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Chemistry World: Hypocrellin A   

2 July 2008 7,885 views 11 Comments

Latest piece for the RSC in the July issue of Chemistry World

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  • antiaromatic says:

    The final deprotection is actually not so surprising at all. It is well known that in 1,3-phenolic ketones, the OMe can be easily deprotected under relatively mild conditions because the product structure is stabilized by an intramolecular hydrogen bond between the phenolic OH and the ketone (six membered ring). There are plenty of cases where simple heating to 60 C with pTSA will do the trick while the other phenolic OMe’s require BBr3 for deprotection.

  • TWYI says:

    I selectively take off OMe’s like this all the time. BCl3 and AlCl3 work well, leaving all phenolic OMe’s not H-bonded alone.

  • Madforit says:

    i need help:i’m trying to do a methilenation(metilentriphenilphosphoniumbromide,sodium hydride,DMSO)on a beta keto ester..any references?Thanks a lot.

  • Diego says:

    Madforit: is that the Eschenmoser Reaction??
    i work a bit with that kind of chemistry. i have many references that can help.

  • Diego says:

    BTW, great job Paul. keep going..

  • Madforit says:

    no it’s “just”a simple wittig reaction…but i m going mad…probably the phosphorous ilide act as a base for tha active methilene H’s,i don t know,i have a mixuture after column …
    Thanks a lot.

  • Madforit says:

    …not a mixture after column,but something that i can t explain

  • milkshake says:

    you will always have a problem as long as you have ester beta to carbonyl – because of the facile enolisation, and also the product, beta C=C ester will like to equilibrate to conjugated one.

    You can try dimethyl titanocene, it should be less basic than Witting and there is a good Orgsyn procedure for it, from Cp2TiCl2 and Me-Grignard. Also check the Petatis original paper, I think it was in a JACS from early 90s and if I remember it works on enones and tetralones – and those are easily enolisable substrates.

  • Madforit says:

    thanks a lot,the alternative strategies was petasis titanocene or protection of carbonyl mojety..

  • antiaromatic says:

    So, in terms of these wittig reactions, suppose you don’t want to just methyleneate the carbonyl and instead you’d like to use, say, the propyl Wittig derivative. Is there any hope of ever doing such a reaction?

    Also, Madforit, not sure if this helps, but sometimes it’s worth remembering it’s possible to do a wittig in two ways. You could also reduce your ketone, turn into bromine, make wittig salt and attack the aldehyde of whatever you had. It’s def. more steps, but it’s an alternative. I suppose you’d still have to worry about the enolization issue with the ylide, but…..

  • Madforit says:

    any procedure for the selective protection of the ester mojety beta to a ketone?