Larock indole synthesis applications

The preparation of diversely substituted azaindoles is fairly difficult, and there are no generally applicable strategies in the literature. Research by L. Xu et al. showed that 2-substituted-5-azaindoles could be synthesized by the Pd-catalyzed coupling of aminopyridyl iodides with terminal alkynes. The coupling reaction proceeded in good yield under the conditions originally developed by Larock. Therefore, this example can be considered an extension of the Larock indole synthesis. By stopping the reaction early it was shown that the intermediate was an internal alkyne. 

The greatest application of the Sonogashira coupling is of course the Larock indole synthesis, which is reviewed in Section 3.3 in this chapter. A Sonogashira coupling was carried out between protected phenyliodide and alkyne. With JV-methanesulfonyl protection, the coupling product spontaneously cyclized to the indole, which was converted into an indole-based insulin mimic. 

Scheme 3 More Applications of the Larock Indole Synthesis...

There are many variations and improvements on the application of Heck cyclization to the synthesis of indoles (the Mori-Ban indole synthesis) in the past three decades. The cyclization of o-halo-A -allylanilines to indoles is a general and efficient methodology that has been improved by Larock. These improvements include cyclozations of o-halo-A -allylanilines and o-halo-A -acryloylanilines into indoles and oxindoles. For example, the conversion of 16 to 17 can be performed at lower temperature, shorter reaction time, and with less catalyst to give 3-methylindole 17 in 87% yield. 

Internal Alkynes. Several applications of the Larock synthesis of indoles have been published where heterocycles are used as substrate. f " In the Larock method for indol construction, Pd-catalyzed heteroannulation of internal alkynes using ort/to-iodoanilines are used. Similarly, Pd-catalyzed heteroannulation of internal alkynes using ortho-amino-iodopyridine substrates produces azaindoles (Scheme 103). The method provides a convenient access to a structurally diverse range of 5-, 6-, and 7-azaindoles, 304, 305, and... 

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