Cycloaddition reactions are among the most important tools for the construction of cyclic compounds in organic synthesis, since these reactions are vital to access natural products and biologically active compounds. Organocatalysis plays an increasingly pivotal role in these reactions, often allowing several stereocenters to be selectively created and integrated in the target molecule. Among the large number of efficient types of organocatalysts available, the diarylprolinol silyl ethers have been established as one of the most frequently used in aminocatalysis allowing for novel activation modes and reaction concepts. In this review, we will focus on the different activation modes made available by the diarylprolinol silyl ether system with the aim of highlighting their applicability in asymmetric cycloadditions for the assembly of complex molecular architectures.

A Lewis acid-triggered zincation allows the regioselective metalation of various chromones and quinolones. In the absence of MgCl(2), a C(3) zincation is observed, whereas in the presence of MgCl(2) or a related Lewis acid, C(2) zincation occurs. Applications to a natural flavone, isoflavone, and quinolone are shown.

The full functionalization of the pyrazole ring was achieved by successive regioselective metalations using TMPMgCl x LiCl and TMP(2)Mg x 2 LiCl. Trapping with various electrophiles led to trisubstituted pyrazoles. An application to the synthesis of the acaricide Tebufenpyrad is reported.

A novel enantioselective organocatalytic strategy is presented for the synthesis of tetrahydrofurobenzofuran and methanobenzodioxepine natural product core structures. The strategy is based on a pair of divergent reaction pathways in which hydroxyarenes react with γ-keto-α,β-unsaturated aldehydes, catalyzed by a chiral secondary amine. One reaction pathway, which leads to chiral 5,5-fused acetals with two stereocenters-the tetrahydrofurobenzofuran scaffolds-proceeds in moderate yields and up to 96 % ee. The other reaction pathway provides 5,6-bridged methanobenzodioxepine scaffolds with three stereocenters in moderate to good yields and up to 95 % ee. The reaction is remarkable as it can proceed with catalyst loadings as low as 0.25 mol %, providing one of the highest known turnover numbers in iminium ion catalysis. Furthermore, the hemiacetal tetrahydrofurobenzofuran can undergo functionalizations including reduction, oxidation, and allylation. Finally, the effects involved in the substrate control for the divergent pathways, based on both experimental and computational studies, have been investigated. A model involving steric, electronic and stereoelectronic interactions is discussed to rationalize the observed selectivities.

The influence of the metal on the nucleophilic reactivities of indenyl metal compounds was quantitatively determined by kinetic investigations of their reactions with benzhydrylium ions (Ar2 CH(+) ) and structurally related quinone methides. With the correlation equation log k2 =sN (N+E), it can be derived that the ionic indenyl alkali compounds are 10(18) to 10(24) times more reactive (depending on the reference electrophile) than the corresponding indenyltrimethylsilane.