Allylic azides: synthesis, reactivity, and the Winstein rearrangement

Stereocontrol in an intermolecular Schmidt reaction of equilibrating hydroxyalkyl allylic azides

A selective intermolecular Schmidt reaction of equilibrating hydroxyalkyl allylic azides is reported to afford N-hydroxyalk-1-en-3-yl lactams in modest to high yields. For prochiral and chiral ketones, modest to high 1,5-diastereoselectivity was achieved, and the mechanistic analysis is supported by DFT calculation.

Regio- and Stereoselective Iodoamination of Ferrocene-Containing Allenylphosphonates: Synthesis of Multifunctional Tetrasubstituted Allylic Amines and Allylic Azides

A general and practical methodology for the regio- and stereoselective synthesis of multifunctional tetrasubstituted allylic amines and azides based on iodoamination of ferrocene-containing allenylphosphonates with anilines and sodium azide is described. A tetrasubstituted olefin moiety, as well as an iodine atom, a phosphonate, and a ferrocene group, are installed to the allylic amine motif simultaneously in moderate to good yields.

Synthesis of Three-Dimensional Ring Fused Heterocycles by a Selective [4 + 2] Cycloaddition Between Bicyclic Thiazolo 2-Pyridones and Arynes

A selective [4 + 2] cycloaddition reaction of thiazolo-2-pyridones with arynes has been demonstrated. The developed protocol allows rapid access to highly functionalized, structurally complex thiazolo-fused bridged isoquinolones in high yields, which are susceptible to further late-stage functionalization.

Progress toward the Total Synthesis of Jogyamycin Using a Tandem Ichikawa/Winstein Rearrangement

Jogyamycin is a densely functionalized aminocyclopentitol that displays potent antiprotozoal activity. Herein, we report a route toward this natural product that utilizes an unprecedented transformation involving a tandem Ichikawa-Winstein rearrangement to install the C-1/C-2 diamine core. Attempts to further functionalize the C-3/C-4 alkene en route to jogyamycin are also discussed.

Synthesis, molecular docking studies of formononetin derivatives as potent Bax agonists for anticancer activity

Formononetin as a Bax agonist exhibits anticancer effects. To identify novel Bax agonist, 18 new structurally modified formononetin derivatives were synthesised and their anticancer activities were evaluated in the A549 and Beas-2b cell lines. The results indicated that 7a elicited the most potent inhibitory effect against the A549 cell line, with an IC50 value of 0.87 μM, and no obvious toxicity to Beas-2b cells. These results indicated that 7a was 40-fold and 6.94-fold more efficacious than Formononetin and Doxorubicin, respectively. Additionally, western blot and immunofluorescence assays demonstrated that 7a downregulated the protein expression of Bcl-2 and upregulated the expressions of Bax to promote A549 apoptosis, the obtained results also suggested that 7a had the potential to be developed into a lead compound that can be applied in the prevention and treatment of lung cancer.

Catalytic Enantioselective Construction of Chiral γ-Azido Nitriles through Nitrile Group-Promoted Electrophilic Reaction of Alkenes

An efficient approach for the construction of enantioenriched γ-azido nitriles through the chiral sulfide-catalyzed asymmetric electrophilic thioazidation of allylic nitriles is disclosed. A wide range of electron-deficient and -rich aryl, heterocyclic aryl, and alkyl substituents are suitable on the substrates of allylic nitriles. The regio-, enantio-, and diastereoselectivities of the reactions are excellent. As versatile platform molecules, the obtained chiral γ-azido nitriles can be easily converted into high-value-added chiral molecules that are not easily accessed by other methods. Control experiments revealed that the allylic nitrile group is important for control of the reactivity and enantioselectivity of the reaction leading to a broad substrate scope.

Synthesis of a New α-Azidomethyl Styrene from Safrole via a Dearomative Rearrangement

There is a growing interest in developing more efficient synthetic alternatives for the synthesis of nitrogen-containing allylic compounds. This article presents a straightforward two-step protocol to produce 5-(3-azidoprop-1-en-2-yl)benzo[d][1,3]dioxole 4 from the natural product safrole. The method yielded the expected α-azidomethyl styrene 4, in good yield, via a dearomative rearrangement.

