Photochemie von tricyclischen β, γ‐γ′, δ′‐ungesättigten Ketonen. 50. Mitteilung über Photoreaktionen

Photochemistry of tricyclic β, γ‐γ′, δ′‐unsaturated ketones The easily available tricyclic ketone 1 (cf. Scheme 1 ) with a homotwistane skeleton yielded upon direct irradiation the cyclobutanone derivative 3 by a 1,3‐acyl shift. Further irradiation converted 3 into the tricyclic hydrocarbon 4 . However, acetone sensitized irradiation of 1 gave the tetracyclic ketone 5 by an oxa‐di‐π‐methane rearrangement. Again with acetone as a sensitizer the ketone 5 was quantitatively converted to the pentacyclic ketone 6 . The conversion 5 → 6 represents a novel photochemical 1,4‐acyl shift. The possible mechanisms are discussed (see Scheme 7 ). The tricyclic ketone 2 underwent similar types of photoreactions as 1 ( Scheme 2 ). Unlike 5 the tetracyclic ketone 9 did not undergo a photochemical 1,4‐acyl shift. The epoxides 10 and 14 derived from the ketones 1 and 2 , respectively, underwent a 1,3‐acyl shift upon irradiation followed by decarbonylation, and the oxa‐di‐π‐methane rearrangement ( Schemes 3 and 4 ). The diketone 18 derived from 1 behaved in the same way ( Scheme 5 ). The tetracyclic diketone 21 cyclized very easily to the internal aldol product 22 under the influence of traces of base ( Scheme 5 ). Upon irradiation the γ, δ‐unsaturated ketone 24 underwent only the Norrish type I cleavage to yield the aldehyde 25 ( Scheme 6 ).