Optically active difluoroboron complexes of both a conventional urobilin derivative (3) and a nonracemizable urobilin analogue (4) have been synthesized for the first time, and their structures have been determined, in solution, by means of 1H{1H}NOE difference experiments. Whereas the BF2 chelate 5 occurs in a “stretched” 9Z,4(5)-ac, 5(6)-sc,10(11)sp,14(15)- sc,15(16)-ac conformation in both dimethylformamide and methylene chloride solutions, the preferred conformation (6 or 7) of the difluoroboron complex of 4 is dependent on the solvent, thus giving rise to opposite CD curves in the aforementioned solvents. As revealed by X-ray diffraction analysis, however, only conformer 6 is present in the solid state, in which extended intermolecular hydrogen bonding involving the lactam groups takes place, independently of the solvent used for crystallization. Conformer 7, on the contrary, is stabilized, in solution, by intramolecular NH···F bonds, as evidenced by measurements of “through-space” 1H−19F spin−spin coupling.

The total synthesis of racemic cis-2,3,18',18*-tetrahydroprotobiliverdin I X a dimethyl ester (19b), which is identical with the dimethyl ester of rac-4, is described (Scheme 2). Under virtually neutral conditions, in solution, this bile pigment isomerized within a few min to racemic Z-phycocyanobilin dimethyl ester (rue-5b). Likewise, acid-catalyzed ally1 rearrangement of 3-vinyl-substituted cis-and trans-2,3-dihydrodipyrrin-1 (10H)-ones l l c and 13c, respectively, yielded the corresponding ethylidene derivatives. In this case, however, the E-isomer was formed stereoselectively from both substrates. The above results prove that, if protobiliverdin

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The optically active urobilin model compound 7 was synthesized, in which Me groups instead of H-atoms are bound to the asymmetric centers, thus preventing loss of chirality by tautomerization. The key intermediate of the eleven-step synthesis of 7 is the 1,4,5,2 0-tetrahydro-lO-hydroxy-l-oxo-l1H-dipyrrin-9-carboxylate ruc-2, which could be resolved into enantiomers by fractional crystallization of the corresponding methyl N-[1 -(naphth-1-yl)ethyl]carbamates 3 and 4. The absolute configuration of enantiomerically pure (-)-2 was determined by X-ray diffraction analysis of its camphor-10-sulfonate 5. As the CD spectrum of the urobilin analogue 7 obtained from (-)-(R)-2 displays a positive Cotton effect, the present results prove, in connection with previous work, that substitution of Me groups for the H-atoms bound to the asymmetric centers of a chiral urobilin chromophore do not influence the relationship between absolute configuration of the latter and its helicity.Urobilinoids (urobilins and stercobilins) are the final products of heme catabolism in humans and mammalians [2]. This class of bile pigments deserve particular interest, however, since the existence of the urobilin chromophore was demonstrated in both algal [3] [4] and bacterial [S] accessory pigments of the phycoerythrin type, which are involved in the process of oxygenic photosynthesis in these organisms.Both urobilins and stercobilins are characterized by the presence of two asymmetric C-atoms (C(4) and C( 16) in the partially hydrogenated 22H-bilin-l,19(21H,24H)-dione chromophore). Thus, symmetrically substituted urobilins (4,5,15,16-tetrahydro-22H-bilin-I, 19(21H,24H)-diones) may be either chiral or sigmasymmetric depending on whether the relative configurations at both asymmetric C-atoms are the same or opposite, respectively (cf. 7). Chiral urobilins are characterized by the extremely high specific rotation of their hydrochlorides (e.g.[a]? = +5000 for natural d-wobilin IXa [6]), which was attributed to the presence of an inherently dissymmetric coiled dipyrrin chrornophore whose chirality is preserved by intramolecular H-bonding [7]. In this model, the helicity of the dipyrrin chromophore is determined by the absolute configurations of the asymmetric atoms C(4) and C(16). Recently, it could be proved that a (S,S)-configurated urobilin molecule displays a negative Cotton effect in the absorption range of the dipyrrin chromophore [8]. Until now, however, the precise structure of the optically active i, Part 16: 111.

Dedicated to Prof. Dr. Charles W. Jeffordon the occasion of his 65th birthday (25. VII.94) Following the conventional methodology used for the synthesis of bile pigments, some oxa-and thia-deazabiliverdin analogues were synthetized for the first time. Both UVjVIS spectroscopic and 'H{'H)-NOE difference studies reveal that the oxa-deaza-biliverdin analogue 8a occurs in apolar solvents in a partly 'stretched' conformation, whereas the corresponding thia derivative 8b behaves rather like a genuine bile pigment. Unexpectedly, the helical-shaped conformation is also preponderant in a trioxa-trideaza-biliverdin analogue, which could be only characterized in its protonated form 14.Bile-pigment chromophores of phycobiliproteins -the light-harvesting pigments which are integral part of the photosynthetic apparatus of cyanobacteria and some microalgae ~ are characterized by two main spectroscopic properties: an enhanced absorption of VIS light with respect to the absorption in the near-UV range ( i e . E~~~/ E , ,~ z 4 for the not denatured biliprotein) and a high quantum yield of photoluminescence. Both properties contrast with those of the isolated bile-pigment chromophores which prefentially occur in helical-shaped conformations, in solution [2] [3]. It can be shown experimentally [4] that at least the enhanced extinction of the VIS absorption band is caused by the presence of 'stretched' conformations of the chromophore, as originally suggested by theoretical calculations [5] [6]. On the other hand, the low quantum yield of fluorescence of protobiliverdin dimethyl ester (@ = 1.1 .in EtOH at room temperature [7]) may be explained by radiationless deactivation due either to rotation around exocyclic bonds [8] or to intramolecular proton jump between the N-atoms of the dipyrrin moiety of the helical-shaped molecules in the excited state [9]. The latter mechanism, however, seems to be a negligible deexcitation process in bilindiones [lo]. Moreover, at room temperature, a part of the fluorescence arises from 'stretched' biliverdin chromophores which emit with a quantum yield of about two orders of magnitude higher than the corresponding coiled conformers