Furan formation from 2- oxiranes

Scheme 11.55 Mechanism of furan formation from 2-(l-alkynyl)oxiranes.

We owe to Kato and his colleagues a considerable advance in furan copper reagents. They have demonstrated the formation of the lithium di(3-furyl) cuprate species 87 which is highly reactive and possesses hard properties that suit it to reaction at hard centers, mainly carbonyl carbon.223 The reagent is easily prepared in situ from 3-furyllithium and Cu2I2. Simple copper derivatives do not react with ketones, but this cuprate reacts well and quantitatively with acid chlorides. It also reacts well with some epoxides (oxirans). Moreover, there is another form prepared in the presence of... 

An interesting synthesis of JH1 170 starting from furan was published by Cavill et al.145). The use of a glycol precursor (193) prior to the oxirane formation guarantees the generation of a uniform 10,11-epoxide, in contrast to the usual m-chloroper-benzoic acid oxidation145) (Scheme 34). 

Cesium fluoride promoted oxirane preparation starting from bis(sulfonyl fluorides) has been reported. In general, epoxide formation predominates over furan synthesis. Investigations of cyclohexane derivatives 1 show trans elimination to form 2 this still holds true when the syn chlorine is substituted by a bromine.Epoxide 4 is a versatile, general, enan-liomerically pure building block that is best synthesized by substitution of the mesylate 3. ... 

An example from 1992 of epoxidation of a strained dihydrofuran derivative by molecular oxygen is shown in Equation (4) (92JOC2467). It is believed that an initially formed hydroperoxide at the ring junction adjacent to the furan oxygen is responsible for oxirane formation. 

---