At 05:21 -0400 16/9/07, Jean-Claude Bradley wrote: >The issue of shift prediction in different solvent environments could be very >useful. In monitoring reactions by H NMR we have observed significant >reproducible shifts over the course of a reaction, presumably because of a >change in the composition of the solution.
See my previous post! By the way, we have tried our protocol on three other natural products. On the obtusallenes, we are finalising a paper to Org Lett on their structural revision (having had to deal with Spin Orbit corrections to 13C shifts. Turns out that this has a small impact on TMS (yes, SO-coupling to Si can be about 1-2 ppm!!!) and hence the referencing standards that are used. The second example is also interesting. I attended an organic chemistry conference during the summer. One plenary lecturer showed a product with stereochemistry that "instinct" (I really cannot put it more firmly than that) suggested to me was wrong. A MOPAC calculation revealed the isomer he showed was about 60 kcal/mol higher than a more obvious candidate (by the way,the La Clair hexacyclinol energy was about 100 kcal/mol higher than the Rychnovsky structure. Nature rarely builds that much energy into its biosynthetic pathways, other than of course the ATP one. No-one has yet mentioned that the structure could have been questioned on these grounds alone!!). I calculated the 13C shifts for my proposed structure, and now eagerly await to learn if they match that measured (along with the one shown in the lecture). Come the day soon when ALL of this will be doable on an iPhone!! (I am not jesting by the way). The third example is perhaps potentially typical as well. A student at Imperial has spent 3 years making a natural product. Of course, the penultimate step refused to crystallise, and two diastereoisomers are isolated. They do not know which is the correct one. The 13C prediction will almost certainly reveal the answer. -- Henry Rzepa. +44 (020) 7594 5774 (Voice); +44 (0870) 132 3747 (eFax); [EMAIL PROTECTED] (iChat) http://www.ch.ic.ac.uk/rzepa/ Dept. Chemistry, Imperial College London, SW7 2AZ, UK. (Voracious anti-spam filter in operation for received email. If expected reply not received, please phone/fax). ------------------------------------------------------------------------- This SF.net email is sponsored by: Microsoft Defy all challenges. Microsoft(R) Visual Studio 2005. http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/ _______________________________________________ Blueobelisk-discuss mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/blueobelisk-discuss
