Specific Organosilicon Compounds

There are several classes of organosilicon compounds lacking organic analogues and groups of compounds with specific properties based on the distinguished characteristics of silicon.

Silicon can form stable penta-8,12 and hexa-coordinated129,135 organic derivatives which is possible for carbon only in some special cases. One of such class of compounds with penta-coordinated silicon is silatranes (22) having a wide variety of biological effects.6 9,12,13

Silatranes bearing aryl7,8 and 2-thienyl254 groups at the silicon atom have been shown to be very toxic compounds (LD50 0.3-1.0 mgkg-1). 1-(4-Chloro-phenyl)-silatrane255 has been proposed as a single-dose rodenticide.256 259 It has several distinct features: self-detoxification, which minimizes or eliminates the hazard of persistent or long-lasting residues, no secondary poisoning hazard and low dermal toxicity.260

Alkyl, alkenyl and alkoxysilatranes are compounds exhibiting a low toxicity (LD50 2000-3000 mg kg-1).6 8 Methyl,9 chloromethyl9,261 and ethoxysilatrane9 have been found to stimulate collagen biosynthesis in cartilaginous tissues of chick embryos in vitro (increasing the formation of peptide-bound 14C-hydroxyproline from the 14C-proline of the incubation medium). These compounds did not change the activity of collagen prolylhydrolase but stimulated protein biosynthesis (the incorporation of 14C-proline increased 1.5-fold).9

Some other biological properties for these low toxic silatranes have been described: anti-tumour,8,12,74,262 hepatoprotective,263 hypocholesterolemic,264 intestine peristalsis stimulating,265 wound and bone fracture healing8,266 and plant growth stimulating.8

Silicones (oligo and polysiloxanes) are the most important organosilicon compounds used in medical and medicinal application, for which there are no carbon analogues. They are widely used in plastic and reconstructive surgery, as implanted prosthetic devices, shunts and catheters. Silicone fluids for their anti-coagulant and anti-foaming properties (simethicone) have been used as lubricants in medical devices and anti-flatulents (also in veterinary medicine). They are also frequently used as depot forms for pharmaceuticals, in cosmetic and other formulations for topical administration (e.g. anti-perspirants).6,7,267

For a long time, these silicones without functional groups have been assumed to be biologically inert.6,7,27

Unexpectedly, some cyclosiloxanes were found to be active estrogens.11,268 The most active compound was 2,6-cis-diphenylhexamethylcyclotetrasiloxane (23).269 272 It entered clinical studies for the treatment of prostatic

272 275

carcinoma.272 275

There are substantial differences in the chemical properties of hydroxyl derivatives of carbon (carbinols) and silicon (silanols). Silanols are stronger acids than the corresponding carbinols, and silanediols are more stable than geminal organic diols that readily eliminate water to give the corresponding ketones. These differences have been used to design biologically active silanols.

Diphenylsilanediol Ph2Si(OH)2 has been found to exhibit anti-convulsant properties.11,276,277 Unfortunately its use as an anti-epileptic agent was prevented by toxic side effects.278

Such simple silanediols did not inhibit hydrolases. Therefore dipeptides and tripeptides containing a more complex silanediol group that mimic angiotensin-converting enzyme (ACE), HIV protease and thermolysin inhibitors were synthesized. One of them, 24, inhibited ACE with an IC50 value as low as 14 nM presumably acting as an active site zinc chelator.282

Methylsilanetriol complexes with sodium and potassium salts of various hydroxycarboxylic and amino acids (salicylic, citric, ascorbic, nicotinic, ben-zylpenicillin) have been reported as useful for the treatment of certain cardiovascular diseases,6,7,283 286 cancers287 and virus infections.288,289 The compound formed by the reaction of methylsilanetriol with potassium salicylate (25) (Conjonctyl®) has been proposed for the treatment of painful polycystic mastopathies.290

A specific property of trimethoxysilanes RSi(OMe)3 is their ready hydrolysis to silanetriols RSi(OH)3, which bind to a wide range of surfaces such as glass, ceramic' cotton' wool' leather' polyester' rubber and wood. If the substituent R contains a quaternary ammonium group, the compound exhibits anti-microbial activity in solution. It binds strongly to the surface of materials and imparts an anti-microbial activity to the treated surface. The practically used compound is dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride (26).291 295 It has a broad spectrum of activity against Gram positive and Gram negative bacteria, fungi and algae.291 Silicon rubber coated with 26 showed antimicrobial activity against adhering bacteria both in vitro and in vivo.296

{MeO)3SiCH2CH2CH2NMe2C18H37 Cl" (PhCh^O^SiMeCh^Ch^CI

Another specific reaction of organosilicon compounds unknown for the carbon analogues is the ^-elimination of 2-halogenoethylsilanes yielding ethylene. Based on this property, (2-chloroethyl)bis(benzyloxy)methylsilane (27) has been proposed as an ethylene-releasing plant growth regulator.7,22,297 300

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