NMR examination of fossil fuels Part I: solid samples

Nuclear Magnetic Resonance (NMR) spectroscopy¹ is one of the most valuable experimental methods available for the analysis of the chemical structure and composition of fuel products. This article describes applications of NMR in the analysis of solid fuels. Part II will describe applications of NMR in the liquid phase.²     

The total synthesis of joubertinamine

The recently described N-demethyl-seco-mesembrane alkaloid joubertinamine¹¹ (1) and the 3a-aryloctahydroindole alkaloid mesembranone (17) are accessible via a common synthetic pathway.     

The structure of PR toxin,a mycotoxin from Penicillium roqueforti

An earlier report from our laboratories described the isolation, partial characterization, and biological activity of a new mycotoxin from cultures of Penicillium roqueforti, and the name “PR toxin” was tentatively assigned to the compound. On the basis of further chemical and spectral evidence we propose that PR toxin has the structure shown in formula 1.¹     

Base induced rearrangements of α-halosulfones α-sulfonyl carbanions with bridgehead electrofugal centre

The KOtBu-induced Ramberg-Bäcklund reaction of the trans bicyclic halosulfones 1 and 2 is studied in DME and DMSO. Strong indications are obtained that the reaction proceeds via the intermediate Z-episulfone with retention-inversion (involving exo-S-geometry).¹ Base induced epimerization at the nucleofugal centre (from axial to equatorial halogen) is demonstrated in DMSO.     

The total synthesis of rac-o-methyljoubertiamine

The Claisen-Eschenmoser[3.3]sigmatropic rearrangement¹ of appropriately functionalized 3-aryl-2-cyclohexenols provides a ready synthesis of mesembrane alkaloids.^2,3 Herein we describe the total synthesis of rac-O-methyljoubertiamine⁴¹ (1) as part of the development of a general synthetic route to these alkaloids, involving the aforementioned rearrangement as the key synthetic step.     

Concerning the physical solution of the inverse vibrational problem

The physical solution of the inverse vibrational problem suggested by Jordanov and Nikolova is discussed in detail and some errors corrected.¹     

Synthesis of 6,7-dehydro-5-oxo-prostaglandin I1: A stable analog of prostacyclin

Stable analogs of prostacyclin, 6,7-dehydro-5-oxo-prostaglandin I₁ and derivatives, were synthesized from readily available 6,7-dehydro-5-hydroxy-prostaglandin F_1α methyl ester²     

Heterodiene syntheses—XXIV: Changes in the reactivity of 2-oxoindolin-3-ylidene derivatives with ethylvinylether induced by electron-withdrawing groups. The importance of the lumo coefficients

Ethylvinylether reacts in acetonitrile with 1 - acetyl - 2 - oxoindolin - 3 - ylidene derivatives β,β -disubstituted with electron-withdrawing substituents. When the β-substituents act by inductive effect alone, a regiospecific β-attack² occurred giving rise, through a 1,4 - cycloaddition, to 2,3 - dihydropyran[2,3-b]indoles. As a by product Michael adduct can be obtained. When the β-substituent act by conjugative effect also, a regiospecific α-attack² occurred giving rise to spiro - dihydropyrane or - cyclobutane - 2 - oxoindolines depending on the nature of the substituents.In a less polar solvent, the latter ones are formed together with the adduct arising from the regioisomeric β-attack.The overall reactivity can be rationalized in terms of LUMO coefficients of the hetcrodiene and of different stabilization offered by the solvent to the various reaction pathways.     

Analysis of anti-hormones

Residues of anti-hormones in biological materials are routinely determined at the ppb-level¹ by specific detection through high-performance thin-layer chromatography with fluorimetric detection or capillary gas chromatography with electron-capture detection backed up by a selective single step extraction.     

A convenient procedure for smooth palladium-catalyzed allylic alkylation by sodium dimethyl malonate and cyclopentadienide. A new synthesis of allylic substituted cyclopentadienes.

The use of the Pd(dba)¹₂/dpe catalytic system in allylic alkylation allows sodium dimethyl malonate to react with allylic acetates at room temperature. According to this procedure, a novel synthesis of allylic substituted cyclopentadienes is described through the use of cyclopentadienide anion as a nucleophile.     

Catalyse asymetrique avec des complexes chiraux de rhodium-diop—V: Effets des substituants lors de la reduction d'acides N-acylamino cinnamiques

Asymmetric synthesis of phenylalanine and its derivatives can be accomplished by asymmetric reduction of N-acylamino-cinnamic acid derivatives with CIRh(+)DIOP² as catalyst. The optical yield is influenced by the stereochemistry of the double bond, the para-substituent on the N-benzoyl group and the esterification of the carboxy group. The mechanism of the reduction is discussed.     

The molecular structure of 4a, 5, 8, 8a-tetrahydro-11, 14-dimethoxy-7-methyl-4a-(3-methyl-2-butenyl)-5, 8a-o-benzeno-1, 4-naphthoquinone

A novel natural product, microphyllone, has been isolated from Ehretia microphylla² together with baurenol and ursolic acid. Spectroscopic techniques, derivative formation and finally X-ray diffraction have been utilized in the structure elucidation of microphyllone. The structure appears to be 4a, 5, 8,8a - tetrahydro -11,14- dihydroxy-7-methyl-4a-(3-methyl-2-butenyl)-5, 8a-o-benzeno-1, 4-naphthoquinone.     

