ORGN 77 |
| Imines 2 prepared from oxazine amine 1 (prepared from (S)-glycidyl phthalimide in 5 high yielding steps), undergo acid-catalysed cyclisation to give oxabispidine acetals 3, which may be converted to oxabispidines 4 in two steps. The reaction is highly stereoselective, affording a product consistent with a chair-chair transition state where the imine E-geometry is retained, and nucleophile capture takes place on the least hindered face. The reaction may be extended to aliphatic aldehydes bearing α-hydrogens by incorporation of benzotriazole into the reaction with 1, stabilising the imine as its benzotriazole adduct. In this manner 5-tosyloxy pentanal was condensed with 1 to give benzotriazole adduct 5, which underwent double cyclisation to generate 6, the precursor to 7, an oxabispidine analogue of O'Brien's (-)-sparteine surrogate.
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Asymmetric Reactions and Syntheses
1:00 PM-5:00 PM, Sunday, 10 September 2006 Moscone Center -- Room 131, Oral
Division of Organic Chemistry |
