Synthesis of peptide isocyanates and isothiocyanates

Full text of DOI:10.1021/jo960038n

J . Org. Chem. 1996, 61, 3929-3934
3929
these conditions, we have also prepared peptide isothio-
cyanates, a second unreported class of compounds.5
Syn th esis of P ep tid e Isocya n a tes a n d
Isoth iocya n a tes
Resu lts
J ames S. Nowick,* Darren L. Holmes, Glenn Noronha,
Eric M. Smith, Tram M. Nguyen, and Sheng-Lin Huang
Addition of a solution of phosgene in toluene to a
solution of a peptide (as the hydrochloride salt, 1) in an
ice-cooled mixture of methylene chloride and saturated
aqueous sodium bicarbonate solution affords the peptide
isocyanate 2 (eq 1). In a typical reaction procedure, 2.0
Department of Chemistry, University of CaliforniasIrvine,
Irvine, California 92717-2025
Received J anuary 4, 1996
Peptide isocyanates are a hitherto unreported class of
compounds that we required as building blocks for the
synthesis of artificial â-sheets.1 In 1950, Goldschmidt
and Wick reported that amino acid ester hydrochlorides
could readily be converted to the corresponding amino
acid ester isocyanates by refluxing in toluene while
sparging with gaseous phosgene.2 When they attempted
to convert a peptide (glycylgylcine ethyl ester hydrochlo-
ride) to its isocyanate, the peptide hydantoin was formed.
The authors postulated that the peptide isocyanate
formed and subsequently cyclized to the hydantoin.
These studies raised the question that the isocyanates
might be inherently unstable and unsuitable for use as
synthetic building blocks. Because the reaction condi-
tions used by Goldschmidt and Wick involved high
temperatures (110 °C) and the liberation of hydrogen
chloride, we decided to investigate whether peptide
isocyanates could be prepared using milder reaction
conditions.
equiv of phosgene is used and the reaction is complete
after 10 min of rapid stirring. Using these reaction
conditions, we have prepared a variety of peptide isocy-
anates (Table 1). Cases studied include dipeptides (Table
1, entries 1-4), tripeptides (Table 1, entries 5 and 6), and
monopeptide amides (Table 1, entries 7, 8). To achieve
a satisfactory yield of phenylalanine methylamide iso-
cyanate (entry 7), 1.1 equiv of phosgene was used (vide
infra). Amino acid ester isocyanates can also be prepared
by this method (Table 1, entry 9). (Because the procedure
is so convenient and sodium bicarbonate is nontoxic and
environmentally benign, we now consider it preferable
to our earlier method of preparing amino acid ester
isocyanates using pyridine.3) When thiophosgene is used
in place of a solution of phosgene in toluene, the corre-
sponding peptide isothiocyanates are formed (Table 1,
entries 10 and 11).
Triphosgene (Cl3COCO2CCl3) is a popular alternative
to phosgene.6,7 Use of triphosgene instead of phosgene
(0.7 mmol/mmol peptide hydrochloride, Table 1, entries
12 and 13) gave isocyanates 2a and 2b in yields similar
to those obtained using phosgene. The isocyanates
produced using triphosgene were contaminated with ca.
10% of unreacted triphosgene, as evidenced by a peak at
1828 cm-1 in the infrared spectrum. We prefer to use
phosgene, because excess phosgene is removed upon
evaporation of the solvent, while unreacted triphosgene
is not volatile and is reactive toward nucleophiles (1
mmol of triphosgene can react with 12 mmol of an
amine). Although phosgene is toxic, its solution is easily
and safely dispensed by syringe in a fume hood, and over
a dozen co-workers in our laboratories have used this
reagent hundreds of times without incident.
The peptide isocyanates and isothiocyanates that are
produced by this procedure are generally formed in near-
quantitative mass recovery and in ca. 60-100% purity,
as indicated by 1H NMR spectroscopic analysis. Whereas
amino acid ester isocyanates are sufficiently volatile that
they are conveniently purified by Kugelrohr distillation,3
peptide isocyanates and isothiocyanates are not. In lieu
of purification, we chose to trap these compounds with
N-ethylaniline as urea and thiourea derivatives 3, which
were readily purified to analytical purity (eq 2). Thus,
the yields reported in Table 1 are the isolated yields of
In 1992, we reported that amino acid ester hydrochlo-
rides could readily be converted to amino acid ester
isocyanates by treatment with a solution of phosgene in
toluene and pyridine in methylene chloride at 0 °C.3 The
commercial availability (Fluka) and ease of handling of
a phosgene solution rendered these reaction conditions
particularly attractive. Because free hydrogen chloride
is not formed, these conditions are compatible with acid-
sensitive functionality, such as tert-butyl ether groups,
which cannot tolerate the conditions of Goldschmidt and
Wick. When the hydrochloride salts of peptides were
subjected to these reaction conditions, the corresponding
peptide isocyanates were generated, along with variable
quantities of the peptide hydantoins. Upon further
investigation, we discovered that modified Schotten-
Baumann conditions afforded the isocyanates reproduc-
ibly and in satisfactory yield, while minimizing the
formation of hydantoins and other side products.4 Using
(1) (a) Nowick, J . S.; Smith, E. M.; Noronha, G. J . Org. Chem. 1995,
60, 7386. (b) Nowick, J . S.; Holmes, D. L.; Mackin, G.; Noronha, G.;
Shaka, A. J .; Smith, E. M. J . Am. Chem. Soc. 1996, 118, 2764.
(2) (a) Goldschmidt, S.; Wick, M. Z. Naturforsch. B 1950, 5, 170. (b)
Goldschmidt, S.; Wick, M. Liebigs Ann. Chem. 1952, 575, 217.
(3) Nowick, J . S.; Powell, N. A.; Nguyen, T. M.; Noronha, G. J . Org.
Chem. 1992, 57, 7364.
(4) For related examples, see: (a) Gazzard, E. G.; Greenshields, J .
N.; Phillipson, J . M. Br. Patent 1 152 877, 1969; Chem. Abstr. 1969,
71, 48896. (b) Botta, A.; Krimm, H. Ger. Patent 1 913 273, 1970; Chem.
Abstr. 1970, 73, 120074.
(5) One reported attempt to prepare
a peptide isothiocyanate
afforded the corresponding peptide thiohydantoin: Bo¨hme, H.; Martin,
F.; Strahl, J . Arch. Pharm. (Weinheim, Ger.) 1980, 313, 10.
(6) Eckert, H.; Forster, B. Angew. Chem., Int. Ed. Engl. 1987, 26,
894.
(7) Majer, P.; Randad, R. S. J . Org. Chem. 1994, 59, 1937.
S0022-3263(96)00038-2 CCC: $12.00 © 1996 American Chemical Society

3930 J . Org. Chem., Vol. 61, No. 11, 1996
Notes
Ta ble 1. Con ver sion of P ep tid es to Isocya n a tes a n d
purified ureas and thioureas 3 formed from the corre-
sponding peptide hydrochlorides.
Isoth iocya n a tes
The peptide isocyanates are moderately stable to short-
term storage at ambient temperature. Valylalanine
methyl ester isocyanate (2b) underwent ca. 50% decom-
position upon storage, either neat or as a solution in
CDCl3, for 1 week at 20 °C. The predominant decompo-
sition product was identified as hydantoin 4b (vide infra).
