Tunichromes, synthesis

This photochemically mediated cisjtrans isomerization of enamines has been used to synthetic advantage by Nakanishi and coworkers in a synthesis of the tunichromes Mml and Mm22. These workers observed that the stereochemistry of the enamine formed by the Emmons reaction shown in Scheme 1 could be changed from EjZ = 8/1 to EjZ =1/2 on irradiation. The undesired E isomer could then be separated and recycled. It should be noted, however, that cis/trans isomerization is more typically an undesired competitive relaxation pathway for most of the photochemical transformations described in this chapter. 

The biosynthesis of tunichromes is not yet fully understood, but it seems likely that these compounds are formed by the degradation of polymers of DOPA and TOPA obtained by ribosomal synthesis. Several labeling experiments have shown that phenylalanine and tyrosine are incorporated in vivo in tunichrome An-1 of Ascidia cerat-odes, the only tunichrome present in this species (He et al, 1992 Robinson et al, 1996). Figure 28.34 summarizes a hypothesis that explains the formation of tunichromes Pm-1 and An-1. 

Kim, D., Li, Y, Horenstein, B.A., and Nakanishi, K. (1990) Synthesis of tunichromes Mm-1 and Mm-2, blood pigments of the iron-assimilating tunicate, Molgula manhattensis. Tetrahedron Lett., 31, 7119-7122. 

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