Synthesis of β-methyl-β-alanine-L-proline-XAA tripeptides by Yb(OTf)3 catalysed Michael addition of amines to N-crotonyl-L-proline-XAA: A versatile route to cyclic β-methyl-β-alanine-derived tripeptides via ring closing metathesis

Article abstract of DOI:10.1016/S0040-4039(02)01237-6

N-Crotonyl-L-proline derived peptides can be transformed to the corresponding β-methyl-β-alanine-L-proline peptides by Yb(OTf)₃ catalysed Michael addition of aliphatic amines to the crotonyl residue. The Michael adducts derived from addition of allylamine are versatile precursors to the corresponding cyclic peptides obtainable via ring closing metathesis.

Full text of DOI:10.1016/S0040-4039(02)01237-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 6467–6471
Synthesis of b-methyl-b-alanine-
Yb(OTf)3 catalysed Michael addition of amines to
N-crotonyl- -proline-XAA: a versatile route to cyclic
b-methyl-b-alanine-derived tripeptides via ring closing metathesis
L-proline-XAA tripeptides by
L
Biswajit Saha,a Debasis Das,b Biswadip Banerjia and Javed Iqbala,b,
*
aDepartment of Chemistry, Indian Institute of Technology, Kanpur 208016, India
bDr. Reddy’s Research Foundation, Bollaram Road, Miyapur 500 050, Hyderabad, India
Received 2 April 2002; revised 5 June 2002; accepted 24 June 2002
Abstract—N-Crotonyl-
L
-proline derived peptides can be transformed to the corresponding b-methyl-b-alanine-L-proline peptides
by Yb(OTf)3 catalysed Michael addition of aliphatic amines to the crotonyl residue. The Michael adducts derived from addition
of allylamine are versatile precursors to the corresponding cyclic peptides obtainable via ring closing metathesis. © 2002 Elsevier
Science Ltd. All rights reserved.
Peptides derived from b-amino acids are resistant to
proteolytic degradation and therefore of considerable
importance in medicinal chemistry.1 A peptide bond
derived from a- and b-amino acids also leads to
molecules with interesting conformational properties.2
In an ongoing project in our laboratory,3 we required
Scheme 1, amide 1 underwent smooth Michael addition
in high yields (70–75%) to the corresponding amide 2,
however, the reaction was not diastereoselective as mix-
tures of both isomers were obtained in equal propor-
tion. In order to improve the diastereoselectivity during
the Michael addition, we carried out additions using
peptides consisting of a b-amino acid and
L
-proline to
the corresponding N-crotonoyl-L-proline dipeptides 3
probe their proteolytic stabilities towards aspartyl
(Scheme 2), reasoning that these may preorganise via a
g-turn leading to a less flexible structure which will
result in a facial bias for Michael addition. An 1H
NMR study of peptides 3 indicated the presence of an
intramolecular hydrogen bond suggesting that 3a and
3b are indeed preorganised via a g-turn. 3a and 3b were
subjected to Yb(OTf)3 catalysed Michael addition using
allylamine leading, after N-protection, to N-butoxycar-
bonyl (Boc) peptides 4a and 4b in good yields (Scheme
2).
proteases.4 In order to develop a synthetic methodology
for a dipeptide, derived from b-methyl-b-alanine-
L-pro-
line we used N-crotonoyl- -proline as a precursor for
L
their synthesis. This communication describes our pre-
liminary results on Yb(OTf)3 catalysed synthesis of
b-methyl-b-alanine-L-proline containing dipeptides.
Yb(OTf)3 is known to catalyse the Michael addition of
amines to crotonate esters.5 In order to probe the
synthetic scope of this reaction, we explored the
Michael addition of different alkyl amines to N-cro-
tonoyl-L
-proline6 methyl ester under Yb(OTf)3 cataly-
sis. Accordingly, allylamine, benzylamine and
3-bromobenzylamine were subjected to Michael addi-
tion with N-crotonoyl-
presence of catalytic (10 mol%) Yb(OTf)3 leading to the
L
-proline methyl ester 1 in the
formation of b-methyl-b-alanine-L-proline containing
amides 2 (Scheme 1).7 As is evident from the results in
* Corresponding author. Present address: Director, Regional
Research Laboratory, Trivandrum 695019, India; e-mail:
javediqbaldrf@hotmail.com
Scheme 1. Yb(OTf)3-catalysed Michael addition of amines.
