Unexpected Catalytic Reactions of Silyl-Protected Enol Diazoacetates with Nitrile Oxides That Form 5-Arylaminofuran-2(3H)-one-4-carboxylates

Abstract

Silyl-protected enoldiazoacetates undergo dirhodium(II) catalyzed reactions with nitrile oxides to form acid-labile ketenimines via dipolar cycloaddition of nitrile oxides to a donor/acceptor cyclopropene and Lossen rearrangement of the dipolar adduct; acid catalysis converts the ketenimine to the furan product.

Substrate-Dependent Divergent Outcomes from Catalytic Reactions of Silyl-Protected Enol Diazoacetates with Nitrile Oxides: Azabicyclo[3.1.0]hexanes or 5-Arylaminofuran-2(3H)-ones

“…Subsequent rearrangements of 6 lead to the formation of 3 ( Lossen rearrangement) 9 and, presumably, 4 (Scheme 2). We anticipated a substituent-dependent correlation between 3 and 4 if the pathways to them are linked through 6 , and Figure 2 clearly shows linear correlation between the two processes (log [ 3 ]/[ 4 ] versus σ).…”

“…The 6 → 3 conversion occurs through ring opening of 6 (with aryl migration from carbon to nitrogen) to a ketenimine intermediate that has been intercepted and shown to undergo acid-catalyzed ring closure to 3 . 9 Could a similar ring opening/ring closure process be occurring in the formation of 4 from 6 ?…”

“…14 The general procedure with corresponding data for the synthesis of 5-arylaminofuran-2(3 H )-one-4-carboxylates ( 3 ) has been reported. 9 …”

“…8 Recently, we reported an unexpected catalytic reaction of enoldiazoacetate 1a with nitrile oxides having electron-donating substituents that form 5-arylaminofuran-2(3 H )-one-4-carboxylates 3 in high yield (Scheme 1). 9 However, a constitutionally isomeric product was detected when nitrile oxides having halide or electron-withdrawing substituents were used. Spectral analyses were insufficient to completely identify the structure of this compound, so crystals of the product from the reaction with o -nitrobenzonitrile- N -oxide ( 2g ) that resulted in the formation of only one isomer were used to obtain its X-ray structure (Figure 1).…”

Cycloaddition reactions of enoldiazo compounds

“…176,177 Unstable compounds 72 , formed by [2+3]-cycloaddition between catalytically generated donor–acceptor cyclopropenes and nitrile oxides, were proposed as plausible intermediates; and consecutive rearrangements were found to be highly substrate-dependent. Electron-donating substituents of the nitrile oxides directed the formation of ketenimine intermediates via the Lossen rearrangement, furnishing multifunctionalized 5-aminofuran-2(3 H )-ones 73 ; 176 whereas the electron-withdrawing substituents favored the Neber rearrangement to deliver azirine intermediates, which further produced 2-oxa-6-azabicyclo[3.1.0]hexan-3-one derivatives 74 . 177 …”