Stereocontrol in the intramolecular Buchner reaction of diazoketones

Abstract

Rhodium(II) acetate catalysed intramolecular Buchner cyclisation of a series of diazoketones 1 proceeds with excellent diastereoselectivity to produce the trans substituted azulenones 2, which exist as a rapidly equilibrating cycloheptatriene–norcaradiene system, from which the norcaradiene tautomers can be efficiently trapped as PTAD cycloadducts 4. The cyclisation–cycloaddition sequence can be conducted in either a stepwise or a tandem process, leading to the pentacyclic systems 4 as a single diastereomer in each case. In the reaction of diazoketone 1f intramolecular cyclopropanation competes with cyclisation to the aromatic ring.