On formation of thiohydantoins from amino acids under acylation conditions

Article abstract of DOI:10.1021/jo052576i

Reactions of glycine, alanine, and phenylalanine with acetic anhydride and ammonium thiocyanate give the 1-acetyl-2-thiohydantoins 2a-c. These results appear to contradict prior literature reports pertaining to this reaction.

Full text of DOI:10.1021/jo052576i

(NMR) evidence provides support for the results that reaction
2 affords products 2, shown above, and not structures 3. First,
the proton chemical shift of the N3H in DMSO-d6 appears in
the region of 12.6 ppm, markedly downfield with respect to
the chemical shifts typically associated with cyclic thioamides
(ca. 9.15 ppm).4 Second, no 2D COSY C5H-to-N1H correlation
cross-peaks were observed for 2a-c, whereas a corresponding
cross-peak was observed for compound 1 as well as in the
COSY spectrum of products 4 (reaction 3). Finally, a new NH
signal for the deacylated products gave chemical shifts consistent
with cyclic thioamides.
On Formation of Thiohydantoins from Amino
Acids under Acylation Conditions
Samuel Reyes and Kevin Burgess*
Texas A & M UniVersity, Chemistry Department,
P.O. Box 30012, College Station, Texas 77842
burgess@tamu.edu
ReceiVed December 14, 2005
Reactions of glycine, alanine, and phenylalanine with acetic
anhydride and ammonium thiocyanate give the 1-acetyl-2-
thiohydantoins 2a-c. These results appear to contradict prior
literature reports pertaining to this reaction.
Reactions of aryl isothiocyanates with (N-termini-unmasked)
peptides represent the Edman degradation for peptide sequenc-
ing.1 The ultimate reaction products of these transformations,
2-thiohydantoins, are also conveniently formed from amino acid
esters and aryl isothiocyanates, as shown in reaction 1.2 This
synthetic approach is useful for the preparation of heterocycles
from peptides and, further, has potential application in combi-
natorial chemistry.3 Hence, researchers in the field may also
consider similar reactions applied to unprotected amino acids
under acylating conditions (reaction 2). We were interested in
exploring this chemistry, but found the literature pertaining to
these reactions to be confusing, ambiguous, and, in isolated
minor cases, incorrect. This paper is intended to clarify the
situation.
We attempted to prepare compound 3a via reaction 4, but
instead obtained the acyclic thioamide product 5. This material
when treated with a suitable base (K2CO3, MeOH, 25 °C, 1 h)
failed to cyclize to 3a. In any event, it is to be expected that
compounds 3 would tend to lose an acyl group even to weak
nucleophiles, i.e., it would not be particularly stable in the
presence of nucleophiles.
Confusion about the products of reaction 2 arose because the
numbering system for hydantoins and thiohydantoins changed
prior to and after 1907.5 Figure 1 illustrates the difference in
numbering before and after 1907. Both Chemical Abstracts and
IUPAC5,6 have adopted the nomenclature appearing for these
compounds after 1907.
In 1911, Johnson et al. published a preparation for compound
2a via the conditions shown in reaction 2,7 and subsequently
others in the series including 2b and 2c.8,9 They referred to them
(1) Edman, P.; Begg, G. Eur. J. Biochem. 1967, 1, 80-91.
(2) LeTiran, A.; Stables, J. P.; Kohn, H. Bioorg. Med. Chem. 2001, 9,
2693-2708.
(3) Evindar, G.; Batey, R. A. Org. Lett. 2003, 5, 1201-4.
(4) Smith, D. C.; Lee, S. W.; Fuchs, P. L. J. Org. Chem. 1994, 59, 348-
54.
(5) Ware, E. Chem. ReV. 1950, 46, 403-70.
(6) A Guide to IUPAC Nomenclature of Organic Chemistry, Recom-
mendations 1993; Blackwell Scientific Publications: Oxford, 1993.
(7) Johnson, T. B.; Nicolet, B. H. J. Am. Chem. Soc. 1911, 33, 1973-8.
(8) Johnson, T. B. Am. Chem. J. 1913, 49, 197-204.