Under similar conditions, phenylalanylleucine methyl
ester isocyanate (2a ) underwent ca. 10% decomposition.
The instability of these compounds contrasts the stability
of amino acid ester isocyanates3 (e.g., 2i), which were
found to be indefinitely stable if protected from moisture
in airtight vials.
Discu ssion
Two pathways were identified that diminished the
yields of the peptide isocyanates and resulted in the
formation of side products: hydantoin formation and
N-acylation of amide groups by phosgene. Valylalanine
methyl ester hydrochloride, leucylglycine methyl ester
hydrochloride, and valyl-S-methylcystylalanine afforded
ca. 20% of hydantoins 4b, 4d , and 4f in addition to
isocyanates 2b, 2d , and 2f (Table 1, entries 2, 4, and 6).
At present, it is not clear why these cases form hydantoin
and others do not. In optimizing the reaction conditions
to minimize hydantoin formation, the effect of a number
of factors upon the amount of hydantoin formed were
investigated, including the number of equivalents of
phosgene added, the volumes of methylene chloride and
sodium bicarbonate solution used, the temperature (0 °C
vs ambient), the reaction time, the rate of stirring, and
the manner of addition of the phosgene. No definite
correlations were observed, although higher tempera-
tures, excessive stirring rates, and stirring while the
phosgene is added appeared to result in slight increases
in the amount of hydantoin formed. Eventually, we chose
to standardize the conditions used for generation of the
isocyanates by using 2.0 equiv of phosgene, adding the
phosgene in one portion to an ice-cooled solution of
peptide hydrochloride in equal volumes of methylene
chloride and saturated aqueous sodium bicarbonate
without stirring, and then stirring at 600 rpm with a
mechanical stirrer for 10 min (reaction conditions A,
Table 1). Although these conditions gave consistent and
reproducible yields of isocyanates and side products, we
have also successfully generated isocyanates with mag-
netic stirring and with smaller or larger quantities of
phosgene with only minor variations in results.
a
Reaction conditions: A, 2.0 equiv of COCl2 solution in toluene;
B, 1.1 equiv of COCl2 solution in toluene; C, 2.0 equiv of CSCl2;
D, 0.7 equiv of Cl3COCO2CCl3 (triphosgene). Yields are based
b
upon quantity of analytically pure ureas 3a -i and thioureas 3j-k
obtained upon trapping of the unpurified products with N-
ethylaniline. (See text for details).
of phosgene. Reduction of the amount of phosgene to 1.1
equiv helped alleviate this problem, affording the desired
isocyanate 2g in 49% yield (Table 1, entry 7). Leucine
butylamide 1h formed the corresponding isocyanate in
71% yield and generated only 16% of N-acylation product
5h when 2.0 equiv of phosgene was used (Table 1, entry
8). No N-acylation product was detected upon treatment
Peptides bearing sterically unhindered amide groups
formed N-acylation side products in addition to the
desired isocyanates. Phenylalanine methylamide 1g was
the worst offender, forming N-acylated isocyanate 5g as
the predominant product upon treatment with 2.0 equiv

Notes
J . Org. Chem., Vol. 61, No. 11, 1996 3931
of leucylglycine methyl ester 1d with 2.0 equiv of phos-
gene (Table 1, entry 4). Apparently, phosgene only
acylates unhindered amides (e.g., methyl and butyl)
under the reaction conditions used for generation of the
isocyanates.
continue to report the application of peptide isocyanates
to the synthesis of artificial â-sheets, and we will describe
any further improvements in peptide isocyanate forma-
tion therein.
Exp er im en ta l Section
Gen er a l P r oced u r es. Dipeptide methyl ester hydrochlo-
rides 1a -c were prepared by coupling of Cbz-protected amino
acids and amino acid methyl ester hydrochlorides (using either
DCC or the nitrophenyl ester method), followed by hydrogenoly-
sis in 2 M hydrochloric acid in methanol.13 Leucylglycine methyl
ester hydrochloride (1d ) was prepared by hydrogenolysis of Cbz-
protected leucylglycine methyl ester (Sigma) in 2 M hydrochloric
acid in methanol. Tripeptide methyl ester hydrochlorides 1e and
1f were prepared by solid-phase synthesis of the Cbz-protected
tripeptides on Merrifield resin, cleavage from the resin as the
methyl esters (treatment with methanol, dimethylformamide,
and triethylamine), and hydrogenolysis in 2 M hydrochloric acid
in methanol.14 Monopeptide amides 1g and 1h were prepared
by aminolysis of the corresponding amino acid ester hydrochlo-
rides with methylamine or butylamine in methanol. Phosgene
was obtained from Fluka as a 20% (1.93 M) solution in toluene.
Thiophosgene and triphosgene were obtained from Aldrich.
Methylene chloride used in trapping the peptide isocyanates and
isothiocyanates was dried by distillation from CaH2. N-Ethyl-
aniline was distilled under vacuum and stored under nitrogen.
High-resolution mass spectra (HRMS) were obtained by fast-
atom bombardment (FAB) of samples in m-nitrobenzyl alcohol
with Cs+ ions (σ ) (1 mmu). Combustion analyses were
performed by Desert Analytics, Tucson, AZ.
Amino acids and peptides can epimerize during activa-
tion and coupling. Although a number of mechanisms
for epimerization upon treatment with phosgene can also
be envisioned, we have not observed epimerization under
the reaction conditions described in this paper. In our
previous report of a method for preparing amino acid
ester isocyanates, we determined that the isocyanates
produced were enantiomerically pure (>99% ee) by
reacting the isocyanates with enantiomerically pure (R)-
1-phenylethylamine and looking for diastereomeric prod-
ucts by 1H NMR spectroscopy.3 In the current study,
epimerization would generate diastereomeric isocyanates
and diastereomeric urea trapping products. These iso-
1
mers would likely exhibit H NMR resonances that are
distinct from those of unepimerized products. Through-
out our studies of the side products produced, and in
using peptide isocyanates in the synthesis of artificial
â-sheets,1 we have not detected epimers. (Epimerization
was, however, detected upon preparation of large quanti-
ties of a peptide isocyanate for another project using
different reaction conditions.8) These experiences suggest
that peptide isocyanate and isothiocyanate formation
proceeds with little or no epimerization under the reac-
tion conditions described in this paper.
Gen er a l P r oced u r e for th e P r ep a r a tion of P ep tid e
Isocya n a tes a n d Isoth iocya n a tes. [CAUTION: PHOSGENE
AND THIOPHOSGENE ARE VOLATILE AND HIGHLY
TOXICsUSE HOOD.] A 25-mL, one-necked, round-bottomed
flask, equipped with a mechanical stirrer, was charged with 0.50
mmol of hydrochloride salt 1, 5 mL of CH2Cl2, and 5 mL of
saturated aqueous NaHCO3. The biphasic mixture was cooled
(9) For some related examples, see: (a) Nowick, J . S.; Powell, N.
