Thioamide synthesis: Thioacyl-N-phthalimides as thioacylating agents

Full text of DOI:10.1021/jo970528v

3808
J . Org. Chem. 1997, 62, 3808-3809
Sch em e 1
Sch em e 2
Th ioa m id e Syn th esis:
Th ioa cyl-N-p h th a lim id es a s Th ioa cyla tin g
Agen ts
Christopher T. Brain, Allan Hallett, and Soo Y. Ko*,†
Novartis Institute for Medical Sciences, 5 Gower Place,
London WC1E 6BN, U.K.
Received March 24, 1997
Thiopeptides have attracted interest as synthetic tar-
gets owing to their potential as backbone-modified pep-
tide surrogates.1 The thiopeptide bond is isoelectronic
to the parent (oxo)amide bond yet possesses markedly
different physical and chemical properties including
enhanced stability against enzymatic degradation. Thio-
peptides have been hitherto most efficiently prepared by
first converting a preformed dipeptide2 (both termini
protected) to the corresponding thiodipeptide, followed
by incorporation into the peptide sequence by fragment
coupling.3,4 A significant limitation of this approach is
that carboxy-terminus activation of the thiodipeptide unit
often leads to epimerization.5 N-Thioacylation using
what amounts to be an activated thiono acid would avoid
this problem and present a means of incorporating the
thioamide unit in a stepwise manner directly comparable
to N-acylation with an activated acid in conventional
(oxo)peptide coupling. Thiono analogues of conventional
activated acids are, however, unstable and not readily
available. In addition, with the advent of combinatorial
chemistry, stepwise N-thioacylation would become a
prerequisite for the construction of thiopeptide libraries.6
This need for efficient thioacylating agents has stimu-
lated considerable research interest, and several ap-
proaches have been described.7,8 In particular, thioacyl-
N-benzimidazolinones,7a,b developed by Zacharie et al.,
and thioacyl-N-nitrobenzotriazoles,7c reported by Rapo-
port et al., offer practical solutions for the synthesis of
thiopeptides. Our effort in this area has resulted in a
novel and efficient procedure for achieving stepwise
Ta ble 1. Da ta for th e Syn th esis of
Th ioa cyl-N-p h th a lim id es 3
yield of
yield of
enantiomeric
1
2a (%)
3a (%)
purity of 3b (%)
a
b
c
d
e
f
Boc-Phe-NH2
Boc-Leu-NH2
Boc-Phg-NH2
Boc-Val-NH2
Fmoc-Ala-NH2
Boc-Ser(Bn)-NH2
Boc-Pro-NH2
87
99
96
100
77
87
65c
63c
77
99d
>99.5d
>95d
94
97.7e
72c
75
98.4f
85.5g
g
82
94
not determined
Isolated products. b Determined by HPLC (1H NMR (360 MHz)
a
for compound 3c) analysis of ratio of diastereoisomers obtained
by reaction with a homochiral amine.d-g c Recrystallized product.
d
e
g
(S)-R-Methylbenzylamine. D-Alaninol. f H-Ala-NHBn. (R)-R-
Methylbenzylamine.
thioacylation using thioacyl-N-phthalimides, which is
disclosed herein.
We envisaged a general reaction scheme for the
preparation of a thioacylating agent (Scheme 1). Thus,
a primary amide would be first thionated, and subse-
quently, the -NH2 would be converted to a good leaving
group (-NXY). Displacement by an incoming amine
would lead to the desired thioamide. Note that the
thionation step precedes the activation step in this
scheme, which would ensure that sulfur is introduced
under mild (nonracemizing) conditions. The starting
material, a primary amide, is also the most reactive
substrate for Lawesson’s reagent,9 the thionating agent
of choice. On account of the ease of preparation, and
leaving group ability of the phthalimide anion, N-
phthalimidation was chosen as the activation method.10
We explored this general idea with a selection of N-
protected amino acid amides11 (Scheme 2 and Table 1).
Preparation of the thioacylating agents 3 turned out
to be an experimentally facile process. Thus, thionation
of the amino acid amides 1 with Lawesson’s reagent
proceeded smoothly at room temperature and provided
the thioamides 2 in excellent yields. These were easily
converted to the thioacyl-N-phthalimides 3 in good to
excellent yields by treatment with phthaloyl dichloride
at 0 °C. The products were stable toward purification
on a silica column and prolonged storage at 4 °C.
