Intramolecular Diels–Alder reaction of cyclopenta-1,3-diene derivatives generated in situ from 4-(pent-4-enyl)cyclopent-2-enone ethylene ketals
作者:Masakazu Ohkita、Hidetoshi Kawai、Takashi Tsuji
DOI:10.1039/b109614a
日期:2002.1.23
Intramolecular DielsâAlder reaction of cyclopenta-1,3-diene derivatives 16 generated in situ from the corresponding 4-(pent-4-enyl)cyclopent-2-enone ethylene ketals 15 is investigated. 4-[(E)-5-(Ethoxycarbonyl)pent-4-enyl]cyclopent-2-enone ethylene ketal 15a and 4-(5,5-dicyanopent-4-enyl)cyclopent-2-enone ethylene ketal 15b were prepared in 5 steps from cyclopent-2-enone ethylene ketal 5. Upon heating at 120 °C, compound 15a undergoes intramolecular DielsâAlder reaction via the 4-substituted cyclopenta-1,3-dien-2-yl
enol ether 17a to produce tricyclo[5.2.1.0]decan-8-one derivative 19a as the sole product. On the other hand, 15b undergoes intramolecular DielsâAlder reaction at the same temperature via the 5-substituted cyclopenta-1,3-dien-2-yl enol ether 16b to give tricyclo[4.4.0.0]decan-10-one derivative 18b as the major product. The molecular structures of these cyclization products are unequivocally elucidated by X-ray crystallographic analyses. The latter reaction represents, to our knowledge, the first direct trapping of 5-substituted cyclopenta-1,3-diene derivative having a simple three-carbon
tether by DielsâAlder cyclization prior to isomerization by 1,5-hydrogen migration.
对从相应的4-(戊-4-烯基)环戊-2-烯酮乙烯基酯15原位生成的环戊-1,3-二烯衍生物16的分子内Diels–Alder反应进行了研究。4-[(E)-5-(乙氧基碳酰基)戊-4-烯基]环戊-2-烯酮乙烯基酯15a和4-(5,5-二氰基戊-4-烯基)环戊-2-烯酮乙烯基酯15b是通过5个步骤从环戊-2-烯酮乙烯基酯5制备而成。在120°C加热时,化合物15a通过4-取代的环戊-1,3-二烯-2-基烯醇醚17a发生分子内Diels–Alder反应,生成三环[5.2.1.0]脱萘-8-酮衍生物19a作为唯一产物。另一方面,15b在相同温度下通过5-取代的环戊-1,3-二烯-2-基烯醇醚16b发生分子内Diels–Alder反应,生成三环[4.4.0.0]脱萘-10-酮衍生物18b作为主要产物。这些环化产物的分子结构通过X射线晶体学分析得到了明确阐明。在我们看来,后者反应代表了首次通过Diels–Alder环化直接捕捉5-取代的环戊-1,3-二烯衍生物,同时具有简单的三碳连接,在通过1,5-氢迁移异构化之前。