Silyl keteneacetals

Scheme 6.88 Silylenol ethers and silyl keteneacetals that were used to trap the cyclic allene 417. The [2+ 2]-cycloadducts such as 435 were converted into products of the type 436.

Finally, free a-chloro acids have been prepared in high enantiomeric excess by chlorination of silyl keteneacetals using D-(+)-glucose diacetonide as chiral auxiliary (refs. 13,14). The most outstanding features of this very efficient chiral auxiliary are the cheapness, and the easy hydrolysis of the final a-halo esters, that allows the removal of the chiral auxiliary under nonracemising conditions moreover, this compounds is non toxic and is commercially available. 

Mukaiyama aldol reactions. Silyl keteneacetals undergo condensation with aldehydes under the influence of tris(2,4,6-trimethoxyphenyl)phosphine. Triphenylphosphine is slightly less efficient and aliphatic phosphines such as t-BujP are much infenior. 

According to the second method of carbonate block copolymer synthesis, sequential monomer polymerization is proceeded with transformation of the active center. The block copolymers are prepared in three steps. First, the polymerization of one monomer is carried out. After complete conversion of the first monomer the transformation of active centers is performed, and the initiation of the polymerization of the second monomer is proceeded. For example, this approach was applied for obtaining poly(styrene-l7-neopentyl carbonate).After completion of the styrene living polymerization, carbanionic centers were transformed into alkoxide ones via reaction with EO and then the ROP of neopentyl carbonate polymerization was performed. In the case of block copolymers of methyl methacrylate with neopentyl carbonate living PMMA, prepared according to GTP, was used as a macroinitiator for DTC polymerization. A silyl keteneacetal active center was transformed to an alkoxide one. Depending on the functionality of the macroinitiator (A) used for cyclic carbonate polymerization, two types of block copolymers can be obtained A-B or B-A-B. 

A keteneacetal, the Brassard diene, has also been bound to polystyrene-based resin (308). This has been achieved by the reaction of diketene with alcohol functions on polystyrene resin. The ketone function of the resulting acet-oacetate could readily be enolized by TMOF (trimethyl orthoformate) and a catalytic amount of H, giving the methyl enolate. The remaining ester function was then deprotonated with LDA and subsequently silylated with TBDMSCl. The resulting diene (311) was used in many hetero-Diels-Alder reactions (HDA) with aldehydes and ketones as dienophiles, giving a library... 

Reactions of O-silyl O-alkyl keteneacetals Silylacetic acids by 0->C-silyl migration... 

Subst. arylcarboxylic acids from O-silyl O-alkyl keteneacetals and benzynes... 

Startg. 0 Silyl O-alkyl keteneacetal mixed with chlorobenzene and excess NaNH2, stirred at room temp, under N2 for 24 h, then quenched with water o-5cc-butylben-zoic acid. Y 51%. Bromobenzene was also effective as benzyne source. F.e.s. S.M. Ali, S. Tanimoto, J. Chem. Soc. Chem. Commun. 1988, 1465-6 details s. J. Chem. Soc. Perkin Trans. I 1989, 915-8. 

O-silyl O-alkyl keteneacetals 43, 869 Aldehydo-a,P-ethyleneoxo compds. 

O-silyl O-alkyl keteneacetals 44, 877 -, hydrolysis, cobalt(III)-promoted (review) 37, 325s43... 

TiCl4 added dropwise to a stirred soln. of startg. O-silyl O-alkyl keteneacetal and isoamyl nitrite in methylene chloride at 10°, and stirring continued at room temp, for 24 h ethyl a-oximinopropionate. Y 65%. O-Silyl O-alkyl keteneacetals not possessing an a-hydrogen gave a-nitrosocarboxylic acid esters. F.e. and with nitric oxide s. S.M. Ali et al., Synthesis 1988, 805-6. 

Lithium N-isopropylcyclohexylamide O-Silyl O-alkyl keteneacetals from carboxylic acid esters... 

Ethylene-/5-siloxycarboxylic acid esters from carboxylic acid chlorides and 0-silyl 0-alkyl keteneacetals... 

Hydroxycarboxylic acids and esters from aldehydes and ketene silyl hemiacetals or O-silyl O-alkyl keteneacetals respectively... 

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