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Stereoselective Synthesis of Aza Analogues of Isoaltholactone and Goniothalesdiol - New Applications of the Heck-Matsuda Reaction
European Journal of Organic Chemistry
Abstract
The stereoselective synthesis of nitrogen analogues of biologically active isoaltholactone and goniothalesdiol are described. The successful strategy employed a Heck–Matsuda reaction between a chiral endocyclic enecarbamate bearing an ester functionality and arenediazonium tetrafluoroborates in a divergent approach at an early stage of the synthesis. Several aspects related to this critical arylation reaction are discussed to highlight structural features that affect the outcome of the arylation process. The synthesis of (–)-aza-isoaltholactone 6 was successfully accomplished in nine steps from the starting enecarbamate. We also performed the synthesis of the new fully substituted pyrrolidine (–)-(2R,3R,4S,5S)-1-(tert-butoxycarbonyl)-3,4-dihydroxy-5-phenylpyrrolidin-2-ylacrylic acid 28, which is a potential advanced intermediate in the route to aza-altholactone. Moreover, the synthesis of a new nitrogen analogue of goniothalesdiol (+)-33 was accomplished from the protected dihydroxypyrrolidine (–)-27, obtained from an attempted synthesis of aza-altholactone.
... Overall, the synthesis of (-)-287 required 13 steps and proceeded in ca. 5% overall yield from pyroglutamic acid (Scheme 47) [104]. ...
This review covers the chemistry and biological aspects of goniothalamin-related styryl lactones isolated from natural sources. This family of secondary metabolites has been reported to display diverse uses in folk medicine, but only a limited number of these compounds have been throughly investigated regarding their biological profile. Herein, we cover the goniothalamin-related styryl lactones having a C6-C3-C4 framework which appeared in the literature for the first time in the period 2000-2017, and the reports on the synthesis, biological activity and mechanism of action which were published from 2007-2017.
The title isoaltholactone derivative, C 13 H 13 NO 3 , has an NH group in place of the ether-O atom in the five-membered ring of the natural product. The five-membered ring is twisted about the N—C bond linking it to the six-membered ring, which has a half-chair conformation with the O atom connected to the ether-O atom lying above the plane defined by the remaining atoms. The dihedral angle between the mean planes of the rings comprising the fused-ring system is 75.10 (8)°. In the crystal, hydroxy-O—H...N(amine) hydrogen bonding sustains linear supramolecular chains along the a axis. Chains are linked into a three-dimensional architecture via amine-N—H...π(phenyl) and phenyl-C—H...O(hydroxy) interactions. The influence of the amine-N—H...π(phenyl) contact on the molecular packing is revealed by an analysis of the Hirshfeld surface.
In situ generated palladium on aluminum oxide provides an active catalytic system for Matsuda-Heck reactions in gram-scale. The novel catalyst proceeded through a significantly higher catalytic activity compared to the classical Pd/C system. Based on the high catalytic activity the first α,β,β-triarylation of methyl acrylate in good yields could be provided in one-step.
A general route to several styryl-lactone frameworks is presented. Starting from a known enol ether, stereoselective epoxidation and methanolysis yields a series of three distinct hydroxyacetals, each further functionalized by etherification with an allylic vinylsilane fragment. A highly efficient Lewis acid-promoted intramolecular annulation at the tethered oxocarbenium allows direct entry to cis-fused bicyclic ether cores. Further manipulation delivers a variety of additional styryl-lactone motifs and analogs; for example, simple allylic oxidation yields 3-deoxyisoaltholactone, in just 5 steps overall.
Aryl diazonium salts have been widely used in cross-coupling reaction due to its easily available and highly reactivity as arylation reagents, which have drawn more and more attention by chemists in organic synthesis in recent years. The research progress since 2006 on cross-coupling with aryl diazonium salts combined with our work is reviewed, focusing on new applications and new progress of aryl diazonium salts in new method to build carbon-carbon bond, with construction of the natural product skeleton, in the field of heterogeneous catalysis and other aspects of green synthesis.
