Giardina A, Mecozzi T, Petrini M
Dipartimento di Scienze Chimiche, Universita di Camerino, via S. Agostino, 1, I-62032 Camerino,Italy.
J Org Chem. 2000 Dec 1;65(24):8277-82. doi: 10.1021/jo001003x.
Optically active N-[1-(phenylsulfonyl)alkyl]imidazolidin-2-ones react at low temperature in the presence of tin tetrachloride to give acyclic N-acyliminium ions. These electrophilic substrates give addition products upon reaction with pi-nucleophiles. Allyltrimethylsilane affords the corresponding allylated products in good yields and high diastereoselectivity. The stereochemical outcome of this process can be rationalized by taking into account the preference of the intermediate N-acyliminium ion for an E configuration that favors the attack of the nucleophile from the si-si face. Disappointing results are obtained using silyl ketene acetals; conversely trimethylsilyl enol ether of acetophenone gives the corresponding adducts in high diastereoselectivity. The utilization of trimethylsilyl enol ether of 2-acetylfuran is particularly interesting since the corresponding adducts are obtained with good diastereoselectivity and the furan ring could be amenable of further synthetic transformations.
光学活性的N-[1-(苯基磺酰基)烷基]咪唑啉-2-酮在四氯化锡存在下于低温反应，生成开环的N-酰基亚胺离子。这些亲电底物与π-亲核试剂反应时生成加成产物。烯丙基三甲基硅烷能以良好的产率和高非对映选择性得到相应的烯丙基化产物。考虑到中间体N-酰基亚胺离子对有利于亲核试剂从si-si面进攻的E构型的偏好，该过程的立体化学结果可以得到合理的解释。使用甲硅烷基烯酮缩醛得到的结果令人失望；相反，苯乙酮的三甲基硅基烯醇醚以高非对映选择性得到相应的加合物。2-乙酰基呋喃的三甲基硅基烯醇醚的应用特别有趣，因为相应的加合物以良好的非对映选择性得到，并且呋喃环适合进一步的合成转化。
