Recent strategies used in the synthesis of saturated four-membered heterocycles

Roles of Nonadiabatic Processes, Reaction Mechanism, and Selectivity in Cu-Catalyzed [2 + 2] Photocycloaddition of Norbornene and Acetone to Oxetane

Oxetane has been extensively studied for its applications in medicinal chemistry and as a reactive intermediate in synthesis. Experiments report a Cu-catalyzed [2 + 2] photocycloaddition of acetone and norbornene to oxetane, which is proposed to deviate from the conventional Paternò-Büchi reaction. However, its mechanism at the atomic level is not clear. In this study, we used a combination of multistate complete active space second-order perturbation theory (MS-CASPT2) and density functional theory to systematically investigate the reaction mechanism and elucidate the factors contributing to the diastereomeric selectivity. Initially, the formation of the TpCu(Norb) complex is achieved by strong interaction between tris(pyrazolyl)borate Cu(I) (TpCu) and norbornene in the ground state (S0). Upon photoexcitation, TpCu(Norb) eventually decays to the T1 state, in which TpCu(Norb) attacks acetone to initiate subsequent reactions and produces final endo- or exo-oxetane products. All these reactions initially involve the C-C bond formation in the T1 state thereto leading to a ring-opening intermediate. This intermediate then undergoes a nonradiative transition to the S0 state, producing a five-membered ring intermediate, from which the C-O bond is formed, leading to the experimentally dominant exo-product. In contrast, the endo-oxetane formation requires a rearrangement process after the C-C bond is formed because of the large steric effects. As a consequence, the different reaction pathways generating exo- and endo-products exhibit large differences in the free-energy barriers, which results in a diastereomeric selectivity observed experimentally. Additionally, the nonradiative transition is found to play an important role in facilitating these reaction steps. The present computational study provides valuable mechanistic insights into Cu-catalyzed photocycloaddition reactions.

1,6-Dioxo-2-azaspiro[3.4]oct-2-enes and Related Spirocycles: Heterocycles from [3 + 2] Nitrile Oxide Cycloadditions with 2-Methyleneoxetanes, -Thietanes, and -Azetidines

Isoxazolines and 4-membered heterocycles are significant structural motifs in numerous synthetic intermediates and natural products. [3 + 2] Cycloadditions between enol ethers and nitrile oxides have been well studied; however, nitrile oxide cycloadditions with 4-membered heterocycles to give spiroisoxazolines are unreported. Here, we showcase the regio- and diastereoselective [3 + 2] nitrile oxide cycloadditions of 2-methyleneoxetanes, -azetidines, and -thietanes to give an array of 1,6-dioxo-2-azaspiro[3.4]oct-2-enes and related spirocycles. 2D NMR experiments suggested that most of the observed diastereoselectivities were dictated by steric interactions; however, dipolarophiles with H bonding donors reversed the stereochemical outcome. X-ray crystallography confirmed the structural assignments.

Oxetanes in Drug Discovery Campaigns

The oxetane ring is an emergent, underexplored motif in drug discovery that shows attractive properties such as low molecular weight, high polarity, and marked three-dimensionality. Oxetanes have garnered further interest as isosteres of carbonyl groups and as molecular tools to fine-tune physicochemical properties of drug compounds such as pKa, LogD, aqueous solubility, and metabolic clearance. This perspective highlights recent applications of oxetane motifs in drug discovery campaigns (2017-2022), with emphasis on the effect of the oxetane on medicinally relevant properties and on the building blocks used to incorporate the oxetane ring. Based on this analysis, we provide an overview of the potential benefits of appending an oxetane to a drug compound, as well as potential pitfalls, challenges, and future directions.

