Delocalized carotenoid cations in relation to the soliton model

A series of charge-delocalized carotenoid mono- and dications have been prepared by treatment of selected carotenoids with Br?nsted and Lewis acids. The detailed structures of the carbocations were established by NMR studies in the temperature range from -10 to -20 degrees C. The general strategy for structure elucidation by NMR of several cationic components in a mixture is outlined. Bond type and regions of bond inversion were established, as well as the charge distribution, which was determined from the difference in (13)C chemical shift at each carbon. This method gave a more accurate estimate for the partial charges than by using the Spiesecke-Schneider relationship. The resulting charge distribution was used as models for the structure of charged solitons. These carotenoid cations have the most delocalized charge so far determined, and the monocations represent the first experimental structure determination of positively charged solitons. The soliton width determined here is in good agreement with the results of previous AM1 calculations.     

The moment problem associated with the Stieltjes-Wigert polynomials

We consider the indeterminate Stieltjes moment problem associated with the Stieltjes-Wigert polynomials. After a presentation of the well-known solutions, we study a transformation T of the set of solutions. All the classical solutions turn out to be fixed under this transformation but this is not the case for the so-called canonical solutions. Based on generating functions for the Stieltjes-Wigert polynomials, expressions for the entire functions A, B, C, and D from the Nevanlinna parametrization are obtained. We describe T⁽ⁿ⁾(μ) for when μ=μ₀ is a particular N-extremal solution and explain in detail what happens when n→∞.     

Regioselectivity in reactions of alkynylmetal complexes with pyrimidinones

Regioselectivity is observed in $\text{1}\text{1}$-adduct formation between phenylethynyltriisopropoxytitanium and 2(1/H)-pyrimidinones; the new carbon-carbon bond is formed at C(4). In contrast the 3,4- and 3,6-dihydro products are formed together from the corresponding magnesium and lithium reagents, but from the magnesium compound the major product is the 3,6-dihydro isomer. Ethynylmagnesium bromide gives equimolar amounts of the 3,4- and 3,6-dihydro isomers. Dehydrogenation of the products gives the alkynylated aromatic heterocycles.     

Structure ofN,N′-diphenyl-N-(2-pyridmyl)urea hemihydrate: A crystallographic and spectroscopic investigation

N,N-diphenyl-N-(2-pyridinyl)urea hemihydrate (C₁₈H₁₅N₃O·1/2H₂O) has been studied by crystallographic and NMR-spectroscopic methods. The crystals are monoclinic,I2/a with unit-cell parameters at 86K,a=17.820(1),b=14.857(1),c=11.299(1) Å,=99.17(1)° andZ=8. The structure was solved by direct methods which involved the use of seventeenmeasured triplet phases in the starting set. This appears to be the first centrosymmetric structure solution being based extensively on physically acquired X-ray phases. Refinement was by full-matrix least-squares involving 3843F _o and yieldedRR _w=0.042. The shape of the molecules can be described approximately by two, nearly orthogonal planes, one containing pyridine, the urea group and the NH-bonded phenyl ring, the other containing the second phenyl ring. Both the X-ray and the NMR studies show that there is an intramolecular H-bond between the urea N and the pyridine N atoms. When pH is lowered, this H-bond is weakened, and eventually breaks due to protonation of the pyridine ring. The X-ray study indicates that about 20% of the pyridine rings in the crystal are protonated. The structure details of the rings, in particular the valency angles, have been analyzed in relation to the orientation of the rings relative to the main molecular plane, and the electronic effects of the substituants.     

PC-fractions and Szegö polynomials associated with starlike univalent functions

Each classS , 0<1, functions starlike of order , can be associated with a Carathéodory function mapping the unit disk onto a subset of the right halfplane. This Carathéodory function determines a certain continued fraction (PC-fraction) and a family of polynomials orthogonal on the unit circle (Szegö polynomials). We compute the PC-fraction and Szegö polynomials corresponding to eachS and do some investigations on these PC-fractions and Szegö polynomials.     

(22S)-6-O-acetyl-21betaH-hopane-3beta,6beta,22,29-tetrol from oakmoss (Evernia prunastri)

A novel hopanoid triterpene, (22S)-6-O-acetyl-21betaH-hopane-3beta,6beta,22,29-tetraol, was isolated from oakmoss (Evernia prunastri (L.) Ach.), as identified from 1H, 13C, DEPT, COSY, NOESY, HSQC and HMBC NMR, MS and IR spectroscopy. During recrystallisation a new compound, 30-nor-6-O-acetyl-3beta,6beta-dihydroxy-21alphaH-hopan-22-one, was formed by a formal loss of methanol from the dihydroxypropyl moiety. No biological activity was found for the naturally occurring compound upon testing against a series of fish and human pathogenic bacteria.     

An alternative multiple-trapping LC-SPE-NMR system

In this paper, we describe approaches that make RP LC-SPE-NMR simpler, and in our opinion, result in more reliable methods for trapping and subsequent transfer of separated trace-level compounds to the NMR. An SPE unit based on a commercially available, low dead-volume 10 port high-pressure column selector gives the possibility of trapping compounds on nine individual SPEs that have standard fittings. This allows the operator to employ specific stationary phases that are not available as SPEs in commercially available LC-SPE-NMR systems. Multiple trappings of small compounds like monuron, 1-(4-chlorophenyl)-3-methylurea, and 4-chlorophenylurea were easily performed employing a porous-carbon SPE material. The system was optimized to elute the SPE-trapped compounds to the NMR probes in as small a volume as possible using back-flushing. The proper match of NMR probe volume and SPE column inner diameter and elution volume was discussed, as well as the necessity of drying loaded SPEs prior to NMR transfer when using porous-carbon SPE material.     

o-Quinonoid heterocycles: synthesis and crystal structure of 2,3-dicyano-5,7-bismethylthieno[3,4-b]pyrazine

2,3-Dicyano-5,7-bismethylthieno[3,4-b]pyrazine (5), has been obtained as a minor product from reactions of S-nucleophiles with 5,6-di(1-bromoethyl)pyrazine-2,3-dicarbonitrile (3), or from a condensation of 4-hydroxy-2,5-dimethyl-2,3-dihydrothiophen-3-one (4) with diaminomaleonitrile. The molecular structure of compound 5 has been confirmed by X-ray diffraction. A partial separation of charge causes a donor-acceptor type arrangement of the planar molecules in uniform parallel stacks with an interplanar spacing of 3.334(2) A at 100 K.     

Stereochemical Definition of the Natural Product (6R,10R,13R, 14R,16R,17R,19S,20S,21R,24S,25S,28S,30S,32R,33R,34R,36S,37S,39R)‐Azaspiracid‐3 by Total Synthesis and Comparative Analyses

The previously accepted structure of the marine toxin azaspiracid‐3 is revised based upon an original convergent and stereoselective total synthesis of the natural product. The development of a structural revision hypothesis, its testing, and corroboration are reported. Synthetic (6R,10R,13R,14R,16R,17R,19S,20S,21R,24S,25S,28S,30S,32R, 33R,34R,36S,37S,39R)‐azaspiracid‐3 chromatographically and spectroscopically matched naturally occurring azaspiracid‐3, whereas the previously assigned 20R epimer did not.