An Unexpected Formation of a 14‐Membered Cyclodepsipeptide

The treatment of diluted solutions of the hydroxy diamides 6a and 6b in toluene with HCl gas at 100° gave the dimeric, 14‐membered cyclodepsipeptide 10 in up to 72% yield (Scheme 3). The same product was formed from the linear dimer of 6b , the depsipeptide 11 , under the same conditions (cf. Scheme 4). All attempts to prepare the cyclic seven‐membered monomer 9 , starting with different precursors and using different lactonization methods failed, and 10 was the only product which was isolated (cf. Scheme 6). For example, the reaction of the ester 20 with NaH in toluene at 80° led exclusively to the cyclodimer 10 . On the other hand, the base‐catalyzed cyclization of the hydroxy diester 22 , which is the ‘O‐analogue' of 20 , yielded neither the seven‐membered dilactone, nor the 14‐membered tetralactone, but only the known trimer 23 and tetramer 24 of 2,2‐dimethylpropano‐3‐lactone (cf. Scheme 7).     

Hydroxamic acid polymers

A polymer bearing hydroxamic acid groups and having a high affinity for iron(III) was prepared through the following procedure. Acryloylalanine (III), prepared by the reaction of acryloyl chloride with alanine, was treated with N-hydroxysuccinimide in the presence of dicyclohexylcarbodiimide to give the N-hydroxysuccinimide ester (IV). The ester IV was polymerized by using AIBN in dioxane to give polymer V. Treatment of polymer V with methylhydroxylamine in DMF gave the hydroxamic acid polymer II. The water-soluble polymer II was purified by dialysis or by gel-permeation chromatography (GPC) on Sephadex G-25. Analytical GPC on Sephadex G-200 and Sepharose 4B indicate that the average molecular weight of the polymer is in the range of 5 × 10⁵ to 1 × 10⁶. The presence of hydroxamic acid groups is confirmed by the intense red-brown color produced by the addition of iron(III) to a 50% aqueous DMF solution of the polymer under acidic conditions. In pure water the polymer-iron complex precipitates as a tan solid. Iron-binding studies of the polymer reveal that the iron(III) trihydroxamic acid complex FeA₃ forms at low concentrations of iron. At higher iron levels a lower order of stability is apparent, which can be accounted for by the conversion of FeA₃ to FeA₂⁺. In contrast, the FeA₃ complex of the trihydroxamic acid deferoxamine-B is stable at all iron levels. These results are consistent with the polymer structure, which for steric reasons would favor a stable complex, FeA₂⁺, between iron and two adjacent hydroxamic acid groups. An FeA₃ complex would be expected to have a lower stability as a result of either bond angle strain and atomic compression, or a lower probability in bringing a third hydroxamic acid into position to form the octahedral complex.     

Synthesis of unsymmetrical 2,4-dialkylpyrazolo[1,5-a]-1,3,5-triazines

The reaction of 3-aminopyrazole with imidate esters such as ethyl acetimidate, gave N-(pyrazol-3-yl)acetamidine (1) rather than the isomeric 2-acetamidoyl-3-aminopyrazole. Ring closure of 1 with orthoesters such as ethyl propionimidate, afforded unsymmetrically substituted 2.4-dialkylpyrazolo[1,5-a]-1,3,5-triazines such as 4-ethyl-2-methylpyrazolo[1,5-a]-1,3,5-triazine (3). The structure of 1 was confirmed by several alternate syntheses. The unique feature of this two-step synthetic approach to the synthesis of pyrazolo[1,5-a]-1,3,5-triazines is that it is a convenient method of preparing fused triazines based on available pyrazoles rather than the less accessible dialkyltriazines.     

The solid solutions Na3(1−x)Alx2xPO4: room temperature powder neutron diffraction studies of the two compositions x=0.025 and x=0.2

The crystal structures of the high-temperature modifications of sodium and silver orthophosphates have been determined using powder neutron diffraction (PND) data. II-Na₃PO₄ adopts the space group Fm3m with at 400°C. The PO³⁻₄ group is centered around the origin, but it shows high orientational disorder. The sodium ions occupy the and sites. II-Ag₃PO₄, at 650°C, is similar with . The structure of I-Ag₃PO₄ at room temperature

Full-size image (<1K)

has been re-examined by single-crystal X-ray diffraction. The derived model, with R=0.019 for 116 independent reflections, is in agreement with the latest work reported in the literature. The structure of I-Ag₃PO₄ at 375°C, as determined by PND, has , and displays no gross modifications from that observed at 25°C, although the anisotropic nature of the silver sites is markedly more pronounced at this higher temperature. The cation mobility is discussed in relation to the high-temperatures structures.     

