Oxa(aza)benzonorbornadienes are utilized as important synthetic intermediates and useful building blocks in organic synthesis due to their structural features and reactivity. In addition to ring‐opening reactions, the transition‐metal‐catalyzed ring‐retentive addition reactions of oxa(aza)bicyclic alkenes have increasingly drawn the attention of various researchers. Numerous efficient protocols in this field have been investigated that enable facile and efficient transformations of oxa(aza)benzonorbornadienes to related valuable adducts, which are summarized and discussed in detail in this review.

A one-pot three-step tandem approach has been developed and enabled direct transformation of amides into 2-aryl benzoxazoles. This approach demonstrated high tolerance to various of halogen atoms, heterocyclic and aryl substituents. Moreover, after three steps reactions, this method allowed synthesis of 2-aryl benzoxazoles in excellent yield, indicating the high conversion of each step. It showed the feasibility of transition-metal free amide activation and transformation approach in 2-aryl benzoxazoles synthesis.

A novel copper-catalyzed cycloaddition of diaryl disulfides to heterobicyclic alkenes has been developed. The C—S and C—C bonds can be formed simultaneously on the C═C bond of the olefins via a single-step cycloaddition to afford a series of 2,3-dihydrobenzo[b]thiophene derivatives. This reaction exhibits excellent diastereoselectivity and relatively broad substrate scope. Various functional groups attached to the substrates are tolerated in this protocol to give the corresponding exo adducts in moderate yields.

Water-promoted palladium-catalyzed asymmetric ring-opening (ARO) reaction of oxabenzonorbornadienes with a wide variety of alkoxysilanes has been developed in a one-pot fashion, yielding cis-1,2-dihydronaphthalen-1-ols in favourable yields (up to 98%) with gratifying enantioselectivities (up to 98% ee) under mild conditions. To the best of our knowledge, it represents the first example in the ring-opening reactions of oxabicyclic alkenes with alkoxysilanes. Furthermore, the cis-1,2-configuration of product was established by X-ray diffraction analysis, and a possible mechanism for the present catalytic ring-opening reaction was also proposed.

A platinum-catalyzed syn-stereocontrolled ring-opening reaction of oxabicyclic alkenes with arylsulfonyl hydra-zides was developed.This protocol exhibited high efficiency and good functional group tolerance,affording cis-2-aryl-1,2-dihydronaphthalen-1-ols (3) or 2-aryl-naphthalenes (4) as dehydrated products in good to excellent yields under mild condi-tions (up to 89％).In addition,the cis-1,2-configuration of product (1S*,2R*)-6,7-dibromo-2-(p-tolyl)-1,2-dihydronaphthalen-1-ol (3db) was confirmed by X-ray single crystal diffraction analysis.Based on the results,a plausible mechanism for the ring-opening reaction was proposed.Remarkably,arylsulfonyl hydrazides were used as carboanion nucleophiles in the ring-opening reaction via releasing N2 and SO2 in situ.

A cobalt(II)-catalyzed [4+2] annulation of picolinamides with alkynes via C-H bond activation has been developed. The operationally simple annulation reaction allows for the synthesis of acyl-substituted 1H-benzoquinoline bearing multiple aromatic rings (up to 96 % yield) without co-oxidant or other oxidation factors under mild conditions. Several control experiments were carried out. This practical [4+2] annulation provides an efficient route to access highly functionalized compounds.

An iron-mediated ring-opening and rearrangement cascade from 4-alkenylisoxazoles to polysubstituted pyrroles is described. The cascade reaction conditions are compatible with a variety of functional groups, including ketones, esters, amides, and aryl halides. High reactivity is maintained even if the reaction scale is expanded to 0.5 grams. It is the first iron-catalyzed rearrangement of 4-alkenylisoxazoles to pyrroles that is independent of the presence of the alkoxy or amino group at position 5 of the isoxazole substrates. A plausible mechanism of the iron-mediated cascade reaction is proposed.

A novel copper-catalyzed hydrothioetherification of oxa(aza)bicyclic alkenes with potassium thioacetate and aryl or alkyl iodides to synthesize unsymmetrical thioethers has been developed. Notably, the reaction with complete diastereoselectivity went through a syn-selective addition process to give exo-adducts. In addition, this protocol exhibited high efficiency and good functional group tolerance to afford the target thioethers in moderate to good yields. Based on the results of mechanistic investigations, a plausible mechanism was proposed.

A highly efficient Ir-catalyzed ring-opening reaction of oxa(aza)benzonorbornadienes with water was firstly developed, which afforded a mild access to substituted dihydronaphthalene products with excellent yields (up to 99%) within 30 min.

3,5-Disubstituted isoxazoles and isoxazolines undergo an iron-catalyzed reductive ring opening in aged N-methyl-2-pyrrolidone (NMP). 5-Hydroxy-N-methyl-2-pyrrolidone generated in situ via a simple activation of commercial NMP acts as the hydrogen donor in the iron-catalyzed transfer hydrogenation reaction. It is the first example employing a combination of an iron catalyst and 5-hydroxy-N-methyl-2-pyrrolidone as reducing agents in a transfer hydrogenation reaction. The protocol is highly efficient for the synthesis of β-enaminones and 1,3-diketones, providing a versatile route for the preparation of these 1,3-difunctional compounds bearing diversified substitution patterns.

