A-Silyl imines

Acetonitrile ylide was generated from a-silyl imine 63 and silver fluoride. This same ylide could be generated from the reaction of methylene with acetonitrile. The chemo-selective silicon- fluoride ion route seems best, however, offering the advantages of readily available, easily handled reagents. The so-generated ylide was cycloadded to several... 

A new protocol for the stereoselective synthesis of 3-lactams [213] has been reported to be performed by a conrotatory ring closure of l-halo-3-aza-4-alkyl-l, 3-dienes, previously prepared by Staudinger methodology, (for the synthesis and chemistry of A-silyl imines see [214] for [2-1-2] cycloaddition of A-silyl imines and ketenes see [215]) in refluxing toluene (Scheme 96). 

Kulkarni have now reported a potentially general method for the preparation of a-t-butyldimethylsilyl aldehydes by hydrolysis of the a-silyl imines (Scheme 34). These silyl aldehydes serve as stereoselective vinyl cation... 

A-Silyl Imines. When aldehydes were treated first with tris(tri-methylstannyl)amine followed by TBDMSCl, the corresponding V-TBDMS imines were isolated in good yields." These silyl imines reacted with ester enolates to give -lactams (eq 24). 

In a similar type of process, imines have been dimerized to give 1,2-diamines, by a number of procedures, including treatment with TiCLj—In/aqueous EtOH, Zn/aqueous NaOH, Cp2VCl2/Zn/PhMe2SiCl, with Sml2, and I for silylated imines) NbCl4(thf)2. When electroreduction was used, it was even... 

Scheme 27 Addition of nitronates, enolates and silyl ketene acetals to chiral a-amino imines and iminium ions...

Furthermore, a highly efficient route to A-tert-butoxycarbonyl (Boc)-protected p-amino acids via the enantioselective addition of silyl ketene acetals to Al-Boc-aldimines catalyzed by thiourea catalyst 4 has been reported (Scheme 12.2)." From a steric and electronic standpoint, the A-Boc imine substrates used in this reaction are fundamentally different from the A-alkyl derivatives used in the Strecker reaction. 

Glucosaminylurea derivahve 198 was also applied to catalyze the enantioselec-hve Mannich addition [72] of a silyl ketene acetal to the N-Boc-protected imine naphthalene-2-carbaldehyde resulting in the desired P-amino acid ester (Scheme... 

Reactions with imines also occur under similar conditions leading to the formation of difluorinated /1-lactams [196,197]. Formally, this reaction corresponds to a [2+ 2]-cycloaddition across an imine there is also an isolated report of a difluoroketene reaction, shown in Eq. (63) [198]. Useful chemistry of a silyl difluoroketene acetal was reported recently Eq. (64) shows the efficient asymmetric reaction between this reactive carbon nucleophile and aldehydes catalysed by chiral Lewis acids [199]. 

Boron aldol reactions have been used to stereoselectively construct the anti-3-hydroxy-2-methylcarbonyl system from carboxylate esters,58 and to combine a-hetero-substituted thioacetates with aldehydes or silyl imines enantio- and/or diastereo-selectively.59... 

These silyl imines are generated in situ from the corresponding aldehydes resulting in a reduction of the number of reaction steps and simplifies the work-up procedures (figure 8). Another option is to convert the aldehyde into a N-tosyl or a N,N-dimethylsulfamoyl irnine23. 

German, C. Vulpetti, A. Pain, G. Highly enantio- and diastereoselective boron aldol reactions of a-heterosubstituted fhioacetates with aldehydes and silyl imines. Tetrahedron 1997, 53, 5909-5924. 

Regioselectivity control in the double nucleophilic addition of ketene silyl acetals to a-unsaturated imines has been reported.85... 

Silyl ketene imines have been acylated asymmetrically by anhydrides evidence for a silyl-free nitrile anion intermediate is discussed.86 An aza-Baylis-Hillman reaction of N-sul fonated imines is described below. 

If HMPA is used in stoichiometric amount in THF as the solvent, then substitution of the chlorine atom occurs providing the corresponding C-silylated imine, the precursor of the acylsilane pivaloylsilane. This imine can also be regarded as a good precursor of an SMA through its reduction. Reduction of the iminoylchloride into the non-silylated imine is a side reaction. 

In the catalytic presence of tetrakis(triphenylphosphine)palladium imines condenses with disilanes in toluene after 5 h at 120 °C in a sealed tube to give high yields of the corresponding a-silyl-iV-silylamine.216... 

The original assignment of the absolute configuration (3S) for products 12 based on a comparison of the optical rotation value with a reported one for the phenyl derivative [27] must be considered uncertain. It would contradict the preferred attack of the nucleophile from the unshielded back side and the stereoselectivity observed in the reaction of A -galactosyl imines 7 with prochiral bis-silyl ketene acetals [28]. N-Galactosyl P-amino acids 13 are produced from bis-silylketene acetals in high yields and excellent diastereoselectivity (Scheme 10). The configuration at the P-position is R. 

Insertion of isonitriles and carbenoids into the B-B and B-Si bonds have recently been developed as a method for synthesizing geminal bis(boryl) and boryl-silyl compounds (Scheme 24). The insertion of alkyl and aryl isonitriles into the B-Si bond proceeded at room temperature to provide monomeric (boryl)(silyl)imines 227.388,389 Alkylidene... 

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