The novel styryl lactone analog (+/-)-3-deoxyisoaltholactone was synthesized in 5 steps from dihydrofuran. An intramolecular vinylsilane-oxocarbenium condensation thus afforded the central cis-fused bicyclic ether array in near quantitative yield as a single stereoisomer.
Among the broad range of transition metal-catalyzed reactions, the Heck reaction is an attractive methodology frequently being used for the synthesis of natural and biologically active products, since a palladium-catalyzed reaction of this kind can directly create complex molecules under mild conditions from readily available starting materials. This feature article provides the comprehensive summary of currently available applications of this reaction in the total synthesis.
The synthesis of the first fluorine-containing analog of isoaltholactone, 5-trifluoromethylisoaltholactone (I), is described.
Herein, we describe the total syntheses of three bioactive pyrones isolated from Polygala sabulosa (i.e., 1, 4, and 7) and eight isolated from Piper methysticum (i.e., 8–10, 13, 15, and 18–20) using the Heck–Matsuda arylation as the key strategy. The evaluation of this methodology by employing different arenediazonium tetrafluoroborates revealed that the Heck arylation was more efficient when the olefin undergoing arylation possessed the vinyl-2-pyrone structural unit instead of the vinyl dihydro-2-pyrone moiety. The Heck–Matsuda arylation of many of the examined olefins proceeded in a practical manner with total regio- and stereocontrol.
An array of C-aryl and C-vinyl furanosides were prepared in good yields and diastereoselectivities from C-halogeno furanosides either with aryl Grignard or with vinyl Grignard using the convenient Co(acac)3/TMEDA catalytic system. This method is illustrated by the total synthesis of the (-)-isoaltholactone.
A short and efficient stereoselective synthesis of a styryllactone (−)-isoaltholactone has been achieved in seven steps and 33% overall yield, starting from the readily available carbohydrate D-mannose. The key steps of our synthesis involve intramolecular tetrahydrofuran cyclization and one-pot acetonide deprotection-lactonization.
We describe herein a synthetically useful method for the enantioselective intermolecular Heck-Matsuda arylation of acyclic allylic alcohols. Aryldiazonium tetrafluoroborates were applied as arylating agents in the presence of Pd(TFA)2 and a chiral, commercially available, bisoxazoline ligand. The methodology is straightforward, robust, scalable up to a few grams, and of broad scope allowing the synthesis of a range of -aryl-carbonyl compounds in good to high enantioselectivities and yields. This new enantioselective Heck-Matsuda arylation allowed the synthesis of -aryl--lactones and -aryl aldehydes, which play a vital role as key intermediates in the synthesis of the biologically active compounds, such as (R)-baclofen, (R)-rolipram, (S)-curcumene, (S)-dehydrocurcumene, and (S)- tumerone.
The "dba-free" Heck-Matsuda reaction was investigated via direct ESI-MS(/MS) monitoring. Palladium species involved in the reduction of Pd(ii) during a Wacker type reaction and several "dba-free" arylpalladium transient complexes were detected and characterized. Based on these findings, a more comprehensible catalytic cycle for this pivotal reaction is suggested.
We describe herein an efficient and diastereoselective substrate-directable Heck-Matsuda reaction with nonactivated five-membered olefins. The carbamate acts as the main directing group in the arylation process allowing the synthesis of several functionalized aryl cyclopentenes in good to excellent diastereoselectivities (>85:15) and in isolated yields ranging from 41 to 90%. No double bond isomerizations were observed in these Heck reactions, and the newly created benzylic centers were preserved in all cases examined. The substrate directable Heck arylation approach was successfully applied in a straightforward total synthesis of the sphingosine 1-phosphate receptor-subtype 1 (S1P(1)) agonist VPC01091 by a concise and practical route involving 5 steps in 40% overall yield.
The Heck arylation of maleic anhydrides using arenediazonium tetrafluoroborates was investigated. Symmetrical and unsymmetrical 3,4-diarylmaleic anhydrides, some of them showing interesting fluorescent properties, were prepared in one or two steps from cheap and commercially available maleic anhydride. This Heck arylation methodology constitutes a new and direct entry to the synthesis of arylated maleic anhydride materials, natural products and their derivatives, as demonstrated with the total syntheses of the marine alkaloids prepolycitrin A and polycitrin A and with the synthesis of a N-protected fluorescent phenylalanine.
