Silyl dienol ethers oxidation

The highly potent antithrombotic (+)-rishirilide B was synthesized in the laboratory of S.J. Danishefsky. One of the tertiary alcohol functionalities was introduced via the Rubottom oxidation of a six-membered silyl dienol ether with dimethyl dioxirane (DMDO). The oxidation was completely stereoselective, and it was guided by the proximal secondary methyl group. Subsequently, the enone was converted to the enedione, which was used as a dienophile in the key intermoiecuiar Dieis-Aider cycioaddition step. 

Lewis acid catalyzed alkylations of cross-conjugated silyl dienol ethers provide routes to a -alkylated ketones. A short synthesis of the sesquiterpene ( )-ar-turmerone has been accomplished using the cross-conjugated TMS dienol ether of mesityl oxide (equation 9). ... 

Finally, silyl dienol ethers (20) have been shown to react with bromonaphthoquinones (21) in a mode described as a formal oxidative addition process (Scheme 9) <9iJOC9l>. 

The total synthesis of ( )-trichostatin A has been carried out by two routes [67] one uses the y-alkylation of a silyl dienol ether with an acetal which gives the ether in a Mukaiyama reaction. A Wittig olefination followed by DDQ oxidation gives a ketoester which reacts with hydroxylamine to give trichostatin A in 22% overall yield (Scheme 20). 

Belli PaolobeUi, A., Latini, D., and Ruzziconi, R., y-Selectivity in the ceric ammonium nitrate promoted oxidative addition of silyl dienol ethers to silyl enol ethers. Tetrahedron Lett., 34, 721, 1993. 

Oxidation with mepba — -hydroxyketones, using the reaction of isophorone dienol ether (detailed preparation given), and Et3N.HF cleavage of the intermediate silyl ether. 

LDA, LiCl) afforded the dienolate intermediate, which was alkylated with iodoethane. Four additional steps including a reductive cleavage of the chiral auxiliary, the protection of the resulting primary alcohol, the oxidative cleavage of the double bond under Johnson-Lemieux conditions, and the formation of the silyl enol ether led to the desired fragment 187. 

This was followed shortly by a stereo- and enantiocontrolled synthesis of (—)-chimonanthine (154) and calycanthine (150) as well as a second route to meso-chimonanthine (152). The central step in this synthesis features the use of a double Heck cyclization to create vicinal quaternary carbon centers in high yields and with complete stereocontrol 124). The synthesis commenced with a double alkylation of the lithium dienolate of dimethyl succinate 194 and tartrate-derived diiodide 195 to give a diastereomeric mixture of the saturated diesters. Subsequent oxidation of the diesters, followed in succession by aminolysis, A-benzylation, removal of the benzyl ethers, and silylation, provided the cyclization substrate 197, which on Heck cyclization yielded a single product, 198, a pentacyclic bisoxindole, subsequently shown to have the meso relationship of the two oxindole groups. Further manipulations of 198 led eventually to the diazide derivative 199, which can be processed to we.so-chimonanthine (152), following the procedure established in the preceding synthesis (Scheme 14). 

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