Amidations employing mixed (carbonic) anhydrides have long been favoured in peptide synthesis because of their cost-effectiveness and less waste generation. Despite their long history, no study has compared the effects of additives on the activation of mixed anhydrides and carbonic anhydrides. In this study, we investigated the amidation of mixed (carbonic) anhydride in the presence of a base and/or Brønsted acids. The use of NMI·HCl significantly improved the conversion of the mixed carbonic anhydride, while expediting nucleophilic attacks on the desired carbonyl group. In contrast, in the case of mixed carbonic anhydrides, neither the conversion nor the desired nucleophilic attack improved significantly. We developed a C-terminus-free N-methylated peptide synthesis method using mixed carbonic anhydrides in a micro-flow reactor. Fourteen N-alkylated peptides were synthesized in moderate to high yields (55-99%) without severe racemization (<1%). Additionally, a significant enhancement in the amidation between mixed carbonic anhydrides and bis-TMS-protected N-methyl amino acids with the inclusion of NMI·HCl was observed for the first time. In addition, we observed unexpected C-terminal epimerization of the C-terminus-free N-methyl peptides.
2,5-Disubstituted furans are frequently found in pharmaceuticals and bioactive natural products. Nucleophilic substitution reactions on the carbon atom adjacent to the furan ring are useful for producing various furan derivatives. However, the formation of 5-substituted 2-halomethylfuran and the subsequent nucleophilic substitution reactions are often limited by severe undesired reactions caused by the highly reactive halomethylfurans. This paper reports the successful rapid synthesis of various 2,5-disubstituted furans using microflow technology, which suppresses undesired reactions including dimerization and ring opening of the furans. We observed that Brønsted acids had a significant effect on the nucleophilic substitution reaction and the use of HBr and HI gave the best results. A plausible mechanism of the Brønsted acid-mediated nucleophilic substitutions in the developed approach was proposed.
An approach for the synthesis of cyclic phosphotriesters with various ring sizes (5- to 8-membered rings) from phosphorus trichloride and diols was developed. The major challenge in developing this approach is the suppression of the undesired reactions caused by substrates containing multiple highly reactive sites. These undesired reactions were successfully suppressed by microflow technology, which can precisely control the reaction time and temperature. Two optimal conditions were developed, depending on the speed of cyclization. Fifteen cyclic phosphotriesters and their analogs were synthesized. A plausible mechanism for suppressing undesired reactions is proposed.
Correction for 'Micro-flow heteroatom alkylation via TfOH-mediated rapid in situ generation of carbocations and subsequent nucleophile addition' by Yuma Matsuura et al., Chem. Commun., 2024, https://doi.org/10.1039/D3CC06308A.
A rapid nucleophilic substitution reaction was developed using carbocations generated from diarylmethanol and trifluoromethanesulfonic acid. Undesired reactions caused by the carbocations were suppressed, presumably due to the rapid and uniform generation of carbocations and the subsequent rapid and uniform distribution of nucleophiles by the micro-flow technology.
Unsymmetrical H-phosphonates were synthesized by a rapid (<15 s) and mild (20 °C) process in a microflow reactor as the first example of the sequential direct substitution of the chlorine atoms in PCl3 with alkoxyl/aryloxy groups using equivalent amounts of PCl3 and alcohols. The optimal base concentration differed in each step, presumably attributed to differences in the Brønsted basicity of the electrophilic intermediates. Phosphite hydrolysis was observed, and the structure-hydrolysis relationship was quantitatively evaluated.
We developed one-flow syntheses of unsymmetrical sulfamides and N-substituted sulfamate esters by changing a nucleophile and a tertiary amine from inexpensive and commercially available chlorosulfonic acid. In the synthesis of N-substituted sulfamate esters, unexpected symmetrical sulfite formation was suppressed by changing the tertiary amine. The effect of tertiary amines was proposed using linear regression. Our approach rapidly (≤90 s) provides desired products containing acidic and/or basic labile groups without tedious purification under mild (20 °C) conditions.
