Efficient syntheses of thiadiazole peptides

Article abstract of DOI:10.1021/jo100922c

Novel N-(Cbz-aminoacyl)thiosemicarbazides 3a-c were cyclized by treatment with sulfuric acid to give 1,3,4-thiadiazoles 4a-c. Compounds 4a-c reacted with N-(Cbz-aminoacyl)- and -dipeptidoylbenzotriazoles to afford chirally pure 1,3,4-thiadiazol-2-yl-subst

Full text of DOI:10.1021/jo100922c

pubs.acs.org/joc
including antibacterial,5 antimicrobial,6 antiarrhythmic,7 anti-
Efficient Syntheses of Thiadiazole Peptides
cancer,8,9 anti-inflammatory,10 antidepressant11 and anti-HIV12
activities.
Alan R. Katritzky,* Claudia El-Nachef, Kiran Bajaj,
Jonathan Kubik, and Danniebelle N. Haase
In previous efforts to develop new chiral nonproteinogenic
R-amino acids and peptides that could be incorporated into
heterocycles, we synthesized chiral 1,2,4-oxadiazoles13 utiliz-
ing N-protected (R-aminoacyl)benzotriazoles.14 We now re-
port the synthesis of chiral 1,3,4-thiadiazolo-substituted amino
acids and peptides as potential building blocks. Similar com-
pounds with 2-amino-1,3,4-thiadiazoles as coupling units to
peptides are known to be potent APN inhibitors15 and metal-
loprotease inhibitors.16 Moreover, reports exist of the synthesis
of thiadiazoles from amino acids for biological testing,12,17
which disclosed low toxicity and, in the case of phenylalanine
derivatives, anti-inflammatory activity.18 Isothiocyanates are
often used to form thiosemicarbazide intermediates for con-
version to 1,3,4-thiadiazoles.5,12,17,18
Center for Heterocyclic Compounds, Department of
Chemistry, University of Florida, Gainesville,
Florida 32611-7200
katritzky@chem.ufl.edu
Received May 11, 2010
Here, we convert amino acid derivatives 1a-c (1cþ1c0) under
microwave irradiation first into thiosemicarbazides 3a-c
(3cþ3c0); we next cyclized 3 to afford enantiomerically pure
1,3,4-thiadiazoles 4a-c, which in turn provide the first synth-
esis of chirally pure peptidoylaminothiadiazoles.
Compounds 1 and 5 (Tables 3 and 4) were prepared
following established procedures.14,19 Thiosemicarbazides
2 were synthesized from 1-(alkylthiocarbamoyl)benzotria-
zoles and hydrazine20 (Table 5).
Novel N-(Cbz-aminoacyl)thiosemicarbazides 3a-c were
cyclized by treatment with sulfuric acid to give 1,3,4-thiadia-
zoles 4a-c. Compounds 4a-c reacted with N-(Cbz-amino-
acyl)- and -dipeptidoylbenzotriazoles to afford chirally pure
1,3,4-thiadiazol-2-yl-substituted amino acids 6a-c and
dipeptides 7a-c.
N-(Cbz-R-aminoacyl)benzotriazoles 1a-c and (1cþ1c0)
reacted with thiosemicarbazides 2a,b under microwave irra-
diation to yield precursors 3a-c and (3cþ3c0) (68-79%).
The enantiopurity of compound 3c was confirmed by HPLC-
UV; as expected, HPLC analysis of 3c showed a single peak
with nearly the retention time (7.31 min) as that of one of the
two peaks (7.10 and 7.38 min) obtained from the racemic
mixture (3cþ3c0). Compounds 3a-c and the racemic mixture
(3cþ3c0) each underwent concurrent cyclization and depro-
tection of the Cbz group on treatment with concentrated
sulfuric acid12,17 to produce 1,3,4-thiadiazoles 4a-c and
(4cþ4c0) in yields of 53-73% (Scheme 1, Table 1).
Attempts to discover peptide analogues with increased
chemical stability and oral availability have replaced peptide
fragments such as -NHCO-CHR- by a wide variety of
alternative structural moieties. In particular, heterocyclic
moieties attached to peptides and R-amino acids have attrac-
ted considerable interest.1-4
Among important five-membered heterocyclic synthetic
building blocks in medicinal, agricultural, and materials chem-
istry, 1,3,4-thiadiazoles exhibit diverse biological properties,
In the 1H NMR, the characteristic peaks of the NH
protons at δ 9.40 and 8.24 ppm of compound 3c disappeared
when thiadiazole 4c was formed. The removal of the Cbz
protecting group was also evident from the aromatic region
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DOI: 10.1021/jo100922c
r
Published on Web 08/09/2010
J. Org. Chem. 2010, 75, 6009–6011 6009
2010 American Chemical Society

Katritzky et al.
