FeCl3·6H2O, a Catalyst for the Diastereoselective Synthesis of cis-Isoxazolidines from N-Protected δ-Hydroxylamino Allylic Acetates

Hydroxylamines: From Synthetic Intermediates to Synthetic Targets

ConspectusSynergy between the teaching and research activities of a University should be a source of new ideas, each informing the other. A classroom discussion gave rise to our concept of using hydroxylamines as a form of "tethered nitrogen" for alkaloid synthesis. The "tether" temporarily connects a nucleophilic nitrogen atom to the substrate, rendering an intermolecular reaction intramolecular, thus providing stereo- and regiochemical control for C-N bond formation. In the context of the synthesis of 1,3-amino alcohols, this necessitated the synthesis of isoxazolidines. This concept led to the exploration of methods for the synthesis of these heterocycles beyond the well-established 1,3-dipolar cycloadditions. The first two methods developed based upon this concept were palladium catalyzed cyclocarbonylation and silver catalyzed allene cyclization. These new methods were applied to two syntheses of sedamine and the synthesis of three Nuphar alkaloids. Intramolecular aza-Michael addition, combined with the use of cross-metathesis to generate the substrates, gave access to both isoxazolidines and tetrahydro-1,2-oxazines. This new method made possible a synthesis of monomorine with high stereochemical control. The extension to more complex alkaloids required the incorporation of additional chemistry. Combining allene cyclization with iminium ion chemistry allowed the extension to the more complex piperidine alkaloids porantheridine and sedinine. In these cases, successful stereocontrol relied on the ability to predict the conformation of the intermediates and the trajectory of the nucleophilic attack, which may be sterically or stereoelectronically controlled. Inspection of our collection of methods revealed that we had good methods for cis-3,5-disubstituted isoxazolidines and trans-3,6-disubstituted tetrahydro-1,2-oxazines. Inspired by earlier work involving iminium ions, methods were developed to provide the complementary trans-isoxazolidines and cis-oxazines, using an allylation reaction. This, combined with our interest in applications of hydroformylation, led to the synthesis of 5-hydroxysedamine. Venturing away from piperidines, this method was successfully applied to the synthesis of the β-amino acid antibiotic negamycin. Use of N,O-heterocycles as intermediates naturally sparked an interest in N,O-heterocycles as synthetic targets, as there are many natural products that contain the hydroxylamine moiety, often incorporated into an isoxazolidine or 1,2-oxazine ring. As the presence of the hydroxylamine moiety cannot be fully demonstrated using the usual spectroscopic methods, total synthesis can be employed to bridge this logical gap. A synthesis of raistrickindole A, utilizing an intramolecular Mitsunobu reaction of a hydroxamic acid to form a 1,2-oxazine, confirmed the structure of this diketopiperazine natural product. A synthesis of preisomide, using an aza-Michael addition to form the required 1,2-oxazine, also confirmed the structure of this natural product.

Leveraging Alkyne-Oximium Cyclization for the Stereoselective Synthesis of Isoxazolidine

A TMSOTf-mediated highly diastereoselective synthesis of isoxazolidine bearing three contiguous stereocenters is described. This method utilizes O-propargyl hydroxylamines as a novel building block for the rapid assembly of the isoxazolidines via alkyne-oximium cyclization. The strategy could be used in the synthesis of enantiomerically enriched isoxazolidines. Reductive cleavage of the N-O bond efficiently gives the corresponding 1,3-aminodiols with precise control over all stereocenters formed.

Palladium/Xu-Phos-catalyzed asymmetric carboamination towards isoxazolidines and pyrrolidines

An efficient palladium-catalyzed enantioselective carboamination reaction of N-Boc-O-homoallyl-hydroxylamines and N-Boc-pent-4-enylamines with aryl or alkenyl bromides was developed, delivering various substituted isoxazolidines and pyrrolidines in good yields with up to 97% ee. The reaction features mild conditions, general substrate scope and scalability. The obtained products can be transformed into chiral 1,3-aminoalcohol derivatives without erosion of chirality. The newly identified Xu-Phos ligand bearing an ortho-OiPr group is responsible for the good yield and high enantioselectivity.

Organic synthesis with the most abundant transition metal–iron: from rust to multitasking catalysts

The promising aspects of iron in synthetic chemistry are being explored for three-four decades as a green and eco-friendly alternative to late transition metals. This present review unveils these rich iron-chemistry towards different transformations.

A molecular electron density theory study on the [3+2] cycloaddition reaction of 5,5-dimethyl-1-pyrroline N-oxide with 2-cyclopentenone

In the present work, the [3 + 2] cycloaddition reaction of 5,5-dimethyl-1-pyrroline N-oxide (Nit-5) and 2-cyclopentenone (CPN-6), experimentally reported by Tamura et al., was theoretically studied using the newly introduced molecular electron density theory (MEDT). Based on the experimental findings, this reaction takes place in an O3-C4 regio- and an exo-stereospecific fashion to give corresponding [3 + 2] exo cycloadduct as the sole product. The results of the potential energy surface analysis indicated that the experimentally reported product is more favorable both thermodynamically and kinetically relative to other possible adducts. In complete agreement with the experimental outcomes, the conceptual density functional theory reactivity indices explained the reactivity and regioselectivity of the reaction. Calculation of global electron density transfer of the energetically most preferred transition state indicated that the electron density fluxes from Nit-5 as a nucleophilic species toward CPN-6 as an electrophilic species. Analysis of the molecular electrostatic potential map of the most favorable transition state showed that approach of Nit-5 and CPN-6 locates the oppositely charged regions over each other leading to attractive forces between two reagents rationalizing the exo stereoselectivity predominance. The molecular mechanism of the reactions was specified using electron localization function analysis over some relevant points along the intrinsic reaction coordinate profile of the most favorable transition state and the results indicated that this zwitterionic-type [3 + 2] cycloaddition reaction proceeds through a two-stage one-step mechanism. In fact, while the O3-C4 single bond is initialy formed between two fragments through donation of some electron density from the O3 oxygen lone electron-pairs of Nit-5 toward the C4 carbon atom of CPN-6, the delayed C1-C5 single bond begins to form via C1- to -C5 coupling of pseudodiracal centers created on theses atoms over the course of reaction.

