Beschreibung:
<jats:title>Abstract</jats:title><jats:p>In a single photochemical operation (<jats:italic>λ</jats:italic>≥350 nm) an easily accessible indanone derivative was converted into a structurally complex precursor of the protoilludane sesquiterpenes. The product (60 % yield) contains all 15 carbon atoms of the skeleton in the required connectivity and was transformed into the natural product atlanticone C (9 steps, 6 % overall yield). In addition, it was shown that other protoilludanes, such as Δ<jats:sup>6</jats:sup>‐protoilludene and paesslerin A, can be prepared in a concise fashion via the photochemical key intermediate. The photochemical reaction cascade comprises an <jats:italic>ortho</jats:italic> photocycloaddition, a thermal disrotatory ring opening and a regioselective disrotatory [4π] photocyclization.</jats:p>