Recent Progress in Catalytic Asymmetric Pictet-Spengler Reaction to Construct 1,1-Disubstituted Scaffolds

Abstract

1,1-disubstituted tetrahydroisoquinolines (THIQs) and tetrahydro-β-carbolines (THBCs) are key intermediates in the synthesis of various alkaloid natural products. These 1,1-disubstituted scaffolds are typically constructed via the Pictet-Spengler reaction. The asymmetric catalytic variant of this reaction enables access to stereochemically pure compounds with significant biological properties. This review focuses on recent progress in organocatalyzed and enzyme-catalyzed asymmetric Pictet-Spengler reactions to generate these 1,1-disubstituted motifs.

Graphical Abstract

1. Introduction

Alkaloids bearing tetrahydroisoquinoline (THIQ) or tetrahydro-β-carboline (THBC) motifs have attracted a lot of attention due to their promising biological activities.¹,^2–6 These privileged scaffolds are not only present in natural products but also in many active pharmaceutical ingredients and drug candidates. For example, cipargamin is a synthetic antimalarial drug currently in phase II clinical trials while FR115427 displays anticonvulsant properties in preclinical studies (Fig. 1A).^{7, 8} THIQs and THBCs are commonly constructed through the Pictet-Spengler (PS) reaction, which involves the condensation of a β-aryl ethylamine with an aldehyde or ketone, followed by intramolecular cyclization.^{9, 10} In the case of indoles, the iminium ion generated from the hemiaminal intermediate under the acidic conditions can undergo cyclization at either the C2- or C3-position of the indole ring (Figure 1B).¹¹ The spiroindolenine formed via the C3- attack will then rearrange readily to yield the same THBC product. ^12–14 However, recent studies have revealed the possibility that this spiroindolenine intermediate may have divergent roles in catalytic asymmetric PS reactions. Depending on the shape of the potential energy surface (PES), spiroindolenines may act as either productive intermediates that can be transformed to protonated tetrahydro-β-carbolines via dynamics effects over the caldera-shaped PES, or as off-cycle intermediates with C3 attack being a reversible process.¹⁵

Figure 1.

(A) THIQ or THBC in natural products or synthetic pharmaceuticals. (B) The mechanism of the Pictet-Spengler reaction.

Developing methodology for stereoselective PS reactions is of significant importance since each stereoisomer is likely to interact differently in biological systems, and only a particular stereoisomer would exhibit the desired pharmacological effects (Fig. 1A).¹⁶ Although many examples utilizing chiral auxiliaries or organocatalysts have been reported to synthesize chiral 1-substituted THIQs or THBCs as key intermediates in the asymmetric total synthesis of natural products and biologically active compounds,^17–24 stereoselective PS reactions with unactivated ketones using chemical means are still quite challenging due to their low reactivity and steric bulk.²⁵ Alternatively, biocatalysis offers a more cost-effective and sustainable solution to achieve high chemo-, regio-, and stereo- selectivity through precise control of the reaction at the active site of the enzyme, where substrates and co-factors interact within a chiral three-dimensional environment. ²⁶ Since a variety of Pictet-Spenglerases involved in the biosynthesis of many alkaloids have been characterized,²⁷ exploiting those enzymes to deliver stereochemically pure THIQs and THBCs has become an attractive approach.^{28, 29} This short review aims to highlight recent progress on catalytic asymmetric PS reactions to generate 1,1-disubstituted THIQs and THBCs, including organocatalyzed and enzyme-catalyzed PS reactions. Stoichiometric chiral silane promoted enantioselective PS reaction and the use of optically pure amines or carbonyl compounds to achieve stereochemically pure products will not be covered.^30–32 Additionally, biocatalytic formation of 1,1-disubstituted tetrahydroimidazopyridine (THIP) and the chemical synthesis of other quaternary carbon-centered 5,6 dihydroindolo[1,2-a]quinoxalines (DHIQs) and phenyl-4,5 dihydropyrrolo[1,2-a]quinoxalines (PDHPQs) will be briefly discussed.

2. Construction of 1,1-Disubstituted Tetrahydroisoquinoline (THIQ) Motif

Given the scarcity of chemocatalytic stereoselective PS reactions with ketones to yield 1,1-disubstituted THIQs, this section focuses on biocatalytic PS transformations to deliver such motif.

Found in a number of plant species, norcoclaurine synthase (NCS) is a Pictet-Spenglerase that catalyzes the stereospecific condensation between two L-tyrosine derived substrates, dopamine (1) and 4-hydroxyphenylacetaldehyde (4-HPAA) (2). The enzymatic reaction form (S)-norcoclaurine (3), which serves as a biosynthetic precursor to all benzylisoquinoline alkaloids (Figure 2).^33–35

Figure 2.

Enzymatic PS reaction catalyzed by norcoclaurine synthase (NCS).