In addition to 2a-c, other plausible products from reaction
2 include the isomeric 3-acylthiohydantoins 3. Spectroscopic
10.1021/jo052576i CCC: $33.50 © 2006 American Chemical Society
Published on Web 02/14/2006
J. Org. Chem. 2006, 71, 2507-2509
2507

mixture was stirred for 1 h at 25 °C, after which a reflux condenser
was attached. The temperature was raised to 40 °C, and stirring
was continued for another 1 h. At this time, TLC revealed all of
the benzylisothiocyanate has been consumed, and a new spot
appeared. The solution was cooled to room temperature and the
solution evaporated in vacuo. The residue was redissolved in 15
mL of CH2Cl2, washed with 10 mL of water and 10 mL of brine,
and dried over Na2SO4. The colorless solution was concentrated
and dried in vacuo to give 1 as a white, tiny crystalline material
(0.410 g, 52%). 1H NMR (500 MHz, DMSO-d6): δ 10.46 (s, 1H),
7.34-7.24 (m, 5H), 4.89 (d, J ) 15.0 Hz, 1H), 4.84 (d, J ) 15.0
Hz, 1H), 4.38 (q, J ) 7.1 Hz, 1H), 1.29 (d, J ) 7.1 Hz, 3H). 13C
NMR (125 MHz, DMSO-d6): δ 182.2 (C), 175.6 (C), 136.5 (C),
128.5 (CH), 127.42 (CH), 127.40 (CH), 54.7 (CH), 43.7 (CH2),
16.3 (CH3). MS (ESI, m/z): 576 (M + H)+. NMR data are in
accordance with the literature.2
FIGURE 1. Numbering system on hydantoins and thiohydantoins
before and after 1907.
as 3-acyl-2-thiohydantoins, following the accepted nomenclature
before 1907. Following Johnson’s studies, another group, more
than 65 years later, reported the transformation of glycine into
“3-acyl-2-thiohydantoin” but did not draw a structure or give
spectral data.10 The reported melting point of the material that
they isolated was 175 °C, close to the one we recorded, 173-
174 °C, for 2a; consequently, it seems likely that the product
of this reaction was 2a, 1-acyl-2-thiohydantoin. In 1993,11
Marton et al. used Johnson’s conditions to prepare compounds
2a and 2c and many other related compounds; they drew the
correct structures and used the name that is now (IUPAC)
accepted for them, i.e., 1-acyl-2-thiohydantoins. However, two
other reports, one in 198712 and the other in 2002,13 mention
syntheses similar to that for compound 2a but draw structure 3
and use the IUPAC name for that latter structure, i.e., 3-acyl-
2-thiohydantoin. In both reports, this compound was an inter-
mediate, and the acyl group was removed via hydrolysis, so
the possible structural misassignment was of no great conse-
quence to the authors. However, it is potentially important to
researchers who use these reactions, and we feel that these
structures were not correctly assigned. We postulate that the
compound prepared in the 2002 report was 2a, and not 3a, based
on the fact that the NH chemical shift given was 12.56 ppm in
DMSO-d6. Unfortunately, the 1987 paper gave NMR data
obtained in CDCl3/DMSO-d6 mixtures. The NH chemical shift
which appeared at 3.90 ppm was surprising since this signal
was observed at such a high field. When the proton NMR of
our sample of 2a was recorded in (∼1 mL, 2:1) CDCl3/DMSO-
d6, the N3H was observed at 12.31 ppm. These values do not
correspond well, but we think it is more likely that an incorrect
chemical shift value was inadvertently reported in the 1987 paper
than it is that they actually prepared a compound possessing
structure 3a.
General Procedure for the Synthesis of Acyl-2-thiohydan-
toins. Synthesis of 1-Acetyl-2-thiohydantoin (2a). Glycine (1.0
g, 13.3 mmol) and NH4SCN (1.038 g, 13.3 mmol) were ground
together using a mortar and pestle. The mixed solid was transferred
in a 50 mL round-bottom flask, acid anhydride (7.5 mL, 79.3 mmol)
was added, and the mixture was heated in an oil bath at 100 °C for
30 min, by which time all solids were already dissolved. The light
orange solution was poured into an ice/water mixture (20 mL) and
stored in a freezer overnight. The resulting light orange solid was
filtered and washed with cold water and dried under vacuum to
afford 2a as a light orange solid which appeared as a tiny crystalline-
like material (1.08 g, 51%). Mp: 173-174 °C (lit.7 mp 175-176
1
°C). H NMR (500 MHz, DMSO-d6): δ 12.58 (s, 1H), 4.40 (s,
2H), 2.68 (s, 3H). 13C NMR (125 MHz, DMSO-d6): δ 182.5 (C),
170.4 (C), 169.3 (C), 52.2 (CH), 26.6 (CH3). MS (ESI, m/z): 576
(M + H)+.
