A One-Pot Formal [4 + 2] Cycloaddition Approach to Substituted Piperidines, Indolizidines, and Quinolizidines. Total Synthesis of Indolizidine (−)-209I

Heating a mixture of substituted N-benzyl γ-chloropropylamines, conjugated alkynoates or alkynones, sodium carbonate, and a catalytic amount of sodium iodide in i-PrOH at 70−83 °C delivers substituted piperidines in good yields. This transformation goes through a cascade Michael addition/alkylation process and represents a facile one-pot formal [4 + 2] cycloaddition approach to piperidine ring. Using secondary cyclic γ-chloropropylamines as substrates, this process produces substituted indolizidines or quinolizidines. On the basis of this approach, indolizidine (−)-209I is elaborated in 11 steps from methyl 2-hexenoate.