First isolated in 2001, NCS from Thalictrum flavum (TfNCS) was later heterologously expressed in E. coli successfully and its structural features as well as the catalytic mechanism have been studied extensively in the past two decades.^36–41 The crystal structure of TfNCS in its complex with dopamine and the nonreactive substrate analogue 4-hydroxybenzaldehyde has been solved at 2.1 Å resolution. ³⁹ Although TfNCS behaves predominantly as a monomer in solution,⁴⁰ the X-ray structure revealed a dimeric assembly in the asymmetric unit. Each monomer has an accessible cleft which is located between the seven-stranded antiparallel β-sheets and the three α-helices, which extend through the protein matrix to form a 23.4-Å long tunnel (Fig. 3A).³⁹ The K122A variant of TfNCS was found to be completely inactive, suggesting that Lys122 is the key catalytic residue while the rest of the active site is shaped by polar residues including Tyr 108, Glu110 and Asp141. Deeper in the cavity, the side chain of Lys122 protrudes toward the substrate tunnel forming a “hook” capable of accepting the carbonyl group of the aldehyde substrate (Fig. 3B).³⁹

Figure 3.

(A) The crystal structure of NCS monomer in complex with dopamine (salmon) and an inactive substrate analogue 4-hydroxybenzaldehyde (yellow) (PDB ID: 2VQ5). (B) Close-up view of NCS active site.

Crystallographic data and recent computational studies suggested that dopamine binds first to the active site followed by the aldehyde substrate.^{40, 41} Residues Glu110 and Asp141 catalyze the formation of the iminium ion. Deprotonation of dopamine 3-OH by Lys122 initiates the electrophilic addition and deprotonation of the quinone by E110 delivers the chiral tetrahydroisoquinoline product (Figure 4). ^{40, 41} Computational modelling attributed the product enantioselectivity to a steric repulsion between the 4-hydroxybenzyl substituent and Leu72 which raises the energy of the disfavored transition state at the rate-limiting rearomatization step, resulting in the (S)-product with the natural substrates (Figure 4).⁴¹

Figure 4.

Proposed dopamine-first mechanism for NCS based on crystallographic data and computational studies.

With respect to the substrate scope, the 3-hydroxy motif in dopamine is crucial for successful transformation while a large number of aldehydes are accepted by TfNCS.^42–45 As for the carbonyl partner, enzymatic PS reactions with unactivated ketones remained underexplored until 2017 when Hailes, Ward and coworkers reported the first TfNCS-catalyzed PS reactions to construct chiral 1,1-disubstituted and spiro THIQs.⁴⁶ Using an N-terminal truncated wild type TfNCS (Δ29TfNCS) for better heterologous production,⁴⁷ 4-hydroxyphenylacetone, a structurally similar substrate to 4-HPAA,was first tested with lysates of Δ29TfNCS-expressing E. coli in analytical scale reactions. The desired m/z signal from HPLC-MS supported the formation of the corresponding 1,1-disubstituted THIQ. Subsequently, a range of ketones was screened using both wild type and variants of Δ29TfNCS. The enzyme variants were rationally chosen based on the dopamine-first mechanism and prior docking studies with the natural aldehyde substrate 4-HPAA.^{45, 48} In addition to 4-hydroxyphenylacetone, other unnatural substrates including arylacetones and substituted cyclohexanones were also accepted by wild type Δ29TfNCS and its variants. Sterically demanding ethyl ketones and less reactive conjugated ketones gave no reactivity. The isolated yields and the product stereochemistry were determined after preparative scale (50 μmol) in vitro enzymatic reactions. (S)-4 was obtained in 87% isolated yield and 95% ee, which indicated NCS can perform stereoselective PS reactions with ketones (Fig. 5). Interestingly, when the chiral ketone (3R)-methyl cyclohexanone was used, a single stereoisomer, (1’R, 3R)-8, was formed, contrasting with the typical (1S)-selectivity observed with aldehydes and the previous methyl ketone phenyl acetone (Fig. 5). Among the enzyme variants screened, the Ala-79 residue located at the active site entrance loop proved to be the most important residue to influence the enzyme’s tolerance for ketone substrates. Amino-acid substitutions A79I and A79F improved the conversions for phenylacetone and cyclohexanones respectively.

Figure 5.

(A) Selected examples of unactivated ketones accepted by TfNCS. (B) α-ketoacids accepted by CjPR10A. ^a=purified enzyme (1.6 mg/mL)

Four years later, the same groups fully explored the ketone substrate promiscuity of TfNCS to access challenging non-natural THIQs.⁴⁹ To prevent side reactions arising from catechol oxidation, the reaction conditions were optimized to include the addition of the antioxidant sodium ascorbate.⁵⁰ Other than phenyl acetones and cyclohexanones, aliphatic, cyclic, α-substituted cyclic, bicyclic and diketones were accepted by wild type Δ29TfNCS and its variants. Regardless of the variants used, increasing the alkyl chain would decrease the THIQ yield due to steric hindrance, while the presence of similarly sized groups on either side of the carbonyl moiety results in poor stereodifferentiation within the active site (5 vs 6, Fig. 5A). However, the enantioselectivity of compound 7 improved to 69% ee after methyl ether installation, which may be attributed to potential hydrogen-bonding interactions between the side chain and the active site residues. Remarkably, both racemic and (R)-2-methyl cyclohexanone were accepted as substrates to yield the same diastereomer (1’S, 2R)-9, which was rarely accessible through chemical methods. Cyclic ketones bearing a heteroatom, such as 1-N-Boc-3-piperidone, were readily converted to the corresponding 1,1-disubstituted THIQ 10 with good enantioselectivity (Fig. 3A). In addition, challenging bicyclic ketones including 6-chloro-β-tetralone were accepted by wild-type TfNCS, generating the complex spiro-THIQ 11 with 90% ee (Fig 5A).

