Rh(I)-Catalyzed Ring Opening of an IMDAF-Derived Oxabicyclo Cycloadduct as the Key Step in the Synthesis of (±)-epi-Zephyranthine

Alkynes as Privileged Synthons in Selected Organic Name Reactions

BACKGROUND

Alkynes are actually basic chemicals, serving as privileged synthons for planning new organic reactions for assemblage of a reactive motif, which easily undergoes a further desirable transformation. Name reactions, in organic chemistry are referred to those reactions which are well-recognized and reached to such status for being called as their explorers, discoverers or developers. Alkynes have been used in various name reactions. In this review, we try to underscore the applications of alkynes as privileged synthons in prevalent name reactions such as Huisgen 1,3-dipolar cycloaddtion via Click reaction, Sonogashira reaction, and Hetero Diels-Alder reaction.

OBJECTIVE

In this review, we try to underscore the applications of alkynes as privileged synthons in the formation of heterocycles, focused on the selected reactions of alkynes as a synthon or impending utilization in synthetic organic chemistry, which have reached such high status for being included in the list of name reactions in organic chemistry.

CONCLUSION

Alkynes (including acetylene) are an unsaturated hydrocarbon bearing one or more triple C-C bond. Remarkably, alkynes and their derivatives are frequently being used as molecular scaffolds for planning new organic reactions and installing reactive functional group for further reaction. It is worth mentioning that in general, the terminal alkynes are more useful and more frequently being used in the art of organic synthesis. Remarkably, alkynes have found different applications in pharmacology, nanotechnology, as well as being known as appropriate starting precursors for the total synthesis of natural products and biologically active complex compounds. They are predominantly applied in various name reactions such as Sonogashira, Glaser reaction, Friedel-crafts reaction, Castro-Stephens coupling, Huisgen 1.3-dipolar cycloaddtion reaction via Click reaction, Sonogashira reaction, hetero-Diels-Alder reaction. In this review, we tried to impress the readers by presenting selected name reactions, which use the alkynes as either stating materials or precursors. We disclosed the applications of alkynes as a privileged synthons in several popular reactions, which reached to such high status being classified as name reactions. They are thriving and well known and established name reactions in organic chemistry such as Regioselective, 1,3-dipolar Huisgen cycloaddtion reaction via Click reaction, Sonogashira reaction and Diels-Alder reaction.

Asymmetric Total Syntheses of (-)-α-Lycorane, (-)-Zephyranthine, and Formal Synthesis of (+)-Clivonine

An asymmetric route to (-)-α-lycorane and (-)-zephyranthine, and a formal total synthesis of (+)-clivonine were achieved. A pivotal intermediate, which serves as a potent precursor for the divergent syntheses of these natural products, was accessed by a diastereoselective Pd-catalyzed cinnamylation of an N-tert-butanesulfinyl imine.

Gold-Catalyzed Cyclization of Furan-Ynes bearing a Propargyl Carbonate Group: Intramolecular Diels-Alder Reaction with In Situ Generated Allenes

Gold-catalyzed cyclization of various furan-ynes with a propargyl carbonate or ester moiety results in the formation of a series of polycyclic aromatic ring systems. The reactions can be rationalized through a tandem gold-catalyzed 3,3-rearrangement of the propargyl carboxylate moiety in furan-yne substrates to form an allenic intermediate, which is followed by an intramolecular Diels-Alder reaction of furan and subsequent ring-opening of the oxa-bridged cycloadduct. It was found that the steric and electronic properties of phosphine ligands on the gold catalyst had a significant impact on the reaction outcome. In the case of 1,5-furan-yne, the cleavage of the oxa-bridge in the cycloadduct with concomitant 1,2-migration of the R(1) group occurs to furnish anthracen-1(2H)-ones bearing a quaternary carbon center. For 1,4-furan-yne, a facile aromatization of the cycloadduct takes place to give 9-oxygenated anthracene derivatives.

Synthesis of hydronaphthalenes through coupling of enyne carbonyl compounds that contain pendant alkene groups with Fischer carbene complexes

The coupling of enyne carbonyl compounds that contain pendant alkene groups with Fischer carbene complexes to afford furans that contain pendant alkene groups is described. Subsequent intramolecular Diels-Alder reactions are effective in select cases, resulting in hydronaphthalene systems after dehydration. Although the Diels-Alder event is thermodynamically unfavorable, the overall transformation of alkene-furans to dihydronaphthalenes is a favorable process.

