Vinylsilanes, ozonolysis

Vinylsilanes, ozonolysis, 275-6 Vimses, ozone disinfection, 616 Visible spectrophotometry see UV-visible spectrophotometry... 

Methylcyclohexenone 281 upon oxidation with Mn(OAc)3 in benzene under reflux gave 282, which reacted with phenylmagnesium chloride and CuBr-Me2S to form two isomeric ketones 283 and 284. Further, 283 has been transformed to vinylsilane 285 followed by its hydrolysis to form the free alcohol 286, which in turn was alkylated with methoxyallyl bromide to give 287. Oxalic acid-mediated deprotection of 287 led to the formation of the ketone 288. Ozonolysis of 288 in methanol afforded the fused 1,2,5-trioxepine 289 in low yields (Scheme 66) <1997BML2357>. 

Known bicyclo[4.3.1]enone 15758 was converted into vinylsilane 158 with bis(trimethylsilyl)methyl lithium.55 Diene 158 underwent selective ozonolysis at the cis-olefin under conditions to produce differentially oxidized termini 90 alde-hydo-ester 159 was homologated with a phosphine oxide anion91 to enol 160. Subsequent hydrolysis of 161 provided substrate 162, which after tandem ozonolysis-acidification gave racemic 6,9-desmethyl analogue 155. Unfortunately, initial efforts failed to resolve 155 into its two optical isomers with cellulose triacetate.92 However, the antimalarial activity of racemate 155 is intriguing, as discussed in a later section. 

As shown in Scheme 17,180 was available from the common, totally synthetic, intermediate 17. Using reported methodology for the introduction of vinylsilanes,57 17 was reacted smoothly with (methoxy dimethylsilyl) trimethylsilyl methyl-lithium in pentane to afford (EIZ) vinyl silane 220 in 54% yield. The main by-product in this reaction was ketone 17, which could be recycled thus, based on recovered 17, the yield of 220 was 93%. Hydrolysis of ketal 220 occurred without protodesilylation upon exposure to aqueous oxalic acid absorbed onto silica gel to give ketone 221 in 80% yield. Upon low-temperature ozonolysis of 221 in methanol, a remarkably stable dioxetane 223 was produced, as evidenced in the H NMR... 

Variation in substituents generates a variety of analogues of artemisinin.59 62 Ozonolysis is carried out at low temperatures (-70°C to -80°C) in a reaction solvent chosen to assure compatibility with both ozone and the vinylsilane. Generally, lower alcohols such as methanol, or haloalkanes such as dichloromethane, or a combination of both types of solvent are used. The reaction is carried... 

Suggest a mechanism for the formation of compound 8, obtained upon ozonolysis of vinylsilane 7. 

Vinylsilanes may be prepared from the corresponding ketones by formation of the hydrazone followed by Shapiro reaction, quenching the vinylic anion with chlorosilanes. An equally effective process derives the vinylsilane from the vinyl chloride, in turn prepared simply from the ketone. The crucial oxidative transformation may be achieved in two ways. Firstly, ozonolysis in dichloromethane/methanol at approximately 0 C followed by reductive work-up affords the a-hy xy ketone, e.g. (96) to (97), via the intermediates (98) and (99). The outcome of the reaction varies with the solvent and work-up conditions, but using the combination indicated, good yields of the desired products are available. 

Recently Avery et al [94] have developed a stereoselective total synthesis of (+)-artemisinine starting from (R)-(+)-pulegone (92). Elaboration of 92 gives the known sulphoxide 93, which was allowed to undergo dianion alkylation and desul-phurisation to yield disubstituted-cyclohexanone (95). Homologation of the latter afforded the aldehyde % in two steps. This product was then converted into the silyl acetate (97), which underwent Tandem Claisen ester-enolate rearrangement to give the vinylsilane 98. Ozonolysis and cyclisation of 98 provided 7 (Scheme 10). 

Ozonolysis of vinylsilanes. Biichi and Wtiest ozonized the vinylsilane 1 expecting to obtain the norketone 2 as the major product. Instead the major product was an a-hydroperoxy aldehyde (3). They then investigated ozonolysis... 

Ketones can be isomerized by a 1,2-carbonyl shift throu a vinylsilane intermediate (Scheme 12). The main products of ozonolysis of vinylsilanes are a-... 

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