The regioselectivity of various enantiocomplementary ω-transaminases was evaluated for the stereoselective monoamination of designated 1,5-diketones; excellent conversions, enantio- and regioselectivities were observed. The resulting amino-ketones underwent spontaneous intramolecular ring closure to afford Δ1-piperideines, which served as precursors for the cis- and anti-piperidine scaffold as demonstrated for the synthesis of the alkaloids dihydropinidine and epi-dihydropinidine. Key to the success of accessing the trans-piperidines was a Lewis acid mediated conformational change of the Δ1-piperideines in the reduction step. Thus, all four diastereomers of 2,6-disubstituted piperidines could successfully be prepared.
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