Silyl ketene acetals/zinc iodide

Additive Pummerer reactions of the type described above using racemic a,P-unsaturated sulfoxides can be accomplished using the following electrophiles acyl chlorides [205,206], dithioacetic acid [207], acetic anhydride [208], mineral acids/alcohols [209], phosphorus pentachloride [210], silyl ketene acetals/zinc iodide [211], thionyl chloride [212], oxalyl chloride [213], trifluoroacetic acid and its anhydride [214-218], triflic anhydride/sodium acetate [219], and dichloroketene (see below). Selected recent examples of work in this area are presented here. 

Diastereoselection is also observed in the catalyzed [titanium tetrachloride (TiCI4)13, trimethyl-silyltrifluoromethanesulfonate (TMSTf)l4, zinc iodide (Znl2)15] reactions of silyl ketene acetal 1 with imines 2, The ami configuration of the product 3 dominates. 

Further versatility of this approach has been realized with contrasting Lewis acid promoted additions of silyl ketene acetals, (191) to (194), to ethyl propynoate (Scheme 42). In fact, the tandem 1,4-conjugate addition-electrophile trapping protocol is feasible when titanium(IV) tetrachloride is employed. In situ functionalization of the intermediate titanate enoate (259), with select electrophiles, affords a-substituted enoates (260) to (262). On the other hand, the zinc iodide and zirconium(IV) tetrachloride protocols afford directly -y-alkoxycarbonyl-a-trimethylsilylenoates (263) and [2 + 2] adducts (264), respectively.100... 

The matched double stereoselective reaction between the chiral C-phenyl A -( 1-phenylethyl) nitrone and the chiral silyl ketene acetals 558 in acetonitrile/dichloromethane at 20 °C under zinc iodide-catalyzed conditions resulted in only the single diastereomer 559 in 98% yield. Exposure of 559 to acid afforded the isoxazolidinone 560 in 74% yield, together with the chiral auxiliary (li ,2 S )-2-phenylcyclohexanol (Scheme 135) <1997JOC6672>. 

The si facial preference displayed by the reagent is enhanced in reactions proceeding through Lewis acid-catalyzed open transition states.Thus, when reacted with the ketene silyl acetal shown (eq 2) under zinc iodide catalysis, a 96 4 ratio of products was obtained. The corresponding uncatalyzed reaction led to an 85 15 mixture of the same products in similar yield. ... 

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