Acid-catalyzed cyclization of farnesol and nerolidol

Biomimetic tail-to-head terpene cyclizations using the resorcin[4]arene capsule catalyst

The tail-to-head terpene (THT) cyclization is a biochemical process that gives rise to many terpene natural product skeletons encountered in nature. Historically, it has been difficult to achieve THT synthetically without using an enzyme. In this protocol, a hexameric resorcin[4]arene capsule acts as an artificial enzyme mimic to carry out biomimetic THT cyclizations and related carbocationic rearrangements. The precursor molecule bears a leaving group (usually an alcohol or acetate group) and undergoes the THT reaction in the presence of the capsule catalyst and HCl as a cocatalyst. Careful control of several parameters (including water content, amount of HCl cocatalyst, temperature and solvent) is crucial to successfully carrying out the reaction. To facilitate the application of this unique capsule-catalysis methodology, we therefore developed a very detailed procedure that includes the preparation and analysis of all reaction components. In this protocol, we describe how to prepare two different terpenes: isolongifolene and presilphiperfolan-1β-ol. The two procedures differ in the water content required for efficient product formation, and thus exemplify the two common use cases of this methodology. The influence of other crucial reaction parameters and means of precisely controlling them are described. A commercially available substrate, nerol, can be used as simple test substrate to validate the reaction setup. Each synthetic procedure requires 5-7 d, including 1-5 h of hands-on time. The protocol applies to the synthesis of many complex terpene natural products that would otherwise be difficult to access in synthetically useful yields.

Chemical Synthesis of Peptides and Proteins Bearing Base-Labile Post-Translational Modifications: Evolution of the Methods in Four Decades

The S-palmitoylation on Cys residue and O-acetylation on Ser/Thr residues are two types of base-labile post-translational modifications (PTMs) in cells. The lability of these PTMs to bases and nucleophiles makes the peptides/proteins bearing S-palmitoyl or O-acetyl groups challenging synthetic targets, which cannot be prepared via the standard Fmoc-SPPS and native chemical ligation. In this review, we summarized the efforts towards their preparation in the past 40 years, with the focus on the evolution of synthetic methods.

Qualitative profiling of mono- and sesquiterpenols in aglycon libraries from Vitis vinifera L. Gewürztraminer using multidimensional gas chromatography–mass spectrometry

In grape berries (Vitis vinifera L.), sesquiterpenes are mainly accumulated as hydrocarbons in the epicuticular wax layer of grapes, whereas monoterpenes, which are predominantly present as alcohols, are glycosylated and are stored as glycosides in the vacuoles of grape berry cells. In this study, extensive analysis of grape berry hydrolysates by means of comprehensive two-dimensional gas chromatography–time-of-flight–mass spectrometry demonstrated that glycosylated sesquiterpene alcohols show very little structural diversity when compared to the sesquiterpene hydrocarbon fraction in the cuticle and are glycosylated to a rather low extent when compared to monoterpenols. Twenty-four enzymatically released terpenols were found in hydrolysates of the aromatic white wine variety Gewürztraminer (V. vinifera subsp. vinifera) after previous solid-phase extraction and headspace solid-phase microextraction. The detection of only three sesquiterpene alcohols, namely farnesol, nerolidol and drimenol, shows that most sesquiterpene hydrocarbons do not have a related hydroxylated structure in grapes. Nevertheless, the presence of the acyclic aglycone farnesol and nerolidol may be of importance for the wine aroma, since these structural isomers can be converted into numerous sesquiterpenes by nonenzymatic acid-catalyzed reactions during wine production. Grape-derived glycosidically bound sesquiterpene alcohols, therefore, represent, in addition to free sesquiterpene hydrocarbons, another pool of compounds that may influence the aroma profile of wines.

Dissecting Sesquiterpene Profiles of Lemberger Red Wines Using Ex Vivo Tissue Deuterium-Labeling and Comprehensive Two-Dimensional Gas Chromatography-Time-of-Flight-Mass Spectrometry

By means of ex vivo tissue deuterium-labeling using the stable isotope-labeled precursor [6,6,6-²H₃]-(±)-mevalonolactone and microvinification experiments, we were able to show for the first time that the three sesquiterpene hydrocarbons, guaiazulene, δ-selinene, and selina-3,7(11)-diene, in Lemberger red wines do not originate from acid-catalyzed cyclization of yeast-derived farnesol and nerolidol. The three aforementioned sesquiterpene hydrocarbons could be unambiguously identified as grape-derived secondary metabolites and can therefore be considered as variety-specific marker compounds. The analysis of sesquiterpene hydrocarbons in red wine samples was performed by solid-phase extraction-headspace solid-phase microextraction-comprehensive two-dimensional gas chromatography-time of flight-mass spectrometry. The developed methodology paves the way for an analytical verification of grape variety labeling in wine authenticity control.

