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Blocking OH groups is not a problem. Sorbitan contains 1 primary and 3 secondary OH groups. You can block them all, e.g. by reaction with acetic anhydride or benzoyl chloride in pyridine at room temperature. You can block selectively the primary OH by the reaction of sorbitan with trityl chloride (or mono- or dimethoxytrityl chloride) in pyridine – these reagents under controlled conditions do not react with secondary OH groups. If you block the remaining three OH groups in this trityl derivative with acetic anhydride and then remove trityl group under acidic conditions, you will get sorbitan with blocked secondary OH groups and free CH2OH. You can block all hydroxyls in form of two isopropylidene groups by the reaction with acetone under acidic conditions (e.g. in the presence ot TsOH). The methods above are the most common; the choice of reactions and reagents is broad. But of course the choice will depend on your particular goal, especially if you want to go into polymer chemistry.
An alternative approach to the chemistries outlined by Igor above is to transform the hydroxyls into urethanes by reacting with with a slight excess of alkyl or aryl isocyanates (eg, 1-phenylethyl isocyanate).
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