Synthesis of Chiral Tetrahydrofurans via Catalytic Asymmetric [3 + 2] Cycloaddition of Heterosubstituted Alkenes with Oxiranes

Cycloaddition of N-arylnitrones with donor-acceptor oxiranes via C-C bond cleavage to construct 1,5,2-dioxazinanes

Highly functionalized 1,5,2-dioxazinanes could be smoothly produced via a Sc(OTf)3-catalyzed chemoselective [3 + 3] cycloaddition of various N-arylnitrones with a series of donor-acceptor oxiranes. This reaction involves in situ generation of 1,3-dipoles through Sc(OTf)3-catalyzed C-C bond cleavage of oxiranes and moderate to high yields were obtained for most substrates. This transformation features C-C bond cleavage of donor-acceptor oxiranes, accessible starting materials and mild reaction conditions.

Highly Chemoselective Synthesis of Purino[3,2-c]oxazoles via the Asymmetric Dearomative [3+2] Cycloaddition of Purines with Donor-Acceptor Oxiranes

A Ni(II)/bisoxazoline-catalyzed asymmetric dearomative [3+2] cycloaddition of substituted purines with donor-acceptor oxiranes was developed. This reaction, which proceeds via highly chemoselective C-C bond cleavage of the oxiranes, accesses chiral purino[3,2-c]oxazole compounds (≤99% ee after enrichment via crystallization). The electronic effects of the purine ring determine the reactivity of the substrate. The general applicability of this method was illustrated by gram-scale synthesis, the diverse transformations of the product, and the promising biological activities of selected derivatives.

Intramolecular Phosphine-Promoted Knoevenagel Based Redox-Reaction

A Knoevenagel based redox-reaction promoted by intramolecular phosphine sources is presented for the first time. The influence of different diketones, aldehydes, bases and acids was investigated. The effects of different substituents were evaluated based on their electronical influence on the diketone structure. With the obtained results a mechanism is proposed, giving information about transition states formed during the reaction, which can lead to different products. This type of an internal redox transformation with a phosphine oxide moiety remaining in the molecule after the redox reaction represents a new type of reaction.

Asymmetric Cycloadditions of Acyclic Carbonyl Ylides with Aldehydes Catalyzed by a Chiral Binaphthyldiimine-Ni(II) Complex: Enantioselective Synthesis of 1,3-Dioxolanes and Mechanistic Studies by DFT Calculations

Chiral binaphthyldiimine-Ni(II)-catalyzed asymmetric 1,3-dipolar cycloaddition reactions between acyclic carbonyl ylides generated from donor-acceptor oxiranes and aldehydes are reported. Both aromatic and aliphatic aldehydes could be used as dipolarophiles, providing cis-1,3-dioxolanes with high diastereo- and enantioselectivities. On the basis of mechanistic studies, a monomeric chiral Ni(II) complex was hypothesized to act as the active species for the cycloaddition. The high levels of asymmetric induction are satisfactorily explained by a concerted-asynchronous endo Si-face approach of the aldehyde.

Chemo- and regioselective ring-opening of donor-acceptor oxiranes with N-heteroaromatics

The first ring-opening of D-A oxiranes with N-heteroaromatics in a chemoselective C-C bond cleavage manner was achieved. In the presence of 5 mol% of Y(OTf)3 as the catalyst, diverse N-heteroaromatics, including benzotriazoles, purines, substituted benzimidazole, imidazole and pyrazole, reacted well with various D-A oxiranes, providing acyclic nucleoside analogues containing a N-glycosidic bond in up to 97% yield and up to >95 : 5 regioselectivity. Through simple transformation, the Ganciclovir analogue could also be obtained.

Asymmetric catalytic [4+3] cycloaddition of ortho-quinone methides with oxiranes

Catalytic enantioselective [4+3] cycloaddition reaction between o-quinone methides and oxiranes was achieved by using a chiral N,N'-dioxide/Tb^III complex as the catalyst, affording medium-sized hydrodioxepine derivatives in high yields (up to 99%) with good to excellent diastereo-(up to 94 : 6 dr) and enantioselectivities (up to 97% ee). The topographic steric maps and distribution of the buried volume (% V_Bur) of the catalysts via Cavallo's SambVca 2 tool were collected to effectively represent the chiral pocket of metal complexes of chiral N,N'-dioxides.

