Pictet-Spengler Research Articles

Construction of pyrrole- and indole-fused CF3-piperazine derivatives

A series of pyrrole- and indole-fused trifluoromethyl-functionalized piperazine derivatives were constructed in moderate to good yields with excellent chemoselectivities via a Pictet-Spengler reaction under mild and operationally simple conditions. The synthetic utility of this protocol was further extended by the facile preparation of indole-fused CF3-1,4-diazocane and enantioenriched CF3-piperazines via a vinylogous Pictet-Spengler reaction and an asymmetric Pictet-Spengler reaction, respectively.

Stereocomplementary Chemoenzymatic Pictet–Spengler Reactions for Formation of Rare Azepino-indole Frameworks: Discovery of Antimalarial Compounds

Strictosidine synthase (STR1) catalyzes a Pictet–Spengler reaction (PSR) forming strictosidine, a likely biosynthetic key to all higher plant monoterpenoid indole alkaloids. Increasing the biocatal...

Enzymatic Pictet-Spengler Reaction: Computational Study of the Mechanism and Enantioselectivity of Norcoclaurine Synthase.

The Pictet-Spengler (PS) reaction, i.e., the acid-catalyzed condensation between β-arylethylamine and an aldehyde or a ketone and the subsequent ring closure, is an important reaction in organic chemistry. A number of enzymes (called Pictet-Spenglerases, PSases) have been identified to catalyze this reaction, usually with very high enantioselectivity, making these enzymes of potential value in biocatalysis. PSases catalyze the key step in the biosynthesis of indole and benzylisoquinoline alkaloids of plant origin, some of which have pharmacological importance. However, the reaction mechanisms and the origins of the selectivity are not fully understood. Herein, we report a quantum chemical investigation of the mechanism and enantioselectivity of norcoclaurine synthase (NCS), an enzyme that catalyzes the PS condensation between dopamine and 4-hydroxyphenylacetaldehyde (4-HPAA). A large model of the active site is designed on the basis of a recent crystal structure, and it is used to calculate the detailed energy profile of the reaction. Good agreement is obtained between the calculated energies and available experimental information. Both the "dopamine-first" and the "HPAA-first" binding modes of the substrates reported in the literature are shown to be energetically accessible in the enzyme-substrate complex. However, it is demonstrated that only the dopamine-first pathway is associated with feasible energy barriers. Key active site residues are identified, and their roles in the catalysis are discussed and compared to site-directed mutagenesis experiments. Very importantly, the calculations are able to reproduce and rationalize the observed enantioselectivity of NCS. A detailed analysis of the geometries of the intermediates and transition states helps to pinpoint the main factors controlling the selectivity.

Substitution of the Dimethylamino Group in Gramines and One-Pot Cyclization to Tetrahydro-β-carbolines Using a Triazine-Based Activating Agent

A new method for the substitution of 3-[(dimethylamino)methyl]indoles (gramines) with malonate-based nucleophiles was developed using 2-chloro-4,6-dimethoxy-1,3,5-triazine (CDMT) as the activating agent for the dimethylamino group. The reaction was completed in 1.5-6 h at room temperature in the presence of a tert-amine base and lithium salt. CDMT afforded superior results to methyl iodide, a common activating agent for the dimethylamino group in Mannich bases, particularly in the reactions of 1-substituted gramines. The reactivity of the possible intermediates, bis(indol-3-ylmethyl)dimethylammonium salts, was examined to obtain mechanistic insights on the reaction. This substitution method with CDMT enabled the sequential transformation of gramines: substitution with ( N-alkylidene)aminomalonates followed by the Pictet-Spengler reaction under acidic conditions afforded 1,2,3,4-tetrahydro-β-carboline derivatives in one pot.

Divergent Total Synthesis of Chaetoglines C to F.

The first total syntheses of chaetoglines C-F via a bioinspired and divergent synthetic strategy are reported. Chaetolines C and D were obtained from the condensation of hemiacetal and tryptophan methyl ester building blocks followed by functional group transformations. The synthesis of chaetogline E employed the diastereoselective Pictet-Spengler reaction, and the tetrahydro-carboline skeleton was further utilized as a precursor for an oxidative aromatization reaction to introduce the β-carboline moiety of chaetogline F.

