We provided a simple and efficient method to conctruct the cis-fused-5,6,3- sesquiterpenes marasmane skeleton via photo-induced decarbonylative 1,3- rearrangement of bicyclo[2.2.2]octenones which were synthesized from the intermolecular Diels-Alder reaction of N-phenylmaleimide 8 or 2,2-dimethylcyclopent-4-ene-1,3-dione 10 with masked o-benzoquinones 2a-m.