Rh (I) -catalyzed Cyclizations via Rhodacycle Intermediates : Application to the Synthesis of (±) -Epiglobulol

Rh (I)-catalyzed hydroacylation of 4, 6-dienals proceeded smoothly, giving cycloheptenone derivatives. It was found that the size of the ring formed in this hydroacylation was dependent on the presence of a substituent at C 7 position and the geometry of the olefin. During ongoing investigation of this reaction, we found that Rh (I) -catalyzed cycloisomerization of 1, 3-dienes with alkenes in a tether gave cyclopentene derivatives in good yields. It was interesting that the existence of a heteroatom between a 1, 3-diene moiety and alkene in the tether affected the reaction course and that only a [4+2] cycloaddition product was produced. These two reactions proceeded using the same cationic Rh (I) catalyst under almost the same reaction conditions. We therefore investigated a new cascade reaction by a combination of these reactions. It was found that this reaction gave the bicyclo [5.3.0] decenone derivative in a stereoselective manner, and the Thorpe-Ingold effect played an important role in the second cycloisomerization step of this cascade cyclization. By the use of this cascade cyclization as a key step, the synthesis of (±) -epiglobulol has been accomplished.