52nd International Chemistry Olympiad, Istanbul, TURKEY
Preparatory problems: THEORETICAL
Problem 7. Which is (±)-Trikentrin A? Although the indole skeleton is ubiquitous in nature, annulated indoles at any of the benzenoid positions are uncommon. The trikentrins and the structurally similar herbindoles represent fascinating such examples of 6,7-annulated indole or polyalkylated cyclopent[g]indole natural products. The trikentrins were isolated from the marine sponge Trikentrion flabelliforme and possess antibacterial activity. Possible structures for trikentrin A are shown in the Figure below. In this problem, we will find out which of these structures is trikentrin A.
There are several ways to synthesize trikentrin A. Two routes below involve aryne-based and hydrovinylation strategies and both finally lead to the formation of trikentrin A. The first step for problems 8.1 and 8.2 is the Bartoli reaction or Bartoli indole synthesis, which is the organic reaction of ortho-substituted nitroarenes with vinyl Grignard reagents to yield substituted indoles. In particular, it is the most efficient route to 7-substituted indoles.
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