Cycloaddition across the benzofuran ring as an approach to the morphine alkaloids

The intramolecular Diels-Alder reaction of several amidofurans tethered onto a benzofuran ring was examined as a strategy for the synthesis of morphine. Bromo substitution on the furan ring did not provide sufficient activation to allow the cycloaddition to take place across the aromatic benzofuran. However, the presence of a large o-methylbenzyl group on the amido nitrogen atom causes the reactive s-trans conformation of the amidofuran to be highly populated, thereby facilitating its Diels-Alder cycloaddition across a tethered benzofuran.

A general synthetic entry to the pentacyclic strychnos alkaloid family, using a [4 + 2]-cycloaddition/rearrangement cascade sequence

The total synthesis of (+/-)-strychnopivotine, (+/-)-tubifolidine, (+/-)-strychnine, and (+/-)-valparicine is reported. The central step in the synthesis consists of an intramolecular [4 + 2]-cycloaddition/rearrangement cascade of an indolyl-substituted amidofuran that delivers an aza-tetracyclic substructure containing the ABCE-rings of the Strychnos alkaloid family. A large substituent group on the amide nitrogen atom causes the reactive s- trans conformation of the amidofuran to be more highly populated, thereby facilitating the Diels-Alder cycloaddition. The reaction also requires the presence of an electron-withdrawing substituent on the indole nitrogen for the cycloaddition to proceed. The cycloaddition/rearrangement cascade was remarkably efficient given that two heteroaromatic systems are compromised in the reaction. Closure to the remaining D-ring of the Strychnos skeleton was carried out from the aza-tetracyclic intermediate by an intramolecular palladium-catalyzed enolate-driven cross-coupling between the N-tethered vinyl iodide and the keto functionality. The cycloaddition/rearrangement approach was successfully applied to (+/-)-strychnopivotine (2), the only Strychnos alkaloid bearing a 2-acylindoline moiety in its pentacyclic framework. A variation of this tactic was then utilized for a synthesis of the heptacyclic framework of (+/-)-strychnine. The total synthesis of (+/-)-strychnine required only 13 steps from furanyl indole 18 and proceeded in an overall yield of 4.4%.

Amaryllidaceae and Sceletium alkaloids

A great number of natural products, especially alkaloids, which exhibit a range of biological activities including acetylcholinesterase inhibition and antineoplastic, cardiovascular and immunostimulatory activities, have been isolated from the plants of the Amaryllidaceae family. this review summarizes isolation, biological activity, and synthetic studies of these alkaloids. The primary biosynthetic pathways of each type of alkaloids are also proposed.

Rhodium(I)-catalyzed nucleophilic ring-opening reactions of oxabicyclo adducts derived from the [4 + 2]-cycloaddition of 2-imido-substituted furans

A series of 2-imido-substituted furans containing tethered unsaturation were prepared by the addition of the lithium carbamate of furan-2-ylcarbamic acid tert-butyl ester to a solution of the mixed anhydride of an appropriately substituted 3-butenoic acid. The initially formed imido furans undergo a rapid intramolecular [4 + 2]-cycloaddition at room temperature to deliver the Diels-Alder cycloadducts in good to excellent yield. Isolation of the highly labile oxabicyclic adduct is believed to be a consequence of the lower reaction temperatures employed as well as the presence of the extra carbonyl group, which diminishes the basicity of the nitrogen atom, thereby retarding the ring cleavage/rearrangement reaction generally encountered with related systems. By using a Rh(I)-catalyzed ring opening of the oxabicyclic adduct with various nucleophilic reagents, it was possible to prepare highly functionalized hexahydro-1H-indol-2(3H)-one derivatives in good yield. The major stereoisomer obtained possesses a cis-relationship between the nucleophile and hydroxyl group in the ring-opened product. The stereochemistry was unequivocally established by X-ray crystallographic analysis. Coordination of Rh(I) to the alkenyl pi-bond followed by a nitrogen-assisted cleavage of the carbon-oxygen bond occurs to furnish a pi-allyl rhodium(III) species. Addition of the nucleophile then occurs from the least hindered terminus of the resulting pi-allyl rhodium(III) complex. Proton exchange followed by rhodium(I) decomplexation ultimately leads to the cis-diastereomer.

