Department of Chemistry and Biochemistry, University of California, Los Angeles, 607 Charles E. Young Drive East, Los Angeles, CA 90095-1569, USA.
Chem Asian J. 2011 Aug 1;6(8):2101-6. doi: 10.1002/asia.201100190. Epub 2011 Jul 7.
From our investigations on phosphine-catalyzed [4+2] annulations between α-alkyl allenoates and activated olefins for the synthesis of cyclohexenes, we discovered a hexamethylphosphorous triamide (HMPT)-catalyzed [4+2] reaction between α-alkyl allenoates 1 and arylidene malonates or arylidene cyanoacetates 2 that provides highly functionalized cyclohexenes 3 and 4 in synthetically useful yields (30-89%), with moderate to exclusive regioselectivity, and reasonable diastereoselectivity. Interestingly, the [4+2] annulations between the α-alkyl allenoates 1 and the olefins 2 manifested a polarity inversion of the 1,4-dipole synthon 1, depending on the structure of the olefin, thus providing cyclohexenes 3 exclusively when using arylidene cyanoacetates. The polarity inversion of α-alkyl allenoates from a 1,4-dipole A to B under phosphine catalysis can be explained by an equilibrium between the phosphonium dienolate C and the phosphorous ylide D.
从我们对膦催化的α-烷基丙二烯酸酯与活化烯烃之间的[4+2]环加成反应的研究中，我们发现了一种六甲基磷三酰胺（HMPT）催化的α-烷基丙二烯酸酯 1 和亚芳基丙二酰基或亚芳基亚氰基乙酸酯 2 之间的[4+2]反应，该反应以中等至高的区域选择性和合理的非对映选择性提供了高度官能化的环己烯 3 和 4，产率为 30-89%。有趣的是，α-烷基丙二烯酸酯 1 和烯烃 2 之间的[4+2]环加成反应表现出 1,4-偶极子 1 的极性反转，这取决于烯烃的结构，因此当使用亚芳基亚氰基乙酸酯时，仅提供环己烯 3。在膦催化下，α-烷基丙二烯酸酯从 1,4-偶极子 A 到 B 的极性反转可以通过磷翁烯醇化物 C 和磷叶立德 D 之间的平衡来解释。