Diastereoselective Synthesis of 2,3,4-Trisubstituted Tetrahydrofurans via Thermally Reactive 1,5-Diene-tert-butyl Carbonates

We report that 3,3-dicyano-1,5-dienes bearing tert-butyl carbonates can be thermally converted to 2,3,4-trisubstituted tetrahydrofurans. The transformation relies on two thermally reactive functional groups, a 1,5-diene and a tert-butyl carbonate, that react cooperatively to yield the furan scaffolds by thermal Cope rearrangement, Boc deprotection, and oxy-Michael addition. Described herein is background related to the discovery, optimization, and scope of the key transformation and representative functional group interconversion chemistry for the tetrahydrofuran scaffolds.

Chiral ScIII-N,N'-Dioxide-Catalyzed 1,3-Dipolar Cycloaddition of Diaziridines with Chalcones

A highly enantioselective 1,3-dipolar cycloaddition of meso-diaziridines with chalcones was realized by utilizing the Sc^III-N,N'-dioxide complex as the catalyst. In this transformation, the 1,3-dipole intermediates generated from the C-N bond cleavage of diaziridine were trapped by chiral N,N'-dioxide/scandium(III) complex activated chalcones to undergo enantioselective 1,3-dipolar cycloaddition. A range of chiral 1,5-diazabicylo[3.3.0]octane derivatives were readily synthesized in good yields with high diastereo- and enantioselectivities.

An Overview of Saturated Cyclic Ethers: Biological Profiles and Synthetic Strategies

Saturated oxygen heterocycles are widely found in a broad array of natural products and other biologically active molecules. In medicinal chemistry, small and medium rings are also important synthetic intermediates since they can undergo ring-opening and -expansion reactions. These applications have driven numerous studies on the synthesis of oxygen-containing heterocycles and considerable effort has been devoted toward the development of methods for the construction of saturated oxygen heterocycles. This paper provides an overview of the biological roles and synthetic strategies of saturated cyclic ethers, covering some of the most studied and newly discovered related natural products in recent years. This paper also reports several promising and newly developed synthetic methods, emphasizing 3-7 membered rings.

Cyclization Reaction of Donor-Acceptor Oxiranes with N,N'-Disubstituted Thioureas: A Domino Process to trans-Dihydropyrimidines

An unprecedented cyclization reaction of donor-acceptor oxiranes with N,N'-disubstituted thioureas to construct trans-dihydropyrimidines is presented. Preliminary reaction mechanism studies demonstrated that the reaction underwent sequential cycloaddition/amine ester exchange/oxygen-sulfur exchange/desulfuration/Michael addition process. A wide range of trans-dihydropyrimidines were produced with high yields up to 94% by using this method.

(3+3)-Annulation of Donor-Acceptor Cyclopropanes with Diaziridines

The first example of (3+3)-annulation of two different three-membered rings is reported herein. Donor-acceptor cyclopropanes in reaction with diaziridines were found to afford perhydropyridazine derivatives in high yields and diastereoselectivity under mild Lewis acid catalysis. The disclosed reaction is applicable for the broad substrate scope and exhibits an excellent functional group tolerance.

Accessing polysubstituted oxazolidines, pyrrolidines and imidazolidines by regioselective [3 + 2] annulations of ketenimines with donor–acceptor oxiranes and aziridines

Diverse five-membered rings result from formal [3 + 2] cycloadditions of ketenimines with carbonyl and azomethine ylides.

An enantioselective conjugate addition reaction of 3-substituted benzothiophen-2-ones and 2-phthalimidoacrylates

A highly enantioselective conjugate addition reaction of 3-substituted benzothiophen-2-ones to 2-phthalimidoacrylates has been developed using a bifunctional tertiary-amine thiourea catalyst.