Development of a highly 'atom-efficient' production process for 2-alkyl-substituted benzenesulfonic acids by arylation of olefins with 2-diazoniobenzenesulfonate catalyzed by a homogeneous Pd-complex and subsequent hydrogenation of the resulting styrenes with an in situ generated
heterogeneous Pd-catalyst.
The novel aryl platynecines 3a/3b were synthesized from endocyclic 4-aryl enecarbamates 8a/8b by a concise and practical route. The synthesis was based on an efficient preparation of 4-aryl enecarbamates 8a/8b from 3-pyrrolines by means of a Heck arylation using aryldiazonium tetrafluoroborates, followed by a highly stereoselective cycloaddition of the 4-aryl enecarbamates 8a/8b to 2-chloroethyl ketene to afford exclusively the 7-endo-(2-chloroethyl) cyclobutanones 12a/12b in good yields (67% and 65%). Baeyer-Villiger oxidation of the cyclobutanones 12a/12b occurred with high regioselectivity to furnish the aza-lactones 13a/13b in 96% and 90% yields. Reduction of lactones 13a and 13b with LiAlH 4 gave the desired aryl platynecines 3a/3b. The total synthetic sequence involved 6 steps and provided the aryl platynecines 3a/3b in an overall yield of 41% and 38%, respectively. These compounds are the first examples of necine bases bearing an aromatic substituent on the azabicyclo[3.3.0]octane framework and incorporate some of the key structural element of the pharmacologically active 1,3,4-trisubstituted pyrrolines which act as antagonists of the chemokine receptor CC5.
The scope of the Heck arylation of cyclic and acyclic enol ethers with arenediazonium salts was evaluated.
Arylation of 2,3-dihydrofuran yielded 2-aryl-2,5-dihydrofurans as the major adducts (>99:1) except when using n-Bu 4 NHSO 4 as additive or 4-NO 2 PhN 2 BF 4 as arenediazonium salt. 2,3-Dihydropyran provided mixtures of the three possible isomeric Heck adducts. Arylation of n-butylvinylether with arenediazonium bearing electron-donating groups resulted in substituted acetophenones as almost exclusive products in good overall yields. Substituted 4H-chromenes provided 2-aryl-2H-chromenes in moderate yield when applying the Pd(OAc) 2 /2,6-di-t-butyl-4-methylpyridine catalytic system, which were applied in the synthesis of flavonoids.
The Heck-Matsuda arylation of chiral 2-(S)-hydroxymethyl dihydrofurans (endocyclic enolethers) and its derivatives, employing arenediazonium tetrafluoroborates, was developed into a highly efficient, practical and diastereoselective synthetic process. This methodology was applied to the total synthesis of the styryllactone (-)-isoaltholactone in seven steps with an overall yield of ∼25%, from the readily available chiral 2-hydroxymethyldihydrofuran. The strategy permits the synthesis of several other aromatic analogues of isoaltholactone.
Heck arylation of N-acyl-3-pyrrolines and an N-acyl-tetrahydropyridine with aryldiazonium tetrafluoroborates under phosphine-free conditions proceeded smoothly to give alpha-hydroxycarbamates (hemiaminals) or alpha-alkoxycarbamates which were oxidized to the desired gamma- and delta-lactams. Acidic hydrolysis of the gamma-lactams produced a series of arylated GABA derivatives, including baclofen, a useful therapeutic drug. in only three steps with ail overall yield of 63 76%, Starting from N-acyl-tetrahydropyridine, aryl-delta-lactams and higher homologues of baclofen can be obtained.
François-Xavier Felpin was born in Villefranche-sur-Saône (France) in 1977. He received his Ph.D. degree in 2003 from the University of Nantes under the supervision of Professor Jacques Lebreton working on synthesis of alkaloids. After his Ph.D., he was engaged in a post-doctoral position with Professor Robert S. Coleman at The Ohio State University working on the synthesis of Mitomycin. In 2004 he joined the University of Bordeaux as an Assistant Professor and he received his habilitation in 2009. His research interests include heterogeneous and homogeneous sustainable catalysis as well as medicinal chemistry.
