Dioxaspiro rearrangement

It is surprising that silicagel D 11-10 shows better results than zeolites in the gas phase rearrangement reaction. In the case of 1,6 dioxaspiro[2,5]octane, excellent results were obtained in the presence of this silicagel as well as of pentasil zeolites (20). In the present case, the molecular dimensions of the starting material seem to be to bulky, due to the additional methyl groups, and cannot enter the pore volume of the middle sized weakly acid boron pentasil zeolite. 

In the case of cyclic bromoacetals the thermal reaction gave bromo esters. Thus, 1 -bromo-5,8-dioxaspiro[3.4]octane (3) underwent thermal ring contraction to 2-bromoethyl cyclo-propanecarboxylate (4), quantitatively, on heating neat at 200°C. This rearrangement did not occur from the homologous a-bromocyclopentanone and a-bromocyclohexanone. ... 

In a 5-mL round-bottom flask equipped with a reflux condenser and a magnetic stirring bar was placed l-bromo-5,8-dioxaspiro[3.4]octane (1 g, 5.2 mmol). The flask was immersed in an oil-bath at 200°C for 5 min. After cooling the reaction mixture, its NMR spectrum showed the rearranged pure product yield 1 g (100%). 

Rearrangements of oxaspiro compounds such as l,5-dioxaspiro-(2,6) octane and... 

---