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

Over the past decade, the most significant, conceptual advances in the field of fluorination were enabled most prominently by organo- and transition-metal catalysis. The most challenging transformation remains the formation of the parent C-F bond, primarily as a consequence of the high hydration energy of fluoride, strong metal-fluorine bonds, and highly polarized bonds to fluorine. Most fluorination reactions still lack generality, predictability, and cost-efficiency. Despite all current limitations, modern fluorination methods have made fluorinated molecules more readily available than ever before and have begun to have an impact on research areas that do not require large amounts of material, such as drug discovery and positron emission tomography. This Review gives a brief summary of conventional fluorination reactions, including those reactions that introduce fluorinated functional groups, and focuses on modern developments in the field.

/pdf/introduction-of-fluorine-and-fluorine-containing-functional-1i8kwiad20.pdf

Introduction of Fluorine and Fluorine-Containing Functional Groups

The design and implementation of cascade reactions is a challenging facet of organic chemistry, yet one that can impart striking novelty, elegance, and efficiency to synthetic strategies. The application of cascade reactions to natural products synthesis represents a particularly demanding task, but the results can be both stunning and instructive. This Review highlights selected examples of cascade reactions in total synthesis, with particular emphasis on recent applications therein. The examples discussed herein illustrate the power of these processes in the construction of complex molecules and underscore their future potential in chemical synthesis.

Cascade reactions in total synthesis.

1,2-Amino Alcohols and Their Heterocyclic Derivatives as Chiral Auxiliaries in Asymmetric Synthesis

Introduction Cationic domino reactions Anionic domino reactions Radical domino reactions Pericyclic domino reactions Photochemically induced domino processes Transition metal catalysis Domino reactions initiated by oxidation or reduction Enzymes in domino reactions Multicomponent reactions Special techniques in domino reactions

Domino Reactions in Organic Synthesis

[2,3]-Wittig sigmatropic rearrangements in organic synthesis

Asymmetric Catalytic Alkylation of Aldehydes with Diethylzinc Using a Chiral Binaphthol-Titanium Complex

The in situ reaction of dibromodifluoromethane, triphenylphosphine, and aldehydes in the presence of zinc dust affords good yields of gem-difiuoroolefins. Reduction of the difluoroolefins gives selectively the corresponding monofluoro-olefins in excellent yields. The scope and limitation of these procedures are described and compared with those of previous methods.

/pdf/convenient-procedures-for-conversion-of-carbonyl-compounds-3hys9zd5ow.pdf

Takeshi Nakai

Papers

[2,3]-Wittig sigmatropic rearrangements in organic synthesis

Asymmetric Catalytic Alkylation of Aldehydes with Diethylzinc Using a Chiral Binaphthol-Titanium Complex

CONVENIENT PROCEDURES FOR CONVERSION OF CARBONYL COMPOUNDS TO gem-DIFLUOROOLEFINS AND THEIR SELECTIVE REDUCTIONS TO MONOFLUOROOLEFINS

Asymmetric Catalytic Cyanosilylation of Aldehydes Using a Chiral Binaphthol-Titanium Complex

Perfluoro-enolate chemistry: facile generation and unique reactivities of metal F-1-propen-2-olates