Synthesis of ketomethylene amino pseudopeptide analogues via reductive amination of glyoxals derived from α-amino acids

Article abstract of DOI:10.1016/S0960-894X(99)00637-X

The reductive amination of an amino acid derived glyoxal, with the free amino group of a protected amino acid or oligopeptide fragment, has been developed as a simple and efficient method for the preparation of ketomethylene amino pseudooligopeptide isost

Full text of DOI:10.1016/S0960-894X(99)00637-X

Bioorganic & Medicinal Chemistry Letters 10 (2000) 153±155
Synthesis of Ketomethylene Amino Pseudopeptide Analogues via
Reductive Amination of Glyoxals Derived from ꢀ-Amino Acids
a
a
a,
b
Michelle Groarke, Basil Hartzoulakis, M. Anthony McKervey, * Brian Walker
and Carvell H. Williams c
aSchool of Chemistry, The Queen's University, Belfast BT9 5AG, UK
bSchool of Pharmacy, Medical Biology Centre, The Queen's University, Belfast BT9 7BL, UK
cSchool of Biology and Biochemistry, Medical Biology Centre, The Queen's University, Belfast BT9 7BL, UK
Received 17 August 1999; accepted 15 November 1999
AbstractÐThe reductive amination of an amino acid derived glyoxal, with the free amino group of a protected amino acid or
oligopeptide fragment, has been developed as a simple and ecient method for the preparation of ketomethylene amino pseudo-
oligopeptide isosteres AaÉ(COCH NH)Aa. Trichlorosilane±DMF is the reagent of choice for the reduction. # 2000 Elsevier Science
2
Ltd. All rights reserved.
The importance of proteolytic enzymes in a wide spec-
trum of biological functions has created a need for more
potent and speci®c inhibitors. One class of useful and
those containing É(COCH ), despite the consequent
2
increase in spatial separation of amino side chains at
0
1
positions P and P . Two methods are currently avail-
1
1
2
versatile inhibitors is the nonhydrolyzable ketomethy-
lene oligopeptide analogues of natural substrates.2
Ketomethylene pseudopeptides can be formed via the
incorporation of a methylene unit in the peptide amide
able for the synthesis of ketomethylene isosteres. The
®rst relies on the alkylation of an oligopetide amine by
an amino acid derived chloromethyl ketone.2 The sec-
,3
ond methodology is a modi®cation of the Dakin±West
2,10,11
2
,3
bond as in A, or by a direct replacement of the -NH-
±7
procedure.
Unfortunately, both these methods
4
function as in B
(Fig. 1). One potentially important
involve conditions that cannot easily be adapted to
peptide synthesis especially in the presence of sensitive
protective groups.
di€erence between these two classes of inhibitors is that
the aminoketone A retains the ability of hydrogen bond
8
formation through the nonamide -NH- group.
We have devised a simple route to ketomethylene amino
peptides of type A from N-protected amino acids 1
The aim of the work described here was to synthe-
size putative peptide-based inhibitors of proteinases
involved in the processing of the Alzheimer's disease-
associated amyloid precursor protein (APP). Our strat-
egy was to replace each of the scissile peptide bonds,
corresponding to the a-, b-, and g-secretase cleavage
sites in APP, with non-hydrolyzable isosteres. As part of
this approach, we attempted the synthesis of keto-
methylene amino-containing of type A, which span the
which is based on the use of novel N-protected amino
glyoxals 31
2,13
in reductive amination of selected amines
(Scheme 1). The route is outlined in Scheme 1 and the
results obtained with combinations of two Cbz-pro-
tected amino acids and four amines, the latter including
a Phe-Val derivative, are summarised in Table 1. The
N-protected amino acid was ®rst transformed into
the terminal diazoketone 2 using standard, well estab-
14,15
0
P and P sites around the scissile amide bonds. This
isostere, É(COCH NH), is believed to act as a transi-
lished methodology. The diazoketone was then
oxidised to the N-protected amino glyoxal 3 using
2
tion state analogue in the inhibition of serine proteases.
In a study of its use in inhibitors of the serine protease
furin, it was found to yield inhibitors more potent than
dimethyldioxirane (DMD) in acetone as oxidant, a
ꢀ
process that was rapid (ca. 10 min at 0 C), quantitative
and clean.
9
The acetone solvent was removed by rotary evaporation
and replaced by dichloromethane, ethanol or dimethyl-
formamide and to the resulting solution was added the
*Corresponding author. Fax: +44-01232-225544; e-mail: tmckervey
@quchem.com
0960-894X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(99)00637-X

