Direct Synthesis of Dibenzocyclooctadienes via Double Ortho Friedel-Crafts Alkylation by the Use of Aldehyde-Trimethylsilyl Iodide Adducts

'The development of a new, direct method for the preparation of dibenzo[a,e]cycloocta-1,5-dienes from phenylacetaldehyde by a double ortho Friedel-Crafts alkylation is described. When benzaldehyde (2b) is treated with trimethylsilyl iodide (3), a,n-diiodotoluene (6) is formed in high yield via the intermediacy of a-iodobenzyl trimethylsilyl ether (4b). Treatment of phenylacetaldehyde (%a) with trimethylsilyl iodide (3) under the same conditions gikes rise to a different reaction pathway, affording initially the aldehyde iodohydrin trimethylsilyl ether (4a), which is transformed on standing into a mixture of three products. The major product of this mixture, isolated in slightly over 50?? yield, is the interesting bicyclic ether 3,7-epoxydibenzocycloocta-1,5-diene (8); the minor products are 2-phenylnaphthalene (9) and 2-iodo-3-phenyltetralin (10). The bicyclic ether 8 can be easily transformed by dissolving metal reduction followed by oxidation into the ketone dibenzocycloocta-1,5-dien-3-one ( I ) , which has been converted into a large variety of biologically active compounds. A possible mechanism for the reaction is discussed. The reaction of the acetaldehyde-trimethylsilyl iodide adduct (4c) with 2-phenylethyl trimethylsilyl ether (2 1) affords 1 -methylisochroman (25), the expected product of the proposed mechanistic pathway. The potential utility of these a-iodoalkyl trimethylsilyl ethers (4) is also discussed.