Preparation of Contiguous Bisaziridines for Regioselective Ring-Opening Reactions
Summary
Contiguous bisaziridines containing non-activated and activated aziridines were synthesized by asymmetric organocatalytic aziridinations and then subjected to chemoselective ring-opening reactions under acidic or basic conditions. The non-activated aziridine ring opens with less reactive nucleophiles under acidic conditions, whereas the activated aziridine ring opens with more reactive nucleophiles under basic conditions.
Abstract
Aziridines, a class of reactive organic molecules containing a three-membered ring, are important synthons for the synthesis of a large variety of functionalized nitrogen-containing target compounds through the regiocontrolled ring-opening of C-substituted aziridines. Despite the tremendous progress in aziridine synthesis over the past decade, accessing contiguous bisaziridines efficiently remains difficult. Therefore, we were interested in synthesizing contiguous bisaziridines bearing an electronically diverse set of N-substituents beyond the single aziridine backbone for regioselective ring-opening reactions with diverse nucleophiles. In this study, chiral contiguous bisaziridines were prepared by organocatalytic asymmetric aziridination of chiral (E)-3-((S)-1-((R)-1-phenylethyl)aziridin-2-yl)acrylaldehyde with N-Ts-O-tosyl or N-Boc-O-tosyl hydroxylamine as the nitrogen source in the presence of (2S)-[diphenyl(trimethylsilyloxy)methyl]pyrrolidine as a chiral organocatalyst. Also demonstrated here are representative examples of regioselective ring-opening reactions of contiguous bisaziridines with a variety of nucleophiles such as sulfur, nitrogen, carbon, and oxygen, and the application of contiguous bisaziridines to the synthesis of multi-substituted chiral pyrrolidines by Pd-catalyzed hydrogenation.
Introduction
Rational design of small organic molecules with diverse reactive sites that precisely control product selectivity is a key goal in modern organic synthesis and green chemistry1,2,3,4,5,6,7,8. To achieve this goal, we were interested in the modular synthesis of aziridines. Aziridines are of interest to most organic chemists, owing to their structurally important framework9 with an electronically diverse set of N-substituents that can lead to regioselective ring-opening reactions with multiple nucleophiles10,11,12,13,14,15,16,17,18,19, and varied pharmacological activities such as antitumor, antimicrobial, and antibacterial properties. Despite the advances in aziridine chemistry, non-activated aziridine and activated aziridine have independent syntheses and ring-opening reactions in the literature20.
Therefore, we aimed to synthesize contiguous bisaziridines comprising both the non-activated and activated aziridines. These contiguous bisaziridines can be used to systematically rationalize a chemoselective ring-opening pattern based on the following electronic properties of the two different aziridines and their reactivity to nucleophiles20,21,22,23,24: a) activated aziridines, in which the electron-withdrawing substituents conjugatively stabilize the negative charge on the nitrogen, readily react with multiple nucleophiles to allow ring-opened products; b) non-activated aziridines, in which the nitrogen is bound to the electron-donating substituents, are considerably inert toward nucleophiles; hence, a pre-activation step with a suitable activator (mainly Brønsted or Lewis acids) is required to afford the ring-opened products in high yields20,21,25,26.
The present study describes the rational design of contiguous bisaziridines as chiral building blocks via transition-metal-free organocatalysis and the synthesis of diverse nitrogen-rich molecules utilizing predictive modeling tools for ring-opening reactions of bisaziridines. This study aims to stimulate the advancement of practical methods for the construction of nitrogen-enriched bioactive compounds and natural products and the polymerization of aziridines.
Protocol
Representative Results
Discussion
The formation of an inseparable mixture of diastereomers has occasionally been observed during the course of organocatalytic aziridination of chiral 3-[1-(1-phenylethyl)aziridin-2-yl)]acrylaldehyde, when N-Boc-O-tosyl or N-Ts-O-tosyl hydroxylamine was used as the nitrogen source. Further, the yield of contiguous bisaziridine product decreased when the amount of diaryl silyl ether prolinol as catalyst was increased from 7 mol% to 20 mol%47,<sup class="…
Disclosures
The authors have nothing to disclose.