Computational docking studies revealed that the ketone acceptance requires the iminium intermediate to adopt a folded productive conformation in the active site along with a favorable interaction between the catalytic residue Lys122 and dopamine meta-OH. Furthermore, in silico modelling by Chagas et al. demonstrated the substrate promiscuity depends strongly on carbonyl electrophilicity as well as steric constraints imposed within the active site.⁵¹

Another NCS isoform from Coptis japonica, CjNCS2 (also known as CjPR10A), which shares 61% identity and 76% similarity with TfNCS, also accepts 4-HPAA and dopamine to yield (S)-norcoclaurine (3). ⁵² Interestingly, wild-type CjNCS2 can convert activated α-keto acids such as 4-hydroxyphenylpyruvic acid and pyruvic acid with dopamine into the corresponding 1,1-disubstituted THIQs, although no information regarding stereoselectivity was reported (Fig. 5B).⁵²

Overall, its ability to accept ketones makes NCS a powerful biocatalyst for constructing stereoselective 1,1-disubstituted THIQs under mild and cofactor-free conditions that are challenging to achieve through chemical approaches.^53–57 NCS-catalyzed stereoselective PS reactions also provide new opportunities for integration into cascade biocatalytic transformations and efficient synthesis of isoquinoline alkaloids.

3. Construction of 1,1-Disubstituted Tetrahydro-β-carboline (THBC) Motif

3.1. Enantioselective Pictet-Spengler Reactions

Catalytic asymmetric PS reactions with aldehydes to generate THBC scaffolds have attracted significant attention since the early 2000s, with extensive studies conducted by Jacobsen, List, Hiemstra and others.^{19, 20, 58–73} In contrast, the analogous transformations employing ketones remain relatively underexplored with only a few examples reported. Chiral organocatalysts such as thioureas, phosphoric acids, prolyl-ureas and squaramides have been applied to the construction of chiral 1,1-disubstituted THBCs. The best catalysts which are utilized in the following examples are listed in Figure 6.

Figure 6.

Chiral organocatalysts described in this section.

Early approaches involved the generation of highly electrophilic cyclic N-acyliminium ions followed by enantioselective PS cyclization to deliver chiral 1,1-disubstituted THBC lactams. In 2007, Jacobsen and coworkers reported the cyclization of β-indolyl ethyl hydroxylactams catalyzed by thiourea 12 to afford highly enantioenriched indolizidinones and quinolizidinones (Fig. 7).⁷⁴ Hydroxylactams were formed either by imide reduction using NaBH₄ or by imide alkylation with organolithium reagents. The authors found that acidic addit-ives required for the formation of N-acyliminium ions such as chlorotrimethylsilane or the combination of HCl and 3 Å molecular sieves gave high conversion and enantioselectivity. The presence of water decreased catalytic activity and lower substrate concentration substantially improved the reaction yields. Under the optimal conditions, seven 1,1-disubstituted THBC lactams were generated with 85–98% ee including one synthetically versatile α, β-unsaturated compound (Fig. 7).

Figure 7.

Selected product scope of asymmetric PS reaction catalyzed by 12.

Experimental observations and DFT computational analyses indicated the reaction proceeded through an S_N1-type pathway (Figure 8). The H-bond donor catalyst 12 promoted enantioselective cyclization through cooperative substrate activation, wherein two molecules of the thiourea catalyst jointly assist the dissociation of the chloride counterion en route to forming the chiral complex.^{75, 76} The resulting anion-bound thiourea then induced high levels of enantioselectivity through counterion interactions.

Figure 8.

Proposed reaction mechanism of asymmetric PS reaction with 12.

Similarly, the Dixon group has reported several N-acyliminium-mediated enantioselective PS cyclization cascades through chiral Brønsted acid catalyzed condensation of tryptamines with different carbonyl partners.^{77, 78} In the presence of BINOL-derived phosphoric acid (R)-14 or (R)-17, tryptamine and its derivatives opened up the lactone electrophile to form an N-acyliminium ion intermediate, followed by cyclization to the final product (Figure 9A). With respect to the substrate scope, tryptamines with electron withdrawing or electron donating groups at 4,5,6 and 7 positions are tolerated, generating chiral tetracyclic products with improved enantioselectivity. A range of chiral γ- lactams (up to 99% ee) were synthesized in the presence of (R)-14, while (R)-17 was optimal to catalyze the formation of δ-lactams (n=2). Only one diastereomer was observed when disubstituted enol lactones were applied. The diastereoselectivity in the enantioenriched product was attributed to dynamic kinetic asymmetric cyclization (k1>k2) which arose from matched substrate and catalyst pairing (Figure 9A). The enol lactones can also be generated in situ from gold(I)-catalyzed cycloisomerization of alkynoic acids, allowing a powerful enantioselective multicatalyst reaction cascade.⁷⁹

Figure 9.

