Patrick O’Leary

Asymmetric synthesis of cis-7-methoxycalamenene via the intramolecular Buchner reaction of an α-diazoketone.

The asymmetric synthesis of cis-7-methoxycalamenene 1 has been accomplished using the intramolecular Buchner reaction of α-diazoketone 7 as the key step in the synthetic route. Upon reduction of the equilibrating azulenone structure 8, the resulting azulenol 9 rearranges to dihydronaphthalene 10 containing the 6,6-membered bicyclic ring system characteristic of 1, by means of an acid-catalyzed aromatization process. Transformation of 10 to 1 is accomplished through a three-step reaction sequence.

Stereocontrol in the intramolecular Buchner reaction of diazoketones

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.