/pdf/self-assembled-monolayers-of-thiolates-on-metals-as-a-form-1neb0hj3k4.pdf

Self-assembled monolayers of thiolates on metals as a form of nanotechnology.

About Supramolecular Assemblies of π-Conjugated Systems

Asymmetric enamine catalysis.

Most enzymatic transformations have a synthetic counterpart. Often though, the mechanisms by which natural and synthetic catalysts operate differ markedly. The catalytic asymmetric aldol reaction as a fundamental C-C bond forming reaction in chemistry and biology is an interesting case in this respect. Chemically, this reaction is dominated by approaches that utilize preformed enolate equivalents in combination with a chiral catalyst.1 Typically, a metal is involved in the reaction mechanism.1d Most enzymes, however, use a fundamentally different strategy and catalyze the direct aldolization of two unmodified carbonyl compounds. Class I aldolases utilize an enamine based mechanism,2 while Class II aldolases mediate this process by using a zinc cofactor.3 The development of aldolase antibodies that use an enamine mechanism and accept hydrophobic organic substrates has demonstrated the potential inherent in amine-catalyzed asymmetric aldol reactions.4 Recently, the first small-molecule asymmetric class II aldolase mimics have been described in the form of zinc, lanthanum, and barium complexes.5,6 However, amine-based asymmetric class I aldolase mimics have not been described in the literature.7 Here we report our finding that the amino acid proline is an effective asymmetric catalyst for the direct aldol reaction between unmodified acetone and a variety of aldehydes. Recently we developed broad scope aldolase antibodies that show very high enantioselectivities, have enzymatic rate accelerations, and use the enamine mechanism of class I aldolases.4 During the course of these studies, we found that one of our aldolase catalytic antibodies (Aldolase Antibody 38C2, Aldrich) is an efficient catalyst for enantiogroup-differentiating aldol cyclodehydrations of 2,6-heptanediones to give cyclohexenones, including the Wieland-Miescher ketone.8,9 These intramolecular reactions are also catalyzed by proline (Hajos-Eder-Sauer-Wiechert reaction)10 and it has been postulated that they proceed via an enamine mechanism.11 However, the proline-catalyzed direct intermolecular asymmetric aldol reaction has not been described. Further, there are no asymmetric small-molecule aldol catalysts that use an enamine mechanism.7 Based on our own results and Shibasaki’s work on lanthanum-based small-molecule aldol catalysts,4,6 we realized the great potential of catalysts for the direct asymmetric aldol reaction. We initially studied the reaction of acetone with 4-nitrobenzaldehyde. Reacting proline (30 mol %) in DMSO/acetone (4:1) with 4-nitrobenzaldehyde at room temperature for 4 h furnished aldol product (R)-1 in 68% yield and 76% ee (eq 1). This result

https://www.scripps.edu/barbas/pdf/List00JACS.pdf

Proline-Catalyzed Direct Asymmetric Aldol Reactions

In studying the evolution of organic chemistry and grasping its essence, one comes quickly to the conclusion that no other type of reaction plays as large a role in shaping this domain of science than carbon-carbon bond-forming reactions. The Grignard, Diels-Alder, and Wittig reactions are but three prominent examples of such processes, and are among those which have undeniably exercised decisive roles in the last century in the emergence of chemical synthesis as we know it today. In the last quarter of the 20th century, a new family of carbon-carbon bond-forming reactions based on transition-metal catalysts evolved as powerful tools in synthesis. Among them, the palladium-catalyzed cross-coupling reactions are the most prominent. In this Review, highlights of a number of selected syntheses are discussed. The examples chosen demonstrate the enormous power of these processes in the art of total synthesis and underscore their future potential in chemical synthesis.

Palladium-catalyzed cross-coupling reactions in total synthesis.

Synthesis and Biological Evaluation of a Fluorinated Analogue of the . beta.‐Adrenergic Blocking Agent Metoprolol.

This chapter describes recent advances in the development of chiral vanadium and niobium catalysts for enantioselective carbon–carbon bond-forming transformations. The catalyst prepared from a vanadium salt or a niobium salt with a chiral multidentate ligand is a powerful tool for the efficient synthesis of optically enriched compounds such as cyanohydrins, amino alcohols and 1,1′-bi-2-naphthols, which can be readily transformed to biologically active compounds and chiral ligands.

Chapter 9:Vanadium- and Niobium-catalysed Enantioselective Reactions

A New and Selective Metal-Catalyzed Baeyer-Villiger Oxidation Procedure.

Palladium and CO: Carbonylation of 1 with [Pd(tfa)2(±)-L1] (tfa = trifluoroacetate) affords the spirofuranone 2 with inversion of the stereochemistry at C17 in 96 % yield. C17-epi-1 also gave the same product 2 with retention of the stereochemistry at C17. Labelling studies show that (13)CO was incorporated into the C5' position of the furanone ring. The first asymmetric version of this new reaction was achieved.

Carbonylation of Propargyl Carbamates with Palladium(II) Bisoxazoline Catalysts: Efficient Synthesis of 5-Methoxy-3(2H)-furanones.

Synthesis of two kinds of new bis(isoxazoline) ligand, achiral 2,2'-bis(isoxazolinyl)propane and (R)-2,2'-bis(isoxazolinyl)-1,1'-binaphthyl bearing an axial chirality is described. Both of the bis(isoxazoline) ligands accelerated the Pd(II)-catalyzed Wacker-type cyclization of alkenyl alcohol. The isoxazoline ligands were found to be essential to promote the cyclization.

Hiroaki Sasai

Papers

Synthesis and Biological Evaluation of a Fluorinated Analogue of the . beta.‐Adrenergic Blocking Agent Metoprolol.

Chapter 9:Vanadium- and Niobium-catalysed Enantioselective Reactions

A New and Selective Metal-Catalyzed Baeyer-Villiger Oxidation Procedure.

Carbonylation of Propargyl Carbamates with Palladium(II) Bisoxazoline Catalysts: Efficient Synthesis of 5-Methoxy-3(2H)-furanones.

Synthesis and Character of New Bis(isoxazoline) Ligands.