Total Syntheses of (-)-Secologanin, (-)-5-Carboxystrictosidine, and (-)-Rubenine.

The first enantioselective total syntheses of (-)-secologanin (1), (-)-5-carboxystrictosidine (2), and (-)-rubenine (3) were accomplished in 10, 9, and 14 steps, respectively. The key transformation in the synthesis of 1 was a sequential anti-selective organocatalytic Michael reaction/Fukuyama reduction/spontaneous cyclization to form an optically active dihydropyran ring. In addition, the secologanin tetraacetate (16), which is a potential key intermediate for the bioinspired divergent syntheses of monoterpenoid indole alkaloids, was prepared in gram-scale in seven steps. The total syntheses of 2 and 3, which are classified as glycosylated monoterpenoid indole alkaloids, were achieved through bioinspired transformations such as a diastereoselective Pictet-Spengler reaction, a site- and stereoselective epoxidation, and a site-selective epoxide ring-opening followed by a lactonization reaction.

Biomimetic Phosphate-Catalyzed Pictet-Spengler Reaction for the Synthesis of 1,1'-Disubstituted and Spiro-Tetrahydroisoquinoline Alkaloids.

Tetrahydroisoquinoline (THIQ) alkaloids are an important group of compounds that exhibit a range of bioactivities. Here, a phosphate buffer-catalyzed Pictet–Spengler reaction (PSR) using unreactive ketone substrates is described. A variety of 1,1′-disubstituted and spiro-tetrahydroisoquinoline alkaloids were readily prepared in one-step and high yields, highlighting the general applicability of this approach. This study features the role of phosphate in the aqueous-based PSR and provides an atom-efficient, sustainable route to new THIQs.

Enantioselective Synthesis of Hydantoin and Diketopiperazine-Fused Tetrahydroisoquinolines via Pictet-Spengler Reaction.

An enantioselective synthesis of iso-, isothio-, and isoselenohydantoin and diketopiperazine-fused tetrahydroisoquinolines from l-Dopa was reported. The route consists of an Pictet-Spengler reaction of ( S)-2-amino-3-(3,4-dimethoxyphenyl)propanoates with various aldehydes to afford diastereomeric tetrahydroisoquinolines. Next step, the tetrahydroisoquinolines were further reacted with iso-, isothio-, or isoselenocyanates to construct hydantoin. Similarly, the diketopiperazine moiety was constructed by subjecting tetrahydroisoquinolines to a condensation reaction with chloroacetyl chloride followed by nucleophilic addition with various primary amines.

Metal free one pot synthesis of β-carbolines via a domino Pictet-Spengler reaction and aromatization

A convenient and efficient metal free, atom economical flexible synthesis of β-carbolines involving a domino Pictet-Spengler reaction and aromatization in oxygen atmosphere in N-methyl-2-pyrollidone (NMP) is described. Variety of aryl, heteroaryl and aliphatic aldehydes were found to be good substrates for this methodology. Several β-carbolines (6a-6t) and β-carboline methyl esters (7a-7e) were synthesized using this methodology.The same reaction carried out in argon atmosphere in the presence of catalytic amount of acid in NMP furnished, tetrahydro-β-carbolines (4a-4g).

Grob-Type Fragmentation Releases Paracyclophane Ring Strain in a Late-Stage Precursor of Haouamine A.

A ketene [2 + 2]-addition, an intramolecular aldol reaction, a Suzuki-Miyaura coupling, and a chemoselective lactam reduction were used to prepare a late-stage precursor of haouamine A. Exposure to acid led to a Grob-type fragmentation of the strained 3-aza[7]paracyclophane ring, followed by a tandem Pictet-Spengler reaction of the intermediate iminium ion and conversion to a novel 1,4a-propanocyclopenta[ b]pyridine. This cascade reaction might also be relevant for the mechanism of action of the natural product.

