Stereospecific mono- and difluorination of the C7-bridge of norbornenes

Fluorinated norbornenes are very desirable monomers in the semiconductor and high-temperature polyimide industries. We describe herein a synthetic strategy for the stereospecific mono- or difluorination of the C7-carbon in norbornene systems beginning with 7-ketonadic anhydride 1. In particular, anti-7-fluoro methyl diester 4 and its 7,7-difluoronadic analog 7 can be prepared from 1 in 3 or 4 steps: saponification, reduction (for 4), esterification, fluorination with DAST. In addition, anti-7-fluoro-syn-7-fluoromethylnadic diester 16 is obtained from epoxide 14, and dimethyl 7,7-difluorobicyclo[2.2.2]oct-5-ene-2,3-dicarboxylate (17) from ketone 15. Anchimeric assistance of the norbornene double bond guides the introduction of attacking fluoride anions stereospecifically anti to the olefinic linkage.

A synthetic strategy for the stereospecific mono- or difluorination of the C7-carbon in norbornene systems by beginning with 7-ketonadic anhydride is described.Figure optionsDownload as PowerPoint slideHighlights
► C7-fluorination of norbornenes can be effected beginning with 7-ketonadic analogs.
► DAST fluorination of the 7-ketonadic analogs generate 7,7-difluoro derivatives.
► Reduction of 7-keto to 7-alcohol followed by DAST yields 7-fluoro derivatives.
► Both 7-syn and 7-anti alcohol react with DAST giving the same 7-anti fluoride.
► Anchimeric assistance guides fluorination stereoselectively anti to the double bond.