Ketene aminals stereochemistry

The silyl ketene acetal rearrangement can also be carried out by reaction of the ester with a silyl triflate and tertiary amine, without formation of the ester enolate. Optimum results are obtained with bulky silyl triflates and amines, e.g., f-butyldimethylsilyl triflate and (V-methyl-Af, /V-dicyclohcxylaminc. Under these conditions the reaction is stereoselective for the Z-silyl ketene acetal and the stereochemistry of the allylic double bond determines the syn or anti configuration of the product.243... 

Treatment of acid chlorides with tertiary amines produces ketenes. In this case an intramolecular 2 + 2] cycloaddition is possible. The stereochemistry is trivial the cis ring junction is the only... 

In the rearrangement of allyl fluoroacetates, trialkylsilyl triflates have been introduced as a new reagent for the Z-selective generation of silyl ketene acetals485. Thus, when (T)-crotyl fluoroacetates are treated at ambient temperatures with a trialkylsilyl triflate in the presence of a tertiary amine, rearranged products with a svn relationship are preferentially obtained. The ketene acetal intermediates cannot be isolated and the geometry has been deduced from the stereochemistry of the products. The selectivity of this process improves in the order triisopropyl > ferf-butyldimethylsilyl > rerf-hexyldimethylsilyl > trimethylsilyl a triethylsilyl (see Table 11). 

L-Daunosamine (166) (R=H), (3-amino-2,3,6-trideoxy-L-/yxo-hexose), is an essential component of both natural and unnatural anthracycline antitumor agents. The 1,3-addition of ketene silyl acetal 164 to the chiral nitrone (Z)-[(4i )- ra 5-2,2,5-trimethyl-l,3-dioxolan-4-yl]methylene[(l S)-l-phenylethyl] amine 7V-oxide (163), prepared by the conversion of 141 to 103 [62] and then reaction of 103 with the hydroxylamine 162, provides the 0-silylated addition product 165 in quantitative yield with an anti relative stereochemistry at C-3 and C-4 (anti syn =>100 1). Efficient transformation of 165 to 166 (R=COPh) is achieved in three steps [63] (Scheme 39). 

Shibasaki [89a] has reported an asymmetric synthesis of (-h) PS-5 by using the boron enolate-imine condensation reaction. The most notable features of this approach were that the correct absolute stereochemistry at C3-C4 of the p-lactam ring was produced in a highly diastereoselective fashion and imines derived from aliphatic amines could be used in this reaction in a similar way to the silyl ketene acetal approach (Scheme 32). 

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