Silanes 3-hydroxy

Scheme 9 demonstrates the further synthetic application of the thus obtained N,0-acetals. Substitution of the alkoxy or acyloxy group by nucleophiles like enol ethers, enol esters, enamines, other electron-rich olefins, CH-acidic compounds, electron-rich aromatics, isocyanides, trimethylsilyl cyanide, organometallics, vinyl and allyl silanes, hydroxy functions, or trialkylphosphites either catalyzed by Lewis acids or proton acids leads to the product of the amidoalkylation reaction (path a). In the presence of stereocenters as control elements, diasteroselective amidoalkylation reactions can be performed as shown in a large number of examples. On the other side, as Nyberg showed for the first time [196], elimination with formation of enecarbamates [208] and enamides [196,208,209] followed by reaction with electrophiles or nucleophiles (path b) also is possible. 

Hydroxy derivatives of silanes in which the hydroxyl groups are attached to a silicon atom are named by adding the suffices -ol, -diol, -triol etc., to the name of the parent compound. Examples are ... 

The synthesis of f-i-i-crotanecine is accomplished in 10 steps in a 10.2% overall yield, as shown in Scheme 8.42. The key step in the asymmetric synthesis is a Lewis acid-promoted, tandem inter [4-i-2 /intra [3-i-2 cycbaddidon between a ffumaroyloxyxiitroalkene and chiral fi-silylvinyl ether, in which the snbsdtuted silanes are used as hydroxy synthons. ... 

Of particular interest are reactions between a-hydroxy ketones and allyl(trifluoro)silanes which proceed with participation of the hydroxy group via a pentacoordinated allylsilane, with intramolecular delivery of the allyl group to the ketone. Excellent stereoselectivity is obtained at up to three contiguous stereogenic centers66. 

Oxo-5-phenyl-pentansaure und 6-Oxo-6-phenyl-hexansaure werden mit Triathyl-silan zu 5-Hydroxy-5-phenyl-pentansaure bzw. 6-Hydroxy-6-phenyl-hexans dure redu-ziert. 

In 1975, the fabrication of a chiral electrode by permanent attachment of amino acid residues to pendant groups on a graphite surface was reported At the same time, stimulated by the development of bonded phases on silica and aluminia surfaces the first example of derivatized metal surfaces for use as chemically modified electrodes was presented. A silanization technique was used for covalently binding redox species to hydroxy groups of SnOj or Pt surfaces. Before that time, some successful attemps to create electrode surfaces with deliberate chemical properties made use of specific adsorption techniques... 

In Entry 5, the carbanion-stabilizing ability of the sulfonyl group enables lithiation and is then reductively removed after alkylation. The reagent in Entry 6 is prepared by dilithiation of allyl hydrosulfide using n-bulyl lithium. After nucleophilic addition and S-alkylation, a masked aldehyde is present in the form of a vinyl thioether. Entry 7 uses the epoxidation of a vinyl silane to form a 7-hydroxy aldehyde masked as a cyclic acetal. Entries 8 and 9 use nucleophilic cuprate reagents to introduce alkyl groups containing aldehydes masked as acetals. 

The stereochemistry was controlled by Lewis acid-induced addition of these allylic silanes to aldehydes. The reaction of the silane with O-protected (S)-3-hydroxy-2-methylpropanal provides 15. The silane reacted with the benzyl-protected analog to provide 16. 

Substituted (5R,6A,)-6-(dimethyl(phenyl)silyl)-2-phenyldihydropyrazolo[l,2- ][l,2,4]triazole-l,3(2//,5//)-dione 716, synthesized via the [3+2] annulation of a-substituted allylic silanes 715 with PTAD, were oxidized to the corresponding hydroxy substituted urazoles 717. This work shows that allylsilanes with a single substituent at the allylic carbon undergo exclusive stereoselective [3+2] annulation (Scheme 114) <2007TL6671>. 

However, most asymmetric 1,3-dipolar cycloaddition reactions of nitrile oxides with alkenes are carried out without Lewis acids as catalysts using either chiral alkenes or chiral auxiliary compounds (with achiral alkenes). Diverse chiral alkenes are in use, such as camphor-derived chiral N-acryloylhydrazide (195), C2-symmetric l,3-diacryloyl-2,2-dimethyl-4,5-diphenylimidazolidine, chiral 3-acryloyl-2,2-dimethyl-4-phenyloxazolidine (196, 197), sugar-based ethenyl ethers (198), acrylic esters (199, 200), C-bonded vinyl-substituted sugar (201), chirally modified vinylboronic ester derived from D-( + )-mannitol (202), (l/ )-menthyl vinyl ether (203), chiral derivatives of vinylacetic acid (204), ( )-l-ethoxy-3-fluoroalkyl-3-hydroxy-4-(4-methylphenylsulfinyl)but-1 -enes (205), enantiopure Y-oxygenated-a,P-unsaturated phenyl sulfones (206), chiral (a-oxyallyl)silanes (207), and (S )-but-3-ene-1,2-diol derivatives (208). As a chiral auxiliary, diisopropyl (i ,i )-tartrate (209, 210) has been very popular. 

Intermolecular cross aldolization of metallo-aldehyde enolates typically suffers from polyaldolization, product dehydration and competitive Tishchenko-type processes [32]. While such cross-aldolizations have been achieved through amine catalysis and the use of aldehyde-derived enol silanes [33], the use of aldehyde enolates in this capacity is otherwise undeveloped. Under hydrogenation conditions, acrolein and crotonaldehyde serve as metallo-aldehyde enolate precursors, participating in selective cross-aldolization with a-ketoaldehydes [24c]. The resulting/ -hydroxy-y-ketoaldehydes are highly unstable, but may be trapped in situ through the addition of methanolic hydrazine to afford 3,5-disubstituted pyridazines (Table 22.4). 

Silyltitanation of 1,3-dienes with Cp2Ti(SiMe2Ph) selectively affords 4-silylated r 3-allyl-titanocenes, which can further react with carbonyl compounds, C02, or a proton source [26]. Hydrotitanation of acyclic and cyclic 1,3-dienes functionalized at C-2 with a silyloxy group has been achieved [27]. The complexes formed undergo highly stereoselective addition with aldehydes to produce, after basic work-up, anti diastereomeric (3-hydroxy enol silanes. These compounds have proved to be versatile building blocks for stereocontrolled polypropionate synthesis. Thus, the combination of allyltitanation and Mukayiama aldol or tandem aldol-Tishchenko reactions provides a short access to five- or six-carbon polypropionate stereosequences (Scheme 13.15) [28],... 

Cleavage of Si—C bonds (12,243-245). This oxidation can be used to convert vinylsilanes in three steps to syn- or anti-1,2-diols. Thus Grignard reagents cleave epoxides of vinylsilanes selectively to (3-hydroxy silanes, which can be oxidized with retention of configuration to 1,2-diols. When applied to an (E)-vinylsilane, the sequence results in the syn-l,2-diol the an -l,2-diol is obtained from a (Z)-vinylsilane by the same reactions. 

Silanes can react with acceptor-substituted carbene complexes to yield products resulting from Si-H bond insertion [695,1168-1171]. This reaction has not, however, been extensively used in organic synthesis. Transition metal-catalyzed decomposition of the 2-diazo-2-phenylacetic ester of pantolactone (3-hydroxy-4,4-dimethyltetrahydro-2-furanone) in the presence of dimethyl(phenyl)silane leads to the a-silylester with 80% de (67% yield [991]). Similarly, vinyldiazoacetic esters of pantolactone react with silanes in the presence of rhodium(II) acetate to yield a-silylesters with up to 70% de [956]. 

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