A Two-Directional Synthesis of (±)-Perhydrohistrionicotoxin

Enantioselective Synthesis of Cyclic Nitrones by Chemoselective Intramolecular Allylic Alkylation of Oximes

The enantio- and chemoselective iridium-catalyzed N-allylation of oximes is described for the first time. Intramolecular kinetic resolution provides access to cyclic nitrones and enantioenriched aliphatic allylic alcohols. Salient features of this transformation are its ability to employ E/Z-isomeric mixtures of oxime starting materials convergently and high functional group tolerance. The implementation of N-allylation/1,3-dipolar cycloaddition reaction sequences furnishes tricyclic isoxazolidines in highly enantio- and diastereoselective fashion. The synthetic utility of the approach is demonstrated by the efficient, formal synthesis of the marine natural product (+)-halichlorine.

E- and Z-, di- and tri-substituted alkenyl nitriles through catalytic cross-metathesis

Nitriles are found in many bioactive compounds, and are among the most versatile functional groups in organic chemistry. Despite many notable recent advances, however, there are no approaches that may be used for preparation of di- or trisubstituted alkenyl nitriles. Related approaches which are broad in scope and can deliver the desired products in high stereoisomeric purity are especially scarce. Here, we describe the development of several efficient catalytic cross-metathesis strategies, which provide direct access to a considerable range of Z- or E-disubstituted cyano-substituted alkenes or their corresponding trisubstituted variants. Depending on the reaction type, a molybdenum-based monoaryloxide pyrrolide (MAP) or chloride (MAC) complex may be the optimal choice. The utility of the approach, enhanced by an easy-to-apply protocol for utilization of substrates bearing an alcohol or a carboxylic acid moiety, is highlighted in the context of applications to synthesis of biologically active compounds.

Studies on the formal [3 + 2] cycloaddition of aziridines with alkenes for the synthesis of 1-azaspiroalkanes

The Lewis acid-mediated [3 + 2] cycloaddition of N-sulfonyl- and N-sulfamoylaziridines with alkenes provides a rapid and efficient access to 1-azaspiro[4.n]alkanes. Experimental studies have been combined with DFT calculations to explore the mechanism of the reaction. They demonstrate that the nature of the electron-withdrawing nitrogen protecting group has a very limited influence on the course of the reaction and, particularly, on the initial formation of the 1,3-zwitterionic species through C-N bond cleavage, which has been found to be the rate-determining step. Compared to N-sulfonylaziridines, N-sulfamoylaziridines have proved to be more synthetically useful synthons that afford crystalline polycyclic structures in good yields. A short sequence of catalytic C(sp(3))-H amination-cyclization-[3 + 2] cycloaddition has then been successfully designed to afford the homologue 1-azaspiro[5.n]alkanes, thereby illustrating the higher versatility of sulfamates in these cycloadditions.

Copper-catalyzed oxidative dimerizations of 3-N-hydroxy-aminoprop-1-enes to form 1,4-dihydroxy-2,3-diaminocyclohexanes with C2 symmetry

This work describes the one-step construction of complex and important molecular frameworks through copper-catalyzed oxidations of cheap tertiary amines. Copper-catalyzed aerobic oxidations of N-hydroxyaminopropenes to form C2 -symmetric N- and O-functionalized cyclohexanes are described. Such catalytic oxidations proceed with remarkable stereocontrol and high efficiency. Reductive cleavage of the two NO bonds of these products delivers 1,4-dihydroxy-2,3-diaminocyclohexanes, which are important skeletons of several bioactive molecules.

Causation in a cascade: the origins of selectivities in intramolecular nitrone cycloadditions

The factors controlling chemo-, regio-, and stereoselectivity in a cascade of reactions starting from a bis(cyanoalkenyl)oxime and proceeding via nitrone cycloadditions have been unraveled through a series of density functional theory calculations with several different functionals. Both kinetic and thermodynamic control of the reaction cascade are important, depending upon the conditions. Kinetic control was analyzed by the distortion/interaction model and found to be dictated by differences in distortions of the cycloaddends in the transition states. A new mechanism competing with that originally proposed in the application of these reactions to the histrionicotoxin synthesis was discovered in these studies.

Two-directional synthesis as a tool for diversity-oriented synthesis: Synthesis of alkaloid scaffolds

Two-directional synthesis represents an ideal strategy for the rapid elaboration of simple starting materials and their subsequent transformation into complex molecular architectures. As such, it is becoming recognised as an enabling technology for diversity-oriented synthesis. Herein, we provide a thorough account of our work combining two-directional synthesis with diversity-oriented synthesis, with particular reference to the synthesis of polycyclic alkaloid scaffolds.

