Synthesis of 3-amino-4-fluoropyrazoles.

Fluorinated pyrazoles bearing additional functional groups that allow further functionalization are of considerable interest as building blocks in medicinal chemistry. The developed synthetic strategy for new 3-amino-4-fluoropyrazoles consists of a monofluorination of ß-methylthio-ß-enaminoketones using 1-(chloromethyl)-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) (Selectfluor) toward the corresponding monofluorinated enaminoketones, followed by condensation with different hydrazines.

Short synthesis of the seed germination inhibitor 3,4,5-trimethyl-2(5H)-furanone.

3,4,5-Trimethyl-2(5H)-furanone, a new seed germination inhibitor with very promising agrochemical applications, was efficiently synthesized from 2,3-dimethylmaleic anhydride via nucleophilic addition of methyllithium followed by reduction using sodium borohydride. This two-step synthesis is straightforward and high-yielding and permits the large-scale preparation of the seed germination inhibitor.

Synthesis of 4-substituted 3,3-difluoropiperidines.

Synthetic strategies toward 4-substituted 3,3-difluoropiperidines were evaluated. 4-Alkoxymethyl- and 4-aryloxymethyl-3,3-difluoropiperidines were synthesized via 1,4-addition of ethyl bromodifluoroacetate to 3-substituted acrylonitriles in the presence of copper powder, followed by borane reduction of the cyano substituent, lactamization, and reduction of the lactam. This method was applied to establish the synthesis of N-protected 3,3-difluoroisonipecotic acid, a fluorinated gamma-amino acid. 4-Benzyloxy-3,3-difluoropiperidine was prepared using an analogous methodology and was converted to N-protected 3,3-difluoro-4,4-dihydroxypiperidine, a compound with high potential as a building block in medicinal chemistry.

Synthesis of 5-amino- and 5-hydroxy-3,3-difluoropiperidines.

Synthetic routes toward new 5-amino- and 5-hydroxy-3,3-difluoropiperidines, which are of high interest as building blocks in medicinal chemistry, are described. The key step involves the N-halosuccinimide-induced cyclization of 2,2-difluoro-4-pentenylamines toward 5-halo-3,3-difluoropiperidines, which were used to synthesize 5-amino-3,3-difluoropiperidine. In a second strategy, iodolactonization of 2,2-difluoro-4-pentenoic acid gave the corresponding γ-lactone, which was transformed into 5-hydroxy-3,3-difluoropiperidine.

New synthesis of 3-fluoropyrroles.

5-Alkoxymethyl-2-aryl-3-fluoro-1H-pyrroles and 2-aryl-3-fluoro-1H-pyrrole-5-carbaldehydes were efficiently prepared from the corresponding 2-aryl-5-(bromomethyl)-1-pyrrolines via electrophilic alpha,alpha-difluorination of the imino bond, using Selectfluor (1-chloromethyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bistetrafluoroborate) and subsequent aromatization by dehydrofluorination. This methodology provides a new and easy entry toward various new 3-fluorinated pyrroles.

New entries toward 3,3-difluoropiperidines.

Difluoropiperidines attract considerable interest from organic and medicinal chemists, but their synthesis is often problematic. This paper describes a new synthetic pathway toward valuable 3,3-difluoropiperidines starting from suitable delta-chloro-alpha,alpha-difluoroimines. The latter imines can be synthesized via electrophilic fluorination of the corresponding delta-chloroimines using NFSI (N-fluorodibenzenesulfonimide) in acetonitrile. After hydride reduction of the imino bond and subsequent intramolecular substitution of the chloride atom, new 3,3-difluoropiperidines were obtained in good yields. In addition, this methodology was applied to establish the first synthesis of N-protected 3,3-difluoropipecolic acid, a new fluorinated amino acid.

Synthesis of 3-substituted 2-fluoro- and 2,2-difluoroaziridines.

A new route for the synthesis of stable 3-alkyl- and 3-aryl-2(,2)-(di)fluoroaziridines was developed by hydride reduction of novel alpha-bromo- and alpha-chloro-alpha(,alpha)-(di)fluoroketimines and subsequent ring closure of beta-fluorinated beta-chloro- and beta-bromoamines. This is the first report on the synthesis of 2,2-difluoroaziridines sensu stricto.

Direct electrophilic alpha-fluorination of imines: efficient synthesis of mono- and difluoroimines.

[reaction: see text] A mild and efficient procedure to synthesize alpha-fluoro- and alpha,alpha-difluoroimines was developed. Various N-alkylimines derived from acetophenones were successfully monofluorinated using NFSI (N-fluorobenzenesulfonimide) in a mixture of CH(3)CN and DMF at 0 degrees C. Alternatively, the same procedure without DMF gave rise to difluorinated imines when performed at room temperature. The obtained alpha- and alpha,alpha-difluorinated imines were subsequently reduced to give the corresponding beta-fluoro- and beta,beta-difluoroamines in good yield.

New synthesis of fluorinated pyrazoles.

A new synthesis of fluorinated pyrazoles, a class of compounds with potential in medicinal chemistry, is described. The treatment of benzoylfluoroacetonitrile with hydrazine yielded the expected new 3-amino-4-fluoropyrazole, while the analogous reaction of α-cyano-α,α-difluoroketones with hydrazine in refluxing isopropanol surprisingly gave rise to 3-unsubstituted 4-fluoropyrazoles via an unprecedented mechanism. The isolation of intermediate hydrazine adducts led to a mechanistic rationale for this transformation.

Gold-catalyzed synthesis of 2-aryl-3-fluoropyrroles.

The gold-catalyzed cyclization and dehydrofluorination of gem-difluorohomopropargylamines provides a novel access to 2-aryl-3-fluoropyrroles. Difluorinated homopropargylamines are prepared by the addition of gem-difluoropropargyllithium reagents to arylated N-tosylimines.