The configuration of the sulfoxide oxygen insyn- andanti-thiobinupharidine sulfoxide was determined by employing the13C nmr sulfoxidation increments of C-6. Establishment of the13C nmr line assignments for C-6 included the study of the C-6 and C-6′ deuterated thiobinupharidine and the corresponding sulfoxides. Thermolysis ofsyn-thiobinupharidine sulfoxide in DMSO and subsequent reduction with sodium borodeuteride in methanol yielded thiobinupharidine, labelled with deuterium only at C-6, and thionuphlutine B, labelled with deuterium at both C-6 and C-6′. Treatment of theantisulfoxide in xylene or DMSO resulted in no thiobinupharidine or thionuphlutine B. Similarly,syn-neothiobinupharidine sulfoxide gave neothiobinupharidine and a new thiaspirane, thionuphlutine C. These two compounds were not produced fromanti-neothiobinupharidine sulfoxide heated in xylene.