Thermodynamic and Structural Effects of Macrocyclization as a Constraining Method in Protein-Ligand Interactions

The thermodynamic and structural effects of macrocyclization as a tactic for stabilizing the biologically-active conformation of Grb2 SH2 binding peptides were investigated using isothermal titration calorimetry and x-ray crystallography. 23-Membered macrocycles containing the sequence pYVN were slightly more potent than their linear controls; however, preorganization did not necessarily eventuate in a more favorable binding entropy. Structures of complexes of macrocycle 7 and its acyclic control 8 are similar except for differences in relative orientations of corresponding atoms in the linking moieties of 7 and 8. There are no differences in the number of direct or water-mediated protein-ligand contacts that might account for the less favorable binding enthalpy of 7; however, an intramolecular hydrogen bond between the pY and pY+3 residues in 8 that is absent in 7 may be a factor. These studies highlight the difficulties associated with correlating energetics and structure in protein-ligand interactions.

Total synthesis of isokidamycin

The synthesis of isokidamycin, which represents the first total synthesis of a bis-C-aryl glycoside natural product in the pluramycin family, has been completed. The synthesis features the use of a silicon tether as a disposable regiocontrol element in an intramolecular Diels-Alder reaction between a substituted naphthyne and a glycosyl furan and a subsequent O→C-glycoside rearrangement.

Binding of flexible and constrained ligands to the Grb2 SH2 domain: structural effects of ligand preorganization

Structures of the Grb2 SH2 domain complexed with a series of pseudopeptides containing flexible (benzyl succinate) and constrained (aryl cyclopropanedicarboxylate) replacements of the phosphotyrosine (pY) residue in tripeptides derived from Ac-pYXN-NH(2) (where X = V, I, E and Q) were elucidated by X-ray crystallography. Complexes of flexible/constrained pairs having the same pY + 1 amino acid were analyzed in order to ascertain what structural differences might be attributed to constraining the phosphotyrosine replacement. In this context, a given structural dissimilarity between complexes was considered to be significant if it was greater than the corresponding difference in complexes coexisting within the same asymmetric unit. The backbone atoms of the domain generally adopt a similar conformation and orientation relative to the ligands in the complexes of each flexible/constrained pair, although there are some significant differences in the relative orientations of several loop regions, most notably in the BC loop that forms part of the binding pocket for the phosphate group in the tyrosine replacements. These variations are greater in the set of complexes of constrained ligands than in the set of complexes of flexible ligands. The constrained ligands make more direct polar contacts to the domain than their flexible counterparts, whereas the more flexible ligand of each pair makes more single-water-mediated contacts to the domain; there was no correlation between the total number of protein-ligand contacts and whether the phosphotyrosine replacement of the ligand was preorganized. The observed differences in hydrophobic interactions between the complexes of each flexible/constrained ligand pair were generally similar to those observed upon comparing such contacts in coexisting complexes. The average adjusted B factors of the backbone atoms of the domain and loop regions are significantly greater in the complexes of constrained ligands than in the complexes of the corresponding flexible ligands, suggesting greater thermal motion in the crystalline state in the former complexes. There was no apparent correlation between variations in crystal packing and observed structural differences or similarities in the complexes of flexible and constrained ligands, but the possibility that crystal packing might result in structural variations cannot be rigorously excluded. Overall, it appears that there are more variations in the three-dimensional structure of the protein and the ligand in complexes of the constrained ligands than in those of their more flexible counterparts.

Enantioselective synthesis of (+)-isolysergol via ring-closing metathesis

The first enantioselective synthesis of (+)-isolysergol was completed in 12 steps from commercially available materials by a novel approach that features a late stage microwave-mediated, diastereomeric ring-closing metathesis catalyzed by a chiral molybdenum catalyst to simultaneously form the D ring and set the stereocenter at C(8).