A.; Martinez, E. J .; Smith, E. M.; Noronha, G., J . Org. Chem. 1992,
57, 3763. (b) Hagihara, M.; Anthony, N. J .; Stout, T. J .; Clardy, J .;
Schreiber, S. L. J . Am. Chem. Soc. 1992, 114, 6568. (c) Simon, R. J .;
Kania, R. S.; Zuckermann, R. N.; Huebner, V. D.; J ewell, D. A.;
Banville, S.; Ng, S.; Wang, L.; Rosenberg, S.; Marlowe, C. K.; Spell-
meyer, D. C.; Tan, R.; Frankel, A. D.; Santi, D. V.; Cohen, F. E.;
Bartlett, P. A. Proc. Natl. Acad. Sci. U.S.A. 1992, 89, 9367. (d)
Zuckermann, R. N.; Kerr, J . M.; Kent, S. B. H.; Moos, W. H. J . Am.
Chem. Soc. 1992, 114, 10646. (e) Bunin, B. A.; Ellman, J . A. J . Am.
Chem. Soc. 1992, 114, 10997. (f) Cho, C. Y.; Moran, E. J .; Cherry, S.
R.; Stephans, J . C.; Fodor, S. P. A.; Adams, C. L. Sundaram, A.; J acobs,
J . W.; Schultz, P. G. Science 1993, 261, 1303. (g) Hutchins, S. M.;
Chapman, K. T. Tetrahedron Lett. 1994, 35, 4055. (h) Carell, T.;
Wintner, E. A.; Bashir-Hashemi, A.; Rebek, J ., J r. Angew. Chem., Int.
Ed. Engl. 1994, 33, 2059. (i) Carell, T.; Wintner, E. A.; Rebek, J ., J r.
Angew. Chem., Int. Ed. Engl. 1994, 33, 2061. (j) Bunin, B. A.; Plunkett,
M. J .; Ellman, J . A. Proc. Natl. Acad. Sci. U.S.A. 1994, 91, 4708. (k)
Virgilio, A. A.; Ellman, J . A. J . Am. Chem. Soc. 1994, 116, 11580. (l)
Nowick, J . S.; Abdi, M.; Bellamo, K. A.; Love, J . A.; Martinez, E. J .;
Noronha, G.; Smith, E. M.; Ziller, J . W. J . Am. Chem. Soc. 1995, 117,
89. (m) Hutchins, S. M.; Chapman, K. T. Tetrahedron Lett. 1995, 36,
2583. (n) Carell, T.; Wintner, E. A.; Sutherland, A. J .; Rebek, J ., J r.;
Dunayevskiy, Y. M.; Vouros, P. Chem. Biol. 1995, 2, 171. (o) Moran,
E. J .; Wilson, T. E.; Cho, C. Y.; Cherry, S. R.; Schultz, P. G. Biopolymers
1995, 37, 213. (p) Burgess, K.; Linthicum, D. S.; Shin, H. W. Angew.
Chem., Int. Ed. Engl. 1995, 34, 907. (q) Nowick, J . S.; Mahrus, S.;
Smith, E. M.; Ziller, J . W. J . Am. Chem. Soc. 1996, 118, 1066.
(10) (a) Losse, G.; Go¨dicke, W. Chem. Ber. 1967, 100, 3314. (b) Ram,
R. N.; Ashare, R.; Mukerjee, A. K. Chem. Ind. 1983, 569. (c) Blag-
brough, I. S.; Mackenzie, N. E.; Ortiz, C.; Scott, A. I. Tetrahedron Lett.
1986, 27, 1251.
Con clu sion s
These studies offer a practical method for the prepara-
tion of peptide isocyanates and peptide isothiocyanates,
two classes of compounds that have not previously been
reported. The yields of these compounds are generally
good, in spite of the side reactions that can occur.
Because the isocyanates cannot easily be purified, it
would still be desirable to improve this method to allow
the generation of isocyanates without side products. The
clean reactions of isocyanates and isothiocyanates with
nucleophiles, such as amines and alcohols, render these
compounds particularly attractive as synthetic building
blocks for combinatorial chemistry and the creation of
unnatural biopolymers, two areas that are undergoing
rapid developments.9 Other reactions of potential inter-
est include condensation with carboxylic acids to form
amides2,10 and cycloaddition reactions to form â-lactams11
and other heterocycles.12 In our own laboratories, we will
(8) Epimerization was detected when multigram quantities of
phenylalanylphenyalanine methyl ester hydrochloride were converted
to the corresponding isocyanate by treatment with 13 equiv of phosgene
in an ice-cooled mixture of aqueous sodium bicarbonate and methylene
chloride. The phenylalanylphenylalanine methyl ester isocyanate was
trapped with an amine, the products of reaction were separated by
preparative HPLC, and epimeric ureas were identified by 1H NMR
spectroscopy. Subsequent studies involving the preparation of this
isocyanate have not resulted in the detection of epimers. When this
isocyanate was prepared under the reaction conditions described in
this paper, digested with HCl to form phenylalanine hydrochloride,
and then analyzed by polarimetry, no loss of optical activity was
detected. Further studies are required to corroborate this anomalous
result and to determine whether factors such as large excesses of
phosgene or large-scale reactions lead to epimerization.
(11) (a) Perelman, M.; Mizsak, S. A. J . Am. Chem. Soc. 1962, 84,
4988. (b) Opitz, G.; Koch, J . Angew. Chem., Int. Ed. Engl. 1963, 2,
152.
(12) Ulrich, H. Cycloaddition Reactions of Heterocumulenes; Aca-
demic: New York, 1967.
(13) (a) Bodansky, M. Principles of Peptide Synthesis, 2nd ed.;
Springer-Verlag: New York, 1993. (b) Bodansky, M.; Bodansky, A. The
Practice of Peptide Synthesis, 2nd ed.; Springer-Verlag: New York,
1994.
(14) Stewart, J . M.; Young, J . D. Solid Phase Peptide Synthesis, 2nd
ed.; Pierce Chemical Company: Rockford, IL, 1984.

3932 J . Org. Chem., Vol. 61, No. 11, 1996
Notes
to 0 °C in an ice bath while stirring for ca. 10 min at 600 rpm.
Stirring was stopped, the layers were allowed to separate, and
phosgene (520 µL of a 1.93 M solution in toluene, 1.00 mmol),
thiophosgene (76 µL, 1.00 mmol), or triphosgene (a solution of
100 mg in 1 mL of methylene chloride, 0.34 mmol) was added
in a single portion via syringe to the lower (organic) phase.
Stirring was resumed immediately, and the ice-cooled reaction
mixture was stirred for 10 min at 600 rpm. The layers were
then separated, the aqueous phase was extracted with three
5-mL portions of CH2Cl2, and the combined organic layers were
dried over Na2SO4, filtered, and concentrated by rotary evapora-
tion. The crude isocyanate or isothiocyanate 2 was further freed
of solvent under vacuum (0.1 mmHg), weighed, characterized
by 1H NMR and IR spectroscopy, and then trapped with
N-ethylaniline as described in the following procedure.
3 H), 2.30-2.24 (m, 1 H), 1.62 (d, J ) 7.4 Hz, 3 H), 1.05 (d, J )
7.0 Hz, 3 H), 0.96 (d, J ) 6.8 Hz, 3 H); HRMS m/ e for
C
10H17N2O4 (M + H)+ calcd 229.1188, found 229.1195.
