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We describe a catalyst, oxidant, and coupling‐reagent free strategy to access 4‐membered heterocycles, representing a unique example of visible‐light triggered intramolecular cyclization of propargylic alcohols and amines to access oxetanones and azetidinones respectively. Despite the direct 4‐endo‐dig cyclization from these starting materials has proven to be unfavorable, the formation of key p‐quinone methide intermediacy allows an efficient bypass for regioselective 4‐exo‐trig cyclization, resulting in the formation of the desired 4‐membered heterocycles. This mild and operationally simple protocol facilitates the synthesis of products with distinct substitution patterns such as quaternary α‐carbons, and enables late‐stage functionalization. [ABSTRACT FROM AUTHOR]

A novel process for the reductive magnesiation of 2-substituted oxetanes and the metalative cyclization of ω-alkynyl oxetanes is developed using n-propylmagnesium chloride in the presence of an iron catalyst. The generated intermediate organomagnesium compounds react with electrophiles. The reactions of 2,3-disubstituted oxetanes and their subsequent allylation with allyl halides in the presence or absence of copper(I) cyanide as the catalyst is studied with a unique switching of stereoselectivity being observed in the absence or presence of copper(I) cyanide . In addition, it is found that the metalative cyclization of 3-substituted 2-alkynyl oxetanes proceeds in an anti-selective manner starting from both syn- and anti-substrates. In all cases, the stereochemistry at the 2-position of the oxetanes is lost during the reactions suggesting the involvement of a radical process. [ABSTRACT FROM AUTHOR]

The first catalytic enantioselective cycloaddition reaction to α,β,β-trisubstituted nitroolefins is reported. For this purpose, nitroolefin oxetanes were employed in the reaction with 2,4-dienals promoted by trienamine catalysis. This methodology provides a facile and efficient strategy for the synthesis of highly functionalized chiral spirocyclohexene-oxetanes with two adjacent tetrasubstituted carbon atoms in high yields and excellent selectivities. This strategy also enabled access to chiral spirocyclohexene-cyclobutanes and -azetidines. Additionally, the obtained scaffolds can undergo diverse transformations leading to complex structures with up to four stereocenters, and we demonstrate that the nitro group, under nucleophilic conditions, can be applied for ring-opening of the oxetane. [ABSTRACT FROM AUTHOR]

Gold-catalyzed [4+2] cycloadditions between ynamides and oxetanes are described; these reactions involve oxetanes and gold-π-ynamides as nucleophiles and electrophiles, respectively. Excellent cycloaddition regioselectivities are achieved over a reasonable range of ynamide and oxetane substrates. For azetidines, their [4+2] cycloadditions with ynamides are implemented more efficiently with silver hexafluoroantimonate, which is also compatible with various ynamides and azetidines. These two cycloadditions provide facile accesses to six-membered heterocycles such as 6-amino-3,4-dihydro-2 H-pyrans and 2-amino-1,4,5,6-tetrahydropyridines. [ABSTRACT FROM AUTHOR]

One ring to bring them all: 4,5‐Spirocycles derived from 3‐oxetanone and β‐heteroatom‐substituted amino compounds undergo a Lewis acid mediated reaction cascade to form saturated nitrogen heterocycles. The unique reactivity of 3‐oxetanone facilitates access to biologically important morpholines, piperazines, and thiomorpholines with an otherwise difficult‐to‐access substitution pattern from readily available starting materials. [ABSTRACT FROM AUTHOR]

Abstract: An efficient and straightforward three-step synthetic route toward 2,4-disubstituted-3-bromooxetanes 5 with the trans–trans contiguous stereogenic centers is developed from functionalized chalcones 3 via NaBH4-mediated reduction of chalcones 3, followed by NBS-mediated electrophilic cyclization of the resulting trans-cinnamic alcohols 4 in good yield. Skeleton 3 is prepared by Claisen–Schmidt condensation of substituted arylaldehydes 1 and aryl methyl ketones or tert-butyl methyl ketones 2. The synthetic route obtained high yields, and the total reaction procedure took only one day. The substituent effect of skeleton 3, various reaction conditions, and plausible mechanism were well-investigated. [Copyright &y& Elsevier]

Treatment of a titanatrane complex with trimethylsilyl chloride and magnesium powder in tetrahydrofuran generated a low-valent titanium species. This species catalyzed the radical ring opening of epoxides and oxetanes to produce the corresponding less substituted alcohols. The reagent also catalyzed the deallylation and depropargylation of allylic and propargylic ethers, respectively, to provide the parent alcohols. [ABSTRACT FROM AUTHOR]

This account summarizes the Njardarson group ring expansion journey from early vinyl oxirane explorations to the latest vinyl oxetane ring expansion discoveries. The evolution of the program and incredible success of Cu(hfacac)2 as catalyst is detailed. Application of these new ring expansion reactions to natural product, pharmaceutical and commodity chemical targets is discussed as well as our intriguing mechanistic journey and interesting entry into the world of chiral counterion catalysis. [ABSTRACT FROM AUTHOR]

On treatment with boron trifluoride etherate as a Lewis acid, acetophenone and benzophenone derivatives bearing a prenyl (or a related) side chain in ortho-position were shown to undergo intramolecular carbonyl-olefin metathesis, in the absence of any transition- metal catalyst. The cationic cyclization process is supposed to proceed via an oxetane intermediate, which fragments to give the cyclization product (indene or 1,2-dihydronaphthalene) and a ketone (acetone) as a stoichiometric by-product. Several substrates were shown to afford the metathesis products with up to 93% yield. [ABSTRACT FROM AUTHOR]

The title compounds have been prepared from previously synthesized 2-endo-acetoxy-3-endo-hydroxy-7-oxabicyclo[2.2.1]hept-5-ene and 3-acetoxy-4-hydroxy-2,5-divinyl-furan via intramolecular electrophilic cyclization. Both types of compounds constitute new functionalized oxetane ring systems. [ABSTRACT FROM AUTHOR]

Abstract: Oxetan-3-one reacts with 4-metallated sydnones to provide functionalised precursors to a variety of novel pyrazoles. Intermolecular cycloadditions were found to proceed in variable yield, but intramolecular cycloadditions were very efficient and provide an effective means to generate spiro-substituted heterocyclic intermediates. The potential of these compounds for further elaboration has been highlighted by cross-coupling reactions. [Copyright &y& Elsevier]