39 四員環の特性を活用したパラジウム錯体による新規分子内連続反応の開発と天然物合成への応用(口頭発表の部)

抄録

Palldium mediated cascade reaction has emerged as one of the most important methods for the preparation of a wide range of molecular frames. Here, we report two types of novel palladium mediated intramolecular cascade reactions triggered by strain release of four membered ring and their application to the natural product synthesis. 1) Palladium(0)-catalyzed cascade insertion-ring expansion reaction Palldium(0)-catalyzed cascade reaction of allenylcyclobutanols 5 and aryl halides 6 gave rise to aryl substituted cyclopentanones 7-9. The reaction pathway was presumed via the ring transformation of the π-allylpalladium intermediate in situ generated by the carbopalladation of allene. We also examined the intramolecular version of this cascade reaction. Consequently, substrates 10a,b and 12a,b, where the allene and vinyl iodide units were tethered by carbon chains, were successfully converted to the corresponding 5,7 and 5,8 ring systems 11 and 13-15. This sequential process provides a unique entry into the biologically important natural products having 5,7- and 5,8-fused ring frameworks. 2) Palldium(II)-mediated cascade ring expansion-insertion reaction The treatment of the isopropenylcyclobutanols 20a,b with palladium(II) complex afforded hydrindans 21-24 by the successive ring expansion and intramolecular insertion reactions. In this cascade reaction, diastereoselectivity of the products was found to be largely dependent on solvents employed. 3) Asymmetric synthesis of (+)-equilenin We applied the above reaction to synthesis of one of steroids, (+)-equilenin (25). In this process, novel synthetic method of chiral cyclobutanone was developed by using chiral (salen)manganese(III) catalyst 33. Thus, the aryl substituted cyclopropylidene 29 was converted to the chiral cyclobutanone 28 by asymmetric epoxidation-ring expansion reaction in the presence of 33. The isopropenylcyclobutanol 27, prepared from 28, was diastereoselectively converted to the trans-naphthohydrindan 26 using palladium(II) acetate in HMPA-THF. Asymmetric synthesis of (+)-equilenin (25) was completed by the transformation of the product 26.