Stereoselectivity control by oxaspiro rings during Diels-Alder cycloadditions to cross-conjugated cyclohexadienones: the syn oxygen phenomenon.
Stereoselectivity control by oxaspiro rings during Diels-Alder cycloadditions to cross-conjugated cyclohexadienones: the syn oxygen phenomenon.
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DOI：
10.1021/ja047027t
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发表时间：
2004-12
影响因子：
15
通讯作者：
K. Ohkata;Y. Tamura;Brandon B Shetuni;Ryukichi Takagi;Wataru Miyanaga;S. Kojima;L. Paquette
中科院分区：
文献类型：
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作者：
K. Ohkata;Y. Tamura;Brandon B Shetuni;Ryukichi Takagi;Wataru Miyanaga;S. Kojima;L. Paquette
研究了螺环交叉共轭环己二酮与不同反应活性的双烯在Diels-Alder加成反应中的非对映界面选择性。该研究包括乙醚系列1a-c以及内酯/酮对2a/2b。在所有情况下，首选的[4+2]环加成途径包括从pi-表面syn到氧原子的成键。还考察了4-取代-4-甲基-2，5-环己二烯酮(单环体系)，发现它们在4位上优先从两个基团吸电子较多的面上成键。测定了1a和2a与环戊二烯的环加成反应的动力学参数。对于1a合成9a，溶剂的速率加速曲线为Cf(3)CH(2)OH>；&Gt；CH(3)CN；对于由2a合成21a，其速率加速曲线分别为Cf(3)CH(2)OH>；&Gt；CH(3)CN。这种极性分布对产物分布没有重大影响，这一现象也反映在4-取代-4-甲基-2，5-环己二烯酮在可比条件下的行为中。这些实验事实的理论评估是在HF/6-31G水平上进行的。根据Cieplak模型的定义，表面选择性可以通过双烯的HOMO和双烯亲分子的LUMO之间的二次相互作用以及由于更多的给电子键而新形成的键和4-反C-C sigma-轨道之间的有效超共轭来理解。
The diastereofacial selectivity operating in Diels-Alder additions involving spirocyclic cross-conjugated cyclohexadienones with dienes of varying reactivity has been investigated. The study has included the ether series 1a-c as well as the lactone/ketone pair 2a/2b. In all cases, the preferred [4+2] cycloaddition pathway consisted of bonding from that pi-surface syn to the oxygen atom. 4-Substituted-4-methyl-2,5-cyclohexadienones (monocyclic systems) were also examined and found to undergo bond formation preferentially from the face bearing the more electron-withdrawing of the two groups at the 4 position. Kinetic parameters were determined for the cycloaddition of 1a and 2a to cyclopentadiene. The rate acceleration profile of solvents was in the order CF(3)CH(2)OH >> CH(3)CN approximately CH(2)Cl(2) for the production of 9a from 1a and CF(3)CH(2)OH >> CH(2)Cl(2) > CH(3)CN for the production of 21a from 2a, respectively. This spread in polarity had no major impact on product distribution, a phenomenon also reflected in the behavior of 4-substituted-4-methyl-2,5-cyclohexadienones under comparable conditions. Theoretical assessment of these experimental facts was undertaken at the HF/6-31G level. The facial selectivity is understandable in terms of the secondary interaction between the HOMO of the diene and LUMO of the dienophile as well as the effective hyperconjugation between the newly forming bond and the 4-anti-C-C sigma-orbital due to the more electron-donating bond, as defined by the Cieplak model.