The Pd(0)-catalyzed reaction of aryl diazonium salts with monosubstituted alkenes [1] was found to be an interesting alternative to the well-known Pd-catalyzed arylhalide alkene coupling (Heck type reaction) or the copper mediated reaction of aryl diazonium salts with alkenes (Meerwein arylation) [2]. The reaction can be run without isolation of the diazonium salt in presence of only 0.5 to 1 mol% of the Palladium catalyst in a one pot procedure, in high yield and under mild conditions. The resulting styrene is reduced in a subsequent hydrogenation step with an in situ generated heterogeneous Pd-catalyst. The combination of three reaction steps without isolation of intermediates and the virtually complete recovery of the Pd-metal at the end of the reaction sequence makes this process [4] extremely efficient.
The reagent formed by combining diethyl azodicarboxylate (DEAD) and triphenylphosphine (TPP) could be utilized in the intermolecular dehydration between an alcohol and various acidic components such as carboxylic acids, phosphoric diesters, imides, and active methylene compounds. By the use of DEAD and TPP, diols and hydroxy acids gave cyclic ethers and lactones, respectively. The reaction of nucleosides with DEAD and TPP afforded triphenylphosphoranylnucleosides. Alcohols reacted with 2,6-di-t-butyl-4-nitrophenol in the presence of DEAD and TPP to give aci-nitroesters which converted into the corresponding carbonyl compounds.
A concise asymmetric total synthesis of goniothalesdiol A was accomplished using protecting-group-free strategy, in which silyl-Prins cyclization was used as the key step.
The palladium-catalyzed reaction of arenediazonium tetrafluoroborates with methyl 4-hydroxy-2-butenoate in MeOH under mild conditions gives 4-arylbutenolides usually in good to high yields through a domino vinylic substitution/cyclization process. The reaction tolerates a variety of useful substituents including the whole range of halogen substituents, nitro, ether, cyano, keto, and ester groups and can be performed as a one-pot process generating the arenediazonium salt in situ. By using this method, the marine antibiotic rubrolide E has been synthesized via an expeditious and efficient sequential protocol that omits the isolation of the butenolide intermediate (two operative steps, 52% overall yield).
An efficient and practical total synthesis of (+)-goniothalesdiol and its 2,5-epi analogue is described herein. The key features include a diastereoselective reduction of C-5 keto with Zn(BH4)(2) to generate the desired stereochemistry at C-5. The tetrahydrofuran backbone of natural goniothalesdiol was synthesized under basic conditions via epoxide formation, followed by in situ 5-exo opening of the epoxide ring with 7-benzoyloxy oxygen upon debenzoylation. For the 2,5-epi analogue, the tetrahydrofuran ring was formed via acid-catalyzed acetonide deprotection followed by concomitant S(N)2 displacement of the O-mesyl group at C-5 center with C-2-gamma-oxygen.
Enantiopure (1R,2S)-erythro- and (1S,2S)-threo-isomers of four new aryl-pyrrolidyl alcohols 5aH, 5aMe, 5bH and 5bMe have been obtained in five steps from (−)-(S)-proline and fully characterized. The oxidation of alcohol 8 into aldehyde 9 was the most difficult step and racemization occurs during Swern oxidation but this difficulty can be overcome by using SO3/pyridine as oxidant. Diastereomer I of the protected amino alcohol 10a crystallized and was shown to be the (1R,2S)-erythro-isomer (e-10a-I) using X-ray crystallography. Therefore the (1R,2S)-erythro structure was assigned to all compounds obtained from e-10a-I, and, as a consequence, the (1S,2S)-threo structure was assigned to diastereomers II.
A flexible stereoselective route to synthesize both enantiomers of the highly functionalized substituted tetrahydrofurans and α,β-unsaturated-δ-lactones, goniothales diol, altholactone, and isoaltholactone, from readily available cinnamyl alcohol is described. This approach derived its asymmetry from Sharpless catalytic asymmetric epoxidation and Sharpless asymmetric dihydroxylation reactions. The resulting diols were produced in high enantiomeric excess and were cyclized in a stereoselective manner in the presence of a catalytic amount of camphor sulfonic acid.