An air-promoted radical addition reaction of various thiols to oxa(aza)bicyclic alkenes is reported, leading to the corresponding exo adducts in good to excellent yields (up to 99%) in water without the use of any metal catalyst, photoredox catalyst or radical initiator reagent under mild conditions. The reaction has a wide scope of oxa(aza)bicyclic alkenes and thiols, which provides an easy and clean way for the direct preparation of thioethers. A proposed mechanism of an exo free radical attacking the double bond of an oxa(aza)bicyclic alkene was investigated by DFT calculations of stepwise reaction pathways and control experiments.

A new iron(III)-promoted reductive ring-opening and isomerization reactions of C4-alkynylisoxazoles in the oxidized N-methylpyrrolidone (NMP*) was reported. By changing the substituents at the C3 and C5 position of the isoxazole ring, the reaction selectively produced 2-alkynylenaminones and polysubstituted furans in good to excellent yields. It was very interesting that large group at the C3 or C5 position of the isoxazole ring was a steric hindrance tertiary butyl group (t-Bu), the reaction proceeded ring-opening rather than isomerization reaction in the presence of FeCl3. It was noteworthy that the NMP* as a solvent and a hydrogen donor was essential for both reactions. A plausible mechanism for the reductive ring-opening and isomerization was also proposed.

A novel copper-catalyzed complete diastereoselective 1,2-difunctionalization of oxabicyclic alkenes has been developed. Two C–S bonds were constructed simultaneously on the oxabenzonorbornadienes leading to β-thiocyanato thioethers through the three-component (oxabicyclic alkenes, aryl iodides, and potassium thiocyanate), one-pot reaction. Various functional groups attached to the substrates were tolerated in this protocol to afford the corresponding β-thiocyanato thioether products in moderate yields.

A new nickel-catalyzed syn-stereocontrolled ring-opening of oxa- and azabicyclic alkenes with dialkylzinc reagents was developed, which afforded the corresponding cis-2-alkyl-1,2-dihydronaphthalen-1-ols and 1,2-alkyl amide derivatives in moderate to excellent yields (up to 99% yield) under mild conditions. In this work, we successfully avoided obtaining hydride addition byproducts arising from -hydride elimination on an ethyl nickel species. Furthermore, a plausible mechanism for the ring-opening reaction was also proposed.

A novel ICl/AgNO3 co-catalyzed radical oxidation of diaryl- and alkylarylalkynes into 1,2-diketones is reported. The reaction proceeded smoothly under mild conditions and generated 1,2-diketones in moderate to good yields with a good tolerance of functional groups. Furthermore, the obtained C4-(1,2-diketoaryl)isoxazoles could react smoothly with 1,2-diaminobenzene to form C4-(3-arylquinoxalin-2-yl)isoxazoles. At last, a new one-pot strategy for the synthesis of quinoxalines from 1,2-diphenylethynes and 1,2-diaminobenzene is also reported.

An efficient three-component cycloaddition of oxa(aza)bicyclic alkenes/norbornene in the presence of NaN3 and arylsulfonyl chlorides was developed, affording the corresponding aziridine products in good yields (up to 82%) with moderate to good endo/exo selectivities (up to >99:1 endo/exo). Further studies showed that the cycloaddition of oxa(aza)bicyclic alkenes in the presence of NaN3 and chloroalkanes could afford the exo-cycloadduct 1,2,3-triazolines in good to excellent yields (up to 95%). Compared with the existing methodologies, the current protocol demands very simple and mild reaction conditions and is a metal-free catalyzed reaction. In addition, a plausible mechanism for the cycloaddition reaction was also proposed.

A new 1,3-dipolar cycloaddition of oxa(aza)bicyclic alkenes with nitrones has been developed without any catalyst and additive under mild conditions. The proposed concerted mechanism is investigated by DFT calculations of the reaction pathways.

A Pd-catalyzed Sonogashira cross-coupling reaction for the synthesis of C4-alkynylisoxazoles from 3,5-disubsitituted-4-iodoisoxazoles and terminal alkynes was described, which could afford the corresponding products with high yield (up to 98%). The results indicated that the steric effect from the group at the C3 position of the isoxazole had greater influence on the cross-coupling reaction than that from the group at the C5 position. In addition, the group at the C3 position of the isoxazole showed negligible electronic effects on the cross-coupling reaction. Furthermore, a gram-scale reaction of the Sonogashira coupling reaction was also investigated. Finally, a plausible mechanism for the Sonogashira coupling reaction was proposed.

A novel palladium-catalyzed ring-opening reaction of oxabicyclic alkenes with arylsulfonyl hydrazides was first developed. In this work, we provide an efficient one-pot reaction to afford the corresponding cis-2-aryl-1,2-dihydronaphthalen-1-ols and 2-aryl-naphthalenes in moderate to excellent yields (up to 95%) under an open-air condition. Various types of functional groups attached to the substrates were tolerated well in this method. Among them, the cis-1,2-configuration of product 3ag was confirmed by X-ray crystallographic analysis. In addition, a plausible mechanism for ring opening was also proposed.