Enantioselective Pictet-Spengler-type cyclizations of hydroxylactams: H-bond donor catalysis by anion binding

Asymmetric Synthesis of (R)-1-Alkyl-Substituted Tetrahydro-ß-carbolines Catalyzed by Strictosidine Synthases

Stereoselective methods for the synthesis of tetrahydro-ß-carbolines are of significant interest due to the broad spectrum of biological activity of the target molecules. In the plant kingdom, strictosidine synthases catalyze the C-C coupling through a Pictet-Spengler reaction of tryptamine and secologanin to exclusively form the (S)-configured tetrahydro-ß-carboline (S)-strictosidine. Investigating the biocatalytic Pictet-Spengler reaction of tryptamine with small-molecular-weight aliphatic aldehydes revealed that the strictosidine synthases give unexpectedly access to the (R)-configured product. Developing an efficient expression method for the enzyme allowed the preparative transformation of various aldehydes, giving the products with up to >98 % ee. With this tool in hand, a chemoenzymatic two-step synthesis of (R)-harmicine was achieved, giving (R)-harmicine in 67 % overall yield in optically pure form.

Lewis Acid-Mediated Room-Temperature Cascade Reaction of 3-Hydroxyisoindolin-1-one with Alkynes

An efficient and mild synthesis of a variety of 3-(2-oxopropyl)-isoindolinone derivatives via a BF₃·Et₂O catalyzed cascade reaction among 3-hydroxyisoindolin-1-one and phenylacetylene was achieved. Various isoindolinone derivatives were obtained in good to excellent yields. The process, which avoided several drawbacks such as the requirement of concentrated protic acids and metal catalysts, protecting group of nitrogen, high temperature, and multistep synthesis, includes C(sp³)-OH cleavage, C-C coupling, and hydration of alkyne.

Anion binding by p-aminoazobenzene-derived aromatic amides: spectroscopic and electrochemical studies

The synthesis and complexing properties of p-aminoazobenzene-derived mono-, bis-, and trisamides were described. Ligands 3 and 4 bind anions, including fluorides, chlorides, bromides, acetates, benzoates, dihydrogen phosphates, hydrogen sulfates, and p-toluenesulfonates, in chloroform forming 1 : 1 complexes. The highest value of stability constant was evaluated for the 4-F^- complex (log K = 5.63 ± 0.21). On the basis of ¹H NMR, and FTIR spectroscopy, the possible nature of the ligand-anion interactions was proposed. The E ⇄ Z isomerization process of tripodal amide 4 in chloroform was studied. The effect of anions on Z to E thermal back isomerization was investigated.

Enantioselective Synthesis of Pyrroloindolines via Noncovalent Stabilization of Indole Radical Cations and Applications to the Synthesis of Alkaloid Natural Products

While interest in the synthetic chemistry of radical cations continues to grow, controlling enantioselectivity in the reactions of these intermediates remains a challenge. Based on recent insights into the oxidation of tryptophan in enzymatic systems, we report a photocatalytic method for the generation of indole radical cations as hydrogen-bonded adducts with chiral phosphate anions. These noncovalent open-shell complexes can be intercepted by the stable nitroxyl radical TEMPO· to form alkoxyamine-substituted pyrroloindolines with high levels of enantioselectivity. Further elaboration of these optically enriched adducts can be achieved via a catalytic single-electron oxidation/mesolytic cleavage sequence to furnish transient carbocation intermediates that may be intercepted by a wide range of nucleophiles. Taken together, this two-step sequence provides a simple catalytic method to access a wide range of substituted pyrroloindolines in enantioenriched form via a standard experimental protocol from a common synthetic intermediate. The design, development, mechanistic study, and scope of this process are presented, as are applications of this method to the synthesis of several dimeric pyrroloindoline natural products.

Alkaloid Synthesis Using Organocatalysts

This chapter covers the literature since the advent, in the 21st century, of total syntheses of alkaloids using enantio- or diastereoselective organocatalytic reactions to construct the alkaloid scaffolds. The details of these alkaloid syntheses are described separately for each basic skeleton, including indole, indoline, oxindole, and piperidine alkaloids.

Four-coordinated see-sawN-(aryl)-2-(propan-2-ylidene)hydrazinecarbothioamide complexes of nickel(ii), copper(ii) and zinc(ii) and their propensity for catalytic cyclisation

Four-coordinated metal complexes of two hydrazinecarbothioamide-derived ligands with divalent nickel, copper and zinc are described and the selective reactivity difference of copper(ii) to cyclise the ligand or to form a complex is described.

