Val-Ala Dipeptide Isosteres by Hydrocyanation of α′-Amino α,β-Unsaturated Ketones − Control of Stereoselectivity by the N-Protecting Group
作者:Fabio Benedetti、Federico Berti、Gianpiero Garau、Ivan Martinuzzi、Stefano Norbedo
DOI:10.1002/ejoc.200200626
日期:2003.5
diastereoisomeric hydroxyethylene isosters of the Val-Ala dipeptide were synthesized from α,β-unsaturated ketones 1 derived from N-Boc- and N,N-dibenzyl-L-valine. The enones were hydrocyanated with diethylaluminum cyanide to give the corresponding β-cyano ketones with the stereoselectivity depending on the protecting group. N-Boc protected enone 1a gave a 1:1 mixture of anti and syn adducts 4a, 5a while the corresponding
Val-Ala 二肽的三种非对映异构羟基亚乙基异构体是由衍生自 N-Boc- 和 N,N-二苄基-L-缬氨酸的 α,β-不饱和酮 1 合成的。烯酮用二乙基氰化铝氢氰化得到相应的β-氰基酮,其立体选择性取决于保护基团。N-Boc保护的烯酮1a产生反和顺加合物4a、5a的1:1混合物,而相应的N,N-二苄基化合物1c产生反、顺加合物4c、5c的6:1混合物。所得氰基酮的硼氢化物还原也受保护基团的控制,导致 N-Boc 和 N,N-二苄基化合物的立体选择性相反。如此获得的氰基醇在几个步骤中被转化成两个系列的Val-Ala二肽的对映体纯羟基亚乙基异构体。在第一个系列中,羟基和异构体的 N 端通过形成恶唑烷在内部受到保护;在第二个系列中,羟基和 C 端被保护为内酯。两个恶唑烷(28, 29),对应于syn,syn 和syn,anti 4-hydroxy-5-amino acid isosters,三个内酯(23-25),对应于syn