Molecular Rearrangements of (−)-Modhephene and (−)-Isocomene to a (−)-Triquinane

Designing a Propellane-based Nonlinear Optically Active System Absorbing in Three Different Wavelength Regions

The aim of this work is to demonstrate the possibility of using propellane in designing a molecule that can absorb in three different wavelength regions and their nonlinear optical (NLO) activity can be fine-tuned by varying the three wings. We considered 22 tailor-made propellane derivatives consisting of phenyl, naphthyl, and biphenyl wings for this purpose. Using the state-of-the-art linear and quadratic response methods within TD-DFT and RI-CC2 theories and a suitable generalized few-state model that quantifies the effect of orientation of different transition moments on NLO properties, we discussed how and why the linear and nonlinear optical activity of propellane vary when the three wings are assembled successively to construct a full-propellane.

Occurrence, synthesis and applications of natural and designed [3.3.3]propellanes

Covering: 1978 to 2019 The synthetically challenging [3.3.3]propellane core has caught a lot of attention over the last 50 years. This comprehensive review details all synthetic strategies reported in the period 1978-2019 to facilitate the synthesis of carbocyclic [3.3.3]propellanes. The described strategies span from acid-catalyzed rearrangements and photo-mediated cycloadditions of ketones, heteropropellanes and dispiroundecanes to thermal rearrangements of acetylenes and alkenes. Other approaches, such as radical reactions with halogenated alkenes, domino cyclizations, the smart use of epoxide-carbonyl rearrangements and intramolecular palladium-catalyzed ring contractions are discussed as well. A special section is dedicated to triptindanes, a subclass of [3.3.3]propellanes which are of interest to material sciences.

Propellanes-From a Chemical Curiosity to "Explosive" Materials and Natural Products

Propellanes are a unique class of compounds currently consisting of well over 10 000 representatives, all featuring two more or less inverted tetrahedral carbon atoms that are common to three bridging rings. The central single bond between the two bridgeheads is significantly weakened in the smaller entities, which leads to unusual reactivities of these structurally interesting propeller-like molecules. This Review highlights the synthesis of such propellanes and their occurrence in material sciences, natural products, and medicinal chemistry. The conversion of [1.1.1]propellane into bridgehead derivatives of bicyclo[1.1.1]pentane, including oligomers and polymers with bicyclo[1.1.1]penta-1,3-diyl repeat units, is also featured. A selection of natural products with larger propellane subunits are discussed in detail. Heteropropellanes and inorganic propellanes are also addressed. The historical background is touched in brief to show the pioneering work of David Ginsburg, Günther Snatzke, Kenneth B. Wiberg, Günter Szeimies, and others.

Theoretical and experimental analysis of the reaction mechanism of MrTPS2, a triquinane-forming sesquiterpene synthase from chamomile

Terpene synthases, as key enzymes of terpene biosynthesis, have garnered the attention of chemists and biologists for many years. Their carbocationic reaction mechanisms are responsible for the huge variety of terpene structures in nature. These mechanisms are amenable to study by using classical biochemical approaches as well as computational analysis, and in this study we combine quantum-chemical calculations and deuterium-labeling experiments to elucidate the reaction mechanism of a triquinane forming sesquiterpene synthase from chamomile. Our results suggest that the reaction from farnesyl diphosphate to triquinanes proceeds through caryophyllyl and presilphiperfolanyl cations and involves the protonation of a stable (-)-(E)-β-caryophyllene intermediate. A tyrosine residue was identified that appears to be involved in the proton-transfer process.

Essential oils from the roots of Echinops bannaticus Rochel ex Schrad. and Echinops sphaerocephalus L. (Asteraceae): chemotaxonomic and biosynthetic aspects

Herein, we report, for the first time, the results of detailed chemical and statistical analyses of the essential oils from the roots of two Echinops species (Asteraceae), E. bannaticus Rochel ex Schrad., and E. sphaerocephalus L., from the weed flora of Serbia. Among 106 and 81 constituents, respectively, S-containing polyacetylene compounds and triquinane sesquiterpenoids made up ca. 80% of the oils. Several of these compounds are reported here as new metabolites for the two species or even for the genus Echinops. A multivariate statistical comparison of the essential-oil composition data for these two and additional six taxa of this genus available from the literature permitted an examination of the mutual relationships of the taxa within this morphologically highly uniform genus. Principal component analysis (PCA) and agglomerative hierarchical clustering revealed a grouping of E. bannaticus and E. sphaerocephalus (both belonging to the section Echinops), and their close relationship with E. grijsii, suggesting a circumscription of this Chinese taxon to the section Echinops. PCA Correlation matrix offered valuable insight into the biosynthetic links between essential-oil constituents, and these agreed excellently with the currently proposed ones for the polyacetylene S-containing compounds, triquinanes, and monoterpenes.

Biosynthesis via carbocations: theoretical studies on terpene formation

This review describes applications of quantum chemical calculations in the field of terpene biosynthesis, with a focus on insights into the mechanisms of terpene-forming carbocation rearrangements arising from theoretical studies.

Wagner-Meerwein rearrangement of a [3.3.3]- to a [4.3.3]propellane: deuterium tracer and conformational analysis

Formation of (-)-[4.3.3]propellane 4 from (-)-14-hydroxymodhephene (2) proceeds through a Wagner-Meerwein rearrangement via C3--C4 bond-shift to give a stable intermediate, dimethylcyclohexadienyl cation A, which undergoes deprotonation. Herein, this mechanism was investigated by using a deuterium labeled substrate at the C-14 methylene group of (-)-2, which was incorporated into the C-4 position of (-)-[4.3.3]propellane 4. The stereostructure of (-)-4 was investigated by applying a combination of NMR experimental and theoretical approaches.