Synthesis of some members of the hydroxylated phenanthridone subclass of the Amaryllidaceae alkaloid family

The total synthesis of several members of the hydroxylated phenanthridone subclass of the Amaryllidaceae alkaloid family has been carried out. (+/-)-Lycoricidine and (+/-)-7-deoxypancratistatin were assembled through a one-pot Stille/intramolecular Diels-Alder cycloaddition cascade to construct the core skeleton. The initially formed [4+2]-cycloadduct undergoes nitrogen-assisted ring opening followed by a deprotonation/reprotonation of the resulting zwitterion to give a rearranged hexahydroindolinone on further heating at 160 degrees C. The stereochemical outcome of the IMDAF cycloaddition has the side arm of the tethered vinyl group oriented exo with respect to the oxygen bridge. The resulting cycloadduct was used for the stereocontrolled installation of the remaining functionality present in the C-ring of the target molecules. Key features of the synthetic strategy include (1) a lithium hydroxide induced tandem hydrolysis/decarboxylation/elimination sequence to introduce the required pi-bond in the C-ring of (+/-)-lycoricidine, and (2) conversion of the initially formed Diels-Alder adduct into an aldehyde intermediate which then undergoes a stereospecific decarbonylation reaction mediated by Wilkinson's catalyst to set the trans-B-C ring junction of (+/-)-7-deoxypancratistatin.

Total synthesis of (+/-)-strychnine via a [4 + 2]-cycloaddition/rearrangement cascade

A new strategy for the synthesis of the Strychnos alkaloid (+/-)-strychnine has been developed and is based on an intramolecular [4 + 2]-cycloaddition/rearrangement cascade of an indolyl-substituted amidofuran. The critical D-ring was assembled by an intramolecular palladium-catalyzed enolate-driven cross-coupling of an N-tethered vinyl iodide. [reaction: see text].

Application of the aza-Achmatowicz oxidative rearrangement for the stereoselective synthesis of the Cassia and Prosopis alkaloid family

cis-2-Methyl-6-substituted piperidin-3-ol alkaloids of the Cassia and Prosopis species are readily prepared by a combination of an aza-Achmatowicz oxidative rearrangement and dihydropyridone reduction followed by a stereoselective allylsilane addition to a N-sulfonyliminium ion. The stereochemical outcome of the reduction reaction can be attributed to steric hindrance between the pseudoaxially oriented 2,6-substituents and the equatorially approaching hydride reagent which explains the exclusive formation of the cis-alcohol by axial approach of the hydride. The unsaturation present in the (E)-methyl-pent-3-enoate side chain was removed by catalytic reduction, and the remaining ester group was converted to the corresponding Weinreb's amide. This key intermediate was utilized for the synthesis of azimic acid, deoxocassine, cassine, and spicigerine. The facile preparation of (S)-N-tosylamidofuran 16 and its conversion to the chiral Achmatowicz oxidation product 18 provide a formal chiral synthesis of these alkaloids.

Synthesis of the erythrina alkaloid 3-demethoxyerythratidinone. Novel acid-induced rearrangements of its precursors

[reaction: see text] A new strategy for the synthesis of 3-demethoxyerythratidinone has been developed and is based on an extraordinarily facile intramolecular Diels-Alder reaction of a 2-imido-substituted furan. During the course of the synthesis, several novel acid-induced rearrangement reactions were encountered.

An efficient synthesis of (+/-)-Lycoricidine featuring a Stille-IMDAF cycloaddition cascade

[reaction: see text] A highly efficient total synthesis of (+/-)-Lycoricidine is described. The synthesis features the ready preparation of the Lycoricidine skeleton by a Stille-IMDAF cycloaddition cascade. The resulting cycloadduct is then used for the stereocontrolled installation of the other functionality present in the C-ring of the target molecule.

Chemical and biological aspects of Narcissus alkaloids

This chapter discusses the chemical and biological aspects of Narcissus alkaloids. Numerous alkaloids have been isolated from Narcissus speciesasaresult of the continuing search for novel alkaloids with pharmacological activity in the Amaryllidaceae family. The alkaloids isolated from this genus, classified in relation to the different skeleton types. The different Narcissus wild species and intersectional hybrids, grouped into subgenera and sections, with their corresponding alkaloids, arranged according to their ring system are listed. The biosynthetic pathways of Narcissus alkaloids includes: (1) enzymatic preparation of the precursors, (2) primary cyclization mechanisms, (3) enzymatic preparation of intermediates, (4) secondary cyclization, diversification, and restructuring. The chapter discusses proton nuclear magnetic resonance (1H NMR), carbon nuclear magnetic resonance (13C NMR), and mass spectrometry (MS) for Narcissus alkaloids. A list of the different Narcissus alkaloids, their spectroscopic properties, and literature with the most recent spectroscopic data is given. Several Narcissus extracts shows the following activities: antiviral, prophage induction, antibacterial, antifungal, antimalarial, insecticidal, cytotoxic, antitumor, antimitotic, antiplatelet, hypotensive, emetic, acetylcholine esterase inhibitory, antifertility, antinociceptive, chronotropic, pheromone, plant growth inhibitor, and allelopathic.