Reaction of 102 mg of 2b with N-ethylaniline (125 µL, 0.99
mmol) yielded 119 mg (69% from 1b) of 3b as a colorless viscous
oil: IR (CHCl3) 3427, 1741, 1672, 1651 cm-1; 1H NMR (500 MHz,
CDCl3) δ 7.45 (t, J ) 7.7 Hz, 2 H), 7.34 (t, J ) 7.4 Hz, 1 H), 7.29
(d, J ) 7.3 Hz, 2 H), 7.04 (d, J ) 7.0 Hz, 1 H), 4.66 (d, J ) 8.6
Hz, 1 H), 4.51 (quintet, J ) 7.2 Hz, 1 H), 4.22 (dd, J ) 8.5, 6.5
Hz, 1 H), 3.75-3.70 (m, 2 H), 3.72 (s, 3 H), 1.98 (octet, J ) 6.7
Hz, 1 H), 1.39 (d, J ) 7.2 Hz, 3 H), 1.10 (t, J ) 7.1 Hz, 3 H),
0.92 (d, J ) 6.8 Hz, 3 H), 0.77 (d, J ) 6.8 Hz, 3 H); HRMS m/ e
for C18H28N3O4 (M + H)+ calcd 350.2080, found 350.2085. Anal.
Calcd for C18H27N3O4: C, 61.87; H, 7.79; N, 12.03. Found: C,
61.65; H, 7.80; N, 11.75.
Gen er a l P r oced u r e for Tr a p p in g P ep tid e Isocya n a tes
a n d Isoth iocya n a tes w ith N-Eth yla n ilin e. A 25-mL, one-
necked, round-bottomed flask fitted with a nitrogen inlet adapter
and containing peptide isocyanate or isothiocyanate 2 from the
above procedure was evacuated and filled with nitrogen three
times. Methylene chloride (5 mL) was added, the isocyanate or
isothiocyanate was allowed to dissolve, and N-ethylaniline (125
µL, 0.99 mmol) was added in a single portion. After 20 h, the
reaction mixture was concentrated by rotary evaporation, and
the residue was purified by column chromatography on silica
gel (EtOAc-CH2Cl2 or EtOAc-CHCl3) or recrystallization
(EtOAc-hexanes) to afford pure urea or thiourea 3.
Va lyla la n in e Meth yl Ester Isocya n a te (2b) a n d Ur ea 3b
(P h N(E t )CONH -(S)-CH (i-P r )CONH -(S)-CH (CH 3)CO2CH 3)
(Usin g Tr ip h osgen e). Reaction of valylalanine methyl ester
hydrochloride (1b, 123 mg, 0.515 mmol) with triphosgene (a
solution of 112 mg in 1 mL of methylene chloride, 0.38 mmol)
yielded 112 mg (95%) of crude isocyanate 2b as a colorless oily
solid. 1H NMR analysis of this product revealed 8% of hydantoin
4b and 15% of an additional impurity. Additional impurity
(identified by examination of the spectra of the crude product):
1H NMR (500 MHz, CDCl3) δ 3.76 (s, 3 H), 1.01 (d, J ) 6.6 Hz,
3 H), 1.00 (d, J ) 6.7 Hz, 3 H).
Reaction of 112 mg of 2b with N-ethylaniline (125 µL, 0.99
mmol) yielded 120 mg (67% from 1b) of 3b as a colorless viscous
oil.
P h en yla la n ylleu cin e Meth yl Ester Isocya n a te (2a ) a n d
Ur ea 3a (P h N(Et)CONH-(S)-CH(CH2P h )CONH-(S)-CH(i-
Bu )CO2CH3). Reaction of phenylalanylleucine methyl ester
hydrochloride (1a , 164 mg, 0.499 mmol) with phosgene (520 µL
of a 1.93 M solution in toluene, 1.00 mmol) yielded 159 mg
(100%) of crude isocyanate 2a as a white solid: IR (CHCl3) 3417,
Ala n ylp h en yla la n in e Meth yl Ester Isocya n a te (2c) a n d
Ur ea 3c (P h N(Et)CONH-(S)-CH(CH3)CONH-(S)-CH(CH2P h )-
CO2CH3). Reaction of alanylphenylalanine methyl ester hy-
drochloride (1c, 143 mg, 0.499 mmol) with phosgene (520 µL of
a 1.93 M solution in toluene, 1.00 mmol) yielded 133 mg (97%)
of crude isocyanate 2c as a pale tan oil: IR (CHCl3) 3415, 2268,
2241, 1743, 1686 cm-1; 1H NMR (500 MHz, CDCl3) δ 7.32-7.26
(m, 3 H), 7.09 (d, J ) 7.1 Hz, 2 H), 6.64 (d, J ) 7.3 Hz, 1 H),
4.85 (appar q, J ) 6.6 Hz, 1 H), 4.11 (q, J ) 7.0 Hz, 1 H), 3.75
(s, 3 H), 3.19 (dd, ABX pattern, J AB ) 13.9 Hz, J AX ) 5.7 Hz, 1
H), 3.09 (dd, ABX pattern, J AB ) 13.9 Hz, J BX ) 6.3 Hz, 1 H),
1.48 (d, J ) 6.9 Hz, 3 H).
Reaction of 133 mg of 2c with N-ethylaniline (125 µL, 0.99
mmol) yielded 157 mg (79% from 1c) of 3c as a colorless viscous
oil: IR (CHCl3) 3427, 1738, 1676, 1645 cm-1; 1H NMR (300 MHz,
CDCl3) δ 7.46-7.12 (m, 11 H), 4.79 (appar q, J ) 6.8 Hz, 1 H)
4.58 (d, J ) 7.5 Hz, 1 H), 4.45 (quintet, J ) 7.2 Hz, 1 H), 3.78-
3.68 (m, 2 H), 3.68 (s, 3 H), 3.14 (dd, ABX pattern, J AB ) 13.9
Hz, J AX ) 5.8 Hz, 1 H), 3.03 (dd, ABX pattern, J AB ) 13.9 Hz,
J BX ) 7.0 Hz, 1 H), 1.20 (d, J ) 7.0 Hz, 3 H), 1.09 (t, J ) 7.1 Hz,
3 H); HRMS m/ e for C22H28N3O4 (M + H)+ calcd 398.2080, found
398.2086. An analytically pure sample was obtained by repeated
chromatography: Anal. Calcd for C22H27N3O4: C, 66.48; H, 6.85;
N, 10.57. Found: C, 66.38; H, 6.71; N, 10.64.
