Silicon compounds protocols

The temporary silicon connection method introduced by Stork is a useful protocol for the preparation of silaoxacycles. The C-Si bond in the silaoxacycles is readily convened to C-OH with retention of stereochemistry by oxidative procedures. Utimoto and co-workers have developed a novel method for the synthesis of triols using the temporary silicon connection method [95BCSJ625], Treatment of the silacycle 184 with tin hydride furnished a mixture of silacycles, which were converted to the triacetate 187 without isolation. The endo isomer 185 was formed as the major compound. 

N-(l-Alkoxyalkyl)-aniides or -carbamates (2 X = OR), most frequently used as stable precursors for A -acyliminium ions, are usually prepared by one of the following routes (equations 7-13). For five- or six-membered cyclic cases a simple acid-catalyzed solvolysis of the hydroxy compound provides the alk-oxy derivative (equation 7). A silicon-assisted approach involves the TMSOTf-catalyzed reaction of bis(trimethylsilyl)formamide with aldehydes (equation 8). /V-(l-Trimethylsilyloxyalkyl)formamides are thus formed in good yields, which on TMSOTf-catalyzed solvolysis lead to the /V-( 1-alkoxyalkyl)form-amides. A third method is based on the NaBH4 reduction of imidates (equation 9), and has proved useful for a total synthesis of the insect poison pederine. Addition of reactive acid derivatives to imines constitutes another method (equations 10 and 11). Acylation with acid chlorides followed by treatment with ethanol in the presence of base leads to N-(l-alkoxyalkyl)amides. A one-step protocol using diethyl dicarbonate provides the corresponding carbamates. 2... 

The previous examples demonstrate that intermolecular mannosylations proceed with high p-selectivity only with donors that are structurally disposed to direct Sn2-type substitution at the anomeric carbon and disfavor the formation of oxocarbonium ion intermediates. Because of the large number of variables, the steric outcome of intermolecular mannosylations is predictable only in closely related systems. Stork has proposed a conceptually new, intramolecular mannosylation protocol that has initially been implemented in the first stereocontrolled synthesis of a p-C-mannoside [108]. The starting compound is the phenylseleno mannoside 34 in which a 2-phenylethynyl group is anchored to 0-2 through a silicon tether. Radical-initiated removal of the anomeric phenylseleno group (— 35) in concert with cycliza-tion of the phenylethynyl group on to the anomeric radical provides 36 from which fluoride-mediated detachment of the silicon connector affords p-C-mannoside 37 (Scheme 12). 

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