INTRAMOLECULAR FORMAL [3+2] CYCLOADDITION OF ALKENES AND BENZYLIC CATIONS - STEREOSELECTIVE SYNTHESIS OF 1,2,3,4,4A,9A-HEXAHYDROFLUORENES

The Lewis acid-promoted intramolecular formal [3 + 2]-atom 'cycloaddition' of alkenes with benzylic cations derived from benzylic alcohols and quinone methides affords products in good yield and with remarkable stereoselectivity. Benzylic alcohol 20 affords hexahydrofluorene 36 with three new stereogenic centers in 73% yield as a 10:1 mixture of diastereomers. The scope and limitations of these reactions were explored by varying the substitution pattern on the benzylic cation, the cyclization initiators, and the alkene terminators.