1
2266, 1741, 1684 cm-1; H NMR (500 MHz, CDCl3) δ 7.34 (t, J
) 7.2 Hz, 2 H), 7.29 (t, J ) 6.9 Hz, 1 H), 7.25 (d, J ) 8.0 Hz, 2
H), 6.62 (d, J ) 8.1 Hz, 1 H), 4.61 (td, J ) 8.4, 5.3 Hz, 1 H), 4.39
(dd, J ) 8.5, 4.2 Hz, 1 H), 3.73 (s, 3 H), 3.35 (dd, ABX pattern,
J AB ) 13.9 Hz, J AX ) 4.1 Hz, 1 H), 3.01 (dd, ABX pattern, J AB
)
13.9 Hz, J BX ) 8.5 Hz, 1 H), 1.65-1.60 (m, 1 H), 1.55-1.48 (m,
2 H), 0.923 (d, J ) 6.1 Hz, 3 H), 0.917 (d, J ) 6.1 Hz, 3 H).
Reaction of 159 mg of 2a with N-ethylaniline (125 µL, 0.99
mmol) yielded 204 mg (93% from 1a ) of 3a as white crystals
from ethyl acetate-hexanes: mp 98-100 °C; IR (CHCl3) 3427,
1741, 1674, 1643 cm-1; 1H NMR (500 MHz, CDCl3) δ 7.37-7.30
(m, 3 H), 7.23-7.20 (m, 3 H) 7.04-7.02 (m, 2 H), 6.98 (d, J )
7.1 Hz, 2 H), 6.91 (d, J ) 7.6 Hz, 1 H), 4.58 (q, J ) 7.2 Hz, 1 H),
4.53-4.49 (m, 1 H), 4.45 (d, J ) 7.4 Hz, 1 H), 3.71 (s, 3 H), 3.73-
3.63 (m, 2 H), 3.02 (dd, ABX pattern, J AB ) 14.0 Hz, J AX ) 6.0
Hz, 1 H), 2.90 (dd, ABX pattern, J AB ) 14.0 Hz, J BX ) 8.0 Hz, 1
H), 1.62-1.50 (m, 3 H), 1.06 (t, J ) 7.0 Hz, 3 H), 0.90 (d, J ) 5.9
Hz, 3 H), 0.88 (d, J ) 5.9 Hz, 3 H); HRMS m/ e for C25H34N3O4
(M + H)+ calcd 440.2549, found 440.2547. Anal. Calcd for
C
25H33N3O4: C, 68.31; H, 7.57; N, 9.56. Found: C, 68.47; H,
7.48; N, 9.53.
Leu cylglycin e Meth yl Ester Isocya n a te (2d ) a n d Ur ea
3d (P h N(Et)CONH-(S)-CH(i-Bu )CONHCH2CO2CH3). Reac-
tion of leucylglycine methyl ester hydrochloride (1d , 119 mg,
0.499 mmol) with phosgene (520 µL of a 1.93 M solution in
toluene, 1.00 mmol) yielded 105 mg (92%) of crude isocyanate
2d as a colorless oil. 1H NMR analysis of this product revealed
25% of hydantoin 4d as an impurity. 2d : IR (CHCl3) 3427, 2268,
1749, 1687 cm-1; 1H NMR (500 MHz, CDCl3) δ 6.81 (br s, 1 H),
4.17 (dd, J ) 8.3, 5.7 Hz, 1 H), 4.07 (d, J ) 5.2 Hz, 2 H), 3.78 (s,
3 H), 1.87-1.80 (m, 1 H), 1.77-1.72 (m, 2 H), 0.99 (d, J ) 6.6
Hz, 3 H), 0.97 (d, J ) 6.5 Hz, 3 H). 4d (identified by examination
P h en yla la n ylleu cin e Meth yl Ester Isocya n a te (2a ) a n d
Ur ea 3a (P h N(Et)CONH-(S)-CH(CH2P h )CONH-(S)-CH(i-
Bu )CO2CH3) (Usin g Tr ip h osgen e). Reaction of phenylalanyl-
leucine methyl ester hydrochloride (1a , 164 mg, 0.499 mmol)
with triphosgene (a solution of 104 mg in 1 mL of methylene
chloride, 0.35 mmol) yielded 162 mg (102%) of crude isocyanate
2a as a colorless viscous oil that crystallized upon standing.
Reaction of 162 mg of 2a with N-ethylaniline (125 µL, 0.99
mmol) yielded 199 mg (91% from 1a ) of 3a as a colorless glassy
solid.
of the spectra of the crude product): IR (CHCl3) 1782, 1726 cm-1
;
Va lyla la n in e Meth yl Ester Isocya n a te (2b) a n d Ur ea 3b
(P h N(Et)CONH-(S)-CH(i-P r )CONH-(S)-CH(CH3)CO2CH3).
Reaction of valylalanine methyl ester hydrochloride (1b, 118 mg,
0.494 mmol) with phosgene (520 µL of a 1.93 M solution in
toluene, 1.00 mmol) yielded 102 mg (90%) of crude isocyanate
2b as a colorless oil. 1H NMR analysis of this product revealed
16% of hydantoin 4b as an impurity. 2b: IR (CHCl3) 3415, 2262,
1743, 1684 cm-1; 1H NMR (500 MHz, CDCl3) δ 6.80 (br s, 1 H),
4.61 (appar q, J ) 6.9 Hz, 1 H), 4.04 (br d, J ) 1.7 Hz, 1 H),
3.78 (s, 3 H), 2.42-2.35 (m, 1 H), 1.44 (d, J ) 7.1 Hz, 3 H), 1.08
(d, J ) 6.8 Hz, 3 H), 0.91 (d, J ) 6.7 Hz, 3 H). 4b (isolated by
column chromatography of crude 3b): IR (CHCl3) 3460, 1780,
1H NMR (500 MHz, CDCl3, partial data) δ 6.32 (br s, 1 H), 4.25
(s, 2 H), 3.76 (s, 3 H), 1.62-1.58 (m, 2 H), 1.00 (d, J ) 6.6 Hz,
3 H).
Reaction of 105 mg of 2d with N-ethylaniline (125 µL, 0.99
mmol) yielded 87 mg (50% from 1d ) of 3d as a pale yellow oily
solid: IR (CHCl3) 3429, 1745, 1674, 1651 cm-1 1H NMR (300
;
MHz, CDCl3) δ 7.45 (t, J ) 7.4 Hz, 2 H), 7.36 (d, J ) 7.3 Hz, 1
H), 7.32-7.23 (m, 3 H), 4.49-4.38 (m, 2 H), 4.06 (dd, ABX
pattern, J AB ) 18.2 Hz, J AX ) 5.5 Hz, 1 H), 3.95 (dd, ABX
pattern, J AB ) 18.1 Hz, J BX ) 5.4 Hz, 1 H), 3.74 (s, 3 H), 3.77-
3.67 (m, 2 H), 1.64-1.50 (m, 2 H), 1.38-1.22 (m, 1 H), 1.10 (t, J
) 7.1 Hz, 3 H), 0.90 (d, J ) 4.7 Hz, 3 H), 0.88 (d, J ) 5.2 Hz, 3
H); HRMS m/ e for C18H28N3O4 (M + H)+ calcd 350.2080, found
1747, 1720 cm-1
4.75 (q, J ) 7.3 Hz, 1 H), 3.97 (dd, J ) 3.4, 0.9 Hz, 1 H), 3.74 (s,
;
1H NMR (500 MHz, CDCl3) δ 5.46 (s, 1 H),