1
54
M. Groarke et al. / Bioorg. Med. Chem. Lett. 10 (2000) 153±155
Figure 1.
Scheme 1.
Table 1.
Entry
ac
a
b
Glyoxal
Amine
Product
Time (h)
Yield (%)
1
90
bc
1
90
c
4
4
72
65
d
e
6
62d
aAldimine formation.
b
Isolated yields after column chromatography.
Condensation performed in dry CH Cl
2
c
2 3
, followed by addition of DMF±SiHCl .
d
Corrected for recovered aldimine which was isolated after column chromatography.

M. Groarke et al. / Bioorg. Med. Chem. Lett. 10 (2000) 153±155
155
Ê
amine (Table 1) and 4A molecular sieves. Aldimine for-
mation 4 was complete in 1±6 h depending on the nature
of the amine and the solvent. Reactions with n-butyl-
amine and benzylamine were conducted in dichloro-
methane whereas ethanol (entries c and d) was a better
solvent for reactions of the amino acid-derived amines.
The condensation necessary to produce e was conducted
in dimethylformamide. To complete the synthesis, the
aldimine 4 was reduced. Sodium cyanoborohydride
was the ®rst reagent used for the reduction step due to
its well documented application in reductive amina-
References
1
2
. Babine, R. E.; Bender, S. L. Chem. Rev. 1997, 97, 1359.
. Natarajan, S.; Gordon, E. M.; Sabo, E. F.; Godfrey, J. D.;
Weller, H. N.; Pluscec, J.; Rom, M. B.; Cushman, D. W. Bio-
chem. Biophys. Res. Comm. 1984, 124, 141.
3. Wallace, D. A.; Bates, S. R. E.; Walker, B.; Kay, G.; White,
J.; Guthrie, D. J. S.; Blumsom, N. L.; Elmore, D. T. Biochem.
J. 1986, 239, 797.
4
1
. Jennings-White, C.; Almquist, R. G. Tetrahedron Lett.
982, 23, 2533.
1
6,17
5. Holladay, M. W.; Salituro, F. G.; Rich, D. H. J. Med.
Chem. 1987, 30, 374.
6
Claeson, G. J. Med. Chem. 1992, 35, 3364.
7
8
M.; Didierjean, C.; Aubry, A. J. Am. Chem. Soc. 1998, 120,
9444.
9. Angliker, H. J. Med. Chem. 1995, 38, 4014.
10. Buchanan, G. L. Chem. Soc. Rev. 1988, 17, 91.
11. Buenning, P.; Riordan, J. F. Israel J. Chem. 1981, 21, 43.
tion.
However, with our substrates, NaCNBH was
3
not suciently selective in that it produced a mixture of
the desired aminoketone 5 and the corresponding amino
alcohol 6, the latter as a mixture of diastereoisomers. A
similar lack of chemoselectivity was also observed when
. Cheng, L.; Goodwin, C. A.; Schully, M. F.; Kakkar, V. V.;
. Ho€man, R. V.; Kim, H-O. J. Org. Chem. 1995, 60, 5107.
. Vanderesse, R.; Grand, V.; Limal, D.; Vicherat, A.; Marraud,
1
8,19
sodium triacetoxyborohydride
was employed as the
reducing agent. However, the desired selectivity could
be achieved through the use of trichlorosilane±dimethyl-
20
formamide as the reducing agent. The reduction
y
ꢀ
proceeded smoothly at 0 C and no other products
could be detected. Our methodology was tried with a
number of amines and the results are summarized in
1
2. Darkins, P.; McCarthy, N.; McKervey, M. A.; Ye, T. J.
Chem. Soc. Chem. Comm. 1993, 15, 1222.
3. Lynas, J. F.; Harriott, P.; Healy, A.; McKervey, M. A.;
Walker, B. Bioorg. Med. Chem. Lett. 1998, 8, 373.
1
1
13
