From: Witek Mozga (mozga$##$trimen.pl)
Date: Fri Apr 12 2002 - 01:56:44 EDT
Hi ,
Methylation of nitrogen in an urethane protected aminoacid with a
free carboxyl group proceeds well in THF in the presence of
excess NaH and excess MeI. I think that double anion
RN(-)CH`RCOO(-)
forms and then it is methylated. But as I could observed there is no
methylation on carboxyl, only nitrogen reacts despite excess of
methyl iodide. I used to think that ester forms but during work up it
hydrolyzes rapidly to give the free aminoacid. I tried to take a small
sample from reaction mixture, acidify it and detect the ester on the
HPLC but I failed. The only peaks were the substrate and the
product RNMeCHR`COOH. Another evidence that the ester does
not exist in the mixture is lack of racemisation. If it did exist the
aminoacid could racemize through enolate. (maybe this is the
reason why not to use methyl sulphate in this reaction?). Does
anyone have any idea why the ester doesn`t form here? Is oxygen
too hard a nucleophile to react with such a soft specie like MeI?
Regards,
Witek
http://www.trimen.pl/witek/
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