Cascade Cope/Winstein Rearrangements: Synthesis of Azido-Cycloheptadienes from Dialkenylcyclopropanes Possessing a Vinyl Azide

cis-1,2-Dialkenylcyclopropanes incorporating a vinyl azide, generated by Knoevenagel condensations between the corresponding cyclopropanecarbaldehydes and α-azido ketones, undergo cascade Cope and Winstein [3,3]-sigmatropic rearrangements, under mild conditions. The sequence allows access to diversely substituted 1,4-cycloheptadienes armed with a secondary allylic azide with up to three stereocenters.

Radical-Mediated Functionalization of Internal Alkenes: Synthesis of Multisubstituted Allylic and Homoallylic Azides

Radical-mediated functionalization of alkenes provides a powerful tool for transformation of simple alkenes into numerous value-added products. The precedent radical functionalization of alkenes is mainly restricted to terminal alkenes, while the conversion of internal alkenes generally remains challenging, as the increased steric congestion on alkenes significantly conflicts with the intermolecular addition of radicals. Herein, we describe an efficient photoredox catalytic functionalization of internal trisubstituted alkenes, leading to a plethora of valuable multifunctionalized allylic and homoallylic azides, which are otherwise difficult to obtain. The azide products serve as versatile feedstock for construction of useful heterocycles. Allylic or homoallylic azides are selectively generated in the transformation, regulated by the regioselective deprotonation process. This method also features mild reaction conditions and high product diversity.

Alternative approaches utilizing click chemistry to develop next-generation analogs of solithromycin

The marked rise in bacterial drug resistance has created an urgent need for novel antibacterials belonging to new drug classes and ideally possessing new mechanisms of action. The superior biological activity of solithromycin against streptococci and other bacteria causative of community-acquired pneumonia pathogens, compared to telithromycin and other macrolides encouraged us to extensively explore this class of antibiotics. We, thus, present the design and synthesis of a novel series of solithromycin analogs. Three main strategies were pursued in structure-activity relationship studies covering the N-11 side chain and the desosamine motif, which are both chief elements for establishing strong interactions with the bacterial ribosome as the molecular target. Minimal inhibitory concentration assays were determined to assess the in vitro potency of the various analogs in relation to solithromycin. Two analogs exhibited improved activity compared to solithromycin against resistant strains, which can be assessed in further pre-clinical studies.

Intramolecular amination via acid-catalyzed rearrangement of azides: a potent alternative to intermolecular direct electrophilic route

Although acid-catalyzed intramolecular rearrangement of organic azides is an attractive route to amines, its mechanism and synthetic prospective are still debated. Herein, through computational and experimental studies, we demonstrated that azide intramolecular rearrangement could serve as a potent synthetic route to a sought-after amine functionality including preparation of difficult to access and valuable heterocyclic amines. Using quantum chemical calculations at MP2/aug-cc-pVTZ and B3LYP/aug-cc-pVDZ levels, we discovered that this reaction proceeds via a concerted transition state with nitrogen elimination and alkyl/aryl migration occurring at the same time. Two conformers of protonated azides - syn- and anti- - were shown to precede corresponding transition states. It was shown that the reaction follows Curtin-Hammett scenario as the energy gap required for conformer interconversion was substantially lower than activation barrier of either transition state. Intramolecular amination via azide rearrangement was predicted to be a selective process with migratory aptitude increasing in a row alkyl<Ar-EWG<Ar-EDG (EWG - electron withdrawing group; EDG - electron donating group), which was supported by experimental results. We demonstrated experimentally that organic azides can be generated from stilbenes in situ and selectively undergo rearrangement to corresponding amines in a cascade fashion via amino-dealkenylation reaction.

Selective hydroboration of equilibrating allylic azides

The iridium(I)-catalyzed hydroboration of equilibrating allylic azides is reported to provide only the anti-Markovnikov product of alk-1-ene isomers in good yields and with good functional group tolerance.