Chiral Thioureas Promote Enantioselective Pictet-Spengler Cyclization by Stabilizing Every Intermediate and Transition State in the Carboxylic Acid-Catalyzed Reaction

An investigation of the mechanism of benzoic acid/thiourea co-catalysis in the asymmetric Pictet-Spengler reaction is reported. Kinetic, computational, and structure-activity relationship studies provide evidence that rearomatization via deprotonation of the pentahydro-β-carbolinium ion intermediate by a chiral thiourea·carboxylate complex is both rate- and enantioselectivity-determining. The thiourea catalyst induces rate acceleration over the background reaction mediated by benzoic acid alone by stabilizing every intermediate and transition state leading up to and including the final selectivity-determining step. Distortion-interaction analyses of the transition structures for deprotonation predicted using density functional theory indicate that differential π-π and C-H···π interactions within a scaffold organized by multiple hydrogen bonds dictate stereoselectivity. The principles underlying rate acceleration and enantiocontrol described herein are expected to have general implications for the design of selective transformations involving deprotonation of high-energy intermediates.

Structural Effect of Thioureas on the Detection of Chemical Warfare Agent Simulants

The ability to rapidly detect, identify, and monitor chemical warfare agents (CWAs) is imperative for both military and civilian defense. Since most CWAs and their simulants have an organophosphonate group, which is a hydrogen (H)-bond acceptor, many H-bond donors have been developed to effectively bind to the organophosphonate group. Although thioureas have been actively studied as an organocatalyst, they are relatively less investigated in CWA detection. In addition, there is a lack of studies on the structure-property relationship for gas phase detection. In this study, we synthesized various thioureas of different chemical structures, and tested them for sensing dimethylmethylphosphonate (DMMP), a CWA simulant. Molecular interaction between DMMP and thiourea was measured by ¹H NMR titration and supported by density functional theory (DFT) calculations. Strong H-bond donor ability of thiourea may cause self-aggregation, and CH-π interaction can play an important role in the DMMP detection. Gas-phase adsorption of DMMP was also measured using a quartz crystal microbalance (QCM) and analyzed using the simple Langmuir isotherm, showing the importance of structure-induced morphology of thioureas on the surface.

Reductive Etherification via Anion-Binding Catalysis

Reductive condensations of alcohols with aldehydes/ketones to generate ethers are catalyzed by a readily accessible thiourea organocatalyst that operates in combination with HCl. 1,1,3,3-tetramethyldisiloxane serves as a convenient reducing reagent. This strategy is applicable to challenging substrate combinations and exhibits functional group tolerance. Competing reductive homocoupling of the carbonyl component is suppressed.

Phenolic Activation in Chiral Brønsted Acid-Catalyzed Intramolecular α-Amidoalkylation Reactions for the Synthesis of Fused Isoquinolines

An organolithium addition-intramolecular α-amidoalkylation sequence on N-phenethylimides has been developed for the synthesis of fused tetrahydroisoquinoline systems using 1,1'-bi-2-naphthol (binol)-derived Brønsted acids. This transformation is the first in which activated benzene derivatives are used as internal nucleophiles, instead of electron-rich heteroaromatics, generating a quaternary stereocenter. Phenolic substitution on the aromatic ring of the phenethylamino moiety and the use of binol-derived N-triflylphosphoramides as catalysts are determinants to achieve reasonable levels of enantioselection, that is, up to 75% enantiomeric excess, in the α-amidoalkylation step. The procedure is complementary to the intermolecular α-amidoalkylation process, as opposite enantiomers are formed, and to the Pictet-Spengler cyclization, which allows the formation of tertiary stereocenters.

Enantioselective Iridium-Catalyzed Allylic Cyclizations

A method for the enantioselective synthesis of carbo- and heterocyclic ring systems enabled through the combination of Lewis acid activation and iridium-catalyzed allylic substitution is described. The reaction proceeds with branched, allylic alcohols and carbon nucleophiles as well as heteronucleophiles to give a diverse set of ring systems in good yields and with high enantioselectivities. The utility of the method is highlighted by the asymmetric syntheses of erythrococcamides A and B.

