Silyl sulfates, hydrolysis

The diols (97) from asymmetric dil droxylation are easily converted to cyclic sii e esters (98) and thence to cyclic sulfate esters (99).This two-step process, reaction of the diol (97) with thionyl chloride followed by ruthenium tetroxide catalyzed oxidation, can be done in one pot if desired and transforms the relatively unreactive diol into an epoxide mimic, ue. the 1,2-cyclic sulfate (99), which is an excellent electrophile. A survey of reactions shows that cyclic sulfates can be opened by hydride, azide, fluoride, thiocyanide, carboxylate and nitrate ions. Benzylmagnesium chloride and thie anion of dimethyl malonate can also be used to open the cyclic sulfates. Opening by a nucleophile leads to formation of an intermediate 3-sidfate aiuon (100) which is easily hydrolyzed to a -hydroxy compound (101). Conditions for cat ytic acid hydrolysis have been developed that allow for selective removal of the sulfate ester in the presence of other acid sensitive groups such as acetals, ketals and silyl ethers. 

A recent notable finding in this field is Mukaiyama aldol reactions in aqueous medium (THF H20 = 9 1) catalyzed by metal salts. Lewis acids based on Fe(II), Cu(II), and Zn(II), and those of some main group metals and lanthanides are stable in water. Remarkably, the aldol reaction shown in Sch. 29 occurs more rapidly than the hydrolysis of the silyl enol ether [137]. In the presence of surfactants (dodecyl sulfates or dodecane sulfonate salts), reactions of thioketene silyl acetals with benzaldehyde can be performed in water [138]. 

If the substituents are not stable during acid hydrolysis or methanolysis, the complementary substitution pattern can be determined after permethylation, as already noted. Examples are organic (acetates) and inorganic esters (sulfates), and benzyl and silyl ethers. " Such an approach was applied as early as 1964 to vinyl starches."" In contrast to the direct determination of the substituent pattern, incomplete methylation and consequently erroneous evaluations cannot be recognized from the product pattern obtained in the indirect methods. If the average DS is known from an independent determination, it allows at least a certain control. Therefore, the absence of OH absorption should be carefully monitored by IR spectroscopy. 

GC methods are summarized in Tables 2 and 3. Sulfates of alcohols and ethoxylated alcohols are readily decomposed by acid hydrolysis to yield the free alcohols or ethoxylated alcohols. The trimethylsilyl derivatives of these are easily analyzed by gas chromatography (24). Derivatization of the alcohols may be omitted if they have reasonable volatility (25). Alternatively, the surfactant may be decomposed by hydriodic acid, giving alkyl iodide derivatives of the starting alcohols for characterization by gas chromatography (26). Hydrolysis can be combined in the same step with derivatization. Reaction of alkyl sulfates with silylating agents such as BSTFA/1% TMCS results in cleavage of the sulfate and formation of the trimethylsilyl ethers of the alcohols (27). GC or GC-MS analysis will... 

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