Substituted indoles are important as drugs. A number of valuable indoles have been synthesized via nucleophilic substitution at the 3'-position of indoles. However, the preparation of an indolylmethyl electrophile containing a tertiary carbon at the 3'-position and its subsequent nucleophilic substitution are challenging owing to the instability of the electrophile. Herein, we demonstrated the rapid one-flow synthesis of indoles via sequential 1,2-addition/nucleophilic substitution of indolyl-3-carbaldehydes. The use of a microflow technology helped in suppressing the undesired reactions caused by the unstable intermediates, resulting in significantly higher yields and reproducibility compared to those under batch conditions. A crown ether was effective when 1-alkylindole-3-carboxaldehyde was used as a substrate. However, the crown ether exerted a detrimental effect when 1H-indole-3-carboxaldehyde was used. A total of 15 structurally diverse indole derivatives were obtained in generally acceptable to good yields.
Short peptides are extremely important as drugs and building blocks for the syntheses of longer peptides. Both solid- and liquid-phase peptide syntheses suffer from a large number of synthetic steps, high cost, and/or tedious purification. Here, we developed a rapid, mild, inexpensive, and column-chromatography-free peptide chain elongation via a one-flow, three-component coupling (3CC) approach that is the first to use α-amino acid N-carboxy anhydrides (α-NCAs) both as electrophiles and nucleophiles. We demonstrated the high-yielding and column-chromatography-free syntheses of 17 tripeptides, as well as a gram-scale synthesis of a tripeptide. The total synthesis of beefy meaty peptide was achieved by repeating the 3CC approach with the addition of only one column chromatographic purification. We also demonstrated a one-flow tripeptide synthesis via in situ preparation of α-NCA starting from three readily available protected amino acids. With this study, we achieved dramatic reductions in both time and cost compared with typical solid-phase synthesis.
Although cyclic peptides have become increasingly important as drugs, the most conventional peptide cyclization method using moderately active coupling agents suffers from a lot of waste and high cost as well as long reaction times and burdensome purification. Herein, we report an unconventional approach to peptide cyclization that uses acylammonium species generated from inexpensive and less wasteful Me2 NBn and ClCO2 i-Pr. Using this approach, we observed the desired rapid activation of the C-terminus of cyclization precursors by an acylammonium ion for rapid and epimerization/dimerization-free cyclization of synthetically challenging peptides, including a difficult cyclization involving N-methyl amide bond formation. The ease of purification, productivities, and reaction mass efficiencies of our approach were significantly superior to those in previous reports. We synthesized a previously reported versicotide D analogue, and our data indicated that its assigned stereostructure should be revised.
Peptides, Cyclic , Peptides , Cyclization , Peptides/chemistry , Peptides, Cyclic/chemistry
PCl3 and POCl3 are the most important sources of phosphorus-containing compounds. They are also used for large-scale industrial productions. However, chemical reactions using the highly reactive PCl3 and POCl3 tend to result in overreactions. In addition, the reactions are usually exothermic and therefore their utilization sometimes involves significant risk. That is why some phosphorylating reagents with mild electrophilicity such as phosphoramidites have been developed. Although these mild electrophiles are very useful for the highly selective synthesis of organophosphorus compounds, problems persist: the high cost of the reagents, the generation of large amounts of waste, and the requirement of long reaction times and high temperatures. Continuous-flow technology is one of the most promising solutions for these problems. In particular, precise control of reaction times and temperatures enabled by micro-flow technology suppresses undesired reactions and allows the safe operation of exothermic reactions using highly reactive PCl3 and POCl3 . This Review describes recently reported reactions of PCl3 and POCl3 using continuous- and micro-flow technologies.
