Lewis Acid-Catalyzed Formal (4+2)-Cycloaddition between Cross-Conjugated Azatrienes and Styrylmalonates: The Way to Functionalized Quinolizidine Precursors

Quinolizidine and azaphenalene alkaloids are common in nature and exhibit a pharmaceutical activity, which stirs up increased interest in expanding the range of methods for the synthesis of the corresponding derivatives. In this work, we attempted to adapt our previously presented method for the synthesis of tetrahydropyridines to the preparation of potential precursors for these heterocycles as a separate development of a necessary intermediate stage. To this end, we studied the reactions of β-styrylmalonates with N-protected cross-conjugated azatrienes in the presence of Sn(OTf)₂. Moreover, the regioselectivity of the process involving unsymmetrically substituted azatrienes was estimated. The diene character of vinyltetrahydropyridines was studied in detail with the participation of PTAD. Finally, for the Ts-protected highly functionalized vinyltetrahydropyridines synthesized, a detosylation method to give new desired azadiene structures as precursors of the quinolizidine core was suggested.

From Glycals to Nitrogen Heterocycles and Carbocycles via "Cleavage-Intramolecular Recombination Strategy"

Glycals (carbohydrate enol-ethers) have enjoyed profound applications in organic synthesis for more than a century. They not only serve as versatile glycosyl donors or as substrates for Ferrier rearrangement, but also find extensive synthetic applications especially as a "chiral pool" for accomplishing the synthesis of a variety of natural and biologically important compounds. As cyclic enol ethers, they demonstrate high reactivity and are among the most and variously transformable monosaccharide derivatives. The uniqueness of the reactivity of glycals is that they can be synthetically tuned to get a library of derivatives through stereo- and regioselective introduction of a variety of functional groups at C1, C2, C3 as well as C4 carbons of the sugar. We have developed a practical approach for stereoselective mono- and diamination of glycals and over the years utilized these scaffolds for the synthesis of a variety of biologically important nitrogen heterocycles and carbocycles through a "Diversity Oriented Approach". Our synthetic strategy in this direction mainly relied on the cleavage of ring O-C bond of the sugar followed by an "intramolecular recombination" reaction. Utilizing this strategy, we have accomplished the synthesis of several biologically important natural products, their analogues and related unnatural derivatives. Examples of such compounds reported from our group include polyhydroxypyrrolidines, DMDP, anisomycin, steviamine, pochonicine, conduramines, bulgecinine, aminocyclitols, azepanes, 4-hydroxy-D-proline, azanucleosides and their analogues. A personal account highlighting these syntheses is presented here.

Conversion of glycals into vicinal-1,2-diazides and 1,2-(or 2,1)-azidoacetates using hypervalent iodine reagents and Me3SiN3. Application in the synthesis of N-glycopeptides, pseudo-trisaccharides and an iminosugar

Glycals react with PIFA (or PIDA)–TMSN₃in presence of TMSOTf to form sugar derived 1,2-diazides and vicinal azidoacetates. Synthesis of 2-azido-N-glycopeptides, pseudotrisaccharides, and a piperidine triol derivative is reported.

Convenient stereocontrolled amidoglycosylation of alcohols with acetylated glycals and trichloroethoxysulfonamide

A regio- and stereo-controlled, rhodium(II)-catalyzed amidoglycosylation of alcohols has been developed using O-acetylated glycals, trichloroethoxysulfonamide, and iodosobenzene. This one-pot amidoglycosylation was applied to a variety of primary and secondary alcohols to afford the β-O-glycosides with acceptable yields up to 84%. The reaction would proceed via stereoselective intermolecular aziridination of the glycal from the α-face followed by S_N2 reaction with alcohol at C-1 from the β-face to give 1,2:2,3-di-trans-substituted isomer only.