The Ambidextrous Pictet–Spengler Reaction: Access to the (+)- or (–)-Enantiomers of the Bioactive C-19 Methyl-Substituted Sarpagine/Macroline/Ajmaline Alkaloids from Either d- or l-Tryptophan

Depicted in the following is the proof of concept, which is important to illustrate the full potential of the ambidextrous Pictet–Spengler (P-S) reaction. Previously, both d-tryptophan and l-tryptophan were employed to synthesize the key intermediates toward the natural enantiomers of C-19 methyl-substituted alkaloids. Now the enantiomeric series of the same key intermediates could also be synthesized from both d- or l-tryptophan in high yield and optical purity via this P-S/Dieckmann protocol. One can make either the natural or the unnatural alkaloids from either of the chiral starting amino acid esters, stereo- and enantiospecifically for biological screening.

Efficient Pictet–Spengler Bioconjugation with N‐Substituted Pyrrolyl Alanine Derivatives

AbstractWe discovered N‐pyrrolyl alanine derivatives as efficient reagents for the fast and selective Pictet–Spengler reaction with aldehyde‐containing biomolecules. Other aldehyde‐labeling methods described so far have several drawbacks, like hydrolytic instability, slow reaction kinetics or not readily available labeling reagents. Pictet–Spengler cyclizations of pyrrolyl 2‐ethylamine substituted at the pyrrole nitrogen are significantly faster than with analogues substituted at the α‐ and β‐ position. Functionalized N‐pyrrolyl alanine derivatives can be synthesized in only 2–3 steps from commercially available materials. The small size of the reagent, the high reaction rate, and the easy synthesis make pyrrolyl alanine Pictet–Spengler (PAPS) an attractive choice for bioconjugation reactions. PAPS was shown as an efficient strategy for the site‐selective biotinylation of an antibody as well as for the condensation of nucleic‐acid derivatives, demonstrating the versatility of this reagent.

Efficient Pictet-Spengler Bioconjugation with N-Substituted Pyrrolyl Alanine Derivatives.

We discovered N-pyrrolyl alanine derivatives as efficient reagents for the fast and selective Pictet-Spengler reaction with aldehyde-containing biomolecules. Other aldehyde-labeling methods described so far have several drawbacks, like hydrolytic instability, slow reaction kinetics or not readily available labeling reagents. Pictet-Spengler cyclizations of pyrrolyl 2-ethylamine substituted at the pyrrole nitrogen are significantly faster than with analogues substituted at the α- and β- position. Functionalized N-pyrrolyl alanine derivatives can be synthesized in only 2-3 steps from commercially available materials. The small size of the reagent, the high reaction rate, and the easy synthesis make pyrrolyl alanine Pictet-Spengler (PAPS) an attractive choice for bioconjugation reactions. PAPS was shown as an efficient strategy for the site-selective biotinylation of an antibody as well as for the condensation of nucleic-acid derivatives, demonstrating the versatility of this reagent.

Application of Electrocyclic Ring-Opening and Desymmetrizing Nucleophilic Trappings of meso-6,6-Dibromobicyclo[3.1.0]hexanes to Total Syntheses of Crinine and Haemanthamine Alkaloids.

The thermally induced electrocyclic ring-opening of C2-symmetric ( meso) 6,6-dibromobicyclo[3.1.0]hexanes such as 10 in the presence of the chiral, nonracemic 1°-amine 28 afforded a ca. 1:1 mixture of the diastereoisomeric and chromatographically separable 1-amino-2-bromo-2-cyclohexenes 37 (42%) and 38 (45%). Each of these was elaborated over 13 steps, including Suzuki-Miyaura cross-coupling, radical cyclization, and Pictet-Spengler reactions, into (-)- or (+)-crinane (1 or ent-1, respectively). Variations on these protocols were applied to the total syntheses of (+)- and (-)-11-hydroxyvattitine [(+)- and (-)-3], (+)- and (-)-bulbispermine [(+)- and (-)-4], (+)- and (-)-haemanthamine [(+)- and (-)-5], (+)- and (-)-pretazettine [(+)- and (-)-6], and (+)- and (-)-tazettine [(+)- and (-)-7] as well as (±)-hamayne [(±)-8] and (±)-apohaemanthamine [(±)-9]. A number of these alkaloids were synthesized for the first time.

One-pot tandem cyclisation to pyrrolo[1,2-a][1,4]benzodiazepines: a modified approach to the Pictet-Spengler reaction.

