Low resolution, electron impact mass spectra of 6,8-dioxa-3-thiabicyyc)o[3.2.l]octane and eight C-methyl or -phenyl derivatives are, in the main, similar to those of the corresponding 3,6,8-trioxa homologs. Oxygenation on C-2 or on the sulfur atom decreases the number of prominent fragments in the scheme of decomposition and apparently affords greater opportunity for formation of small, stable, acylium ions, but the added oxygen atoms generally do not direct major new modes of fragmentation.6,8-Dioxabicyclo[3.2.l]octane derivatives are encountered frequently in studies involving natural products. Several alkylated derivatives have been identified' as components of pheromone aggregates, and 1,6-anhydrohexopyranoses (hydroxylated derivatives) occur frequently in investigations dealing with sugars. We have reported earlier detailed analyses of the fragmentation of two members of the latter class, namely 1,6-anhydro-2,3-0 -isopropylidene-/ 3 -~-talopyranose3 and its 3,4-0-isopropylidene isomer: based on high resolution measurements using specifically deuterated analogs. In preceding reports in this series' of studies, we have examined homologous and isomeric groups of alkylated derivatives of 3,6,8-trioxabicycl0[3.2.1]octane~~~ and of 3,7,9-and 4,7,9-trioxabicyclo[4.2. llnonane' in order to identify the principal pathways whereby EI mass spectral decomposition proceeds; a large element of analogy relates the decomposition modes of these three groups of molecules, although the determination was incomplete for the last-mentioned heterocycles because differently placed alkyl groups exerted diverse directing effects upon modes of fragmentation. Introduction of an oxygenated substituent at C-4 of the 3,6,8-trioxabicyclo[3.2.1]octanes likewise affected the decomposition, introducing prominent, new pathways.Whereas the mass spectrometric literature abounds with reports about oxygenated heterocycles, there is a paucity7 of data for mixed heterocycles containing both oxygen and sulfur atoms. 6,8-Dioxa-3-thiabicyclo[3.2.1]octane (1) has been known for a decade; its preparation (Scheme 1, method A) by the action of sodium sulfide on 2,4-di-(p-tolylsulfonyloxy-* or ~h l o r o -~ methyl)-1,3-dioxolane was extended" ret For Part XXII, see Ref. 1. cently to the preparation of 2 and 5-7. One of us'l reported earlier a convenient synthesis of 2 directly from 1-thioglycerol and 1-chloropropanone (Scheme 1, method B), which was later extended by reaction of 3-bromo-2-butanone (giving" 5 and 6) and abromoacetophenone (phenacyl bromide) (giving13 9), or several a-hydroxyketones (giving" 2, 5, 6 and 8) with the thiol; however, 2 prepared by the latter method is contaminated by isolable amounts of 3 and Method A: Y = C1, Br, or OTs