Chiral phosphoric acid catalyzed PS reactions proceeding via in situ generated cyclic N-acyliminium ions.

Following the same idea, the Dixon group then extended the carbonyl partner to γ- or δ-ketoacids and γ- or δ-ketoesters, which are more readily accessible compared to enol lactones (Figure 9B). The same chiral Brønsted acids (R)-14 and (R)-17 were the best ones to yield high levels of enantioselectivity. While mechanistic studies conducted by the Dixon group did not address the origin of the reaction’s stereoselectivity, the Goodman group later carried out computational studies and proposed a model to rationalize it.⁸⁰ Using tryptamine and (2-oxocyclohexyl) acetic acid as the model reaction, it was found that the N-acyliminium ion was bound to the catalyst in a bidentate fashion. Stable hydrogen-bonding interactions between the two terminal oxygen atoms of the chiral phosphate group and the N1 and C2 hydrogens of the would lead to a highly ordered transition state TS I, allowing the triphenylsilyl groups to influence the stereoselectivity (Figure 9B). Given the ability of chiral phosphoric acid in sequential catalysis with the ruthenium catalyst, You and coworkers showcased a highly efficient synthesis of chiral 1,1-disubstituted THBC fused γ-lactam scaffolds through a ring closing metathesis (RCM)/isomerization/PS cascade reaction in 2012 (Figure 9C).⁸¹ Under the optimized reaction conditions, the Hoveyda-Grubbs II metathesis catalyst facilitated ring closure of the starting diene followed by phosphoric acid-catalyzed alkene isomerization. The N-acyliminium ion derived from the corresponding enamine then underwent a PS cyclization with (S)-14 to form products in excellent yields (82–98%) and enantioselectivity (68–99% ee). In a related work, Dixon and coworkers also utilized N-sulfonyliminium intermediates to develop a highly enantioselective PS cyclization cascade catalyzed by gold (I) and a chiral phosphoric acid, through which complex sulfonamide scaffolds were accessed in up to 85% yield and 96% ee. ⁸²

In addition to N-acyliminium ions, nitrones have also been reported as intermediates in asymmetric PS cyclizations.^{83, 84} Most examples employ aldonitrones whereas the Snyder group described the first enantioselective variant using ketonitrones activated by commercially available acyl chlorides to furnish 1,1-disubstituted THBCs.⁸⁵ Starting from tryptophol,⁸⁶ the corresponding ketonitrone was formed via a three-step synthesis and activated with benzoyl bromide to give the reactive N-acyloxyiminium ion. Subsequent nucleophilic attack in the presence of chiral thiourea 13 generated products with aza-quaternary stereocenters (Fig. 10). The presence of molecular sieves proved essential to suppress hydrolysis of the N-acyloxyiminium ion, which would diminish product yields. Instead of the typical anion-bonding mechanism (Figure 8), the authors proposed that the catalyst itself could interact with both the carbonyl of the N-acyloxyiminium species and the phenyl group of the activating benzoyl moiety, thereby influencing the product chirality. Good enantioselectivities were achieved for ketonitrones possessing electronically diverse benzene rings while heteroatom substituted nitrone afforded only moderate selectivity. Interestingly, nearly all ketonitrones were obtained as single isomers except for alkyl-alkyl substituted nitrones, which existed as mixtures of E-/Z-isomers that, when separated, exhibited opposite enantioselectivity (Fig. 10).

Figure 10.

Chiral thiourea catalyzed Pictet-Spengler cyclization of ketonitrones.

In 2011, Bencivenni and coworkers developed the first catalytic asymmetric PS reaction of isatins with BINOL-derived phosphoric acid (S)-15 to construct THBCs with spiro quaternary stereocenters (Figure 11.⁸⁷ Different substituents on the aryl rings of the isatin and the tryptamine fragments were well tolerated. N-met-hylindole tryptamine and N-substituted isatins could also be employed, but the more sterically hindered 1-benzyl isatin resulted in decreased enantioselectivity. Unlike other asymmetric processes involving the indole nucleus (Figure 9B), the stereochemical outcome of this reaction could be explained by hydrogen-bonding interactions between the oxygen atom of the P=O moiety and the indole C2 hydrogen as well as the stabilizing interactions between the carbonyl group and imine nitrogen atom of the ketimine intermediate. In addition, steric constraints and bidentate coordination of (S)-15 rigidified the ketimine conformation, thus enabling cyclization from the Si-face of the ketimine to deliver the corresponding (S)-enantiomer. The Franz group developed a similar approach independently in the same year, but employed a different 3,3’-substitution on the binaphthyl system to induce stereoselectivity in the spirocyclization..⁸⁸ Notably, chiral phosphoric acids (S)-15 and (R)-16 yielded the same (S)-enantiomer of spiroindoline despite the opposite axial chirality. Under (R)-16 catalyzed conditions, improved enantioselectivity for halide-substituted isatins were observed compared to (S)-15. The geminally disubstituted tryptamine (R₂ = CO₂Me) was unreactive with both catalysts, demonstrating that steric hindrance would inhibit the formation of the iminium-phosphate ion complex responsible for stereoinduction (Figure 11). N-propargyl isatin was tolerated and further diversified through copper-catalyzed azide-alkyne cycloaddition chemistry to furnish the triazole-containing spirocycle 26 (Figure 11).⁸⁹

Figure 11.