In this Letter, we report, for the first time, the development of an efficient method for the intramolecular Heck reaction of arenediazonium salts in the synthesis of benzofuran and indole derivatives. In addition, this methodology allowed the synthesis of a series of dihydrobenzofuran acetic acid derivatives via a domino Heck–Matsuda coupling–carbonylation reaction.
The first total synthesis of goniothalesdiol, a dihydroxylated tetrahydrofuran isolated from Goniothalamus borneensis (Annonaceae), and its 7-epimer is described starting with d-mannitol. The key step of these syntheses is palladium(II)-catalyzed oxycarbonylation of unsaturated triols with d-lyxo and d-xylo configuration, respectively, which allowed efficient construction of the tetrahydrofuran ring with excellent threo-selectivity at the newly formed stereogenic centre.
The reaction of a series of monoterpenic olefins and Δ4-octalins with dimethyldioxirane led to the corresponding epoxides in excellent yields. Remarkable diastereoselectivity was observed for the Δ4-octalins. The procedure consists simply in stirring the substrate, NaHCO3 and acetone, at 0°C, with dropwise addition of an aqueous solution of oxone.
A total synthesis of (+)-goniothalesdiol, a 3,4-dihydroxy-2,5-disubstituted tetrahydrofuran isolated from Goniothalamus borneensis (Annonaceae), and its 7-epimer is reported using oxycarbonylation methodology for construction of polyhydroxylated substituted heterocycles. Diastereoselectivity of addition of organometallic reagents to 2,3-O-isopropylidene-d-threose derivatives using theoretical calculations based on the semiempirical PM5 was studied.
Twelve natural products, including two new compounds, goniothalesdiol 1a and goniothalactam 2, were isolated from the bark of the Malaysian tree Goniothalamus borneensis (Annonaceae) and evaluated for their cytotoxic activity. Their structures were elucidated by spectroscopy including 2D-NMR spectroscopy. 1a, which was semisynthesised from (+)-goniothalenol (3a), was related to several cytotoxic styryl-lactones (4–8) also isolated from this species.
Tetra-n-butylammonium per-ruthenate (Bun4N)(RuO4) and tetra-n-propylammonium per-ruthenate (Prn4N)(RuO4), with N-methylmorpholine N-oxide, function as mild catalytic oxidants for the high yield conversion of alcohols to aldehydes and ketones and are competitive with more conventional reagents.
When n-valeric acid was treated with allyl diethyl phosphite and diethyl azodicarboxylate, allyl valeriate and diethyl N-(diethyl)phosphoryl hydrazodicarboxylate were obtained in good yields. Similarly ethyl benzoate was obtained in a nearly quantitative yield by the reaction of benzoic acid with triethyl phosphite and diethyl azodicarboxylate. The reaction of carboxylic acid with triphenyl phosphine and diethyl azodicarboxylate in the presence of an alcohol resulted in the formation of the corresponding esters of the carboxylic acid, triphenyl phosphine oxide, and diethyl hydrazodicarboxylate. The mechanisms of these reactions are also discussed.
Review: Dieser Übersichtsartikel beschreibt Herstellung und Eigenschaften verschiedener Reagentien aus Dimethylsulfoxid und Elektrophilen und deren Verwendung zur Oxidation von Alkoholen zu Carbonylverbindungen und zur Synthese von Sulfiliminen und Sulfoximinen.
A new Horner-Emmons reagent, ethyl diphenylphosphonoacetate 1 was prepared from triethyl phosphonoacetate, PCl5, and phenol in 60% overall yield. Horner-Emmons reactions of 1 with aldehydes in the presence of Triton® B or NaH in THF gave the Z-unsaturated esters in 89–93% selectivity in almost quantitative yields. Furthermore, 1 showed up to 99% Z-selectivity under Still's condition (KHMDS/18-crown-6).
