for a dipeptide (Ala-Ala) and the analogous didepsi-peptide
(Figure 1A). Previous A-to-E studies utilized one or a few
commercially available R-hyrdroxy acids to replace all the
amino acids, often necessitating side chain structural alter-
ations that complicate data interpretation, especially when
these changes alter the hydrophobic core and hence thermo-
dynamic stability. It is highly desirable to utilize R-hydroxy
acids with the same side-chain as the R-amino acid residue
being replaced to eliminate side chain effects in H-bonding
studies.
9
(Ala-Lac) are virtually identical. Furthermore, amides and
esters both strongly prefer a trans conformation, are planar,
and have similar bond angles and lengths.9
-12
As a result,
depsi-peptides are isostructural to proteins with an all amide
backbone.
5
Depsi-peptides can be synthesized recombinantly or
chemically.3 The recombinant approach requires charging
tRNA with the desired R-hydroxy acid, while the chemical
synthesis approach necessitates the availability of appropriate
,4
A number of approaches have been reported for the
5
13-23
protected R-hydroxy acids. Chemical synthesis approaches
synthesis of chiral R-hydroxy acids.
Asymmetry can be
to depsi-peptides have been amply described in the litera-
achieved either by using chiral starting materials or chiral
catalysts; however, none of the approaches affords all 19 of
the R-hydroxy acid R-amino acid equivalents with complete
3
,4
ture. Typically, solid-phase peptide synthesis utilizing
appropriately protected N-Boc-R-amino acids and R-hydroxy
acids is employed. The side-chain protecting groups (when
necessary) on both the R-hydroxy acids (Figure 1) and the
1
4,21-23
stereocontrol.
Of the previously reported approaches,
diazotization of chiral R-amino acids with sodium nitrite in
acid appears to be the most efficient route for the stereo-
specific synthesis of R-hydroxy acids containing hydrocarbon
1
5
side-chains. Herein, we attempt to prepare all 19 of the
R-hydroxy acids using diazotization chemistry and report the
scope and limitations of this approach. Alternative stereo-
controlled approaches are reported to deliver the protected
R-hydroxy acids that cannot be synthesized efficiently by a
diazotization strategy.
Numerous publications report variable conditions for the
diazotization of L-R-amino acids using sodium nitrite in
1
5-18,20,24-26
acid.
Variables include the concentration and
identity of the aqueous acid and the sodium nitrite stoichi-
ometry relative to the starting material. All but one of the
optimizations herein led to the use of 2 equiv of sodium
nitrite in 20% aqueous acetic acid (Method A). Diazotization
of Arg(Tos) (Method B) is the exception, requiring glacial
acetic acid for an efficient reaction. While the literature
consensus suggests that a higher stoichiometry of sodium
nitrite should be utilized, we found that 2 equiv relative to
R-amino acid was sufficient, resulting in milder reaction
conditions and less sodium nitrite to decompose at the end
of the reaction. Acetic acid was preferable to other acids
used previously, including hydrochloric acid, because chlo-
ride ion and related counterions can be nucleophilic enough
(
11) Ohyama, T.; Oku, H.; Yoshida, M.; Katakai, R. Biopolymers 2001,
8, 636-642.
12) Aravinda, S.; Shamala, N.; Das, C.; Balaram, P. Biopolymers 2002,
5
6
(
4, 255-267.
Figure 1. Nineteen R-hydroxy acids suitably protected for solid-
phase depsi-peptide synthesis using a t-Boc strategy, employing
diisopropylcarbodiimide (DIC), 1-hydroxybenzotriazole hydrate
(13) Kunz, H.; Lerchen, H. G. Tetrahedron Lett. 1987, 28, 1873-1876.
(14) Wang, Z.; La, B.; Fortunak, J. M.; Meng, X. J.; Kabalka, G. W.
Tetrahedron Lett. 1998, 39, 5501-5504.
(
15) Winitz, M.; Bloch-Frankenthal, L.; Izumiya, N.; Birnbaum, S. M.;
(HOBt), and N-ethylmorpholine (NEM) to activate the carboxyl
Baker, C. G. et al. J. Am. Chem. Soc. 1956, 78, 2423-2430.
(16) Takahashi, S.; Yanagida, Y. (Kanegafuchi Chemical Industry Co.,
Ltd., Japan). Eur. Pat. Appl. EP430234, 1991; pp 8 pp.
group: (A) commercially available or previously synthesized
(
denoted by an asterisk) R-hydroxy acids and (B) R-hydroxy acids
(
17) Shin, I.; Lee, M.-r.; Lee, J.; Jung, M.; Lee, W.; Yoon, J. J. Org.
Chem. 2000, 65, 7667-7675.
18) Naito, T.; Nakagawa, S. (Bristol-Myers Co., USA). Br. Patent GB
466001, 1977; pp 5 pp.
synthesized herein.
(
1
(
19) Tang, L.; Deng, L. J. Am. Chem. Soc. 2002, 124, 2870-2871.
R-amino acids must be stable to the TFA deprotection step
used to liberate the free amine in order to elongate the peptide
chain with high fidelity. Only 6 of the 19 required R-hydroxy
acids for A-to-E substitutions are commercially available
(20) Yoneta, T.; Shibahara, S.; Seki, S.; Fukatsu, S. (Meiji Seika Kaisha,
Ltd., Japan). U.S. Patent, 1981; pp 6 pp. Cont.-in-part of U.S. Patent
4290972 Ser. No. 805, abandoned.
(
21) Corey, E. J.; Link, J. O.; Shao, Y. Tetrahedron Lett. 1992, 33, 3435-
438.
22) Ramachandran, P. V.; Brown, H. C.; Pitre, S. Org. Lett. 2001, 3,
17-18.
(23) Chang, J.-W.; Jang, D.-P.; Uang, B.-J.; Liao, F.-L.; Wang, S.-L.
Org. Lett. 1999, 1, 2061-2063.
3
(
(
9) Wiberg, K. B.; Laidig, K. E. J. Am. Chem. Soc. 1987, 109, 5935-
943.
10) Ingwall, R. T.; Goodman, M. Macromolecules 1974, 7, 598-605.
5
(
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Org. Lett., Vol. 6, No. 4, 2004