‘Five at One Stroke’: Proline and Small Peptides in the Stereoselectivede novo Synthesis and Enantiotopic Functionalization of Carbohydrates

Organocatalysis applied to carbohydrates: from roots to current developments

Organocatalysis emerged in the last decade as a powerful tool for the synthesis of complex molecules. In the field of carbohydrates, it found widespread use in the synthesis of rare and non-natural carbohydrate derivatives. Additionally, it has also found important application in the stereoselective functionalization of the anomeric carbon in glycosylation reactions. These efforts culminated in the development of different types of catalysts operating through distinct activation modes that allow the selective synthesis of α- or β-glycosides even on daunting substrates. All these advances starting from its first examples in carbohydrate synthesis to the current developments in glycosylation reactions are reviewed.

Caged Proline in Photoinitiated Organocatalysis

Organocatalysis is an emerging field, in which small metal-free organic structures catalyze a diversity of reactions with a remarkable stereoselectivity. The ability to selectively switch on such pathways upon demand has proven to be a valuable tool in biological systems. Light as a trigger provides the ultimate spatial and temporal control of activation. However, there have been limited examples of phototriggered catalytic systems. Herein, we describe the synthesis and application of a caged proline system that can initiate organocatalysis upon irradiation. The caged proline was generated using the highly efficient 4-carboxy-5,7-dinitroindolinyl (CDNI) photocleavable protecting group in a four-step synthesis. Advantages of this system include water solubility, biocompatibility, high quantum yield for catalyst release, and responsiveness to two-photon excitation. We showed the light-triggered catalysis of a crossed aldol reaction, a Mannich reaction, and a self-aldol condensation reaction. We also demonstrated light-initiated catalysis, leading to the formation of a biocide in situ, which resulted in the growth inhibition of E. coli, with as little as 3 min of irradiation. This technique can be broadly applied to other systems, by which the formation of active forms of drugs can be catalytically assembled remotely via two-photon irradiation.

Analysis of the intermediary metabolism of a reductive chemoautotroph

All extant life forms depend, directly or indirectly, on the autotrophic fixation of the dominant elements of the biosphere: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. We have earlier presented the canonical network of reactions that constitute the anabolism of a reductive chemoautotroph. Separating this network into subgraphs reveals several empirical generalizations: (1) acetate (acetyl-CoA), pyruvate, phosphoenol pyruvate, oxaloacetate, and 2-oxoglutarate serve as universal starting points for all pathways leading to the universal building blocks-20 amino acids and 4 ribonucleotide triphosphates; (2) all pathways are anabolic; (3) all reactions operate by complete utilization of outputs with no molecules left behind as waste, ensuring conservation of information; (4) the core metabolome of 120 compounds is acidic, consisting of compounds containing phosphoric or carboxylic acid or both; and (5) the core network is both brittle-vulnerable to a single break-and robust-having persisted for 4 billion years. Preliminary analysis of the chemical reactions and resultant structures reveals (a) a sparseness among possible molecular structures; (b) subdomains in the network; and (c) restriction of anabolism to a small set of rudimentary organic reactions with limited diversity in chemical mechanisms. These generalizations have implications for biogenesis and trophic ecology.

5-(Pyrrolidin-2-yl)-1H-tetrazole and 5-[(Pyrrolidin-2-yl)methyl]-1H-tetrazole: Proline Surrogates with Increased Potential in Asymmetric Catalysis

Catalytic enantioselective methodology has dramatically been enriched by the re-discovery of the simple amino acid proline as a chiral catalyst in the year 2000. Although no catalyst offers such a simple and broad access to quite complex reaction products, as does proline, its synthetic potential is not unrestricted, what is especially connected to its poor solubility in organic media. Exchange of the carboxylic moiety by a tetrazole unit leads to proline surrogates, that by far outperform proline with respect to yield, enantioselectivity, reaction time, substrate and solvent scope, catalyst loading, and stoichiometry of the compounds used in excess. These factors are discussed and critically compared with selected representative proline-catalyzed reactions.