Preparation of a Stable Bicyclic Aziridinium Ion and Its Ring Expansion toward Piperidines and Azepanes

Gilman reagent toward the synthesis of natural products

With the ever-increasing scope of organocuprates, a well-established Gilman reagent has been considered as an unprecedented synthetic tool in modern organic chemistry. The broad research profile of the Gilman reagent (R2CuLi in THF or Et2O) is owing to its propensity to carry out three kinds of reactions, i.e., epoxide ring opening reactions, 1,4-conjugate addition reactions, and SN2 reactions in a regioselective manner. This review examines the applications of Gilman reagent in the total synthesis of both abundant and scarce natural products of remarkable synthetic pharmaceutical profile reported since 2011. The presented insights will be of a vital roadmap to general organic synthesis and it will contribute to the development of new natural products and their analogues in future drug discovery.

Regioselective ring opening of aziridine for synthesizing azaheterocycle

Aziridine had different regioselective ring openings depending on the functional group of its alkyl substituent. In the case of the alkyl group bearing γ-ketone at the C2 substituent of aziridine, the ring opening by the hydroxy nucleophile from H2O occurred by attacking the aziridine carbon at the C2 position. This reaction proceeded efficiently in the presence of CF3CO2H. Interestingly, the same starting aziridine ring bearing the alkyl substituent at the C2 position with the γ-silylated hydroxy group instead of γ-ketone led to the ring-opening reaction by the same oxygen nucleophile at the unsubstituted C3 position, with the breakage of the bond between aziridine N1 nitrogen and carbon at C3. These reaction products were cyclized to afford substituted pyrrolidine and piperidine rings with representative examples of congeners of pseudoconhydrine and monomorine.

Synthesis of Functionalized Azepines via Cu(I)-Catalyzed Tandem Amination/Cyclization Reaction of Fluorinated Allenynes

An efficient method for the selective preparation of trifluoromethyl-substituted azepin-2-carboxylates and their phosphorous analogues has been developed via Cu(I)-catalyzed tandem amination/cyclization reaction of functionalized allenynes with primary and secondary amines.

Green synthesis and investigation of antioxidant and antimicrobial activity of new schiff base of pyrimidoazepine derivatives: application of Fe3O4/CuO/ZnO@MWCNT MNCs as an efficient organometallic nanocatalyst

In this study, we synthesized schiff base of pyrimidoazepine derivatives in high yields using multicomponent reactions of isatins, alkyl bromides, activated acetylenic compounds, guanidine and aldehydes in the presence of Fe₃O₄/CuO/ZnO@ Multi Walled Carbon Nanotubes (MWCNT) as a high performance catalyst in water at room temperature. The Fe₃O₄/CuO/ZnO@MWCNT synthesizes using Petasites hybridus rhizome water extract as a green media and moderate base. As well Fe₃O₄/CuO/ZnO@MWCNT magnetic nanocomposites show a good improvement in the yield of the product and displayed significant reusable activity. Investigation of antioxidant ability of synthesized compounds using radical trapping of diphenyl-picrylhydrazine and ferric reduction power experiment is another purpose in this research. Also, the antimicrobial activity of some synthesized compounds proved by employing the disk diffusion test on Gram-positive and Gram-negative bacteria. This procedure has some benefits such as short reaction time, product with excellent yields, simple catalyst and products separation.

Employing of Fe3O4/CuO/ZnO@MWCNT MNCs in the solvent-free synthesis of new cyanopyrroloazepine derivatives and investigation of biological activity

In this research, we synthesized the Fe₃O₄/CuO/ZnO@MWCNT magnetic nanocomposites using water extract of Petasites hybridus rhizome, and the high performance of synthesized catalyst was confirmed by using in the solvent-free multicomponent reactions of isatoic anhydride, N-methylimidazole, alkyl bromides, activated acetylenic compounds and 2-aminoacetonitrile at ambient temperature for the production of new cyanopyrroloazepine derivatives in high yields. This catalyst could be used several times in these reactions and have main role in the yield of product. The synthesized cyanopyrroloazepines have NH groups in their structure and for this reason have good antioxidant activity. Also, employing Gram-positive and Gram-negative bacteria in the disk diffusion procedure confirmed some cyanopyrroloazepines antimicrobial effect. The results showed that synthesized cyanopyrroloazepine prevented the bacterial growth. This used process for preparation of new cyanopyrroloazepine has some improvements such as low reaction time, product with high yields, simple separation of catalyst and products.

