Chemistry of the chippiine/dippinine/tronocarpine class of indole alkaloids.

The chippiines/dippinines/tronocarpine are a family of biologically and structurally interesting polycyclic tryptamine-derived indole alkaloids isolated from the leaf and bark extracts of plants belonging to the Tabernaemontana genus. To date, 14 members of this family have been isolated and characterized. This review discusses the isolation, structure determination, biological activity, and proposed biosynthesis of these metabolites. In addition, synthetic studies on the alkaloids are described including approaches to tronocarpine and dippinine B core intermediates and total syntheses of (+)-dippinine B and (+)-tronocarpine.

The Alstoscholarisine Alkaloids: Isolation, Structure Determination, Biogenesis, Biological Evaluation, and Synthesis.

The alstoscholarisines are a small family of biologically and structurally interesting polycyclic monoterpenoid indole alkaloids isolated from the leaf extracts of Alstonia scholaris. The alkaloids can be divided into three different subtypes based upon their structures and putative biogenesis: (1) (-)-alstoscholarisines A-E, (2) (+)-alstoscholarisine G, and (3) (+)-alstoscholarisines H-J. This review discusses the isolation, structure determination, biological activity, and proposed biosynthesis of these metabolites. In addition, synthetic studies on the alkaloids are described including total syntheses of racemic alstoscholarisines A-E, a total synthesis of (-)-alstoscholarisine A, and a synthesis of racemic alstoscholarisine H.

Synthesis of Alstoscholarisines A-E, Monoterpene Indole Alkaloids with Modulating Effects on Neural Stem Cells.

A divergent synthetic strategy has been developed for stereoselective total syntheses of alstoscholarisines A-E, monoterpenoid indole alkaloids which are modulators of adult neuronal stem cells. A pivotal step includes an intermolecular Michael addition of an indole-2-acetic acid methyl ester enolate to an α,ß-unsaturated N-sulfonyllactam to form the C15, C16 bond of the alkaloids. Other features of the strategy involve a selective partial reduction of an intermediate N-sulfonyllactam followed by cyclization to a bridged aminal system that serves as a key precursor for all five of the alkaloids as well as the use of an allyl group as a masked aldehyde equivalent.

Total Syntheses of the Monoterpenoid Indole Alkaloids (±)-Alstoscholarisine†B and C.

Total syntheses of the monoterpenoid indole alkaloids (±)-alstoscholarisine B and C were accomplished starting from a readily available indole-2-acetic ester and an α,ß-unsaturated N-sulfonyllactam.

Synthesis of the tetracyclic skeleton of the Lycopodium alkaloid lycopladine H via a pivotal double hydroformylation/intramolecular reductive amination sequence.

A synthesis of the complete tetracyclic framework of the structurally unique Lycopodium alkaloid lycopladine H has been accomplished using a strategy involving a double alkene hydroformylation/intramolecular reductive amination to form the azocane and spiro-piperidine moieties of the natural product.

Construction of the Myrioneuron alkaloids: a total synthesis of (±)-myrioneurinol.

A strategy has been developed that culminated in a stereoselective total synthesis of the tetracyclic antimalarial Myrioneuron alkaloid myrioneurinol. The synthesis relies on three highly diastereoselective reactions, including an intramolecular chelation-controlled Michael spirocyclization of an N-Cbz-lactam titanium enolate to an α,ß-unsaturated ester for construction of the A/D-ring system and the attendant C5 (quaternary), C6 relative stereochemistry; a malonate enolate conjugate addition to a nitrosoalkene in order to install the appropriate functionality and establish the configuration at C7; and an intramolecular aza-Sakurai reaction to form the B-ring and the accompanying C9 and C10 stereocenters.

Total synthesis of the tetracyclic antimalarial alkaloid (±)-myrioneurinol.

The first total synthesis of the tetracyclic antimalarial Myrioneuron alkaloid (±)-myrioneurinol has been accomplished using three highly diastereoselective reactions as pivotal steps: 1) an intramolecular Michael addition of a benzyloxycarbonyl-protected lactam titanium enolate to an α,ß-unsaturated ester for construction of the spirocyclic C5 quaternary center and the a/d rings, 2) a malonate anion conjugate addition to a transient nitrosoalkene to install the requisite functionality and configuration at the C7 position, and 3) an intramolecular sulfonyliminium aza-Sakurai reaction to form the b ring and the attendant C9/C10 configuration of the natural product.

Convergent approach to the tetracyclic core of the apparicine class of indole alkaloids via a key intermolecular nitrosoalkene conjugate addition.

Readily available methyl 3-formylindol-2-ylacetate and N-tosyl-4-chloro-3-piperidone oxime have been used to construct the tetracyclic skeleton of the apparicine class of monoterpene indole alkaloids in only four steps in 80% overall yield. Key transformations in this convergent approach involve use of an intermolecular ester enolate/nitrosoalkene conjugate addition to form the C-15/16 bond, followed by a reductive cyclization to construct the C-ring of the tetracycle.

