13 C‐NMR. Spectra, Structure and Reactivity of Cyclic Oxocarbons

The structures of cyclic polyketones, their hydrates and reduction products were studied in different solvents by 13 C‐NMR. spectroscopy. In dimethylsulfoxide solution rhodizonic acid (1) , croconic acid (2) and squaric acid (3) exhibit signal averaging. In anhydrous tetrahydrofuran 1 and 2 could be observed as non‐dynamic species. The spontaneous reactions of dodecahydroxycyclohexane (5) and decahydroxycyclopentane (‘leuconic acid’) (6) and the sequence of formation of ring‐contracted products were investigated. Key intermediates could be clearly identified which support the mechanism proposed earlier involving a benzylic acid type rearrangement followed by decarboxylation and subsequent redox reactions.