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Diisobutylaluminium Hydride (DIBAL‐H) Promoted Secondary Rim Regioselective Demethylations of Permethylated β‐Cyclodextrin: A Mechanistic Proposal
Author(s) -
Xiao Sulong,
Yang Ming,
Sinaÿ Pierre,
Blériot Yves,
Sollogoub Matthieu,
Zhang Yongmin
Publication year - 2010
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.200901230
Subject(s) - chemistry , regioselectivity , intramolecular force , reagent , demethylation , cyclodextrin , alcohol , hydride , medicinal chemistry , stereochemistry , organic chemistry , catalysis , hydrogen , biochemistry , gene expression , dna methylation , gene
Diisobutylaluminium hydride (DIBAL‐H) promotes secondary rim regioselective bis‐de‐ O ‐methylation at the 2 A ‐ and 3 B ‐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 2 A ,3 B ‐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.