Lithiation-induced migrations from nitrogen to carbon in terminal aziridines

Synthetic Methods for Azaheterocyclic Phosphonates and Their Biological Activity: An Update 2004-2024

The increasing importance of azaheterocyclic phosphonates in the agrochemical, synthetic, and medicinal field has provoked an intense search in the development of synthetic routes for obtaining novel members of this family of compounds. This updated review covers methodologies established since 2004, focusing on the synthesis of azaheterocyclic phosphonates, of which the phosphonate moiety is directly substituted onto to the azaheterocyclic structure. Emphasizing recent advances, this review classifies newly developed synthetic approaches according to the ring size and providing information on biological activities whenever available. Furthermore, this review summarizes information on various methods for the formation of C-P bonds, examining sustainable approaches such as the Michaelis-Arbuzov reaction, the Michaelis-Becker reaction, the Pudovik reaction, the Hirao coupling, and the Kabachnik-Fields reaction. After analyzing the biological activities and applications of azaheterocyclic phosphonates investigated in recent years, a predominant focus on the evaluation of these compounds as anticancer agents is evident. Furthermore, emerging applications underline the versatility and potential of these compounds, highlighting the need for continued research on synthetic methods to expand this interesting family.

Synthesis of Selenoaziridines: A Study on Stereochemical Outcomes of the Reaction of Aziridine Radicals and Anions Generated from Iodoaziridines

The synthesis of a new functional group in the form of selenyl-substituted aziridines is described. Selenoaziridines are stereoselectively prepared by functionalization of intact aziridine precursors involving radical and anionic intermediates. Radicals are generated from cis-N-Ts iodoaziridines by activation of the C-I bond using alkoxides as a source of single electrons. These form predominantly trans-substituted selenoaziridines dependent on the size of the diselenide. cis-Aziridinyllithiums generated by Li-I exchange also react with diselenides stereospecifically to form a range of cis-selenoaziridines. Proposals for the stereochemical outcome are presented.

Multiple-functionalizations of terminal alkynes with sodium sulfinates and tert-butyl nitrite: facile synthesis of 2H-azirines

A new, catalyst-free tandem annulation route to 2,2-disulfonyl-2H-azirines via multiple-functionalizations of terminal alkynes with sodium sulfinates and tert-butyl nitrite is described.

Enantiomeric Variability of Distaminolyne A. Refinement of ECD and NMR Methods for Determining Optical Purity of 1-Amino-2-Alkanols

Sample configurations of distaminolyne A (1a); isolated from the ascidians Pseudodistoma opacum and P. cereum, and collected at different sites in New Zealand, were investigated by two methods: Exciton coupled electronic circular dichroism (EC ECD) of the corresponding N,O-dibenzoyl derivative 1b; and chiral reagent derivatization of 1a with (S)- and (R)-α-methoxyphenylacetic acid (MPA), followed by ¹H-NMR analysis. Configuration and optical purity of 1a (%ee) was found to vary depending on the geographic distribution of ascidian colonies. An improved method for preparing N,O-diarenoyl derivatives of 1a was optimized. The EC ECD method was found to be complementary to the MPA-NMR method at different ranges of %ee.

Synthesis and kinetic resolution of substituted tetrahydroquinolines by lithiation then electrophilic quench

