Studies of cyclic acetals. Part XV. Use of alkyl substituents as pseudoisotopic labels: influence of a 4-hydroxy-substituent upon the electron-impact mass spectrum of alkylated derivatives of 3,6,8-trioxabicyclo[3.2.1]octane

Abstract

Low-resolution, electron-impact mass spectra are tabulated for derivatives of the title compound having methyl substitutions in the following positions: 4,5 (1); 1,5 (2); 1,4,4 (3); 1,4,4,5 (4); 4,4,5,7,7 (5), for 5-methyl-3,6,8-trioxabicyclo[3.2.1]octan-4-ol (6) and its analogues in which the hydroxy-group is replaced by OMe (8), OEt (10), OCMe₃(11), and OAc (12), and for 4,5-dimethyl-3,6,8-trioxabicyclo[3.2.1]octan-4-ol (7) and its 4-OMe analogue (9). A substantial portion of the decomposition subsequent to electron impact is rationalized according to a few, common fragmentation pathways, which are identified by analysis of mass-number shifts caused by changes in substitution; variations in relative abundance of major fragments as a function of substitution are consistent with conventional considerations of carbonium-ion stability. The presence of an oxygenated substituent at C-4 of these trioxabicyclo-octanes provides two additional fragmentation modes, whose relative prominence is also rationalized on the basis of general principles of stabilization of charges.