Recent trends in the Chemistry of fluorinated five and six-membered heterocycles

Copper-catalyzed intermolecular formal (5 + 1) annulation of 1,5-diynes with 1,2,5-oxadiazoles

One-carbon homologation reactions based on one-carbon insertion into the N-O bond of heterocycles have received tremendous interest over the past decades. However, these protocols have to rely on the use of hazardous and not easily accessible diazo compounds as precursors, and examples of the relevant asymmetric catalysis have not been reported. Here we show that a copper-catalyzed intermolecular formal (5 + 1) annulation of 1,5-diynes with 1,2,5-oxadiazoles involving one-carbon insertion into the heterocyclic N-O bond via non-diazo approach. This method enables practical and atom-economic synthesis of valuable pyrrole-substituted oxadiazines in generally moderate to good yields under mild reaction conditions. In addition, the possibility of such an asymmetric formal (5 + 1) annulation also emerges.

Expedient cobalt-catalyzed stereospecific cascade C-N and C-O bond formation of styrene oxides with hydrazones

Cobalt-catalyzed cascade C-N and C-O bond formation of epoxides with hydrazones is described to furnish oxadiazines using air as an oxidant. The catalyst plays a dual role as a Lewis acid followed by a redox catalyst to accomplish the C-H/O-H cyclization. Optically active styrene oxide can be reacted enantiospecifically (>99% ee).

Ag-Catalyzed Trifluoromethylative Ring Expansion of Isatins and Isatin Ketimines with Trifluorodiazoethane

A general method for the construction of trifluoromethylated 2-quinolinones has been established herein by using a trifluoromethylative ring expansion of isatin with trifluorodiazoethane. The strategy provides a platform for the rapid synthesis of a wide range of substituted 3-hydroxy-4-trifluoromethyl-2-quinolinones. This operationally simple and robust Ag-catalyzed protocol successfully transforms isatin ketimines to 3-amino-4-trifluoromethylquinolinones in excellent yields. The utility of this novel method is further illustrated by the conversion of the products into various synthetically and medicinally relevant molecules.

Facile Rh(III)-Catalyzed Synthesis of Fluorinated Pyridines

A Rh(III)-catalyzed C-H functionalization approach was developed for the preparation of multisubstituted 3-fluoropyridines from α-fluoro-α,β-unsaturated oximes and alkynes. Oximes substituted with aryl, heteroaryl, and alkyl β-substituents were effective coupling partners, as were symmetrical and unsymmetrical alkynes with aryl and alkyl substituents. The first examples of coupling α,β-unsaturated oximes with terminal alkynes was also demonstrated and proceeded with uniformly high regioselectivity to provide single 3-fluoropyridine regioisomers. Reactions were also conveniently set up in air on the benchtop.

Regioselective access to CF3S-substituted dihydrofurans from homopropargylic alcohols with trifluoromethanesulfenamide

A facile access to 4-((trifluoromethyl)thio)-2,3-dihydrofurans from unprotected homopropargylic alcohols in high regioselectivity is reported.

Direct Nucleophilic Difluoromethylation of Aromatic Isoxazoles Activated by Electron-Withdrawing Groups Using (Difluoromethyl)trimethylsilane

The activation of aromatic diaryl isoxazoles with strong electron-withdrawing groups, such as the nitro, triflyl and the phenylsulfonyl groups, at the 4-position has enabled the first regio- and diastereoselective difluoromethylation at the 5-position of isoxazoles by nucleophilic addition using (difluoromethyl)trimethylsilane, Me3SiCF2H, to provide difluoromethylated isoxazolines in good yields. Conjugated styryl-4-nitroisoxazoles were also nicely converted into the corresponding CF2H adducts with high regio- and excellent diastereoselectivities. Since the trifluoromethylated analogues of the corresponding diaryl-isoxazolines are effective ectoparasiticides, represented by fluralaner, should a series of difluoromethylated isoxazolines be obtained, they would be of great importance as promising drug candidates in this field.