A universal temperature controlled membrane interface for the analysis of volatile and semi-volatile organic compounds

A universal temperature controlled membrane interface (TCMI) has been constructed for hollow-fibre membranes. The membrane temperature is controllable in the range -70 to 250 degrees C using an electric heater and a flow of cooled nitrogen or helium gas. Volatile and semi-volatile organic compounds may be detected either by continuous diffusion across the membrane or by in-membrane pre-concentration followed by thermal desorption into the detector. The TCMI interface is demonstrated in combination with mass spectrometry and GC-MS, for the determination of VOCs and SVOCs in aqueous and air samples and for the on-line monitoring of a bioreactor.     

Evaluation of protocols for reproducible electrospray in-source collisionally induced dissociation on various liquid chromatography/mass spectrometry instruments and the development of spectral libraries

Mass spectral libraries provide a tool for identifying unknown compounds using both molecular weight and fragmentation information. Mass spectrometers with electrospray ionisation (ESI) and atmospheric chemical ionisation (ApCI) sources have the capability to produce data of this type using in-source collisionally induced dissociation (CID), and in-source CID libraries can be created. Due to the variation in electrospray source design from different instrument manufacturers, the production of reproducible in-source CID spectra that can be used in libraries for all instrument types is not a trivial task. To date, the evaluation of the production of in-source CID libraries has tended to focus on similar instruments from one manufacturer. The studies have also tended to focus on specific compound classes, with a limited molecular weight range.This report describes the findings from the investigation of protocols for the creation of mass spectral libraries using ESI in-source CID on six instruments from four different manufacturers. The overall goal was to create a spectral library for the identification of unknowns. The library could then be applied across all manufacturers' electrospray instruments.Two different experimental protocols were attempted. The first used a tuning compound to establish standard ESI source conditions, with fixed fragmentation potentials. The second involved the attenuation of the [M + H](+) ion to a known degree. A diverse range of compounds (pharmaceutical, photographic, pesticides) was tested to establish the reproducibility of the spectra on the six instruments. Both protocols produced spectra on the various instruments that in many cases were very similar. In other examples, the spectra differed not only in their relative ion abundances, but also in terms of the spectral content. Important observations regarding the effect of ion source design are also reported.The degree of spectral reproducibility was calculated off-line by comparing the five most abundant ions (20% for each ion that matches) from each spectrum on each instrument. This approach was adopted, as we do not possess a software package that met our requirements for spectral comparison. Match factors (% fit) were calculated by comparing each spectrum against the spectra recorded for the same compound and then for all other compounds, on each instrument. The % fit values derived by the off-line approach gave a clear view of the spectral reproducibility from instrument to instrument and also discriminated the spectra of the various compounds from each other. The applicability of this approach was tested using a blind trial in which several compounds were presented as unknowns, their in-source CID spectra recorded and the five-ion approach used for identification.     

Voltammetric Reduction of alpha- and gamma-[S2W18O62]4- and alpha-, beta-, and gamma-[SiW12O40]4-: isomeric dependence of reversible potentials of polyoxometalate anions using data obtained by novel dissolution and conventional solution-phase processes

Comparative studies on the voltammetric reduction of the alpha and gamma isomers of Dawson [S(2)W(18)O(62)](4)(-) and alpha, beta, and gamma forms of Keggin [SiW(12)O(40)](4)(-) polyoxometalate anions have been undertaken. For the six reversible one-electron [S(2)W(18)O(62)](4)(-)(/5)(-)(/6)(-)(/7)(-)(/8)(-)(/9)(-)(/10)(-) processes in acetonitrile, reversible potentials (E(0)(')) were found to be independent of isomeric form within experimental error (+/-5 mV). However, because both the alpha and gamma* isomers of [Bu(4)N](4)[S(2)W(18)O(62)] are insoluble in water, solid-state voltammetric studies with microcrystals adhered to electrode surfaces in contact with aqueous Et(4)NCl and Bu(4)NCl electrolyte media were also possible. Although no isomeric distinction was again detected in the solid-state studies, it was found that reduction of adhered solid by four or more electron equivalents led to rapid dissolution. When Et(4)NCl was the electrolyte, this dissolution process coupled with potential cycling experiments enabled conventional solution-phase data to be obtained in water for the analogous six one-electron reduction steps previously detected in acetonitrile. A strong medium effect attributed to Lewis acidity effects was apparent upon comparison with E(0)(') data obtained in water and acetonitrile. In contrast, with the [SiW(12)O(40)](4)(-) system, E(0)(') values for the [SiW(12)O(40)](4)(-)(/5)(-)(/6)(-)(/7)(-) processes in acetonitrile exhibited a larger (about 70 mV) dependence on isomeric form, and the isomerization step, [gamma-SiW(12)O(40)](6)(-)--> [alpha-SiW(12)O(40)](6)(-), was detected on the voltammetric time scale. The influence of isomeric form on reversible potential data is considered in terms of structural and charge density differences exhibited in the [S(2)W(18)O(62)](4)(-) and [SiW(12)O(40)](4)(-) systems studied in this paper and published data available on the alpha, beta, gamma, and gamma isomers of [As(2)W(18)O(62)](6)(-) and [P(2)W(18)O(62)](6)(-) Dawson anions and Keggin systems.     