DBU-Catalyzed Diastereo/Regioselective Access to Highly Substituted Spiro-oxetane Oxindoles via Ring Annulation of Isatins and Allenoates

A facile and efficient method for the diastereo/regioselective synthesis of highly functionalized spiro-oxetane oxindoles has been described. The 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)-catalyzed reaction proceeds via spiro-annulation of isatins and allenoates. The reaction is compatible with a wide range of isatins containing electron-donating groups (EDGs) and electron-withdrawing groups (EWGs) with various allenoates affording the corresponding products in acceptable yields. It is noteworthy that this is the first protocol for constructing structurally diverse motifs of highly functionalized spiro-oxetane oxindoles of pharmaceutical relevance.

Horner-Wadsworth-Emmons reaction as an excellent tool in the synthesis of fluoro-containing biologically important compounds

Selective introduction of a double bond motif into a multifunctional organic compound is always a big challenge. The Horner-Wadsworth-Emmons reaction is one of the most reliable, simple, and stereoselective olefination methods, widely used in organic chemistry. To the best of our knowledge, no review article on the application of HWE reaction in the synthesis of fluoroorganic compounds with direct biological interest has been published in recent years. The importance of the HWE reaction should be emphasised due to its simplicity and stereoselectivity. Under mild conditions and in one step, valuable compounds can be obtained. The HWE reaction is primarily a great tool in the synthesis of fluoroolefins that are, among others, peptide bond mimetics. Therefore, it can serve as an indispensable approach to access peptide bioisosteres and, consequently, analogues of numerous enzyme inhibitors. The protocol may be utilized to obtain florinated vinylphosphonate, vinylsulfone or sulfonate derivatives, which exhibit biological activity. In this review article, we would like to summarize the HWE reaction output of the last 12 years (since 2010).

Synthesis of functionalized spirocyclic oxetanes through Paternò-Büchi reactions of cyclic ketones and maleic acid derivatives

A telescoped three-step sequence to functionalised spirocyclic oxetanes is reported, involving Paternò-Büchi reactions between maleic acid derivatives and cyclic ketones. p-Xylene suppresses the competing alkene dimerization that has plagued previous work, allowing access to 35 novel spirocyclic oxetanes that cannot be prepared using existing methodologies, and which represent versatile intermediates for further elaboration.

Isocyanide-Based Multicomponent Reactions in Water: Advanced Green Tools for the Synthesis of Heterocyclic Compounds

Reaction rate acceleration using green methods is an intriguing area of research for chemists. In this regard, water as a "green solvent" plays a crucial role in the acceleration of some organic transformations and reveals exclusive selectivity and reactivity in comparison with conventional organic solvents. In particular, multicomponent reactions (MCRs) as sustainable tools lead to the rapid generation of small-molecule libraries in water and aqueous media due to the prominent role of the hydrophobic effect. MCRs, as diversity-oriented synthesis (DOS) methods, have great efficiency with simple operations, atom, pot, and step economy synthesis, and mechanistic beauty. Among diverse classes of MCRs, isocyanide-based multicomponent reactions (I-MCRs), as sustainable and versatile reactions, have gained considerable attention in the synthesis of diverse heterocycle rings, especially in drug design because of the peculiar nature of isocyanide as a particular active reactant. I-MCRs that are performed in water are mild, environmentally friendly, and easily controlled, and have a reduced number of workup, purification, and extraction steps, which fit well with the advantages of "green" chemistry. Performing these powerful organic transformations in water and aqueous media is accompanied by acceleration owing to negative activation volumes, which originate from connecting several reactants together to generate a single product. It should be noted that the combination of MCR strategy and aqueous phase reaction is of growing interest for the development of sustainable synthetic techniques in organic conversions. However, an exclusive account focusing on the recent progress in eco-friendly I-MCRs for the construction of heterocycles in water and aqueous media is particularly lacking. This review highlights the progress of various kinds of I-MCRs in water and aqueous media as benign methods for the efficient construction of vital heterocyclic scaffolds, with a critical discussion of the subject in the period 2000-2021. We hope that this themed collection will be of interest and beneficial for organic and pharmaceutical chemists and will inspire more reaction development in this fascinating field.