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Sequential Desymmetrization–Fluorination: Enantioselective Synthesis of Fluorinated Cyclitols
Author(s) -
Purser Sophie,
Odell Barbara,
Claridge Timothy D. W.,
Moore Peter R.,
Gouverneur Véronique
Publication year - 2006
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200600861
Subject(s) - enantioselective synthesis , chemistry , electrophile , steric effects , stereochemistry , desymmetrization , substituent , electrophilic fluorination , double bond , conformational isomerism , epoxide , dihydroxylation , silylation , organic chemistry , catalysis , molecule
An asymmetric synthesis of enantioenriched, highly functionalized fluorinated carbocycles has been developed based on an enantioselective Sharpless dihydroxylation of cyclohexadienylsilanes, combined with a diastereoselective electrophilic fluorodesilylation. Several parameters define the level of diastereocontrol for the fluorination step. These include the relative stereochemistry of the starting endocyclic allylsilanes and the structural features of the reactants. As expected for an S E 2′ mechanism, the fluorodesilylation occurred with clean transposition of the double bond, with the attack of the electrophilic fluorinating agent taking place preferentially anti to the silyl group. For the fluorination step, the best selectivities were observed for the monocyclic anti , syn benzyl‐protected precursors and for the syn , syn starting allylsilane. Full NMR spectroscopic analysis of the fluorinated monocyclic compounds, which possess an endocyclic double bond flanked by two electronegative groups (the methoxy group and the fluorine substituent) revealed that a subtle combination of steric (1,3‐axial/pseudoaxial interaction) and stereoelectronic effects (π–σ* interaction) favours the preferential conformers featuring the methoxy group in a pseudoaxial position.