ORGLIST: Re: S-alkylation of 2-thiouracil

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From: by way of Joao Aires de Sousa (r.ragno$##$
Date: Wed Mar 19 1997 - 03:38:58 EST

On Tue, 18 Mar 1997, Mark Rosen wrote:

> At 08:50 AM 3/18/97 +0100, you wrote:
> >Dear colleagues,
> >I've synthesized many 6-substituted-2-alkylthio-4-oxo-pyrimidines by
> >S-alkylation of 2-thiouracils with alkyl halides and potassium carbonate
> >in DMF, but when I tried to introduce the cyclopropyl group in the same
> >way (i.e. with cyclopropyl iodide) I failed. Do you have any suggestion?
> >I'll summarize to the list the answers I'll get.
> >Thank you in advance,
> >
> >Gianluca " Blue Merlin" Sbardella
> >
> I would imagine this is due to the fact that the sulfur nucleophile
> displaces the halogen in an SN2 type mechanism. Thus the sulfur
> approaches the from the opposite side of the halide. This is precluded
> in the cyclopropyl system. Furthermore, SN2 type reactions occur with
> inversion of stereochemistry (if relevent), showing that the relative
> position of the substituants at the carbon in question must invert. This
> again cannot occur in a cyclic system.
 Dear Mark,
thank you for your answer.
As regards to cyclic systems, the reaction was succesful with cyclohexyl
iodide as well as cyclopentyl iodide. I believe that the peculiar
structure of cyclopropyl ring (banana-like orbitals...) prevents the
nucleophile to approach sufficiently. Any other suggestions?
Thank you again,
Gianluca "Blue Merlin" Sbardella
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    * Dr. Gianluca Sbardella E-mail: r.ragno$##$ *
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