The Heck–Matsuda arylation of 2-aza and 2-oxo-substituted acrylates is described. Several reaction conditions were evaluated including the influence of solvents, temperature, catalysts, and stoichiometry. While the oxygenated system was successfully arylated in benzonitrile with Pd2(dba)3 as catalyst, the aza-acrylate furnished methoxylated products. The methoxylated products were subjected to an elimination/reduction protocol to obtain the corresponding N,O-protected phenylalanine derivatives. A one-pot procedure for the preparation of phenylalanine derivatives from 2-aza-substituted acrylates is presented.
An efficient and practical strategy has been developed for the construction of the antipode of 3,4-dihydroxy-2,5-disubstituted tetrahydrofuran, goniothalesdiol, isolated from the bark of the Malaysian tree. The synthetic process is based on the convenient manipulation via Lewis acid-induced deoxygenation of the highly functionalized lactone derived from D-glucuronolactone.
A small, but extraordinary diverse class of bioactive styryl-lactones has been isolated from several species of the genus Goniothalamus (Annonnaceae). The interesting biological properties, in particular antitumoral, and structural diversity of these lactones have prompted several asymmetric syntheses. They may be classified in two types: synthesis from enantiomerically pure material (carbohydrate...) or with asymmetric methodologies (dihydroxylation, epoxidation...). This review will discuss the different strategies implemented to prepare these styryl-lactones.
The total synthesis of the 3-(3,4,5-trimethoxyphenyl)-pyrrolidine, a new and conformationally constrained mescaline analogue, was accomplished in a concise and efficient manner. The synthetic route encompassed only 4 steps from the starting N-Cbz-3-pyrrolidine, in 46% overall yield. The route features a highly effective Heck arylation of the non-activated olefin N-Cbz-3-pyrrolidine with the 3,4,5-trimethoxybenzene diazonium tetrafluoroborate. The hemiaminal (lactamol) Heck adduct was converted to the mescaline analogue in a sequence of reactions: (a) dehydration of the intermediate hemiaminal 3 with trifluoroacetic acid anhydride, (b) hydrogenation/hydrogenolysis of the endocyclic enecarbamate 6 with H2-Pd/C, and (c) formation of the rather stable mescaline analogue in the form of a hydrochloride salt. The target molecule constitutes a new mescaline analogue with potential activity towards 5-HT2 dopamine receptors.
During the last years, the stereoselective synthesis of a small group of styryl lactones has increased the interest in synthetic organic chemistry in which many research groups have focused much of their efforts. This review gives an overview of the different approaches for the total synthesis of novel styryl lactones, which were found to possess marginal to significant cytotoxicities against several human tumors reported to date.
Syntheses of both 5,6,10-triepi-actinobolin and the antibiotic actinobolin are described in which a homochiral diene prepared from L-threonine is employed as a key component in a Diels-Alder reaction with an acetylenic dienophile. While the Diels-Alder reaction of this diene with methyl propiolate furnished the cycloadduct required for the synthesis of (+)-actinobolin as the minor diastereomer, the completion of the synthesis required but seven additional steps. The steric and stereoelectronic features responsible for the π-facial course of this cycloaddition reaction are discussed along with the various steps required to complete the syntheses of the title compounds.
Preparation de dioxyde-2,2 de dioxathiolannes-1,3,2(I) a partir de glycols et reactions des sulfates cycliques(I) avec divers nucleophiles (H − , N 3 − , F − , PhCO 2 − , NO 3 − , SCN − , PhCH 2 − )
New and recyclable chiral amino alcohols are prepared and used as chiral ligands in catalytic asymmetric aryl transfer reactions.
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An efficient and stereoselective protocol for the preparation of β,β‐disubstituted acrylates in good to high yields by means of a Heck–Matsuda arylation was accomplished. The method employs a base‐ and ligand‐free Heck arylation reaction of methyl cinnamate using both electron‐deficient and electron‐rich arenediazonium salts as electrophiles. The Heck reaction displays a remarkable electronic dependence regarding the diastereoselectivity of the arylation process, which correlates with the electronic nature of the arenediazonium salts employed. A rationale for the observed diastereoselectivity is presented. The overall methodology provides a convenient route to 3‐arylindanones and 4‐aryltetralones allowing the concise formal total syntheses of the therapeutically important psychoactive compounds (±)‐indatraline and (±)‐sertraline.
