Cyclic siliconides

Another, similar, procedure is for steroids with ris-diol groupings, and uses a mixture of DMCS, pyridine and benzene in the ratio of 1 1 8 [35], or even more simply DMCS 1 1 in pyridine [36], for 1 h at room temperature. The proportions of reagents to steroid are not critical, although an excess of DMCS is essential. The reaction mixure may be directly analysed by GC. 

Alternatively, the steroid (10is dissolved in 1,1,3,3-tetramethyldisilazane (10/rl) and DMCS (10/il of a 20% solution in benzene) and incubated for 12 h in a sealed tube at 40 °C. Again, samples may be injected directly, or the reaction mixture may be evaporated to dryness, the derivative being dissolved in carbon disulphide for analysis [39], 

A few mg of the steroids in a reaction vial are treated with 20 fi of a hexane solution containing 2% DMDAS and 2% triethylamine for 2 h at 40 °C, and the products are analysed directly [37]. A similar procedure, on the micro scale, may be used for salicylic and anthranilic acids and other ortho-hydroxy aromatic adds (40). 

Diols and o-hydroxy acids can also be converted to the cyclic di-t-butylsilyl derivatives, which have certain advantages, particularly for GC-MS. The diols or hydroxy acids (100 mg) are dissolved in a mixture of acetonitrile (30 fil) and N-methylmorpholine (20 (A), or alternatively just N-methylmorpholine alone (50 fil), and treated with 3.5 //I of di-t-butyldichlorosilane and 1 /il of a 0.3% solution, in acetonitrile, of the silyl transfer catalyst 1-hydroxybenzotriazole for 15 h at 80 °C [41], 

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Cyclic siliconides are analogous to acetonides in structure, and can most readily be made with dichlorodimethyl-silane (DMCS) in pyridine [35,36], Cyclic siliconide derivatives of steroids can similarly be made by reacting the dihydroxyacetone side chain of, e.g., cortisone, cortisol or betamethasone with dimethyldiacetoxysilane (DMDAS) and triethylamine [37] as shown in Figure 5. Other reagents for preparing cyclic siliconides include tetramethoxysilane and triethoxysilane [38]. 

SUiconides. The steroidal cis-diol, A5-3/3-acetoxy-16a,17a-dihydroxy-pregnene-20-one (1), is converted by reaction with dimethyldichlorosilane in pyridine into a cyclic silyldioxy compound (2). In analogy with acetonides, the name siliconide is suggested for such a derivative.1... 

Protection. The success of the pinacol cyclization is due to the formation of a siliconide , the silicon equivalent of an ace-tonide. Reaction of a diol with (1) in the presence of base also provides these cyclic silicon compounds (eq 2). However, the labile dimethylsiliconide does not afford a widely applicable form of protection for this functional group, although it has found some use with pericyclic reactions. ... 

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