Enantioselective Pictet-Spengler reactions of isatins for the synthesis of spiroindolones.

Nakamura and coworkers documented the first enantioselective PS reaction of acyclic α-ketoesters with tryptamines.⁹⁰ Excellent yields (up to 99%) and enantioselectivity (up to 99%) were obtained using chiral imidazoline-phosphoric acid 21 as the catalyst. Further derivatization of the ester-substituted THBC product such as LiALH₄-mediated reduction was accomplished without loss of enantiopurity. Compared to other BINOL-derived catalysts, the imidazoline moiety facilitated intermolecular hydrogen bonds with the tryptamine motif during the reaction. According to DFT calculations, the indole nucleophilicity and ketimine electrophilicity were enhanced by H-bonding interactions with the imidazoline and the phosphoric acid groups of 21 respectively. The absolute R configuration of the 1,1-disubstituted THBC product arose from minimized steric repulsion between the ester group and the imidazoline substituents, enabling Re-face cyclization at the indole C2 position (Figure 12A).

Figure 12.

Chiral phosphoric catalyzed enantioselective Pictet-Spengler reaction between tryptamine and acyclic α-ketoesters and its application in the total synthesis of (+)-tabertinggine.

In 2022, the She group successfully employed a chiral phosphoric acid to catalyze an asymmetric PS reaction/lactamization cascade in the total synthesis of iboga-type indole alkaloid (+)-tabertinggine (34).⁹¹ After identifying (R)-15 as the optimal catalyst, screening of different alkoxyl groups suggested that isopropyl ester 28 was the best reaction partner in the reaction. The synthesis began with the formation of tetracyclic lactam 29 in 96% yield and 98% ee after recrystallization. Boc protection of the indole nitrogen and subsequent aldol condensation with aldehyde 31 generated a hydroxy lactam intermediate. Elimination followed by reduction afforded compound 32, which was reduced with LiAlH₄ and deprotected with 2N HCl to deliver hydroxy ketone 33. Subsequent Swern oxidation and base-mediated intramolecular aldol condensation afforded the final product. Remarkably, (+)-tabertinggine 34 was synthesized in 9 steps with 26.2% overall yield and 97% ee on a gram scale.

More recently, Terada and coworkers established an approach for the synthesis of enantioenriched trifluoromethyl s-ubstituted THBCs (Figure 13A). ⁹² In the presence of chiral phosphoric acid 18, trifluoromethyl-substituted ynones served as the carbonyl partner to access these 1,1-disubstituted THBCs. Notably, a bulky TIPS group tethered to the alkyne was crucial for suppressing undesired 1,4-addition. Substrate scope studies revealed broad acceptance of electron-rich and electron-neutral tryptamines to furnish products in high yields and enantioselectivities, whereas electron-withdrawing CF₃ group caused the reaction to stall at the imine stage, reducing the yield drastically. In such cases, cyclization of the imine in the presence of 18 would restore product formation. The synthetic utility of this method was further exemplified by a seven-step synthesis of a CF₃-substituted analog of (-)-harmicine starting from 35 (Figure 13B).

Figure 13.

Chiral phosphoric acid catalyzed enantioselective Pictet-Spengler reaction for the synthesis of CF₃-substituted THBCs.

Urea-based catalysts offer an alternative solution to achieve stereocontrol in PS reactions for the synthesis of 1,1-disubstituted THBCs. Zhong and coworkers demonstrated the use of cinchona alkaloid-derived thiourea to construct tetrahydro-β-spirooxindoles bearing spirocyclic quaternary stereocenters. ⁹³ Activation through hydrogen bonding between chiral catalyst 20 and the isatin carbonyl promoted nucleophilic attack by N-benzyl tryptamine, generating iminium intermediate 36 after dehydration. A Si-face-selective intramolecular cyclization assisted by hydrogen-bonding stabilization furnished the (S)-enantiomer (Figure 14A). Subsequent biological assays demonstrated anticancer activity for several of the synthesized (S)-spirooxindoles with the 5-fluoro d- erivative identified as the most potent candidate.

Figure 14.

Thiourea and prolyl-urea catalyzed enantioselective Pictet-Spengler reactions.

