Oxygen-17 n.m.r. spectra of C(2)-substituted thioxanthones, thioxanthone sulphoxide, and thioxanthone sulphone

Abstract

The ¹⁷O n.m.r. spectra of a series of ¹⁷O-enriched 2-substituted thioxanthen-9-ones are reported. Analysis of the ¹⁷O chemical shifts using the dual substituent parameter (DSP) method shows that the carbonyl oxygen chemical shift correlates precisely with σ_I and σ_R(+) even though the carbonyl group is meta to the C(2)-substituent; interaction between them is apparently mediated by the heterocyclic sulphur. The carbonyl ¹³C chemical shifts do not correlate well with the C(2)substituent.

Enrichment with oxygen-17 was accomplished using ¹⁷O-enriched water in hydrochloric acid–dioxane. Under these conditions, thioxanthenone S-oxide reacts to produce small amounts of enriched thioxanthenone and thioxanthenone S,S-dioxide in addition to enriched starting material.