Abstract
Bicyclo [3. 2. 2] nonane derivatives are prepared from tropones by Diels-Alder reaction with ethylene and from bicyclo [2. 2. 2] oct-5-en-2-ones by Tiffeneau-Demjanov ring enlargements. The methods developed for the construction of [m-n] fused-ring skeletons are the photochemical [3, 3] -sigmatropic rearrangement of bicyclo [3. 2. 2] nona-3, 6-dien-2-ones into endo-7-bicyclo [4. 1.0] hept-2-enylketenes and the photochemical [1, 3] -acyl migration of bicyclo [3. 2. 2] non-6-en-2-ones into bicyclo [4. 3. 0] non-4-en-7-ones. The pinacoltype rearrangement of 1-methoxybicyclo [3. 2. 2] non-6-en-2-ols derived from the corresponding ketones by Grignard reactions has been developed in order to establish the formal bridgehead-substitution of these ketones and/or to exchange the saturated bridges each other. The natural products synthesized on the basis of these methods are the [3-6] fused-ring sesquiterpenes (±) -sesquicarene (27), (±) -sirenin (28), and (±) -isosesquicarene (37), the [5-6] fused-ring furano-sesquiterpenes (±) -pinguisone (59) and (±) -deoxopinguisone (60), the [6-7] fused-ring sesquiterpene (±) -widdrol (92), and the antitumor antibiotic polyketide (±) -ptilocaulin (96).
