Silicon-directed carbon-carbon bond forming reactions
<p>It is shown that O-silyl ketene N,O-acetals react with aldehydes, without catalysis, to form aldol-type products with high diastereoselectivity. For example, the prolinolderived O-silyl ketene N,O-acetal 7 reacts with benzaldehyde to form the anti aldol product 9 with >98% diastereoselec...
| Summary: | <p>It is shown that O-silyl ketene N,O-acetals react with aldehydes, without catalysis, to form aldol-type products with high diastereoselectivity. For example, the prolinolderived O-silyl ketene N,O-acetal 7 reacts with benzaldehyde to form the anti aldol product 9 with >98% diastereoselectivity. Mechanistic studies are presented which support a scheme involving formation of trigonal bipyrarnid (tbp) G, pseudorotation to form tbp
T_(boat), and rate-determining C-C bond formation to produce 9. </p>
<p>It is shown that aldehydes react with pyridine, benzeneselenol, dichlorodimethylsilane, and an allylic alcohol to form O-allyloxy dimethylsilyl hemiselenoacetals in high yield. These derivatives undergo efficient free radical-mediated cyclization upon treatment with BU_3SnH, as exemplified by the transformation of 7→8. Together these steps provide an efficient method for carbon-carbon bond formation between the carbonyl carbon of an aldehyde and the terminus of an allylic alcohol.</p>
<p>2,3-Epoxy alcohols are transformed to C2-inverted cyclic carbonates upon treatment with cesium carbonate under an atmosphere of carbon dioxide (e.g., 5→6).The reaction is completely stereospecific, within experimental error, and forms only 5-membered ring cyclic carbonates.</p>
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