JOCNote
TABLE 1. Preparation of Substituted Amino Acid Thiosemicarbazides 3 and 1,3,4-Thiadiazoles 4
compounds 3
compounds 4
reactants 1
R2
entry
3a
3b
3c
(3cþ3c0)
yield (%)
mp (°C)
entry
4a
4b
4c
(4cþ4c0)
yield (%)
mp (°C)
[R]25
D
Z-L-Val-Bt
Z-L-Phe-Bt
Z-L-Ala-Bt
Z-DL-Ala-Bt
isopropyl
cyclohexyl
cyclohexyl
cyclohexyl
68
74
79
72
140-142
164-165
193-195
189-190
68
53
73
67
60-62
þ13.0
þ5.1
-12.9
0.0
158-159
138-139
133-135
TABLE 2. Preparation of 1,3,4-Thiadiazolo-Substituted Amino Acids 6 and Dipeptides 7
reactants 4
reactants 1 and 5
products
conditions
yield (%)
mp (°C)
4a
4b
4c
4a
4b
4c
(4cþ4c0)
Z-L-Phe-Bt, 1b
Z-L-Val-Bt, 1a
Z-L-Trp-Bt, 1d
Z-L-Ala-L-Phe-Bt, 5a
Z-L-Ala-L-Phe-Bt, 5a
Z-L-Val-L-Met-Bt, 5b
Z-L-Val-DL-Met-Bt, (5bþ5b0)
6a
6b
6c
7a
7b
7c
(7cþ7c0)
TEA, DMF, 2.5 h
TEA, DMF, 3 h
THF, 2.5 h
THF, 2 h
THF, 3 h
62
60
60
57
67
65
61
74-75
205-206
112-114
208-209
229-230
237-238
233-234
THF, 0.5 h
THF, 1.5 h
SCHEME 1
SCHEME 2
of the 1H NMR spectrum, where the multiplet at δ 7.40-7.33
ppm for the precursor 3c disappeared in the product thiadia-
zole 4c. The enantiomeric purity of compound 4c was con-
firmed by HPLC studies; thus using a Chiralcel-OD column,
4c showed a single peak (11.47 min), whereas racemic
mixture (4cþ4c0) showed two peaks (11.87 and 12.54 min),
one of which had nearly the same retention time as the single
peak from 4c. To justify the small difference in retention times,
we ran the HPLC of the 1:1 ratio mixture of 4c and (4cþ4c0)
and observed two peaks at 11.43 and 12.26 min in a ratio of 3:1,
respectively, which confirmed the previous analysis. The chiral
integrity of 4a and 4b is proven by the chiral purity of 7a and 7b,
which is discussed later in this paper. The measured optical
rotations (Table 1) support this conclusion.
(12.17 min) nearly the same that one of the two peaks
(retention times of 11.63 and 12.16 min) disclosed by the
diastereomeric mixture (7cþ7c0). We further analyzed a 1:1
mixture of 7c and (7cþ7c0) and as expected observed an
intensity ratio of 1:3 for the two peaks at 11.72 and 12.30 min;
this confirms the above HPLC conclusions.
The HPLC evidence of 7c confirms the 13C evidence for the
chiral integrity of 7c; by analogy, we conclude that the 13C NMR
spectra for 6a-c and for 7a-b are evidence for their chiral purity.
In conclusion, novel N-(Cbz-aminoacyl)thiosemicarbazides
3a-c and racemate (3cþ3c0) underwent concurrent cyclization
and deprotection with retention of chirality to give 2,5-disub-
stituted 1,3,4-thiadiazoles 4a-c and (4cþ4c0) each possessing a
free amino group. The free amino group in the substituted
thiadiazoles 4a-c coupled with diverse acylbenzotriazoles
under microwave conditions to give chirally pure thiadiazolyl
amino acids 6a-cand dipeptides 7a-c and (7cþ7c0) (57-67%).
Considering the high selectivity and the fact that there is no
other known method to make such compounds, this method
represents a promising route to the preparation of various thia-
diazole-substituted peptides.
1,3,4-Thiadiazoles 4a-c were each reacted with acylben-
zotriazoles 1 and 5 under microwave irradiation to afford
1,3,4-thiadiazolo-substituted amino acids 6a-c and dipep-
tides 7a-c, respectively (Scheme 2, Table 2).
