Remote allylic silyloxy groups as stereocontrol elements in intramolecular oxymercurations of gamma-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.