Thermolysis of 2-acyloxy-Δ³-1,3,4-oxadiazolines. Evidence for a preferred sense of cycloreversion to carbonyl ylides and for fast 1,4-sigmatropic ylide rearrangement

Abstract

Thermolysis of 2-acyloxy-2,5,5-trialkyl-A3-1 ,3,4-oxadiazolines in benzene solution at 80°C furnishes acyloxy-substituted en01 ethers (hemiacylals) in high yield. Mixtures of cis:trans isomers of such oxadiazolines afford mixtures of isomeric hemiacylals in nearly the same ratio. Those and other results are rationalized in terms of cycloreversion of the oxadiazolines to carbonyl ylides that are not equilibrated during their lifetimes and undergo primarily 1,4-sigmatropic H-migration. Some fragmentation of the ylides to anhydrides and carbenes was also observed. A consistent mechanistic account includes concerted suprafacial (415 + 215) cycloreversion in the sense that places a large ylide substituent at C-l or at C-3, preferentially exo.A smaller preference for the cycloreversion that places the acetoxy group at C-1 in the endo position, when the steric effect of the alkyl group at C-1 is small, can be inferred. The possibility that the overall 1,4-H shift is the result of sequential 1,7-antarafacial and 1,4-suprafacial shifts, in some cases, is considered.Key words: carbonyl ylide; 1,4-sigmatropic rearrangement; cycloreversion, thermal, of oxadiazolines; ylide, carbonyl; oxadiazoline, thermolysis of. [Traduit par la revue] Introduction 1,4-Sigmatropic H-migrations (1) in ylides, including azomethine ylides (2-4) and carbonyl ylides (5-7), are well known. Many details of those rearrangements, however, are poorly understood. First, there is the question of whether ylide equilibration, by rotation about one or both carbon-heteroatom bonds, is faster or slower than sigmatropic rearrangement. Second, the preferred geometry (E or 2 ) about the newly formed double bond of the product of H-migration is not known. Third, the sense of sigmatropic rearrangement (toward C-1 or C-3) in unsymmetric systems is not established for a variety of cases. Finally, there is little information in the literature concerning substitution patterns that cause suppression of the sigmatropic rearrangement in favour of alternative unimolecular processes such as fragmentations to carbonyl compounds and carbenes.These questions are illustrated in Scheme 1 for a simple carbonyl ylide to which they would apply.We now report the synthesis and thermolysis of a series

Spiro fused β-lactam cyclopropenes

“…By analogy to the mechanism of thermolysis of other A3-1 ,3,4-oxadiazolines (5,6) we propose that the first step is a concerted, suprafacial, 1,3-dipolar cycloreversion to N2 and a carbonyl ylide (6), Scheme 3. Again, on the basis of analogy, 6 might be expected to undergo 1,4-sigmatropic rearrangement to 7 (7,8), electrocyclization to oxirane 8 (9-1 l ) , and fragmentation to carbonyl compounds and carbenes (8,9 , 12), Scheme 3. Compound 7, which should be stable under the reaction conditions, was not detectable by 'H nmr spectroscopy.…”

Intra‐ and intermolecular reactions of nucleophilic carbenes

“…15 ) of Scheme 4 for dioxycarbenes, and according to Scheme 5 for alkoxyaminocarbenes (Ref. [17][18][19][20], and that mechanism is assumed to apply in the absence of evidence to the contrary. In a subsequent step the ylide fragments to carbene and carbonyl compound, Scheme…”