REMOTE ALLYLIC SILYLOXY GROUPS AS STEREOCONTROL ELEMENTS IN INTRAMOLECULAR OXYMERCURATIONS OF GAMMA-HYDROXYALKENES

The diastereoselectivity in intramolecular oxymercurations of gamma-hy droxyalkenes bearing a remote allylic oxy substituent has been investi gated, It was found that the best selectivity was obtained by employin g a combination of (Z)-alkene geometry and a tert-butyldiphenylsilyl p rotecting group attached to the remote allylic oxygen as in 4a-g. Cycl ization, using mercuric acetate in dichloromethane, of all the (Z)-alk enols gave the syn, diastereomer, 5a-g, as the major product. For exam ple, 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 b y replacing dichloromethane with acetonitrile. Under these conditions the ratio of 5b to 6b increased to 19:1. Cyclization of (E)-alkene 9 g ave very poor diastereoselection. These syn-selective intramolecular o xymercurations were exploited in enantioselective syntheses of two dia stereomers of methyl nonactate.