Q8(b). Epoxidation of a silyl dihydropyran; the Shapiro reaction

Reading of the question: in (i) the printed ring has an extra stroke between the ring oxygen and C-8a, which would make the oxygen trivalent. It is read as the cyclic enol ether 8-methyl-2-(trimethylsilyl)-4a,5,6,7,8,8a-hexahydro-4H-chromene, with a single O–C8a bond.

(i) Epoxidation of the 2-silyl dihydropyran, then acid

Scheme: The electron-rich vinylsilane enol ether is epoxidised; acid unmasks the C–Si carbon as a lactone carbonyl.

1. Formation of X. The C2=C3 bond carries the ring oxygen (a +M donor) and is very electron-rich. The peroxy acid transfers its terminal oxygen in one concerted step through the cyclic “butterfly” transition state (Prilezhaev reaction): the π bond attacks the O–O σ*, the O–H proton moves to the carbonyl oxygen, and m-chlorobenzoic acid is released. Addition is syn, from the less hindered face.
2. X = the 2,3-epoxy-2-(trimethylsilyl)tetrahydropyran (an α,β-epoxysilane that is also an epoxy ether).
3. Acid step. The epoxide oxygen is protonated and water opens it at C-2 (the carbon bearing Si and the ring O), leaving OH on C-3. Si on C-2 and OH on C-3 are now anti to each other; Peterson-type elimination of Me₃SiOH gives the enol of a lactone, which tautomerises to the C=O.
4. Y = 8-methyl-octahydro-2H-chromen-2-one, a δ-valerolactone fused to the cyclohexane. The C–Si carbon has become the lactone carbonyl: the 2-silyl dihydropyran is a masked lactone.

(ii) Shapiro reaction

Scheme: The less substituted α-carbon (C-5) is deprotonated; the new C=C forms on that side.

1. First BuLi removes the acidic N–H proton of the tosylhydrazone.
2. Second BuLi removes an α-H syn to the N–NTs group from the less substituted α-carbon, the ring CH₂ (C-5), not the CH carrying the ethyl group (C-2): the dianion forms under kinetic control.
3. The dianion expels toluenesulfinate (Ts⁻) to give a vinyldiazenide anion, which loses N₂ (irreversible) and leaves a vinyllithium at C-1.
4. H₃O⁺ protonates the C–Li bond. X = 3-ethyl-4-methylcyclopent-1-ene: the C=C lies between the former C=N carbon and C-5, the less substituted alkene. (Under protic Bamford–Stevens conditions the more substituted alkene would form.)

Conclusion. (i) X = the α,β-epoxysilane (mCPBA, concerted butterfly epoxidation of the electron-rich C=C); Y = the fused δ-lactone after acid opening and loss of Me₃SiOH. (ii) Shapiro reaction: X = 3-ethyl-4-methylcyclopentene, the less substituted alkene, via a vinyllithium.

Application. Vinylsilanes and their epoxides serve as masked carbonyl groups in annulation chemistry, and the Shapiro reaction is used to make vinyllithium reagents for C–C bond formation (for example in Nicolaou's Taxol synthesis).