Phosphorus Compounds , Phosphorus Compounds/chemistry , Temperature
Although highly reactive (1H-indol-3-yl)methyl electrophiles such as (1H-indol-3-yl)methyl halides are potential precursors for the synthesis of various indole derivatives, some researchers have reported difficulties in their preparation due to concomitant undesired dimerization/oligomerization. Nevertheless, there have been some reports on the preparation of (1H-indol-3-yl)methyl halides. To resolve this contradiction, all the previously reported preparations of (1H-indol-3-yl)methyl halides were examined. However, we could not reproduce any of these preparations, and we revised several structures of indole derivatives. Here we show the rapid (0.02 s) and mild (25 °C) generation of an (1H-indol-3-yl)methyl electrophile that enables the rapid (0.1 s) and mild (25 °C) nucleophilic substitution in a microflow reactor. Eighteen unprotected indole analogues can be successfully synthesized using the developed microflow nucleophilic substitution with various nucleophiles.
A detailed protocol is described for the continuous-flow synthesis of N-methylated peptides. N-Methylated peptides are very important class of bioactive compounds compared with normal peptides because they can enhance oral bioavailability, cell membrane permeability, and stability against enzymatic degradation. In our developed flow synthesis, a variety of N-methylated dipeptides is obtained in high yields without severe racemization from equivalent amounts of amino acids. The addition of a strong Brønsted acid is critical to generate the highly reactive N-methylimidazolium cation species to accelerate the amidation. The developed approach enabled the synthesis of a bulky peptide with a higher yield in a shorter amount of time compared with the results of conventional amidation.
Dipeptides , Peptides , Acylation , Amino Acids , Biological Availability , Cations , Imidazoles
We demonstrate a sequential nucleophilic substitution of highly electrophilic and inexpensive phosphorus trichloride with three different alcohols in a continuous-flow reactor. A variety of alcohols including ones that contained acid- and/or basic-labile functionalities were rapidly reacted. A over nucleophilic substitution that occurred during reaction of the second alcohol was suppressed by the addition of imidazole. Density functional theory calculations of the sequential nucleophilic substitutions of alcohols were performed both with and without imidazole, and Berry pseudorotation was suggested as a rate-limiting step in both cases. Herein, we discuss the reasons for the decreased selectivity in the absence of imidazole as well as those for improved selectivity in the presence of imidazole during the second nucleophilic substitution.
Alcohols , Phosphorus Compounds , Chlorides , Imidazoles
This study demonstrated the rapid dual activation (10 s, 20 °C) of a combination of an α-amino acid N-carboxyanhydride and alkyl chloroformate in the synthesis of a urethane-protected α-amino acid N-carboxyanhydride in a micro-flow reactor. The key to success was the combined use of two amines that activated both substrates with proper timing. Three amines, i-Pr2NEt, Me2NBn, or N-ethylmorpholine, were used with pyridine in accordance with the steric bulkiness of a side chain in the α-amino acid N-carboxyanhydride. A variety of 16 urethane-protected α-amino acid N-carboxyanhydrides were synthesized in high yields. The role of amines was investigated based on the measurement of the time-dependent (0.5 to 10 s) decrease of α-amino acid N-carboxyanhydrides and alkyl chloroformates in the presence of amines via flash mixing technology using a micro-flow reactor. It was suggested that the in situ generated acylpyridinium cation was highly active and less prone to causing undesired decomposition compared with the acylammonium cation examined in this study. Thus, even at a very low concentration, the acylpyridinium cation facilitated the desired coupling reaction.
Peptides , Urethane , Amides , Amines , Amino Acids/chemistry , Esters , Peptides/chemistry , Urethane/chemistry
Lactams are cyclic amides that are indispensable as drugs and as drug candidates. Conventional lactamization includes acid-mediated and coupling-agent-mediated approaches that suffer from narrow substrate scope, much waste, and/or high cost. Inexpensive, less-wasteful approaches mediated by highly electrophilic reagents are attractive, but there is an imminent risk of side reactions. Herein, a methods using highly electrophilic triphosgene in a microflow reactor that accomplishes rapid (0.5-10â s), mild, inexpensive, and less-wasteful lactamization are described. Methodsâ A and B, which use N-methylmorpholine and N-methylimidazole, respectively, were developed. Various lactams and a cyclic peptide containing acid- and/or heat-labile functional groups were synthesized in good to high yields without the need for tedious purification. Undesired reactions were successfully suppressed, and the risk of handling triphosgene was minimized by the use of microflow technology.
