Passerini and Ugi reactions of benzyl and acetyl protected isocyanoglucoses

Isonitriles: Versatile Handles for the Bioorthogonal Functionalization of Proteins

The property of the isonitrile group to enable the simultaneous α-addition of a strong electrophile and a nucleophile has always attracted the attention of organic chemists. Its versatility is augmented when recognizing that its high structural compactness, the inertia to most of the naturally occurring functional groups, and relatively prolonged physiological and metabolical stability, convert it into the smallest bioorthogonal group. The discovery and optimization of the isonitrile-tetrazine [4+1] cycloaddition as an alternative tool for the development of ligation and decaging strategies and the recently reported reaction of isonitriles with chlorooximes bring new opportunities for the utilization of this functional group in biological systems. Although several approaches have been reported for the synthesis of isonitrile-modified carbohydrates and polysaccharides, its incorporation in proteins has been barely explored. Besides compiling the reported methods for the assembly of isonitrile-modified proteins, this Mini-Review aims at calling attention to the real potential of this modification for protein ligation, decaging, immobilization, imaging, and many other applications at a low structural and functional cost.

Structure and Reactivity of Glycosyl Isocyanides

3D structural information was obtained from mono-, di- and trisaccharide formamide and isocyanide derivatives by analysis of their X-ray crystal structure and NMR spectroscopy. The isocyanide anomeric effect was observed. Data mining of the Cambridge Structural Database (CSD) was performed and statistically confirmed our findings. Application of the glycoside isocyanides in the synthesis of novel glycoconjugates as drug-like scaffolds by MCR chemistry underscores the usefulness of the novel building blocks.

CeCl₃.7H2O/NaI-promoted direct synthesis of 1,3-benzoxazine-2-thione n-nucleosides under microwave irradiation

A microwave-assisted efficient and direct synthetic route to 1,3-benzoxazine-2-thione N-nucleos ides via CeCl₃.7H2O/NaI catalyzed cycloisomerization of salicylaldehyde 4-(β-D-ribofuranosyl) thiosemicarbazones followed by reductive dehydrazination is reported.

Carbohydrate-steroid conjugation by Ugi reaction: one-pot synthesis of triple sugar/pseudo-peptide/spirostane hybrids

The one-pot synthesis of novel molecular chimeras incorporating sugar, pseudo-peptide, and steroidal moieties is described. For this, a new carbohydrate-steroid conjugation approach based on the Ugi four-component reaction was implemented for the ligation of glucose and chacotriose to spirostanic steroids. The approach proved wide substrate scope, as both mono and oligosaccharides functionalized with amino, carboxy, and isocyano groups were conjugated to steroidal substrates in an efficient, multicomponent manner. Two alternative strategies based on the hydrazoic acid variant of the Ugi reaction were employed for the synthesis of tetrazole-based chacotriose-diosgenin conjugates resembling naturally occurring spirostan saponins. This is the first time that triple sugar/pseudo-peptide/steroid hybrids are produced, thus opening up an avenue of opportunities for applications in drug discovery and biological chemistry.

Diastereoselective synthesis of fused [1,3]thiazolo[1,3]oxazins and [1,3]oxazino[2,3-b][1,3]benzothiazoles

An efficient diastereoselective synthesis of 7-ethyl 5,6-dialkyl 7H-[1,3]thiazolo[2,3-b][1,3]oxazin-5,6, 7-tricarboxylates and 2-ethyl 3,4-dialkyl 2H-[1,3]oxazino [2,3-b][1,3]benzothiazole-2,3,4-tricarboxylates via reaction of thiazole and benzothiazole with dialkyl acetylenedicarboxylates in the presence of ethyl pyruvate is described.

Mineral supported facile synthesis of novel 4-hydroxybenzoxazin-2-thione N-nucleosides

One-pot montmorillonite K-10 clay-supported reactions of substituted/unsubstituted salicylaldehyde and ribosyl/deoxyribosyl thioureas expeditiously yielded novel N-nucleosides, 4-hydroxy-3,4-dihydro-3-(beta-D-ribofuranosyl or beta-D-2'-deoxyribofuranosyl)-2''-benz[e]-1,3-oxazin-2-thione via cycloisomerization of aldehyde intermediate under solvent-free microwave irradiation conditions.

