Silylation, conditions

There are rarely more than eight alkaloids present in a species, and the possibility of the same GC retention time is not normally a problem. In cases where overlap has been observed it has proved possible to identify both components from the mass spectrum of the mixture. The crude alkaloid extracts are treated with trimethylsilyldiethylamine to form volatile TMS derivatives of the hydroxylated components. The presence of a free phenolic or hydroxyl group is then detected by an ion with m/e 73 [(CH3)3Si+]. Positional isomers [e.g., erysovine (5) and erysodine (7)] are resolved although a- and /1-erythroidine are not. The presence of fi-erythroidine (60) can be estimated since it shows some enol content under the silylation conditions and gives rise to a monotrimisyl derivative with m/e 345 (15). 

Early reviews on the mass spectra of nucleotides, more particularly of mononucleotides, " covered trimethylsilyl and methyl derivatives of nucleotides. The silylation of nucleotides, for instance, takes place on the base, on the sugar hydroxyls, and on the free phosphate hydroxyls under mild (room temperature) and basic (pyridine) silylation conditions (BSTFA/TMCS). The fragmentation of derivatives obtained from both procedures has been well studied. Several typical fragments ([M — CHj]", [base -I- 2H], silylated and desilylated sugar, S-TMSOH, etc.), as well as a weak M+ ion have been recorded. The FD spectrum of an underivatized... 

Thiol esters can be converted to the corresponding 0-silyl ketene thioacetals. The thiol ester (56), under different silylating conditions, can be converted stereoselectively into either the ( )- or the (Z)-isomers of (57). ... 

If necessary, aldehyde 21 could easily be isolated and subjected to the standard silylation conditions to regenerate 18. By maintaining the reaction temperature between -5 to 0 °C, formation of 21 was minimized. If the temperature of the reaction mixture during the quench and work-up was not strictly held below 10 °C, significant quantities of 22 were isolated (currently we have around 2 kg of this compound). This is formed by P-elimination either of silyl alcohol from 18 or of water from... 

The generality of these silylation conditions was then briefly explored. A variety of silyl ethers were synthesized from various alcohols and one equivalent of the corresponding trialkylsilyl chloride in hydrocarbon solvent. One equivalent of imidazole was utilized in each experiment. A partial listing of the results are tabulated in Figure 9. The yield and... 

The use of molar equivalents of methylboronic add and steroid diol affords only the cyclic ester. Acyclic boronate esters will be formed at remote hydroxyl groups if excess reagent is used. While these latter derivatives display undesirable GC properties, the acyclic boronate groups are readily displaced by trimethylsilyiation the cyclic methaneboronate will be unaffected under the mild silylation conditions described above. n-Butyl boronate—methoxime—TMS derivatization has been... 

Pyridinium salts of some sugar sulfates can be desulfated and silylated under certain silylating conditions. ... 

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