Photocycloaddition-cyclobutane rearrangement to spiro cyclopentanones: application in a formal synthesis of (± )-α-cedrene

A novel four-step sequence has been developed for the construction of cyclopentanone at the carbonyl carbon of cyclic ketones 5 leading to the synthesis of functionalised spiro[4.n] systems 9. The key steps involve a pinacol-type rearrangement of ethoxycyclobutane derivatives 8 obtained from copper(I)-catalysed photocycloaddition of the dienes 7 prepared from the cyclic ketones 5. The methodology also works well on cyclic ketones with an additional functional group. For example, the ketones 13 and 20 produce the spiro ketone 19 ideally suited for synthesis of the acorane sesquiterpenes. The synthetic potential of this protocol has been demonstrated by a formal synthesis of α-cedrene 3