Synthetic Applications of Aziridinium Ions

Nonactivated aziridine with an electron-donating group at the ring nitrogen should be activated to an aziridinium ion prior to being converted to cyclic and acyclic nitrogen-containing molecules. This review describes ways to generate aziridinium ions and their utilization for synthetic purposes. Specifically, the intra- and intermolecular formation of aziridinium ions with proper electrophiles are classified, and their regio- and stereoselective transformations with nucleophiles are described on the basis of recent developments.

Alkylative Aziridine Ring-Opening Reactions

In this study, the highly strained three-membered aziridine ring was successfully activated as the aziridinium ion by alkylation of the ring nitrogen with a methyl, ethyl or allyl group, which was followed by ring opening with external nucleophiles such as acetate and azide. Such alkylative aziridine ring opening provides an easy route for the synthesis of various N-alkylated amine-containing molecules with concomitant introduction of an external nucleophile at either its α- or β-position.

Alkylative Ring-Opening of Bicyclic Aziridinium Ion and Its Application for Alkaloid Synthesis

Alkylative ring-opening of bicyclic aziridinium ion generated from 4-hydroxybutylaziridine with organocopper reagent was achieved successfully to afford 2-alkylsubstituted piperidine in high or moderate yield. This method allowed carbon-carbon bond formation of "non-activated" aziridine via aziridinium ion ring-opening in regio- and stereo-selective manner for the first time. This newly developed reaction was applied for an efficient synthesis of alkaloid with the representative example of conine and epiquinamide.

Synthesis of Highly Substituted Azepanones from 2 H-Azirines by a Stepwise Annulation/Ring-Opening Sequence

Bicyclic aziridines possessing a 1-azabicyclo[4.1.0]heptan-2-one core were prepared from 2 H-azirines by a stepwise annulation sequence involving a diastereoselective allylindanation, an N-acylation, and a ring-closing metathesis to construct the six-membered ring. After hydrogenation or functionalization of the olefin, regioselective ring opening of the resulting azabicyclic compounds with carboxylic acids (or sulfur nucleophiles) afforded highly substituted azepanones possessing an ester moiety or a trifluoromethyl group and a tetrasubstituted carbon at the α and β positions of the nitrogen atom, respectively.

Theoretical insight into the regioselective ring-expansions of bicyclic aziridinium ions

Transient bicyclic aziridinium ions are known to undergo ring-expansion reactions, paving the way to functionalized nitrogen-containing heterocycles. In this study, the regioselectivity observed in the ring-expansion reactions of 1-azoniabicyclo[n.1.0]alkanes was investigated from a computational viewpoint to study the ring-expansion pathways of two bicyclic systems with different ring sizes. Moreover, several nucleophiles leading to different experimental results were investigated. The effect of solvation was taken into account using both explicit and implicit solvent models. This theoretical rationalization provides valuable insight into the observed regioselectivity and may be used as a predictive tool in future studies.

Total Synthesis and Absolute Stereochemical Assignment of Microgrewiapine A and Its Stereoisomers

Total synthesis of both enantiomers of (-)-(2 S,3 R,6 S)- and (+)-(2 R,3 S,6 R)-microgrewiapine A along with (+)-microcosamine A and (-)-6- epi-microgrewiapine A from chiral 1-(α-methylbenzyl)-aziridine-2-carboxylate was accomplished for the first time. Key steps involved in this synthesis include one-pot reductive ring-opening of aziridine, debenzylation, intramolecular N-alkylation to obtain the key piperidine ring, and Julia-Kociensky olefination. The absolute configuration of natural microgrewiapine A is assigned as (+)-(2 R,3 S,6 R), which is opposite to the originally proposed structure by comparing optical rotation data of both synthetic enantiomers.

Atom-Economical and Metal-Free Synthesis of Multisubstituted Furans from Intramolecular Aziridine Ring Opening

Multisubstituted furans were prepared from dialkyl 2-(aziridin-2-ylmethylene)malonate and/or 1,3-dione through aziridine ring opening by the internal carbonyl oxygen with the assistance of BF3·OEt2, followed by aromatization. This synthetic method is free from any metal and is atom-economical with all of the atoms in the starting material retained in the final product.