A Metal-Free Protocol for Aminofunctionalization of Olefins Using TsNBr2

N,N-Dibromo-p-toluene sulfonamide (TsNBr2) has been found to be an effective reagent for various aminofunctionalization reactions. This reagent behaves both as an electrophilic bromine source as well as amine to react with olefins under different conditions to yield aminoether, imidazoline, diamine and amino bromine. The reaction proceeds rapidly under mild conditions with high regioselectivity. Olefins react with TsNBr2 in moist THF to form δ-amino ether at room temperature. Treatment of TsNBr2 with olefin in MeCN at room temperature produced imidazoline in high yield. Further modification of the reaction condition resulted in the development of a one-step procedure for the synthesis of N-acetyl,N'-tosyl diamine derivatives directly from olefin. When the olefin was treated with 2.4 mol equiv of TsNBr2 in the presence of K2CO3, N,N'-ditosyl diamine derivative was obtained in moderate yield. Instantaneous formation of aminobromine was observed when an olefin was treated with the reagent in dry CH2Cl2 at room temperature.

Amino-functionalized iminocyclitols: synthetic glycomimetics of medicinal interest

A review on the syntheses and biological activities of unnatural glycomimetics highlighting the effect of replacement of hydroxyl groups of natural iminosugars by amino functionalities is presented.

Versatile on-resin synthesis of high mannose glycosylated asparagine with functional handles

Here we present a synthetic route for solid phase synthesis of N-linked glycoconjugates containing high mannose oligosaccharides which allows the incorporation of useful functional handles on the N-terminus of asparagine. In this strategy, the C-terminus of an Fmoc protected aspartic acid residue is first attached to a solid phase support. The side chain of aspartic acid is protected by a 2-phenylisopropyl protecting group, which allows selective deprotection for the introduction of glycosylation. By using a convergent on-resin glycosylamine coupling strategy, an N-glycosidic linkage is successfully formed on the free side chain of the resin bound aspartic acid with a large high mannose oligosaccharide, Man8GlcNAc2, to yield N-linked high mannose glycosylated asparagine. The use of on-resin glycosylamine coupling provides excellent glycosylation yield, can be applied to couple other types of oligosaccharides, and also makes it possible to recover excess oligosaccharides conveniently after the on-resin coupling reaction. Useful functional handles including an alkene (p-vinylbenzoic acid), an alkyne (4-pentynoic acid), biotin, and 5-carboxyfluorescein are then conjugated onto the N-terminal amine of asparagine on-resin after the removal of the Fmoc protecting group. In this way, useful functional handles are introduced onto the glycosylated asparagine while maintaining the structural integrity of the reducing end of the oligosaccharide. The asparagine side chain also serves as a linker between the glycan and the functional group and preserves the native presentation of N-linked glycan which may aid in biochemical and structural studies. As an example of a biochemical study using functionalized high mannose glycosylated asparagine, a fluorescence polarization assay has been utilized to study the binding of the lectin Concanavalin A (ConA) using 5-carboxyfluorescein labeled high mannose glycosylated asparagine.

An efficient and cost-effective preparation of di-O-acetyl-D-rhamnal

We have developed a synthetic route to the frequently utilized deoxysugar building block di-O-acetyl-D-rhamnal originating from the inexpensive starting material methyl α-D-glucopyranoside. Our approach proceeds in five steps with minimal column chromatography purification needed to afford the title compound in good overall yield. 2009 Elsevier Ltd. All rights reserved.

Methods for direct alkene diamination, new & old

The 1,2-diamine moiety is a ubiquitous structural motif present in a wealth of natural products, including non-proteinogenic amino acids and numerous alkaloids, as well as in pharmaceutical agents, chiral ligands and organic reagents. The biological activity associated with many of these systems and their chemical utility in general has ensured that the development of methods for their preparation is of critical importance. While a wide range of strategies for the preparation of 1,2-diamines have been established, the diamination of alkenes offers a particularly direct and efficient means of accessing these systems. The purpose of this review is to provide an overview of all methods of direct alkene diamination, metal-mediated or otherwise.