Stereocontrolled total syntheses of isodomoic acids G and H via a unified strategy

Copper catalyzed Shono-type oxidation of proline residues in peptide

Since the initial report in 1975, the Shono oxidation has become a powerful tool to functionalize the α position of amines, including proline derivatives, by electrochemical oxidation. However, the application of electrochemical Shono oxidations is restricted to the preparation of simple building blocks and homogeneous Shono-type oxidation of proline derivatives remains challenging. The late-stage functionalization at proline residues embedded within peptides is highly important as substitutions about the proline ring are known to affect biological and pharmacological activities. Here, we show that homogenous copper-catalyzed oxidation conditions complement the Shono oxidation and this general protocol can be applied to a series of formal C-C coupling reactions with a variety of nucleophiles using a one-pot procedure. This protocol shows good tolerance toward 19 proteinogenic amino acids and was used to functionalize several representative bioactive peptides, including captopril, enalapril, Smac, and endomorphin-2. Last, peptide cyclization can also be achieved by using an appropriately positioned side-chain hydroxyl moiety.

Total synthesis of (-)-domoic acid, a potent ionotropic glutamate receptor agonist and the key compound in oceanic harmful algal blooms

The stereo-controlled total synthesis of (-)-domoic acid is described. The critical construction of the C1'-C2' Z-configuration was accomplished by taking advantage of an unsaturated lactam structure. The side chain fragment was introduced in the final stages of synthesis through a modified Julia-Kocieński reaction, aiming for its efficient derivatization.

Preparation of domoic acid analogues using a bioconversion system, and their toxicity in mice

Domoic acid (1, DA), a member of the natural kainoid family, is a potent agonist of ionotropic glutamate receptors in the central nervous system. The chemical synthesis of DA and its derivatives requires considerable effort to establish a pyrrolidine ring containing three contiguous stereocenters. Recently, a biosynthetic cyclase for DA, DabC, was identified. This enzyme cyclizes the linear precursor of isodomoic acid A (IA) to IA, a bioactive DA analogue. In this study, we developed a bioconversion system to obtain DA analogues from linear substrates prepared by simple chemical synthesis using DabC expressed in Escherichia coli, in vivo. Three IA analogues with various substitutions at the C7'-geranyl terminus were prepared using this system: two minor natural analogues, 7'-methyl-IA (5) and 7'-hydroxy-IA (6), and one new unnatural analogue, 7'-amide-IA (7). In addition, the toxicity of these DA analogues in mice was examined by intracerebroventricular injection. Most of the mice injected with 5 (3 nmol) and 6 (3 nmol) did not show any adverse symptoms, whereas the mice injected with 7 (3 nmol) showed typical symptoms induced by DA (1, 0.7 nmol) and IA (2, 3 nmol). These results suggest that the 7'-carbonyl group in the side chain of IA (2) is crucial for its toxicity. The docking studies of DA, IA (2), 5, 6, and 7 to GluK1 supported these results.

Stereodivergent routes in organic synthesis: marine natural products, lactones, other natural products, heterocycles and unnatural compounds

Enantio- and diastereodivergent routes to marine-origin natural products with different sizes of cyclic ethers and lactones have been used in order to assign stereochemical features. Kainoid amino acids such as isodomoic acids have been synthesized using diastereodivergent routes. The bis(indole) alkaloid dragmacidin F has been prepared by enantiodivergent strategies as well as furanoterpenes and the tetracyclic agelastatin A. Natural products containing five-membered lactones like quercus lactones, muricatacins, goniofufuranones, methylenolactocins and frenolicin B have been synthesized using stereodivergent routes. Macrolides are very abundant lactones and have been mainly prepared from the corresponding seco-acids by lactonization, such as lasiodiplodin, zaeralanes, macrosphelides and haloprins, or by ring-closing metathesis, such as aspercyclides, microcarpalides, macrolides FD-891 and 892, and tetradic-5-en-9-olides. Other natural products including cyclic ethers (such as sesamin, asarinin, acetogenins, centrolobines and nabilones), alcohols (such as sulcatol), esters (such as methyl jasmonates), polycyclic precursors of fredericamycin, amino alcohols (such as ambroxol and sphingosines), isoprostanes, isofurans, polyketide precursors of anachelins, brevicomins, gummiferol, shikimic acid and the related compounds, and the pheromone disparlure have been synthesized stereodivergently. Heterocyclic systems such as epoxides, theobroxides and bromoxones, oxetan-3-ones, 5- to 8-membered cyclic ethers, azetidones, γ-lactams, oxazolidinones, bis(oxazolines), dihydropyridoisoindolines and octahydroisoquinolines have been prepared following stereodivergent routes. Stereodivergent routes to unnatural compounds such as alkenes, dienes, allenes, cyclopropanes, alcohols, aldols, amines, amino alcohols, β-amino acids, carboxylic acids, lactones, nitriles and α-amino nitriles have been considered as well.

