Conrotatory ring opening of cis-fused 3-aminocyclobutenes. X-ray analysis of a cis,trans-2H-thiocin

Abstract

The thermal valence isomerization of (hetero) cyclobutenes to (helero) 1,3-butadienes is a classic example of a stereospecific reaction, the conrotatory mode of which is in agreement with the principle of conservation of orbital symmetry in electrocyclic reactions.1,2 A large amount of work has been carried out on the thermal isomerization of compounds in which the cyclobutene moiety is cis annulated to another ring system. It is generally accepted that when the annulated ring possesses less than eight atoms, ring opening must occur by way of the symmetry-forbidden disrotatory mode3 or by homolytic4 or heterolytic5 pathways, all

Chapter 4. Reaction mechanisms. Part (i) Pericyclic reactions

“…anti-Markovnikov-type addition where there is little steric Stephenson isotope [2H12]- (53) has been applied to the addition of nitrosopentafluorobenzene to obtain the intra-and inter-molecular isotope effect^.^'" The lack of an intermolecular isotope effect compared with the large isotope effects with E- (53) and gern- (53), and the substantial difference in isotope effects for E-(53) compared with 2-(53), are regarded as strong evidence against cleavage of the allylic C-H(D) bond in the rate-determining step. Several important papers have appeared this year concerned with the transitionstate geometry and mechanisms of ene reactions, Lewis acid-catalysed ene reactions, and superene reaction^.^^.^' Kinetic isotope-eff ect data for the addition of mesoxalic esters to allylben~ene~~" suggest that a symmetrically structured (2 + 2) chargetransfer complex is formed in a preliminary step, followed by a pseudopericyclic transition state similar to that proposed previously41b for superene reactions.…”

Chiral Nonracemic Synthesis and Reactivity of Two New Endocyclic Enamines in the Phenyloxazolopiperidine Series

“…When heated in a polar aprotic solvent, the electron-rich carbon−carbon double bond of the cyclic enamine reacts with the electron-deficient alkyne to give an intermediate cyclobutene. Spontaneous ring opening of the unstable cyclobutene affords ring-enlarged compounds according to a well-investigated mechanism …”

“…Spontaneous ring opening of the unstable cyclobutene affords ring-enlarged compounds according to a well-investigated mechanism. 20 We chose this reaction not only as an efficient method to compare the reactivity of enamines 2a, 2b, and 3 with that of compound 1 21 but also because there has been increasing interest recently in the synthesis of eightmembered ring systems such as azocines. This diverse class of compounds often possesses biological activity 22 and has been widely used in synthetic chemistry.…”