ketones with 1,3-dienes in the presence of CeCl3–SnCl2 in tetrahydrofuran is found to give the corresponding [3+4] cycloadduct in fair to good yields under mild conditions. Furan and cyclopentadiene serve as highly efficient receptors of the oxyallyl intermediate to give bicyclic cycloadducts. The reaction of 2,4-dibromo-3-pentanone with isoprene gives both [3+4] and [3+2] cycloadducts. [3+2] Cycloaddition