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

The structures of cyclic polyketones, their hydrates and reduction products were studied in different solvents by ¹³C-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.