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)01237-6

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B. Saha et al. / Tetrahedron Letters 43 (2002) 6467–6471
Scheme 2. Yb(OTf)3-catalysed Michael addition of allylamine.
Thus we wanted to constrain these peptides containing
a b-methyl-b-alanine- -proline bond, by cyclisation. The
However, a careful analysis of the reaction mixture by
1H NMR indicated it to be a mixture (60:40) of
diastereomers meaning that the g-turn present in 3 is not
able to exert any meaningful control during Michael
addition of amines. In order to further improve the
diastereoselectivity, we synthesised peptides capable of
existing as b-turns. It is known that a b-turn preorganises
peptides into conformationally constrained, less flexible
conformations. Such preorganised b-turns in a peptide
may provide a better facial selectivity in the crotonoyl
group leading to high diastereoselectivity during
Yb(OTf)3 catalysed Michael addition of alkyl amines.
L
cyclic peptides derived from such residues are attractive
compounds as pharmaceutical probes for proteases. Also
a comparative study of the acyclic as well as the
corresponding cyclic peptide may provide vital informa-
tion regarding the ‘bioactive conformation’ of such
structures.
In view of the importance of the cyclic peptides, we
undertook a systematic study on the cyclisation of these
peptides by RCM.10 Thus the dipeptides 4 were con-
verted to the corresponding allylamides 7 via routine
synthetic manipulation (Scheme 4) and the
diastereomers were separated by column chromatogra-
phy. In view of the probability that the allylamides 7
exist with a b-turn, we first explored their cyclisation
using Grubbs’ ruthenium catalyst. Surprisingly no
cyclised peptide 8 was observed with either of the
diastereomers (7a and 7b) under RCM conditions.
Molecular dynamics simulation studies showed that
allylamides 7 are organised via a conformation that
does not permit the two terminal double bonds to come
into close proximity for cyclisation (Fig. 1). Accord-
ingly, we turned our attention to the corresponding
allyl esters 9 with the premise that these may exist in a
favourable conformation (Fig. 1). Thus both the
diastereomers of 9 (9a and 9b) were separated by
column chromatography and subjected to RCM stud-
ies. Our assumption was vindicated as both 9a and 9b
underwent smooth RCM with Grubbs’ catalyst leading
to the corresponding cyclic peptides 10a and 10b,
respectively. Similarly the diastereomers 9c and 9d were
separated and subjected to RCM to afford the corre-
sponding cyclic peptides 10c and 10d, respectively. It is
noteworthy that all the diastereomers i.e. 9a–d under-
went RCM with equal ease and efficiency (Table 1).
Thus the corresponding dipeptide N-crotonoyl-L-pro-
line-leucine–glycine ethyl ester 5 was synthesised accord-
ing to standard amide coupling procedures and the
existence of g- and b-turns8 in this peptide was probed
by deuterium exchange studies9 using CD3OD in CDCl3
(Scheme 3). There was no deuterium exchange for the
bonded amide proton even after 5 hours, clearly indicat-
ing the presence of strong intramolecular hydrogen
bonding in 5. When 5 was treated with allylamine or
benzylamine under Yb(OTf)3 catalysed conditions, fol-
lowed by protection, N-Boc dipeptides 6a and 6b were
obtained in good yields (Scheme 3). However the
diastereoselectivity in this reaction had improved only
marginally (70:30) indicating only a limited role for the
preorganised structure, in influencing the transition state
of the Michael addition. The peptides 4 and 6 could be
separated into pure diastereomers and in spite of the
moderate selectivity during the Michael addition, this
methodology provided an efficient route for the synthesis
of both diastereomers of b-methyl-b-alanine-L-proline-
XAA tripeptides. With allylamine at the b-amino acid
terminus in these peptides we realised that the introduc-
tion of another double bond at the other end of the
peptide would lead to a suitable precursor for ring closing
metathesis (RCM).
