1,1-Dioxonaphtho[1,2-b]thiophene-2-methyloxycarbonyl (α-Nsmoc) and 3,3-dioxonaphtho[2,1-b]thiophene-2-methyloxycarbonyl (β-Nsmoc) amino-protecting groups

Article abstract of DOI:10.1021/jo062397g

Of the three theoretically possible, Bsmoc-related, naphthothiophene sulfone-based amino-protecting groups, the two most readily available derivatives, the α- and β-Nsmoc analogues, have been examined as substitutes for the Bsmoc residue in cases where the latter lead to oily protected amino acids or amino acid fluorides. All of the naphtho systems gave easily handled solid amino acid derivatives. The intermediate sulfone alcohol 11 used as the key reagent for introduction of the α-Nsmoc protecting group was readily made from α-tetralone (Scheme 1). The corresponding β-analogue 17 was made similarly on a small scale, but due to the high cost of β-tetralone, an alternate route involving reaction of rhodanine with a-naphthaldehyde was used for large-scale work (Scheme 2). All proteinogenic amino acids were converted to their α- and β-Nsmoc derivatives. Deblocking studies showed that the reactivity toward deblocking by piperidine followed the order α-Nsmoc > Bsmoc > β-Nsmoc. ¹H NMR experiments showed that deblocking of the two new systems was mechanistically similar to that previously established for the Bsmoc derivative in that the reaction is initiated by Michael addition to the β-carbon atom of the α,β-unsaturated sulfone system. Application of α- and β-Nsmoc amino acids to the solid-phase synthesis of two model peptides was examined. An advantage of the α-Nsmoc system over the long-known Bsmoc system proved to be the milder conditions needed for the deblocking step relative to the Bsmoc case, which is itself more readily deblocked than the classic Fmoc analogue.

Full text of DOI:10.1021/jo062397g

1,1-Dioxonaphtho[1,2-b]thiophene-2-methyloxycarbonyl (r-Nsmoc)
and 3,3-Dioxonaphtho[2,1-b]thiophene-2-methyloxycarbonyl
(â-Nsmoc) Amino-Protecting Groups†
Louis A. Carpino,* Adel Ali Abdel-Maksoud,‡ Dumitru Ionescu,§ E. M. E. Mansour,| and
Mohamed A. Zewail
Department of Chemistry, UniVersity of Massachusetts, Amherst, Massachusetts 01003
carpino@chem.umass.edu
ReceiVed NoVember 21, 2006
Of the three theoretically possible, Bsmoc-related, naphthothiophene sulfone-based amino-protecting
groups, the two most readily available derivatives, the R- and â-Nsmoc analogues, have been examined
as substitutes for the Bsmoc residue in cases where the latter lead to oily protected amino acids or amino
acid fluorides. All of the naphtho systems gave easily handled solid amino acid derivatives. The
intermediate sulfone alcohol 11 used as the key reagent for introduction of the R-Nsmoc protecting group
was readily made from R-tetralone (Scheme 1). The corresponding â-analogue 17 was made similarly
on a small scale, but due to the high cost of â-tetralone, an alternate route involving reaction of rhodanine
with R-naphthaldehyde was used for large-scale work (Scheme 2). All proteinogenic amino acids were
converted to their R- and â-Nsmoc derivatives. Deblocking studies showed that the reactivity toward
1
deblocking by piperidine followed the order R-Nsmoc > Bsmoc > â-Nsmoc. H NMR experiments
showed that deblocking of the two new systems was mechanistically similar to that previously established
for the Bsmoc derivative in that the reaction is initiated by Michael addition to the â-carbon atom of the
R,â-unsaturated sulfone system. Application of R- and â-Nsmoc amino acids to the solid-phase synthesis
of two model peptides was examined. An advantage of the R-Nsmoc system over the long-known Bsmoc
system proved to be the milder conditions needed for the deblocking step relative to the Bsmoc case,
which is itself more readily deblocked than the classic Fmoc analogue.
Introduction
addition by means of a primary or secondary amine, most often
piperidine. This process of addition to an R,â-unsaturated system
Recently1 we described an amino-protecting group, the Bsmoc
residue 1, which is deblocked under conditions of Michael
† Common names: R- and â-Napthothiophenesulfone-2-methyloxycarbonyl.
Other abbreviations used: ACP ) acyl carrier protein decapeptide (64-75);
Aib ) R-aminoisobutyric acid; R-Nsm ) 1,1-dioxonaphtho[1,2-b]thiophene-2-
methyl; R-Nsmoc ) 1,1-dioxonaphtho[1,2-b]thiophene-2-methyloxycarbonyl;
â-Nsm ) 3,3-dioxonaphtho[2,1-b]thiophene-2-methyl; â-Nsmoc ) 3,3-diox-
onaphtho[2,1-b]thiophene-2-methyloxycarbonyl; Boc ) tert-butyloxycarbonyl;
Bsm ) 1,1-dioxobenzo[b]thiophene-2-methyl; Bsmoc )1,1-dioxobenzo[b]-
thiophene-2-methyloxycarbonyl; DCC ) dicyclohexylcarbodiimide; DCM )
dichloromethane; DIEA ) diisopropylethylamine; DMAP ) 4-(dimethylamino)-
pyridine; DMF ) N,N-dimethylformamide; Fmoc ) 9-fluorenemethyloxycar-
contrasts with the standard â-elimination process involved in
the deblocking of the Fmoc2 and related base-sensitive amino
‡ On leave of absence from the Protein Research Department, Genetic
Engineering Institute, Mubarek City for Scientific Research, Borg El-Arab,
Alexandria 21934, Egypt.
bonyl; N-HATU
)
1-[bis(dimethylamino)methylene]-1H-1,2,3-triazolo[4,5-
§ Fulbright Scholar. On leave of absence from the Department of Organic
Chemistry, Faculty of Chemistry, University of Bucharest, 70346 Bucharest,
Romania.
b]pyridinium hexafluorophosphate 3-oxide; N-HBTU )1-[bis(dimethylamino)-
methylene]-1H-1-benzotriazolium hexafluorophosphate 3-oxide; MMPP ) mag-
nesium monoperoxyphthalate hexahydrate; NBS ) N-bromosuccinimide; PAL-
PEG-PS ) 5-[4-(aminomethyl)-3,5-dimethoxyphenoxy]valeric acid linker on
poly(ethylene glycol)/polystyrene support; PCA ) p-chloroaniline; TFA )
trifluoroacetic acid; THF ) tetrahydrofuran.
| Department of Chemistry, Faculty of Science, Alexandria University,
Alexandria 21321, Egypt.
Peptide Chemistry Department, National Research Center, El-Tahrir St.,
Dokki, Cairo 12622 Egypt.
10.1021/jo062397g CCC: $37.00 © 2007 American Chemical Society
Published on Web 02/08/2007
J. Org. Chem. 2007, 72, 1729-1736
1729

Carpino et al.
SCHEME 1
protecting groups. Because of its unique deblocking chemistry,
the Bsmoc group provides a number of advantages over Fmoc
chemistry in terms of speed and completeness of deblocking,3
selectivity, etc. In addition, a remarkable reactivity difference
has emerged between Bsmoc and Fmoc amino acids with the
Bsmoc derivatives being more reactive in acylation reactions
than their Fmoc counterparts to an extent which becomes greater
and greater as the steric requirements between the coupling
components increase.4 These various advantages of Bsmoc
chemistry justify its continued evaluation and the amelioration
of some limited deficiencies of the Bsmoc system itself.