1-Acetyl-5-methyl-2-thiohydantoin (2b). L-Alanine was used
to prepare this material which followed the procedure for 2a. After
filtration and washing, the off-white solid was dried under vacuum
1
to yield 2b (1.30 g, 68%). Mp: 164-166 °C (lit.9 166 °C). H
NMR (500 MHz, DMSO-d6): δ 12.63 (s, 1H), 4.67 (ddd, J ) 7.1,
7.1, 7.1 Hz, 1H), 2.70 (s, 3H), 1.42 (d, J ) 7.1 Hz, 3H). 13C NMR
(125 MHz, DMSO-d6): δ 182.3 (C), 173.9 (C), 169.7 (C), 59.8
(CH), 27.4 (CH3), 15.9 (CH3). MS (APCI): 173 (M + H)+.
1-Acetyl-5-benzyl-2-thiohydantoin (2c). L-Phenylalanine was
used to prepare this material which followed the procedure for 2a.
After filtration and washing, the white solid was dried under vacuum
1
to afford 2c (0.850 g, 71%). Mp: 168-169 °C (lit.8 170 °C). H
NMR (500 MHz, DMSO-d6): δ 12.43 (s, 1H), 7.26 (m, 3H), 6.98
(m, 2H), 4.99 (dd, J ) 5.9, 2.7 Hz, 1H), 3.38 (dd, J ) 13.9, 5.9
Hz, 1H), 3.12 (dd, J ) 13.8, 2.7 Hz, 1H), 2.69 (s, 3H). 13C NMR
(125 MHz, DMSO-d6): δ 182.3 (C), 172.6 (C), 170.02 (C), 134.2
(C), 129.2 (CH), 128.4 (CH), 127.3 (CH), 63.5 (CH), 34.4 (CH2),
27.4 (CH3). MS (APCI): 249 (M + H)+.
Any one-step reaction that transforms amino acids into the
types of heterocycles commonly found in pharmaceuticals is
of interest in contemporary medicinal chemistry. Reaction 2
represents one such transformation, but potential practitioners
should exercise care when interpreting the chemical literature
in this area.
General Procedure for Deacylation. Synthesis of 2-Thio-
hydantoin (4a). A suspension of 2a (0.148 g, 0.94 mmol) and ∼5
mL of 3 M HCl in a microwave tube was sealed and heated under
microwave irradiation using a CEM (Discover) microwave at 150
°C for 5 min. The resulting clear, yellow solution was extracted
with 4 × 5 mL of EtOAc. The combined EtOAc extracts were
dried over Na2SO4, concentrated, and dried under vacuum to give
Experimental Section
1
4a as a light orange solid (0.081 g, 74%). H NMR (500 MHz,
Synthesis of 3-Benzyl-5-methyl-2-thiohydantoin (1). To a
mixture of L-alanine methyl ester hydrochloride salt (0.500 g, 3.58
mmol) and Et3N (0.5 mL, 3.58 mmol) in 10 mL of CH2Cl2 was
slowly added benzyl isothiocyanate (0.475 mL, 3.58 mmol). The
DMSO-d6): δ 11.66 (s, 1H), 9.86 (s, 1H), 4.08 (s, 2H). 13C NMR
(125 MHz, DMSO-d6): δ 183.4 (C), 174.6 (C), 50.3 (CH2). NMR
data matched those reported in the literature.14
5-Methyl-2-thiohydantoin (4b). Compound 2b was used to
prepare the title compound, which followed the above procedure.
After drying of the almost colorless solution, 4b appear as white
solid (66 mg, 87%). 1H NMR (500 MHz, DMSO-d6): δ 11.64 (s,
(9) Johnson, T. B. J. Biol. Chem. 1912, 11, 97-101.
