Cycloaddition/rearrangement

In the overall cycloaddition-rearrangement process [64], the C-3 - C-8a relative stereochemistry of the indolizidinone obtained by rearrangement of the isoxazolidine derives from the cycloaddition step and is not affected during the rearrangement. This allowed the control of two out of three chiral centers in a synthetic protocol for a synthesis of the amphibian alkaloid ( + )-Gephyro-toxin 223AB (Scheme 46) [65c]. 

Cycloaddition reactions of furans are still widely used as key steps in the construction of complex molecules including natural products. As an example, the intramolecular Diels-Alder cycloaddition of 2-amido substituted furans provides a useful tool for the synthesis of fused, nitrogen-containing poly-heterocycles. Thus, thermolysis of 3-substituted amidofuran produces tricyclic indolinone 39 as a 2 1 mixture of diastereomers via amidofuran cycloaddition-rearrangement methodology, which serves as a key intermediate in the total synthesis of ( )-dendrobine, a major component of the Chinese ornamental orchid Dendrobium nobile . 

The one step domino cycloaddition-rearrangement process of A-aryl nitrones 78 to BCP (79) afforded spirocyclopropane-annelated pyridones 81 and benzazocinones 82 in good overall isolated yields <00SL1034>. 

Aldol reaction Condensation with imines Cycloaddition Rearrangments... 

Dipolar cycloaddition occurred preferentially at the electron-rich double bond of 22 to give the unstable triazoline 23, which on thermolysis led to extrusion of nitrogen and rearrangement to give the cyclopentenoid compound 25. The 1,3-dipolar cycloaddition-rearrangement sequence was subsequently extended to ultrasonic conditions. 

J(P1)2797>. A similar cycloaddition-rearrangement sequence accounted for the formation of oxazolo[4,5-r/ [l,2,3]tri-azoles 203 as side products from the reaction between triazolium imides 201 and N-substituted benzaldimines, leading to the imidazo[4,5-t/ [l,2,3]triazole 204 (Equation 23) (see Section 10.05.10.3). The oxazolotriazoles 203 were presumed to arise as a consequence of hydrolysis of the imine to the parent aldehydes under the reaction conditions <2003ARK110>. 

Finally, a rather unusual cycloaddition-rearrangement reaction produced the 1,2,5-triazabicyclo[4.2.0]octane derivative (135) in 64% yield (80JOC4587). 

Ozonolysis of a double bond leads first to a so-called primary ozonide 40 through 1,3-dipolar cycloaddition. Rearrangement of primary ozonide 40 with ring cleavage produces a carbonyl oxide 42 and a carbonyl compound 41, which then recyclize to secondary ozonide 43. The reaction terminates with a redox process involving... 

The salts 139 and 140a were desilylated with CsF to generate transient selenadiazoliumylmethanide (ylide) intermediates 245 and 248 at ambient temperature in dichloromethane (Scheme 32) <2001J(P1)394>. 4,5-Diphenyl-l,2,3-selenadiazol-3-ium-3-ylmethanide 245 undergoes cycloaddition-rearrangement reaction with excess... 

Abstract Major advances in transition-metal catalyzed reactions have taken place since the discovery of N-heterocydic carbenes (NHCs). This review provides a summery of recent M-NHC catalyzed reactions including cycloadditions, rearrangements, coupling reactions, polymerizations, and the additions of H-X. 

Padwa A, Dimitroff M, Liu B (2000) Formal Synthesis of ( )-Dendrobine Use of the Amidofuran Cycloaddition/Rearrangement Sequence. Org Lett 2 3233... 

The synthetic merits of the cycloaddition-rearrangement sequence in the construction of multi-ring frameworks are further illustrated by a concise... 

Certain of the bridged A -norcarenes depicted by 149, and formed by [2-1-4] cycloaddition, rearrange to cycloheptadienes or -trienes under the reaction conditions . For example, tetrachlorocyclopropene and cyclopentadiene give 153 in 95% yield by facile two-electron disrotatory cyclopropyl-allyl opening of the primary [2 -H 4] endo adduct (equation 57). The cycloadditions of simple cyclopropenes with dienes such as... 

Phenyl- and 4-methyl-4H-l,2,4-triazole-3,5-dione are often used as reactive dienophiles with subtituted norbornadienes and other polycyclic 1,4-dienes. 2-Trimethylsilyl-, 2-chloro-, 2-cyano- and 2-methoxycarbonylnorbornadiene react with 4-phenyl-4i/-l,2,4-triazole-3,5-dione in dichloromethane at room temperature to give the corresponding homo-Diels-Alder adducts 18, accompanied by [2 + 2] cycloaddition, rearrangement, insertion and polymerization products. 

In a preliminary chapter, entitled Introduction, the underlying principles of physical organic chemistry, as applied to stereoselective reactions, are succintly recalled. The three subsequent chapters describe the chiral auxiliaries, reagents, catalysts and ligands that are most commonly used in asymmetric synthesis. The remaining chapters are devoted to the description and delineation of the scope of the main classes of asymmetric organic reactions. These indude protonations and deprotonations alkylations and related reactions additions to C=0, C=N and C=C double bonds cycloadditions rearrangements and transition metal-catalyzed reactions. 

Cyclopentenecarboximidates were produced by the reaction of p-bromobenzenesulphonyl azide with cyclohexa-1,4-dienol ethers, the latter having been prepared by Birch reduction the cycloaddition-rearrangement results from selective attack at the more... 

Boonsombat J, Zhang H et al (2008) A general synthetic entry to the pentacyclic strychnos alkaloid family, using a [4 + 2]-cycloaddition/rearrangement cascade sequence. J Org Chem 73 3539-3550... 

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