â-carbon atom of the R,â-unsaturated sulfone unit due to the
peri hydrogen atom shown.
Linear system 2 was not examined in view of the sparseness
of the chemical literature on potential precursors for this system
which contrasts with the rich chemistry recorded for the two
bent systems related to 3 and 4. The two sulfide alcohols 5 and
6, promising intermediates for the synthesis of protective
systems 3 and 4, respectively, have been reported6 and require
only oxidation to the corresponding sulfone alcohols for
conversion to the two Bsmoc-related protectants.
The two sulfide alcohols 5 and 6 were prepared by adaptations
of the methods previously described. The R-derivative 5 was
obtained readily from inexpensive R-tetralone 7 according to
Scheme 1.7 Oxidation via the commercially available magne-
sium salt of perphthalic acid or use of sodium perborate8 gave
the sulfone alcohol 11.
Results and Discussion
One limitation is that four Bsmoc amino acids (and eight
Bsmoc amino acid fluorides) could be obtained only as
difficultly handled oils or foamy materials. Substitutes were
sought among the higher molecular weight benzo derivatives.
This led to consideration of the three monobenzo derivatives
2-4. With regard to the key deblocking step, systems 2 and 3
Although the isomeric alcohol 17 could be obtained similarly
from â-tetralone, this compound is currently rather expensive,
and we examined alternative sources for the â-analogue of key
ester 10. The method chosen involved a standard rhodanine route
to mercapto carboxylic acid 14 which underwent iodine-
catalyzed cyclization to acid 15 according to the method of
Campaigne and Cline9 (Scheme 2). An attempt to reduce acid
15 directly to alcohol 6 by means of lithium aluminum hydride
in ether failed, possibly due to its very low solubility. The acid
was therefore esterified to give 16, which was reduced readily,
and the remainder of the synthesis proceeded normally.
With the two sulfone alcohols now readily available, each
was converted to the appropriate chloroformates 18 and 19 by
reaction with phosgene or triphosgene. Protected amino acid
derivatives were made from the chloroformates by the Bolin
technique,10 which involves prior treatment of the amino acid
with trimethylsilyl chloride. Table 1 collects the various amino
are expected to be directly comparable to the Bsmoc residue,
whereas system 4 was predicted to be somewhat less reactive
due to possible steric retardation5 of attack by piperidine at the
(1) Carpino, L. A.; Ismail, M.; Truran, G. A.; Mansour, E. M. E.; Iguchi,
S.; Ionescu, D.; El-Faham, A.; Riemer, C.; Warrass, R. J. Org. Chem. 1999,
64, 4324.
(2) Carpino, L. A. Acc. Chem. Res. 1987, 20, 401.
(3) For the use of exactly 1 equiv of a secondary amine (4-piperidinopi-
peridine) to deblock a sensitive Bsmoc paclitaxel derivative, see: Greenwald,
R. B.; Zhao, H.; Reddy, P. J. Org. Chem. 2003, 68, 4894.
(4) (a) Carpino, L. A.; Philbin, M.; Ismail, M.; Truran, G. A.; Mansour,
E. M. E.; Iguchi, S.; Ionescu, D.; El-Faham, A.; Riemer, C.; Warrass, R.;
Weiss, M. S. J. Am. Chem. Soc. 1997, 119, 9915. (b) Carpino, L. A.; Krause,
E.; Sferdean, C. D.; Schu¨mann, M.; Fabian, H.; Bienert, M.; Beyermann,
M. Tetrahedron Lett. 2004, 45, 7519.
acids made. For full characterization data, see the Supporting
Information. As noted, for both series, all were obtained in solid
form including those for proline, tryptophan, serine tert-butyl
(5) (a) Carpino, L. A.; Philbin, M. J. Org. Chem. 1999, 64, 4315. (b)
McDowell, S. T.; Stirling, C.J.M. J. Chem. Soc. B 1967, 343. (c) McDowell,
S. T.; Stirling, C.J.M. J. Chem. Soc. C 1967, 351.
1730 J. Org. Chem., Vol. 72, No. 5, 2007

R-Nsmoc and â-Nsmoc Amino-Protecting Groups
SCHEME 2
ether, and aspartic acid tert-butyl ester, for which in the Bsmoc
series this was not possible. In some cases, the acids exhibited
wide melting point ranges, although even then no difficulty was
encountered in obtaining analytically pure material by recrys-
tallization from DCM/hexane. Only in the cases of asparagine,
lysine and glutamine was it necessary to use column chroma-
tography to obtain pure samples. The acids were all converted
to their acid fluoride derivatives via standard cyanuric fluoride
methodology,11 and again, all proved to be obtainable in solid
form. Using either the acid fluoride as coupling species or
coupling to H-Ala-OMe via N-HATU/DIEA, it was shown that
no significant loss of configuration accompanied the synthesis
and use of R-Nsmoc R-phenylglycine (see the Supporting
Information). A similar result had previously been reported for
Bsmoc-Phg-OH (see ref 1, p 4333).
the same 1H NMR technique used previously involving model
carbamate substrate 20a. Treatment of these derivatives of
p-chloroaniline (PCA) with 2 equiv of piperidine in CDCl3-
DMSO-d6 showed that under the conditions and at the concen-
trations used, which were the same in all three cases, rough
halftimes for release of p-chloroaniline were 2.0, 2.4, and 7.4
min in the cases of the R-Nsmoc, Bsmoc, and â-Nsmoc systems,
respectively. Similar halftimes were measured on the basis of
the disappearance of the starting urethanes by following the
decrease for the CH2O peaks near δ 5.2 in the three cases. The
lowered reactivity of the â-Nsmoc system is in accord with the
peri-hindrance mentioned above. The enhanced reactivity of the
R-Nsmoc system relative to the Bsmoc case may be due to the
inductive effect of the added benzo substituent since steric
factors are much the same for these two systems. The course
of the reaction for the R-Nsmoc case is outlined in Scheme 3.12
The decrease in the intensity of the NMR peak at δ 5.24 due to
the CH2O group of 20b is paralleled by increase of the olefinic
peaks and the methine proton position at δ 6.13, 6.45, and 5.08,
respectively, signaling the formation of initial piperidine adduct
21. At the same time, aromatic peaks at δ 6.59 and 6.98 appear
due to liberated PCA. Adduct 21 then undergoes piperidine-
catalyzed rearrangement to give stable adduct 22 with its CH2N
peak appearing at δ 3.35.
