2-Bromocyclobutanone acetals

Bromocyclobutanone acetals (249) are quantitatively converted to cyclopropane-carboxylic esters (250) and alkyl bromides, or to bromoesters in the case of cyclic acetals, upon simple heating in a sealed tube at 200°C (equation 168) . 

On simple heating in a sealed tube at 200°C 2-bromocyclobutanone acetals undergo quantitative ring contraction to cyclopropanecarboxylic esters 1 and alkyl bromides 2. ... 

The ambivalent nature of the acetal function must be underlined effectively it can act equally either as an electron-withdrawing substituent (Section 4.1.2.2.4. the ring contraction of 2-hydroxycyclobutanone acetals) or as an electron-donating substituent in these rearrangements of 2-bromocyclobutanone acetals. ... 

As the reaction of 2-bromocyclobutanone with 1,2-phenylenediamine was carried out in 80% acidic aqueous methanol, ring contraction of a 2-bromocyclobutanone hydrate, hemiacetal or dimethyl acetal to cyclopropanecarboxylic acid (Section 4.1.2.2.4), followed by a Philipps-type benzimidazole synthesis could explain the formation of the 2-cyclopropylbenzimidazole (8). However, the reaction proceeded equally well in an aprotic solvent such as chloroform. Therefore, direct ring contraction of a 2-bromo-l,l-(l,2-phenylenediamino)-... 

---