Compounds 3a , 3b, and 3e were crystalline.
† Samsung Advanced Institute of Technology, Taejon 305-380, South
Korea.
(1) Review: Spatola, A. F. In Chemistry and Biochemistry of Amino
Acids, Peptides and Proteins; Weinstein, B., Ed.; Marcel Dekker: New
York, 1983; Vol. 7, Chapter 5, pp 267-357.
(2) Synthesis of a more complex multithiopeptide incorporating n
adjacent thioamide linkages would require thionation of a preformed
peptide of (n + 1) residues.
(3) (a) Clausen, K.; Thorsen, M.; Lawesson, S.-O.; Spatola, A. F. J .
Chem. Soc., Perkin Trans. 1 1984, 785-798. (b) Thorsen, M.; Yde, B.;
Pedersen, U.; Clausen, K.; Lawesson, S.-O. Tetrahedron 1983, 39,
3429-3435.
(4) Direct thionation of a peptide usually leads to a mixture of
regioisomeric thiopeptides unless a strong steric/conformational bias
is present; for example, see: Seebach, D.; Ko, S. Y.; Kessler, H.; Ko¨ck,
M.; Reggelin, M.; Schmieder, P.; Walkinshaw, M. D.; Bo¨lsterli, J . J .;
Bevec, D. Helv. Chim. Acta 1991, 74, 1953-1990.
(5) Unverzagt, C.; Geyer, A.; Kessler, H. Angew. Chem., Int. Ed.
Engl. 1992, 31, 1229-1230.
(6) This aspect will be discussed fully in a later publication.
(7) (a) Zacharie, B.; Sauve´, G.; Penney, C. Tetrahedron 1993, 49,
10489-10500. (b) Belleau, B.; Brillon, D.; Sauve´, G.; Zacharie, B. U.S.
Patent No. 5,138,061, Aug 11, 1992. (c) Reported during the prepara-
tion of this paper: Shalaby, M. A.; Grote, C. W.; Rapoport, H. J . Org.
Chem. 1996, 61, 9045-9048.
(8) (a) Høeg-J ensen, T.; Olsen, C. E.; Holm, A. J . Org. Chem. 1994,
59, 1257-1263. (b) Katritzky, A. R.; Moutou, J .-L.; Yang, Z. Synthesis
1995, 1497-1505. (c) Katritzky, A. R.; Moutou, J .-L.; Yang, Z. Synlett
1995, 99-100. (d) Le, H.-T.; Mayer, M.; Thoret, S.; Michelot, R. Int. J .
Peptide Protein Res. 1995, 45, 138-144. (e) DeBruin, K. E.; Boros, E.
E. J . Org. Chem. 1990, 55, 6091-6098. (f) Elmore, D. T.; Guthrie, D.
J . S.; Kay, G.; Williams, C. H. J . Chem. Soc., Perkin Trans. 1 1988,
1051-1055.
(9) For a review of Lawesson’s reagent see: Cava, M. P.; Levinson,
M. I. Tetrahedron 1985, 41, 5061-5087.
(10) A literature survey revealed the following references to (thio-
acyl)-N-phthalimides: (a) Goerdeler, J .; Stadelbauer, K. Chem. Ber.
1965, 98, 1556-1561. (b) Goerdeler, J .; Horstmenn, H. Chem. Ber.
1960, 93, 670-678; (c) 1960, 93, 663-670.
(11) Unless indicated otherwise, amino acid symbols represent the
L-enantiomers.
S0022-3263(97)00528-8 CCC: $14.00 © 1997 American Chemical Society

Communications
J . Org. Chem., Vol. 62, No. 12, 1997 3809
Ta ble 2. Rea ction s of Th ioa cyl-N-p h th a lim id es 3 w ith
The enantiomeric purities of the thioacyl-N-phthalim-
ides 3 were determined by reaction with the two enan-
tiomers of a chiral amine: R-methylbenzylamine for
3a ,b,c,f, alaninol for 3d , or alanine benzylamide for 3e.
