Oxidation of tetrathiolanes: isolation of a vic-disulfoxide, 5-(1-adamantyl)-5-tert-butyltetrathiolane 2,3-dioxide and its decomposition to the dithiirane 1-oxide and ‘S2O’

Oxidation of 5-(1-adamantyl)-5-tert-butyltetrathiolane (3a) with dimethyldioxirane (DMD) at −78 or −20°C gave (1R*,3S*)- and (1R*,3R*)-3-(1-adamantyl)-3-tert-butyldithiirane 1-oxides (cis-4a and trans-4a, respectively) as the final products. The reaction was revealed to proceed through stepwise oxidation to the corresponding 2-oxide and then to the 2,3-dioxide 5, a vic-disulfoxide; the latter is isolated in pure form by low-temperature recrystallization and is fairly stable at room temperature in the crystalline state. The structure of 5 was determined by X-ray crystallography. The 2,3-oxide 5 decomposes in solution above −10°C to cis-4a, trans-4a, and ‘S2O’ as the principal products. The reactive sulfur species, S2O, is trapped by 2,3-dimethyl-1,3-butadiene to give 4,5-dimethyl-3H,6H-1,2-dithiin 1-oxide. In the absence of the diene, S2O disproportionates to SO2 and ‘S3’, which is trapped by norbornene to give exo-norbornane trithiolane (exo-3,4,5-trithiatricyclo[5.2.1.02,6]decane).