The 13C NMR spectrum of the diastereomeric mixture
(7cþ7c0) derived from the coupling of the diastereomeric
mixture of dipeptides (5bþ5b0) with enantiomerically pure
thiadiazole 4c showed two peaks at δ 14.7 and 14.5 ppm for
the methionine methyl carbons, while a single peak at δ 14.6
ppm was observed for the pure chiral compound 7c.
HPLC-UV analysis of compound 7c and the diastereo-
meric mixture (7cþ7c0) further supported the above conclu-
sions; thus 7c showed a single peak with a retention time
6010 J. Org. Chem. Vol. 75, No. 17, 2010

Katritzky et al.
JOCNote
TABLE 3. Synthesis of N-Protected (R-Aminoacyl)benzotriazoles
1a-d
General Procedure for the Preparation of 1,3,4-Thiadiazoles
4a-c and (4cþ4c0). Concentrated sulfuric acid (2 mL) was added
dropwise to (2.0 mmol) of thiosemicarbazides 3a-c and (3cþ3c0).
The resulting mixture was stirred at room temperature for 8-12 h.
The solution was quenched with ice and then extracted with
methylene chloride (2 ꢀ 10 mL). The aqueous portion was col-
lected and neutralized with sodium hydroxide and then extracted
with methylene chloride (3 ꢀ 10 mL). The organic layers were
combined, washed with water (1 ꢀ 10 mL) and brine (1 ꢀ 10 mL),
and then dried over anhydrous magnesium sulfate. The solvent
was removed in vacuo to yield 1,3,4-thiadiazoles 4a-c and
(4cþ4c0).
lit mp
(°C)
reactants
products
yield (%) mp (°C)
Z-L-Val-OH Z-L-Val-Bt (1a)
Z-L-Phe-OH Z-L-Phe-Bt (1b)
87
85
85
83
88
107-108 73-74
149-151 151-152
115-117 114-115
116-118 112-113
99-100 100-101
Z-L-Ala-OH
Z-L-Ala-Bt (1c)
Z-DL-Ala-OH Z-DL-Ala-Bt (1cþ1c0)
Z-L-Trp-OH Z-L-Trp-Bt (1d)
TABLE 4. Preparation of Dipeptidoylbenzotriazoles 5a,b and (5bþ5b0)
(S)-5-(1-Aminoethyl)-N-cyclohexyl-1,3,4-thiadiazol-2-amine
(4c): White microcrystals (73%, 138-139 °C); 1H NMR
(300 MHz, CDCl3) δ 5.82 (br s, 1H), 4.40-4.30 (m, 1H), 3.29
(br s, 1H), 2.22-1.96 (m, 2H), 1.78-1.55 (m, 5H), 1.49 (dd, J =
6.6, 2.4 Hz, 3H), 1.44-1.10 (m, 5H); 13C NMR (75 MHz,
CDCl3) δ 169.7, 166.3, 56.5, 48.1, 33.2, 25.6, 24.9, 24.6. Anal.
Calcd for C10H18N4S: C, 53.06; H, 8.02; N, 24.75. Found: C,
52.79; H, 8.18; N, 24.51.
reactants
products
yield (%)
mp (°C)
Z-L-Ala-L-Phe-OH
Z-L-Val-L-Met-OH
Z-L-Val-DL-Met-OH Z-L-Val-DL-Met-Bt
Z-L-Ala-L-Phe-Bt (5a)
Z-L-Val-L-Met-Bt (5b)
68
70
79
148-149a
145-147
143-145
(5bþ5b0)
aLiterature mp of 5a is 148-149 °C.19
TABLE 5. Preparation of Thiosemicarbazides 2a,b
General Procedure for the Preparation of 1,3,4-Thiadiazole
Derivatives 6a-c, 7a-c, and (7cþ7c0). 1,3,4-Thiadiazoles 4
(0.3 mmol) and acylbenzotriazoles 1 or 5 (0.3 mmol) were dis-
solved in anhydrous THF (4 mL). The reaction mixture was
exposed to microwave irradiation at 70 °C, 65 W until comple-
tion of the reaction (monitored by TLC). After the solvent was
evaporated, ethyl acetate was added (4 mL) to give a white
precipitate. This solid was filtered and dried to afford 1,3,4-
thiadiazolo-substituted amino acids 6a-c and dipeptides
7a-c and (7cþ7c0). In the case of 6a and 6b, anhydrous
DMF and triethylamine proved to be the best conditions for
a shorter reaction time and a short flash column with a
gradient elution of hexanes/ethyl acetate was used to purify
the product.