From Alkynes to Heterocycles through Metal-Promoted Silylformylation and Silylcarbocyclization Reactions

Oxygen and nitrogen heterocyclic systems are present in a large number of natural and synthetic compounds. In particular, oxa- and aza-silacyclane, tetrahydrofuran, benzofuran, cycloheptadifuranone, cycloheptadipyrrolone, pyrrolidine, lactone, lactam, phthalan, isochromanone, tetrahydroisoquinolinone, benzoindolizidinone, indoline and indolizidine scaffolds are present in many classes of biologically active molecules. Most of these contain a C=O moiety which can be easily introduced using carbonylative reaction conditions. In this field, intramolecular silylformylation and silylcarbocyclization reactions may afford heterocyclic compounds containing a carbonyl functional group together with a vinylsilane moiety which can be further transformed. Considering these two aspects, in this review a detailed analysis of the literature data regarding the application of silylformylation and silylcarbocyclization reactions to the synthesis of several heterocyclic derivatives is reported.

Kainic Acid-Based Agonists of Glutamate Receptors: SAR Analysis and Guidelines for Analog Design

A comprehensive survey of kainic acid analogs that have been tested for their biological activity is presented. Specifically, this review (1) gathers and compares over 100 kainoids according to a relative activity scale, (2) exposes structural features required to optimize affinity for kainate receptors, and (3) suggests design rules to create next-generation KA analogs. Literature SAR data are analyzed systematically and combined with the most recent crystallographic studies. In view of the renewed interest in neuroactive molecules, this review aims to help guide the efforts of organic synthesis laboratories, as well as to inform newcomers to KA/GluK research.

Polyfunctional Lithium, Magnesium, and Zinc Alkenyl Reagents as Building Blocks for the Synthesis of Complex Heterocycles

New conjunctive β-silylated organometallic reagents of Li, Mg, and Zn have been prepared and used for an expeditive construction of various polyfunctionalized 5-, 6-, and 7-membered heterocycles, such as furans, pyrroles, quinolines, benzo[b]thieno-[2,3-b]pyridine, naphthyridines, fused pyrazoles, and 2,3-dihydro-benzo[c]azepines. The latent silyl group has been converted into various carbon-carbon bonds in most heterocycle types.

Why You Really Should Consider Using Palladium-Catalyzed Cross-Coupling of Silanols and Silanolates

The transition metal-catalyzed cross-coupling of organometallic nucleophiles derived from tin, boron, and zinc with organic electrophiles enjoys a preeminent status among modern synthetic methods for the formation of carbon-carbon bonds. In recent years, organosilanes have emerged as viable alternatives to the conventional reagents, with the added benefits of low cost, low toxicity and high chemical stability. However, silicon-based cross-coupling reactions often require heating in the presence of a fluoride source, which has significantly hampered their widespread acceptance. To address the "fluoride problem", a new paradigm for palladium-catalyzed, silicon-based cross-coupling reactions has been developed that employs a heretofore underutilized class of silicon reagents, the organosilanols. The use of organosilanols, either in the presence of Brønsted bases or as their silanolate salts, represents an operationally simple and mild alternative to the fluoride-based activation method. Organosilanols are readily available by many well-established methods for introducing carbon-silicon bonds onto alkenes, alkynes, arenes and heteroarenes. Moreover, several different protocols for the generation of alkali metal salts of, vinyl-, alkenyl-, alkynyl-, aryl-, and heteroarylsilanolates have been developed and the advantages of each of these methods have been demonstrated for a number of different coupling classes. This review will describe the development and implementation of cross-coupling reactions of organosilanols and their conjugate bases, silanolates, with a wide variety of substrate classes. In addition, application of these transformations in the total synthesis of complex natural products will also be highlighted. Finally, the unique advantages of organosilicon coupling strategies vis a vis organoboron reagents are discussed.

An alkyne hydrosilylation–Hiyama coupling approach to highly functionalised 1,3-dienes

A high yielding and completely stereoselective hydrosilylation–Hiyama protocol has been established for the synthesis of highly functionalised E,E-dienes.

Recent advances in stereoselective synthesis of 1,3-dienes

The aim of this review is to present the latest developments in the stereoselective synthesis of conjugated dienes, covering the period 2005-2010. Since the use of this class of compounds is linked to the nature of their appendages (aryls, alkyls, electron-withdrawing, and heterosubstituted groups), the review has been categorized accordingly and illustrates the most representative strategies and mechanisms to access these targets.