Fragmentation of cyclobutane derivatives upon electron impact: transformation pathways elucidated by mass spectrometric methods and semiempirical quantum chemical calculations

The fragmentation behaviour of cyclobutane derivatives (carboxylic acids, esters, cyclobutanediols, silyl ethers of the hydroxy compounds) was first studied by the GC/MS method under electron impact (EI) ionization, Then, in order to establish the sequence of transformations, MS/MS, and in situ deuteration as well as accurate mass measurements, were performed, Analysis of these measurements revealed characteristic transformation pathways: (i) the substituents were easily cleaved, (ii) in most cases the cyclobutane ring was opened with the rupture of the C-C bonds opposite to each other, (iii) upon ring opening, the most substituted ring C-C bond was cleaved first and preferentially (iv) primary fragmentation was followed by secondary reactions including further cleavage of the ions as well as rearrangement reactions. For cyclobutanediols and their silyl ethers symmetric ring scission was observed providing only one primary fragment with mit half that of the molecular weight. This type of fragmentation was consistent with semiempirical quantum chemical calculation