Scheme 3. Yb(OTf)3-catalysed Michael addition of amines to peptide 5.

B. Saha et al. / Tetrahedron Letters 43 (2002) 6467–6471
6469
Scheme 4. RCM of tripeptides 7 and 9.
Figure 1. Energy minimised structures show preference for the allyl esters 9 as compared to allyl amides 7 in RCM cyclisation.
Table 1. Ru-alkylidine catalysed RCM on the diastereomers of tripeptides 9
Ph
Ph
O
O
O
O
O
O
N
H
O
N
H
N
H
O
N
N
H
N
N
O
N
"Ru"b
56%a
O
O
N
O
"Ru"
55%
O
O
O
O
N
Boc
9a
O
S
Boc
S
S
N
S
N
Me
Me
Boc
Me
Me
N
Boc
10a
9c
10c
Ph
O
Ph
O
O
O
O
N
H
O
N
H
O
N
H
N
H
N
N
N
O
O
"Ru"
58%
O
"Ru"
61%
O
O
O
N
O
O
R
R
Boc
Boc
N
R
N
R
N
Me
Me
Boc
Me
Boc
Me
9b
10b
10d
9d
a) yield of the isolated cyclic peptides b) "Ru"= Cl2(Cy3P)2Ru=CHPh

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B. Saha et al. / Tetrahedron Letters 43 (2002) 6467–6471
Scheme 5. Assignment of the configuration of the b-amino acid in 9a. Reagents and conditions: (i) LiOH, THF:H2O, rt; (ii)
isobutyl chloroformate, Et3N, CH2N2 (excess), CH2Cl2 at 0°C; (iii) Ag(OAc), Et3N, MeOH, rt; (iv) LiOH, THF:H2O; (v) EDC,
HOBT, O-allyl-L-phenylalanine-L-proline, Et3N, CH2Cl2, 0°C to rt.
G. C., Ed; Chapman and Hall: London, 1985;; (b) Jones,
J. H. (Sen. Reporter), Amino Acids and Peptides; Specialist
Periodical Report; The Royal Society of Chemistry, 1992;
Vol. 23;; (c) Gellman, S. H. Acc. Chem. Res. 1998, 31, 173;
(d) Sibi, M. P.; Deshpande, P. K. J. Chem. Soc., Perkin
Trans. 1 2000, 1461; (e) Abele, S.; Seebach, D. Eur. J. Org.
Chem. 2000, 1; (f) Juaristi, E. Enantioselective Synthesis of
i-Amino Acids; Wiley-VCH: New York, 1997; (g) Nico-
laou, K. C.; Dai, W. M.; Guy, R. K. Angew. Chem., Int.
Ed. Engl. 1994, 33, 15 and references cited therein.
2. (a) Seebach, D.; Abele, S.; Gademann, K.; Guichard, G.;
Hintermann, T.; Jaun, B.; Matthews, J. L.; Schreiber, S.
Helv. Chim. Acta 1998, 81, 932–982; (b) Gellman, S. H.
Acc. Chem. Res. 1998, 31, 173–180; (c) Barchi, J. J., Jr.;
Huang, X.; Appella, D. H.; Christianson, L. A.; Durell, S.
R.; Gellman, S. H. J. Am. Chem. Soc. 2000, 122, 2711–
2718.
The cyclic peptides were obtained as a mixture of E and
Z isomers in which the former isomer was the major
product (E:Z=3:1).
In order to ascertain the absolute stereochemistry of the
chiral centre of the b-amino acid, we have carried out the
chemical correlation study described in Scheme 5. We
synthesised N-allyl-N-Boc-
L
-alanine 11 starting from
L
-alanine and made the corresponding diazoketone 12 by
mixed anhydride protocols using isobutyl chloroformate
and diazomethane in dichloromethane. Arndt–Eistert
homologation on 12 using AgOAc gave b-amino ester 13,
which was hydrolysed to afford the corresponding b-
amino acid 14. Coupling the pure acid 14 with
L-proline-
L
-phenylalanine allyl ester yielded the pure diastereomer.
