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 aci ds, esters, cyclobutanediols, silyl ethers of the hydroxy compounds) w as first studied by the GC/MS method under electron impact (EI) ioniza tion, Then, in order to establish the sequence of transformations, MS/ MS, and in situ deuteration as well as accurate mass measurements, wer e performed, Analysis of these measurements revealed characteristic tr ansformation 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 su bstituted ring C-C bond was cleaved first and preferentially (iv) prim ary fragmentation was followed by secondary reactions including furthe r cleavage of the ions as well as rearrangement reactions. For cyclobu tanediols and their silyl ethers symmetric ring scission was observed providing only one primary fragment with mit half that of the molecula r weight. This type of fragmentation was consistent with semiempirical quantum chemical calculation. (C) 1998 John Wiley & Sons, Ltd.