✍ Scribed by Roland Grandel; Uli Kazmaier
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Deprotonation of N-protected alanine esters with LDA, and lates of N-sulfonylated alanine esters, which give excellent results with both aliphatic and aromatic aldehydes. Em-subsequent addition of various metal salts, most likely results in the formation of chelated metal enolates. Aldol reactions ploying the SES-protected derivatives which show the same good yield and diastereoselectivity as the corresponding Ts-of these enolates with aldehydes afford the anti isomers of αmethyl α-amino-β-hydroxy acid derivatives in a highly dia-protected esters, allows the preparation of the free α-methylserines. stereoselectiv fashion. Best results are obtained with tin eno-Product anti/syn [a] Yield [b] [%]
widely used synthetic method which should proceed with high diastereoselectivities because of the fixed geometry of 1 rac-1 1.2 equiv. TiCl(OiPr) 3 rac-3 72:28 76 the chelated amino acid ester enolates. 2 rac-1 2.5 equiv. TiCl(OiPr) 3 rac-3 92:8 87 Recently we presented our results of the aldol reactions 3 rac-2 1.2 equiv. TiCl(OiPr) 3 rac-4 65:35 86 4 rac-2 2.5 equiv. TiCl(OiPr) 3 rac-4 65:35 90 of titanium enolates of N-(benzyloxycarbonyl)-protected 5 rac-2 1.2 equiv. SnCl 2 rac-4 60:40 70 amino acid esters with aliphatic aldehydes. [9] With these 6 rac-2 2.5 equiv. SnCl 2 rac-4 98:2 80 substrates, high diastereoselectivities were obtained by adding 2.5 equiv. of TiCl(OiPr) 3 . Unfortunately aromatic alde-[a] Based on HPLC of the crude reaction mixture. Ϫ [b] Yield of isolated mixture of diastereomers.
hydes did not show any diastereoselectivity in these aldol
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Isolated yield of the major diastereoisomer.
Chemoselective reduction of the carbonyl group in ~-acyl-N-[Bis(methylthio)methylene]alaninates la--¢ and phenylalaninates I d,e allowed for the diastereoselective synthesis of both the syn and the anti isomers of the corresponding N-protected ot,a-disubstituted [3-hydroxy-ot-amino esters 2. The ste
Diastereoselective Reduction of α-Acyl-N-(bis(methylthio) methylene)alaninates and Phenylalaninates: Synthesis of α,α -Disubstituted β-Hydroxy α-Amino Esters.