Treatment of N-Boc-2-aryl-1,2,3,4-tetrahydroquinolines with n-butyllithium in THF at -78 °C resulted in efficient lithiation at the 2-position and the organolithiums were trapped with a variety of electrophiles to give substituted products. Variable temperature NMR spectroscopy gave kinetic data that showed that the rate of tert-butoxycarbonyl (Boc) rotation was fast (ΔG^‡ ≈ 45 kJ mol^-1 at -78 °C) and in situ ReactIR spectroscopy showed fast lithiation at -78 °C. By carrying out the lithiation in the presence of the chiral ligand sparteine, kinetic resolutions with very high levels of enantioselectivity were achieved. The resulting enantioenriched N-Boc-2-aryltetrahydroquinolines were converted to 2,2-disubstituted products without significant loss in enantiopurity. Most electrophiles add at the 2-position and the chemistry provides a way to access tetrahydroquinolines that are fully substituted alpha to the nitrogen atom. Notably, either enantiomer of the 2,2-disubstituted tetrahydroquinolines can be obtained with high selectivity from the same enantiomer of the chiral ligand. Unusually, when methyl cyanoformate was used as the electrophile, substitution occurred in the ortho position of the aryl ring attached at C-2. This change in regioselectivity on changing the electrophile was probed by deuterium isotope studies and by DFT calculations which suggested that the binding of the cyanoformate altered the structure of the intermediate organolithium. Secondary amine products can be prepared by removing the Boc group with acid or by inducing the Boc group to rearrange to the 2-position in the presence of triethylborane and this carbonyl N-to-C rearrangement occurs with retention of configuration from the intermediate enantiomerically enriched organolithium species.

Recent advances in the chemistry of metallated azetidines

The almost unexplored four-membered azetidines represent a particularly interesting class of molecules, among the family of saturated nitrogen heterocycles. This review reports recent developments in direct metal-based functionalization of the azetidine ring, focusing on the regio- and stereoselectivity of these reactions.

Synthesis of trifluoromethyl γ-aminophosphonates by nucleophilic aziridine ring opening

Phosphonated derivatives of trifluoromethyl aziridine were obtained with good yield from aziridine-2-carbaldehyde by two distinct methods, which resulted in different diastereoselectivities. Using thiols as nucleophiles ring opening reactions of trifluoromethylated derivatives of aziridine-2-phosphonates proceeded regio- and diastereoselectively, giving rise to γ-amino-γ-trifluoromethyl phosphonates.

Two Scaffolds from Two Flips: (α,β)/(β,γ) CH2/NH "Met-Im" Analogues of dTTP

Novel α,β-CH2 and β,γ-NH (1a) or α,β-NH and β,γ-CH2 (1b) "Met-Im" dTTPs were synthesized via monodemethylation of triethyl-dimethyl phosphorimido-bisphosphonate synthons (4a, 4b), formed via a base-induced [1,3]-rearrangement of precursors (3a, 3b) in a reaction with dimethyl or diethyl phosphochloridate. Anomerization during final bromotrimethylsilane (BTMS) deprotection after Mitsunobu conjugation with dT was avoided by microwave conditions. 1a was 9-fold more potent in inhibiting DNA polymerase β, attributed to an NH-group interaction with R183 in the active site.

Synthesis of chiral N-phosphoryl aziridines through enantioselective aziridination of alkenes with phosphoryl azide via Co(II)-based metalloradical catalysis

The Co(II) complex of a new D 2-symmetric chiral porphyrin 3,5-DiMes-QingPhyrin, [Co(P6)], can catalyze asymmetric aziridination of alkenes with bis(2,2,2-trichloroethyl)phosphoryl azide (TcepN3) as a nitrene source. This new Co(II)-based metalloradical aziridination is suitable for different aromatic olefins, producing the corresponding N-phosphorylaziridines in good to excellent yields (up to 99%) with moderate to high enantioselectivities (up to 85% ee). In addition to mild reaction conditions and generation of N2 as the only byproduct, this new metalloradical catalytic system is highlighted with a practical protocol that operates under neutral and non-oxidative conditions.