Notes
J . Org. Chem., Vol. 61, No. 11, 1996 3933
350.2074. Anal. Calcd for C18H27N3O4: C, 61.87; H, 7.79; N,
12.03. Found: C, 61.47; H, 7.63; N, 11.79.
impurities comprising ca. 25%. 2g: IR (CHCl3) 3444, 2268, 1677
cm-1; 1H NMR (500 MHz, CDCl3) δ 7.36-7.22 (m, 5 H), 6.29 (br
s, 1 H), 4.33 (dd, J ) 8.6, 4.4 Hz, 1 H), 3.37 (dd, ABX pattern,
P h en yla la n ylisoleu cylleu cin e Meth yl Ester Isocya n a te
(2e) a n d Ur ea 3e (P h N(Et)CONH-(S)-CH(CH2P h )CONH-
(S)-CH((S)-s-Bu )CONH(S)-CH(i-Bu )CO2CH3). Reaction of
phenylalanylisoleucylleucine methyl ester hydrochloride (1e, 217
mg, 0.491 mmol) with phosgene (520 µL of a 1.93 M solution in
toluene, 1.00 mmol) in 10 mL of CH2Cl2 and 10 mL of saturated
J AB ) 13.8 Hz, J AX ) 4.1 Hz, 1 H), 2.97 (dd, ABX pattern, J AB
)
13.7 Hz, J BX ) 9.0 Hz, 1 H), 2.83 (d, J ) 4.6 Hz, 3 H). 5g
(identified by examination of the spectra of the crude product):
1
IR (CHCl3) 2241, 1751, 1718 cm-1; H NMR (300 MHz, CDCl3)
δ 7.40-7.16 (m, 5 H), 5.11 (dd, J ) 8.7, 3.5 Hz, 1 H), 3.47 (s, 3
H), 3.21 (dd, ABX pattern, J AB ) 13.6 Hz, J AX ) 3.4 Hz, 1 H),
2.82 (dd, ABX pattern, J AB ) 13.6 Hz, J BX ) 8.7 Hz, 1 H).16
Reaction of 96 mg of 2g with N-ethylaniline (125 µL, 0.99
mmol) yielded 79 mg (49% from 1g) of 3g as a colorless oil after
purification by preparative reversed-phase HPLC (C18 column,
80:20 methanol-water): IR (CHCl3) 3444, 1668 cm-1; 1H NMR
(500 MHz, CDCl3) δ 7.38-7.31 (m, 3 H), 7.25-7.20 (m, 3 H),
7.04-6.90 (m, 4 H), 6.63 (appar q, J ) 4.8 Hz, 1 H), 4.58-4.50
(m, 2 H), 3.73-3.65 (m, 1 H), 3.65-3.57 (m, 1 H), 3.00 (dd, ABX
pattern, J AB ) 13.9 Hz, J AX ) 5.9 Hz, 1 H), 2.88 (dd, ABX
pattern, J AB ) 13.9 Hz, J BX ) 7.5 Hz, 1 H), 2.71 (d, J ) 4.8 Hz,
3 H), 1.03 (t, J ) 7.1 Hz, 3 H); HRMS m/ e for C19H24N3O2 (M +
15
aqueous NaHCO3 yielded 191 mg (90%) of crude isocyanate
2e as a white solid: IR (CHCl3) 3413, 2266, 2240 (sh), 1741,
1672 cm-1 1H NMR (500 MHz, CDCl3) δ 7.34-7.27 (m, 3 H),
;
7.23 (d, J ) 7.1 Hz, 2 H), 7.18 (d, J ) 8.7 Hz, 1 H), 6.80 (d, J )
7.5 Hz, 1 H), 4.56 (appar q, J ) 8.2 Hz, 1 H), 4.40 (t, J ) 8.2 Hz,
1 H), 4.35 (dd, J ) 8.7, 3.7 Hz, 1 H), 3.73 (s, 3 H), 3.30 (dd, ABX
pattern, J AB ) 13.8 Hz, J AX ) 3.6 Hz, 1 H), 2.96 (dd, ABX
pattern, J AB ) 13.8 Hz, J BX ) 8.7 Hz, 1 H), 1.87-1.77 (m, 1 H),
1.71-1.60 (m, 2 H), 1.62-1.54 (m, 1 H), 1.52-1.42 (m, 1 H),
1.13-1.02 (m, 1 H), 0.93-0.88 (m, 12 H).
Reaction of 191 mg of 2e with N-ethylaniline (125 µL, 0.99
mmol) yielded 221 mg (81% from 1e) of 3e as white crystals from
ethyl acetate-hexanes: mp 118-120 °C; IR (CHCl3) 3423, 3307,
1741, 1664 cm-1; 1H NMR (500 MHz, CDCl3) δ 7.38-7.32 (m, 3
H), 7.22-7.17 (m, 3 H), 6.97-6.93 (m, 4 H), 6.83 (d, J ) 8.1 Hz,
1 H), 6.67 (d, J ) 8.1 Hz, 1 H), 4.60-4.56 (m, 1 H), 4.46 (dt, J
) 8.1, 5.9 Hz, 1 H), 4.38 (d, J ) 6.1 Hz, 1 H), 4.29 (dd, J ) 8.6,
5.4 Hz, 1 H), 3.74-3.67 (m, 1 H), 3.72 (s, 3 H), 3.64-3.57 (m, 1
H), 3.06 (dd, ABX pattern, J AB ) 14.1 Hz, J AX ) 5.5 Hz, 1 H),
2.88 (dd, ABX pattern, J AB ) 14.1 Hz, J BX ) 8.4 Hz, 1 H), 2.07-
1.97 (m, 1 H), 1.68-1.60 (m, 3 H), 1.45-1.36 (m, 1 H), 1.09-
1.01 (m, 1 H), 1.04 (t, J ) 7.1 Hz, 3 H), 0.96 (d, J ) 6.1 Hz, 3 H),
0.93 (d, J ) 6.1 Hz, 3 H), 0.90 (t, J ) 7.4 Hz, 3 H), 0.86 (d, J )
6.8 Hz, 3 H); HRMS m/ e for C31H45N4O5 (M + H)+ calcd
553.3390, found 553.3399. Anal. Calcd for C31H44N4O5: C,
67.37; H, 8.02; N, 10.14. Found: C, 67.08; H, 7.95; N, 9.95.
Va lyl-S-m eth ylcystyla la n in e Meth yl Ester Isocya n a te
(2f) a n d Ur ea 3f (P h N(Et)CONH-(S)-CH(i-P r )CONH-(S)-
CH(CH2SCH3)CONH-(S)-CH(CH3)CO2CH3). Reaction of va-
lyl-S-methylcystylalanine methyl ester hydrochloride (1f, 178
mg, 0.500 mmol) with phosgene (520 µL of a 1.93 M solution in
toluene, 1.00 mmol) yielded 159 mg (92%) of crude isocyanate
2f as a pale yellow solid. 1H NMR analysis of this product
revealed 19% of hydantoin 4f as an impurity. 2f: IR (CHCl3)
3405, 3320, 2262, 1741, 1670 cm-1; 1H NMR (300 MHz, CDCl3)
δ 7.30 (d, J ) 8.6 Hz, 1 H), 7.24 (d, J ) 7.4 Hz, 1 H), 4.63 (q, J
) 7.0 Hz, 1 H), 4.53 (quintet, J ) 7.2 Hz, 1 H), 4.06 (d, J ) 3.4
Hz, 1 H), 3.76 (s, 3 H), 2.95-2.80 (m, 2 H), 2.40-2.25 (m, 1 H),
2.19 (s, 3 H), 1.43 (d, J ) 7.2 Hz, 3 H), 1.07 (d, J ) 6.8 Hz, 3 H),
0.91 (d, J ) 6.7 Hz, 3 H). 4f (identified by examination of the
H)+ calcd 326.1868, found 326.1874. Anal. Calcd for C19H23
-
N3O2: C, 70.13; H, 7.12; N, 12.91. Found: C, 70.26; H, 7.07; N,
12.75.