Intercepting the Banert cascade with nucleophilic fluorine: direct access to α-fluorinated NH-1,2,3-triazoles

The treatment of propargylic azides with silver(i) fluoride in acetonitrile was found to yield α-fluorinated NH-1,2,3-triazoles via the Banert cascade. The reaction was regioselective and the products result from an initial [3,3] rearrangement. The reaction is demonstrated on >15 examples with yields ranging from 37% to 86%.

Enantioselective Nickel-Catalyzed Alkyne-Azide Cycloaddition by Dynamic Kinetic Resolution

The triazole heterocycle has been widely adopted as an isostere for the amide bond. Many native amides are α-chiral, being derived from amino acids. This makes α-N-chiral triazoles attractive building blocks. This report describes the first enantioselective triazole synthesis that proceeds via nickel-catalyzed alkyne-azide cycloaddition (NiAAC). This dynamic kinetic resolution is enabled by a spontaneous [3,3]-sigmatropic rearrangement of the allylic azide. The 1,4,5-trisubstituted triazole products, derived from internal alkynes, are complementary to those commonly obtained by the related CuAAC reaction. Initial mechanistic experiments indicate that the NiAAC reaction proceeds through a monometallic Ni complex, which is distinct from the CuAAC manifold.

A cascade reaction of cinnamyl azides with vinyl sulfones directly generates dihydro-pyrrolo-pyrazole heterocycles

This report describes the direct synthesis of dihydro-pyrrolo-pyrazole heterocycles from allylic azides and methyl vinyl sulfone. The product results from a complex cascade reaction that is operationally straightforward, with aromatization being the result of a concomitant elimination step. A variety of azides could participate in this reaction (12 examples) and the isolated yields of the desired product ranged from 51%-72%. Lastly the ethylene sulfone group could be removed by heating the product in pyrrolidine.

Highly selective hydrosilylation of equilibrating allylic azides

The Pt-catalyzed hydrosilylation of equilibrating allylic azides is reported. The reaction provides only one out of four possible hydrosilylation products in good yields and with very high chemoselectivity (alk-1-ene vs. alk-2-ene), regioselectivity (linear vs. branched), and excellent functional group tolerance.

A Cascade Reaction of Cinnamyl Azides with Acrylates Directly Generates Tetrahydro-Pyrrolo-Pyrazole Heterocycles

Developing reactions to generate complex and modular building blocks in a concise and direct fashion remains a contemporary synthetic challenge. This work describes a stereoselective cascade reaction between allylic azides and acrylates that directly generates tetrahydro-pyrrolo-pyrazole ring systems. These products contain up to four contiguous stereocenters, two of which may be tetrasubstituted carbon atoms attached to a nitrogen atom. Over 30 examples are provided with an average isolated yield of 71% (ranging from 40% to 94%). The reaction was easily scaled to use more than one gram of starting material, and the products can be readily diversified.

Stereocontrolled Microwave-Assisted Domino [3,3]-Sigmatropic Reactions: A Winstein-Overman Rearrangement for the Formation of Differentiated Contiguous C-N Bonds

A domino [3,3]-sigmatropic rearrangement sequence employing a sequential reversible allylic azide rearrangement followed by an irreversible Overman reaction provides a new route to the formation of two contiguous C-N bonds. The reaction occurs in a stereocontrolled fashion in two steps from readily available alkenyl epoxides via initial azide anion ring opening of the epoxides.

H-bond donor-directed switching of diastereoselectivity in the Michael addition of α-azido ketones to nitroolefins

The development of catalyst-controlled stereodivergent asymmetric catalysis is important for providing facile access to all stereoisomers of chiral products with multiple stereocenters from the same starting materials. Despite progress, new design strategies for diastereodivergent asymmetric catalysis are still highly desirable. Here we report the potency of H-bond donors as the governing factor to tune diastereoselectivity in a highly diastereoselective switchable enantioselective Michael addition of α-azido ketones to nitroolefins. While a newly developed bifunctional tertiary amine, phosphoramide, preferentially afforded syn-adducts, an analogous squaramide catalyst selectively gave anti-adducts. The resulting multifunctional tertiary azides can be converted to spiro-pyrrolidines with four continuous stereocenters in a one-pot operation. Mechanistic studies cast light on the control of diastereoselectivity by H-bond donors. While the squaramide-catalyzed reaction proceeded with a transition state with both squaramide N–H bonds binding to an enolate intermediate, an unprecedented model was proposed for the phosphoramide-mediated reaction wherein an amide N–H bond and an alkylammonium ion formed in situ interact with nitroolefins, with the enolate stabilized by nonclassical C–H⋯O hydrogen-bonding interactions.