In order to compare deblocking rates for the two new
protecting systems with those for the Bsmoc residue, we used
TABLE 1. r- and â-Nsmoc Amino Acidsa,b
R-derivative
â-derivative
amino acid
mpc (°C)
yield (%)
mpc (°C)
yield (%)
Gly
Ala
Val
Leu
Ile
Pro
Phe
Met
191-193
120-125
138-139
75-96
90
76-151
89-109
89-115
78-105
76-123
91-146
81-158
74-143
119-204
79-139
88-119
96-162
85-151
77-164
86-149
98-159
184-187
77-164
238-240
89.2
82.7
73.3
83.8
84.0
80.0
91.8
73.5
85.7
78.5
86.3
87.2
83.0
52.0
68.5
80.3
77.8
47.8
56.3
86.2
75.4
84.6
88.5
77.6
89.2
80.8
85.6
82.0
82.8
81.8
84.0
46.1
72.0
78.8
84.4
49.7
56.8
148-150
192-195
81-135
55-117
107-160
76-135
85-107
134-136
67-142
69-128
83-125
93-146
181-182
170-200
120-160
Trp
Ser(tBu)
Thr(tBu)
Tyr(tBu)
Asp(O-tBu)
Glu(O-tBu)
Lys(BOC)
Phg
Aib
Asn(Trt)
Gln(Trt)
Comparable reactions occur with the â-Nsmoc analogue 20c,
although complete liberation of PCA is slower as noted above.
(6) Tedjamulia, M. L.; Tominaga, Y.; Castle, R. N.; Lee, M. L. J.
Heterocycl. Chem. 1983, 20, 1143.
(7) Clarke, K.; Gregory, D. N.; Scrowston, R. M. J. Chem. Soc., Perkin
Trans. 1 1973, 2956.
(8) McKillip, A.; Kemp, D. Tetrahedron 1989, 45, 3299.
(9) Campaigne, E.; Cline, R. E. J. Org. Chem. 1956, 21, 32 and 39.
(10) Bolin, D. R.; Sytwu, I.-I.; Humiec, F.; Meienhofer, J. Int. J. Pept.
Protein Res. 1989, 33, 353.
(11) Carpino, L. A.; Sadat-Aalaee, D.; Chao, H. G.; DeSelms, R. H. J.
Am. Chem. Soc. 1990, 112, 9651.
a All compounds were characterized on the basis of elemental analysis
((0.3%, C, H, N) and consistent IR and 1H NMR spectral data. Complete
data are presented in the Supporting Information. b All compounds were
recrystallized from DCM/hexane. c In many cases, wide melting point ranges
were observed. In spite of the mp behavior, analytically pure samples were
obtained in all cases.
(12) A similar course for the Bsmoc deblocking process was previously
recorded. See the Supporting Information of ref 4a, page S13.
J. Org. Chem, Vol. 72, No. 5, 2007 1731

Carpino et al.
SCHEME 3
It proved to be difficult to isolate pure samples of either adduct
21 or 22 from the urethane-based reaction mixtures, but when
chloride 23, available from alcohol 11 by treatment with thionyl
chloride, was substituted for the urethane in the reaction with
piperidine it was possible to isolate stable adduct 22 in pure
form. When piperidine was substituted by 2-methylpiperidine,
R-Nsmoc (crude purity 87.5%) and 8 min for â-Nsmoc (crude
purity 83.5%).
In previous studies,1,4a it was shown that one could selectively
deblock the Fmoc group from dipeptide ester 24 using a
hindered amine such as tert-butylamine with only minimal attack
at the Bsm residue. Indeed, treatment of 24 with 30% tert-
butylamine in acetonitrile for 10 min followed by in situ
coupling via Fmoc-Gly-OH in the presence of N-HATU
(60 min) gave after workup and column chromatography
tripeptide 25 in a yield of 72.4%. Application of the same
methodology to the â-Nsm analogue of 24 gave the correspond-
ing â-Nsm analogue of 25 in a yield of only 31.8%. This
suggests that factors other than steric hindrance toward attack
at the R,â-unsaturated sulfone units of these systems are
important in determining the relative sensitivity for attack at
the Fmoc vs the Bsm or â-Nsm ester functions.
Because of their greater availability and the closer cor-
respondence of their deblocking sensitivity to the simple Bsmoc
derivatives, the R-Nsmoc amino acid derivatives are recom-
mended over the â-isomers as substitutes for the few Bsmoc
amino acids which are not available in solid form. In addition,
the full complement of R-Nsmoc derivatives described here are
potentially of special utility in view of their even greater
sensitivity toward deblocking compared to the already highly
sensitive Bsmoc analogues.
both intermediate and final adducts could be isolated in
analytically pure form. With 20% 2-methylpiperidine in DMF,
HPLC analysis showed that after 1 min the initial adduct (2-
methylpiperidine analogue of 21) was present in a yield of
93.9%. After 3 h, rearrangement had occurred to give the 2-Me
analogue of 22 in 97.3% yield. With only 1 equiv of 2-meth-
ylpiperidine only the initial adduct could be observed, and it
was still the only isomer visible even after 24 h.
As a test for the use of the new R- and â-Nsmoc protectants
in solid-phase peptide synthesis, two model peptides leucine
enkephalin (H-Tyr-Gly-Gly-Phe-Leu-NH2)13 and ACP65-74 (H-
Val-Gln-Ala-Ala-Ile-Asp-Tyr-Ile-Asn-Gly-NH2)14 were as-
sembled. For the R-Nsmoc system in the case of leucine
enkephalin, with a PAL-PEG-PS resin, optimum deblocking
times varied from 20 s to 5 min depending on the concentration
of the piperidine reagent (Table 2). For the â-Nsmoc system,
in line with its reduced sensitivity toward bases, best results
were observed when 20% piperidine was used with a deblocking
time of 5 min (96.9% purity of the crude pentapeptide). In
contrast, with the 5%/5 min system which gave excellent results
for the R-Nsmoc system (crude purity 97%), the crude pen-
tapeptide purity was only 67.8%. For assembly of the ACP
decapeptide using N-HATU in place of N-HBTU, comparable
results were obtained for both protectants provided that 20%
piperidine was used with a deblocking time of 1 min for
Experimental Section
Ethyl Naphtho[1,2-b]thiophene-2-carboxylate (10). N-Bromo-
succinimide (15 g, 84.2 mmol) was added in portions with stirring
to a boiling solution of ethyl 4,5-dihydronaphtho[1,2-b]thiophene-
TABLE 2. Synthesis of Leucine Enkephalin via r-Nsmoc Amino
Acidsa
concentration (%)
of piperidine in DMF
deblocking
time (min)
crude purity by
HPLC analysis (%)
2
5
5
5
5
1
20 s
1
3
5
85.9
87.7
96.6
94.2
97.0
100
(13) (a) Carpino, L. A.; Chao, H. G.; Tien, J.-H. J. Org. Chem. 1989,
54, 4302. (b) Carpino, L. A.; Gao, H.-S.; Ti, G.-S.; Segev, D. J. Org. Chem.
1989, 54, 5887. (c) Carpino, L. A.; Cohen, B. J.; Lin, Y-Z.; Stephens, K.
E., Jr.; Triolo, S. A. J. Org. Chem. 1990, 55, 251.
(14) (a) Arshady, R.; Atherton, E.; Clive, D. L. J.; Sheppard, R. C. J.
Chem. Soc., Perkin Trans. 1 1981, 529. (b) Schno¨lzer, M.; Jones, A.;
Alewood, P. F.; Kent, S. B. H. Anal. Biochem. 1992, 204, 335. (c) Carpino,
L. A.; El-Faham, A.; Minor, C. A.; Albericio, F. J. Chem. Soc., Chem.
Commun. 1994, 201.
