Scaffolding Catalysts: Highly Enantioselective Desymmetrization Reactions

Total Synthesis of (±)-Phyllantidine: Development and Mechanistic Evaluation of a Ring Expansion for Installation of Embedded Nitrogen-Oxygen Bonds

The development of a concise total synthesis of (±)-phyllantidine (1), a member of the securinega family of alkaloids containing an unusual oxazabicyclo[3.3.1]nonane core, is described. The synthesis employs a unique synthetic strategy featuring the ring expansion of a substituted cyclopentanone to a cyclic hydroxamic acid as a key step that allows facile installation of the embedded nitrogen-oxygen (N-O) bond. The optimization of this sequence to effect the desired regiochemical outcome and its mechanistic underpinnings were assessed both computationally and experimentally. This synthetic approach also features an early-stage diastereoselective aldol reaction to assemble the substituted cyclopentanone, a mild reduction of an amide intermediate without N-O bond cleavage, and the rapid assembly of the butenolide found in (1) via use of the Bestmann ylide.

Making the Silylation of Alcohols Chiral: Asymmetric Protection of Hydroxy Groups

The non-enzymatic kinetic resolution of racemic mixtures of alcohols by silylation had been unknown before the turn of the century. This stands in stark contrast to established acylation techniques. The same applies to the related desymmetrization of diols. This might come as a surprise, given the significance of silyl ethers as protecting groups in multistep synthesis of complex molecules. The situation changed after a seminal report by Ishikawa nearly twenty years ago. Since then, enantioselective silylation of alcohols has matured and grown into an independent research field with organocatalytic and transition-metal-catalyzed approaches providing powerful solutions. This Minireview summarizes these recent advances with particular emphasis on the stereoselective dehydrogenative coupling of alcohols and hydrosilanes.

Exploring Cooperative Effects in Oxidative NHC Catalysis: Regioselective Acylation of Carbohydrates

The utility of oxidative NHC catalysis for both the regioselective and chemoselective functionalization of carbohydrates is explored. Chiral NHCs allow for the highly regioselective oxidative esterification of various carbohydrates using aldehydes as acylation precursors. The transformation was also shown to be amenable to both cis/trans diol isomers, free amino groups, and selective for specific sugar epimers in competition experiments. Efficiency and regioselectivity of the acylation can be improved upon using two different NHC catalysts that act cooperatively. The potential of the method is documented by the regioselective acylation of an amino-linked neodisaccharide.

Copper-Catalyzed Intramolecular Desymmetric Aryl C-O Coupling for the Enantioselective Construction of Chiral Dihydrobenzofurans and Dihydrobenzopyrans

O-Heterocyclic structures such as 2,3-dihydrobenzofurans are key motifs in many natural compounds and pharmaceuticals. Enantioselective formation of chiral dihydrobenzofurans and analogues was achieved through a copper-catalyzed desymmetrization strategy with a chiral cyclic 1,2-diamine. A broad range of substrates are compatible with this Cu(I)-diamine catalytic system and afford the desired coupling products with chiral tertiary or quaternary carbon centers in high yields and good to excellent enantioselectivities under mild conditions.

New paradigms for the chiral synthesis of inositol phosphates

Paradigms found: Inositol phosphates are biomolecules found ubiquitously in eukaryotes, in which they play a number of vital biological roles. Their enantioselective synthesis has recently received a boost from two complementary phosphorylation methods that could change the way they are synthesised, and hopefully provide invaluable chemical biology tools to further our understanding of this large family.

Organocatalyzed enantioselective desymmetrization of diols in the preparation of chiral building blocks

Desymmetrization of diols is a powerful tool to the synthesis of chiral building blocks. Among the different approaches to perform discrimination between both enantiotopic hydroxyl groups, the organocatalytic approach has gained importance in the last years. A diverse range of organocatalysts has been used to efficiently promote this enantioselective transformation and this Minireview examines the different contributions in this field.