Silyl derivatives compounds

Replacement of heterocyclic rings in nucleosides by ring systems which do not occur in nature represents another approach to compounds which may have activity against viral and neoplastic diseases. One of the early successes in this category involves replacement of a pyrimidine ring by a triazine. The synthesis starts with a now classical glycosidation of a heterocycle as its silylated derivative (146) with a protected halosugar (145), in this case a derivative of arabinose... 

Silicon-transition metal chemistry is a relatively new area. The work of Hein and his associates (1941) on Sn—Co derivatives established the possibility of forming bonds between a Group IVB metal and a transition element 139), but it was another fifteen years before CpFe(CO)2SiMej 203), the first of many silyl derivatives, was synthesized. The interest in these compounds derives from (1) comparison with the corresponding alkyl- and Ge-, Sn-, and Pb- transition metal (M) complexes, including the role of ir-back-bonding from filled d orbitals of M into empty d orbitals on Si (or other Group IVB metal), and (2) expectation of useful catalytic properties from such heteronuclear derivatives. 

Reaction of lithiated allylbenzotriazole 452 with chloromethyltrimethylsilane yields silyl derivative 464 which can be further alkylated to give compound 465 (Scheme 76) <1999JOC1888>. Upon heating, product 465 is readily converted to diene 466 via vicinal elimination of benzotriazolyl and silyl substituents. Additions of lithiated silyl derivative 464 to carbonyl groups of aldehydes lead to alcohols 463 which readily eliminate benzotriazole and silane to furnish 2-(l-hydroxyalkyl)butadienes 466 (R1 = 1-hydroxyalkyl). 

The Peterson olefination reaction involves the addition of an a-silyl substituted anion to an aldehyde or a ketone followed by the elimination of silylcarbinol either under acidic (awP -elimination) or basic (syn-elimination) conditions to furnish olefins178. Thus, Peterson olefination, just like Wittig and related reactions, is a method for regioselective conversion of a carbonyl compound to an olefin. Dienes and polyenes can be generated when the Peterson reaction is conducted using either an ,/l-unsaturated carbonyl compound or unsaturated silyl derivatives as reaction partners (Table 20)179. 

Intramolecular Trapping of Bis-N,N-Oxyiminium Cations Special y-functionalized nitro compounds (372) were constmcted with the aim of performing intramolecular trapping of bis-7V,7V-siloxyiminium cations prepared in process of double silylation of (372). Monosilylation of the latter compounds can afford different silyl derivatives (373a-c) (Scheme 3.212) (486, 487). 

If the alkoxy group at the C-6 atom in nitroso acetals (475) is absent, silyl derivatives (476) are generated instead of nitroso ketones A, and methanolysis of compounds (476) gives rise to y-hydroxy oximes (479). 

The resulting radicals R efficiently alkylate BENAs (495) at the (3-C atom to give silyl derivatives of oximes (496) in good yields. The latter readily undergo deoximation in the presence of 1 M hydrochloric acid to give the corresponding carbonyl compounds (494). Thus, a convenient procedure was developed for the synthesis of carbonyl compounds (494) from secondary AN (493) through the intermediate terminal BENA (495) (527). 

Main Rearrangements of BENA In previous Sections (3.5.4.1. and 3.5.4.2.), a-hydroxy oximes (503) and their bis-silyl derivatives (504) were considered as undesired by-products, formed in the synthesis and chemical transformations of BENA. The aim was to minimize the amount of these impurities. On the other hand, oximes (503) are convenient precursors of various useful products, such as p-amino alcohols (530), amino acids (531), a-hydroxycarbonyl compounds (532) and various heterocyclic systems (533). 

Here, six-membered cyclic nitronate (575) is initially assembled from the simple reagents according to known procedures, and this nitronate is silylated to give the intermediate A, whose [4+ 2]-cycloreversion affords silyl derivative of enoxime (574). Finally, desilylation of the latter compound gives rise to enoxime... 

Previous derivatization of the extract is necessary to improve the stability of the compounds and the sensitivity and precision of subsequent GC-MS analysis. Silyl derivatives formed for example with MSTFA [43], halogenated alkene derivatives produced with heptafluorobutyric anhydride (HFBA) [36] or pentafluoropropionic acid [58] or anhydride (PFPA), as well as acetate derivatives formed using acetic anhydride [48] have been widely employed. 

These compounds could not be isolated in pure form. They appear during the final work-up of the reaction solution as viscous liquids which are colorless to light yellow and which mix well under all relative concentrations with the usual solvents and show no tendency whatsoever to crystallize. From the integration of the P( H -NMR spectra of these mixtures, a ratio of 10 3 was established for the isomeric compounds 108 and 109. This agrees with the ratio in the phosphides initially present (44). Although all the n-tetraphosphides so far described can be transformed into the corresponding silyl derivatives by reaction with MejSiCl, in the silylation of the phosphide 104 the following peculiarity is encountered. 

The a,p-unsaturated amides 180-188a have all been used in 1,3-dipolar cycloadditions with nitrile oxides, and some of them represent the most diastereoselective reactions of nitrile oxides. The camphor derivative 180 of Chen and co-workers (294), the sultam 181 of Oppolzer et al. (295), and the two Kemp s acid derived compounds 186 (296) and 187 (297) described by Curran et al. (296) are excellent partners for diastereoselective reactions with nitrile oxides, as very high diastereos-electivities have been observed for all of them. In particular, compound 186 gave, with few exceptions, complete diastereoselection in reactions with a wide range of different nitrile oxides. Good selectivities were also observed when using compounds 183 (298) and 184 (299-301) in nitrile oxide cycloadditions, and they have the advantage that they are more readily available. Curran and co-workers also studied the 1,3-dipolar cycloaddition of 187 with silyl nitronates. However, compared to the reactions of nitrile oxides, lower selectivities of up to 86% de were obtained (302). 

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