Rearrangement, vinylogous mechanism

Both V and VII are highly unstable species, as is copper carbenoid IV. It is conceivable that both evolutionary alternatives follow downhill energy profiles. Whatever the particular mechanism, the transformation of I into II has been termed the vinylogous Wolff rearrangement, since it was taken as a homolog of the classical Wolff transposition. 

Hydrolytic studies with other K-region arene oxides 3, 20, 4, and 109 have been reported. ° ° A comparative investigation of the mechanism of the solvolysis and rearrangement of K- and non-K-region arene oxides showed that dihydrodiols were not produced from non-K-region arene oxides and the exclusively formed phenols resulted mainly from the vinylogous benzylic carbonium ions. 

Based upon studies involving the interaction of 6-acetylmethylenepenicil-lanic acid (27) with TEM-1 yff-lactamase, as well as its chemical reactivity with sodium methoxide and hydroxylamine, Arisawa and Adam [45] proposed a mechanism for the inhibition of TEM-1 )8-lactamase by (27) Scheme 6.10). They proposed the initial formation of a rather stable inactivated enzyme (55). This was followed by either slow hydrolysis of the acyl bond, releasing active enzyme, or rearrangement to (56). The stability of this intermediate was explained on the grounds of its pyrrolic structure the conjugated ester behaves as a vinylogous urethane, which is known to be resistant to hydrolysis [72]. 

Not all dienone-phenol rearrangements are acid catalyzed. The 4-hydroxy-4-methyl-2,5-cyclohexadi-enones (38 R = H) and (38 R = Me) react in refluxing aqueous alkali to afford the methylhydroquinones (39 R = H) and (39 R = Me 95%), respectively. This is understandable as migration in an electron-rich, vinylogous a-ketol system (40). A different mechanism operates in the rearrangement of the decala-dienone (41), induced by sodium hydroxide-aqueous methanol, to the tetralindiol (42 76%) which appears to proceed by a retro-aldol-aldol sequence, by way of the aldehyde (43). ... 

Condensation of the vinylogous carbamate (40) with acetaldehyde and isocyanic acid afforded the pyrimidine derivative (41) which was converted to the urea derivative (42) by Curtius rearrangement in 65% overall yield. Treatment of (42) with acetic acid at 50° afforded exclusively the desired product (39), whereas treatment with trifluoroacetic acid gave a mixture of (39) and stereoisomer (43). The stereochemistry was established by comparing the coupling constants involving H-5 and H-6 with those of saxitoxin (1). Different cyclization mechanisms were suggested for the acetic acid and trifluoroacetic acid catalyzed cyclization to account for the different stereochemistry of the products. 

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