In 2022, Zhu and coworkers reported the first enantioselective PS reactions of tryptamines with α-ketoamides using a newly developed prolyl-urea catalyst 19.⁹⁴ Unlike traditional thioureas, this new class of organocatalyst possesses only a single H-bond donor motif. Owing to this weak activating ability of 19, o-nitrobenzoic acid was required as a co-catalyst. The scope of the reaction is remarkably broad: α-ketoamides derived from benzylamine, cyclohexylamine, tert-butylamine and aniline as well as those bearing additional functionalities such as esters, silyl ethers, carbamates, secondary amides and diverse alkyl residues were all well tolerated, affording the corresponding 1,1-disubstituted THBCs in high yields (up to 99%) and excellent enantioselectivities (up to 97%). Control experiments indicated that hydrogen-bonding interactions with the indole nitrogen, the iminium nitrogen and the amide N-H of the α-ketoamide were essential for both the reactivity and enantioselectivity. A kinetic isotopic effect suggested that rearomatization of the pentahydro-β-carbolinium intermediate may be both the rate- and the enantioselectivity- determining step. The authors proposed that the favored transition state (TS IV), stabilized by a network of H-bonds and a π-π interaction between the naphthalene moiety of 19 and o-nitrobenzoic acid, accounted for the formation of the major (S)-enantiomer. In contrast, the alternative transition state suffered from several steric clashes and was therefore disfavored. Various post-transformations were conducted including the conversion from (+)-38 to 39, which contained the key structural motifs of peganumine A. It is also worth mentioning that the Christmann group achieved the asymmetric synthesis of (+)- and (-)-peganumine A in a protecting-group free sequence using a disulfonimide (DSI)-catalyzed PS reaction with cyclic α-ketoamides as the key step.⁹⁵

Chiral squaramides have also been employed in enantioselective PS reactions. These catalysts typically incorporate a chiral scaffold featuring a basic moiety (e.g., a tertiary amine), while the two N-H groups engage in strong hydrogen bonds with the substrates, thus functioning as highly effective bifunctional hydrogen-bonding catalysts.⁹⁶ In 2020, the Zhu group accomplished a concise five-step asymmetric synthesis of (-)-arborisidine and (-)-19-epi-arborisidine by applying a chiral pyrenylpyrrolidino squaramide catalyst 22.⁹⁷ After thorough catalyst screening, (2R)-pyrenylpyrrolidino-L-leucine squaramide 22 was identified as the optimal catalyst, converting tryptamine and 2,3-pentanedione to the correspondi-ing THBC (40) in 53% yield with 86% ee after in situ N-Boc protection. Intramolecular oxidative coupling followed by a one-pot imine reduction and N-Boc removal afforded (+)-42 in 42% yield over two steps. Vinyl Grignard addition to the carbonyl delivered (+)-43 with 99% ee. Finally, an aza-Cope/Mannich/oxidation/epimerization cascade performed at room temperature provided the target molecule (-)-arborisidine (45) in 61% overall yield. Alternatively, the cascade could be broken down into two separate steps wherein the aza-Cope/Mannich/oxidation sequence at 0 ºC first delievered (-)-19-epi-arborisidine, which was subsequently transformed into (-)-arborisidine (45) by epimerization (Figure 15).

Figure 15.

Squaramide 22 catalyzed enantioselective Pictet-Spengler reaction in the asymmetric synthesis of of (-)-arborisidine and (-)-19-epi-arborisidine.

The same research group developed a catalytic asymmetric PS reaction of α-ketoesters with an alanine-derived squaramide catalyst, which served as the key step in their seven-step synthesis of (+)-alstratine A, a hexacyclic cagelike monoterpene indole alkaloid.⁹⁸ In the presence of chiral squaramide (S)-23 or (S)-24, a large number of 1-alkyl-1-methoxycarbonyl THBCs were constructed in high yields (up to 99%) and excellent enantioselectivities (up to 99% ee). The wide substrate scope encompassed substituted tryptamines, and both linear and branched aliphatic chains could be introduced. Functionalized substrates featuring ethers, heteroaryl groups, acetals, azides, alkenes and alkynes were also tolerated, providing versatile handles for further derivatization. Similar to their previous work,⁹⁴ rearomatization of the pentahydro-β-carbolinium intermediate was proposed to be the rate-determining step, as suggested by the large primary kinetic isotope effect of the reaction (KIE = 4.5). The reaction enantioselectivity was rationalized by invoking a Curtin-Hammett-controlled dynamic kinetic resolution, wherein the E-ketiminium isomer stabilized by an internal hydrogen bond is more reactive and the resulting pentahydro-β-carbolinium ion B undergoes faster rearomatization, leading to the formation of the observed (S)-enantiomer (Figure 16A). The utility of this reaction was highlighted in the first asymmetric synthesis of (+)-alstratine A (52). Since the (R)-alanine derived catalysts were required to access the natural enantiomer, (R)-24 in combination with the co-catalyst salicylic acid was chosen to catalyze the PS reaction of 27 and 28, forming (R)-47 in 98% yield and 92% ee (Figure 16B). Chemoselective N-crotylation and N-benzoylation afforded 48 which was subsequently hydrolyzed and oxidized to yield α, β-unsaturated aldehyde 50. Heating of 50 induced an intramolecular inverse-electron-demand hetero-Diels-Alder to product 51. Owing to the instability of 49 and 50 on the silica column, no purification was performed throughout the 3-step sequence from 48. Treatment of crude 51 with potassium carbonate to remove the benzoyl group followed by acid-catalyzed intramolecular aminal formation furnished the natural product (+)-alstratine A (52).

Figure 16.

Alanine-derived squaramide-catalyzed enantioselective Pictet-Spengler reaction of α-ketoesters and its application in alstratine A synthesis.

