De-O-silylation

In contrast, the synthetic pathway using trimethylsilyl triflate and trimethylsilyl, azide followed by hydrogenation, de-O-silylation, condensation with thiocarbonyl diimidazole and spontaneous cyclization of an intermediate isothiocyanate, afforded a (3-anomer in good yield (Scheme 32). 

Je nach Struktur des Substrates kann bei diesem Verfahren auch zweifache Aminome-thylierung (bzw. auch deren Folgereaktionen) eintreten, wie die Bildung von 2-Benzoyl-1,3-bis-[dimethylamino -propan als Hauptprodukt bei der Umsetzung des O-Silyl-enols von Acetophenon unter den gleichen Bcdingungen zeigt. 

Ether cleavage. Alumina containing 3% H2O mediates de-O-silylation with or without solvent. With microwave irradiation the reaction time shortens dramatically. 

Die Kondensation von O-Silyl-enolen mit Nitrosobenzol in Benzol und Zugabe des so erhaltenen Reaktionsgemisches zu Lithium-alanat in Ether ergibt Amino-alkohole1. 

Takano et al. used the Ireland-Claisen rearrangement of an allyl lactate in the total synthesis of calcitriol lactone (Scheme 4.76) [74]. The rearrangement proceeded with a 6.7 1 diastereoselectivity via O-silylation of the intermediate li-che-lated Z-enolate. Takano noted that the corresponding benzyl ether gave significantly lower de (70%) than the PMB ether. This is presumably due to the PMB ether s greater Lewis basicity and hence its greater propensity to coordinate a lithium cation. 

As a first extension of the new overall enantioselective a-alkylation of 2-ke-toesters we developed an efficient asymmetric synthesis of 3-substituted cyclic hemiketals of (o-hydroxy-2-oxoesters [42]. As is depicted in scheme 15, the metalated SAMP-hydrazones were trapped with a number of O-benzyl- and O-silyl-protected co-hydroxy-l-iodoalkanes, which after hydrazone cleavage and deprotection afforded the cyclic hemiketals (de, ee > 98%), structurally modified deoxygenated analogs of ulosonic acids. To our knowledge only two methods have been reported for the synthesis of similar simple tetrahydropyran derivatives [43,44]. 

The same strategy can be applied to the bridged cyclodextrin 82, giving the 6A,6D-bridge,6C-hydroxyl a-cydodextrin 94 in 52% yield (Scheme 9.43) [78]. The bridge can be removed using a palladium-catalyzed double de-O-aUylation to give the 6A,6C,6D-trihydroxy a-cydodextrin 92, or cyclodextrin 93 if the hydroxyl has been protected with a silyl ether before. 

In 2001, De Luca and GiacomeUi " reported a new simple and high-yielding one-flask synthesis of Weinreb amides from carboxylic acids and A-protected amino acids that uses different 1,3,5-triazine derivatives (such as 236) as the coupling agents (Scheme 104). The method allows the preparation of Weinreb amides 237 and hydroxamates as O-benzyl and 0-silyl hydroxamates that can be easily transformed into hydroxamic acids. 

Conversion of Enolates to Silyl Enol Ethers 3/O-Trimethylsilyl-de-hydrogenation... 

Dimerization of Silyl Enol Ethers or of Lithium Enolates 3/O-De-trimethylsilyM / C-coupling... 

Des-oxo-bis(dithiolene)M(IV) (dithiolene = L = bdt or edt M = Mo or W) complexes, M(OSiPh2f-Bu)(L)21 have been prepared by silylation of the corresponding [MO(L)2]2 complex (see Fig. 6) and characterized by X-ray crystallography. Each anion possesses a square-pyramidal stereochemistry in which the metal atom is coordinated by an axial silyloxide and two dithiolene ligands (79). The Mo—O bond is 0.14 A longer and the average Mo—S bond... 

Amino-l-hydroxyethyl)phosphonic acid occurs in the plasma membrane of Acanthamoeba castellani and the 2R isomer is formed, in that organism, by the hydroxy-lation of (2-aminoethvl)phosphonic acid This biosynthesis step in vitro has been studied by Hammerschmidt" who synthesized various chiral deuterium-labelled derivatives of both compounds using the isotopically labelled 2-benzyloxyethanal in Abramov reactions to obtain, initially, the dimethyl (2-benzyloxy-l-hydroxyethyl)phosphonate (362). This ester was resolved through the diastereoisomeric carbamates 363 the separated carbamates were sequentially de-l-O-protected, silylated at the a-HO group, debenzylat-ed and, by means of the Mitsunobu reaction, converted into dimethyl [2-eizido- -(tert-butyldimethylsilyloxy)ethyl]phosphonates. Subsequently, standard reactions were used to transform the latter into the diastereoisomeric, isotopically labelled (2-amino-1-hydroxy-ethyl)phosphonic acid. 

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