Trimethylsilyl enolate ozonolysis

Reductive removal of the halogen was achieved with tributyltin hydride and subsequent ozonolysis gave aldehyde 221. An aldol condensation of 221 with the trimethylsilyl enol ether of methyl propionate, followed by Jones oxidation... 

A section on cardenolides would not be complete without mentioning the total synthesis of (+)-digitoxinin by Stork and his co-workers. Protected Wieland-Miescher ketone 66 was converted into compound 67 via formation of the trimethylsilyl enol ether ozonolysis of the enol ether gave a mixture of hydroxyketones, which was reduced to the corresponding diol and then cleaved with sodium periodate to yield dialdehyde intermediate 67. Compound 67 was then elaborated into Diels-Alder precursor 68 in two steps. After completion of the Diels-Alder reaction, four additional steps led to the preparation of intermediate 69, which was ideally set up for a vinyl radical cyclization. Key intermediate 70 was then elegantly converted into (+)-digitoxinin (71). 

Further oxidation reactions of trimethylsilyl enol ethers have been reported. For example, Mo02(acac)2 and Bu OOH can be used very effectively to cleave these systems to carbonyl compounds. This procedure is quoted as being more convenient and certainly more selective than the alternative ozonolysis method (Scheme 7). Trimethylsilyl enol ethers also react with silver carboxylate-iodine... 

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. 

Nitrosolysis of camphor ethyl acetal with ethanolic ethyl nitrite in sulphur dioxide yields the orthoester oxime (205) which is rapidly dehydrated by excess acetal to the orthoester nitrile which then reacts with sulphur dioxide to form the ester nitrile and diethyl sulphite.Further papers in this section include the full paper on ozonolysis of silyl ethers (Vol. 5, p. 33), another synthesis of camphor-enol trimethylsilyl ether (cf. Vol. 6, p. 41)/°° the conversion of camphor oxime with Grignard reagents into the corresponding imine with no aziridine formation/° the preparation of (206) by treating bornylene with trichloroacetyl isocyanate/ the oxidation of thiocam-phor to the 5-oxide and alkylation in the presence of thallium(i) ethoxide to a/S-unsaturated sulphoxides/ and the free-radical C-3 alkylation of camphor with alkenes. " ... 

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