A gram-scale batch and flow total synthesis of perhydrohistrionicotoxin

The total synthesis of the spiropiperidine alkaloid (-)-perhydrohistrionicotoxin (perhydro-HTX) 2 has been accomplished on a gram scale by employing both conventional batch chemistry as well as microreactor techniques. (S)-(-)-6-Pentyltetrahydro-pyran-2-one 8 underwent nucleophilic ring opening to afford the alcohol 10, which was elaborated to the nitrone 13. Protection of the nitrone as the 1,3-adduct of styrene and side-chain extension to the unsaturated nitrile afforded a precursor 17, which underwent dipolar cycloreversion and 1,3-dipolar cycloaddition to give the core spirocyclic precursor 18 that was converted into perhydro-HTX 2. The principal steps to the spirocycle 18 have successfully been transferred into flow mode by using different types of microreactors and in a telescoped fashion, allowing for a more rapid access to the histrionicotoxins and their analogues by continuous processing.

Stereoselective formation of fused tricyclic amines from acyclic aldehydes by a cascade process involving condensation, cyclization, and dipolar cycloaddition

The preparation of tricyclic amines from acyclic precursors is described using a cascade of tandem reactions involving condensation of an aldehyde with a primary amine, cyclization (with displacement of a halide), and then in situ deprotonation or decarboxylation to give an azomethine ylide or nitrone followed by intramolecular dipolar cycloaddition. The methodology is straightforward, and the aldehyde precursors are prepared easily and quickly in high yield using nitrile alkylations followed by DIBAL-H reduction. The relative ease of reaction of various substrates with different tether lengths between the aldehyde and the halide or dipolarophile has been studied. Several primary amines including simple amino acids such as glycine, alanine, and phenylalanine and derivatives such as glycine ethyl ester and also hydroxylamine have been investigated. High yields are obtained in the formation of different tricyclic ring sizes; the dipolar cycloaddition necessarily creates a five-membered ring, and we have investigated the formation of five- and six-membered rings for the other two new ring sizes. In all cases, yields are high (except when using glycine when the tether to the terminal alkene dipolarophile leads to a six-membered ring), and most efficient is the formation of the tricyclic product in which all five-membered rings are formed. Examples with an alkyne as the dipolarophile were also successful. In all the reactions studied, the products are formed with complete regioselectivity and remarkably with complete stereoselectivity. The key step involves the formation of three new rings and potentially up to four new stereocenters in a single transformation. The power of the chemistry was demonstrated by the synthesis of the core ring systems of the alkaloids (+/-)-scandine and (+/-)-myrioneurinol and the total syntheses of the alkaloids (+/-)-aspidospermine, (+/-)-quebrachamine, and (+/-)-aspidospermidine.

A regio- and diastereoselective intramolecular nitrone cycloaddition for practical 3- and 2,3-disubstituted piperidine synthesis from gamma-butyrolactone

A fast and efficient route for diversity-oriented synthesis of 3- and 2,3-disubstituted piperidines, featuring an intramolecular nitrone cycloaddition with high regio- and diastereoselectivity, was achieved in six steps and 36-66% overall yield from commercially available gamma-butyrolactone or 1,4-butanediol. A new N-alkenyl nitrone enoate was used in this intramolecular nitrone cycloaddition, and the regioselectivity, diastereoselectivity, and reversibility of this cycloaddition were investigated.

Thirty-five years of synthetic studies directed towards the histrionicotoxin family of alkaloids

This article brings together for the first time reviews of all the synthetic attempts towards the spirocyclic histrionicotoxin alkaloids published since the discovery of the group in 1971. This covers 5 total syntheses of the fully unsaturated parent alkaloid HTX-283A, 7 total syntheses of perhydrohistrionicotoxin, 15 total syntheses of other members of this alkaloid family, 25 formal syntheses, and 19 partial syntheses involving the successful formation of the core azaspirocyclic structure but lacking advancement towards the target structure.

Verticilide: Elucidation of Absolute Configuration and Total Synthesis

Verticilide (1) is a 24-membered cyclic depsipeptide isolated from the culture broth of Verticillium sp. FKI-1033. It inhibits ryanodine binding to ryanodine receptor (RyR) and has insecticidal activity. The stereochemistry of 2-hydroxyheptanoic acid in verticilide was elucidated by chiral HPLC analysis of the degradation product 6 and synthetic (+) and (-)-6. We also describe the practical total synthesis of verticilide. [reaction: see text].

