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The drimane sesquiterpenoids drimenin, cinnamolide, dendocarbin A, and polygodial were purified from the Canelo tree (Drimys winteri) and chemically characterized by spectroscopic methods. The pharmacological activity of these natural compounds were determined on hα4β2, hα3β4, and hα7 nicotinic acetylcholine receptors (AChRs) by Ca2+ influx measurements. The results established that drimane sesquiterpenoids inhibit AChRs with the following selectivity: hα4β2 > hα3β4 > hα7. In the case of hα4β2 AChRs, the following potency rank order was determined (IC50’s in μM): drimenin (0.97 ± 0.35) > cinnamolide (1.57 ± 0.36) > polygodial (62.5 ± 19.9) ≫ dendocarbin A (no activity). To determine putative structural features underlying the differences in inhibitory potency at hα4β2 AChRs, additional structure–activity relationship and molecular docking experiments were performed. The Ca2+ influx and structural results supported a noncompetitive mechanism of inhibition, where drimenin interacted with luminal and nonluminal (TMD-β2 intrasubunit) sites. The structure–activity relationship results, i.e., the lower the ligand polarity, the higher the inhibitory potency, supported the nonluminal interaction. Ligand binding to both sites might inhibit the hα4β2 AChR by a cooperative mechanism, as shown experimentally (nH > 1). Drimenin could be used as a molecular scaffold for the development of more potent inhibitors with higher selectivity for the hα4β2 AChR.
Cystine was used as a platform chemical to prepare cyclic and acyclic monomers for entropy-driven ringopening polymerization (ED-ROMP) via olefin or disulfide metathesis and for step-growth polymerization. The olefin ED-ROMP of an olefin/disulfide containing 16-atom macrocycle using the 3rd generation Grubbs catalyst was examined in greater detail. Kinetic studies revealed that the catalyst turned inactive during the polymerization, which limited the achievable (apparent) polymer molar mass to similar to 70 kg mol(-1). Such limitation could be overcome with the disulfide ED-ROMP of the same macrocycle to yield polymers with molar masses of up to 180 kg mol(-1). The step-growth polymerizations of acyclic diene and dithiol monomers via olefin metathesis or oxidation were far less effective and yielded just low molar mass polymers or oligomers; photopolymerization of a thiol-ene monomer produced a polyester with a molar mass of 35 kg mol(-1).
A series of biomass-derived levoglucosenyl alkyl ethers (alkyl = methyl, ethyl, n-propyl, isopropyl, and n-butyl) were synthesized and polymerized by ring-opening olefin metathesis polymerization using the Grubbs catalyst C793 at room temperature. Polymerizations were successfully performed in conventional solvents such as 1,4-dioxane and dichloromethane as well as in polar aprotic "green" solvents such as 2-methyltetrahydrofuran, dihydrolevoglucosenone (Cyrene), and ethyl acetate. The prepared polyacetals with degrees of polymerization of similar to 100 exhibit Schulz-Flory-type molar mass distributions and are thermoplastic materials with rather low glass transition temperatures in the range of 43-0 degrees C depending on the length of the alkyl substituent. Kinetic studies revealed that the polymerization proceeded rapidly to a steady state with a certain minimum monomer concentration threshold. When the steady state was reached, just about half of the [Ru] catalyst had been effective to initiate the polymerization, indicating that the initiation step was a slow process. The remaining catalyst was still active and did no longer react with monomers but with in-chain double bonds, cutting the formed polymer chains into shorter fragments. In the long term, all catalyst was consumed and propagating [Ru] chain ends were deactivated by the elimination of [Ru] from the chain ends to form inactive chains with terminal aldehyde groups.
We have synthesized mesoporous silica (monoliths) with defined surface chemistry by means of a number of addition reactions: (i) coupling of an isocyanate to a surface-immobilized thiol, (ii) addition of an epoxide to a surface-immobilized thiol, (iii) cross-metathesis between two olefins, and (iv) Huisgen [2+3] cycloaddition of an alkyne-functionalized silica monolith with an azide. Functionalization of the mesopores was observed, but there are significant differences between individual approaches. Isocyanate and epoxide additions lead to high degrees of functionalization, whereas olefin metathesis and [2+3] cycloaddition are less effective. We further show that the efficiency of the modification is about twice as high in mesoporous silica particles than in macroscopic silica monoliths.
