Syntheses of 2- and 3-Substituted Morpholine Congeners via Ring Opening of 2-Tosyl-1,2-Oxazetidine

Diastereoselective and diastereoconvergent syntheses of 2- and 3-substituted morpholine congeners are reported. Starting from tosyl-oxazatedine 1 and α-formyl carboxylates 2, base catalysis is utilized to yield morpholine hemiaminals. Their further synthetic elaborations allowed the concise constructions of conformationally rigid morpholines. The observed diastereoselectivities and the unusual diastereoconvergence in the photoredox radical processes seem to be the direct consequence of the avoidance of pseudo A^1,3 strain between the C-3 substituent and the N-tosyl group and the anomeric effect of oxygen atoms.

Synthesis of 1,2-oxazetidines with a free -NH group via photoredox catalysis

A photoredox approach enabling one-step synthesis of oxazetidines with a free -NH group via the combined use of alkyne, thiophenol, and azide has been reported. The synthesized oxazetidine with the free -NH group was stable enough for various late-stage transformations such as methylation, acetylation, tosylation, and ring-opening reaction to afford synthetically useful α-aminoketones.

Pd-Catalyzed Regioselective Intramolecular Allylic C-H Amination of 1,1-Disubstituted Alkenyl Amines

Pd-Catalyzed intramolecular allylic C-H amination of 1,1-disubstituted alkenyl amines with various allylic tethers (X = O, NMs, CH₂) was developed. This process allows for the divergent synthesis of 1,3-X,N-heterocycles through a regioselective allylic C-H cleavage and π-allylpalladium formation. Particularly noteworthy is the use of substrates containing a labile allylic moiety and new simple catalytic systems capable of promoting highly chemo- and regioselective allylic C-H amination by overcoming significant challenges.

Merging strain-release and copper catalysis: the selective ring-opening cross-coupling of 1,2-oxazetidines with boronic acids

An unprecedented ring-opening cross-coupling of 1,2-oxazetidines with readily available arylboronic acids is achieved for the first time by copper catalysis. Unlike the known electrophilic oxygen reactivity in coupling with organometallic reagents, 1,2-oxazetidines were utilized as formaldimine precursors in this protocol. Remarkable features of this reaction include simple operation, inexpensive catalyst, broad scope and high regioselectivity, delivering a wide array of aminomethylation products. The practicality of this reaction was validated in the one-step downstream transformation of the obtained products into synthetically important molecules and late-stage modification of bioactive acids.

A Comparison of Immobilised Triphenylphosphine and 1-Hydroxybenzotriazole as Mediators of Catch-and-Release Acylation Under Flow Conditions

Described herein is a comparative study of immobilised triphenylphosphine (PS-PPh3) and 1-hydroxybenzotriazole (PS-HOBt) to mediate amide couplings under continuous flow. Compared to Appel-type amidations (PS-PPh3), the developed 'catch-and-release' approach (PS-HOBt) afforded near-quantitative amide conversions. Utilising this strategy, sulfonyl chloride amenability enabled facile access to an expanded library of sulfonate and sulfonamides. Post-constructional peptide modification was also demonstrated, affording two Nβ-functionalised pentapeptides in high yields and purities. In contrast to frequently utilised coupling agents, the PS-HOBt resin could be recycled six times without a reduction in efficacy or regeneration requirements.

Merging C-H Activation and Strain-Release in Ruthenium-Catalyzed Isoindolinone Synthesis

The merger of strain-release of 1,2-oxazetidines with carboxylic acid directed C-H activation in catalytic synthesis of isoindolinones is reported for the first time. This reaction opens a new and sustainable avenue to prepare a range of structurally diverse isoindolinone skeletons from readily available benzoic acids. The success of late-stage functionalization of some bioactive acids, and concise synthesis of biologically important skeletons demonstrated its great synthetic potential in drug discovery. Mechanistic studies indicated a plausible C-H activation/β-carbon elimination/intramolecular cyclization cascade pathway.

Synthesis of 1,3-Aminoalcohols and Spirocyclic Azetidines via Tandem Hydroxymethylation and Aminomethylation Reaction of β-Keto Phosphonates with N-Nosyl-O-(2-bromoethyl)hydroxylamine

An unprecedented tandem α-hydroxymethylation and α-aminomethylation reaction of aromatic cyclic β-keto phosphonates with N-nosyl-O-(2-bromoethyl)hydroxylamine in the presence of DBU base has been developed, affording a range of 1,3-aminoalcohols in good yields. The resultant products could be flexibly transformed into the spirocyclic and bispirocyclic azetidines via one step of Mitsunobu reaction. Mechanistic study revealed that hydroxylamine in situ generated the formaldehyde and nosylamide, which in turn triggered the sequential Horner-Wadsworth-Emmons, Michael, and aldol reactions.

N-Nosyl-O-bromoethyl hydroxylamine acts as a multifunctional formaldehyde, formaldimine, and 1,2-oxazetidine surrogate for C–C and C–O bond-forming reactions

Hydroxylamine could chemo- and stereoselectively produce α-hydroxymethyl and α-aminomethyl ketones and chiral α-alkoxyl and α-aminomethyl carboxylates in good yields.

Copper-Catalyzed Electrophilic Etherification of Arylboronic Esters with Isoxazolidines

A copper-catalyzed electrophilic etherification of arylboronic esters is reported. Isoxazolidines are utilized as easily available and stable [RO]⁺ surrogates to give 1,3-amino aryl ethers. The O-selective arylation of isoxazolidines takes place without causing competitive N-arylation. In contrast to previously reported anionic conditions, our copper-catalyzed conditions are mild enough to achieve high functional group tolerance. Preliminary mechanistic studies and DFT calculations support that the reaction proceeds via a transmetalation/oxidative addition pathway, followed by a Lewis acid-promoted reductive elimination to induce the crucial O-selectivity.

N-Tosyl-1,5,2,6-dithiadiazocane: a waste-free electrophilic sulfur reagent for the efficient synthesis of medium-ring S,N-heterocycles

Medium-ring S,N-heterocycles can be easily accessed in a one-pot procedure using an eight-membered dimeric sulfenamide annulation reagent.