20
20 s
a 3 equiv of the amino acid with 3 equiv of N-HBTU and 6 equiv of
DIEA and a coupling time of 30 min.
1732 J. Org. Chem., Vol. 72, No. 5, 2007

R-Nsmoc and â-Nsmoc Amino-Protecting Groups
2-carboxylate7(17.0 g, 65.8 mmol) and dibenzoyl peroxide (20 mg)
in dry CCl4(100 mL) while being irradiated with a 200 W sun lamp.
The hydrogen bromide which was formed was removed in a stream
of nitrogen. When the addition was complete, the mixture was
refluxed for a furthur 45 min (NMR was used to check for reaction
completion, and if the reaction was not finished, a small additional
amount of N-bromosuccinimide was added, the refluxing continued,
and NMR checked again). The mixture was cooled and filtered,
and evaporation of the filtrate gave the crude ester which was
recrystallized from EtOAc/hexane to give 14.5 g (87.5%) of the
pure ester as off-white crystals: mp 86-88 °C (lit.7 mp 86-87 °C);
mL each), dried over MgSO4, and filtered, and the solvent removed
in vacuo. The resulting off-white solid was recrystallized from hot
ethyl acetate/hexane to give 9.2 g (80.1%) of the sulfone alcohol:
mp 169-170 °C; IR (KBr) 1152, 1292 (SO), 3522 (OH) cm-1; 1H
NMR (CDCl3 + TFA) δ 4.91 (s, 2) 7.30 (s,1), 7.4 (d, 1), 7.63 (m,
1), 7.71 (m, 1), 7.91 (d, 1), 8.1 (d, 1), 8.23 (d, 1). Anal. Calcd for
C13H10O3S: C, 63.41; H, 4.06; S, 13.01. Found: C, 63.38; H, 4.11;
S, 12.93.
(b) Via Sodium Perborate Oxidation. To a stirred solution of
26.5 (0.13 mol) of the sulfide alcohol in glacial acetic acid (500
mL) at 45-50 °C was added portionwise during 20 min 95.4 g
(0.62 mol) of sodium perborate tetrahydrate. Stirring was continued
at this temperature until TLC analysis (EtOAc-hexane (3/2 v/v))
indicated completion of the reaction (5-6 h). The acetic acid was
removed in vacuo and the residue stirred with 150 mL of water.
The crude alcohol which separated was filtered, washed with water,
dried in air, and recrystallized from absolute ethanol to give 22.5 g
1
IR (KBr) 1696 cm1(CO); H NMR (CDCl3) δ 1.40-1.47 (t, 3),
4.38-4.49 (q, 2), 7.25-8.37 (m, 7).
Ethyl Naphtho[2,1-b]thiophene-2-carboxylate (16) from â-
Tetralone. Ethyl mercaptoacetate (15.69 g, 14.31 mL, 130.8 mmol)
was added to a cooled, stirred solution of sodium (3.77 g, 0.16
g-atom) in dry ethanol (58 mL). Crude 2-bromo-3,4-dihydronaph-
thalene-1-carboxaldehyde (30 g) prepared according to the method
of Gilchrist and Summersell15 (see also the Supporting Information)
was added portionwise during 30 min at 0-5 °C. The mixture was
stirred overnight at room temperature, refluxed for 30 min, and
poured into 300 mL of water. The ester was filtered, decolorized
with activated charcoal, and recrystallized from ethanol to give
21.5 g (64% based on 1H NMR analysis of the crude starting
aldehyde showing that 25 g of pure material was present) of the
ester: mp 50-52 °C; IR (KBr) 1708 cm-1 (CdO); 1H NMR
(CDCl3) δ 1.35-1.42 (t, 3), 2.99-3.00 (s, 4), 4.30-4.41 (q, 2),
7.13-8.03 (m, 5). Without further purification, the dihydro
compound was treated with NBS as described above for ethyl
naphtho[1,2-b]thiophene-2-carboxylate. The pure ester was obtained
as an off-white solid in 85% yield: mp 83-84 °C; IR (KBr) 1717
cm-1 (CdO); 1H NMR (CDCl3) δ 1.41-1.48 (t, 3), 4.39-4.50 (q,
2), 7.26-8.75 (m, 7). Anal. Calcd for C15H12O2S: C, 70.31; H,
4.68. Found: C, 70.36; H, 4.68.
1
(73.6%) of the pure sulfone alcohol: mp 169-170 °C; H NMR
(CDCl3) δ 2.35 (s, 1), 4.79 (s, 2), 7.17 (t, 1), 7.37-8.29 (m, 6). In
the case of the sample of alcohol derived by MMPP oxidation, the
hydroxyl proton was not observed in the NMR spectrum, presum-
ably because TFA had been added to the CDCl3 solvent.
3,3-Dioxo-2-(hydroxymethyl)naphtho[2,1-b]thiophene (17).
The alcohol was obtained in the same manner as described above
for the R-isomer in 81.3% yield (8.4 g): mp 127-129 °C; IR (KBr)
1
1130, 1279 (SO2) 3508 (OH) cm-1; H NMR (CDCl3 + 1 drop
TFA) δ 4.96 (s, 2), δ 7.26-8.00 (m, 7). Anal. Calcd for
C13H10O3S: C, 63.41; H, 4.06. Found: 63.53; H, 4.05
Ethyl Naphtho[2,1-b]thiophene-2-carboxylate (16) from the
Corresponding Carboxylic Acid. To a stirred solution of 40 g
(175.4 mmol) of naphtho[2,1-b]thiophene-2-carboxylic acid pre-
pared as described previously9 (full details in the Supporting
Information) in 600 mL of absolute ethanol was added dropwise
45 mL of concentrated H2SO4, and the reaction mixture was
refluxed overnight. Ethanol was evaporated at reduced pressure,
and the residue was dissolved in 1200 mL of EtOAc. The organic
solution was washed with water, saturated NaHCO3, saturated NaCl,
and water (3 × 300 mL each). After drying over MgSO4,
evaporation of ethyl acetate and recrystallization of the residue from
EtOAc/hexane gave 35.2 g (78.4%) of the pure ester for which the
mp, IR, and 1H NMR characterization data were the same as
described above for the sample prepared from â-tetralone. The ester
obtained in this way was also reduced to its sulfide alcohol and
the sulfide alcohol oxidized to the sulfone alcohol by the same
methods described above. The physical and spectral data collected
on these samples of the sulfide and sulfone alcohols also agreed
with the data given above and in refs 6 and 9.
