Matteson Reaction under Flow Conditions: Iterative Homologations of Terpenes

Size-Programmable Matteson-Type Annulation: Construction of Spirocycles from Simple Cyclic Ketones

Despite the recent advancement, Matteson-type reactions are almost exclusively used to construct linear molecules. Herein we report an iterative boron-homologation approach to construct various carbocycles from a single precursor. This method utilizes an electron-withdrawing group (EWG) as a handle to enable intramolecular Matteson-type couplings, leading to diastereoselective and enantioselective ring formation. An intriguing role of the Lewis acid additive is identified. This approach proves to be general for preparing carbocycles with different ring sizes and multiple stereocenters. The annulation processes are also scalable, and the products can undergo various transformations to provide synthetically valuable structural motifs. In addition, this method can be extended to the preparation of diverse hard-to-make spirocyclic compounds from simple cyclic ketones. Moreover, an iterative approach to synthesize double spirocycles is also demonstrated.

Conformational Selection in Enzyme-Catalyzed Depolymerization of Bio-based Polyesters

Enzymatic degradation of polymers holds promise for advancing towards a bio-based economy. However, the bulky nature of polymers presents challenges in accessibility for biocatalysts, hindering depolymerization reactions. Beyond the impact of crystallinity, polymer chains can reside in different conformations affecting binding efficiency to the enzyme active site. We previously showed that the gauche and trans chain conformers associated with crystalline and amorphous regions of the synthetic polyethylene terephthalate (PET) display different affinity to PETase, thus affecting the depolymerization rate. However, structural-function relationships for biopolymers remain poorly understood in biocatalysis. In this study, we explored the biodegradation of previously synthesized bio-polyesters made from a rigid bicyclic chiral terpene-based diol and copolymerized with various renewable diesters. Herein, four of those polyesters spanning from semi-aromatic to aliphatic were subjected to enzymatic degradations in concert with induced-fit docking (IFD) analyses. The monomer yield following enzymatic depolymerization by IsPETase S238 A, Dura and LCC ranged from 2 % to 17 % without any further pre-treatment step. The degradation efficiency was found to correlate with the extent of matched substrate and enzyme conformations revealed by IFD, regardless of the actual reaction temperature employed. Our findings demonstrate the importance of conformational selection in enzymatic depolymerization of biopolymers. A straight or twisted conformation of the polymer chain is crucial in biocatalytic degradation by showing different affinities to enzyme ground-state conformers. This work highlights the importance of considering the conformational match between the polymer and the enzyme to optimize the biocatalytic degradation efficiency of biopolymers, providing valuable insights for the development of sustainable bioprocesses.

Chiroptically Active Host-Guest Composites Using a Terpene-Based Micellar Capsule

For the design of a new chiroptically active host-guest system, a bent amphiphilic compound was synthesized using cyclic monoterpenes as key biorelated chiral frameworks. In water, the bent amphiphiles form a terpene-based micellar capsule with a core diameter of ∼2 nm in a spontaneous and quantitative fashion. The resultant chiral capsule shows wide-ranging uptake abilities toward achiral fluorescent dyes in water. Notably, relatively strong CD bands are generated from the resultant host-guest composites, e.g., possessing AIE-active tetraphenylethene and sterically demanding BODIPY dyes, through efficient host-to-guest chirality transfer. The composites also display CPL, with moderate to high emission asymmetry factors (|glum| = up to 3.3 × 10-3).

External Flash Generation of Carbenoids Enables Monodeuteration of Dihalomethanes

In this study, incorporation of one deuterium atom was achieved by H-D exchange of one of the two identical methylene protons in various dihalomethanes (halogen=Cl, Br, and I) through a rapid-mixing microflow reaction of lithium diisopropylamide as a strong base and deuterated methanol as a deuteration reagent. Generation of highly unstable carbenoid intermediate and suppression of its decomposition were successfully controlled under high flow-rate conditions. Monofunctionalization of diiodomethane afforded various building blocks composed of boryl, stannyl, and silyl groups. The monodeuterated diiodomethane, which served as a deuterated C1 source, was subsequently subjected to diverted functionalization methods to afford various products including biologically important molecules bearing isotope labelling at specific positions and homologation products with monodeuteration.