Total syntheses of the monoterpene indole alkaloids (±)-alstilobanine A and E and (±)-angustilodine.

A synthetic strategy has been developed culminating in stereoselective total syntheses of the small class of unusual monoterpenoid indole alkaloids exemplified by alstilobanines A (3) and E (2) and angustilodine (1). A pivotal step includes a novel intermolecular Michael-type addition of an indole ester dianion to a piperidine-derived nitrosoalkene to form the C15, C16 bond of the alkaloids. In addition, an application of the Romo protocol for effecting a stereoselective intramolecular nucleophile-assisted aldol-lactonization was employed, leading to a ß-lactone incorporating the requisite cis-fused 2-azadecalin moiety and also setting the C15, C19, C20 relative stereochemistry of the metabolites. It was then possible to stereoselectively effect an aldolization of a dianion derived from this indole ester ß-lactone intermediate with formaldehyde to introduce the requisite C16 hydroxymethyl group. Further manipulations of the system ultimately led to the three alkaloids in racemic form.

Construction of the azocane (azacyclooctane) moiety of the Lycopodium alkaloid lycopladine H via an intramolecular hydroaminomethylation strategy.

An efficient synthetic strategy has been developed for annulation of an azocane ring onto a bicyclo[2.2.2]octane scaffold via an intramolecular hydroaminomethylation protocol to generate an advanced intermediate bearing three of the four rings of the structurally unique Lycopodium alkaloid lycopladine H (1).

Stereochemical investigation of conjugate additions of carbon- and heteronucleophiles to ring-substituted nitrosocyclohexenes.

Intermolecular Michael-type conjugate additions of some in situ-generated ring-substituted nitrosocyclohexenes with both carbon- and heteronucleophiles have been found to be highly stereoselective, leading predominantly (or exclusively) to products resulting from axial attack on a half-chair conformation of the nitrosoalkene substrate.

Exploratory studies towards a total synthesis of the unusual bridged tetracyclic Lycopodium alkaloid lycopladine H.

A strategy for a total synthesis of the structurally novel Lycopodium alkaloid lycopladine H has been investigated. Key steps that have been tested include: 1.a regioselective Diels-Alder cycloaddition of nitroethylene with an o-quinone ketal to produce the bicyclo[2.2.2]octane moiety of the alkaloid; 2. a stereoselective Henry reaction to generate the requisite functionality and configuration at C-5; 3. a stereoselective catalytic hydrogenation of a trisubstituted alkene to set the C-15 methyl configuration.

Further studies of intramolecular Michael reactions of nitrosoalkenes for construction of functionalized bridged ring systems.

A wide variety of stabilized carbanions have been found to participate as nucleophiles in intramolecular Michael-type conjugate additions to in situ generated nitrosoalkenes to form bridged carbocyclic systems. The vinylnitroso platforms for these cyclizations have been prepared via two key steps involving ring-closing metathesis of vinyl chlorides and regioselective conversion of vinyl chlorides to α-chloroketones with sodium hypochlorite in glacial acetic acid/acetone. An alternative approach to preparation of some cyclization substrates has involved use of more reactive enol ethers as precursors to the requisite α-chloroketones. A sulfonamide anion has also been found to be an effective nucleophile in this type of reaction, leading to formation of a 6-azabicyclo[3.2.1]octane.

Investigation of the stereochemistry of intermolecular conjugate additions of nucleophiles to acyclic nitrosoalkenes.

Michael-type conjugate additions of γ-chiral aldehyde-derived acyclic nitrosoalkenes have been explored using a series of carbon and hetero nucleophiles. In all cases examined, these reactions are stereoselective, leading exclusively to the anti products.

An expedient reductive method for conversion of ketoximes to the corresponding carbonyl compounds.

A wide array of readily prepared pivalates of ketoximes can be converted to the corresponding ketones in good yields by treatment with iron powder in THF containing catalytic amounts of both trimethylsilyl chloride and glacial acetic acid at room temperature for 30 minutes, followed by a brief aqueous workup.

Efficient methodology for alkylation of vinylnitroso compounds with carbon nucleophiles.

A diverse array of nitrosoalkenes derived from both acyclic and cyclic ketones, as well as aldehydes, via the Denmark protocol using alpha-chloro-O-TBS-oximes can be trapped efficiently in situ by a wide variety of potassium ester enolates to afford conjugate addition products in good yields.

Evolution of a strategy for total synthesis of the marine fungal alkaloid (+/-)-communesin F.

A new synthetic strategy for construction of the heptacyclic marine fungal alkaloid (+/-)-communesin F has been devised. Key reactions include an intramolecular Heck cyclization of a tetrasubstituted alkene to generate a tetracyclic enamide bearing one of the quaternary carbon centers (C7) of the alkaloid, an intramolecular reductive cyclization of an N-Boc aniline onto the oxindole moiety to form a pentacyclic framework containing the southern aminal, a stereoselective N-Boc-lactam enolate C-allylation to introduce the second quaternary carbon center (C8), and an azide reduction/N-Boc-lactam-opening cascade leading to the northern aminal.