Oxirane isomerizations stereochemistry

The introduction of fluorine on position 6 can be performed according to various pathways fluorohalogenation, via the oxirane 5a-6a, or electrophilic fluorination (FC103,CF3C00Na/F2, and now F-TEDA-BF4) (Figure 8.47). The stereochemistry of the electrophilic fluorination product is dependent on the reagent used. However, the 6a compound is generally the major one. In acidic medium, it is possible to isomerize the 6/i-F compounds into 6a-F compounds.Methods to introduce... 

The rotation about the carbon-carbon bond in intermediate 30 is fast compared to cyclization. As a result isomeric alkenes are transformed to oxiranes of the same, predominantly trans stereochemistry. A practical process for the epoxidation of a wide range of alkenes was described in the presence of biacetyl as sensitizer to yield epoxides in about 90% yield.268... 

There has recently been an upsurge in the research into the role of metals in the isomerization of oxiranes. This research is in part of a synthetic nature and in part of theoretical significance in that the aim is the understanding of the stereochemistry and mechanism of metal catalysis. 

The experimental results outlined in Eqs. 384 and 385 permit considerations of the stereochemistry of the processes.In every case a cis product is obtained. 4,5-Dihydrooxepine derivatives are formed from cfs-oxiranes in an intramolecular concerted 3,3-sigmatropic reaction, while frans-oxiranes yield not only the dihydro-oxepines but also dihydrofurans, the latter by isomerization via an ylide inte-mediate. 

---