These reactions were carried out in dichloromethane (in
the R-methylbenzylamine cases) or chloroform and pro-
ceeded in high yields (0 °C, 10 min in all cases). HPLC
and 1H NMR analysis of the diastereomeric pairs of
thioamide products indicated that the compounds 3a -e
were highly enantiopure (Table 1). In particular, the
result for the phenylglycine derivative 3c demonstrates
the compatibility of our synthetic procedure with an
amino acid that is especially prone to racemization. The
compound 3f, derived from N-Boc-O-Bn-serine, presented
an anomalous case since the R-methylbenzylthioamides
were obtained in ca. 85% diastereomeric purity. It is
suspected that some epimerization, presumably via
â-elimination/1,4-addition of benzyl alcohol, accompanied
the thioacylation step.
In order to assess their utility in thiopeptide synthesis,
the thioacyl-N-phthalimides were treated with a selection
of amino acid amides, including a dipeptide. The results
are summarized in Table 2. As we anticipated, the
thioacyl-N-phthalimides reacted smoothly with these
nucleophiles in good to quantitative yields under very
mild conditions (chloroform, 0 °C, 10 min). The high
reactivity of our new thioacylating agents was notewor-
thy: thus, sterically demanding couplings as exemplified
by entries 4 and 8 (hindered valine nucleophiles) and
entry 10 (hindered thioacylating agent 3d , derived from
N-Boc-valine) proceeded efficiently. Some explanation for
this high reactivity may be found in the crystalline
conformations of the compounds 3a and 3b, as deter-
mined by X-ray crystallography.12
Am in o Acid Am id es
entry thioacyl-N-phthalimide 3a
R′NH2
yieldb (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
a
a
a
a
a
b
b
b
b
d
e
e
e
e
e
f
H-Ala-NHBnc
H-Ser-NHBnc
H-Phe-Gly-NH2
H-Val-NHBnc
H-Phe-NHBnc
H-Gly-NHBnc
H-Phe-Gly-NH2
98
81
73
69
68
100
79
72
71
77
100
99
76
68
55
60
74
50
d
d
c
H-Val-NH2
H-Phe-NHBnc
H-Phe-NHBnd
H-Gly-NHBnd
H-Ala-NHBnc
H-D-Ala-NHBnc
H-Phe-NHBnc
d
H-Phe-Gly-NH2
H-Gly-NHBnd
H-Gly-NHBnd
H-Ala-NHBnd
g
g
a
b
See Table 1 for structures. Isolated products. c Reaction
performed with the HCl salt in the presence of 1.0 equiv of
d
triethylamine. Reaction performed with the free base.
a negligible degree of epimerization during the thioacy-
lation step.13
In summary, we have developed a new thioacylating
procedure based on thioacyl-N-phthalimides. The re-
agents are very easily prepared in high enantiomeric
purity from readily available starting materials, and the
N-thioacylation step proceeds smoothly in high yields
under mild conditions. This method presents an efficient
means of achieving a stepwise thiocoupling reaction.
Present efforts are directed toward application of this
thioacylation methodology to solid-phase synthesis.
The selectivity of these reagents was illustrated by the
reaction of 3a with serine benzylamide (Table 2, entry
2) which provided exclusively the N-thioacylated product
without need for protection of the hydroxyl group (note
also that reaction of 3d with alaninol (Table 1) provided
only the N-thioacylated product). The high diastereo-
meric purities observed for the coupling products of
compound 3e with the two enantiomers of alanine
benzylamide (Table 2, entries 12 and 13) demonstrated
Ack n ow led gm en t. Technical support from Dr. I.
Linney is gratefully acknowledged.
Su p p or tin g In for m a tion Ava ila ble: Representative ex-
perimental procedures and characterization data for com-
pounds 2 and 3 and the thioacylation products (9 pages).
(12) (a) The phthalimide ring is completely planar, while the thioacyl
group lies ca. 36° out of the plane and, therefore, is not stabilized by
conjugation. (b) The authors have deposited atomic coordinates for
compounds 3a and 3b with the Cambridge Crystallographic Data
Centre. The coordinates may be obtained, on request, from the Director,
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, U.K.
J O970528V
(13) Diastereoisomeric purity of the coupling product of 3e and
D-alanine benzylamide: 98.9%. See also reaction of 3e with L-alanine
benzylamide (Table 1).