Optical rotation and HPLC studies indicated that the
compound obtained by homologation ([h]D −31.3, c 1.15
in CHCl3) (Scheme 5) is identical with the polar
diastereomer 9a ([h]D −37.8, c 0.65 in CHCl3) obtained
by Yb(OTf)3 catalysed Michael addition of allylamine
with 3a. Therefore the polar diastereomers of 4a and 9
were assigned the ‘S’ absolute stereochemistry at the
chiral centre in the b-amino acid residue. Similarly the
‘S’ assignment was made for the other polar diastereomer
9c and the corresponding cyclic peptides (i.e. 10a and
10c). The less polar diastereomers were accordingly
assigned the ‘R’ configuration at the b-amino residue in
9b, 9d, 10b and 10d.
3. Prabhakaran, E. N.; Rajesh, V.; Dubey, S.; Iqbal, J.
Tetrahedron Lett. 2001, 42, 339–342.
4. (a) Babine, R. E.; Bender, S. L. Chem. Rev. 1997, 97, 1359;
(b) Sherin, S.; Abdel-Meguid; Zhao, B.; Murthy, K. H. M.;
Winborne, E.; Choi, J. K.; Desjarlais, R. I.; Minnich, M.
D.; Culp, J. S.; Debouck, C.; Tomaszek, T. A., Jr.; Meek;
Dreyer, G. B. Biochemistry 1993, 32, 7972–7980; (c)
Debouck, C. AIDS Res. Human Retroviruses 1992, 8,
153–164; (d) Slee, D. H.; Lasio, K. L.; Elder, J. H.;
Ollmann, I. R.; Gustchina, A.; Kervinen, J.; Zdanov, A.;
Wlodawer, A.; Wong, C. H. J. Am. Chem. Soc. 1995, 117,
11867–11878; (e) Wiley, R. A.; Rich, D. H. Med. Res. Rev.
1993, 13, 327–384; (f) Owens, R. A.; Gesellchen, P. D.;
Houchins, B. J.; DiMarchi, R. D. Biochem. Biophys. Res.
Commun. 1991, 181, 402–408.
In conclusion, this paper describes a novel synthetic route
for b-methyl-b-alanine-
Yb(OTf)3 catalysed Michael addition of amines to N-
crotonoyl- -proline or N-crotonoyl- -proline peptides,
respectively. The facile addition of allylamine leads to the
dipeptide containing -proline
L-proline derived peptides using
5. (a) Matsubara, S.; Yoshika, M.; Utimoto, K. Chem. Lett.
1994, 827–830; (b) Matsubara, S.; Kodama, T.; Utimoto,
K. Tetrahedron Lett. 1990, 31, 6379–6380.
L
L
b-methyl-b-alanine-
L
6. General procedure for N-crotonoyl-
L
-proline-XAA peptide
-proline
residue which are useful precursors for cyclic peptides
obtainable by RCM.
synthesis: To a stirred solution of N-crotonoyl-
L
acid in THF at 0°C was added triethylamine (1 equiv.),
followed by isobutyl chloroformate (1 equiv.) and the
mixture vigorously stirred for 5 min and then to this was
added XA%A%-allyl amide/ester followed by triethylamine (1
equiv.). The reaction mixture was stirred for 5 h at rt and
after that the solvent was removed, the residue taken up
References
1. (a) Chemistry and Biochemistry of the Amino Acids; Barrett,

B. Saha et al. / Tetrahedron Letters 43 (2002) 6467–6471
6471
in ethyl acetate, washed with sodium bicarbonate and
saturated citric acid solution and finally with brine. Dry-
ing over sodium sulfate and concentration in vacuo
yielded the crude product which was subjected to column
chromatography (silica gel, EtOAc:hexane) to afford the
desired peptide in good yield.
