Q8(c). Photocyclisation of cyclodecanone to 9-decalol

Reading of the question: the starting compound is drawn as a ten-membered ring folded into a figure of eight, with the C=O pointing inwards: cyclodecanone.

Introduction. This is a photochemical transannular cyclisation: an intramolecular hydrogen abstraction by the excited carbonyl oxygen (the Norrish type II primary process) followed by ring closure of the 1,6-biradical (a Norrish–Yang-type cyclisation). The product, decalin-4a-ol (9-decalol), is an isomer of the starting ketone (C₁₀H₁₈O).

Scheme: Transannular H transfer across the ring, then radical–radical coupling.

1. Excitation: the C=O absorbs (about 280–300 nm, n→π*); the singlet crosses to the triplet n,π* state, in which the oxygen has a half-filled n orbital and behaves like an alkoxy radical.
2. Transannular H abstraction: in the ten-membered ring the C-6 hydrogens point across the ring towards the C=O (transannular strain in medium rings). The electrophilic oxygen takes an H atom from C-6: a 1,6-biradical, a ketyl radical (C–OH) at C-1 and an alkyl radical at C-6.
3. Ring closure: after spin inversion the two radical centres, held close by the ring, combine: a new C1–C6 bond splits the ten-membered ring into two six-membered rings. C-1 carries OH and C-6 carries H: 9-decalol (decalin-4a-ol).
4. The geometry of the abstraction favours the cis-fused alcohol (OH and the ring-fusion H on the same side), the main isomer reported.

Conclusion. The reaction is a photochemical intramolecular (transannular) H-abstraction–cyclisation of a ketone (Norrish type II / Yang type), giving 9-decalol (decalin-4a-ol), mainly cis through a triplet n,π* state and a 1,6-biradical.

Application. Transannular photocyclisations of medium-ring ketones are a route to bicyclic alcohols; the related Norrish–Yang reaction (cyclobutanols from γ-H abstraction) is used in natural-product synthesis and explains the photodegradation of ketone-containing polymers.