We report the successful reversal of the diastereoselectivity in an unprecedented Michael addition of α-azido ketones to nitroolefins catalyzed by bifunctional tertiary amines, simply by varying the H-bond donor from phosphoramide to squaramide.

Divergent Mechanisms of the Banert Cascade with Propargyl Azides

Triazoles are privileged heterocycles for a variety of applications. The synthesis of 1H-triazoles can be accomplished by the Banert cascade from propargylic azides. Depending on the substrate and conditions, the Banert cascade can proceed by either a sigmatropic or prototropic mechanism. This report describes the first detailed kinetic analysis of the Banert cascade proceeding by both pathways including substituent effects and KIE. The analysis identified the inflection point in the divergent pathways, allowing future work to predict which Banert products are accessible.

Enantioselective Cu(I)-Catalyzed Cycloaddition of Prochiral Diazides with Terminal or 1-Iodoalkynes

We report an unprecedented highly enantioselective desymmetric Cu(I)-catalyzed 1,3-dipolar cycloaddition of diazides with terminal alkynes and 1-iodoalkynes, affording tertiary alcohols bearing a 1,2,3-triazole moiety in high yield and excellent ee value. PYBOX ligands with a C4 shielding group once again show the promising ability to achieve higher enantioselectivity.

Insight into the factors controlling the equilibrium of allylic azides

Several allylic azides with different double bond substitutions were studied to understand the factors, governing their equilibrium using density functional theory along with the quantum theory of atoms in molecules, non-covalent interactions and natural bond orbital approaches. The results showed that the hydroxyl group or heteroatoms in allylic azides interact with the molecule through an electrostatic weak interaction in each pair of regioisomers. The equilibrium shifts of substituted allylic azides, compared to non-substituted allylic azides, were not attributed to the presence of specific interactions, such as hydrogen bonds. The observed equilibrium shifts stemmed mainly from the strengthening and weakening of negative hyperconjugative interactions, which were affected by the weak interaction involving the proximal substituent in each regioisomer. A good linear correlation was obtained between the hyperconjugative energies of πC Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C→σ*Z_b interactions and the calculated percentages of the secondary azide and tertiary azide in the equilibrium mixture. Also, the effect of the aromatic ring substituent was analysed using such approaches. This study not only provides insights into the factors controlling the stabilities of the substituted allylic azides, but also settles the basis to predict the regioisomer predominance in the equilibrium mixture.

The factors controlling the allyl azides equilibrium has been studied by different theoretical approaches setting the basis to predict the regioisomers predominance in the equilibrium mixture.

Crystal structures of 4-{(E)-3-[(imino-λ5-aza-nyl-idene)amino]-prop-1-en-yl}-N,N-di-methyl-imidazole-1-sulfonamide and 2-[(imino-λ5-aza-nyl-idene)amino]-4-{(E)-3-[(imino-λ5-aza-nyl-idene)amino]-prop-1-en-yl}-N,N-di-methyl-imidazole-1-sulfonamide

The structures of two azide containing imidazole derivatives are reported. Allylic azides are fairly reactive making them attractive starting compounds to convert into amides. The first, C8H12N6O2S, contains one azide group with an Nα-Nβ distance of 1.229 (2) Å and an Nβ-Nγ distance of 1.128 (2) Å. The second, C8H11N9O2S, contains two azide groups with an average Nα-Nβ distance of 1.249 (2) Å and an average Nβ-Nγ distance of 1.132 (2) Å. Each compound contains a bulky protecting group (di-methyl-amino-sulfon-yl) which can be easily removed under mildly acidic conditions.