Construction of Chiral Tetrahydro-β-Carbolines: Asymmetric Pictet-Spengler Reaction of Indolyl Dihydropyridines

A highly efficient synthesis of the enantioenriched tetrahydro-β-carbolines was developed by using a chiral phosphoric acid catalyzed Pictet-Spengler reaction of indolyl dihydropyridines. The reaction proceeds under mild reaction conditions to afford the desired chiral tetrahydro-β-carbolines in good to excellent yields (up to 96 %) and high enantioselectivities (up to 99 % ee). With this method, a formal synthesis of tangutorine and a total synthesis of deplancheine were achieved in a highly efficient manner.

Application of Pictet-Spengler Reaction to Indole-Based Alkaloids Containing Tetrahydro-β-carboline Scaffold in Combinatorial Chemistry

Indole-based alkaloids are well-known in the literature for their diverse biological properties. Polysubstituted optically active tetrahydro-β-carboline derivatives functionalized on C-1 position are the common structural motif in most of the indole-based alkaloids, as well as highly marketed drugs. The stereoselective Pictet-Spengler reaction is one of the currently most important synthetic techniques used for the preparation of these privileged tetrahydro-β-carboline scaffolds. To date, there are numerous research reports that have been published on the synthesis of the tetrahydro-β-carboline scaffold both on solid phase, as well as in solution phase. Moreover rapid growth has been observed for the enantioselective synthesis of tetrahydro-β-carboline scaffold using chiral organocatalysts. In this Review, efforts have been taken to shed light on the latest information available on different strategies to synthesize tetrahydro-β-carboline both on solid phase and in solution phase during the last 20 years. Furthermore, we believe that the present synthetic methodologies covered in this Review will help to improve the status of this privileged tetrahydro-β-carboline scaffold in its use for drug discovery.

Iridium-catalyzed asymmetric hydrogenation of cyclic iminium salts

An iridium-catalyzed asymmetric hydrogenation of cyclic iminium salts has been developed, affording products with up to 96% ee.

Insight into the deprotonation at the half-equivalence point of (thio)amido-benzimidazoles in the presence of anions

Three crystallographic structures highlight the acid–base half-equivalence point of hydrogen-bond donor (thio)amido-benzimidazoles induced by fluoride or benzoate salts with concomitant hydrogen-bonding and deprotonation as a merged synergic process.

Enantioselective total synthesis of (+)-arborescidine C and related tetracyclic indole alkaloids using organocatalysis

Enantioselective synthesis of (+)-arborescidine C was achieved by the key step of Pictet–Spengler cyclization reaction with a Jacobsen-type thiourea organocatalyst.

Enantioselective Aza-Sakurai Cyclizations: Dual Role of Thiourea as H-Bond Donor and Lewis Base

An enantioselective, catalytic aza-Sakurai cyclization of chlorolactams has been developed as an efficient entry into indolizidine and quinolizidine frameworks. Structure-enantioselectivity relationship studies and mechanistic analysis point to a dual role of the catalyst wherein the thiourea moiety of the catalyst is engaged in both anion binding and Lewis base activation of a substrate.

Mechanism-Guided Development of a Highly Active Bis-thiourea Catalyst for Anion-Abstraction Catalysis

We describe the rational design of a linked, bis-thiourea catalyst with enhanced activity relative to monomeric analogues in a representative enantioselective anion-abstraction reaction. Mechanistic insights guide development of this linking strategy to favor substrate activation though the intramolecular cooperation of two thiourea subunits while avoiding nonproductive aggregation. The resulting catalyst platform overcomes many of the practical limitations that have plagued hydrogen-bond-donor catalysis and enables use of catalyst loadings as low as 0.05 mol %. Computational analyses of possible anion-binding modes provide detailed insight into the precise mechanism of anion-abstraction catalysis with this pseudo-dimeric thiourea.

Enantioselective Synthesis of (+)-Peganumine A

A gram-scale enantioselective total synthesis of (+)-peganumine A was accomplished in 7 steps from commercially available 6-methoxytryptamine. Key steps included (a) a Liebeskind-Srogl cross-coupling; (b) a one-pot construction of the tetracyclic skeleton from an ω-isocyano-γ-oxo-aldehyde via a sequence of an unprecedented C-C bond forming lactamization and a transannular condensation; (c) a one-pot organocatalytic process merging two achiral building blocks into an octacyclic structure via a sequence of enantioselective Pictet-Spengler reaction followed by a transannular cyclization. This last reaction created two spirocycles and a 2,7-diazabicyclo[2.2.1]heptan-3-one unit with excellent control of both the absolute and relative stereochemistry of the two newly created quaternary stereocenters.