Acknowledgements
This research was supported by the Korea Basic Science Institute (National Research Facilities and Equipment Center) grant funded by the Ministry of Education (2022R1A6C101A751). This work was also supported by the National Research Foundation of Korea (NRF) grants (2020R1A2C1007102 and 2021R1A5A6002803).
Materials
| (R)-(+)-α,α-Diphenyl-2-pyrrolidinemethanol trimethylsilyl ether | Sigma-Aldrich | 677191 | reagent |
| (R)-1-((R)-1-phenylethyl)aziridine-2-carbaldehyde | Imagene Co.,Ltd. | reagent | |
| (S)-(–)-α,α-Diphenyl-2-pyrrolidinemethanol trimethylsilyl ether | Sigma-Aldrich | 677183 | reagent |
| (S)-2-(diphenyl((trim ethylsilyl)oxy)methyl)pyrrolidine | Sigma-Aldrich | 677183 | reagent |
| (Triphenylphosphoranylidene) acetaldehyde | Sigma-Aldrich | 280933 | reagent |
| 1,2-Dichloroethane | Sigma-Aldrich | 284505 | solvent |
| AB Sciex 4800 Plus MALDI TOFTM (2,5-dihydroxybenzoic acid (DHB) matrix | Sciex | High resolution mass spectra | |
| Acetic acid | Sigma-Aldrich | A6283 | reagent |
| Ammonium chloride | Sigma-Aldrich | 254134 | reagent |
| aniline | Sigma-Aldrich | 132934 | reagent |
| Autopol III digital polarimeter | Rudolph Research Analytical | polarimeter | |
| AVANCE III HD (400 MHz) spectrometer | Bruker | NMR spectrometer | |
| Bruker Ascend 500 (500 MHz) | Bruker | NMR spectrometer | |
| Celite 535 | Sigma-Aldrich | 22138 | For Celite pad |
| Dichloromethane | Sigma-Aldrich | 270997 | solvent |
| Di-tert-butyl dicarbonate | Sigma-Aldrich | 361941 | reagent |
| Ethyl Acetate | Sigma-Aldrich | 270989 | solvent |
| Ethyl nitroacetate | Sigma-Aldrich | 192333 | reagent |
| Imidazole | Sigma-Aldrich | I2399 | reagent |
| INOVA 400WB (400 MHz) | Varian | NMR spectrometer | |
| JMS-700 | JEOL | High resolution mass spectra | |
| Methanol | Sigma-Aldrich | 322415 | solvent |
| N-Boc-O-tosylhydroxylamine | Sigma-Aldrich | 775037 | reagent |
| P-2000 | JASCO | polarimeter | |
| Palladium hydroxide on carbon | Sigma-Aldrich | 212911 | reagent |
| Phenyl-1H-tetrazole-5-thiol | TCI | P0640 | reagent |
| Silica gel | Sigma-Aldrich | 227196 | For flash clromatography |
| Silica gel on TLC plates | Merck | 60768 | TLC plate |
| Sodium acetate | Sigma-Aldrich | S8750 | reagent |
| Sodium azide | Sigma-Aldrich | S2002 | reagent |
| Sodium borohydride | Sigma-Aldrich | 452882 | reagent |
| Sodium carbonate | Sigma-Aldrich | S2127 | reagent |
| tert-Butyldimethylsilyl chloride | Sigma-Aldrich | 190500 | reagent |
| Tetrahydrofuran | Sigma-Aldrich | 401757 | solvent |
| Toluene | Sigma-Aldrich | 244511 | solvent |
| Zinc bromide | Sigma-Aldrich | 230022 | reagent |
| Zinc chloride | Sigma-Aldrich | 429430 | reagent |
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