This review highlights the potential and the versatility of arenediazonium salts as viable alternatives to conventional aryl halides and oxygen-based electrophiles for Pd-catalyzed Heck reactions. It also presents an overview of the field over the last decade, including some historical perspective of the Heck–Matsuda (HM) reaction with general considerations regarding reaction conditions, type of catalysts used, and the application of arenediazonium salts as reagents for the HM arylation of several types of alkenes. Throughout this review, the principal aspects related to reactivity and selectivity are discussed, and when applicable a comparison is made between the HM and the conventional Heck reactions. One-pot procedures that involve generation of the arenediazonium salt in situ from the corresponding aniline have been presented as well as some useful heterogeneous catalytic protocols. Allylic alcohols, conjugated alkenes, unsaturated heterocycles, and unactivated alkenes are capable of being arylated with arenediazonium salts by using simple catalysts such as Pd(OAc)2 or Pd2(dba)3 at room temperature in air and in benign and conventional solvents. In addition to the intramolecular variant of the HM reaction, intermolecular tandem coupling–cyclization processes have also been developed for the construction of a range of oxygen and nitrogen heterocycles. Finally, several applications towards the total synthesis of biologically active natural and nonnatural compounds are highlighted as well as the use of the HM reaction in the synthesis of new chiral, nonracemic ligands.
An advanced ABCD fragment (see picture) constructed on the way to the total synthesis of the potent antitumor agent (+)-spongistatin 2 has also been used in a formal total synthesis of (+)-spongistatin 1. In both cases the critical step was an acid-mediated epimerization in the presence of Ca²⁺ ions to stabilize the axial–equitorial CD spiroketal.
Short, efficient, and stereoselective synthesis of the trans-stilbenes resveratrol, DMU-212, and analogues of both compounds are described. The synthesis of these important anti-cancer agents feature the palladium catalyzed Heck–Matsuda arylation of styrenes with arenediazonium tetrafluoroborates.
The first total synthesis of the natural product (3S,7R)-5,6-dehydro-de-O-methyl centrolobine and various analogues is reported, using a highly regio- and diastereoselective Mizoroki-Heck reaction of phenol diazonium salts and enantiopure dihydropyrans. The assigned relative configuration was confirmed by single-crystal X-ray structure analysis, but a revision of the absolute configuration is proposed based on polarimetric measurement.
β,β-Disubstituted α,β-unsaturated esters may serve as valuable derivatives for the preparation of other highly functionalized systems found in many natural products and marketed drugs. The stereoselective synthesis of unsymmetrical β,β-diarylacrylate compounds possessing two similar aromatic groups remains a substantial challenge. A simple and convenient stereoselective protocol for the preparation of β,β-disubstituted acrylates via a Heck-Matsuda reaction is reported. Good to high yields were accomplished by a "ligand-free" Pd-catalyzed arylation reaction of cinnamate esters with arenediazonium tetrafluoroborates. Both electron-deficient and electron-rich arenediazonium salts could be employed as arylating reagents, and cinnamate esters were generally more reactive when substituted with an electron-donating group. The overall methodology is highly stereoselective, and this attribute was taken advantage of in the asymmetric Cu-catalyzed 1,4 reduction reaction to provide β,β-diarylpropanoates in high enantioselectivities. The synthesis of a 3-aryl indole and a chiral 4-aryl-2-quinolone from β,β-diarylacrylates was achieved by cyclization in the presence of a diphosphine ligated CuH catalyst. A convenient route for the asymmetric formal synthesis of the psychoactive compound (-)-Indatraline is also presented.
Arene diazonium tetrafluoroborates can be synthesized from aromatic acetamides via a sequence of deacetylation, diazotation and precipitation, induced by anion exchange. The reaction is conducted as a convenient one-flask transformation with consecutive addition of the appropriate reagents. Exchange of solvents or removal of byproducts prior to isolation of the product is not required. The arene diazonium salts are isolated from the reaction mixture by simple filtration. Two complementary protocols are presented, and the utility of the reaction is exemplified for a synthesis of the diarylheptanoid natural product de-O-methyl centrolobine.