Approaches to N-methylwelwitindolinone C isothiocyanate: facile synthesis of the tetracyclic core

The synthesis of a functionalized, tetracyclic core of N-methylwelwitindolinone C isothiocyanate is reported. The approach features a convergent coupling between an indole iminium ion and a highly functionalized vinylogous silyl ketene acetal followed by an intramolecular palladium-catalyzed cyclization that proceeds via an enolate arylation.

Synthesis of Diverse Heterocyclic Scaffolds via Tandem Additions to Imine Derivatives and Ring-Forming Reactions

A novel strategy has been developed for the efficient syntheses of diverse arrays of heterocyclic compounds. The key elements of the approach comprise a Mannich-type, multicomponent coupling reaction in which functionalized amines, aromatic aldehydes, acylating agents, and π- and organometallic nucleophiles are combined to generate intermediates that are then further transformed into diverse heterocyclic scaffolds via a variety of cyclization manifolds. Significantly, many of these scaffolds bear functionality that may be exploited by further manipulation to create diverse collections of compounds having substructures found in biologically active natural products and clinically useful drugs. The practical utility of this strategy was exemplified by its application to the first, and extraordinarily concise synthesis of the isopavine alkaloid roelactamine.

Synthesis of β-Heteroaryl Propionates via Trapping of Carbocations with π-Nucleophiles

A variety of heterocyclic alcohols and acetates were coupled with silyl ketene acetals and other π-nucleophiles in the presence of trimethylsilyl trifluoromethanesulfonate to provide an array of substituted β-heteroaryl propionates, including those with contiguous quaternary centers, as well as vinylogs thereof. This reaction also proceeds with high diastereoselectivity when the π-nucleophile bears a chiral auxiliary.

Total synthesis of cribrostatin 6

Fast and furious: Cribrostatin 6, an antimicrobial and antineoplastic agent, was the target of a total synthesis where the longest linear sequence was only four steps. The key step involves a tandem 4pi electrocyclic ring opening, radical cyclization, and homolytic aromatic substitution sequence to afford the tricyclic core of the natural product.

Iminium Ion Cascade Reactions: Stereoselective Synthesis of Quinolizidines and Indolizidines

A novel iminium ion cascade reaction has been developed that allows for the stereoselective synthesis of a variety of substituted aza-fused bicycles. The combination of amino allylsilanes and aldehydes (or ketones) was used to synthesize a number of quinolizidines and indolizidines in an one-pot reaction sequence. This technology has been used to effect the facile syntheses of several indolizidine and quinolizidine natural products including, (±)-epilupinine, (±)-tashiromine, and (-)-epimyrtine. Substrate scope has been examined varying the type of amino allylsilanes (primary, secondary and conjugated) and carbonyl compounds (aldehydes and ketones) to give a variety of fused ring structures. Varying the components chosen allows for the inclusion of synthetically useful functional groups at different positions on the core structure. The methodology has been used to construct the tricyclic core structures present in the cylindricine family and halichlorine.

Innate and adaptive immune responses in contact dermatitis: analogy with infections

Allergic contact dermatitis (ACD) is an inflammatory skin disease of great and steadily increasing importance as an occupational health problem. The disease is induced by chemicals and metal ions which penetrate the skin and form complexes with host proteins. This process is accompanied by a strong, allergen-induced inflammatory reaction and leads to the migration of allergen-carrying dendritic cells (DC) from the skin to regional lymph nodes, where they promote generation of allergen-specific T cells. The latter are the ultimate effector cells of the disease. Re-exposure to the causative agent leads to the recruitment of the T effector cells, which then elicit the typical skin inflammatory reaction at the site of contact. Although DC and effector T cells play a protagonistic role in the sensitization and elicitation phase of ACD, respectively, other cell types including keratinocytes, NK cells, mast cells and B cells contribute to the pathogenesis of the disease. In this review the authors summarize recent findings that identify stress responses and innate immune pathways triggered by contact allergens and review recent data regarding the adaptive T cell response. The new data were collected mainly from studies on contact hypersensitivity (CHS), the corresponding experimental mouse model of human ACD. The elucidation of the molecular events involved in contact allergen-induced innate responses will help to design new treatment strategies and may allow to develop predictive in vitro assays for the identification of contact allergens.

