One-Step Stereochemical Determination of Contiguous Four Acyclic Chiral Centers on the Steroidal Side Chain: A Novel Synthesis of Brassinolide

Article abstract of DOI:10.1021/ja00282a034

The stereoselective synthesis of brassinolide and its enantiomer (22S,23S,24R)-24-epibrassinolide was accomplished.The key feature of this synthesis is based on the stereoselective reduction of the 5-ylidenetetronate derivative to control the stereochemistry of the contiguous four acyclic chiral centers on the steroid side chain in one-step, wherein the stereochemically determinitive step was carried out in a cyclic system.The addition reaction of the dianion of the tetronate derivative to the 20-oxo steroid afforded the adduct, whose syn-dehydration reaction brought about the formation of the desired (Z)-5-ylidenetetronate.The stereoselective reduction of the (Z)-5-ylidenetetronate afforded the key intermediate for the synthesis of brassinolide.On the other hand, the (E)-5-ylideneteronate could also be prepared from the same adduct by manipulation of the dehydration reaction, and this approach led to the synthesis of (22S,23S,24R)-24-epibrassinolide.