Shedding Light on the Diverse Reactivity of NacNacAl with N‐Heterocycles

The aluminum(I) compound NacNacAl (NacNac=[ArNC(Me)CHC(Me)NAr]^?, Ar=2,6‐iPr₂C₆H₃, 1 ) shows diverse and substrate‐controlled reactivity in reactions with N‐heterocycles. 4‐Dimethylaminopyridine (DMAP), a basic substrate in which the 4‐position is blocked, induces rearrangement of NacNacAl by shifting a hydrogen atom from the methyl group of the NacNac backbone to the aluminum center. In contrast, C?H activation of the methyl group of 4‐picoline takes place to produce a species with a reactive terminal methylene. Reaction of 1 with 3,5‐lutidine results in the first example of an uncatalyzed, room‐temperature cleavage of an sp² C?H bond (in the 4‐position) by an Al^I species. Another reactivity mode was observed for quinoline, which undergoes 2,2′‐coupling. Finally, the reaction of 1 with phthalazine produces the product of N?N bond cleavage.     

Water Triggers Hydrogen-Bond-Network Reshaping in the Glycoaldehyde Dimer

Carbohydrates are ubiquitous biomolecules in nature. The vast majority of their biomolecular activity takes place in aqueous environments. Molecular reactivity and functionality are, therefore, often strongly influenced by not only interactions with equivalent counterparts, but also with the surrounding water molecules. Glycoaldehyde (Gly) represents a prototypical system to identify the relevant interactions and the balance that governs them. Here we present a broadband rotational-spectroscopy study on the stepwise hydration of the Gly dimer with up to three water molecules. We reveal the preferred hydrogen-bond networks formed when water molecules sequentially bond to the sugar dimer. We observe that the dimer structure and the hydrogen-bond networks at play remarkably change upon the addition of just a single water molecule to the dimer. Further addition of water molecules does not significantly alter the observed hydrogen-bond topologies.     

Flowing maps to minimal surfaces: Existence and uniqueness of solutions

We study the new geometric flow that was introduced in the paper [12] of Topping and the author that evolves a pair of map and (domain) metric in such a way that it changes appropriate initial data into branched minimal immersions. In the present paper we focus on the existence theory as well as the issue of uniqueness of solutions. We establish that a (weak) solution exists for as long as the metrics remain in a bounded region of moduli space, i.e. as long as the flow does not collapse a closed geodesic in the domain manifold to a point. Furthermore, we prove that this solution is unique in the class of all weak solutions with non-increasing energy. This work complements the paper of Topping and the author [12] where the flow was introduced and its asymptotic convergence to branched minimal immersions is discussed.     

Free limits of Thompson’s group <Emphasis Type="Italic">F</Emphasis>

We produce a sequence of markings S _k of Thompson’s group F within the space G_n{{\mathcal G}_n} of all marked n-generator groups so that the sequence (F, S _k ) converges to the free group on n generators, for n ≥ 3. In addition, we give presentations for the limits of some other natural (convergent) sequences of markings to consider on F within G₃{{\mathcal G}_3}, including (F, {x ₀, x ₁, x _n }) and (F,{x₀,x₁,x₀ⁿ}){(F,\{x_0,x_1,x_0^n\})}.     

Phyteumosides A and B: new saponins with unique triterpenoid aglycons from Phyteuma orbiculare L

Phyteumosides A (1) and B (2), two saponins with unprecedented triterpenoid aglycons, were isolated from the aerial parts of Phyteuma orbiculare (Campanulaceae). Their structures were elucidated by spectroscopic and chemical methods and corroborated by X-ray diffraction analyses of the aglycons obtained through enzymatic hydrolysis. The aglycon of 1 can be considered as an incompletely cyclized onoceroid or gammaceroid triterpene with two additional tetrahydropyran rings arising from oxygen bridges. Compound 2 possesses a new 17-polypodene aglycon.     

Palladium-catalyzed C-H arylation of 2,5-substituted pyrroles

The palladium-catalyzed direct arylation of 2,5-substituted pyrrole derivatives with diaryliodonium salts to generate tri-, tetra-, and penta-substituted pyrrole products is described. The scope and limitations of these transformations are also reported.     

Pullulan hydrogels improve mesenchymal stem cell delivery into high-oxidative-stress wounds

Cell-based therapies for wound repair are limited by inefficient delivery systems that fail to protect cells from the acute inflammatory environment. Here, a biomimetic hydrogel system is described that is based on the polymer pullulan, a carbohydrate glucan known to exhibit potent antioxidant capabilities. It is shown that pullulan hydrogels are an effective cell delivery system and improve mesenchymal stem cell survival and engraftment in high-oxidative-stress environments. The results suggest that glucan hydrogel systems may prove beneficial for progenitor-cell-based approaches to skin regeneration.     

Low frequency temperature forcing of chemical oscillations

The low frequency forcing of chemical oscillations by temperature is investigated experimentally in the Belousov-Zhabotinsky (BZ) reaction and in simulations of the Oregonator model with Arrhenius temperature dependence of the rate constants. Forcing with temperature leads to modulation of the chemical frequency. The number of response cycles per forcing cycle is given by the ratio of the natural frequency to the forcing frequency and phase locking is only observed in simulations when this ratio is a whole number and the forcing amplitude is small. The global temperature forcing of flow-distributed oscillations in a tubular reactor is also investigated and synchronisation is observed in the variation of band position with the external signal, reflecting the periodic modulation of chemical oscillations by temperature.