Applications of multicomponent reactions to the synthesis of diverse heterocyclic scaffolds

The sequencing of multicomponent reactions (MCRs) and subsequent cyclization reactions is a powerful stratagem for the rapid synthesis of diverse heterocyclic scaffolds. The optimal MCR is sufficiently flexible that it can be employed to generate adducts bearing a variety of functional groups that may then be selectively paired to enable different cyclization manifolds, thereby leading to a diverse collection of products. The growing interest in diversity-oriented synthesis has led to increased attention to this paradigm for library synthesis, which has inspired many advances in the design and implementation of MCRs for the construction of diverse heterocyclic scaffolds.

Recent Applications of Imines as Key Intermediates in the Synthesis of Alkaloids and Novel Nitrogen Heterocycles

One of the major challenges in contemporary synthetic organic chemistry is the design and development of new tactics and strategies and their application to concise and efficient syntheses of biologically active natural products. Strategies that utilize reactions that enable the rapid assembly of the skeletal framework of such targets are thus especially attractive. In this context, we have developed novel applications of imine chemistry in Mannich and related reactions, cascade processes, and multicomponent reactions to rapidly assemble structural subunits common to diverse families of alkaloids. The practical utility of these chemistries is evidenced by their use in the execution of facile total syntheses of (±)-epilupinine (1), (±)-tashiromine (2), (-)-epimyrtine (3), and (±)-roelactamine (4) as well as other nitrogen heterocycles of potential biological interest.

Carbonylative cross-coupling of ortho-disubstituted aryl iodides. convenient synthesis of sterically hindered aryl ketones

A mild and general protocol for the carbonylative cross-coupling of sterically hindered ortho-disubstituted aryl iodides is reported. Carbonylative Suzuki-Miyaura couplings of a variety of aryl boronic acids provide an array of substituted biaryl ketones in modest to excellent yield. A carbonylative Negishi coupling that utilizes alkynyl nucleophiles is also described.

The Pauson-Khand Reaction as a New Entry to the Synthesis of Bridged Bicyclic Heterocycles: Application to the Enantioselective Total Synthesis of (-)-Alstonerine

The first application of the Pauson-Khand reaction (PKR) to the synthesis of azabridged bicyclic structures is described. Compounds containing azabicyclo[3.3.1]nonane and azabicyclo[3.2.1]octane rings fused to cyclopentenones were efficiently constructed via the PKR of cis-2,6-disubstituted N-acyl piperidine enyne substrates, many of which can be readily prepared from 4-methoxypyridine in a few steps. Moreover, the PKR of cis-2,6-disubstituted piperazine enynes allowed the preparation of diazabicyclo[3.3.1]nonanes fused to cyclopentenones. This new strategy for the synthesis of azabridged bicyclic frameworks was exploited as a key step in a concise, enantioselective total synthesis of the macroline alklaoid (-)-alstonerine.

Applications of the Ugi reaction with ketones

A convenient synthesis of highly functionalized, α,α-disubstituted amino acid amide derivatives has been accomplished by using cyclic and acyclic ketones as the carbonyl inputs in the Ugi multicomponent reaction. An application of this extension of the Ugi reaction to the synthesis of α,α-divinyl amino acids that may be cyclized via ring-closing metathesis to provide highly substituted pyrrolidines is described.

C-Aryl glycosides via tandem intramolecular benzyne-furan cycloadditions. Total synthesis of vineomycinone B2 methyl ester

A triply convergent total synthesis of vineomycinone B2 methyl ester has been achieved by an approach with a longest linear sequence of 16 steps. The synthesis features the use of silicon tethers as disposable linkers to control the regiochemistry in two tandem Diels-Alder reactions of substituted benzynes and glycosyl furans to provide rapid access to the fully intact anthrarufin core of vineomycinone B2 methyl ester.