Fluorinated (hetero)cycles via ring-closing metathesis of fluoride- and trifluoromethyl-functionalized olefins

Sequential Modification of Pyrrole Ring with up to Three Different Nucleophiles

An umpolung strategy was used for the preparation of highly functionalized 3-pyrrolin-2-ones. This approach involves dearomative double chlorination of 1H-pyrroles to form highly reactive dichloro-substituted 2H-pyrroles. The resulting intermediate reacts selectively with wet alcohols to form the corresponding alkoxy-substituted 3-pyrrolin-2-ones via double nucleophilic substitution in up to 99% yield. The subsequent reaction with different N-, O-, and S-nucleophiles opens access to highly functionalized pyrrolinones bearing additional functionality. The overall outcome of the reported sequence is step-by-step nucleophilic modification of pyrroles with three different nucleophiles. All steps were found to be highly efficient and 100% regioselective. This transformation proceeds under mild conditions and does not require any catalyst to give final products in very high yields. The obtained experimental results are in perfect agreement with the data obtained by theoretical investigation of these reactions.

Selective Synthesis of Monofluorinated Compounds Applying Amine/HF Reagents

For nucleophilic monofluorination, amine/HF reagents such as Et3 N⋅3HF, Pyr⋅9HF (Olah's reagent) and similar combinations belong to the most frequently used fluoride sources, whereupon the selectivity of these reagents can be very different depending of its acidity, the nucleophilicity of the fluoride equivalent, and the structure of the particular substrate. These reagents can be used safely in ordinary chemistry laboratories for nucleophilic substitution reactions by fluoride at sp3 -hybridized carbon centers. For ring opening reactions of epoxides, the regio- and stereoselectivity is very much depending of the nature of the epoxide and the acidity of the HF reagent favoring either SN 1 or SN 2 type reactions. Similarly, the outcome of halofluorination and similar reactions with sulfur or seleno electrophiles can be controlled by the particular combination of the electrophile and the fluoride source. Examples for the application of these reaction types for the synthesis of fluorine-containing analogues of natural products or biologically relevant compounds are in the focus of this personal account.

Computational Elucidation on the Conformational Control of Selectivity in Intramolecular Ring-Closing Metathesis vs Intermolecular Homometathesis

The ring-closing metathesis reaction of diene plays an important role in the construction of cyclic compounds. In this research, density functional theory (DFT) calculations were conducted to elucidate the mechanisms and origins of the selectivity of ring-closing metathesis and homometathesis. The computational results suggest that the selectivity is determined by the substrate conformation. For the ester-tethered substrate, the homometathesis is more favorable, due to the planar structure of ester facilitating the conjugative effect of the formed E-homometathesis product. For the amide-tethered substrate, the ring-closing metathesis product is the only observed product because the steric hindrance of N-substituents disfavors homometathesis.

Photocatalytic Cyclization/Defluorination Domino Sequence to Access 3-Fluoro-1,5-dihydro-2H-pyrrol-2-one Scaffold

We herein report an unprecedented photoinduced cyclization/defluorination domino process of N-allylbromodifluoroacetamide with cyclic secondary amines. Consequently, a wide array of valuable 3-fluoro-1,5-dihydro-2H-pyrrol-2-ones were facilely prepared from readily available starting materials under mild conditions. Preliminary mechanistic investigations suggest that a radical chain propagation and amine-promoted defluorination pathway are presumably involved in this transformation.

Ruthenium Metathesis: A Key Step To Access a New Cyclic Tetrasubstituted Olefin Platform

An rapid and mild synthetic route for the preparation of cyclic tetrasubstituted platforms via ruthenium-catalyzed ring-closing metathesis (RCM) has been developed. This process tolerates a wide range of functionalities such as nitrogen, oxygen, sulfur, silicon, and carbon tethered groups, as well as very challenging fluorine and boron atoms (36 derivatives, up to 96%). This diversity-oriented method was further demonstrated by the postfunctionalization reactions, such as Pd-couplings, N-substitution, and reductive amination introducing a morpholine moiety.

Approaches to Obtaining Fluorinated α-Amino Acids

Fluorine does not belong to the pool of chemical elements that nature uses to build organic matter. However, chemists have exploited the unique properties of fluorine and produced countless fluoro-organic compounds without which our everyday lives would be unimaginable. The incorporation of fluorine into amino acids established a completely new class of amino acids and their properties, and those of the biopolymers constructed from them are extremely interesting. Increasing interest in this class of amino acids caused the demand for robust and stereoselective synthetic protocols that enable straightforward access to these building blocks. Herein, we present a comprehensive account of the literature in this field going back to 1995. We place special emphasis on a particular fluorination strategy. The four main sections describe fluorinated versions of alkyl, cyclic, aromatic amino acids, and also nickel-complexes to access them. We progress by one carbon unit increments. Special cases of amino acids for which there is no natural counterpart are described at the end of each section. Synthetic access to each of the amino acids is summarized in form of a table at the end of this article with the aim to make the information easily accessible to the reader.

Flexible synthesis of polyfunctionalised 3-fluoropyrroles

Convergent synthesis of polyfunctionalised 3-fluoropyrroles.

Short and efficient synthesis of fluorinated δ-lactams

The diastereoselective synthesis of fluorinated δ-lactams.

A general enantioselective route to the chamigrene natural product family

Described in this report is an enantioselective route toward the chamigrene natural product family. The key disconnections in our synthetic approach include sequential enantioselective decarboxylative allylation and ring-closing olefin metathesis to form the all-carbon quaternary stereocenter and spirocyclic core present in all members of this class of compounds. The generality of this strategy is demonstrated by the first total syntheses of elatol and the proposed structure of laurencenone B, as well as the first enantioselective total syntheses of laurencenone C and α-chamigrene. A brief exploration of the substrate scope of the enantioselective decarboxylative allylation/ring-closing metathesis sequence with fully substituted vinyl chlorides is also presented.

Total Syntheses of Conformationally Locked Difluorinated Pentopyranose Analogues and a Pentopyranosyl Phosphate Mimetic

Trifluoroethanol has been elaborated, via a telescoped sequence involving a metalated difluoroenol, a difluoroallylic alcohol, [2,3]-Wittig rearrangement, and ultimately an RCM reaction and requiring minimal intermediate purification, to a number of cyclooctenone intermediates. Epoxidation of these intermediates followed by transannular ring opening or dihydroxylation, then transannular hemiacetalization delivers novel bicyclic analogues of pentopyranoses, which were elaborated (in one case) to an analogue of a glycosyl phosphate.

Ring-Closing Olefin Metathesis for the Synthesis of Benzene Derivatives

Benzene derivatives were synthesized in excellent yield from 1,4,7-trien-3-ols by tandem ruthenium-catalyzed ring-closing olefin metathesis (RCM)/dehydration. The method was extended to the tandem RCM/oxidation process to obtain phenol and aniline derivatives. This method displays many advantages over aromatic-substitution-based classical routes.

RCM-Mediated Synthesis of Trifluoromethyl-Containing Nitrogen Heterocycles

A ring-closing metathesis mediated pathway to trifluoromethyl-containing piperidines is detailed. This involves the development of a synthetic route to a new (trifluoromethyl)allylating reagent via a Diels-Alder/retro-Diels-Alder strategy, its application in the synthesis of a series of trifluormethyl-substituted diolefin precursors for ring-closing metathesis, and eventually the successful cyclization of these precursor molecules into the corresponding functionalized piperidines.