1,1-Dioxonaphtho[1,2-b]thiophene-2-methyl Chloroformate
(R-Nsmoc-Cl) (18) (a) Via Phosgene. CAUTION! Working with
highly toxic gaseous phosgene requires a good hood, special
precautions, and an experienced operator. It is highly recommended
that the safer reagent triphosgene be used as a substitute (see (b)
below). To a stirred solution of 10 g (40.6 mmol) of 1,1-dioxo-2-
(hydroxymethyl)naphtho[1,2-b]thiophene in 150 mL of dry DCM
and 50 mL of dry THF at -78 °C was added in one portion 15 mL
(21.0 g; 212.5 mmol) of phosgene which had been condensed into
a graduated cylinder. The solution was allowed to come to room
temperature and stirred overnight. Excess phosgene and solvent
were removed under reduced pressure with the aid of a water
aspirator, the process being repeated using an additional 200 mL
of DCM and then 200 mL of ether. Recrystallization of the residue
from DCM/hexane gave 11.3 g (90.5%) of the chloroformate as
off-white crystals: mp 134-135 °C; IR (KBr) 1151, 1295 (SO2),
1766 (CO) cm-1; 1H NMR (CDCl3) δ 7.30 (s, 1), 7.41 (d, 1), 7.61
(m, 1), 7.69 (m, 1), 7.90 (d, 1), 8.0 (d, 1), 8.25 (d, 1). Anal. Calcd
for C14H9ClO4S: C, 54.46; H, 2.93; S, 10.38; Cl, 11.48. Found:
C, 54.70; H, 2.85; S, 10.46; Cl, 11.53.
2-(Hydroxymethyl)naphtho[1,2-b]thiophene (5). A solution of
14.0 g (54.7 mmol) of ethyl naphtho[1,2-b]thiophene-2-carboxylate
in 200 mL of dry THF/ether (1:1) was added at room temperature
under N2 to a suspension of 3.11 g (82 mmol) of lithium aluminum
hydride in 60 mL of dry THF/ether (1:1). After being refluxed for
3.5 h, the reaction mixture was cooled and carefully quenched by
adding 50 mL of cold water and enough 20% HCl (∼200 mL) to
dissolve the inorganic salts. The aqueous layer was separated and
extracted with ethyl acetate (3 × 300 mL). The organic layers were
washed with water (3 × 200 mL) and saturated NaHCO3 (1 × 200
mL), dried over MgSO4, and filtered and the solvent evaporated in
vacuo. The residue was recrystallized from benzene/hexane to give
10.2 g (87.2%) of the alcohol: mp 85-86 (lit.6 mp 89-91 °C); IR
1
(KBr) 3252 cm-1(OH); H NMR (CDCl3) δ 2.10 (s, 1), 4.99 (s,
2), 7.32 (s, 1), 7.30 (s, 1), 7.43 (m, 2), 7.54 (m, 2), 7.91 (d, 1), 8.1
(d, 1).
2-(Hydroxymethyl)naphtho[2,1-b]thiophene (6). The reaction
was carried out as described above for 2-(hydroxymethyl)naphtho-
[1,2-b]thiophene. There was obtained 9.5 g (87.5%) of the
alcohol: mp 105-106 °C (lit.6 mp 106 °C); IR (KBr) 3220
cm-1(OH); 1H NMR (CDCl3) δ 2.07 (s, 1), 4.99 (s, 2), 7.24-8.25
(m, 7).
1,1-Dioxo-2-(hydroxymethyl)naphtho[1,2-b]thiophene (11). (a)
Via MMPP Oxidation. To a stirred solution of 10 g (46.7 mmol)
of the sulfide alcohol in 120 mL of methanol at 0 °C was added
portionwise 34.7 g (70 mmol) of monoperoxyphthalic acid,
magnesium salt hexahydrate (Aldrich Chemical Co.) with 30 mL
of water also added slowly over the same period. The reaction
mixture was stirred overnight at room temperature. The white
precipitate was filtered, and the solvent was removed in vacuo.
The residue was dissolved in 400 mL of ethyl acetate, the solution
washed with saturated NaHCO3, saturated NaCl, water (3 × 75
(b) Via Triphosgene. A mixture of the alcohol (10.0 g, 40.7
mmol), triphosgene (6.0 g, 20.2 mmol), and triethylamine hydro-
(15) Gilchrist, T. L.; Summersell, R. J. J. Chem. Soc., Perkin Trans. 1
1988, 2595.
J. Org. Chem, Vol. 72, No. 5, 2007 1733

Carpino et al.
chloride (0.244 g, 1.77 mmol) in 70 mL of dry epoxide-free THF
was heated to 40 °C (outside oil bath temperature) for 3 h. A test
by infrared examination showed that after 1 h some alcohol was
still present, but after 3 h, alcohol was no longer visible. The solvent
was removed in vacuo with a rotary evaporator to give 10.4 g
(82.8%) of the R-Nsmoc-Cl after recrystallization from dry ether.
Triphosgene was used similarly to prepare â-Nsmoc-Cl (85.6%)
and Bsmoc-Cl (88.6%).
residue was recystallized from the appropriate solvent mixture to
give the corresponding R-Nsmoc (or â-Nsmoc)-protected amino
acids.
Note: In the cases of the Lys(Boc), Asn(Trt), and Gln(Trt)
derivatives, the regular Bolin technique was not suitable; therefore,
we modified the procedure by adding 2.5 equiv of TEA in the case
Lys and Gln and 3.5 equiv of TEA in the case of Asn to the amino
acid suspension in 40 mL of dry DCM. Chlorotrimethylsilane (2
equiv) was added, and the mixture was refluxed for 2 h. The
chloroformate was added in one portion at room temperature, and
the mixture was stirred overnight. The solution was washed once
with 10 mL of water, dried over MgSO4, concentrated in vacuo,
and column chromatographed using ethyl acetate/hexane (3/2). The
isolated material was then recrystallized from DCM/hexane.
For details see Table 1 and the Supporting Information.
3,3-Dioxonaphtho[2,1-b]thiophene-2-methyl Chloroformate
(â-Nsmoc-Cl) (19). The reaction was carried out as described in
(a) above for the R-analogue to give 11.7 g (93.8%) of the
chloroformate as cream-colored crystals: mp 135-136 °C; IR
1
(KBr) 1152, 1298 (SO2), 1785 (CO) cm-1; H NMR (CDCl3) δ
5.37 (d, 2), 7.26-8.09 (m, 7). Anal. Calcd for C14H9O4ClS: C,
54.46; H, 2.93; S, 10.38. Found: C, 54.47; H, 3.02; S, 10.38.
1,1-Dioxonaphtho[1,2-b]thiophene-2-methyl N-Succinimidyl
Carbonate (R-Nsmoc-OSu). N-Hydroxysuccinimidyl DCHA salt16
(3.39 g, 11.6 mmol) was added in small portions to a stirred solution
of R-Nsmoc-Cl (3.57 g, 11.6 mmol) in 100 mL of dry DCM. The
reaction mixture was stirred overnight at room temperature after
which the separated precipitate was filtered and washed with DCM.
