Indole-2-carboxylic acid fragments

Moreover, Fukuyama successfully utilized the same chemistry for the assembly of indole-2-carboxylic acid fragments 222,226,228, and 230 during a highly convergent total synthesis of potent antitumor antibiotics, (+ )-duocarmycdns A 223 and SA 224 (Scheme 9.80), and (+ )-yatakemycin 231, respectively (Scheme 9.81) [233, 234]. 

Syntheses of 2-substituted indoles from (2-aminobenzyl)triphenylphosphonium salts have been achieved either by cyclization with carboxylic acid anhydrides in the presence of triethylamine, or acyl chlorides in 2,6-lutidine <02TL2885>. In a related study, the acylated indoles 65 have been prepared from the phosphonium salts 66 in a process involving extrusion of a one-carbon fragment. A mechanistic rationale for these findings was also provided by the authors <02TL8893>. 

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This work, some aspects of which are outlined below, has served to show that melanins are in fact highly irregular polymers in which several different types of monomer unit are linked in a variety of ways. From the results of an extensive series of experiments in which D L-dopa deuteriated separately at the 1, 2, 2, 5 and 6 positions and D L-l-[ C]-dopa were converted into dopa-melanin autoxid-atively or by enzymic oxidation in the presence of mushroom polyphenol oxidase. Swan and his collaborators have conclude that the principal structural fragments in the polymer are probably (0 uncyclised units of dopa (68, 0.1), (ii) indoline carboxylic acid units (69, 0.1), ( /) indole units (70, 0.65) and (iv) pyrrole carboxylic acid units (71, 0.15). The indoline (69) and indole (70) units are presumed to exist in the polymer half in the phenolic and half in the quinonoid forms. It was also suggested that the pyrrole carboxylic unit (71) is probably derived by oxidative fission, during the synthesis of the dopa-melanin, of a benzenoid unit such as (70). 

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