Siliconides

Two hydroxyl groups also can be blocked if siliconides are prepared (Scheme 4.20). 

Acetonides [383,384] and siliconides [385,386] are prepared by the reactions of neighbouring hydroxyl groups, e.g., in positions 16,17 and 17,21, with acetone or dimethylchlorosilane. The reaction with acetone proceeds under acid catalysis with hydrogen chloride or TMCS, as follows. Steroids are dissolved in 10 ml of freshly distilled acetone and 100 n 1 of TMCS are added. The mixture is agitated at room temperature for 2 h, 1 ml of 1 N sodium hydroxide solution is added and the solvent is evaporated at... 

Dimethyldiacetoxysilane can also be used as a reagent for the preparation of siliconides, as it reacts simultaneously with other unprotected hydroxyl groups and blocks them. 

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... 

Silylation.1 The reagent reacts with rf.v-diols and with corticosteroids in the presence of triethylamine to form siliconides (2) analogous to acetonides. It reacts with an isolated hydroxyl group to give a thermally stable silyl derivative of type... 

The acetonides of various pregnane-17,20- and -20,21-diols have been prepared, and their properties studied. The 20,21-acetonides are very readily formed and hydrolysed, with toluene-p-sulphonic acid catalysis. Perchloric acid is required to catalyse formation of 17,20-acetonides these are also more resistant to hydrolysis, especially in the case of the 20j5-epimer, which is subject to more steric hindrance. Acetonide formation (121) is also reported in a 12o,17a-dihydroxy-pregnan-20-one. The term siliconide is proposed for the novel 16o,17a-di-methylsilyldioxy-derivative (122), readily formed by treating the 16a,17a-diol... 

The formation of isopropylidene derivatives (aceto-nides) is also possible, and those of 3,16a-oestradiol and cis (but not trans) epimeric oestriols have been successfully separated [28, 29]. Under normal GLC conditions 17-hydroxycorticosteroids undergo pyrolysis to C-19 ketosteroids. This can be avoided if a thermally stable C-20-21 side chain can be formed by cyclization (see also siliconides and boronates, below) under acid conditions. 

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]. 

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. ... 

Pinacol cyclisation agent. Forms siliconides with cw-diols suitable for gc anal. Liq. d 1.06. Mp —16°. Bp 70°. >VV3150000. 

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