Leu cin e Bu tyla m id e Isocya n a te (2h ) a n d Ur ea 3h (P h N-
(Et)CONH-(S)-CH(i-Bu )CONHBu ). Reaction of leucine bu-
tylamide hydrochloride (1h , 110 mg, 0.494 mmol) with phosgene
(520 µL of a 1.93 M solution in toluene, 1.00 mmol) yielded 106
mg (101%) of crude isocyanate 2h as a colorless oil. 1H NMR
analysis of this product revealed 16 mol % of N-acylation product
1
5h as an impurity. 2h : IR (CHCl3) 3435, 2266, 1676 cm-1; H
NMR (500 MHz, CDCl3) δ: 6.28 (br s, 1 H), 4.09 (dd, J ) 10.1,
3.9 Hz, 1 H), 3.28 (q, J ) 6.8 Hz, 2 H), 1.87-1.65 (m, 3 H), 1.55-
1.48 (m, 2 H), 1.40-1.32 (m, 2 H), 0.98 (d, J ) 8.9 Hz, 3 H), 0.96
(d, J ) 8.9 Hz, 3 H), 0.94 (t, J ) 7.3 Hz, 3 H). 5h (identified by
examination of the spectra of the crude product): 1H NMR (500
MHz, CDCl3, partial data) δ 4.88 (dd, J ) 9.8, 3.6 Hz, 1 H), 3.97
(ddd, J ) 15.8, 9.9, 5.7 Hz, 1 H), 3.92 (ddd, J ) 15.7, 10.1, 5.8
Hz, 1 H).
Reaction of 104 mg of 2h with N-ethylaniline (125 µL, 0.99
mmol) yielded 116 mg (71% from 1h ) of 3h as a colorless oil: IR
(CHCl3) 3435, 1666, 1645 cm-1 1H NMR (500 MHz, CDCl3) δ
;
7.44 (t, J ) 7.6 Hz, 2 H), 7.34 (t, J ) 7.4 Hz, 1 H), 7.22 (d, J )
7.7 Hz, 2 H), 6.72 (br s, 1 H), 4.47 (d, J ) 8.1 Hz, 1 H), 4.30 (td,
J ) 8.3, 5.8 Hz, 1 H), 3.77-3.65 (m, 2 H), 3.28-3.15 (m, 2 H),
1.56-1.44 (m, 4 H), 1.38-1.28 (m, 3 H), 1.10 (t, J ) 7.1 Hz, 3
H), 0.92 (t, J ) 7.3 Hz, 3 H), 0.89 (d, J ) 6.5 Hz, 3 H), 0.87 (d,
J ) 6.6 Hz, 3 H); HRMS m/ e for C19H32N3O2 (M + H)+, calcd
334.2494, found 334.2495. Anal. Calcd for C19H31N3O2: C,
68.43; H, 9.37; N, 12.60. Found: C, 68.22; H, 9.37; N, 12.46.
P h en yla la n in e Meth yl Ester Isocya n a te (2i)3 a n d Ur ea
3i9l (P h N(Et)CONH-(S)-CH(CH2P h )CO2CH3). Reaction of
phenylalanine methyl ester hydrochloride (1i, 111 mg, 0.515
mmol) with phosgene (520 µL of a 1.93 M solution in toluene,
1.00 mmol) yielded 103 mg (98%) of crude isocyanate 2i as a
colorless oil.3
1
spectra of the crude product): IR (CHCl3) 1770, 1718 cm-1; H
NMR (300 MHz, CDCl3, partial data) δ 6.97 (s, 1 H), 4.76 (dd, J
) 10.5, 5.6 Hz, 1 H), 3.74 (s, 3 H), 2.11 (s, 3 H), 1.40 (d, J ) 7.0
Hz, 3 H), 1.09 (d, J ) 6.8 Hz, 3 H), 0.99 (d, J ) 6.8 Hz, 3 H).
Reaction of 159 mg of 2f with N-ethylaniline (125 µL, 0.99
mmol) yielded 161 mg (69% from 1f) of 3f as a colorless glassy
solid: IR (CHCl3) 3415, 3305, 1741, 1659 cm-1 1H NMR (500
;
MHz, CDCl3) δ 7.52-7.46 (m, 3 H), 7.40 (t, J ) 7.4 Hz, 1 H),
7.30 (d, J ) 7.4 Hz, 2 H), 7.03 (d, J ) 7.9 Hz, 1 H), 4.60 (dt, J
) 7.7, 5.0 Hz, 1 H), 4.55 (quintet, J ) 7.3 Hz, 1 H), 4.47 (d, J )
6.1 Hz, 1 H), 4.06 (appar t, J ) 5.8 Hz, 1 H), 3.83-3.76 (m, 1
H), 3.74 (s, 3 H), 3.72-3.64 (m, 1 H), 3.06 (dd, ABX pattern, J AB
Reaction of 103 mg of 2i with N-ethylaniline (125 µL, 0.99
mmol) yielded 156 mg (93% from 1i) of 3i as a colorless oil.9l
P h en yla la n ylleu cin e Meth yl Ester Isoth iocya n a te (2j)
a n d Th iou r ea 3j (P h N(Et)CSNH(S)-CH(CH2P h )CONH(S)-
CH(i-Bu )CO2CH3). Reaction of phenylalanylleucine methyl
ester hydrochloride (1a , 165 mg, 0.502 mmol) with thiophosgene
(76 µL, 1.00 mmol) yielded 160 mg (95%) of crude isothiocyanate
) 14.0 Hz, J AX ) 5.4 Hz, 1 H), 2.85 (dd, ABX pattern, J AB
)
14.0 Hz, J BX )6.2 Hz, 1 H), 2.20-2.13 (m, 1 H), 2.17 (s, 3 H),
1.46 (d, J ) 7.2 Hz, 3 H), 1.12 (t, J ) 7.1 Hz, 3 H), 0.86 (d, J )
6.8 Hz, 3 H), 0.67 (d, J ) 6.7 Hz, 3 H); HRMS m/ e for
2j as a pale tan oil: IR (CHCl3) 3417, 2056, 1741, 1684 cm-1
;
1H NMR (300 MHz, CDCl3) δ 7.37-7.22 (m, 5 H), 6.58 (d, J )