Enantioselective Tail-to-Head Cyclizations Catalyzed by Dual-Hydrogen-Bond Donors

Chiral urea derivatives are shown to catalyze enantioselective tail-to-head cyclization reactions of neryl chloride analogues. Experimental data are consistent with a mechanism in which π-participation by the nucleophilic olefin facilitates chloride ionization and thereby circumvents simple elimination pathways. Kinetic and computational studies support a cooperative mode of catalysis wherein two molecules of the urea catalyst engage the substrate and induce enantioselectivity through selective transition state stabilization.

Discovering Monoterpene Catalysis Inside Nanocapsules with Multiscale Modeling and Experiments

Large-scale production of natural products, such as terpenes, presents a significant scientific and technological challenge. One promising approach to tackle this problem is chemical synthesis inside nanocapsules, although enzyme-like control of such chemistry has not yet been achieved. In order to better understand the complex chemistry inside nanocapsules, we design a multiscale nanoreactor simulation approach. The nanoreactor simulation protocol consists of hybrid quantum mechanics-molecular mechanics-based high temperature Langevin molecular dynamics simulations. Using this approach we model the tail-to-head formation of monoterpenes inside a resorcin[4]arene-based capsule (capsule I). We provide a rationale for the experimentally observed kinetics of monoterpene product formation and product distribution using capsule I, and we explain why additional stable monoterpenes, like camphene, are not observed. On the basis of the in-capsule I simulations, and mechanistic insights, we propose that feeding the capsule with pinene can yield camphene, and this proposal is verified experimentally. This suggests that the capsule may direct the dynamic reaction cascades by virtue of π-cation interactions.

Chemistry of the Secondary Metabolites of Termites

Isolation, structure determination, synthesis, and biochemistry of the low-molecular-weight compounds of the secretion of exocrine glands of termites are described, with an emphasis on pheromones and defensive compounds.

Sesquiterpene Cyclisations Catalysed inside the Resorcinarene Capsule and Application in the Short Synthesis of Isolongifolene and Isolongifolenone

Terpenes constitute the largest class of natural products and serve as an important source for medicinal treatments. Despite constant progress in chemical synthesis, the construction of complex polycyclic sesqui- and diterpene scaffolds remains challenging. Natural cyclase enzymes, however, are able to form the whole variety of terpene structures from just a handful of linear precursors. Man-made catalysts able to mimic such natural enzymes are lacking. Here, we describe the examples of sesquiterpene cyclisations inside an enzyme-mimicking supramolecular catalyst. This strategy allowed the formation of the tricyclic sesquiterpene isolongifolene in only four steps. The mechanism of the catalysed cyclisation reaction was elucidated using ¹³C-labelling studies and DFT calculations.

Synthesis of highly strained terpenes by non-stop tail-to-head polycyclization

Non-stop carbocationic polycyclizations of isoprenoids have been called the most complex chemical reactions occurring in nature. We describe a strategy for the initiation of tail-to-head polycyclization that relies on the sequestration of the counteranion away from the carbocation, which allows full propagation of the cationic charge. If the anion is mobile, Coulombic forces hold this species in close proximity to the carbocation and cause preemptive termination through elimination. Anion sequestration is crucial for effecting the biomimetic synthesis of complex and unstable terpenes, including the highly strained funebrenes. This study illustrates the deleterious role of the counterion in tail-to-head carbocationic polycyclization reactions, which to the best of our knowledge has not been rigorously explored. These observations are also expected to find use in the design and control of cationic polycyclization along biosynthetic pathways that have previously been inaccessible in bulk solvent.

Evolving biosynthetic tangos negotiate mechanistic landscapes

The dependence of polyketide synthase and terpene cyclase mechanistic adaptation on the chemistry of their oligomeric substrates illuminates a convergent evolutionary strategy for shaping cyclization in these otherwise disparate reactions. Evolution of these enzyme families relies on rhythmic tangos, in which the enzymes and substrates together determine product outcome by negotiating decision networks governing intrinsic and induced chemical reactivities.