Cyclopentene-fused indole

The Iwasawa group developed a platinum-catalyzed generation of unsaturated carbene complexes 325 by cyclization of methoxyalkyne-substituted aniline derivatives 321 and elimination of methoxide as depicted in 324 (Scheme 19.86) [159]. The carbene complexes 325 are useful for [3 + 2] cycloaddition with enol ethers 322, leading to efficient formation of cyclopentene-fused indoles 323. In this case, the indolylmetal species 326 would act as the nucleophile to promote the second cyclization with the oxonium moiety. 

Utilizing an alternate mode of Diels-Alder reactivity, Harman has examined the cycloaddition reactions of 4,5-T -Os(II)pentaammine-3-vinylpyrrole complexes with suitably activated dienophiles <96JA7117>. For instance, cycloaddition of the p-vinylpyrrole complex 58 with 4-cyclopentene-l,3-dione, followed by DDQ oxidation affords 59, possessing the fused-ring indole skeleton of the marine cytotoxic agent, herbindole B. 

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