3.2. Diastereoselective Pictet-Spengler Reactions

Tryptophan and its derivatives are commonly used as the aromatic partner in diastereoselective Pictet-Spengler reactions. Chemical strategies to access 1,1-disubstituted THBCs are far less developed than those employing aldehydes. While diastereoselectivity can hypothetically arise when asymmetric ketones are used, in reality, the reaction often prod-duces mixtures with poor diastereoselectivity.^99–103 Furthermore, current chemical conditions for PS reaction with tryptophan and its derivatives require excess acid, which presents potential functional group compatibility issues.

In 2004, the Chu group reported a microwave-accelerated PS reaction of tryptophan with ketones, although the diastereoselectivity was limited (up to 60% de). ¹⁰⁴ Microwave radiation shortened the reaction time drastically and provided a cleaner reaction profile compared to conventional heating. Among the three asymmetric ketones that were screened, only acetophenone exhibited moderate diastereoselectivity, with the best dr observed when the reaction was performed at room temperature (Table 1).

Table 1.

Diastereoselective Pictet-Spengler reactions with ketones (dr = cis isomer: trans isomer).

Entry	Ketone	Room Temperature		Heated at 60 °C		Microwave (60 °C, 300 W)
		Time	Yield; dr	Time	Yield; dr	Time	Yield; dr
1	Acetophenone	49 d	90% 20 : 80	95 h	96% 25 : 75	40 h	67% 23 : 77
2	2-Butanone	18 h	74% 51 : 49	3.5 h	99% 55 : 45	20 min	96% 50 : 50
3	3-Methyl-2-butanone	11.5d	87% 68 : 32	62 h	86% 60 : 40	15 h	76% 55 : 45

In contrast, biocatalysis provides an attractive alternative for accessing 1,1-disubstituted scaffolds stereoselectively due to enzymes’ ability to achieve precise stereochemical control within their active site. Recently, Renata, Abe, Mori and coworkers successfully achieved a diastereoselective PS reaction of tryptophans with α-ketoacids by utilizing KslB, a microbial Pictet-Spenglerase from Kitasatospora setae.¹⁰⁵ Previous in vivo biosynthetic studies had suggested that KslB is a putative enzyme in kitasetaline biosynthesis, catalyzing t-he Pictet-Spengler reaction between L-tryptophan and α-ketoglutarate. ^106–108 The authors first confirmed KslB’s native reactivity through heterologous expression in E. coli and in vitro assays. Contemporaneous with this work, the Liu group independently characterized the function of KslB by reconstituting kitasetaline biosynthesis in vitro.¹⁰⁹ The high turnover numbers (> 438,000) and tolerance to high substrate concentrations (up to 0.1M) on a gram scale without loss of activity established KslB as a versatile biocatalyst. KslB displayed broad acceptance of diverse L-tryptophan derivatives, affording 1,1-disubstituted THBCs in high yields and excellent diastereoselectivity (up to 99% dr) (Figure 171A). Chemical derivatization was performed to determine the stereochemistry of the enzymatic product. The assigned S configuration at C1, which is the opposite of that found in the chemical PS product, highlighted the enzyme’s ability to control the diastereoselectivity of the reaction. Notably, even the presence of a free aldehyde on the tryptophan fragment did not interfere with the PS reaction, highlighting the chemoselectivity of KslB-catalyzed reaction. With respect to the ketone partner, only activated α-ketoacids were accepted while more sterically hindered substrates such as 2-oxocyclohexaneacetic acid and 2-oxocyclohexanebutanoic acid gave only trace amounts of PS products. Remarkably, a tandem enzymatic cascade combining KslB with engineered tryptophan synthase TrpB Pf0A9¹¹⁰ enabled the one-pot construction of complex 1,1-disubstituted THBCs from simple indoles (Figure 17B).

Figure 17.

KslB-catalyzed diastereoselective Pictet-Spengler reactions.

The crystal structures of KslB in complex with L-Tryptophan and with its enzymatic product were solved at resolutions of 2.10 and 2.17 Å, respectively. The asymmetric unit contained two molecules that assembled into a functional dimer, with each monomer consisting of five α-helices and a right-handed, eight-stranded antiparallel β-barrel covered by short helices. The product-bound crystal structure further verified the S-selectivity of the reaction. The active site is formed at the interface between the monomers and Glu 274 is identified as the catalytic residue, acting as a base to activate the amine group of L-tryptophan. The ketone acceptance and diastereoselectivity can be rationalized by multiple hydrogen-bonding interactions between L-tryptophan or α-KG with several active site residues. Specifically, the α-KG binding pocket is defined by interactions of the C1’ and C5’ carboxyl groups of α-KG with Arg256 and Asn93 respectively, under the assumption that the orientation of L-Tryptophan remains fixed during the reaction (Fig. 18B). More recently, the Liu group conducted detailed studies on the mode of substrate recognition and mechanism of stereoselectivity.¹¹¹ Interestingly, the carboxylate groups of α-KG were positioned differently in the co-crystallized, product-bound KslB structure solved by the Liu group compared to the soaked, product-bound KslB structure previously reported by Renata and coworkers. Nevertheless, based on the five KslB crystal structures and computational studies, the α-KG moiety is tightly anchored by two salt-bridge interactions mediated by the positively charged residues Arg256 and Lys264, thereby enforcing a Si-face attack to afford the S-configuration at C1, consistent with the stereochemistry observed by the Renata group (Figure 19).