The filtrate was washed with 10% aqueous citric acid, saturated
NaHCO3, saturated NaCl, and water (30 mL each) and dried over
MgSO4 and the solvent removed in vacuo. The resulting off-white
solid was recrystallized from EtOAc/hexane to give 2.5 g (55.8%)
of the ester as white crystals: mp 188-190 °C; IR (KBr) 1152,
General Procedure for the Preparation of R- and â-Nsmoc
Amino Acid Fluorides.11 To a stirred solution of the amino acid
(1 mmol) in dry DCM (20 mL) and dry pyridine (1 mmol) kept
under an N2 atmosphere was added cyanuric fluoride (5 mmol) at
-20 to -10 °C. A precipitate or emulsion was formed and gradually
increased in amount. After the mixture was stirred at this temper-
ature for 1 h and at room temperature for 2 h, crushed ice was
added followed by 20 mL of DCM. The organic layer was
separated, and the aqueous layer was extracted with DCM (2 ×
20 mL). The combined DCM layers were washed with ice-cold
water (3 × 15 mL) and dried over MgSO4, and the solvent was
removed in vacuo. The residue was recrystallized from an ap-
propriate solvent. In some cases, the reaction mixture had to be
refluxed rather than being held at room temperature for 2 h. The
1
1303 (SO2), 1740 (CdO, imide), 1815 (OCOO) cm-1; H NMR
(CDCl3 + TFA) δ 2.99 (s, 4), 5.4 (s, 2), 7.45 (d, 2), 7.71 (m, 1),
7.82 (m, 1), 7.96 (d, 1), 8.15 (d, 1), 8.22 (d, 1). Anal. Calcd for
C18H13NO7S: C, 55.82; H, 3.35; N, 3.61. Found: C, 55.63; H, 3.29;
N, 3.50.
1
acid fluorides were identified by their IR and H NMR spectra.
Solid-Phase Peptide Synthesis. Peptide segments were synthe-
sized from both R-Nsmoc and â-Nsmoc amino acids and their
corresponding fluorides via manual synthesis with a plastic syringe
being used as a reaction vessel.17 The syringe was fitted with a
sintered disk and connected to a water aspirator.
1,1-Dioxonaphtho[1,2-b]thiophene-2-methyl N-p-Chlorophen-
yl Carbamate (R-Nsmoc-PCA) (20b). A solution of 1 g (4.06
mmol) of 1,1-dioxo-2-(hydroxymethyl)naptho[1,2-b]thiophene and
0.73 g (4.8 mmol) of p-chlorophenyl isocyanate in 20 mL of
benzene was refluxed for 24 h. The precipitate was filtered, washed
with benzene, and recrystallized from EtOAc/hexane to give 0.98 g
(60.5%) of the urethane as off-white crystals: mp 212-214 °C;
IR (KBr) 1143, 1282 (SO2), 1738 (CO) (urethane) cm-1; 1H NMR
(CDCl3 + DMSO-d6) δ 5.24 (s, 2), 7.22-8.20 (m, 11), 9.97 (bs,
1). Anal. Calcd for C20H14ClO4S: C, 60.12; H, 3.50; N, 3.50.
Found: 59.98; H, 3.43; N, 3.52
3,3-Dioxonaphtho[2,1-b]thiophene-2-methyl N-p-Chlorophen-
yl Carbamate (â-Nsmoc-PCA) (20c). The preparation was carried
out as described above for the isomeric alcohol. The urethane was
obtained in a yield of 61.7% as off-white crystals: mp 181-183 °C;
IR (KBr) 1151, 1295 (SO2), 1701 (CO2, urethane) cm-1; 1H NMR
(CDCl3 + DMSO-d6) δ 5.27 (s, 2), 7.21-8.33 (m, 11), 9.94 (bs,
1). Anal. Calcd for C20H14ClO4S: C, 60.12; H, 3.50; N, 3.50.
Found: 59.96; H, 3.60; N, 3.77
General Procedure for the Preparation of 1,1-Dioxonaphtho-
[1,2-b]thiophene-2-methoxycarbonyl Amino Acids (R-Nsmoc-
AAs) and 3,3-Dioxonaphtho[2,1-b]thiophene-2-methoxycarbonyl
Amino Acids (â-Nsmoc-AAs) Using the Bolin Technique.10 To
a suspension of 3.24 mmol of an amino acid in 20 mL of dry DCM
was added in one portion 0.82 mL (6.5 mmol) of dry chlorotri-
methylsilane. The mixture was refluxed for 1.5 h and cooled in an
ice bath. Diisopropylethylamine (1.1 mL, 6.2 mmol) was added
slowly followed by 1 g (3.24 mmol) of R-Nsmoc-Cl (or â-Nsmoc-
Cl). The reaction mixture was allowed to stand at 0 °C for 30 min
and for 3 h at room temperature. The solvent was removed in vacuo
and the resulting oil distributed between 50 mL of ether and 100 mL
of 2.5% NaHCO3 solution. The combined aqueous layers were
acidified to pH 2 with concentrated HCl and extracted with EtOAc
(3 × 40 mL). The extracts were combined and washed with 40 mL
of saturated NaCl and 40 mL of water, dried over MgSO4, and
filtered, and solvent was evaporated in vacuo. The solid or oily
Synthesis of Leucine Enkephalin via R-Nsmoc and â-Nsmoc
Amino Acids. Peptide segments were synthesized using either a
coupling reagent or the isolated acid fluorides. The starting resin
(Fmoc-PAL-PEG-PS, 100 mg, 0.24 mmol/g) was washed with DMF,
DCM, and DMF (3 × 4 mL each). The Fmoc group on the resin
was deblocked using 20% pip/DMF for 7 min followed by washing
with DMF, DCM, and DMF (3 × 4 mL each). Coupling was carried
out using 3 equiv of either N-HATU or N-HBTU as a coupling
reagent along with 3 equiv of R-Nsmoc and â-Nsmoc amino acids
and 6 equiv of DIEA as a base. In the case of the corresponding
R-Nsmoc or â-Nsmoc amino acid fluorides, only 3 equiv of DIEA
was used. In all cases, the coupling time was 30 min. The R-Nsmoc
and â-Nsmoc groups were cleaved for various times using different
pip/DMF ratios. A washing cycle involving DMF, DCM, and DMF
(3 × 4 mL each) followed each deprotection and coupling step.
After the complete sequence of the desired peptide was assembled,
the R-Nsmoc and â-Nsmoc N-terminal groups were deblocked and
the resin washed with DMF, DCM, ethanol, and ether (3 × 4 mL
each). The resin was dried under vacuum and then treated with
4 mL of a mixture of 95% TFA and 5% water for 2 h. The filtrate
was collected, and the resin was further washed with DCM (3 ×
4 mL). The combined filtrates were evaporated in vacuo at room
temperature, the peptide was precipitated by adding cold ether, the
solvent was decanted, and the crude peptide was dried under
vacuum and examined by HPLC analysis using a solvent system
consisting of 5/35 CH3CN 0.1% TFA over 25 min.
Synthesis of Acyl Carrier Peptide ACP65-74 via R- or â-Nsmoc
Amino Acids. Sequence: H-Val-Gln-Ala-Ala-Ile-Asp-Tyr-Ile-Asn-
Gly-NH2. The method followed that described for leucine enkepha-
lin, except that the deblocking time was 1 min using 20% piperidine
in DMF for R-Nsmoc protection and 8 min using 20% piperidine
in DMF for â-Nsmoc protection.
(16) Paquet, A. Can. J. Chem. 1982, 60, 976.
1734 J. Org. Chem., Vol. 72, No. 5, 2007
(17) For a general description of the syringe methodology, see ref 1.