30 min and then refluxed. After 12 h another portion of
catalyst (10 mol%) was added to the reaction mixture and
refluxing continued for another 12 h. After this the
reaction was exposed to air and directly subjected to
column chromatography (silica gel, EtOAc:hexane) to
afford the corresponding cyclic product 10 as a mixture
of E and Z isomers in 50–60% yield. Spectral data for
some selected compounds: 4a: Oil; Yield: 67%; FTIR
7. General procedures for Michael addition reaction of amines
to methyl N-crotonoyl
tion: To a stirred solution of methyl N-crotonoyl
L
-prolinate followed by Boc protec-
-proli-
(CHCl3): 3313, 2976, 1746, 1686, 1528 cm−1 1H NMR
;
L
nate 1 (0.4 g, 1 equiv., 2.0 mmol) and benzylamine (0.22
mL, 0.21 g, 1 equiv., 2.0 mmol) or 3-bromobenzylamine
(generated prior to the reaction by neutralising 3-bro-
mobenzylamine hydrochloride (0.5 g, 1.1 equiv., 2.2
mmol), with an excess of NaHCO3) or allylamine (0.2
mL, 0.15 g, 1.3 equiv., 2.6 mmol) in THF (1.0 mL/mmol)
was added Yb(OTf)3 (0.125 g, 0.1 equiv., 0.2 mmol) at
room temperature and the reaction mixture was stirred
for 12 h. After that the reaction mixture was diluted with
hexane and filtered through a celite pad, the filtrate was
concentrated under vacuum, and passed through a filtra-
tion column (silica gel, 2% MeOH in ethyl acetate). This
mixture was subjected to di-tert-butyl dicarbonate
(Boc2O), (0.45 g, 2.06 mmol), catalytic N,N-dimethyl-
aminopyridine in dichloromethane and after aqueous
work-up dried and evaporated to give a crude residue.
Chromatographic purification (silica gel, EtOAc:hexane)
of the crude material gave the desired amines 2 as a
diastereomeric mixture.
(200 MHz, CDCl3) l: 1.25 (d, 3H, J=5.9 Hz), 1.45 (s,
9H), 1.85–2.01 (m, 3H), 2.25–2.67 (m, 2H), 2.95–3.38 (m,
5H), 3.71 (s, 3H), 3.75–3.91 (m, 2H), 4.1–4.20 (m, 2H),
4.53–4.84 (m, 2H), 5.07–5.19 (m, 2H), 5.72–5.78 (m, 1H),
7.14–7.26 (m, 5H), 7.40 (bs, 1H); CI MS m/z (iso-
butane): 502 (M+1), 470, 444, 402, 370, 277, 170. 9a: Oil;
yield: 76%; [h]D −37.8, (c 0.65, CHCl3); FTIR (CHCl3):
3311, 2976, 1744, 1687, 1527, 1366 cm−1 1HNMR (200
;
MHz, CDCl3): l 1.23 (d, 3H, J=7.2 Hz), 1.47 (s, 9H),
1.77–2.04 (m, 4H), 2.31–2.41 (m, 2H), 3.00 (dd, 1H,
J=7.4, 13.5 Hz), 3.19 (dd, 1H, J=5.9, 13.6 Hz), 3.42 (bs,
2H), 3.68–3.87 (m, 2H), 4.15–4.45 (m, 1H), 4.50–4.76 (m,
3H), 4.81 (dd, 1H, J=6.9, 13.7 Hz), 5.07–5.34 (m, 4H),
5.76–5.92 (m, 2H), 7.13–7.27 (m, 5H), 7.44 (bs, 1H); CI
MS m/z (iso-butane): 528 (M+1), 514, 470, 428, 370, 342,
303, 170. 10a: solid; yield: 56%; mp: 157–158°C; [h]D
−110.0 (c 0.15, MeOH); FTIR (CHCl3): 3314, 3010, 2962,
2928, 1736, 1683, 1516 cm−1 1H NMR (200 MHz,
;
CDCl3): l 1.44 (d, 3H, J=7.2 Hz), 1.51 (s, 9H), 1.70–1.91
(m, 4H), 2.30–2.36 (m, 2H), 2.61–2.76 (m, 1H), 3.2 (dd,
1H, J=5.2, 14.2 Hz), 3.42–3.50 (m, 2H), 3.64–3.94 (m,
2H), 4.12–4.35 (m, 1H), 4.45 (dd, 1H, J=6.7, 11.2 Hz),
4.54–4.72 (m, 2H), 5.01 (dd, 1H, J=7.0, 13.5 Hz), 5.66–
5.70 (m, 1H), 5.78–5.86 (m, 1H), 6.0 (brd, 1H, J=13.3
Hz), 7.1 (d, 1H, J=7.1 Hz), 7.24–7.30 (m, 4H); CI MS
m/z (iso-butane): 500, 484, 426, 400 (100%), 388; 10b:
Solid; yield: 61%; mp: 183–184°C; [h]D −82.6 (c
0.5,CHCl3); FTIR (CHCl3): 3314, 3009, 2962, 2929, 1737,
8. (a) Rose, G. D.; Gierasch, L. M.; Smith, J. A. Turns in
Peptides and Proteins; Adv. Protein Chem.; Academic
Press: New York, 1985; (b) Farmer, P. S. In Drug Design;
Ariens, E. J., Ed.; Academic: New York, 1980; Vol. 10,
pp. 119–143; (c) Feng, Y.; Pattarawarapan, M.; Wang,
Z.; Burgess, K. Org. Lett. 1999, 1, 121; (d) Belvisi, L.;
Bernardi, A.; Manzoni, L.; Potenza, D.; Scolastico, C.