Table 1. H and C NMR analysis of products 5c±5e
indicated that they were single diastereoisomers, con-
1
1
4. Podlech, J.; Seebach, D. Liebigs Ann. 1995, 7, 1217.
5. Ye, T.; McKervey, M. A. Tetrahedron 1992, 48, 8007.
®
rming that racemisation did not occur at any step in
the overall synthesis. The reaction times depended on
the nature of the amino acid side chain R and R . We
are currently investigating the applicability of this
method to resin bound peptides. When this is achieved,
our methodology should be ideal for oligopeptide solid
state synthesis and combinatorial applications.
16. Hutchins, R. O.; Hutchins, M. K. In Reduction of the
CˆN to CHNH by Metal Hydrides, Comprehensive Organic
Synthesis; Trost, B. N.; Fleming, I., Eds.; Pergamon Press:
New York, 1991; Vol. 8.
1
2
1
7. Wyvratt, M. J.; Tristram, E. W.; Ikeler, T. J.; Lohr, N. S.;
Joshua, H.; Springer, J. P.; Arison, B. H.; Patchett, A. A. J.
Org. Chem. 1984, 49, 2816.
1
8. Abdel-Magid, A. F.; Carson, K. G.; Harris, B. D.;
Maryano€, C. A.; Shah, R. D. J. Org. Chem. 1996, 61, 3849.
9. Szardenings, A. K.; Burkoth, T. S.; Look, G. C.;
Acknowledgement
1
Campbell, D. A. J. Org. Chem. 1996, 61, 6720.
20. Kobayashi, S.; Yasuda, M.; Hachiya, I. Chemistry Lett.
1996, 407.
We thank the Wellcome Trust for ®nancial support
044101/Z/95).
(
y
A typical procedure for the reductive amination. Cbz-l-AlaW(COCH NH)-L-Val-OMe (5c). A solution of (S)-valine methyl ester (0.5 mmol, 74 mg)
2
was added to a stirred solution of freshly prepared glyoxal 3c (120 mg, 0.5 mmol) in ethanol (10 mL) containing molecular sieves (1 g). The mixture
was stirred for 4 h under N while the colour changes from colourless to bright yellow. At this stage the aldimine could be isolated in quantitative
2
ꢀ
yield by ®ltration and removal of solvent under high vacuum. The crude imine was dissolved in CH
Cl SiH (50 mL, 0.85 mmol) in DMF:CH Cl
at this temperature, and MeOH (1 mL) was added. Saturated aqueous sodium bicarbonate was then added and any insoluble materials were
removed by ®ltration. The aqueous layer was extracted with CH Cl
(2Â20 mL). The organic layers were combined, dried and evaporated to a€ord
the amine 5c (120 mg, 72%). For further puri®cation, the product was subjected to column chromatography on silica using ethyl acetate:hexane (20±
0%). Compound 5c: H NMR (CDCl
H), 3.61 (2H, q, -COCH NH-), 3.75 (3H, s, -OCH
, 125 MHz) 17.11, 17.49, 17.85, 30.38, 50.65, 52.71, 53.79, 67.14, 127.06±129.86, 135.19, 154.64, 206.17; Acc.Mass calculated
51.1913, found 351.1919.
2
Cl
2
(10 mL) under N
2
. After cooling at 0 C,
3
2
2
(1:3, 3 mL) was added and the solution changed colour to light orange. The mixture was stirred for 4 h
2
2
1
4
3
, 300 MHz) 0.93 (6H, d, (-CH
3
)
2
CH)), 1.25 (3H, d, Ala-CH
3
), 1.98 (1H, m, (CH
3 2
) CH), 3.02 (1H, d, Val-a-
O-), 5.77 (1H, d, -NH-), 7.24±7.34 (6H, m, -Ph);
2
3
), 4.43 (1H, m, Ala-a-H), 5.05 5.01 (2H, dd, -CH
2
13
C NMR (CDCl
3
3