Transannular events

Of late, I have not come across too many unusual reactions that involve silicon (if you have any recent examples – do let me know), which is why I am going a couple of years back to the rescue. One of the reasons I am keen on transannular collapse processes will become evident once you will (hopefully) read the Perspective on macrocycles I am putting together for Chemical Science. Tonight, though, I am showing an eight-membered ring that undergoes a very interesting and unusual contraction to generate the cyclopropyl-containing seven-membered heterocycle shown below. When a silicon-bearing molecule is being “hit” with fluoride anions, one typically expects a fairly mundane silyl group removal. There are, of course, some really useful reactions (such as aldol processes using silyl enol ethers) that accompany the process of desilylation. Here is a good example that is unusual in terms of what goes in the course of desilylation. According to this report by Dowden and coworkers, clean “ablation” of the trimethylsilyl group from the eight-membered ring triggers conjugate addition and generates the cyclopropane ring that you see. What’s more, attempts to induce conjugate addition to the unsaturated amide intermolecularly (e.g. by adding thiophenol and base to the starting 8-membered ring) did not result in anything tractable. Thus, it is clear that the conformation of the 5,6-dihydroazocinone helps to guide the observed cyclopropanation. This example attests to how medium sized rings are full of surprizing features when it comes to uncommon reactivity patterns such as transannular ring formation. hth

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