The '"0 NMR spectra of terminal oxygen atoms in esters, anions and amides of substituted arenesulphinic acids and in esters and amides of substituted arenesulphonic acids were measured. The signals of the terminal 0 appear close to those of the bridge 0. Compared with carbonyl 0, terminal S-0 shows (a) a lower sensitivity to the electronic influences of geminal groups, (b) only a low sensitivity to arene ring substituents and (c) small solvent effects. The difierence between C-and Sbound 0 is discussed in terms of rr-bond character. Dy3+ complexation occurs with the terminal 0 in methyl arenesulphinates.

~~~ ~Whereas tropone (2) and a cyclopropanone 5 show rather normal ketone signals in their "0-NMR spectra, the two cyclopropenones la, and l b exhibit exceptionally high shielding, indicating a structure closer to a -0-formula than to a ketone. For comparison, an enolate and several phenolates have been measured. In order to test the ketonic character, the nucleophilic addition of water was determined by the rate of isotopic 0-exchange between ketone and water; 2 exchanged ca. 20 times slower than acetophenone, whereas l a reacted very much more slowly.Introduction. --Cyclopropenones 1 [l] are quite stable compounds, in spite of their high ring strain (estimated at 67 kcal/mol [2]) and in contrast to the less unsaturated cyclopropanones and cyclopropenes; this has been attributed to resonance stabilisation by polar formulae IS and 1C of which 1C represents a pseudoaromatic HuckeZ(2n +2) system (n = 0; see Schernr). The extent of aromaticity of 1, however, is still controversial.

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The I70-NMR spectra of 5 mesoionic compounds have been measured; in two cases, the peak attribution between endo-and exocyclic 0-atoms has been made unambiguous by synthesizing specifically 0-labelled samples. The signal of the exocyclic 0-atom appears at particularly high field, closer to the values of enolate than of carbonyl 0-atoms. The cyclic 0-atom resonates close to values of furans (or isoxazol, if bound to N). The exocyclic 0-atom is much more susceptible to structure and substituent variations than the endocyclic, an observation which is in favour of a chain-conjugated (non-aromatic) structure 1E.

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