Regioselective synthesis of pyrimidine-fused tetrahydropyridines and pyridines by microwave-assisted one-pot reaction

A regioselective three-component reaction of α,β-unsaturated aldehydes, cyclic 1,3-dicarbonyls and 6-aminouracils in the presence of FeCl₃·6H₂O as catalyst under microwave irradiation has been demonstrated. Three-component reaction of α,β-unsaturated aldehydes like cinnamaldehyde/crotonaldehyde, cyclic 1,3-diketones such as 2-hydroxy-1,4-naphthaquinone/dimedone and 6-aminouracils provides regioselective pyrimidine-fused tetrahydropyridines tethered with cyclic 1,3-diketones. On the other hand, replacing cyclic 1,3-diketones by 4-hydroxycoumarin and keeping all other conditions the same provided a two-component pyrimidine-fused pyridines. The salient features of this methodology are operational simplicity, short reaction time, good-to-moderate yields of the products, easy purification method and regioselective products having medicinally important heterocyclic rings such as pyrimidine, tetrahydropyridine or pyridine.

Isoxazolidine: A Privileged Scaffold for Organic and Medicinal Chemistry

The isoxazolidine ring represents one of the privileged structures in medicinal chemistry, and there have been an increasing number of studies on isoxazolidine and isoxazolidine-containing compounds. Optimization of the 1,3-dipolar cycloaddition (1,3-DC), original methods including electrophilic or palladium-mediated cyclization of unsaturated hydroxylamine, has been developed to obtain isoxazolidines. Novel reactions involving the isoxazolidine ring have been highlighted to accomplish total synthesis or to obtain bioactive compounds, one of the most significant examples being probably the thermic ring contraction applied to the total synthesis of (±)-Gelsemoxonine. The unique isoxazolidine scaffold also exhibits an impressive potential as a mimic of nucleosides, carbohydrates, PNA, amino acids, and steroid analogs. This review aims to be a comprehensive and general summary of the different isoxazolidine syntheses, their use as starting building blocks for the preparation of natural compounds, and their main biological activities.

Iron- and indium-catalyzed reactions toward nitrogen- and oxygen-containing saturated heterocycles

A myriad of natural and/or biologically active products include nitrogen- and oxygen-containing saturated heterocycles, which are thus considered as attractive scaffolds in the drug discovery process. As a consequence, a wide range of reactions has been developed for the construction of these frameworks, much effort being specially devoted to the formation of substituted tetrahydropyrans and piperidines. Among the existing methods to form these heterocycles, the metal-catalyzed heterocyclization of amino- or hydroxy-allylic alcohol derivatives has emerged as a powerful and stereoselective strategy that is particularly interesting in terms of both atom-economy and ecocompatibility. For a long time, palladium catalysts have widely dominated this area either in Tsuji-Trost reactions [Pd(0)] or in an electrophilic activation process [Pd(II)]. More recently, gold-catalyzed formation of saturated N- and O-heterocycles has received growing attention because it generally exhibits high efficiency and diastereoselectivity. Despite their demonstrated utility, Pd- and Au-complexes suffer from high costs, toxicity, and limited natural abundance, which can be barriers to their widespread use in industrial processes. Thus, the replacement of precious metals with less expensive and more environmentally benign catalysts has become a challenging issue for organic chemists. In 2010, our group took advantage of the ability of the low-toxicity and inexpensive FeCl3 in activating allylic or benzylic alcohols to develop iron-catalyzed N- and O-heterocylizations. We first focused on N-heterocycles, and a variety of 2,6-disubstituted piperidines as well as pyrrolidines were synthesized in a highly diastereoselective fashion in favor of the cis-compounds. The reaction was further extended to the construction of substituted tetrahydropyrans. Besides triggering the formation of heterocycles, the iron salts were shown to induce a thermodynamic epimerization, which is the key to reach the high diastereoselectivities observed in favor of the most stable cis-isomers. It is worth noting that spiroketals could be prepared by using this method, which was successfully applied to a synthetic approach toward natural products belonging to the bistramide family. We then turned our attention to heterocycles incorporating two heteroatoms such as isoxazolidines. These frameworks can be found in biologically active natural products, and in addition, they can be transformed into 1,3-amino alcohols, which are of importance in organic chemistry. The use of FeCl3·6H2O allowed the access to a large variety of 3,5-disubstituted isoxazolidines from δ-hydroxylamino allylic alcohol derivatives with good yields and diastereoselectivities in favor of the cis-isomer. Recently, a Lewis acid-catalyzed synthesis of six- and five-membered ring carbonates starting from linear tert-butyl carbonates was reported. In some cases, the mild and chemoselective InCl3 was preferred over FeCl3·6H2O to avoid side-product formation. The resulting cyclic carbonates were easily transformed into 1,3- or 1,2-diols, and a total synthesis of (3S,5S)-alpinikatin was achieved.