The use of the Ugi four-component condensation

This protocol describes a procedure for the Ugi four-component condensation. It describes the general mechanism as well as the effects of the nature of the components on the Ugi reaction. It also describes the effects of the reaction conditions on the reaction, along with special procedures and workup. The experimental procedure is exemplified by a description of the preparation of N-cyclohexyl 2-[N-(2-chloroacetyl)-N-(4-chlorobenzyl)]amino-2-(4-chlorophenyl)acetamide, a typical Ugi product, that is subsequently used for the synthesis of a 2,5-diketopiperazine, an example of an important type of pharmaceutical compound. The experimental procedure is then extended to the synthesis of a 1,5-disubstituted tetrazole via Ugi four-component condensation. The protocol describes the preparation and characterization of the new 1-cyclohexyl-5-(1-phenylamino-2-methyl)propyltetrazole. The total time for the synthesis and isolation of the two example reactions in parallel is 3 d.

Novel synthesis of alpha-PNA monomers by U-4CR

A novel synthesis of alpha-PNA monomers was carried out by U-4CR, followed by photochemical cleavage of the 2-nitrobenzyl group and selective hydrolysis in the presence of 10% HCl in THF. Three of four functional components in the U-4CR were specially protected: cyclohexenyl isocyanide, Boc for protecting the amino group of glycine, and 2-nitrobenzyl group as a photocage (photoremovable protecting group) for ammonia. The amino group of aldehyde-containing adenine is too weak to interfere with the U-4CR, so that it is not necessary to be protected.

Asymmetric multicomponent reactions (AMCRs): the new frontier

Asymmetric multicomponent reactions involve the preparation of chiral compounds by the reaction of three or more reagents added simultaneously. This kind of addition and reaction has some advantages over classic divergent reaction strategies, such as lower costs, time, and energy, as well as environmentally friendlier aspects. All these advantages, together with the high level of stereoselectivity attained in some of these reactions, will force chemists in industry as in academia to adopt this new strategy of synthesis, or at least to consider it as a viable option. The positive aspects as well as the drawbacks of this strategy are discussed in this Review.

Stereospecific synthesis of the alpha- and beta-D-glucopyranosyl ureas

A new, one-pot, two-stage procedure for the preparation of the alpha- and beta-D-glucopyranosyl ureas has been developed. Oxidation of glucopyranosyl isocyanides provides glucopyranosyl isocyanates, which can be trapped in situ with amines to afford good yields of glucopyranosyl ureas. Application of this method establishes the successful synthesis of the hitherto unknown N,N'-di-alpha,alpha- and alpha,beta-D-glucopyranosyl ureas.

Multicomponent Reactions with Isocyanides

Multicomponent reactions (MCRs) are fundamentally different from two-component reactions in several aspects. Among the MCRs, those with isocyanides have developed into popular organic-chemical reactions in the pharmaceutical industry for the preparation of compound libraries of low-molecular druglike compounds. With a small set of starting materials, very large libraries can be built up within a short time, which can then be used for research on medicinal substances. Due to the intensive research of the last few years, many new backbone types have become accessible. MCRs are also increasingly being employed in the total synthesis of natural products. MCRs and especially MCRs with isocyanides offer many opportunities to attain new reactions and basic structures. However, this requires that the chemist learns the "language" of MCRs, something that this review wishes to stimulate.

New synthetic technology for efficient construction of alpha-hydroxy-beta-amino amides via the Passerini reaction

[reaction: see text] The Passerini reaction of N-protected amino aldehydes, isonitriles, and TFA using pyridine-type bases proceeds under mild conditions and directly affords alpha-hydroxy-beta-amino amide derivatives in moderate to high yields. These adducts are readily hydrolyzed to alpha-hydroxy-beta-amino carboxylic acids. Application of these key intermediates to concise syntheses of P(1)-alpha-ketoamide protease inhibitors is illustrated.

The discovery of new isocyanide-based multi-component reactions

Multi-component reactions are finding increasing use in the discovery process of new drugs and agrochemicals. Some years ago they were considered as highly exotic types of organic reactions. Recently, many groups have realized that the field of multi-component reactions is full of new opportunities. These comprise fast and efficient production of small-molecular-weight compound libraries, highly atom-economic chemistry, very large chemical libraries, improvements in total synthesis, and applications in bioconjugate chemistry, as well as in related fields (e.g. chemical biology).