Air- and moisture-stable amphoteric molecules: enabling reagents in synthesis

Researchers continue to develop chemoselective synthesis strategies with the goal of rapidly assembling complex molecules. As one appealing approach, chemists are searching for new building blocks that include multiple functional groups with orthogonal chemical reactivity. Amphoteric molecules that possess nucleophilic and electrophilic sites offer a versatile platform for the development of chemoselective transformations. As part of a program focused on new methods of synthesis, we have been developing this type of reagents. This Account highlights examples of amphoteric molecules developed by our lab since 2006. We have prepared and evaluated aziridine aldehydes, a class of stable unprotected α-amino aldehydes. Structurally, aziridine aldehydes include both a nucleophilic amine nitrogen and an electrophilic aldehyde carbon over the span of three atoms. Under ambient conditions, these compounds exist as homochiral dimers with an aziridine-fused five-membered cyclic hemiaminal structure. We have investigated chemoselective reactions of aziridine aldehydes that involve both the aziridine and aldehyde functionalities. These transformations have produced a variety of densely functionalized nitrogen-containing compounds, including amino aldehydes, 1,2-diamines, reduced hydantoins, C-vinyl or alkynyl aziridines, and macrocyclic peptides. We have also developed air- and moisture-stable α-boryl aldehydes, another class of molecules that are kinetically amphoteric. The α-boryl aldehydes contain a tetracoordinated N-methyliminodiacetyl (MIDA) boryl substituent, which stabilizes the α-metalloid carbonyl system and prevents isomerization to its O-bond enolate form. Primarily taking advantage of chemoselective transformations at the aldehyde functionality, these α-boryl aldehydes have allowed us to synthesize a series of new functionalized boron-containing compounds that are difficult or impossible to prepare using established protocols, such as α-borylcarboxylic acids, boryl alcohols, enol ethers, and enamides. Using α-borylcarboxylic acids as starting materials, we have also prepared several new amphoteric borylated reagents, such as α-boryl isocyanates, isocyanides, and acylboronates. These compounds are versatile building blocks in their own right, enabling the rapid synthesis of other boron-containing molecules.

Generation and electrophile trapping of N-Boc-2-lithio-2-azetine: synthesis of 2-substituted 2-azetines

s-BuLi-induced α-lithiation–elimination of LiOMe from N-Boc-3-methoxyazetidine and further in situ α-lithiation generates N-Boc-2-lithio-2-azetine which can be trapped with electrophiles, either directly (carbonyl or heteroatom electrophiles) or after transmetalation to copper (allowing allylations and propargylations), providing a concise access to 2-substituted 2-azetines.

[Asymmetric syntheses of azacycloalkanes by one-pot [1+2+n] cyclization]

Enantio- and diastereoselective one-pot synthesis of three- to seven-membered cis-azaheterocycles was achieved using a triggered asymmetric conjugate addition reaction of lithium amide with an enoate, followed by C- and N-alkylation of the resulting lithium 3-aminoenolate with α, ω-dihaloalkane, which can be regarded as a [1+2+n] cyclization. The usefulness of the developed methodology was clearly demonstrated by the short-step asymmetric syntheses of azirinomycin ester, nemonapride, and kopsinine.

Solution-dispersible Au nanocube dimers with greatly enhanced two-photon luminescence and SERS

We report the synthesis of 43-nm diameter Au nanocube dimers by using Ag(+) ions as competitive ligands to freeze L-cysteine-induced assembly process of the nanocubes to a desirable stage. Ascribed to the resonant interparticle coupling with an newly arising plasmon band at 710 nm and local field enhancement, the two-photon luminescence intensity of the Au nanocube dimers in solution was over 20 times stronger than that of the monomers in the wavelength range 555-620 nm. Furthermore, by coupling Raman tags onto the nanocube surface, a solution-based surface-enhanced Raman scattering (SERS) of the nanocube dimers had an enhancement factor of over 10 times compared to the isolated nanocubes. To sum up, with high stability in solution and attractive optical properties, the Au nanocube dimers have potential applications in in vivo bio-imaging and solution-based SERS.

Highly cis-selective synthesis of iodo-aziridines using diiodomethyllithium and in situ generated N-Boc-imines

The first preparation of iodoaziridines is described. The addition of diiodomethyllithium to N-Boc-imines affords these novel aziridines in high yields. The reaction proceeds in one-pot via a highly diastereoselective cyclisation of an amino gem-diiodide intermediate.

