P-69 動的速度論分割に基づく不斉Heck反応の開発とDichroanal B合成への応用(ポスター発表の部)

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Recently, it was reported that diterpenes having 1,1,4a-trimethylhydrofluorene skeleton, e.g., standishinal (5) isolated from Thuja standishii, exhibited potent inhibitory activity toward aromatases. However, no effective synthetic methods have been reported to date. Thus, we embarked synthetic study of these diterpenes by using intramolecular asymmetric Heck reaction. A substrate of the Heck reaction (1a) was prepared by addition of 2-methoxyphenyllithium to β-cyclocitral and successive dehydration. Although the distance between Pd and an endo-double bond in the stable form of an aryl palladium intermediate derived from 1a, was too far to proceed the Heck reaction, a tricyclic product (2) was afforded in good yield. This result was explained by a bending conformation of the A-ring in the transition state, where the Pd closed to the double bond. Taking advantage of this reaction, (±)-dichroanal B (4) having a same skeleton as standishinal (5) was synthesized in a short step as shown in Scheme 4. On the other hand, NMR studies of 1a-b at variable temperatures exhibited that the key intermediate of the Heck reaction existed as atropisomers, which were equilibrated at higher temperature. In such cases, an optically active product could be obtained practically by using dynamic kinetic resolution. Therefore, an intramolecular asymmetric Heck reactions using chiral ligands was scrutinized to carry out the dynamic kinetic resolution. In conclusion, the reaction of 1d, which had less hindered protective group of catechol moiety than 1b and 1c, in the presence of S-(-)-bis-(diphenylphosphino)-6,6'-dimethoxy-1,1'-biphenyl gave an excellent result to afford 2d. Further studies on conversion from 2d to optically pure (-)-dichroanal B (4) is in progress.
天然有機化合物討論会の論文
2006-09-15

P-69 動的速度論分割に基づく不斉Heck反応の開発とDichroanal B合成への応用(ポスター発表の部)

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