Lewis Acid Mediated “endo-dig” Hydroalkoxylation–Reduction on Internal Alkynols for the Stereoselective Synthesis of Cyclic Ethers and 1,4-Oxazepanes

Deciphering substitution effects on reductive hydroalkoxylation of alkynyl aminols for stereoselective synthesis of morpholines and 1,4-oxazepanes: total synthesis of tridemorph and fenpropimorph

Acid catalysed reductive etherification of N-propargyl amino alcohols for the stereoselective synthesis of cis-2,5/2,6-disubstituted morpholines and cis-2,6/2,7-disubstituted oxazepanes has been developed. Mechanistic studies revealed that terminal alkynols gave morpholines via a 6-exo-dig hydroalkoxylation-isomerization-reduction cascade. Interestingly, an alkyne hydration-cyclization-reduction sequence is found to be involved in the formation of oxazepanes from alkyl substituted internal alkynols. The strategy was used as a key step in the total synthesis of fungicides tridemorph and fenpropimorph.

Bimetallic Ru/Ru-Catalyzed Asymmetric One-Pot Sequential Hydrogenations for the Stereodivergent Synthesis of Chiral Lactones

Asymmetric sequential hydrogenations of α-methylene γ- or δ-keto carboxylic acids are established in one-pot using a bimetallic Ru/Ru catalyst system, achieving the stereodivergent synthesis of all four stereoisomers of both chiral γ- and δ-lactones with two non-vicinal carbon stereocenters in high yields (up to 99%) and with excellent stereoselectivities (up to >99% ee and >20:1 dr). The compatibility of the two chiral Ru catalyst systems is investigated in detail, and it is found that the basicity of the reaction system plays a key role in the sequential hydrogenation processes. The protocol can be performed on a gram-scale with a low catalyst loading (up to 11000 S/C) and the resulting products allow for many transformations, particularly for the synthesis of several key intermediates useful for the preparation of chiral drugs and natural products.

Recent Advances in the Synthesis of Diarylheptanoids

In the quest for synthesizing biologically important natural products, medicinal chemists embark on an endless journey. This review focuses on the reports published towards the syntheses of diarylheptanoids, classifying them into linear, tetrahydropyran, diarylether, and biphenyl categories. The synthesis methods for each class from 2013 to 2023 are discussed, providing a comprehensive overview of the advancements in the field. Representative natural product examples are highlighted for each category. The review emphasizes the importance of diarylheptanoids in the realms of chemistry and medicine, showcasing their potential as valuable compounds for medicinal and synthetic chemists.

Phenyliodine bis(trifluoroacetate) as a sustainable reagent: exploring its significance in organic synthesis

Iodine-containing molecules, especially hypervalent iodine compounds, have gained significant attention in organic synthesis. They are valuable and sustainable reagents, leading to a remarkable surge in their use for chemical transformations. One such hypervalent iodine compound, phenyliodine bis(trifluoroacetate)/bis(trifluoroacetoxy)iodobenzene, commonly referred to as PIFA, has emerged as a prominent candidate due to its attributes of facile manipulation, moderate reactivity, low toxicity, and ready availability. PIFA presents an auspicious prospect as a substitute for costly organometallic catalysts and environmentally hazardous oxidants containing heavy metals. PIFA exhibits remarkable catalytic activity, facilitating an array of consequential organic reactions, including sulfenylation, alkylarylation, oxidative coupling, cascade reactions, amination, amidation, ring-rearrangement, carboxylation, and numerous others. Over the past decade, the application of PIFA in synthetic chemistry has witnessed substantial growth, necessitating an updated exploration of this field. In this discourse, we present a concise overview of PIFA's applications as a 'green' reagent in the domain of synthetic organic chemistry. A primary objective of this article is to bring to the forefront the scientific community's awareness of the merits associated with adopting PIFA as an environmentally conscientious alternative to heavy metals.

Leveraging cascade alkynyl Prins cyclization towards the stereoselective synthesis of spiro-furan quinazolinone scaffolds

A TfOH-promoted, metal-free protocol has been unveiled for the synthesis of spiro-furan quinazolinones employing alkynol urea derivatives utilizing alkynyl Prins cyclization reaction. This methodology produces highly functionalized spiro-heterocycles in excellent yields with exclusive E-selectivity under ambient conditions. Furthermore, late-stage modifications incorporate bromide and acetyl functionalities into the synthesized spiro-heterocycles.

Hydroalkoxylation-Initiated Cascade on Sulfone-Tethered Aryl Alkynols Gives Cyclic and Spiro-Heterocyclic β-Ketosulfones

Serendipitous formation of cyclic β-ketosulfones is observed when sulfone-tethered arylalkynols are reacted with base. The reaction involves a base-promoted propargyl sulfone to the allene isomerization/intramolecular hydroalkoxylation/retro-oxa-Michael/6-endo-trig Michael addition cascade. Sulfone-tethered alkynyl acrylates gave stereoselective access to a diverse array of spirocyclic β-ketosulfone benzofuran/isochroman/indolines and sulfone-tethered bridged bicyclo[3.3.1]nonane. These cyclic β-ketosulfones could be readily elaborated into benzofuran-fused cyclic sulfones and tetracyclic spiroindoline.

