Stereoselectivity in the Diels‐Alder Cycloadditions of a Facially Dissymmetric Bicyclo[2.2.2]octadiene‐Grafted Maleic Anhydride with Exocyclic Butadienes. Unexpected Facial Selectivity in the Cycloaddition with 2,3,5,6‐tetramethylidenebicyclo[2.2.2]octene

The Diels‐Alder cycloadditions of facially dissymmetric maleic anhydride 1 with facially nonequivalent exocyclic 1,3‐butadienes(dimethylidenebicyclo[2.2.2]octene 3 and 2,3,5,6‐tetramethylidenebicyclo[2.2.2]‐octene ( 4 )) were investigated. In each cycloaddition, the reaction occurred via the course in which 1 added exclusively by its syn‐face (same face as the etheno‐bridge) onto either π‐face of the exocyclic 1,3‐butadiene systems to produce only two of the four possible stereoisomeric monocycloadducts ( 8a / 8b and 9a / 9b ). In the Diels‐Alder cycloaddition of 1 with bis‐exocyclic butadiene 4 , however, both monocycloadducts 9a and 9b underwent subsequent cycloaddition with distinctive facial selectivity to produce the C_s‐symmetric bis‐cyclohexanobarrelene 10a as only bis‐cycloadduct.     

Unexpected 1,3‐Dipolar Cycloaddition Reaction of Thiazolo[3,2‐a]pyrimidine Derivatives and Nitrilimine

The 1,3‐dipolar cycloaddition reactions of nitrilimine with thiazolo[3,2‐a]pyrimidine derivatives was investigated. Bis‐cycloadducts were obtained through a domino 1,3‐dipolar cycloaddition/ring‐opening/ring‐opening/1,3‐dipolar cycloaddition processes. The structures of the products were characterized thoroughly by NMR, IR, MS, elemental analysis together with X‐ray crystallographic analysis.     

Catalytic Asymmetric Synthesis of [2,3]‐Fused Indoline Heterocycles through Inverse‐Electron‐Demand Aza‐Diels–Alder Reaction of Indoles with Azoalkenes

An unprecedented catalytic asymmetric inverse‐electron‐demand aza‐Diels–Alder reaction of indoles with in situ formed azoalkenes is reported. A diverse set of [2,3]‐fused indoline heterocycles were achieved in generally good yields (up to 97 %) with high regioselectivity and diastereoselectivity (>20:1 d.r.), and with excellent enantioselectivity (up to 99 % ee).     

One Stone for Three Birds‐Rhodium‐Catalyzed Highly Diastereoselective Intramolecular [4+2] Cycloaddition of Optically Active Allene‐1,3‐dienes

RhCl(PPh₃)₃‐catalyzed [4+2] intramolecular cycloaddition of optically active axially chiral allene‐dienes afforded cis‐fused [3.4.0]‐bicyclic products with three chiral centers in good yields with an excellent chemo‐ and diastereoselectivity. A pair of enantiomers of such products was generated highly selectively from both enantiomers of starting allene‐dienes, indicating that the axial chirality dictated the absolute configurations of the three in situ generated chiral centers with a very high efficiency of chirality transfer.     

Probing Reaction Mechanism of [1,5]‐Migration in Pyrrolium and Pyrrole Derivatives: Activation of a Stronger Bond in Electropositive Groups Becomes Easier

The [1,5]‐migration reaction has attracted considerable attention from experimentalists and theoreticians for decades. Although it has been extensively investigated in various systems, studies on pyrrolium derivatives are underdeveloped. Herein, a theoretical study on the reaction mechanism of [1,5]‐migration in both pyrrolium and pyrrole derivatives is presented. The results reveal lower activation barriers in [1,5]‐migration of electropositive groups (AuPMe₃ and SnH₃) in pyrrolium derivatives, although the bond dissociation energies of the Au?N bond (98.8 kcal mol^?1) and Sn?N bond (81.7 kcal mol^?1) are larger than that of the N?F bond (57.6 kcal mol^?1). The unexpectedly lower activation barriers (4.5 and 4.9 kcal mol^?1 for AuPMe₃ and SnH₃, respectively) for [1,5]‐migration of electropositive groups, in comparison with the [1,5]‐fluorine shift, can be attributed to aromaticity stabilizing the transition states, as revealed by significantly negative nucleus‐independent chemical shift (NICS) values. Further studies indicate that charge distribution and frontier molecular orbitals also play some roles in [1,5]‐migration of pyrrolium derivatives.     

[3+2] Cycloaddition of Propargylic Alcohols and α‐Oxo Ketene Dithioacetals: Synthesis of Functionalized Cyclopentadienes and Further Application in a Diels–Alder Reaction

Cyclopentadienes are valuable intermediates in organic synthesis and also ubiquitous as the Cp ligands in organometallic chemistry. As part of ongoing efforts to develop novel organic reactions that employ functionalized alkynes, a [3+2] cycloaddition of propargylic alcohols and ketene dithioacetals has been developed, which leads to fully substituted 2,5‐dialkylthio cyclopentadienes in good to excellent yields. In an unusual dethiolating Diels–Alder reaction, the cyclopentadienes were further reacted with maleimides to afford a family of novel fluorescent polycyclic compounds.     

Gold‐Catalyzed 1,2‐Acyloxy Migration/Intramolecular [3+2] 1,3‐Dipolar Cycloaddtion Cascade Reaction: An Efficient Strategy for Syntheses of Medium‐Sized‐Ring Ethers and Amines

A highly efficient strategy for the formation of medium‐sized‐ring ethers and amines based on a gold‐catalyzed cascade reaction, involving enynyl ester isomerization and intramolecular [3+2] cyclization, has been developed. Various multisubstituted medium‐sized‐ring unsaturated ethers and amines were obtained through this transformation. This method represents one of the relatively few transition metal catalyzed intramolecular cycloaddition reactions for medium‐sized ring synthesis.     

An Efficient Synthesis of 3,3′,4,4′‐Tetrahydro‐4,4′‐bibenzo[e][1,3]oxazine‐2,2′‐dione Derivatives with the Aid of Low‐valent Titanium

Synthesis of 3,3′,4,4′‐tetrahydro‐4,4′‐bibenzo[e][1,3]oxazine‐2,2′‐diones via reaction of salicylidendphenylhydrazone and triphosgene with the aid of low‐valent titanium reagent is described. This method has the advantages of accessible starting materials, good yields and short reaction time.