Formal [4+1]- and [5+1]-Annulation by an SN2-Conjugate Addition Sequence: Stereoselective Synthesis of Highly Substituted Carbocycles

K2CO3-mediated reactions of 6-bromo-2-hexenoates and 7-bromo-2-heptenoate with active methylene compounds deliver highly substituted cyclopentane and cyclohexane derivatives, respectively via a sequence of S(N)2 conjugate addition reactions (formal [4 + 1]- and [5 + 1]-annulation) in a diastereoselective manner.