Total Synthesis of Pyrrolidine and Piperidine Natural Products via TMSOTf-Mediated "5/6-endo-dig" Reductive Hydroamination of Enynyl Amines

Stereoselective syntheses of pyrrolidines and piperidines bearing hydrophobic chains have been achieved through a metal free, Lewis acid-mediated 5/6-endo-dig reductive hydroamination cascade of enynyl amines. The brevity of the developed strategy allowed for the collective stereoselective total synthesis of various alkaloids, including (±)-pyrrolidine cis-225H, (±)-epi-197B, (±)-epi-225C, the family of (+)-solenopsins and (+)-isosolenopsins, and the formal synthesis of (±)-bgugaine and (+)-azimic acid.

Lewis-Acid-Catalyzed Reductive Hydroalkoxylation of Propargylic N-Hydroxylamines Gives Stereoselective Access to Isoxazolidines

Lewis-acid-catalyzed 5-endo-dig reductive hydroalkoxylation cascade on propargylic N-hydroxylamine gave expedient, stereoselective access to isoxazolidine derivatives. The developed method provides a new approach toward the synthesis of isoxazolidine, a biologically privileged scaffold. The synthetic potential of the developed methodology was demonstrated by synthesizing 1,3-aminoalcohol, 4-aminotetrahydropyran, and sedamine natural products.

Transposition of an acrylate moiety in TMSOTf-mediated reaction of alkynyl vinylogous carbonates gives heterocyclic dienes

TMSOTf-mediated reaction of alkynyl vinylogous carbonates serendipitously gave 1,4-oxazepine and dihydropyran dienes via transposition of an ethyl acrylate moiety involving intramolecular cascade Prins-type cyclization/retro-oxa-Michael reaction/cycloisomerisation. The developed atom-economical protocol selectively provides an E double bond geometry. Dihydropyran dienes could be reduced diastereoselectively using Et₃SiH/TMSOTf or could be transformed into polycyclic heterocycles by Heck reaction.

Photo-induced copper-catalyzed alkynylation and amination of remote unactivated C(sp3)-H bonds

A method for remote radical C-H alkynylation and amination of diverse aliphatic alcohols has been developed. The reaction features a copper nucleophile complex formed in situ as a photocatalyst, which reduces the silicon-tethered aliphatic iodide to an alkyl radical to initiate 1,n-hydrogen atom transfer. Unactivated secondary and tertiary C-H bonds at β, γ, and δ positions can be functionalized in a predictable manner.

Diastereoselective Construction of trans-2-Alkyl-6-aryl-3,6-dihydro-2H-pyrans via Dehydrogenative Cycloetherification Promoted by DDQ

A diastereoselective synthesis of trans-2-alkyl-6-aryl-3,6-dihydro-2H-pyrans has been described. Dehydrogenative cycloetherification of (E)-(±)-1-aryl-5-hydroxy-1-alkenes promoted by DDQ proceeded cleanly via 6-endo cyclization to afford trans-2-alkyl-6-aryl-3,6-dihydro-2H-pyrans (32 examples) in good yield (up to 89%) and with moderate to excellent diastereoselectivity (up to 99:1). The synthetic utility of the method was illustrated by the second total synthesis of (±)-(2R,6S)-3,4-dehydro-1,7-bis(4-hydroxy phenyl)-4'-de-O-methyl centrolobine and a total synthesis of (±)-centrolobine.

Restoration of catalytic activity by the preservation of ligand structure: Cu-catalysed asymmetric conjugate addition with 1,1-diborylmethane

Reported herein is a novel reaction engineering protocol to enhance the efficiency of a transition metal-catalysed process by strategically preventing ligand degradation. Based on spectroscopic investigations, a decomposition pathway of a chiral phosphoramidite ligand during a Cu-catalysed reaction was identified. The involvement of the destructive process could be minimized under the modified reaction conditions that control the amount of nucleophilic alkoxide base, which is the origin of ligand decomposition. Overall, the strategy has been successfully applied to a new class of asymmetric conjugate addition reactions with bis[(pinacolato)boryl]methane, in which α,β-unsaturated enones are utilised as substrates.

A novel Cu-catalysed asymmetric conjugate addition reaction with bis[(pinacolato)boryl]methane using α,β-unsaturated enones as substrates has been developed on the basis of strategic preservation of the supporting ligand.

Tandem Prins cyclizations for the construction of oxygen containing heterocycles

Tandem Prins cyclization is a versatile method for the synthesis of fused/bridged/spirotetrahydropyran scaffolds. Therefore, it has become a powerful tool for the stereoselective synthesis of oxygen/nitrogen containing heterocycles. Indeed, previous review articles on Prins spirocyclization illustrate the synthesis of spirotetrahydropyran derivatives and the aza-Prins reaction demonstrates its application in the total synthesis of natural products. The current review is devoted specifically to highlight tandem Prins cyclizations for the construction of fused scaffolds and related frameworks with a particular emphasis on recent applications. The mechanistic aspects and the scope of the methods are briefly discussed herein.

