Dedicated to Prof. AIberf Eschenmoser on the occasion of his 60th birthday (28.5.85) The yield of the main product from the reaction of heptamethyl dicyanocobyrinate with 0, in the presence of ascorbic acid has been optimised by systematic variation of the reaction conditions, and the structure of the obtained 'stable yellow corrinoid' has been established by X-ray analysis. From the relationship between the structures of analogous xanthocorrinoids obtained likewise from a series of dicyanocobyrinic-acid derivatives, which has been prepared by systematic modifications of the c-acetic-acid chain, and the functionality of the substituent at C(7). a reaction mechanism is suggested.Stable yellow corrinoids, also termed 'xanthocorrinoids' [2], are formed as by-products of most reactions of cyanocobalamin (vitamin B,,) and its derivatives, which proceed with change of the oxidation state of the complex-bonded Co ion [3b]. They are stable to air and to light, and show, as a common feature, absorption bands at about 460480 nm, a spectral range which is also characteristic for some yellow compounds formed on degradation of vitamin B,, by bacteria (e.g. Aerohucter uerogenes [4] and Pseudomonus rubescens . Moreover, metal-free yellow corrinoids have been obtained from the culture fiitrates of phototrophic bacteria as Rhodopsezidomonus sphueroides or Rh. cupsulufa, when grown in a medium deficient in Co [lo]. However, as the structures of the above mentioned xanthocorrinoids are, with one exception, unknown, it is questionable whether all of them belong to the same class of compounds.Among the different reactions, which lead to the formation of stable yellow corrinoids from vitamin-B,, derivatives (cf. Fig. I ) , we investigated first the formation of such pigments under the conditions of Udenfriend's reaction using heptamethyl Cox, Copdicyanocobyrinate (1) as a substrate. Now, after a series of experiments directed towards the elucidation of the mechanism of the formation of xanthocorrinoids using different dicyanocobyrinic-acid derivatives as substrates has been completed [ 1 11, the experimental ') Part I [I]. 2, ') 4,

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Fluorescence spectra of methanolic solutions of the 2-pyridone derivatives I to XI have been measured at laboratory temperature and interpreted. Influence of the substituents R1 and R2, the 3-cyano group, and that of methylation on quantum yield of the fluorescence has been followed. Preparation of the studied compounds I to XI has been described, and methylation of the compound I has been studied in detail.

The synthesis of the diastereoisomeric l,l'-diaryl-l,l'-bi(isobenzofuran)-3,3'( IH,l'H)-diones 3a-d starting from the readily available 2-aroylbenzoic acids l a 4 is described (Scheme I ) . Of the colourless dimers 3 a 4 , only the sterically congested 3a and 3b dissociate at ambient temperature in solution to the deep red free 3-oxoisobenzofuran-1-yl radicals 4a and 4b, respectively. The radicals 4a, b are extremely persistent in the absence of 0,. The structures of these radicals are confirmed and the coupling constants assigned by ESR and ENDOR spectroscopy and computer simulation of their ESR spectra. The dissociation equilibrium constant at 20" in toluene for 3a is determined to be 1.18' M. By studying the steady-state radical concentration as a function of temperature, the enthalpy and entropy changes for the homolytic dissociation of 3a are determined.