A Concise Total Synthesis of dl-Histrionicotoxin

The synthesis of (+/-)-histrionicotoxin has been achieved in just nine steps using a two-directional synthesis strategy. Key reactions include a two-directional cross-metathesis, a tandem oxime formation/Michael addition/1,4-prototopic shift/[3 + 2]-cycloaddition cascade, a selective Z,Z-bisenyne formation, and a one-pot N-O and bischloroacetylene reduction.

Assembly of the 1-Azaspiro[5.5]undecane Framework Associated with Perhydrohistrionicotoxin via Electrocyclic Ring-Opening of a Ring-Fused gem-Dichlorocyclopropane and Trapping of the Resulting π-Allyl Cation by a Tethered, Nitrogen-Centered Nucleophile

The carbamate-tethered gem-dichlorocyclopropane 27 was prepared, as a mixture of epimers, in ten steps from commercially available β,γ-unsaturated nitrile 9. Upon treatment with silver acetate under a range of reaction conditions, compound 27 underwent electrocyclic ring opening to give the corresponding π-allyl cation that was then trapped by the reaction solvent, chloride ion, and/or acetate ion, and so affording varying mixtures of the chlorocyclohexenes 28, 29, 30, and/or 31. Sequential treatment of the same substrate with LiHMDS (to generate the conjugate base of this carbamate) then silver tetrafluoroborate afforded the chlorocyclohexadiene 32 as the exclusive product of reaction. No spirocyclization product of the type 3 arising from trapping of the intermediate π-allyl cation by the tethered carbamate was observed under any of the reaction conditions examined. In contrast, analogous treatment of the more rigid system 38 afforded compound 39 incorporating the 1-azaspiro[5.5]undecane framework associated with the potent neurotoxin perhydrohistrionicotoxin (2).

S-(3-Chloro-2-oxo-propyl)-O-ethyl xanthate: a linchpin radical coupling agent for the synthesis of heterocyclic and polycyclic compounds

The preparation and use of S-(3-chloro-2-oxo-propyl)-O-ethyl xanthate to directly introduce an alpha-chloroketone motif is described, along with applications for the synthesis of heterocyclic and polycyclic structures.

Formal Synthesis of (−)-Perhydrohistrionicotoxin via Cyclic Amino Acid Ester−Enolate Claisen Rearrangement and Ring-Closing Metathesis

[reaction: see text] We have successfully synthesized an advanced synthetic intermediate, hydroxy lactam 3, which has previously been converted to perhydrohistrionicotoxin. An important feature of this synthesis is the creation of stereogenic centers by using the cyclic amino acid ester-enolate Claisen rearrangement together with a ring-closing metathesis for azaspirocyclic skeleton construction.

Cycloaddition Chemistry of 2-Vinyl-Substituted Indoles and Related Heteroaromatic Systems

[reaction: see text] The intramolecular Diels-Alder cycloaddition reaction (IMDAF) of several N-phenylsulfonylindolyl-substituted furanyl carbamates containing a tethered pi-bond on the indole ring were examined as an approach to the iboga alkaloid catharanthine. Only in the case where the tethered pi-bond contained two carbomethoxy groups did the [4 + 2]-cycloaddition occur. Push-pull dipoles generated from the Rh(II)-catalyzed reaction of diazo imides, on the other hand, undergo successful intramolecular 1,3-dipolar cycloaddition across both alkenyl and heteroaromatic pi-bonds to provide novel pentacyclic compounds in good yield and in a stereocontrolled fashion. The facility of the cycloaddition was found to be critically dependent on conformational factors in the transition state. Ligand substitution in the rhodium(II) catalyst markedly altered the product ratio between [3 + 2]-cycloaddition and intramolecular C-H insertion. The variation in reactivity reflects the difference in electrophilicity between the various rhodium carbenoid intermediates. Intramolecular C-H insertion is enhanced with the more electrophilic carbene generated using Rh(II) perfluorobutyrate.

Combining two-directional synthesis and tandem reactions: desymmetrisation by intramolecular cycloaddition/triazoline fragmentation

A tandem azide formation/intramolecular cycloaddition/triazoline fragmentation/Michael addition, which results in a non-symmetrical quinolizidine from an acyclic symmetrical precursor, is presented.

Combining Two-Directional Synthesis and Tandem Reactions: Synthesis of Trioxadispiroketals

A tandem bromonium ion-promoted cyclization of two pendant hydroxyl groups onto a central furan core provides a highly direct route to both the [5,5,5]- and the [6,5,6]-trioxadispiroketal ring systems.