R-Nsmoc and â-Nsmoc Amino-Protecting Groups
Possible Loss of Configuration Involving the Synthesis and
Coupling of R-Nsmoc-Phg-OH. (a) R-Nsmoc-Phg-Ala-OMe. To
a solution of 140 mg (1 mmol) of H-Ala-OMe‚HCl in 15 mL of
DMF was added 0.52 mL (3.0 mmol) of DIEA, 421 mg (1.0 mmol)
of R-Nsmoc-Phg-OH, and 380 mg (1 mmol) of N-HATU. The
reaction mixture was stirred for 1 h at 0 °C and at room temperature
for 2 h. The solution was diluted with 50 mL of EtOAc and washed
with 5% citric acid, saturated NaHCO3, and saturated NaCl (3 ×
15 mL each) and then dried over MgSO4. The solvent was removed
with a rotary evaporator to give a residue which prior to purification
was shown by 1H NMR analysis to contain 1.46% of the
DL-diastereomer. A sample for elemental analysis was prepared by
recrystallization from DCM/hexane to give 302 mg (59.4%) of the
dipeptide as an off-white solid: mp 182-184 °C; IR (KBr) 1154,
EtOAc/hexane (3/2) to give two substances which were separately
recrystallized from DCM/hexane: (1) 235 mg of a white solid, Rf,
0.33, mp 155-160 °C which could not be obtained in a pure state
but is assigned structure 21 on the basis of its 1H NMR spectrum,
and (2) 130 mg of the title substance as bright yellow crystals: Rf
0.83; mp 237-240 °C; IR (KBr) 1157, 1263 (SO2) cm-1; 1H NMR
(CDCl3) δ 1.75 (m, 6), 2.29 (s, 4), 3.35 (d, 2), 7.27-8.39 (m, 7).
Anal. Calcd for C18H19NO2S: C, 69.01; H, 6.06; N, 4.46. Found:
C, 68.75; H, 5.97, N, 4.41
Isolation of Initial and Final Stable Adducts by Reaction of
1,1-Dioxo-2-(chloromethyl)naphtho[1,2-b]thiophene (23) with
2-Methylpiperidine/DMF. To 0.330 g (1.25 mmol) of 1,1-dioxo-
2-(chloromethyl)naphtho[1,2-b]thiophene (23) was added 20%
2-methylpiperidine in DMF. The reaction mixture was stirred for
1 min or 3 h, and then 100 mL of EtOAc (for each reaction) was
added. The reaction mixture was washed with water and saturated
NaCl (3 × 30 mL in both cases) and dried over MgSO4 and the
solvent removed in vacuo. Analytical samples were obtained by
flash column chromatography using EtOAc/hexane (3/2). From the
1 min reaction mixture the initial adduct was obtained as off-
white crystals from DCM/hexane in 42.5% yield: mp 164-166 °C;
IR (KBr) 1162, 1290 (SO2) cm-1; 1H NMR (CDCl3) δ 1.28-1.69
(m, 8), 2.19 (m, 3), 2.88 (m, 1), 5.45 (t, 1), 5.95 (t, 1), 6.47 (t, 1)
7.26-8.58 (m, 6). Anal. Calcd for C19H21NO2S: C, 69.72; H, 6.41;
N, 4.27. Found: C, 69.54; H, 6.43; N, 4.30.
1
1303 (SO2),1648, 1691, 1741 (CO), 3309 (NH) cm-1; H NMR
(CDCl3) δ 1.38 (d, 3), 3.63 (s, 3), 4.52 (m, 1), δ 5.17 (d, 1), 5.15
(m, 2) 6.25 (dd, 2), 7.21 (d, 1), 7.32-7.41 (m 6), 7.6 (t, 1), 7.7 (m,
1), 7.91 (d, 1), 7.8 (d, 1), 8.27 (d, 1). Anal. Calcd for
C26H24N2O7S: C, 61.41; H, 4.76; N, 5.51. Found: C, 61.51; H,
4.82; N, 5.44.
(b) Establishment of the Authentic Positions for the OMe
1
Singlets in the H NMR Spectra of LL- and LD-R-Nsmoc-Phg-
Ala-OMe. In order to establish the position of the OMe singlet in
1
the H NMR spectrum for the DL-diastereomer (which would be
the same as that for the LD-diastereomer), the same method was
used as described above except that DL-H-Ala-OMe‚HCl was
substituted for the L-isomer. This gave a roughly 50:50 mixture of
the LL- and LD-diastereomers (mp 80 °C) for which the OMe singlets
were found at δ 3.63 and 3.70, respectively.
From the 3 h reaction mixture, the final stable adduct was
obtained as yellow crystals from DCM/hexane in 40.9% yield: mp
127-130 °C; IR (KBr) 1162, 1285 (SO2) cm-1; 1H NMR (CDCl3)
δ 1.18-1.20 (d, 3), 1.59-2.04 (m, 6), 2.26 (s, 2), 3.03 (m, 1),
3.47 (m, 1), 3.8 (m, 1), 7.27-8.38 (m, 7). Anal. Calcd for C19H21-
NO2S: C, 69.72; H, 6.41; N, 4.27. Found: 69.44; H, 6.41; N, 4.17
Fmoc-Phe-Leu-O-â-Nsm. To a solution of 0.747 g (3 mmol)
of Boc-Leu-OH·H2O, 0.738 g (3 mmol) of â-Nsm-OH, and 100 mg
of DMAP in 30 mL of dry DCM was added 0.618 g (3 mmol) of
DCC at 0 °C. After the mixture was stirred at 0 °C for 1 h and at
room temperature overnight, the solvent was removed in vacuo,
90 mL of EtOAc was added, and the mixture was filtered to remove
DCU. The organic layer was washed with citric acid, saturated
NaHCO3, and saturated NaCl (3 × 25 mL each) and dried (MgSO4).
The solvent was removed in vacuo, the residue was stored under
vacuum for 2 h, and then 30 mL of TFA/DCM (50% v/v) was
added and the solution stirred for 2 h at room temperature. After
the Boc group had been completely removed, the solvent mixture
was evaporated in vacuo and the residue was recrystallized from
EtOH/ether (1:100) to give the TFA salt, which was obtained as
cream-colored crystals in 80% yield: mp 161-163 °C; IR (KBr)
(c) Establishment of Minimal Loss of Configuration via Acid
Fluoride Coupling. A second test for the loss of configuration was
made using the acid fluoride (see the Supporting Information) of
R-Nsmoc-Phg-OH prepared in the normal manner.11 The acid
fluoride (346 mg, 0.82 mmol) was added in one portion to a stirred
solution of 104 mg (0.75 mmol) of H-Ala-OMe‚HCl in 10 mL of
DCM in the presence of 0.261 mL (1.5 mmol) of DIEA. The
mixture was stirred at room temperature for 10 min (IR analysis
showed complete disappearance of the acid fluoride band at 1846
cm-1). An additional 25 mL of DCM was added, the DCM layer
washed with 5% citric acid, saturated NaHCO3, and saturated NaCl
solution (2 × 10 mL each), and the solvent dried over MgSO4.
The solvent was evaporated with a rotary evaporator and examined
1
as described above in the OMe singlet region of the H NMR
spectrum. This showed that no more than 0.6% of the DL-
diastereomer could have been present. The acid fluoride was also
coupled to H-Ala-OMe‚HCl by a two-phase method in the presence
of sodium bicarbonate. In this case, somewhat more loss of
configuration was observed (1.23% of the DL-diastereomer). For
the 1H NMR spectra detailing these tests, see the Supporting
Information.
