New cyclization reactions in organic syntheses

Abstract

Recent development in the transition metal-catalyzed cyclization reactions for organic syntheses in the author's laboratories is summarized, which includes (i) novel silylcarbocyclizations (SiCaCs) and carbonylative carbotricyclizations, (ii) intramolecular silylformylations and desymmerization of siloxydiynes by sequential double silylformylation, (iii) efficient total synthesis of (+)-prosopinine, (iv) enantioselective desymmetrization of aminodienes, and (iv) new and efficient routes to 1-azabicyclo[x.y.0]alkane amino acids. All these processes are catalyzed by Rh or Rh-Co complexes, and useful for rapid and efficient construction of a variety of heterocyclic and carbocyclic compounds. Mechanisms of these new carbocyclization and cyclohydrocarbonylation reactions are also discussed.It has been the central focus in modern organic synthesis to develop highly efficient catalytic processes for the syntheses of natural and unnatural compounds of medicinal interest or intermediates useful for functional materials. One of the most attractive approaches to such aims is to apply transition metal-catalyzed cyclization reactions for the transformations of simple starting materials into monocyclic, bicyclic, and polycyclic scaffolds that can be further elaborated into specific targets.This account describes new and useful cyclization processes based on silylcarbocyclization (SiCaC), silylformylation, and cyclohydrocarbonylation reactions, which have recently been developed in these laboratories and will find many applications in organic syntheses. SILYLCARBOCYCLIZATIONS (SiCaCs)The first silylcarbocyclization was discovered serendipitously during our detailed product analysis of the silylformylation of 1-hexyne catalyzed by Rh and Rh-Co carbonyl clusters, which gave dibutylcyclopentenone by incorporating two molecules of 1-hexyne, one molecule of hydrosilane, and one molecule of CO. Following up this discovery, we have investigated the intramolecular version of this reaction using a variety of 1,6-enynes, which led to the discovery of novel silylcarbocyclization (SiCaC) reaction (eq. 1) [1][2][3]. The SiCaC reaction of 1,6-enynes is catalyzed by Rh 4 (CO) 12 and Rh 2 Co 2 (CO) 12 , *Lecture presented at the

From Alkynes to Heterocycles through Metal-Promoted Silylformylation and Silylcarbocyclization Reactions

“…Several synthetic applications of silylcarbocyclization reactions have been previously treated as part of more general reviews, focused on the transition metal-promoted cyclizations [68][69][70] or on the chemistry of hydrosilanes with alkynes [27]; however, a complete and up-to-date overview of SiCAC protocols for the preparation of heterocycles is still missing. Therefore, in the second part of the present review we shall try to provide an exhaustive and critical account of this literature, giving special emphasis on the adopted catalytic systems.…”

“…studies performed in a following paper better clarified the origin of this cyclic product: a metal-promoted silylcarbonylation of 1-hexyne gave the β-silylacryloyl-metal intermediate (I), which in turn then provided a carbometalation on a second 1-hexyne molecule to generate (II); after the following carbocyclization and β-hydride elimination steps, a highly regioselective reduction of intermediate (IV) took place at the less sterically hindered double bond; finally, a hydrogen-metal exchange between species (V) and a further triethylsilane molecule gave the final cyclopentenone product (Scheme 19) [26]. Several synthetic applications of silylcarbocyclization reactions have been previously treated as part of more general reviews, focused on the transition metal-promoted cyclizations [68][69][70] or on the chemistry of hydrosilanes with alkynes [27]; however, a complete and up-to-date overview of SiCAC protocols for the preparation of heterocycles is still missing. Therefore, in the second part of the present review we shall try to provide an exhaustive and critical account of this literature, giving special emphasis on the adopted catalytic systems.…”