The First Fully Planar
<i>C</i>
<sub>5</sub>
‐Conformation of Homooligopeptides Prepared from a Chiral
<i>α</i>
‐Ethylated
<i>α</i>
,
<i>α</i>
‐Disubstituted Amino Acid: (
<i>S</i>
)‐Butylethylglycine (=(2
<i>S</i>
)‐2‐Amino‐2‐ethylhexanoic Acid)
作者:Naoto Imawaka、Masakazu Tanaka、Hiroshi Suemune
DOI:10.1002/1522-2675(20001004)83:10<2823::aid-hlca2823>3.0.co;2-#
日期:2000.10.4
An optically active alpha -ethylated alpha,alpha -disubstituted amino acid, (S)-butylethylglycine (= (2S)-2-amino-2-ethylhexanoic acid; (S)-Beg; (S)-2),was prepared starring from butyl ethyl ketone (1) by the Strecker method and enzymatic kinetic resolution of the racemic amino acid. Homooligopeptides containing (S)-Beg (up to hexapeptide) were synthesized by conventional solution methods. An ethyl cater was used for the protection at the C-terminus, and a trifluoroacetyl group was used for the N-terminus of the peptides. Thr structures of tri- and tetrapeptides 5 and 6 in the solid state were solved by X-ray crystallographic analysis, and were shown to have a bent planar C-conformation (tripeptide) and a fully planar C-5-conformation (tetrapeptide) (see Figs. I and 2, resp.). The IR and H-1-NMR spectra of hexapeptide 8 revealed that the dominant conformation in CDCl3, solution was also a fully planar C-5-conformation, These results show for the first time that the preferred conformation of homopeptides containing a chiral alpha -ethylated alpha,alpha -disubstituted amino acid is a planar C-5-conformation.