Synthesis of the sedum and related alkaloids: A personal perspective

Significant recent contributions to the synthesis of the sedum alkaloids are discussed. Related compounds, such as pinidinol, porantheridine, dumetorine and the tetraponerines are also included. The syntheses are categorised according to the key motif or chemistry employed: isoxazolidines, metathesis, asymmetric aza-Michael, heterocycle lithiation, organocatalysis, aromatic heterocycles and chiral imines.     

Synthesis of sedamine by tethered cyclofunctionalisation

The piperidine alkaloid, (+)-sedamine has been synthesised starting from (S)-1-phenyl-3-butenol using a stereoselective, intramolecular palladium-catalysed cyclocarbonylation reaction of a substituted hydroxylamine.     

Yeast adaptation on softwood prehydrolysate

Several strains and genera of yeast, includingSaccharomyces cerevisiae D₅A,Pachysolen tannophilus, S. cerevisiae K-l,Brettanomyces custersii, Candida shehatae, andCandida acidothermophilum, are screened for growth on dilute acid-pretreated softwood prehydrolysate. Selected softwood species found in forest underbrush of the western United States, which contain predominantly hexosan hemicellulose, were studied. This phase of the work emphasized debarked Douglas fir. The two best initial isolates were gradually selected for improved growth by adaptation to increasing prehydrolysate concentrations in batch culture, with due consideration of nutrient requirements. Microaerophilic conditions were evaluated to encourage tolerance of pretreatment hydrolysate, as well as ethanol product. Adaptation and simultaneous saccharification and fermentation (SSF) results are used to illustrate improved performance with an adapted strain, compared to the wild type.     

The correspondence between the molecular orbital and differential ionization energies methods

The correspondence between Self-Consistent Hückel MO methods and Differential Ionization Energies methods is discussed in terms of the approximations used for the diagonal matrix elements. The two methods are shown to be equivalent if electronic correlation is neglected. Ground-state properties of the hydrogen halides are calculated by these simple methods and shown to be in good overall agreement with experimental data.

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Zusammenfassung Die Übereinstimmung zwischen selbstkonsistenten Hückel MO-Methoden und Methoden der Differentiellen Ionisierungsenergien wird in Termen solcher Näherungen diskutiert, die für die diagonalen Matrixelemente benutzt werden. Es wird gezeigt, daß die beiden Methoden äquivalent sind, wenn die Elektronenkorrelation vernachlässigt wird. Grundzustandseigenschaften der hydrogen halides werden mit diesen einfachen Methoden ausgerechnet und zeigen sich in überall guter Übereinstimmung mit experimentellen Daten.

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Résumé La correspondance entre les méthodes SCF Hückel et d'énergie d'ionisation différentielle est discutée en fonction des approximations utilisées pour les éléments de matrice diagonaux. Les deux méthodes sont équivalentes si la corrélation électronique est négligée. Les propriétés de l'état fondamental des acides halogènés sont calculées par ces méthodes simples et l'on constate un accord raisonnable avec les données expérimentales.

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Work supported by the Research Corporation and the National Science Foundation (Grant GY 2657/67).     

Cycloadditions of methyl dichloroacetate to electrophilic alkenes

Seven MIRC reactions of methyl dichloroacetate with alkenes containing electron-withdrawing groups were used to make 1-chlorocyclopropanecarboxylates.     

Rouse–Bueche theory and the calculation of the monomeric friction coefficient in a filled system

Direct experimental access to the monomeric friction coefficient (ζ₀) relies on the availability of a suitable polymer dynamics model. Thus far, no method has been suggested that is applicable to filled systems, such as filled rubbers or microphase‐segregated A–B–A thermoplastic elastomers (TPEs) at T_g,B < T < T_g,A. Building upon the procedure proposed by Ferry for entangled and unfilled polymer melts, the Rouse–Bueche theory is applied to an undiluted triblock copolymer to extract ζ₀ from the linear behavior in the rubber‐glass transition region, and to estimate the size of Gaussian submolecules. When compared at constant T – T_g, the matrix monomeric friction factor is consistent with the corresponding value for the homopolymer melt. In addition, the characteristic Rouse dimensions are in good agreement with independent estimates based on the Kratky–Porod worm‐like chain model. These results seem to validate the proposed approach for estimating ζ₀ in filled systems. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1437–1442     

Radical‐cured block copolymer‐modified thermosets

Poly(ethylene‐alt‐propylene)‐b‐poly(ethylene oxide) (PEP‐PEO) diblock copolymers were synthesized and added at 4 wt % to 2,2‐bis[4‐(2‐hydroxy‐3‐methacryloxypropoxy)phenyl]propane (BisGMA), a monomer that cures using free radical chemistry. In separate experiments, poly(ethylene glycol) dimethacrylate (PEGDMA) was combined as a secondary monomer with BisGMA and the monomers were loaded with 4 wt % PEP‐PEO. The diblock copolymers self‐assembled into well‐dispersed spherical micelles with PEP cores and PEO coronas. No appreciable change in the final extent of cure of the thermosets was caused by the addition of diblock copolymer, except in the case of BisGMA, where the addition of the block copolymer increased extent of cure by 12%. Furthermore, the extent of cure was increased by 29% and 37% with the addition of 25 and 50 wt % PEGDMA, respectively. Elastic modulus and fracture resistance were also determined, and the values indicate that the addition of block copolymers does not significantly toughen the thermoset materials. This finding is surprising when compared with the large increase in fracture resistance seen in block copolymer‐modified epoxies, and an explanation is proposed. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

On curves of constant torsion I

We give an explicit construction of a closed curve with constant torsion and everywhere positive curvature. We also discuss the restrictions on closed curves of constant torsion when they are constrained to lie on convex surfaces.     

Efficient path tracking methods

Path tracking is the fundamental computational tool in homotopy continuation and is therefore key in most algorithms in the emerging field of numerical algebraic geometry. Though the basic notions of predictor-corrector methods have been known for years, there is still much to be considered, particularly in the specialized algebraic setting of solving polynomial systems. In this article, the effects of the choice of predictor method on the performance of a tracker is analyzed, and details for using Runge-Kutta methods in conjunction with adaptive precision are provided. These methods have been implemented in the Bertini software package, and several examples are described.