Eur. J. Org. Chem. 2000, 2563–2569; (e) Kaul, R.; Ange-
les, A. R.; Jager, M.; Powers, E. T.; Kelly, J. J. Am.
Chem. Soc. 2001, 123, 5206–5212.
1683, 1516 cm−1 1H NMR (500 MHz, CDCl3, l ppm):
;
9. Winkler, J. D.; Piatnitski, E. L.; Mehlmann, J.; Kasparec,
J.; Axelsen, P. H. Angew. Chem., Int. Ed. 2001, 40,
743–745.
1.44 (d, 3H, J=8.2), 1.53 (s, 9H), 1.69–1.90 (m, 4H),
2.30–2.39 (m, 2H), 2.93 (m, 1H), 3.25 (dd, 1H, J=5.2,
14.2), 3.40–3.50 (m, 2H), 3.64–3.94 (m, 2H), 4.14–4.37
(m, 1H), 4.45 (dd, 1H, J=6.6, 11.2), 4.54–4.7 (m, 2H),
5.0 (dd, 1H, J=7.0, 13.7), 5.65–5.71 (m, 1H), 5.78–5.86
(m, 1H), 6.01 (brd, 1H, J=13.3), 7.12 (d, 1H, J=7.1),
7.24–7.30 (m, 4H); CI MS m/z (iso-butane): 500 (M+H)+,
484, 444, 426, 400 (100%), 388, 149; 10c: Oil; yield: 55%;
[h]D −58.9 (c 0.1, CHCl3); FTIR (CHCl3): 3307, 3010,
10. (a) Nguyen, S. T.; Johnson, L. K.; Grubbs, R. H.; Ziller,
J. W. J. Am. Chem. Soc. 1992, 114, 3974; (b) Schuster,
M.; Blechert, S. Angew. Chem., Int. Ed. 1997, 36, 2036–
2056 and References cited therein; (c) Furstner, A.
Angew. Chem., Int. Ed. 2000, 39, 3012–3043 and Refer-
ences cited therein; (d) Phillips, A. J.; Abell, A. D.
Aldrichim. Acta 1999, 32, 75–90; (e) Flink, B. E.; Kym, P.
R.; Katzenellenbogen, J. A. J. Am. Chem. Soc. 1998, 120,
4334–4344. General procedure for RCM: To a stirred
solution of Grubbs’ ruthenium catalyst (10 mol%) in dry
dichloromethane (60 mL) under a nitrogen atmosphere
was added the di-allylated peptide 9 (1 mmol) dissolved
in dry dichloromethane (40 mL) slowly over a period of
1
2965, 2874, 1743, 1685, 1534 cm−1; H NMR (200 MHz,
CDCl3, l ppm): 0.90 (bs, 6H), 1.27 (d, 3H, J=6.8), 1.44
(s, 9H), 1.57–2.5 (m, 9H), 3.5–4.13 (m, 5H), 4.5–4.62 (m,
4H), 5.06–5.6 (m, 4H), 5.73–5.91 (m, 1H), 6.0–6.2 (m,
1H); CI MS m/z (iso-butane): 466, 450, 410, 394, 366,
354, 344.