,4-dimethyl-2,6-dioxocyclohexylidenemethyl)-amino]-α,β-d-glucopyranoside and -α-d-glucofuranoside. Removal of the N-protecting group gave high yields of the free aminodeoxyglyco-pyranosides and -furanosides. In addition to other known glycosides, allyl and benzyl 2-amino-2-deoxy-α-d-glucopyranoside and ethyl and allyl 2-amino-2-deoxy-α-β-glucofuranoside were obtained. An X-ray crystallographic study of 6 indicated that, in the solid state, it has the α-d configuration and that the pyranoside ring adopts the ⁴C₁ conformation.     

Synthesis of 2- (and 6-) -dithian-2-yluracil nucleosides and their conversion into nucleoside derivatives

Addition of 2,2′-anhydro-[1-(3-O-acetyl-5-O-trityl-β-D-arabinofuranosyl)uracil] (1) to excess 2-litho-1,3-dithiane (2)in oxolane at ?78° gave 2-(1,3-dithian-2-yl)-1-(5-O-trityl-β-D-arabinofuranosyl)-4(1H)pyrimidinone (3), O²,2′-anhydro-5,6-di-hydro-6-(S)-(1,3-dithian-2-yl)-5′-O-trityluridine (4), and 2-(1,4-dihydroxybutyl)-1,3-dithiane (5) in yields of 15, 30, and 10% respectively. The structure of 3 was proved by its hydrolysis in acid to give 2-(1,3-dithian-2-yl)-4-pyrimidinone (6) and arabinose, and by desulfurization with Raney nickel to yield the known 2-methyl-1-(5-O-trityl-β-D-arabinofuranosyl)-4(1H)-pyrimidinone (7). Detritylation of 3 without glycosidic cleavage could only be effected by prior acetylation to 1-(2,3-di-O-acetyl-5-O-trityl-β-D-arabinofuranosyl)-2-(1,3-dithian-2-yl)-4(1H)-pyrimidinone (8) which, after treatment with acetic acid at room temperature for 65 h followed by the action of sodium methoxide gave 2-(1,3-dithian-2-yl)-1-β-D-arabinofuranosyl-4(1H)-pyrimidinone (10) in 45% yield. Detritylation of 4 in boiling acetic acid gave 5,6-dihydro-6-(S)-(1,3-dithian-2-yl)-1-β-D-arabinofuranosyluracil (12) and 3-[(S)-1-(1,3-dithian-2-yl)]propionamido-(1,2-dideoxy-β-D-arabinofurano)-[1,2-d]-2-oxazolidinone (13) in 10 and 90% yields, respectively. When 12 was kept in water or methanol for 7 days, quantitative conversion into 13 occurred. Acid hydrolysis of 12 afforded arabinose and 5,6-di-hydro-6-(1,3-dithian-2-yl)uracil (14), which was desulfurized with Raney nickel to the known 5,6-dihydro-6-methyluracil (15). Treatment of 13 with trifluoroacetic anhydride-pyridine yielded 77% of the cyano derivative 17. Similar dehydration of 3-(R)-1-methylpropionamido-(1,2-dideoxy-β-D-arabinofurano)-[1,2-d]-2-oxalidinone (18), obtained by desulfurization of 13, gave 60% of the nitrile 19. Hydrogenation of 19 over platinum oxide in acetic anhydride gave the acetamide derivative 20 in 95% yield. Nitrobenzoylation of 13 gave 3-[(S)-1-(1,3-dithian-2-yl)]cyanomethyl-3,5-di-O-p-nitrobenzoyl-(1,2-dideoxy-β-D-arabinofurano)-[1,2-d]-2-oxazolidinone (22), which was converted in 37% yield by treatment with methyl iodide in dimethyl sulfoxide into the aldehyde 24, characterized as the semicarbazone 25. The purification of 5 and its characterization as 2-(1,4-di-O-p-nitrobenzoylbutyl)-1,3-dithiane (27) is described.     

