Oxetane formation by 1,3-migration of benzyloxy group in 7-oxabicyclo[2.2.1]hept-2-yl cations: Synthesis of 4,7-dioxatricyclo[3.2.1.O-3,O-6]octane and 2-oxabicyclo[2.2.2]octane encrusted in the 1,4 : 5,8-diepoxy-perhydrophenanthrene ring system

Epoxidation of (1RS,2SR,3SR,4RS,6RS,7RS,8RS, 12RS, 13RS)-6,13-bis(benzyloxy)-15,16-dioxapentacyclo[10. 2.1.1(4.7).0(2,11).0(3.8)]hexadec-10-en-8-yl-methanol (12) led to the stereoselective formation of (1RS,2RS,3RS,5RS,6SR, 7RS,8RS, 10RS,11SR, 12SR,16SR)-3,10-bis(benzyloxy)-13,17,18-trioxahexacyclo[10.2.2. 1(2,5).1(8,11).0(1,6).0(7,12)]octadecan-16-ol (13). Epoxidation of the acetate (19) derived from 12 gave an epoxide (20) that generated (1RS,2SR,3RS,4RS, 6RS,7SR,8RS,9SR,11RS,12RS,13RS)-13-benzyloxy-9-hydroxy-15,16,17-trioxahe xacyclo[10.2.1.1(4,7). 1(6,8).0(2,11).0(3,8)]heptadecane-11-methyl acetate (21) under acidic conditions. These reactions imply the formation of 2-alkyl-6-endobenzyloxy-7-oxabicyclo [2.2.1]hept-2-yl cation intermediates (e.g. 14) that are quenched intramolecularly by the hydroxymethyl group to give the corresponding 2-oxabicyclo[2.2.2]octane systems (12-->13). In the case of the acetoxymethyl containing intermediate, the latter cyclization is retarted and a facile 1,3-migration of the endo-benzyloxy group occurs leading to the corresponding 4,7-dioxatricyclo[3.2.1.0(3,6)]octane derivative (20-->21).