Figure 18.

(A) KslB in complex with product (PDB ID: 9K0A). (B) Close-up view of the catalytic site. Product is depicted in salmon. H-bonds are depicted in dashed lines. C1’ carboxyl group of α-KG interacts with S228 and Arg256 through a water-mediated hydrogen bond network (water is depicted as “O”).

Figure 19.

Proposed mechanism of KslB-catalyzed Pictet-Spengler reaction.

4. Miscellaneous

In addition to 1,1-disubstituted THIQs and THBCs, other quaternary center-containing heterocyclic scaffolds can be accessed through the Pictet-Spengler reaction.

Building on the strategy discussed in the previous section,⁷⁴ Jacobson and coworkers achieved an enantioselective N-acyl-PS reaction with pyrrole-derived hydroxylactams.¹¹² Using catalyst 12, a series of pyrroles were converted to the corresponding pyrroloindolizidinones and pyrroloquinolizidinones in moderate to excellent yields and ee (Figure 20A). The C4-selective cyclization rather than the typical C2-reactivity can be controlled by installing the sterically bulky TIPS protecting group on the pyrrole nitrogen. The same group also reported the first example of an iso-PS reaction with an acyclic ketone. The corresponding 4,4-disubstituted tetrahydro-γ-carboline 55 was synthesized in 95% yield and 76% ee under a chiral thiourea/benzoic acid dual catalyst system (Figure 20B).¹¹³

Figure 20.

Stereoselective Pictet-Spengler reactions to generate quaternary carbon-centered scaffolds.

In 2014, Zhang, Antilla and coworkers developed a highly enantioselective synthesis of 5,6 dihydroindolo[1,2-a]quinoxalines (DHIQs) and phenyl-4,5 dihydropyrrolo[1,2-a]quinoxalines (PDHPQs) that features the generation of a fully-substituted stereocenter α to the heteroaromatic ring system (Figure 20C).¹¹⁴ The newly developed H₈-BINOL-type chiral imidodiphosphoric acid catalyst 56 displayed excellent catalytic activity, delivering products in high yields (up to 98%) and enantioselectivities (up to 98%) with only 2 mol% catalyst loading. This reaction exhibited a wide substrate scope towards the synthesis of DHIQs. A variety of substituents on the indole as well as modifications on the aniline moiety were compatible with the reaction. With respect to the carbonyl partner, pyruvates such as phenyl, 2-naphthyl and tert-butyl pyruvates were efficiently converted and even butanedione was accepted to give the corresponding DHIQ in 81% and 93% ee. Similarly, a range of 3- or 5-substituted PDHPQs were accessed from pyrroles carrying 2- or 3- aryl substituents. Importantly, both DHIQs and PDHPQs could be successfully constructed on gram scale with minimal loss in yields and enantioselectivities.

Biocatalytically, microbial Pictet-Spenglerases mikimopine and cucumopine synthases were hypothesized to catalyze the condensation of L-histidine with α-ketoglutaric acid, generating 1,1-disubtituted tetrahydroimidazopyridines (THIPs) (Figure 20D).¹¹⁵ Both enzymes have been expressed recombinantly in E. coli and their native enzymatic activity was confirmed in vitro although no further biocatalytic applications of these synthases has been reported to date.¹¹⁶ Interestingly, Renata and coworkers found that KslB, whose natural substrate is L-tryptophan, can accept L-histidine, delivering the corresponding 1,1-disubtituted THIP, cucumopine (60), in 68% yield without the need for an NADH cofactor, highlighting a more economical alternative.¹⁰⁵

5. Conclusion

Catalytic asymmetric Pictet-Spengler reactions have been applied in the efficient synthesis of chiral THBCs and THIQs, which are prevalent across many natural products and bioactive pharmaceutical compounds. Unlike the well-established aldehyde-based approaches, the construction of 1,1-disubstituted scaffolds is considerably more challenging because of slow ketimine formation and the steric demands of cyclization. Nevertheless, significant advances over the past two decades have enabled access to these 1,1-disubstituted scaffolds, as summarized in this review. Chemocatalytic strategies have employed organocatalysts including chiral phosphoric acids, squaramides and urea-based catalysts to deliver products with high levels of stereoselectivity primarily through counterion-controlled mechanisms or H-bonding interactions. These catalysts have proven effective with highly reactive ketones such as 1,2-dicarbonyls but fail with unactivated ketones, underscoring the importance of developing future approaches to overcome this limitation. Alternatively, with the increasing number of Pictet-Spenglerases being characterized, stereoselective Pictet-Spengler reactions catalyzed by enzymes under mild conditions have become a complementary solution to conventional chemical synthesis. As illustrated by norcoclaurine synthase TfNCS, even unactivated ketones can be accepted, enabling the generation of highly enantioselective 1,1-disubstituted THIQs through protein engineering. Moreover, the integration of KslB into cascade biocatalytic transformations highlights the power of enzymes to construct molecular complexity from simple building blocks. Future progress may involve the development of chemoenzymatic strategies for the synthesis of pharmaceutically relevant alkaloids to further broaden the synthetic utility of this transformation.