Benzyl ((S)-1-(((S)-1-(((S)-1-(5-(Cyclohexylamino)-1,3,4-thi-
adiazol-2-yl)ethyl)amino)-4-(methylthio)-1-oxobutan-2-yl)amino)-
3-methyl-1-oxobutan-2-yl)carbamate (7c): White microcrystals
(60%, 237-238 °C); 1H NMR (300 MHz, DMSO-d6) δ 8.60 (d,
J = 7.8 Hz, 1H), 8.01 (d, J = 7.8 Hz, 1H), 7.59 (d, J = 6.9 Hz,
1H), 7.50-7.22 (m, 6H), 5.17-4.94 (m, 3H), 4.48-4.26 (m, 1H),
3.96-3.80 (m, 1H), 3.52-3.38 (m, 1H), 2.49-2.30 (m, 2H), 2.02
(s, 3H), 2.00-1.86 (m, 4H), 1.86-1.74 (m, 1H), 1.74-1.61 (m,
2H), 1.61-1.50 (m, 1H), 1.44 (d, J = 6.9 Hz, 3H), 1.39-1.10 (m,
5H), 0.88-0.82 (m. 6H); 13C NMR (75 MHz, DMSO-d6) δ
170.9, 170.1, 167.7, 159.9, 156.0, 136.9, 128.2, 127.5, 65.3, 60.1,
53.5, 51.5, 44.3, 32.0, 30.0, 29.3, 25.2, 24.2, 19.5, 19.1, 18.0, 14.6.
Anal. Calcd for C28H42N6O4S2: C, 56.92; H, 7.17; N, 14.22.
Found: C, 56.56; H, 7.45; N, 14.15.
Experimental Section
N-Protected (R-aminoacyl)benzotriazoles 1a-d and N-pro-
tected (dipeptidoyl)benzotriazoles 5a,b were synthesized follow-
ing our established procedure14,19 (Tables 3 and 4).
Thiosemicarbazides 2a,b were synthesized following our es-
tablished procedure20 (Table 5).
General Procedure for the Preparation of 1,3,4-Thiadiazole
Intermediates 3a-c and (3cþ3c0). N-Protected (R-aminoacyl)-
benzotriazoles 1a-c and (1cþ1c0) (2.5 mmol) in tetrahydrofur-
an (5 mL) were each irradiated by microwave at 70 °C, 65 W,
together with thiosemicarbazide 2a or 2b. Upon completion of
the reaction (monitored by TLC) (2-4 h), the solvent was
evaporated and the residue was washed with methylene chloride
or diethyl ether then filtered to afford 3a-c and (3cþ3c0).
(S)-Benzyl (1-(2-(Cyclohexylcarbamothioyl)hydrazinyl)-1-oxo-
propan-2-yl)carbamate (3c): Purified by washing with methylene
chloride to yield the product as white microcrystals (79%,
1
193-195 °C); H NMR (300 MHz, CD3COCD3) δ 9.41 (br s,
1H), 8.25 (br s, 1H), 7.45-7.22 (m, 5H), 7.00 (br s, 1H), 5.22-
5.05 (m, 2H), 4.26-4.12 (m, 1H), 4.12-3.99 (m, 1H), 2.85 (br s,
1H), 2.00-1.87 (m, 2H), 1.80-1.65 (m, 2H), 1.65-1.54 (m, 1H),
1.37 (overlapped d, J = 6.6 Hz, 3H), 1.34-1.04 (m, 5H); 13C
NMR (75 MHz, CD3COCD3) δ 183.1, 173.0, 158.2, 138.3, 129.8,
129.3, 129.0, 67.6, 54.7, 51.6, 33.5, 26.8, 26.4, 26.3, 17.7. Anal.
Calcd for C18H26N4O3S: C, 57.12; H, 6.92; N, 14.80. Found: C,
57.11; H, 7.06; N, 14.92.
Acknowledgment. We thank Dr. C. D. Hall for help with
the preparation of this manuscript.
Supporting Information Available: Compound character-
ization data for 3a, 3b, (3cþ3c0), 4a, 4b, (4cþ4c0), 5b, (5bþ5b0),
6a, 6b, 7a, 7b, and (7cþ7c0). This material is available free of
charge via the Internet at http://pubs.acs.org.
J. Org. Chem. Vol. 75, No. 17, 2010 6011