8.3 Hz, 1 H), 4.64 (dd, J ) 7.2, 4.2 Hz, 1 H), 4.58 (td, J ) 8.6,
5.5 Hz, 1 H), 3.72 (s, 3 H), 3.35 (dd, ABX pattern, J AB ) 13.8,
J AX ) 4.2 Hz, 1 H), 3.19 (dd, ABX pattern, J AB ) 13.9, J BX ) 7.4
C
22H35N4O5S (M + H)+ calcd 467.2328, found 467.2330. An
analytically pure sample was obtained by repeated chroma-
tography: Anal. Calcd for C22H34N4O5S: C, 56.63; H, 7.34; N,
12.01. Found: C, 56.85; H, 7.30; N, 11.87.
P h en yla la n in e Meth yla m id e Isocya n a te (2g) a n d Ur ea
3g (P h N(Et)CONH-(S)-CH(CH2P h )CONHCH3). Reaction of
phenylalanine methylamide hydrochloride (1g, 107 mg, 0.498
mmol) with phosgene (275 µL of a 1.93 M solution in toluene,
0.531 mmol) yielded 96 mg (94%) of crude isocyanate 2g as a
colorless waxy solid. 1H NMR analysis of this product revealed
6 mol % of N-acylation product 5g and a number of other
(16) N-acylation product 5g was further characterized by trapping
with N-ethylaniline to form PhN(Et)CONHCH(CH2Ph)CON(Me)CON-
(Et)Ph, which was isolated by column chromatography: IR (CHCl3)
3435, 1716, 1674, 1657 (sh) cm-1; 1H NMR (500 MHz, CDCl3) δ 7.48-
7.19 (m, 11 H), 7.09 (d, J ) 7.0 Hz, 2 H), 6.97 (d, J ) 7.4 Hz, 2 H), 5.12
(br s, 1 H), 4.57 (d, J ) 8.5 Hz, 1 H), 4.05-3.85 (m, 1 H), 3.78-3.57
(m, 3 H), 3.40-3.25 (m, 1 H), 2.63 (br s, 3 H), 2.62-2.50 (m, 1 H), 1.28
(t, J ) 7.1 Hz, 3 H), 1.03 (t, J ) 7.1 Hz, 3 H); HRMS m/ e for
C28H33N4O3 (M + H)+, calcd 473.2552, found 473.2556. Anal Calcd for
C28H32N4O3: C, 71.16; H, 6.83; N, 11.86. Found: C, 70.80; H, 6.67; N,
11.69.
(15) A larger volume of methylene chloride was required to dissolve
this peptide.

3934 J . Org. Chem., Vol. 61, No. 11, 1996
Notes
Hz, 1 H), 1.65-1.35 (m, 3 H), 0.90 (d, J ) 6.3 Hz, 3 H), 0.88 (d,
J ) 6.4 Hz, 3 H).
solid: mp 98-100 °C; IR (CHCl3) 3426, 3388, 1741, 1678 cm-1
1H NMR (300 MHz, CDCl3) δ 7.52 (t, J ) 7.5 Hz, 2 H), 7.43 (t,
J ) 7.3 Hz, 1 H), 7.22 (d, J ) 7.3 Hz, 2 H), 6.62 (d, J ) 7.1 Hz,
1 H), 5.70 (d, J ) 8.2 Hz, 1 H), 4.91 (dd, J ) 8.1, 6.2 Hz, 1 H),
4.48 (quintet, J ) 7.2 Hz, 1 H), 4.24 (appar q, J ) 7.1 Hz, 2 H),
3.72 (s, 3 H), 2.16-1.98 (m, 1 H), 1.41 (d, J ) 7.2 Hz, 3 H), 1.20
(t, J ) 7.1 Hz, 3 H), 0.87 (d, J ) 6.8 Hz, 3 H), 0.79 (d, J ) 6.9
Hz, 3 H); HRMS m/ e for C18H28N3O3S (M + H)+ calcd 366.1851,
found 366.1853. Anal. Calcd for C18H27N3O3S: C, 59.15; H,
7.45; N, 11.50; S, 8.77. Found: C, 59.41; H, 7.29; N, 11.25; S,
8.92.
;
Reaction of 160 mg of 2j with N-ethylaniline (125 µL, 0.99
mmol) yielded 215 mg (94% from 1a ) of 3j as a pale yellow oil:
IR (CHCl3) 3415, 3381, 1741, 1678 cm-1 1H NMR (500 MHz,
;
CDCl3) δ 7.43-7.36 (m, 3 H), 7.25-7.19 (m, 3 H), 7.09 (dd, J )
7.5, 1.8 Hz, 2 H), 6.94 (br d, J ) 5.2 Hz, 2 H), 6.54 (d, J ) 7.8
Hz, 1 H), 5.58 (d, J ) 7.5 Hz, 1 H), 5.31 (q, J ) 7.2 Hz, 1 H),
4.47 (td, J ) 8.3, 5.1 Hz, 1 H), 4.25-4.10 (m, 2 H), 3.70 (s, 3 H),
3.07 (dd, ABX pattern, J AB ) 14.1 Hz, J AX ) 6.9 Hz, 1 H), 3.00
(dd, ABX pattern, J AB ) 14.1 Hz, J BX ) 7.1 Hz, 1 H), 1.60-1.45
(m, 3 H), 1.15 (t, J ) 7.1 Hz, 3 H), 0.89 (d, J ) 6.3 Hz, 3 H), 0.87
(d, J ) 6.2 Hz, 3 H); HRMS m/ e for C25H32N3O3S (M - H)+ calcd
454.2164, found 454.2175. Anal. Calcd for C25H33N3O3S: C,
65.91; H, 7.30; N, 9.22; S, 7.04. Found: C, 65.57; H, 7.24; N,
9.02; S, 6.68.
Ack n ow led gm en t. This work was supported by the
National Institutes of Health Grant GM-49076, Zeneca
Pharmaceuticals Group, the Upjohn Co., and Hoffman-
La Roche, Inc. T.M.N. thanks Pfizer, Inc., for support
in the form of a Pfizer Undergraduate Summer Fellow-
ship. D.L.H. and E.M.S. thank the National Institute
on Aging for support in the form of a training grant
(National Research Service Award AG00096-12). J .S.N.
thanks the following agencies for support in the form
of awards: the Camille and Henry Dreyfus Foundation
(New Faculty Award), the American Cancer Society
(J unior Faculty Research Award), the National Science
Foundation (Young Investigator Award, Presidential
Faculty Fellowship), and the Arnold and Mabel Beck-
man Foundation (Young Investigator Award).
Va lyla la n in e Meth yl Ester Isoth iocya n a te (2k ) a n d
Th iou r ea 3k (P h N(Et)CSNH-(S)-CH(i-P r )CONH-(S)-CH-
(CH3)CO2CH3). Reaction of valylalanine methyl ester hydro-
chloride (1b, 124 mg, 0.519 mmol) with thiophosgene (76 µL,
1.00 mmol) yielded 101 mg (80%) of crude isothiocyanate 2k as
a pale orange oil. 1H NMR analysis of this product revealed
12% of an impurity. 2k : IR (CHCl3) 3415, 2108, 2056, 1743,
1
1682 cm-1; H NMR (500 MHz, CDCl3) δ 6.91 (d, J ) 6.7 Hz, 1
H), 4.60 (quintet, J ) 7.2 Hz, 1 H), 4.29 (d, J ) 3.4 Hz, 1 H),
3.78 (s, 3 H), 2.55-2.47 (m, 1 H), 1.46 (d, J ) 7.2 Hz, 3 H), 1.13
(d, J ) 6.8 Hz, 3 H), 0.96 (d, J ) 6.7 Hz, 3 H). Impurity
(identified by examination of the spectra of the crude product):
1H NMR (500 MHz, CDCl3, partial data) δ 8.35 (br s, 1 H), 4.45
(br s, 1 H), 4.25 (br s, 1 H), 3.72 (s, 3 H), 2.38 (br s, 1 H), 1.09
(br d, J ) 4.4 Hz, 3 H).
Reaction of 101 mg of 2k with N-ethylaniline (125 µL, 0.99
mmol) yielded 118 mg (62% from 1b) of 3k as a white foamy
J O960038N