Miranda Pedro O, Ramírez Miguel A, Martín Víctor S, Padrón Juan I
Instituto Universitario de Bio-Orgánica Antonio González, Universidad de La Laguna, C/Astrofísico Francisco Sánchez 2, 38206 La Laguna, Tenerife, Spain.
Chemistry. 2008;14(20):6260-8. doi: 10.1002/chem.200800281.
The relative stability of the intermediates involved in the alkyne Prins cyclization and the competitive 2-oxonia-[3,3]-sigmatropic rearrangement was studied. This rearrangement was shown to occur slowly under typical alkyne Prins cyclization conditions when the allenyl oxocarbenium ion that results from the rearrangement is similar to or higher in energy than the starting alkynyl oxocarbenium ion. The formal 2-oxonia-[3,3]-sigmatropic rearrangement may be disfavored by destabilizing the resultant allenyl oxocarbenium ion or by stabilizing an intermediate dihydropyranyl cation. The trimethylsilyl group as a substituent at the alkyne and electron-withdrawing groups (CH2Cl and CH2CN) located at the alpha-position to the carbinol center are shown to be effective. DFT calculations suggest that these substituents stabilize the dihydropyranyl cations, thus leading to a more uniform reorganization of the electronic density in the ring, and do not have a direct effect on the formally positively charged carbon atom.
研究了炔烃普林斯环化反应中涉及的中间体以及竞争性的2-氧杂-[3,3]-σ迁移重排的相对稳定性。当重排产生的烯丙基氧鎓离子的能量与起始炔基氧鎓离子相似或更高时，这种重排在典型的炔烃普林斯环化条件下进行得很慢。形式上的2-氧杂-[3,3]-σ迁移重排可能因使生成的烯丙基氧鎓离子不稳定或使中间体二氢吡喃基阳离子稳定而受到不利影响。结果表明，作为炔烃取代基的三甲基硅基和位于甲醇中心α位的吸电子基团（CH2Cl和CH2CN）是有效的。密度泛函理论计算表明，这些取代基使二氢吡喃基阳离子稳定，从而导致环中电子密度更均匀地重新分布，并且对形式上带正电荷的碳原子没有直接影响。
