Serendipitous synthesis of a ditwistane: a one-step access!

4-tert-Butyl-2-cyclohexen-1-one dimerizes in THF solution via its kinetic enolate, leading to di-tert-butylditwistane 8 in up to 36% yield (−78 °C → room temp., protonolysis, flash chromatography). X-ray crystallography shows that 8 incorporates one R and one S enantiomer of the starting ketone; none of the diastereomeric ditwistanes epi-8, epi’-8 or iso-8 was isolated. This means that the formation of 8 proceeds with mutual kinetic resolution and 100% induced diastereoselectivity.     

MECHANISMS OF 1,2-DIOXETANE DECOMPOSITION: THE ROLE OF ELECTRON TRANSFER

Abstract— The chemiluminescence from the cleavage of a number of 1,6-diaryl-2,5,7,8-tetraoxabicyclo[4.2.0]octanes 1 has been examined. The ease of oxidation of (object) the aryl moiety strongly influences both the stability and chemiluminescence efficiency of these 1,2-dioxetanes. When Ar is difficult to oxidize, 1 is comparable in stability to simple. alkyl-substituted 1,2-dioxe-tanes and affords triplet excited states in moderate yield. Both biradical and concerted cleavage mechanisms have been suggested to explain this behavior. However, when Ar is a readily oxidized group, 1 is substantially destahilized and gives excited singlet states in high yield. In this instance 1 is analogous to a number of bioluminescent systems. Cleavage mechanisms involving intramolecular electron transfer are proposed to account for this observation. In certain cases thermolysis of 1 occurs by both types of mechanism in competition, and the electron transfer mechanism may be selectively catalyzed by polar, protic media.     

Über „Lithovanadate”︁: Zur Kenntnis von Rb2[LiVO4] und Cs2[LiVO4]

On ?Lithovanadates”?: Rb₂[LiVO₄] and Cs₂[LiVO₄] By heating of well ground mixtures of the binary oxides [A₂O, Li₂O, V₂O₅, A : Li: V = 2.2 : 1.1 : 1.0 (A = Rb, Cs); Ni-tube, 750° 25 d] we obtained Rb₂[LiVO₄] and Cs₂[LiVO₄] colourless, orthorhombic single crystals. We found a new type of ?Lithovanadate”?-structure: space group Cmc2₁; a = 587.9(1), b = 1170.1(1), c = 793.3(1) pm, Z = 4 (A = Rb) bzw. a = 610.5(1), b = 1222.6(3), c = 815.5(2) pm, Z = 4 (A = Cs). The structure was determined by four-circle diffractometer data [MoKα -radiation; 997 from 1157 I₀(hkl), R = 7.75%, R_w = 5.54% (A = Rb); 686 from 686 I₀(hkl), R = 6.97%, R_w = 4.20% (A = Cs)] parameters see text. The Madelung part of Lattice Energy, MAPLE, and Effective Coordination Numbers, ECoN, these via Mean Fictive Ionic Radii, MEFIR, have been calculated.     

Structure-function analysis of an enzymatic prenyl transfer reaction identifies a reaction chamber with modifiable specificity

Fungal indole prenyltransferases participate in a multitude of biosynthetic pathways. Their ability to prenylate diverse substrates has attracted interest for potential use in chemoenzymatic synthesis. The fungal indole prenyltransferase FtmPT1 catalyzes the prenylation of brevianamide F in the biosynthesis of fumitremorgin-type alkaloids, which show diverse pharmacological activities and are promising candidates for the development of antitumor agents. Here, we report crystal structures of unliganded Aspergillus fumigatus FtmPT1 as well as of a ternary complex of FtmPT1 bound to brevianamide F and an analogue of its isoprenoid substrate dimethylallyl diphosphate. FtmPT1 assumes a rare α/β-barrel fold, consisting of 10 circularly arranged β-strands surrounded by α-helices. Catalysis is performed in a hydrophobic reaction chamber at the center of the barrel. In combination with mutagenesis experiments, our analysis of the liganded and unliganded structures provides insight into the mechanism of catalysis and the determinants of regiospecificity. Sequence conservation of key features indicates that all fungal indole prenyltransferases possess similar active site architectures. However, while the dimethylallyl diphosphate binding site is strictly conserved in these enzymes, subtle changes in the reaction chamber likely allow for the accommodation of diverse aromatic substrates for prenylation. In support of this concept, we were able to redirect the regioselectivity of FtmPT1 by a single mutation of glycine 115 to threonine. This finding provides support for a potential use of fungal indole prenyltransferases as modifiable bioreactors that can be engineered to catalyze highly specific prenyl transfer reactions.     