Characterization of tautomeric limonoids from the fruits of Melia toosendan

Four nimbolinin-type limonoids, 12α/β-1-O-tigloyl-1-O-deacetyl-nimbolinin B (1), 1-deacetylnimbolinin B (2), nimbolinin B (3) and nimbolinin A (4), were isolated from the fruits of Melia toosendan. 1 was a new compound and existed as a mixture of a pair of tautomers, 12α- (1a) and 12β- (1b). The structures of both tautomers were fully determined by extensive spectroscopic methods including UV, IR, NMR and ESI-MS. Tautomeric behaviors and their relative molar ratios in compounds 1–4 were further investigated using optical rotation, TLC, ¹H NMR and HPLC. Equilibrium equation of nimbolinin was proposed accordingly, with 12α- and 12β-isomers interchanging via a 12-hemiacetal intermediate.     

Aufbau von oligosacchariden mit glycosylfluoriden unter lewissäure-katalyse

Glycosylation of 1,2:3,4-di-O-isopropylidene-α-d-galactopyranose (6), as well as its 6-trimethylsilyl ether 7 with 2,3,4,6-tetra-O-acetyl-β-d-glucopyranosyl fluoride (5) was achieved stereospecifically in a mild and fast manner in the presence of Lewis acids like, e.g., titanium tetrafluoride, to give the β-(1→6)-linked disaccharide derivative 1. By use of 2,3,4,6-tetra-O-benzyl-β-d-glucopyranosyl fluoride (8) or its α anomer 10 and titanium tetrafluoride in acetonitrile with 6 or 7, a fast reaction proceeds preponderantly to yield 1,2:3,4-di-O-isopropylidene 6-O-(2,3,4,6-tetra-O-benzyl-β-d-glucopyranosyl)-α-d-galactopyranose (2). In ether, however, mainly the α-(1→6) anomer was formed. These model systems were used to elucidate the limiting conditions for this procedure, and mechanistic conceptions are discussed. By glycosylation at O-4 of 1,6:2,3-dianhydro-β-d-mannopyranose (11) with the perbenzylated α-fluoride 10 both the α- and the β-d-(1→4) disaccharide derivatives 12 and 14 were obtained, but 5 gave exclusively the β-d-(1→4) compound 16. Opening of the anhydro rings of 12 led to the synthesis of N-acetyl-maltosamine (22). 1,6-Anhydro-2-azido-4-O-benzyl-2-deoxy-β-d-glucopyranose was glycosylated with methyl (2,3,4-tri-O-acetyl-β-d-galactopyranosyl fluoride)uronate under titanium tetrafluoride catalysis to give the β-d-(1→3)-linked disaccharide 16, subsequently transformed into 29.     

2-β-d-ribofuranosylbenzoxazole from 2,5-anhydro-d-allonoimidate,and 1,3-dimethyl-8-β-d-ribofuranosylxanthine from 2,5-anhydro-d-allono-thioimidates and -dithioates

The ability of imidates, thioimidates, and dithioates to react with o-aminophenol (2) and 5,6-diamino-1,3-dimethyluracil (6) was studied, using non-saccharide model compounds, as well as saccharide derivatives. All of the model compounds gave 2-methylbenzoxazole, but only ethyl dithioacetate gave a purine derivative with 6. Methyl 2,5-anhydro-d-allonoimidate hydrochloride reacted with 2 to yield 2-β-d-ribofuranosylbenzoxazole, but failed to react with compound 6. On reaction with compound 6 such fully acylated thioimidates as ethyl and benzyl 2,5-anhydrotri-O-benzoyl- or tri-O-p-toluoyl-d-allonothiomidate hydrochloride yielded amidines that underwent aromatization of the furanose ring. Such monoacylated thioimidates as ethyl or benzyl 2,5-anhydro-6-O-benzoyl--d-allonothioimidate hydrochloride yielded, with compound 6, 8-(5-O-benzoyl-β-d-ribofuranosyl)-1,3-dimethylxanthine, without aromatization. Such dithioates as benzyl 2,5-anhydro-6-O-benzoyl-d-allonodithioate and ethyl 2,5-anhydrotri-O-benzoyl-d-allonodithioate were obtained by treating the corresponding thioimidate with H₂S in pyridine. With compound 6, the first yielded 8-(5-O-benzoyl-β-d-ribofuranosyl)-1,3-dimethylxanthine, which afforded the free C-nucleoside 1,3-dimethyl-8-β-d-ribofuranosylxanthine on treatment with methanolic ammonia.     

