Enantioselectivity and diastereoselectivity in reductive radical cyclization reactions of 3-(ω-iodoalkylidene)-piperidin-2-ones

The E-configurated 3-(ω-iodoalkylidene)-piperidin-2-ones la-e were prepared from the corresponding ω-tert-butyldimethylsilyloxyalkanals 5a-e by aldol condensation with N-tert-butyloxycarbonylpiperidone, subsequent silyl deprotection and iodo-dehydroxylation. In the presence of tributylstannane as the reducing agent and of triethylborane as the initiator the iodides underwent a reductive radical cyclization to the corresponding products rac-9a-e (72-90% yield). The hydrogen abstraction occurred enantioselectively in the presence of the chiral complexing agent 2 in toluene as solvent (up to 88% ee). For substrates 1b and 1c, for which both the radical cyclization and the hydrogen abstraction step were stereogenic, the insignificant change in diastereoselectivity indicated that the cyclization step was not enantioselective. A 1:1 stoichiometry for the complex was proven by Job plot experiments. The enantioselectivity of the reaction la → 9a was concentration dependent.