Abstract
The synthesis of two new 2-phenylisoserines, each bearing a quaternary stereocenter is described. These compounds, which are analogs of the amino acid side chain found in taxol and taxotere, were obtained by addition of the lithium enolates of (2S,5S)-2-isopropyl-5-phenyl-1,3dioxolan-4-one and (2S,5S)-2-tert-butyl-2-methyl-5-phenyl-1,3-dioxolan-4-one to benzylbenzylideneamine in the presence of BF3 . O(Et)2. The diastereomeric mixtures were separated in each case and the absolute configurations thereof were determined by X-ray analysis.
Highlights
The Mannich reaction and transformations involving nucleophilic addition to the C=N of imine derivatives are of significant importance in organic synthesis.[1]
Preparation of chiral 1,3-dioxolan-4-ones (6a-b and 8a-b) and configurational assignments As shown in Scheme 1, the chiral 1,3-dioxolan-4-ones (2R,5S)-6a, (2S,5S)-6a, (2R,5S)-6b and (2S,5S)-6b were prepared from (S)-mandelic acid 4 with isobutyraldehyde 5a or pinacolone 5b using a slight modification of the known procedure.[7]
Methanolysis of 1,3-dioxolan-4-one amines (2S,5R,1′R)-10a and (2S,5R,1′R)-10b In the third part of the present work, we focused our attention on the methanolysis of the derivatives (2S,5R,1′R)-10a and (2S,5R,1′R)-10b, to the corresponding tetrasubstituted isoserine derivatives (2R,3R)-11 and (3R,4R)-12
Summary
The Mannich reaction and transformations involving nucleophilic addition to the C=N of imine derivatives are of significant importance in organic synthesis.[1]. Dioxolan-4-one based lithium enolate methodology it was of interest to study the reaction at low temperature using benzylbenzylideneamine as the electrophile in the presence of Lewis acids. BF3.OEt2 (2 equiv.) proceeds under the influence of the temporary stereogenic acetal C-2 center to give a 7:4 diastereomeric mixture of 1,3-dioxolan-4-one amine (2S,5R,1′S)-10a and (2S,5R,1′R)-10a (Scheme 3).[16] The ratio of these two diastereomers was determined by 1H NMR.
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