Amino acids surrogates: an indirect method for the synthesis of peptides containing the thioasparaginyl residue

The protected pentapeptide, Boc-Tan-Cys(PMB)-Pro-Leu-GlynH₂¹ and related thioasparaginyl compounds were prepared by an indirect method from their corresponding β-cyanoalanyl precursors by treatment with H₂S/NH₃. The optical purities of thioasparagine (Tan) derivatives were evaluated by two different approaches.     

Comparison of high-performance liquid chromatography electrochemical detectors for the determination of aporphines,catecholamines and melatonin

A Bioanalytical Systems detector showed a greater sensitivity for low-level determinations of apomorphine, norepinephrine, epinephrine, dopamine, and melatonin compared to a Brinkmann Instruments system. Modifications of the Bioanalytical Systems flow cell, which have been developed in our laboratories, increase the sensitivity toward certain catecholic compounds, generally decrease baseline noise, and allow for serial coupling of detectors.²     

Radikalkomplexe vom typ Cp(CO)2MnXR. (X = S,Se)

CpMn(CO)₂ER (E = S, Se) compounds [1] belong to the rare radical derivatives of the CpMn(CO)₂ fragment. We describe herein the syntheses, electrochemistry, and organometallic reactions of CpMn(CO)₂SR ¹ (1a: R = t-C₄H₉, 1b: R = 2-adamantyl) and CpM(CO)₂SePh (2).     

The stereochemistry of the adducts derived from the Diels-Alder addition of cyclobutenes and cyclopentadienones: A caveat on some existing PMR methods for the assignment of stereochemistry

The stereochemistry of the adducts formed from the cycloaddition of tetracyclone, phencyclone and 2-methyl-3,4,5-triphenylcyclopenta-2,4-dienone with cis-3,4-dichlorocyclobutene (in one case with 3-chloro-4-methoxycyclobutene) has been evaluated in a rigorous fashion. This allows a firmly-based caveat to be issued regarding the p.m.r. probe for stereochemical evaluation of tetracyclone adducts recently proposed by Coxon and Battiste.¹     

Structure of mildiomycin,a new antifungal nucleoside antibiotic

The chemical structure of mildiomycin (1) active against powdery mildews was determined by chemical degradations and physical analyses to be 2 - [(2R, 5S, 6S) - 2 - (4 - amino - 1,2 - dihydro - 5 - hydroxymethyl - 2 -oxopyrimidin -1 - yl) - 5,6 - dihydro - 5 - L - serylamino - 2H - pyran - 6 - yl] - 5 - (3H⁺ - guanidino) - 2,4 -dihydroxyvalerate as shown in Chart 1.¹     

Methyl radical kinetics: relaxation versus H-atom abstraction in flash-photolyzed mixtures of methyl iodide and diluent gases

Time-resolved mass spectrometry has been used to deduce the de-excitation rate (k_d) of photolytically produced “hot” methyl radicals relative to the H-atom abstraction rate (K_a) from CH₃¹. The rate-constant ratios k_d(M)/k_a f Me = He, Ar, N₂, and CH₃l are 0.22 ± 0.02, 0.36 ± 0.03, 0.86 ± 0.13 and 9.6 ± 0.8, respectively.     

MOGADOC (Molecular Gas Phase Documentation) — An interactive computerised search/retrieval system

For 22 years the Section for Structure Documentation (SSD), University of Ulm, has provided a documentation service covering the literature of molecules studied in the gas phase by electron diffraction, microwave spectroscopy and other techniques. Much of the information which has been accumulated over these years has now been keyboarded to constitute a computerised database. An interactive search/retrieval system, MOGADOC, has been written using the SIMULA¹ language implemented on the UNIVAC 1100/82 computer at the University of Freiburg. MOGADOC enables the user to search the database on the basis of bibliographic, chemical and physical search terms.     

The structure of dehydrated zeolite 3A (Si/Al = 1.01) by neutron profile refinement

A neutron powder diffraction study of a dehydrated commercially available potassium exchanged zeolite A (Linde 3A) has shown that the diffraction pattern can be indexed in cubic space group $\text{Fm}\text{3}\text{c}$. For this sample there is 63% exchange of potassium for sodium (K⁺/Na⁺ = 1.69). Data collected at a neutron wavelength of 2.98 Å shows no evidence of rhombohedral distortion and suggests that the assignment of space group $\text{Fm}\text{3}\text{c}$ is correct. The final structural model is closely analogous to that found for dehydrated sodium zeolite A (J. M. Adams, D. A. Haselden, and A. W. Hewat, J. Solid State Chem.44, 245 (1982); J. J. Pluth and J. V. Smith, J. Amer. Chem. Soc.102, 4074 (1980). Unusual features of previous refinements of potassium containing zeolite A samples, i.e., “zero coordinate” cations (P. C. W. Leung, M. B. Kunz, K. Seff, and I. E. Maxwell, J. Phys. Chem.83, 741 (1979)) or potassium inside the β-cage (J. J. Pluth and J. V. Smith, J. Phys. Chem.83, 741 (1979)) have not been found. Refinements using the same 1.9 Å neutron powder diffraction data were also obtained with the models of Leung et al. and Pluth and Smith (1979) as starting points (denoted LKSM and PS, respectively) and comparison is made with these. The final R factors for the three refinements were R_pw (A. K. Cheetham and J. C. Taylor, J. Solid State Chem.21, 253 (1977)) = 10.24% for the model presented here, R_pw = 10.38% (PS model), and R_pw = 10.61% (LKSM model).