TES enol ether

In the highly stereoselective synthesis of hispidospermidin, the oxygenation of the CIO position was achieved via a Rubottom oxidation by S.J. Danishefsky et al. The tricyclic ketone was first converted to the TES enol ether, which was readily oxidized with mCPBA to give the corresponding a-hydroxy ketone as a single diastereomer. 

Since the Viismeier-Haack formyiation is feasible on electron-rich alkenes such as end ethers, it was a method of choice to prepare an a, 3-unsaturated aldehyde during the total synthesis of (+)-illudin C by R.L. Funk et al." The TES enol ether was treated with several reagent combinations (e.g., PBra/DMF/DCM), but unfortunately only regioisomeric product mixtures were obtained. However, the use of POBrs/DMF/DCM allowed the clean preparation of the desired aldehyde regioisomer in good yield. 

Alternatively, triorganyl-//-silanes can be added across the carbonyl function under hydrosilylation-type conditions, e.g., triethylsilane (442) reacts with aldehydes in the presence of a Ni catalyst to yield the corresponding TES-enol ether 632 (equation... 

The chiral (V-camphanoyl iminium ion 7, prepared by hydride abstraction from 2-camphanoyl-l,2,3,4-tetrahydro-6,7-dimethoxyisoquinoline 6 (see Appendix) with triphenylcarbenium te-trafluoroborate, reacts with silyl enol ethers to give 1-substituted tetrahydroisoquinoline derivatives with reasonable diastereoselectivity, 0°. On addition of titanium(IV) chloride, prior to the addition of the silyl enol ether, the diastereoselectivity gradually rises to an optimum at 2.5 equivalents of the Lewis acid, but the yield drops by 20%. 

Diels-Alder reactionsBoth 1,4-dicyanonaphthalene (DCN) and 2,6,9,10-te-tracyanoanthracene (TCA) have been used as sensitizers to effect photochemical [4 + 2]cycloadditions of electron-rich dienes and electron-rich dienophiles, which do not normally undergo thermal cycloadditions. These cycloadditions are known as triplex Diels-Alder reactions because they are postulated to involve as an intermediate a three-membered complex of sensitizer, dienophile, and diene. This reaction is useful for synthesis of bicyclo[2.2.2]octenes from some silyl enol ethers, alkenes, or arylalkynes. 

Many examples exist wherein a silyl enol ether is deprotected and oxidised in a one-pot procedure using Swem conditions. Godfroid and co-workers report the selective oxidation of primary TMS or TES protected 1,2-diols, 1,3-triols, and polyhydroxy compounds to the corresponding silyloxy aldehydes even when using excess Swem reagents.36... 

Mannich-type Reactions. The reactions of imines with ketene silyl acetals proceed smoothly in the presence of Sc(OTf)3 to afford the corresponding /3-amino ester derivative in moderate yield (eq 6). Sc(OTf)3 shows higher activity than Yb(OTf>3 does in this case. The catalyst can be recovered after the reaction is complete and reused. A Mannich-type reaction of IV-(/3-aminoalkyl)benzotriazoles with sUyl enolates has also been developed. Mannich-type reactions of polymer-supported sUyl enol ethers with imines or of polymer-supported a-iminoaceta-tes with silyl enolates are also catalyzed by Sc(OTf)3. 

On the other hand, methylaluminum bis(4-bromo-2,6-di-te/t-butylphenoxide) can effect the a-alkylation of enol silyl ethers of a variety of ketones, esters and some aldehydes . Use of the (t-Bu)Me2Si group is recommend in the ketene silyl acetal substrates. Use of a less bulky MeaSi or Eta Si group leads to a mixture of monoalkylation products and rearranged a-silyl esters. 

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