Stepped supporting tools were developed and used in the university seminar Organic Chemistry taken by nonmajor chemistry students, which supported self-regulated learning. These supporting tools were also used for accompanying homework, which included a QR code that led to additional supporting tools. The application of stepped supporting tools in the seminars was evaluated by a four-item Likert scale. The students assessed the tools as a helpful instrument for solving tasks in chemistry.
Die Verwendung von QR-Codes in Begleitseminaren zur Vorlesung „Organische Chemie“ für Studierende mit Chemie im Nebenfach wird vorgestellt. Die Hausaufgaben zu den Seminaren wurden mit einem QR-Code versehen. Dieser führt zu weiterführenden Hilfen. Der Einsatz der QR-Codes sowie die Neukonzeption der Seminare wurden evaluiert.
In this communication the development of an online course on the topic organic chemistry for nonmajor chemistry students is described and discussed. For this online course, the existing classroom course was further developed. New elements such as podcasts, task navigators, and a forum for discussing the solving of tasks or problems with the content were added. This new online course was evaluated. Therefore, a questionnaire was developed. This consists of questions with regard to the longtime learning behavior of the students and to the online learning. The results of this evaluation show that a preference for online learning and a preference for classroom teaching can be measured separately in two scales. Students values on the scale representing a preference for online learning correlate positively and significantly with confidence in the choice of the study subject, enthusiasm about the subject, and the ability to organize their learning, learning environment, and time management. They correlate also with the satisfaction concerning the materials provided. Students values for one of those teaching methods also correlate with their rating with regard to their exam preparation. Values representing a preference for online teaching correlate positively with students better feeling of exam preparation. Values representing a preference for classroom teaching show negative correlations with the values representing students similar or even better preparation for the exams as a result of online teaching. It is therefore not surprising that the ratings for the two scales correlate with the wish for a combination of online teaching and classroom teaching in the future. As a solution, a new course concept for the time after the corona virus crisis that suits all students is outlined in the outlook.
Helical chirality is a novel enantioselectivity-inducing property in transition-metal-catalyzed transformations. The principle is illustrated herein for the example of asymmetric olefin metathesis. This work reports the synthesis of the first helically chiral Ru-NHC alkylidene complex from an aminohelicene-derived imidazolium salt, which was ligated to the first generation Hoveyda-Grubbs catalyst. Kinetic data were acquired for benchmark test reactions and compared to an achiral catalyst. The helically chiral Ru-catalyst was evaluated in asymmetric ring-closing metathesis (RCM) and ring-opening metathesis-cross-metathesis (ROM/CM) reactions, which proceeded with promising levels of enantioselectivity. Extensive NMR-spectroscopic investigations and a DFT geometry optimization were performed. These results led to a topographic steric map and calculation of percent-buried-volume values for each quadrant around the metal center.
Asymmetric Synthesis of Nonracemic 2-Amino[6]helicenes and Their Self-Assembly into Langmuir Films
(2018)
Alternative ways of preparing nonracemic 2-amino[6]helicene derivatives were explored. The enantioselective [2 + 2 + 2] cycloisomerization of a nonchiral triyne under Ni(cod)(2)/(R)-QUINAP catalysis delivered the enantioenriched (+)-(P)-2-aminodibenzo[6]helicene derivative in 67% ee. An ultimate "point-to-helical" chirality transfer was observed in the cyclization of enantiopure triynes mediated by Ni(CO)(2)(PPh3)(2) affording (-)-(M)- or (+)-(P)-7,8-bis(p-tolyl)hexahelicen-2-amine in >99% ee as well as its benzoderivative in >99% ee. The latter mode of stereocontrol was inefficient for a 2-aminobenzo[6]helicene congener with an embedded five-membered ring. The rac-, (-)-(M)-, and (+)-(P)-7,8-bis(p-tolyl)hexahelicen-2-amines formed Langmuir monolayers at the air water interface featuring practically identical surface pressure vs mean molecular area isotherms. The corresponding Langmuir-Blodgett films on quartz or silicon substrates were characterized by UV vis/ECD spectroscopy and AFM microscopy, respectively.