Application of Arene-Ruthenium Chemistry to a Formal Total Synthesis of OF 4949 III

A convergent formal total synthesis of OF 4949 III is described. Arene-ruthenium chemistry was used in the construction of the diaryl ether linkage in high yield, and cycloamidation under high dilution conditions (0.005 M) was achieved using DPPA as coupling reagent. SmI(2) was used to reductively remove the 2-iodoethyl ester protecting group in the presence of DMPU or HMPA.     

Structural studies in main group chemistry : XIX. Organotin(IV) derivatives of tetracyanoethylene, 7,7,8,8-tetracyanoquinodimethane and 2,3,5,6-tetrachlorobenzoquinone. The isolation of stable organotin-substituted radicals

The reaction of organotin chlorides with the lithium salt of 7,7,8,8-tetracyanoquinodimethane (TCNQ) or hexaalkylditins with TCNQ yield stable organotin-substituted free radicals of the types R₃SnTCNQ^. (R = Me, n-Pr, n-Bu) and Me₂Sn(TCNQ^.)₂. The reaction of hexaphenylditin with TCNQ yields a (σ → π) charge transfer complex of stoichiometry (Ph₃SnSnPh₃)·TCNQ, whilst [Me₂SnCl(terpyridyl)⁺](TCNQ^-·) was isolated from the reaction of [Me₂SnCl(terpyridlyl)⁺][Me₂SnCl₃^-] and LiTCNQ. The oxidation of hexaalkylditins by tetracyanoethylene (TCNE) yields stable free radicals of the type R₃SnTCNE·, but treatment with 2,3,5,6-tetrachlorobenzoquinone yields either R₃SnOC₆Cl₄O·-p (R = Me) or R₃SnOC₆Cl₄OSnR₃-p (R = n-Bu, Ph). Tin-119 Mössbauer spectroscopy shows that the derivatives R₃SnTCNQ· and R₃TCNE· have trigonally-bipyramidally coordinated tin with planar [SnC₃] skeletons and bridging [TCNQ·] and [TCNE·] groups forming infinite one-dimensional chain structures. Me₃SnOC₆Cl₄O·-p was inferred to possess a similar structure but with oxy bridges forming chains with a Sn---O---Sn---O backbone. Me₂Sn(TCNQ·)₂ has a structure intermediate between tetrahedral and octahedral with a non-linear MeSnMe unit and anisobidentate chelation by two TCNQ groups. The TCNQ derivatives were of two types: (i) “green” or “brown”, indicative of delocalisation of the Ione electron over the cyanoquinone ligand, and (ii) a “blue” form in which spin-pairing of the Ione electron between adjacent organic groups takes place. Me₃SnTCNQ· may exist in both forms depending upon the mode of preparation.     

Coordination properties of a diarylaza crown ether appended with a luminescent [Ru(bipy)3]2+ unit

The [Ru(bipy)(2)(1)](PF(6))(2) (bipy refers to 2,2'-bipyridine) complex, comprising a ruthenium(II) tris(2,2'-bipyridine) luminophore covalently linked to a di[(o-triethyleneglycoxy)phenyl]amine crown ether 1, has been synthesized and fully characterized. The photophysical properties of this metal complex have been examined in solution at ambient temperature. Luminescence from the metal complex is enhanced significantly in the presence of various adventitious cations, including protons. In particular, Li(+) cations bind to the crown ether, as evidenced by (1)H NMR and luminescence spectroscopy. Cation binding serves to decrease the rate of reductive quenching of the triplet state of the metal complex, thereby increasing the extent of luminescence. The solution-phase conformation of [Ru(bipy)(2)(1)](PF(6))(2), with and without encapsulated Li(+), has been examined by 2-D NMR and by molecular dynamics simulations.