OXETANE FORMATION BY 1,3-MIGRATION OF BENZYLOXY GROUP IN 7-OXABICYCLO[2.2.1]HEPT-2-YL CATIONS:SYNTHESIS OF 4,7-DIOXATRICYCLO[3.2.1.0~(3,6)]OCTANE AND 2-OXABICYCLO[2.2.2]OCTANE ENCRUSTED IN THE 1,4:5,8-DIEPOXY-PERHYDROPHENANTHRENE RING SYSTEM

摘要

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,3SR,4RS,6RS,7RS,8RS,12RS,13RS)-13-benzyloxy-9-hydroxy-15,16,17-trioxa-hexacyclo [10.2.1.1~(4.7).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[2.2.1]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).