We have reported a one-pot two-step methodology for the synthesis of highly condensed heterocycles, pyrrolo[1,2-a][1,4]benzodiazepines, by a modified Pictet-Spengler reaction under mild conditions in a short time. Our approach has a few advantages over the conventional two components synthesis as it is step and atom economic, environmentally benign and a convergent synthetic method. We have discussed here the broad substrate scope of this novel methodology.

An amide probe as a selective Al3+ and Fe3+ sensor inside the HeLa and a549 cell lines: Pictet–Spengler reaction for the rapid detection of tryptophan amino acid

Newly synthesized amide-based probe for the selective and specific detection of (i) Al3+ or Fe3+ ions as a cation sensor and (ii) tryptophan as an amino acid sensor.

Questioning the γ-gauche effect: stereoassignment of 1,3-disubstituted-tetrahydro-β-carbolines using 1H-1H coupling constants.

The Pictet-Spengler reaction of tryptophan esters and aldehydes has been widely applied in natural product synthesis and medicinal chemistry. To date, the trans- or cis-configuration of 1,3-disubstituted tetrahydro-β-carbolines (THβCs) formed in this reaction has most often been assigned based on the relative 13C chemical shifts of C1 and C3 in the diastereomers. Although the upfield shifts of C1 and C3 in trans-THβCs relative to cis-THβCs has been attributed to steric compression associated with the "γ-gauche" effect, we show that this effect is not borne out experimentally for other carbons that should suffer this same compression. Thus we developed a robust alternative method for stereochemical assignment based on 1H NMR coupling constants (31 examples) and supported by extensive DFT-based conformational analysis and calculation of 1H-1H coupling constants. DFT calculations of 13C NMR chemical shifts also cast doubt upon the role of the "γ-gauche" effect on C1 and C3 chemical shifts in trans-THβCs.

Synthesis of Novel Isoquinolino[5,4-ab]phenanthridine Derivatives via Pictet–Spengler Reaction

The reaction of 5-amino-2-[2-(dimethylamino)ethyl]-6-phenyl-1H-benzo[de]isoquinoline-1,3(2H)-dione with a series of aldehydes in acidic media (phosphoric acid, trifluoromethanesulfonic acid, and trifluoroacetic acid) is described. This key step is based on the Pictet–Spengler reaction to synthesize ten novel isoquinolino[5,4-ab]phenanthridine derivatives. The most effective acid is phosphoric acid and optimized yields were obtained. The frameworks of target compounds are unique and have never been reported.

Total Synthesis of the Ortho-Hydroxylated Protoberberines ( S)-Govaniadine, ( S)-Caseamine, and ( S)-Clarkeanidine via a Solvent-Directed Pictet-Spengler Reaction.

The common para regioselectivity in Pictet–Spengler reactions with dopamine derivatives is redirected to the ortho position by a simple change of solvents. In combination with a chiral auxiliary on nitrogen, this ortho-selective Pictet–Spengler produced the 1-benzyltetrahydroisoquinoline alkaloids (S)-crassifoline and (S)-norcrassifoline and the bioactive 1,2-dioxygenated tetrahydroprotoberberine alkaloids (S)-govaniadine, (S)-caseamine, and (S)-clarkeanidine with high enantiopurity. Ortho/para ratios up to 89:19 and diastereomeric ratios up to 85:15 were obtained during formation of the B-ring. The general applicability of this solvent-directed regioselectivity was demonstrated with a second Pictet–Spengler reaction as required for C-ring formation of caseamine (o/p = 14:86 in trifluoroethanol) and clarkeanidine (o/p = 86:14 in toluene).

Diverse Approaches and Recent Advances in the Synthesis of Tetrahydroprotoberberines

Racemic and enantioselective synthesis of tetrahydroprotoberberine (THPB) alkaloids, a family of natural products showing a wide range of biological activity, have been based either on extension or enhancement of conventional methods or on new, recently developed tactics. Classical approaches to synthesize the THPB scaffold have utilized Bischler-Napieralski cyclization/reduction or Pictet-Spengler reactions to generate a benzyltetrahydroisoquinoline motif, followed by Mannich type condensation to obtain the tetracyclic THPB core. In recent developments, contiguously tetra-substituted ring D moieties and chiral auxiliaries to install the chiral center have been utilized to synthesize THPBs. The present review summarizes various approaches adopted to synthesize THPBs and their congeners, based on the sequence of formation of rings B and C.