P-24 1,3-ジオキセビン誘導体の特性を基礎とする天然物の合成(ポスター発表の部)

抄録

Heck reaction of 4,5-dihydro-1.3-dioxepins with aryl and alkenyl iodides furnished 5-substituted-4,5-dihydro-1,3-dioxepin derivatives in satisfactory yields. We found that the 5-aryl substituted 4,5-dihydro-1,3-dioxepins thus obtained afforded the corresponding 3-arylbutanals in good yields on Birch reduction. On the other hand, both of the 5-aryl and 5-alkenyl substituted 4.5-dihydro-1,3-dioxepins afforded the corresponding 2.3,4-trisubstituted tetrahydro-3-furfuryl alcohols in one step on exposure to diisobutylaluminum hydride by concurrent ring-contraction and reduction. Based on these two unprecedented reactions, we have developed a new synthetic route to the aromatic bisabolane sesquiterpenes and a diastereocontrolled route to the Furofuran lignans. Thus. 5-(4-methylphenyl)-4,5-dihydro-1,3-dioxepin gave 3-(4-methylphenyl)butanal in an excellent yield from which six aromatic Bisabolane sesquiterpenes could be obtained by conventional transformations. However, the present procedure was found to be unsuitable for the chiral synthesis, because complete racemization took place in the key reductive cleavage reaction in which chirality at the benzylic center was lost. While in the reductive ring-contracting reaction, it was found that the stereochemistry of the three substituents on the tetrahydrofuran could be controlled depending on the Lewis acids used. Thus, starting from the 5-alkenyl-2-aryl-4,5-dihydro-1,3-dioxepin, racemic synthesis of three Furofuran lignans having the same stereochemistry and the first synthesis of racemic asarinin having different stereochemistry have been accomplished by using appropriate Lewis acids. The latter compound has been synthesized for the first time. This procedure could also be modified chirotopically to produce these Furofuran lignans in optically active forms using the alkenyl iodide bearing a chirality controlling group in the molecule.