Total Synthesis of (±)-Communesin F via a Cycloaddition with Indol-2-one

Total Synthesis of (±)-Communesin F via a Cycloaddition with Indol-2-one

Johannes Belmar and Raymond L. Funk

 DOI: http:// dx.doi.org/10.1021/ja307277w

It has been some time since my last entry. I was very busy with moving to the US and starting my master’s thesis. But as you can see after about 12 weeks I am back. I chose a rather short synthesis but there is still some work to be covered within the next weeks which should result in much more detailed write-ups.

The communesins are known to the synthetic community for quite a while and were the targets of extensive research and synthetic studies. The current paper utilized some nice methodology developed by the group and published in an earlier synthesis of perophoramidine. [1]

The synthesis begins with the union of azide 1 and bromooxindole 2. Because their initially reported conditions did not give any product at all it was found that the reaction proceeded smoothly in the presence of substoichiometric amounts of silver carbonate yielding 3. After tosylation the mixture was exposed to methanolysis to produce the backbone structure 4 of communesin F. Methylation with Meerweins’s salt, hydrogenolysis of the azide with subsequent Boc-protection and detosylation furnished amide 5.

 Scheme 1

The bromine was then used as a handle to introduce the prenyl sidechain with a Heck reaction to give an intermediate allylic alcohol. In the presence of mercury salts a cyclization took place constructing the crucial seven-membered amine ring. Deprotection of the amine under mild conditions using TBSOTf was followed by amide formation with some help from trimethylaluminium. The second side-chain was introduced in the usual sequence of deprotonation and subsequent alkylation with iodoacetonitrile.

 Scheme 2

The last ring was closed in a straightforward manner. Reduction of the nitrile to the aldehyde and the amide to the hemiaminal gave tetrahydrofuran 9. Reductive amination and acetylation finally produced communesin F in an overall yield of 6.7 %.

Scheme 3

The mechanism of the key step is pretty straightforward but nevertheless a nice one. After tosylating the oxindole nitrogen the resulting amide can be cleaved by methoxide to give an anilide anion which undergoes intramolecular attack on the indolenine 2 position to close the ring.

Scheme 4

[1] J. Am. Chem. Soc. 2004, 126, 5068

Yeah… that is it for the moment. I found some new interesting stuff to write-up, so I promise I will be up to date from now on J

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3 Responses

  1. The “substoichiometric amounts of silver carbonate” look like 1.6 eq of Ag(+) ions to me.
    Great post, by the way.

  2. thx for mentioning, of course you are right. fortunately silver carbonate is not that expensive 😉
    fixed…

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