Diisobutylaluminium Hydride (DIBAL‐H) Promoted Secondary Rim Regioselective Demethylations of Permethylated β‐Cyclodextrin: A Mechanistic Proposal

Abstract

AbstractDiisobutylaluminium hydride (DIBAL‐H) promotes secondary rim regioselective bis‐de‐O‐methylation at the 2A‐ and 3B‐positions of permethylated β‐cyclodextrin. This result contrasts with the selective bis‐de‐O‐benzylation of perbenzylated cyclodextrins in which regioselective deprotection occurs at the primary rim. To gain an insight into the mechanism of this remarkable contrasting behavior, the two corresponding permethylated cyclodextrins with an alcohol function at either the 2‐ or 3‐position were synthesized. The cyclodextrin with the alcohol at the 3‐position reacts withDIBAL‐H to afford the 2A,3B‐diol whereas the cyclodextrin with the alcohol at the 2‐position is unreactive. This observation allows us to propose a mechanism for the demethylation reaction that involves for each demethylation step two molecules of aluminium reagent, in accord with the previous hypothesis on the debenzylation reaction. The second demethylation step appears to be faster than the first, the coordination of aluminium now being an intramolecular process.