1
1157, 1303 (SO2), 1675, 1754 (CO amide, ester) cm-1; H NMR
(CDCl3 + TFA) δ 0.95 (m, 6), 1.75-1.99 (m, 3), 4.24 (m, 1), 5.37
(s, 2), 7.26-8.09 (m, 10). Without further purification, to 1.42 g
(3 mmol) of the TFA salt in 30 mL of DMF were added 1.7 mL
(9.9 mmol) of DIEA, 1.3 g (3.3 mmol) of Fmoc-Phe-OH, and
1.26 g (3.3 mmol) of N-HATU. The reaction mixture was stirred
for 30 min at 0 °C and at room temperature for 2 h. The solution
was diluted with 120 mL of EtOAc, washed with 5% citric acid,
saturated NaHCO3, and saturated NaCl (3 × 30 mL each), and then
dried over MgSO4. The solvent was removed in vacuo, and the
resulting residue was flash chromatographed using EtOAc/hexane
(3/2) to give the dipeptide as cream-colored crystals from DCM/
hexane in 79.4% yield: mp 128-130 °C; IR (KBr) 1157, 1301
(SO2), 1675 (CO, amide), 1723 (CO ester, urethane), 3359 (NH)
1,1-Dioxo-2-(chloromethyl)naphtho[1,2-b]thiophene (23). To
a solution of 3 g (12.2 mmol) of 1,1 dioxo-2-(hydroxymethyl)-
naphtho[1,2-b]thiophene in 20 mL of DMF at 0 °C was slowly
added 10 mL of thionyl chloride. The reaction mixture was stirred
for 30 min at 0 °C and for 5 h at room temperature and quenched
with 200 mL of ice-water and the off-white precipitate filtered
and dried. Recrystallization from DCM/hexane gave 2.3 g (72.2%)
of the 2-chloromethyl derivative as off-white crystals: mp 158-
1
160 °C; IR (KBr) 1148, 1291 (SO2) cm-1; H NMR (CDCl3) δ
4.61 (d, 2), 7.41 (d, 2), 7.63 (m, 1), 7.71 (m, 1), 7.93 (d, 1), 8.12
(d, 1), 8.27 (d, 1). Anal. Calcd for C13H9ClO2S: C, 58.99; H, 3.40.
Found: C, 58.90; H, 3.42
1
cm-1; H NMR (CDCl3) δ 0.9 (m, 6), 1.48-1.73 (m, 3), 3.09 (d,
2), 4.13-4.69 (m, 5), 5.15 (d, 2), 6.26 (d, 1), 7.16-8.08 (m, 21).
Anal. Calcd for C43H40N2O7S: C, 70.89; H, 5.84; N, 3.84. Found:
C, 70.87; H, 5.56; N, 3.78
1,1-Dioxo-2-(1-piperidinomethyl)naphtho[1,2-b]thiophene (22).
To 0.66 g (2.5 mmol) of 1,1-dioxo-2-(chloromethyl)naphtho[1,2-
b]thiophene was added 20 mL of DMF and 5 mL of piperidine.
The reaction mixture was stirred for 17 h, 200 mL of EtOAc added,
and the solution washed with water and saturated NaCl (3 × 70 mL
each). After drying over MgSO4, the solvent was evaporated in
vacuo and the resulting residue was flash chromatographed using
Fmoc-Gly-Phe-Leu-O-â-Nsm. A solution of 0.728 g (1 mmol)
of Fmoc-Phe-Leu-O-â-Nsm in 10 mL of DMF containing 20%
t-BuNH2 was stirred at room temperature for 3 min. The reaction
was quenched by quickly adding 100 mL of EtOAc, and the solution
was washed with H2O (3 × 25 mL), dried over MgSO4, and
J. Org. Chem, Vol. 72, No. 5, 2007 1735

Carpino et al.
evaporated in vacuo. The resulting oil was dissolved in 7 mL of
DCM and 3 mL of DMF, and 0.3 g (1.0 mmol) of Fmoc-Gly-F
was added in addition to 174 µL (10 mmol) of DIEA. The mixture
was stirred for 45 min at room temperature, and then DCM was
evaporated, 100 mL of EtOAc was added, the solution was washed
with water, saturated Na2CO3, and saturated NaCl (3 × 25 mL each)
and dried over MgSO4. The solvent was evaporated in vacuo, and
the resulting oil was precipitated from DCM/hexane, washed with
hexane, and then chromatographed using EtOAc/hexane (3/2) as
an eluent. The residue was separated and recrystallized from DCM/
hexane to give Fmoc-Gly-Phe-Leu-O-â-Nsm which was obtained
as cream-colored crystals in 31.8% yield: mp 95-120 °C; IR (KBr)
1151, 1301 (SO2), 1664 (CO, amide), 1728 (CO ester and urethane),
the initial adduct 21. At the same time, peaks at δ 6.59 and 6.98
appear from the liberated PCA. The peak at δ 3.35 then builds up
as the initial adduct is converted to the final stable adduct 22. A
similar series of changes was noted for the Bsmoc and â-Nsmoc
derivatives. By measuring at 30 s intervals the integrated areas for
the buildup of PCA at δ 6.59 or the decrease in area for the CH2O
peaks near δ 5.2 and plotting the figures against time shows the
rough half-lives of the R-Nsmoc, Bsmoc, and â-Nsmoc systems to
be 2.0, 2.4, and 7.4 min respectively.
Acknowledgment. We are indebted to the National Science
Foundation (NSF CHE-9003192) and the National Institutes of
Health (GM-09706) for support of this work. Millipore, Inc.,
Perseptive Biosystems, Inc., and Polycarbon, Inc. kindly sup-
plied the N-HATU used in this study. The National Science
Foundation is also thanked for grants used to purchase the high-
field NMR spectrometers used in this research.
1
3348 (NH) cm-1; H NMR (CDCl3) δ 0.85 (d, 6), 1.65 (m, 3),
3.09 (d, 2), 3.83 (d, 2), 4.20 (t, 1), 4.34 (d, 2) 4.57 (m, 1), 4.70 (m,
1), 5.21 (d, 2), 5.48 (t, 1), 6.57 (d, 2), 7.17-8.12 (m, 20). Anal.
Calcd for C45H43N3O8S: C, 68.80; H, 5.47; N, 5.34. Found: C,
68.50; H, 5.62; N, 5.30
Relative Reactivities of Bsmoc, R-Nsmoc, and â-Nsmoc PCA
Derivatives. In an NMR tube, 10 mg of carbamate was dissolved
in a mixture of 0.5 mL of CDCl3 and 0.4 mL of DMSO-d6. The
Supporting Information Available: Additional experimental
details for the synthesis of sulfone alcohols 11 and 17, full char-
acterization data for all R- and â-Nsmoc amino acids and selected
spectral data for these and other new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
1
initial H NMR spectrum was recorded at 500 MHz, 4.9 µL (2
equiv) of piperidine was added, and the progress of the reaction
was followed by changes in the NMR spectra. Thus, for the
R-Nsmoc system the peak at δ 5.24 (CH2O) decreases in intensity
as peaks at δ 6.13, 6.45, and 5.08 appear due to the formation of
JO062397G
1736 J. Org. Chem., Vol. 72, No. 5, 2007