Acylation of Tropane Alkaloids Displaying Reversed Diastereoselectivities under Enzymatic versus Chemical Conditions
2014
Affolter, Olena | Baro, Angelika | Laschat, Sabine | Fischer, Peter
Lipase-mediated monoacetylation of 6,7-dihydroxytropinones 4 gave acetates 5, ent-5 which were analyzed as Mosher esters 9a, b by ¹H NMR spectroscopy. However, the hydroxy groups in 4 were not differentiated by lipases. Reduction of the keto function and subsequent silylation afforded a mixture of endo/exo-TBS ethers 11, which were dihydroxylated to give the corresponding diols endo/exo-12. In chemical acetylation a change of the endo/exo ratio in favor of the endo-derivative endo-13 was observed, whereas the formation of the exo-acetate exo-13 dominated in lipase-catalyzed acylation reactions. A mechanistic proposal is given.
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