K. Senten et al. / Tetrahedron Letters 42 (2001) 9135–9138
Table 2. Synthesised dipeptide diphenyl phosphonates
9137
ProP(OPh)2
AlaP(OPh)2
Xaaa
Purityb (8) (%)
Purityc (2) (%)
Yieldd (2) (%)
Purityb (8) (%)
Purityc (2) (%)
Yieldd (2) (%)
Ala
Asn
Asp
Gly
His
58e
87e
66e
64e
31e
96
99
98
36
59
48
61
7f
76e
80
91
77
91
51
23
32
60
27f
21
53e
78
94
94
100f
30e
85
100f
97
Ile
Lys
Phe
Pro
Ser
ThiaPro
Tyr
Val
66e
83e
100f
95
51f
58
89
97
95
83
13
55
18
90
68e
75e
84
95
60
41
56e
90
94
95
95
65
13
69e
90
41
a Side chain protection for the synthesis of 8 is as follows: Asn(Trt), Asp(OtBu), His(Boc), Lys (Boc), Ser(tBu), Tyr(tBu).
b The HPLC purity of 8 at 214 nm before a possible purificatione by preparative TLC.
c The HPLC purity of 2 at 214 nm after a possiblef purification by HPLC.
d This is the total yield of both coupling and deprotection after a possiblef purification by HPLC.
e Those compounds were purified by preparative TLC before deprotection.
f Those compounds were purified by preparative reverse-phase HPLC after deprotection.
The p-nitroanilides 7 were generally more than 95%
pure and could be deprotected without further purifica-
tion. The diphenyl phosphonates 8 were somewhat less
pure, probably because of a lower stability, and some
needed purification using preparative TLC before pro-
ceeding to deprotection. Deprotection with 50% trifl-
uoroacetic acid (TFA) in dichloromethane afforded the
target compounds 1 and 2. Only 8 of the 35 synthesised
compounds needed preparative HPLC purification in
order to assure the more than 90–95% purity needed for
biological evaluation. Generally it was noted that inter-
mediate purification of the protected dipeptides pre-
vented purification of the final compounds.
Diphenyl 1-aminoethane phosphonate hydrobromide,
alanine p-nitroanilide hydrochloride and proline p-
nitroanilide hydrobromide were prior to their use con-
verted into their free base form by basic extraction.
Diphenyl pyrrolidine-2-phosphonate hydrochloride was
used as such: coupling was mediated by adding an
equivalent amount of triethylamine to the reaction mix-
ture. After stirring at room temperature overnight the
polymer-bound polyamine (1.5 mmol) was added and
stirring was continued for 5 h. The reaction mixture
was filtered and the amide product (7 or 8) was col-
lected in the filtrate. The resins are washed two times
with 4 ml of dichloromethane. The combined fractions
were evaporated under reduced pressure. The purity of
the compounds was checked by TLC and reverse phase
HPLC. Some compounds were purified by preparative
TLC as indicated in Tables 1 and 2. The protected
compounds were dissolved in 4 ml of a TFA/
dichloromethane (1:1) mixture. The solution was stirred
for 3 h and the volatile part was removed under
reduced pressure. The series of the diphenyl alanine
phosphonates and the p-nitroanilides were treated with
dry diethyl ether, the precipitated products were
washed with ether and lyophilised from tert-butanol/
water (4:1). The series of the diphenyl proline phospho-
nates were lyophilised immediately after evaporation of
the volatile part.
Concludingly we can state that the dipeptide p-
nitroanilides 1 and the dipeptide diphenyl phosphonates
2 could be easily and rapidly obtained using polymer-
assisted solution-phase synthesis, starting from prod-
ucts that are either commercially available or easily
accessible.
3. General synthetic procedure
Parallel synthesis was performed using the Quest 210
Organic Synthesizer (Argonaut Technologies). Boc-pro-
tected amino acids, N-cyclohexylcarbodiimide,N%-
methylpolystyrene
resin
(PS-Carbodiimide)
and
1
Characterisation of all compounds was done with H
tris-(2-aminoethyl)-amine polystyrene resin were pur-
chased from Novabiochem. The amino acid p-
nitroanilides were from Bachem.
NMR, mass spectrometry and analytical reverse-phase
HPLC.
Protected amino acid (3) (0.375 mmol), HOBt (0.425
mmol) and PS-Carbodiimide (0.75 mmol) were added
to a dry reaction vessel. Dichloromethane (4 ml) was
added and the mixture was stirred for 10 min prior to
the addition of the amine compound (5 or 6) (0.25
mmol), dissolved in 0.8 ml of dichloromethane.
Acknowledgements
This work received support from The Fund for Scien-
tific Research, Flanders (Belgium) (F.W.O.) and The