Chiral Template Mediated Diastereoselective Intramolecular Diels−Alder Reaction Using Furan as a Diene. Toward the Synthesis of Enantiopure Trisubstituted Tetrahydroepoxyisoindolones

[reaction: see text] Chiral precursors to the intramolecular Diels-Alder reaction using furan as a diene (IMDAF) have been prepared by diastereoselective addition of furyllithium to imines attached to perhydro-1,3-benzoxazines derived from (-)-8-aminomenthol. The acylation of the furylamines with alpha,beta-unsaturated acyl chlorides and subsequent IMDAF reaction under thermal conditions provided the corresponding cycloadducts as single diastereoisomers. The hydrolytic elimination of the chiral template yielded enantiopure perhydroepoxyisoindolones with up to five stereocenters.

Intramolecular Diels−Alder Reactions of Optically Active Allenic Ketones: Chirality Transfer in the Preparation of Substituted Oxa-Bridged Octalones

The intramolecular Diels-Alder reaction of allenic ketones containing a furyl unit (IMDAF) to generate oxatricyclic systems in good yields is described. The alkene dienophiles 1ab give poor yields of the cycloadducts 2ab, presumably due to the facile retro Diels-Alder reaction. However, the analogous allenic dienophile 7 afforded the desired cycloadduct 8 in 91% yield on treatment with dimethylaluminum chloride. When the allene bears an alkyl substituent on the terminal carbon, complete diastereoselectivity is seen in the IMDAF, e.g. cyclization of 14 gave only the cycloadduct 15 in 80% yield presumably due to greater steric hindrance in the transition state II as compared to that in I. Finally we report complete chirality transfer of the stereochemistry of an allene to the carbon framework of the oxatricyclic system. Thus, the optically active allenic ketone 20 afforded only the desired cycloadduct 21 with the correct absolute stereochemistry needed for the synthesis of the arisugacin class of natural products.

Studies on the Synthesis of (±)-Stenine: A Combined Intramolecular [4 + 2]-Cycloaddition/Rearrangement Cascade

Several cyclic 2-(methylthio)-5-amidofurans containing tethered unsaturation were prepared via the reaction of dimethyl(methylthio)sulfonium tetrafluoroborate (DMSTF) with beta-alkoxy-gamma-dithiane lactams. Thermolysis of these furans resulted in an intramolecular Diels-Alder reaction (IMDAF). The resulting oxa-bridge cycloadducts underwent a subsequent 1,2-methylthio shift to form tricyclic lactams in high yield. Furan 9, annealed to an azepine ring, underwent the IMDAF reaction at or below room temperature. Conformational effects imposed by the placement of a carbonyl group within the tether, combined with a rotational bias about the C(2)-N bond, enhances the rate of the IMDAF reaction of the seven-ring system so that it occurs readily at 25 degrees C. The feasibility of using the cascade sequence in the context of a total synthesis of the Stemona alkaloid (+/-)-stenine was explored. The eventual synthesis of (+/-)-stenine was carried out by an intramolecular Diels-Alder reaction of a 2-amido-5-methylthio-substituted furan containing a trans-pent-3-enoic acid methyl ester side chain in order to create the desired azepinoindole skeleton. This was followed by a series of reductions to set the syn-anti stereochemical relationship at the incipient ring fusion sites present in stenine. All six stereocenters at the azepinoindole core were derived in high stereoselectivity from the functionality present in the rearranged cycloadduct 10. Compound 10 was converted to stenine in 11 additional steps via a sequence that features a Crabtree's-catalyst directed hydrogenation, iodolactonization, and a Keck allylation.

A Stereoselective and Atom-Efficient Approach to Multifunctionalized Five- and Six-Membered Rings via a Novel Michael-Initiated Intramolecular Diels−Alder Furan Reaction

[reaction: see text] A variety of key precursors to the intramolecular Diels-Alder reaction of furan diene (IMDAF) have been prepared via a very facile 1,4-addition of O-, S-, N-, and C-centered nucleophiles possessing unsaturated tether to beta-furyl nitroethylene. Subsequent IMDAF reaction of the 1,4-adducts, carried out under thermal conditions, provided five- and six-membered carbocycles and heterocycles fused to an easily cleavable oxabicycloheptene moiety. The structure and stereochemistry of the cycloadducts were determined by 2D-NMR experiments and further confirmed by X-ray crystallography. The salient features of the strategy include high degree of stereoselectivity (>80:20) in the cycloaddition, atom and step economy, and generation of multiple chiral centers and functionalities. The feasibility of the cleavage of the oxa bridge in the cycloadducts to afford novel multifunctional molecules has also been demonstrated.