From 1-Vinylcyclopropanol and Vinylogous Derivatives

First of all, it has been shown that the 1-vinylcyclopropanols 207 undergo ring expansion to cyclobutanones 209 with a large variety of electrophilic reagents. Thus, with hydrobromic and perbenzoic acid and with t-butylhypochlorite, 2-alkyl-, -2-hydroxymethyl- and 2-chloroalkylcyclobutanones 209, respectively, were obtained, through 1-hydroxycyclopropylcarbinyl cation intermediates 208 Eq. (65) 10,133). 

With paraformaldehyde and dibenzylamine hydrochloride in refluxing ethanol 1-vinylcyclopropanol 68 underwent a Mannich-type reaction to yield the corresponding 2-(2-dibenzylaminoethyl)cyclobutanone 210, Eq. (66)10). 

When 2-methyl-1-vinylcyclopropanol 211 was treated with concentrated sulfuric acid at 0 °C, the rearrangement took place to the extend of 40 % within 5 min to give a mixture of 2,4-trans- 212, 2,4-cis- 213, 2,3-trans- 214 and 2,3-cis-dimethylcyclo-butanone 215 Eq. (67)I33). 

In a non polar solvent, the acid-catalyzed ring expansion of 211 took a different course. Thus, when the cyclopropanol 211 was treated with dry HBr in methylene chloride at 0 °C for 5 min, only the 2,3-dimethylcyclobutanones 214 and 215 were formed in 83% yield with a trans/cis ratio (214/215) of 3 1. It has been verified that under all the acidic conditions employed, the 2,3 and 2,4-isomers did not interconvert  

The thermal rearrangement in a sealed tube at 100 °C of the [Ds]-labelled cyclopropanol 217 (prepared from perdeutero-a,a dichloroacetone, (vide supra, Sect. 4.2, Eq. (23)) led to the expected 2,2,3,3,5-pentadeuteriospiro[3.4] octan-l-one 218, Eq. (69)41  

---