The influence of alkene geometry and remote substitution on the stereochemical outcome of intramolecular oxymercurations leading to five-membered rings is described.
描述了烯烃的几何形状和远程取代对导致五元环的分子内氧化汞的立体化学结果的影响。
Remote Allylic Silyloxy Groups as Stereocontrol Elements in Intramolecular Oxymercurations of γ-Hydroxyalkenes
The diastereoselectivity in intramolecular oxymercurations of gamma-hydroxyalkenes bearing a remote allylic oxy substituent has been investigated, It was found that the best selectivity was obtained by employing a combination of (Z)-alkene geometry and a tert-butyldiphenylsilyl protecting group attached to the remote allylic oxygen as in 4a-g. Cyclization, using mercuric acetate in dichloromethane, of all the (Z)-alkenols gave the syn, diastereomer, 5a-g, as the major product. For example, cyclization of 4b gave syn diastereomer 5b and anti diastereomer 6b in a ratio of 7:1. It was found that this ratio could be improved by replacing dichloromethane with acetonitrile. Under these conditions the ratio of 5b to 6b increased to 19:1. Cyclization of (E)-alkene 9 gave very poor diastereoselection. These syn-selective intramolecular oxymercurations were exploited in enantioselective syntheses of two diastereomers of methyl nonactate.