Amides , Peptides , Carboxylic Acids , Lactams , Phosgene/analogs & derivatives
Photodynamic therapy (PDT) is a non-invasive, selective, and cost-effective cancer therapy. We previously reported that thiophene-based organic D-π-A sensitizers consist of an electron-donating (D) moiety, a π-conjugated bridge (π) moiety, and an electron-accepting (A) moiety, and are readily accessible and stable templates for photosensitizers that could be used in PDT. In addition, acrylic acid acceptor-containing photosensitizers exert a high level of phototoxicity. This study was an investigation into 1) the possibility of increasing phototoxicity by introducing another carboxyl group or by replacing a carboxyl group with a pyridinium group, and 2) the importance of an alkene in the acrylic acid acceptor for phototoxicity. A review of the design, synthesis, and evaluation of sensitizers revealed that neither dicarboxylic acid nor pyridinium photosensitizers enhance phototoxicity. An evaluation of a photosensitizer without an alkene in the acrylic acid moiety revealed that the alkene was not indispensable in the pursuit of phototoxicity. The obtained results provided new insight into the design of ideal D-π-A photosensitizers for PDT.
Acrylates/chemistry , Antineoplastic Agents/chemistry , Photosensitizing Agents/chemistry , Thiophenes/chemical synthesis , Acrylates/metabolism , Alkenes/chemistry , Antineoplastic Agents/pharmacology , Cell Membrane Permeability , Cell Survival/drug effects , Dicarboxylic Acids/chemistry , HeLa Cells , Humans , Molecular Structure , Photochemotherapy , Photosensitizing Agents/pharmacology , Serum Albumin, Human/metabolism , Singlet Oxygen/chemistry , Thiophenes/pharmacology
Photochemical conversion of isoxazole I to 5-hydroxyimidazoline VII proceeded via the trapping of the photogenerated acylazirine III with amines under UV light irradiation. This is the first report of the efficient synthesis of 5-hydroxyimidazolines that are not readily accessible by other means. 5-Hydroxyimidazolines were also converted into multisubstituted imidazoles in one step by treatment with trifluoroacetic anhydride (TFAA) and 2,6-lutidine in dichloromethane.
The development of a robust amide-bond formation remains a critical aspect of N-methylated peptide synthesis. In this study, we synthesized a variety of dipeptides in high yields, without severe racemization, from equivalent amounts of amino acids. Highly reactive N-methylimidazolium cation species were generated inâ situ to accelerate the amidation. The key to success was the addition of a strong Brønsted acid. The developed amidation enabled the synthesis of a bulky peptide with a higher yield in a shorter amount of time compared with the results of conventional amidation. In addition, the amidation can be performed by using either a microflow reactor or a conventional flask. The first total synthesis of naturally occurring bulky N-methylated peptides, pterulamides I-IV, was achieved. Based on experimental results and theoretical calculations, we speculated that a Brønsted acid would accelerate the rate-limiting generation of acyl imidazolium cations from mixed carbonic anhydrides.
Dipeptides/chemical synthesis , Hydrochloric Acid/chemistry , Imidazoles/chemistry , Stereoisomerism
Rapid dual activation (≤3.3 s) of both ß-amino acid N-carboxy anhydride and alkyl chloroformate for the synthesis of a ß-amino acid-derived scaffold was demonstrated. The key to success was the use of rapid mixing enabled by using a micro-flow reactor. The one-flow synthesis of 22 ß-amino acid derivatives was achieved.
Amino Acids/chemistry , Chemistry Techniques, Synthetic , Carbon Dioxide/chemistry , Isocyanates/chemistry
