Reactions of cage-annulated 2-methyl-5-(trifluoromethanesulfonyloxy)furanswith lithium diisopropylamide. Evidence for nucleophilic reactivity of LDA

Reaction of 3-trifluoromethanesulfonyloxy-5-methyl-4-oxahexacyclo[5.4.1.0(2 ,6).0(3,10).0(5,9).0(8,11)]dodecane (6a) with lithium diisopropylamide (LDA , 6 equivalents) in dry THF affords 11-methylenepentacyclo[5.4.0.0(2,6).0(3 ,10).0(5,9)]undecan-8-one (3a) in low yield (11%). This reaction is shown t o proceed via formation of an intermediate cage-annulated hemiaminal (7a), which in turn may have resulted via a highly unusual nucleophilic attack by (i-Pr)(2)N:(-) upon exo-11-diisopropylamino-endo-11-methylpentacyclo[5.4.0 .0(2,6).0(3,10).0(5,9)]undecan-8-one (8a). The corresponding reaction of 3- trifluoromethanesulfonyloxy-5,9,10-trimethyl-4-oxahexacyclo[5.4.1.0(2,6).0( 3,10).0(5,9).0(8,11)]dodecane (6b) with LDA (6 equivalents) in dry THF prod uced 1,7-dimethyl-11-methylenepentacyclo[5.4.0.0(2,6).0(3,10).0(5,9)]undeca n-8-one (3b) in 10% isolated yield.