Non-projectability of polytope skeleta

We investigate necessary conditions for the existence of projections of polytopes that preserve full k-skeleta. More precisely, given the combinatorics of a polytope and the dimension e of the target space, what are obstructions to the existence of a geometric realization of a polytope with the given combinatorial type such that a linear projection to e-space strictly preserves the k-skeleton. Building on the work of Sanyal (2009), we develop a general framework to calculate obstructions to the existence of such realizations using topological combinatorics. Our obstructions take the form of graph colorings and linear integer programs. We focus on polytopes of product type and calculate the obstructions for products of polygons, products of simplices, and wedge products of polytopes. Our results show the limitations of constructions for the deformed products of polygons of Sanyal and Ziegler (2010) and the wedge product surfaces of Rörig and Ziegler (2011) and complement their results.     

Line Shift Investigations for Different Isotopomers of Carbon Monoxide

Line shift coefficients for five lines of five different isotopomers in the fundamental band of CO in the spectral region near 2058 cm⁻¹were measured using a three channel lead salt diode laser spectrometer. The study includes the linesP(3) of¹³C¹⁷O,R(3) of¹³C¹⁸O,P(9) of¹²C¹⁸O,P(10) of¹³C¹⁶O, andP(21) of¹²C¹⁶O, and covers collisions with N₂, O₂, H₂, D₂, He, Ne, Ar, Kr, and Xe. Line shifts of the isotopomers¹³C¹⁶O,¹²C¹⁸O,¹³C¹⁸O, and¹³C¹⁷O were determined for the first time. Within the experimental uncertainty no significant dependence of the shift effect on the isotopomer was found. TheR-branch line under study shows a smaller line shift coefficient than aP-branch line with a similar rotational quantum number. With increasing mass of the noble gas perturber the absolute size of the shift coefficient increases. Moreover self- and nitrogen-broadening coefficients for the isotopomer lines were determined. Compared to previous measurements no significant deviations between different isotopomers were observed.     

A variational approach to the construction and Malliavin differentiability of strong solutions of SDE’s

In this article we develop a new approach to construct solutions of stochastic equations with merely measurable drift coefficients. We aim at demonstrating the principles of our technique by analyzing strong solutions of stochastic differential equations driven by Brownian motion. An important and rather surprising consequence of our method which is based on Malliavin calculus is that the solutions derived by Veretennikov (Theory Probab Appl 24:354–366, 1979) for Brownian motion with bounded and measurable drift in $\mathbb{R }^{d}$ are Malliavin differentiable. Further, a strength of our approach, which does not rely on a pathwise uniqueness argument, is that it can be transferred and applied to the analysis of various other types of (stochastic) equations: We obtain a Bismut–Elworthy–Li formula (Elworthy and Li, J Funct Anal 125:252–286, 1994) for spatial derivatives of solutions to the Kolmogorov equation with bounded and measurable drift coefficients. To derive the formula, we use that our approach can be applied to obtain Sobolev differentiability in the initial condition in addition to Malliavin differentiability of the associated stochastic differential equations. Another application of our technique is the construction of unique solutions of the stochastic transport equation with irregular vector fields. Moreover, our approach is also applicable to the construction of solutions of stochastic evolution equations on Hilbert spaces.