Taming Highly Unstable Radical Anions and 1,4-Organodilithiums by Flow Microreactors: Controlled Reductive Dimerization of Styrenes

The reduction of styrenes with lithium arenide in a flow microreactor leads to the instantaneous generation of highly unstable radical anions that subsequently dimerize to yield the corresponding 1,4-organodilithiums. A flow reactor with fast mixing is essential for this reductive dimerization as the efficiency and selectivity are low under batch conditions. A series of styrenes undergo dimerization, and the resulting 1,4-organodilithiums are trapped with various electrophiles. Trapping with divalent electrophiles affords precursors for useful yet less accessible cyclic structures, for example, siloles from dichlorosilanes. Thus, we highlight the power of single-electron reduction of unsaturated compounds in flow microreactors for organic synthesis.

Continuous‐Flow Divergent Lithiation of 2,3‐Dihalopyridines: Deprotolithiation versus Halogen Dance

We describe herein the first halogen dance (HD) in continuous flow on 2-chloro-3-bromopyridine by selectively trapping a (pyridin-4-yl)lithium species that is known to undergo the halogen-dance process. In addition, this lithiated intermediate was trapped at lower temperature before the HD occurs. The HD process was extended to fluoro-iodopyridines by using various electrophiles to afford 28 examples with yields ranging from 42 to 97 % with very short residence times. Finally, scale up of the reaction was demonstrated, affording a promising space-time yield (STY) of 4.2 kg.h^-1 .L^-1 .

Synthesis of odorants in flow and their applications in perfumery

Continuous flow technology is a key technology for sustainable manufacturing with numerous applications for the synthesis of fine chemicals. In recent years, the preparation of odorants utilizing the advantages of flow reactors received growing attention. In this review, we give an overview of selected methods for the synthesis of odorants in flow, including heterogeneously catalyzed reactions, gas reactions, and photochemical C–H functionalization processes. After a brief introduction on types of odorants, the presented odorant syntheses are ordered according to the main odor families “fruity”, “green”, “marine”, “floral”, “spicy”, “woody”, “ambery”, and “musky” and their use and importance for perfumery is briefly discussed.

Odor Characteristics of Novel Non-Canonical Terpenes

Several non-canonical, methylated terpenes have been described as products of genetically modified Escherichia coli recently, and the aroma properties of 28 odor-active methylated derivatives of prenol, isoprenol, bornane, camphene, carene, citronellol, fenchol, geraniol, limonene, linalool, terpineol, and farnesol were characterized for the first time in the current study. Twelve methylated monoterpenes exhibited a particularly intense and pleasant odor and were therefore chosen for the determination of their respective odor thresholds (OTs) in comparison to their non-methylated equivalents. In addition to the determination of OTs based on the literature value for the internal standard, (2E)-decenal, the threshold values of the compounds with individually determined OTs of the participants were calculated. This enabled a more precise identification of the OTs. Among the non-canonical terpenes, the lowest OTs in the air were found for 2-methyllinalool (flowery, 1.8 ng L⁻¹), 2-methyl-α-fenchol (moldy, 3.6 ng L⁻¹), 2-methylgeraniol (flowery, 5.4 ng L⁻¹), 2-methylcitronellol (citrus-like, 7.2 ng L⁻¹), and 4-methylgeraniol (citrus-like, 16 ng L⁻¹). The derivatives of geraniol, linalool, and citronellol showed very pleasant odor impressions, which could make them interesting for use as flavoring agents in the flavor and fragrance industry.

Flow synthesis of an α-amino boronic ester as a key precursor of bortezomib drug

The flow synthesis of the optically active α-amino boronate precursor of the bortezomib drug is described, including a key diastereoselective Matteson rearrangement.

Forgotten and forbidden chemical reactions revitalised through continuous flow technology

Continuous flow technology has played an undeniable role in enabling modern chemical synthesis, whereby a myriad of reactions can now be performed with greater efficiency, safety and control. As flow chemistry furthermore delivers more sustainable and readily scalable routes to important target structures a growing number of industrial applications are being reported. In this review we highlight the impact of flow chemistry on revitalising important chemical reactions that were either forgotten soon after their initial report as necessary improvements were not realised due to a lack of available technology, or forbidden due to unacceptable safety concerns relating to the experimental procedure. In both cases flow processing in combination with further reaction optimisation has rendered a powerful set of tools that make such transformations not only highly efficient but moreover very desirable due to a more streamlined construction of desired scaffolds. This short review highlights important contributions from academic and industrial laboratories predominantly from the last 5 years allowing the reader to gain an appreciation of the impact of flow chemistry.