“…By far the most susceptible unsaturated group wdth regard to ring-chain tautomerism is the carbonyl function. Appropriately constituted aldehydes may isomerize to their cyclic, hydroxy 91,180,181,260,301); to the cyclic hemiacetal or "ketol" if X = OH (36,74,80,81,106,114,123,169,216,218,219,220,224,228,246,294,300,319); or to the cyclic hydroxy lactone ("lactol") if X = CO2H (7,12,25,51,73,107,110,111,127,170,187,209,210,211,212,213,214,217,221,227,229,248,252,268,269,270,…”