Direct Nucleophilic Difluoromethylation of Aromatic Isoxazoles Activated by Electron-Withdrawing Groups Using (Difluoromethyl)trimethylsilane

The activation of aromatic diaryl isoxazoles with strong electron-withdrawing groups, such as the nitro, triflyl and the phenylsulfonyl groups, at the 4-position has enabled the first regio- and diastereoselective difluoromethylation at the 5-position of isoxazoles by nucleophilic addition using (difluoromethyl)trimethylsilane, Me3SiCF2H, to provide difluoromethylated isoxazolines in good yields. Conjugated styryl-4-nitroisoxazoles were also nicely converted into the corresponding CF2H adducts with high regio- and excellent diastereoselectivities. Since the trifluoromethylated analogues of the corresponding diaryl-isoxazolines are effective ectoparasiticides, represented by fluralaner, should a series of difluoromethylated isoxazolines be obtained, they would be of great importance as promising drug candidates in this field.

Diastereoselective additive trifluoromethylation/halogenation of isoxazole triflones: synthesis of all-carbon-functionalized trifluoromethyl isoxazoline triflones

Highly functionalized 5-trifluoromethyl-2-isoxazoline derivatives featuring a triflyl (SO2CF3) group at the 4-position were successfully synthesized via diastereoselective trifluoromethylation and halogenation of isoxazole triflones using the Ruppert- Prakash reagent. The trifluoromethylation is quite general in terms of the substrates including 3,5-diaryl isoxazole triflones and 3-aryl-5-styrylisoxazole triflones to provide products in high yields with excellent diastereoselectivities. The highly functionalized 5-trifluoromethyl-2-isoxazoline derivatives are expected to be a new class of antiparasiticides. Thus the triflyl group both activates isoxazoles and the 4-postion of CF3 adducts, and has a potential biological function.

Selective C-H fluorination of pyridines and diazines inspired by a classic amination reaction

Fluorinated heterocycles are prevalent in pharmaceuticals, agrochemicals, and materials. However, reactions that incorporate fluorine into heteroarenes are limited in scope and can be hazardous. We present a broadly applicable and safe method for the site-selective fluorination of a single carbon-hydrogen bond in pyridines and diazines using commercially available silver(II) fluoride. The reactions occur at ambient temperature within 1 hour with exclusive selectivity for fluorination adjacent to nitrogen. The mild conditions allow access to fluorinated derivatives of medicinally important compounds, as well as a range of 2-substituted pyridines prepared by subsequent nucleophilic displacement of fluoride. Mechanistic studies demonstrate that the pathway of a classic pyridine amination can be adapted for selective fluorination of a broad range of nitrogen heterocycles.

Synthesis and biological activity of 5-fluorotubercidin

The electrophilic fluorination of 4-chloropyrrolo[2,3-d]pyrimidine (1) was studied culminating a 59% conversion of compound 1 to 4-chloro-5-fluoropyrrolo[2,3-d]pyrimidine (2) using Selectfluor. This transformation proceeded via the 4-chloro-5,6-dihydro-5-fluoro-6-hydroxypyrrolo[2,3-d]pyrimidine (3) in a 9:1 trans:cis ratio. The trans isomer of compound 3 was studied by 1H NMR and 19F NMR, and the 5-H tautomer (4) was observed as another intermediate. A modified Vorbruggen procedure of compound 2 and tetra-O-acetylribose gave 4-chloro-5-fluoro-7-(2,3,5,-tri-O-benzoyl-beta-D-ribofuranosyl)pyrrolo[2,3-d]pyrimidine (6) in a 65% yield. Treatment of compound 6 with ammonia (l) in dioxane gave 5-fluorotubercidin (7). No antibacterial activity was observed. An MTT assay (Promega) against Huh-7 liver cells, normal mouse spleen cells stimulated with Con A (a T-cell mitogen), and normal mouse spleen stimulated with LPS (a B-cell mitogen) showed no significant toxicity. Increased activity of 7 over tubercidin was observed against L-1210 cells and toxicity in fibroblast cells was reduced.

Phenoxydifluoromethyl substituted nitrogen heterocycles. Synthesis and heterocyclization reactions of ethyl 4,4-difluoro- 4-phenoxyacetoacetate

Ethyl 4,4-difluoro-4-phenoxyacetoacetate was obtained and studied as a precursor to new heterocyclic compounds. 6-Hydroxypyrimidine, 1,3-dihydro-1,5- benzodiazepin-2-one, quinolin-2-one and 6-hydroxypyrazolo[3,4-b]pyridine derivatives containing phenoxydifluoromethyl groups were synthesized. These results make it possible to introduce aryloxydifluoromethyl substituents for the design of biologically active heterocycles.