Nickel(II) complexes of terdentate or symmetrical tetradentate Schiff bases: Evidence of the influence of the counter anions in the hydrolysis of the imine bond in Schiff base complexes

Two sets of ligands, set-1 and set-2, have been prepared by mixing 1,3-diaminopentane and carbonyl compounds (2-acetylpyridine or pyridine-2-carboxaldehyde) in 1:1 and 1:2 ratios, respectively, and employed for the synthesis of complexes with Ni(II) perchlorate, Ni(II) thiocyanate and Ni(II) chloride. Ni(II) perchlorate yields the complexes having general formula [NiL₂](ClO₄)₂(L = L¹ [N³-(1-pyridin-2-yl-ethylidene)-pentane-1,3-diamine] for complex 1 or L²[N³-pyridin-2-ylmethylene-pentane-1,3-diamine] for complex 2) in which the Schiff bases are monocondensed terdentate, whereas Ni(II) thiocyanate results in the formation of tetradentate Schiff base complexes, [NiL(SCN)₂] (L = L³[N,N′-bis-(1-pyridin-2-yl-ethylidine)-pentane-1,3-diamine] for complex 3 or L⁴ [N,N′-bis(pyridin-2-ylmethyline)-pentane-1,3-diamine] for complex 4) irrespective of the sets of ligands used. Complexes 5 {[NiL³(N₃)₂]} and 6 {[NiL⁴(N₃)₂]} are prepared by adding sodium azide to the methanol solution of complexes 1 and 2. Addition of Ni(II) chloride to the set-1 or set-2 ligands produces [Ni(pn)₂]Cl₂, 7, as the major product, where pn = 1,3-diaminopentane. Formation of the complexes has been explained by the activation of the imine bond by the counter anion and thereby favouring the hydrolysis of the Schiff base. All the complexes have been characterized by elemental analyses and spectral data. Single crystal X-ray diffraction studies confirm the structures of three representative members, 1, 4 and 7; all of them have distorted octahedral geometry around Ni(II). The bis-complex of terdentate ligands, 1, is the mer isomer, and complexes 4 and 7 possess trans geometry.     

Investigations on the Δ23-, Δ24(28)- and Δ25-Sterols of zea mays

The sterols of Zea mays shoots were isolated and characterized by TLC, HPLC, GC/MS and ¹H NMR techniques. In all, 22 4-demethyl sterols were identified and they included trace amounts of the Δ²³-, Δ²⁴- and Δ²⁵-sterols, 24-methylcholesta-5,E-23-dien-3β-ol, 24-methylcholesta-5,Z-23-dien-3β-ol, 24-methylcholesta-5,25-dien-3β-ol, 24-ethylcholesta-5,25-dien-3β-ol and 24-ethylcholesta-5,24-dien-3β-ol. In the 4,4-dimethyl sterol fraction, cycloartenol and 24-methylenecycloartanol were the major sterol components but small amounts of the Δ²³-compound, cyclosadol, and the Δ²⁵-compound, cyclolaudenol, were recognized. These various Δ²³- and Δ²⁵-sterols may have some importance in alternative biosynthetic routes to the major sterols, particularly the 24β-methylcholest-5-en-3β-ol component of the C₂₈-sterols. Radioactivity from both [2-¹⁴C]MVA and [methyl-¹⁴C]methionine was incorporated by Z. mays shoots into the sterol mixture. Although 24-methylene and 24-ethylidene sterols were relatively highly labelled, the various Δ²³- and Δ²⁵-sterols contained much lower levels of radioactivity, which is possibly indicative of their participation in alternative sterol biosynthetic routes. (24R)-24-Ethylcholest-5-en-3β-ol (sitosterol) had a significantly higher specific activity than the 24-methylcholest-5-en-3β-ol indicating that the former is synthesized at a faster rate.