(10) Thielemann, H. Z. Chem. 1978, 18, 174.
(11) Marton, J.; Enisz, J.; Hosztafi, S.; Timar, T. J. Agric. Food Chem
1993, 41, 148-152.
(12) Villemin, D.; Ricard, M. Synth. Commun. 1987, 17, 283-9.
(13) Davis, R. A.; Aalbersberg, W.; Meo, S.; Moreira da Rocha, R.;
Ireland, C. M. Tetrahedron 2002, 58, 3263-3269.
(14) Osz, E.; Szilagyi, L.; Marton, J. J. Mol. Struct. 1998, 442, 267-
274.
2508 J. Org. Chem., Vol. 71, No. 6, 2006

1H), 10.01 (s, 1H), 4.23 (dddd, J ) 1.1, 7.1, 7.1, 7.1 Hz, 1H), 1.24
(d, J ) 7.1 Hz, 3H). 13C NMR (125 MHz, DMSO-d6): δ 182.0
(C), 177.3 (C), 56.3 (CH), 16.1 (CH3). NMR data matched those
reported in the literature.14
5-Benzyl-2-thiohydantoin (4c). Compound 2c was used to
prepare the title compound which followed the above procedure.
After drying of the colorless liquid, 4c appear as white solid (0.35
g, 84%). 1H NMR (500 MHz, DMSO-d6): δ 11.44 (s, 1H), 10.07
(s, 1H), 7.27 (m, 2H), 7.22 (m, 1H), 7.17 (m, 2H), 4.56 (t, J ) 5.1
Hz, 1H), 2.98 (dd, J ) 5.0, 2.8 Hz, 2H). 13C NMR (125 MHz,
DMSO-d6): δ 182.3 (C), 175.8 (C), 135.0 (C), 129.7 (CH), 128.3
(CH), 127.0 (CH), 61.5 (CH), 35.7 (CH2). NMR data matched those
reported in the literature.14
dried over Na2SO4. The solution was concentrated and dried in
vacuo to give 5 as an off white solid (0.475 g, 75%). H NMR
1
(500 MHz, DMSO-d6): δ 11.36 (s, 1H), 10.85 (t, J ) 5. 7 Hz,
1H), 4.37 (d, J ) 5.7 Hz, 2H), 3.66 (s, 3H), 2.09 (s, 3H). 13C NMR
(125 MHz, DMSO-d6): δ 181.2 (C), 172.3 (C), 169.0 (C), 52.1
(CH2), 46.2 (CH3), 23.8 (CH3). HRMS (ESI): calcd for C6H10N2O3S
191.0412 (M + H)+, found 191.0497.
Acknowledgment. Support for this work was provided by
the NIH (MH 070040) and by the Robert A. Welch Foundation.
The TAMU/LBMS-Applications Laboratory, directed by Dr.
Shane Tichy, is also thanked.
Synthesis of 5-Acetyl-4-thiohydantoic Acid Methyl Ester (5).
To a mixture of glycine methyl ester hydrochloride salt (0.50 g,
3.98 mmol) and Et3N (0.350 mL, 3.98 mmol) in 15 mL of dry
CH2Cl2 was slowly added acyl isothiocyanate (0.559 mL, 3.98
mmol). The mixture was stirred for 1 h at 25 °C, after which a
reflux condenser was attached. The temperature was raised to 40
°C, and stirring was continued for another 1 h. After 1 h, a TLC of
the reaction showed a faint major spot (Rf ) 0.41, 40% EtOAc/
hexanes). The red brown mixture was cooled to room temperature
and then evaporated in vacuo. The residue was redissolved in 15
mL of CH2Cl2, washed with 10 mL water and 10 mL brine, and
Note Added after ASAP Publication. The general procedure
for the synthesis of compound 2a was incomplete in the version
published ASAP February 14, 2006; the corrected version was
published ASAP February 16, 2006.
Supporting Information Available: NMR spectra (1H, 13C, and
COSY) of compounds 1, 2a-c, 4a-c, and 5. This material is
available free of charge via the Internet at http://pubs.acs.org.
JO052576I
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