"Chemical Equation Presented" What a (strain) relief! The first broadly applicable, catalytic, and stereoselective vinyl oxirane ring expansion is described (see scheme; hfacac = hexafluoroacetylacetonate). The stereoselectivity was influenced by several reaction parameters, and kinetic studies support a mechanistic proposal involving the in situ formation of a more reactive catalytic species. This ring-expansion reaction has been employed in the asymmetric total synthesis of (+)-goniothalesdiol.
A highly efficient palladium-catalyzed Heck reaction of allylic esters with arenediazonium salts is described. The reaction proceeds under mild conditions, with excellent to total regio- and stereochemical control and with retention of the traditional leaving group. Furthermore, the generality of the present methodology is illustrated by the short total synthesis of the natural kavalactones, yangonine, (+/-)-methysticin, and (+/-)-dihydromethysticin.
Natural products have been the single most productive source of leads for the development of drugs. Over a 100 new products are in clinical development, particularly as anti-cancer agents and anti-infectives. Application of molecular biological techniques is increasing the availability of novel compounds that can be conveniently produced in bacteria or yeasts, and combinatorial chemistry approaches are being based on natural product scaffolds to create screening libraries that closely resemble drug-like compounds. Various screening approaches are being developed to improve the ease with which natural products can be used in drug discovery campaigns, and data mining and virtual screening techniques are also being applied to databases of natural products. It is hoped that the more efficient and effective application of natural products will improve the drug discovery process.
[structure: see text] In this Letter we describe the first total synthesis of mycothiazole, a polyketide thiazole from a marine sponge. Key steps include our CMD oxidation for the conversion of thiazolidine 11 to thiazole 12 and the Nagao acetate aldol reaction of 5 with aldehyde 4 to construct the chiral secondary alcohol. The skipped diene was constructed by the standard Stille coupling, and the conjugated diene was synthesized by lithium(I)- and copper(I)-mediated Stille coupling.
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Total enantioselective synthesis of the natural (-)-codonopsinine was accomplished in seven steps with an overall yield of approximately 16% starting from the five-membered endocyclic enecarbamate 4. The total synthesis features a highly efficient and stereoselective Heck arylation of endocyclic enecarbamate 4 with p-methoxybenzenediazonium tetrafluoroborate and a stereoselective epoxidation/epoxide opening sequence as key steps.
New Horner-Emmons reagents, ethyl (diarylphosphono)acetates 1, were prepared from triethyl phosphonoacetate, PCl(5), and the corresponding phenols. The reaction of 1 with several kinds of aldehydes in the presence of Triton B or NaH in THF solvent revealed that these reagents are useful for the synthesis of Z-unsaturated esters. Among the reagents examined, ethyl(di-o-tolylphosphono)-, [bis(o-ethylphenyl)phosphono]-, and [bis(o-isopropylphenyl)phosphono]acetates (1k-m) were found to be the most effective, giving Z-unsaturated esters with 93-99% selectivity.
[reaction: see text] An efficient, user-friendly procedure for the oxidation of alcohols using IBX is described. Simply heating a solution of the alcohol in the presence of suspended IBX followed by filtration and removal of the solvent gives excellent yields of the corresponding carbonyl compounds. We illustrate this procedure with a panel of primary and secondary alcohol substrates and note that it allows recycling and reuse of the oxidant.
[reaction: see text] The diastereoselectivity of the Heck arylation of several chiral, nonracemic, five-membered endocyclic enecarbamates with aryldiazonium tetrafluoroborates was evaluated. The cis selectivity observed for some enecarbamates bearing coordinating groups was explored in the concise synthesis of the (2S,5R)-(+)-phenylproline methyl ester, a scaffold for the nonpeptide cholecystokinin antagonist (+)-RP 66803, and in the synthesis of Schramm's potent antiprotozoan C-azanucleoside.
An improved procedure for the synthesis of anti aldols from protected erythrulose derivatives is reported. The preparation of functionalized d3 and d4 synthons with various stereochemical arrays by means of this methodology is described and subsequently applied to a stereoselective formal synthesis of the natural metabolite goniothalesdiol.