Ketals, cyclopropenone 4 + 3 cycloaddition reactions

S CONTENTS Introduction to the Series An Editor s Foreword, Albert Padwa. Preface, Dennis P. Curran. Intramolecular 1,3-Dipolar Cycloaddition Chemistry, Albert Padwa and Allen M. Schoffstall. Stereochemical and Synthetic Studies of the Intramolecular Diels-Alder Reaction, William R. Roush. Thermal Reaction of Cyclopropenone Ketals, Key Mechanistic Features, Scope and Application of the Cycloaddition Reactions of Cyclopropenone Ketals and p - Delocalized Singlet Vinyl Carbenes Three Carbon 1,1-/1,3-Dipoles, Dale L. Boger and Christine E. Brotherton-Pleiss. Index. 

In addition to the representative [3 + 2] cycloaddition reactions shown in Table I, the delocalized singlet vinyl carbenes have been shown to participate as tt2a components of non-Hnear cheletroplc [tt 2S + it 2a] cycloadditions to provided cyclopropanes with an observable endo effect, and asw2s components of [tt4s + i2s] cycloadditions with selected dienes to provide cyclo-heptadienes, (Scheme 1). This thermal reactivity of cyclopropenone ketals... 

The total synthesis of the rubrolone aglycon was accomplished in the laboratory of D.L. Boger as part of the ongoing research to explore the cycloaddition reaction of cyclopropenone ketals. The key step in the production of the seven-membered C-ring was the intermolecular Diels-Alder reaction of an electron-rich diene with the very strained dienophile. The cycloaddition took place in excellent yield (97%) and with complete disastereoselectivity. 

Boger DL, Brotherton CE. Thermal, four-carbon + three-carbon cycloaddition reaction of cyclopropenone ketals. Total synthesis of deacetamidocolchiceine formal total synthesis of colchicine. J. Org. Chem. 1985 50(18) 3425-3427. 

Diels-Alder reactions.1 This cyclopropenone ketal undergoes [4 -I- 2] cycloaddition with electron-deficient or electron-rich dienes at 25° when the reaction... 

Cydopropenone ketals, of which cyclopropenone 1,3-propanediol ketal (1) is a representative and unusually stable example, have proven to be useful equivalents of the 1,3-dipole (1) 1n a regiospeclfic three-carbon + two-carbon cycloaddition with electron-deficient olefins, (eq 1). Table I shows representative results of a study of this reaction.7... 

Cycloaddition.5 Reaction of cyclopropenone ketals with alkenes bearing only one electron-withdrawing group results in unstable cyclopropane ketene ketals, which are not isolated, but rather are converted into ds-disubstituted cyclopropanes by acid hydrolysis. 

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