Sulfur a and nitrogen in this small mlecule aren't indipendent in the attempt to protect the sulfur you will block the nitrogen too

See also strusctures here ...

http://www.ias.ac.in/chemsci/Pdf-Jan2008/143.pdf

Thiols are MOM-protected using bromochloromethane and methanol under phase-transfer catalysis with benzyltriethylammonium chloride. Under this condition alcs. and carboxylic acids are recovered unchanged as reported earlier

---A simple method for the selective MOM protectionof thiols, Toste, F. Dean; Still, Ian W. J., Synlett (1995), (2), 159-60.

You may apply this condition too and see what happen

Good luck

Sorry gentlement I do not probably understand completely what your answers refers to The Methimidazole molecule Fariba refers as to willinf to S protect, indeed isn't a thiol properly speaking, it may behave as a thiol but it isn't a thiol (1-methyl-1,3-dihydro-2H-imidazole-2-thione). For sure it would have a partial character of the tautomeric thiiolic form but to the expenses of the conjugated nitrogen which will loose its proton, that it is what I meant on the first answer.

If is that what Fariba wants then you all righ there are a lot of procedures that can be used ...

Regards

Hi Fariba,

numerous examples of MOM-protection (selective to S- vs N-atom) is described in Bull. Korean Chem. Soc. 2003, Vol. 24, No. 11 , pp.1689-1691 - methimazole is also included.

As catalyst, authors use low-valent Ti, but I am sure that ordinary P2O5 will work perfect in combination with dimethoxymethane CH3OCH2OCH3 too ( Org. Synth. Coll., 9, 539 (1998) - via inet available from www.orgsyn.org ).

However, two problems will still present - selective alkylation and deprotection.