Synthesis of Enantiomerically Pure Unsaturated .alpha.-Amino Acids Using Serine-Derived Zinc/Copper Reagents
作者:Michael J. Dunn、Richard F. W. Jackson、Joerg Pietruszka、Debra Turner
DOI:10.1021/jo00112a048
日期:1995.4
Treatment of the serine-derived organozinc reagent 2, in benzene/dimethylacetamide, with a THF solution of CuCN . 2LiCl gives rise to a zinc/copper reagent 6 which reacts directly with allylic halides and tosylates to give the corresponding enantiomerically pure substitution products 9 in 32-65% yield (11 examples). The reaction proceeds by formal S(N)2' displacement of the leaving group. Reaction with propargyl halides gives the corresponding terminal allene 12a. The zinc reagent 2 may also be prepared directly from protected iodoalanine 1 in THP by the Knochel method. Reaction with propargylic tosylates as electrophiles gives rise to the corresponding protected terminal allenic amino acids in 51-81% yield (four examples); use of enantiomerically enriched propargylic tosylates results in the formation of diastereoisomerically enriched allenic products. Reactions of the zinc/copper reagent 6 with a range of representative electrophiles is explored; use of relatively reactive electrophiles is necessary to obtain satisfactory yields. Finally, the possibility of using the serine-derived iodide 20, in which the carboxylic acid is protected as a methyl ester, is established: This reagent offers advantages over the corresponding benzyl ester protected reagent 6 for the synthesis of unsaturated amino acids.