Abstract
Aldol condensation between 3-keto and aldehyde sugars have been studied. The enolate 3 reacts with aldehydes preferentially from the β-face to give axial C2 condensation products. However, if the reaction conditions are not controlled properly some C2 epimerization may be observed. With respect to the aldehyde partner, facial selectivity can be predicted on the basis of the preferred, permissible chelation pattern. Thus α/β chelation (which occurs whenever possible) produces Cram products, while α-chelation produces anti-Cram products. For the major (C2-axial) products, the stereochemistry around the newly forged C2C7 bond can be determined by the magnitude of J 2,7= 8–10 Hz for the C7-(S) condensation product, and ~5 Hz for the C7-(R) analogue. The C3-carbonyl group of the aldol products can be reduced with DIBAL and LiALH 4 to give products bearing equatorial and axial C3-OH groups, respectively. On the other hand, LiBHEt 3 gives axial alcohols only. The ease of formation of an isopropylidene ring between the equatorial C3-OH and C7-OH can be used to deduce the C7 orientation. For example, with the C7-(R) isomer 25 (eq), an isopropylidene ring was readily formed whereas for the C7-(S) analogue 28, ring formation was not observed.
Published Version
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