The chemistry of small ring compounds. Part 30. Reactions of arylcyclopropanone hemiacetals with bases and acids

The arylcyclopropanone methyl hemiacetals (aryl = C 6 H 5 , p ‐ClC 6 H 4 and p ‐CH 3 C 6 H 4 ) are quantitatively isomerised by sodium methoxide or tert ‐amines to propionic esters. The ring fission is rapid and occurs exclusively at the C 1 ‐C 2 position, leading to products derived from the most stable incipient carbanion. The hemiacetals react with acids via a Woodward‐Hoffmann cleavage of the C 1 ‐C 3 cyclopropyl bond, affording a mixture of 1‐ and 3‐substituted arylacetones, the ratio of which depends on the nature of the acid. This acid‐catalysed C 2 ‐C 3 ring‐fission only occurs when an aryl group is present. The unsubstituted cyclopropanone hemiacetal isomerises in (weak) acid to the propionic ester (C 1 ‐C 2 rupture). The transformation of arylcyclopropanone methyl hemiacetals in acidified methanol to α‐aryl‐α‐methoxy‐acetones, is first order in the hemiacetal. From the rate constants a Hammett ρ value of −0.7 is obtained, thus showing a small interaction of the aryl group with the remote reaction centre. This is considered to support a mechanism in which arylcyclopropanone is an essential intermediate. Subsequent protonation of this ketone with simultaneous C 2 ‐C 3 bond rupture gives the substituted 2‐hydroxyallylic cation. For this step an approximate ρ value of −2.9 is calculated, thus indicating that this step is much more sensitive to the presence of a ( para ) substituent in the aromatic ring. Further evidence for the occurrence of an intermediate arylcyclopropanone is the alkoxy exchange in the starting material when CD 3 OD or n ‐propanol is used as solvent. The proposed mechanism is also consistent with the observed stability of the corresponding arylcyclopropanone acetals under similar acidic conditions.