Synthetic Applications of 3-Haloindolenine Intermediates

Several synthetic methods employ the 3-halo indolenine intermediates generated by halogenations as intermediates. Danishefsky and coworkers found that the chlor-oindolenine from the methyl ester of W, W -dibenzyltryptophan can be converted to 2-substituted products using a variety of nucleophiles, including allylic boranes and staimanes, enamines and ester silyl enol ethers [68]. 

This method has proven especially useful for the synthesis of a variety of natural products containing the prenyl (3-methyl-2-butenyl) or ferf-prenyl (1,1-dimethyl-2-propenyl) groups, as will be discussed in Sect. 4.2.3) 

One of the first indole derivatives to find a utilitarian application was the 6,6 -dibromo derivative of indigo, which was extracted from a Mediterranean mollusk to provide the dye Tyrian purple, long known to be a symbol of royalty. The parent compound, indigo, was also one of the first dyes to be manufactured by synthesis. 

In recent years, the enzymes responsible for the introduction of halogen in biological systems have been identified. One group of vanadium bromoperoxidases is found in marine algae [69-71]. 7-Chlorination of tryptophan is involved in the biosynthesis of materials such as pyrrolnitrin and rebeccamycin. The enzyme is a flavin-dependent monooxygenase that generates hypochlorite ion. The structure of the enzyme is such that the OCl ion is conducted to the indole binding site over a distance of about 10 A [72, 73]. 

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