Palladium-catalyzed cross-coupling of aziridinylmetal species, generated by sulfinyl-magnesium exchange, with aryl bromides: reaction optimization, scope, and kinetic investigations

A series of novel, highly substituted N-PMP aziridines have been accessed in high yields by palladium-catalyzed cross-coupling of intact aziridines. The cross-coupling employed aryl bromides and tertiary organometallic aziridines, generated from sulfinylaziridines by sulfinyl-magnesium exchange and transmetalation to the aziridinylzinc with zinc chloride. A wide range of electron-rich and electron-poor aryl bromides were utilized to afford the functionalized aziridine products as single diastereoisomers with retention of configuration at the reacting center. Assessment of the reaction kinetics showed zero-order in both the aziridine species and the aryl bromide. This indicated that the rate-determining step was reductive elimination from the palladium(II) species bearing both the aziridine and aryl groups to form the hindered C-C bond. The intermediate aziridinylzinc species underwent a progressive, background degradation that led to a plateau in yield and afforded reduced yields in substrates with ortho-substituted aryl groups, which are less reactive due to the additional steric demands.

General entry to asymmetric one-pot [N + 2 + n] cyclization for the synthesis of three- to seven-membered azacycloalkanes

Enantio- and diastereoselective one-pot synthesis of three- to seven-membered cis-azaheterocycles was achieved using a triggered asymmetric conjugate addition reaction of lithium amide with an enoate, followed by alkylation of the resulting lithium enolate with α,ω-dihaloalkane and N-alkylation. Isomerization of cis-azaheterocycles with a base yielded the trans-product, constituting a one-pot synthesis of cis-azacycles and a two-step synthesis of trans-azacycles. The four-step asymmetric synthesis of nemonapride highlights the general utility of the method.

α,β-Aziridinylphosphonates by lithium amide-induced phosphonyl migration from nitrogen to carbon in terminal aziridines

N-Phosphonate terminal aziridines undergo lithium 2,2,6,6-tetramethylpiperidide-induced N- to C-[1,2]-anionic phosphonyl group migration under experimentally straightforward conditions, to provide a stereocontrolled access to synthetically valuable trans-α,β-aziridinylphosphonates. The utility of this chemistry has been demonstrated in the asymmetric synthesis of a β-aminophosphonate.

Lithiation-electrophilic substitution of N-thiopivaloylazetidine

The fourth protocol: the rarely studied N-thiopivaloyl group plays a crucial role in mediating efficient α lithiation and incorporation of diverse electrophiles onto an azetidine ring; in the presence of chiral ligands, this chemistry also provides the first example of an enantioselective electrophilic substitution on a four-membered ring.

Synthesis of optically active arylaziridines by regio- and stereospecific lithiation of N-bus-phenylaziridine

Alpha,alpha-disubstituted aziridines can be produced in good yields by selective lithiation of N-tert-butylsulfonyl-2-phenylaziridine (n-BuLi/TMEDA, Et(2)O) at the benzylic position and subsequent trapping with a range of electrophiles. Repetition of the lithiation/electrophilic trapping sequence provides a stereocontrolled route to trisubstituted aziridines. Using (R)-N-tert-butylsulfonyl-2-phenylaziridine, the alpha,alpha-disubstituted aziridines can be produced as single enantiomers (er >98:2), indicating that the intermediate organolithium is configurationally stable. Efficient aziridine ring-opening reactions leading to 1,2-diamines and 1,4-diamines are also reported.

Terminal aziridines by alpha-deprotonation/electrophile trapping of N-protected aziridine

N-tert-Butylsulfonyl and N-tert-butylsulfinyl aziridine undergo alpha-lithiation/electrophile trapping providing a new entry to terminal aziridines. With N-tert-butylsulfinyl aziridine complete asymmetric induction is observed alpha to nitrogen.