Q4(a). Epoxidation, then intramolecular opening by a nitrile anion

Introduction. The substrate has a disubstituted (E) C=C and an α-hydrogen next to CN. mCPBA epoxidises the alkene; LDA then forms an α-cyano carbanion, which opens the epoxide intramolecularly by SN2 (epoxynitrile cyclisation).

Scheme: The carbanion attacks the nearer epoxide carbon; the C–O bond breaks with inversion and the alkoxide is protonated on work-up.

1. Epoxidation. mCPBA transfers O to the C=C in one concerted step (butterfly TS); the reaction is stereospecific, so the E-alkene gives the trans-epoxide.
2. Carbanion. LDA (bulky, non-nucleophilic) removes the H on the carbon bearing CN; the anion is stabilised by the nitrile (C⁻-C#N <→ C=C=N⁻). At -78°C LDA does not attack the nitrile or the epoxide.
3. Ring closure. The carbanion can reach either epoxide carbon. Attack at the nearer carbon forms a four-membered ring with the C–O bond breaking in line with the attacking carbanion (4-exo-tet, allowed). Attack at the far carbon would make a five-membered ring, but the C–O bond would then be inside the forming ring and the back-side approach cannot be linear (5-endo-tet, disfavoured, Baldwin).
4. The attacked carbon is inverted; the oxygen ends up on the side chain as an alkoxide, protonated by H₃O⁺.

Conclusion. Final product: 1-ethyl-2-(1-hydroxyethyl)cyclobutane-1-carbonitrile (C₉H₁₅NO), from trans-epoxide formation and intramolecular SN2 opening (4-exo-tet) with inversion at the ring carbon.

Application. Stork's epoxynitrile cyclisation makes strained cyclobutanes and cyclopropanes in one step, a route used in terpene synthesis (for example to grandisol-type cyclobutanes).