HFIP-Promoted Synthesis of Substituted Tetrahydrofurans by Reaction of Epoxides with Electron-Rich Alkenes

In the present work, the employment of fluorinated alcohols, specifically 1,1,1,3,3,3-hexafluoroisopropanol (HFIP), as solvent and promoter of the catalyst-free synthesis of substituted tetrahydrofuranes through the addition of electron-rich alkenes to epoxydes is described. The unique properties of this fluorinated alcohol, which is very different from their non-fluorinated analogs, allows carrying out this new straightforward protocol under smooth reaction conditions affording the corresponding adducts in moderate yields in the majority of cases. Remarkably, this methodology has allowed the synthesis of new tetrahydrofuran-based spiro compounds as well as tetrahydrofurobenzofuran derivatives. The scope and limitations of the process are also discussed. Mechanistic studies were also performed pointing towards a purely ionic or a SN2-type process depending on the nucleophilicity of the alkene employed.

Synthesis of chiral 1,4-oxazepane-5-carboxylic acids from polymer-supported homoserine

The preparation of novel 1,4-oxazepane-5-carboxylic acids bearing two stereocenters is reported in this article. Fmoc-HSe(TBDMS)-OH immobilized on Wang resin was reacted with different nitrobenzenesulfonyl chlorides and alkylated with 2-bromoacetophenones to yield N-phenacyl nitrobenzenesulfonamides. Their cleavage from the polymer support using trifluoroacetic acid (TFA) led to the removal of the silyl protective group followed by spontaneous lactonization. In contrast, TFA/triethylsilane (Et₃SiH)-mediated cleavage yielded 1,4-oxazepane derivatives as a mixture of inseparable diastereomers. The regioselectivity/stereoselectivity depended on the substitution of the starting 2-bromoacetophenones and was studied in detail. Catalytic hydrogenation of the nitro group improved the separability of the resulting diastereomeric anilines, which allowed us to isolate and fully characterize the major isomers.

The preparation of novel 1,4-oxazepane-5-carboxylic acids bearing two stereocenters is reported in this article.

Synthesis of Benzofused Dioxabicycle Scaffolds via a Catellani Strategy

Reported is a modular strategy for the preparation of the unique benzofused dioxabicycle scaffolds involving a Catellani reaction of aryl iodides, epoxides, and terminal alkynes and an oxa-cyclization. This is a mild, scalable, chemoselective, and atom-economical protocol, compatible with various functionalized aryl iodides, epoxides, and terminal alkynes. With the ability to build up the molecular complexity rapidly and efficiently from feedstock chemicals, this method will have wide applications in organic synthesis.

ZnI2/Zn(OTf)2-TsOH: a versatile combined-acid system for catalytic intramolecular hydrofunctionalization and polyene cyclization

A mild and efficient combined-acid system using a zinc(ii) salt [ZnI2 or Zn(OTf)2] and p-toluene sulfonic acid (TsOH) was investigated for catalytic cationic cyclizations, including intramolecular hydrocarboxylation, hydroalkoxylation, hydroamination, hydroamidation, hydroarylation and polyene cyclizations. This reaction provides easy access to five- and six-membered O- and N-containing saturated heterocyclic compounds, tetrahydronaphthalene derivatives and polycyclic skeletons in excellent yield with perfect Markovnikov selectivity and under mild conditions. The operational simplicity, broad applicability, and use of inexpensive commercially available catalysts make this protocol superior to existing methodologies.

Expeditious diastereoselective synthesis of medium ring heterocycle-fused chromenes via tandem 8/9-endo-dig and 8-exo-dig hydroalkoxylation-formal-[4 + 2] cycloaddition

The first examples of highly diastereoselective tandem 8/9-endo-dig and 8-exo-dig hydroalkoxylation-formal-[4 + 2] cycloaddition are described for the synthesis of medium ring heterocycle-fused chromenes. TMS-alkynols preferred the exo-dig mode of hydroalkoxylation over the endo-dig mode leading to spiro-cyclic chromenes. The method could be used for the synthesis of linearly-fused ladder-like polyethers. A thia-heterocycle-fused chromene could be transformed into a complex bridged tricyclic ketal by a tandem carbene-insertion-[2,3]-sigmatropic shift.

TMSOTf mediated ‘5/6-endo-dig’ reductive hydroamination for the stereoselective synthesis of pyrrolidine and piperidine derivatives

A TMSOTf mediated 5/6-endo-dig reductive hydroamination cascade on internal alkynylamines gave expedient, stereoselective access to pyrrolidine and piperidine derivatives. We also demonstrate that a protecting group on nitrogen has a profound effect on the reactivity as well as diastereoselectivity.