Cascade reactions silyl enolates

The influence of catalytic C,C-coupling of six-membered cyclic nitronates can be used, under more drastic conditions, for the constmction of nitrogen-containing heterocycles via cascade chemical transformations (see, e.g., the coupling reaction of nitronate (368) with silyl enolates (369a-d) in the presence of trimethylsilyl triflate (154) (Scheme 3.211)). 

Diastereo- and enantio-selective cascade of Michael addition and lactonization between various silyl enolates derived from phenyl carboxylates and -unsaturated ketones were successfully carried out by using an efficient organic catalyst, a cinchoni- dine-derived chiral quaternary ammonium phenoxide. In this asymmetric domino reaction, the corresponding tnms-3,4-dihydropyran-2-oncs were obtained in high yields with almost complete diastereoselectivities and good to excellent enantioselectivities.161... 

Silyl allenes such as 48 and enol acetates such as 50 were particularly effective substrates for these reactions. The oxidative cascade reaction of trisubstituted alkene substrate 52 demonstrated a powerful application of ETIC chemistry to form bicycle 53 through consecutive carbon-carbon and carbon-oxygen bond construction. Highly diastereoselective reactions were observed when the nucleophile was... 

Furthermore, the asymmetric PS reaction has been applied as a key step in the cascade reaction to build up chiral polycyclic compounds. In 2009, Dixon and coworkers [71] developed a chiral phosphoric acid 8f-catalyzed cyclization cascade between tryptamines and enol lactones, leading to the products in good yields and with good to excellent enantioselectivities. This was further extended to the reaction of tryptamines and ketoacids, which were more readily available than enol lactones [72]. By using diarylprolinol silyl ether as catalyst, the other cascade transformations involving Michael addition, iminium formation and PS cyclization were also efficiently realized [73]. 

Exposure of 270 to the TBSOTf/DIPEA reagent couple once more triggered a cascade of reactions, namely chemoselective thiolactone enolization, intramolecular aldolization, and hydroxyl silylation, which ultimately gave rise to 3,4-trans and 3,4-cis bicyclic thiolactones 271 and 272. Final unmasking of the pseudoanomeric thiol function and terminal hydroxymethyl moiety was performed on the individual precursors 271 and 272 to deliver, as expected, the corresponding thiocarbasugars 273 and 274 in high isolated yields. 

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