Concerning selectivity in the oxidation of peptides by dioxiranes. Further insight into the effect of carbamate protecting groups

Article abstract of DOI:10.1021/jo100855h

(Figure Presented) With use of methyl(trifluoromethyl)dioxirane (TFDO), the oxidation of some tripeptide esters protected at the N-terminus with carbamate or amide groups could be achieved efficiently under mild conditions with no loss of configuration at the chiral centers. Expanding on preliminary investigations, it is found that, while peptides protected with amide groups (PG = Ac-, Tfa-, Piv-) undergo exclusive hydroxylation at the side chain, their analogues bearing a carbamate group (PG = Cbz-, Moc-, Boc-, TcBoc-) give competitive and/or concurrent hydroxylation at the terminal N-H moiety. Valuable nitro derivatives are also formed as a result of oxidative deprotection of the carbamate group with excess dioxirane. A rationale is proposed to explain the dependence of the selectivity upon the nature of the protecting group.

Full text of DOI:10.1021/jo100855h

pubs.acs.org/joc
Concerning Selectivity in the Oxidation of Peptides by Dioxiranes.
Further Insight into the Effect of Carbamate Protecting Groups
Cosimo Annese,† Lucia D’Accolti,† Marta De Zotti,‡ Caterina Fusco,† Claudio Toniolo,‡
Paul G. Williard,§ and Ruggero Curci*,†,§
†
ꢀ
Dipartimento Chimica, CNR-ICCOM, Universita di Bari, v. Amendola 173, 70126 Bari, Italy,
‡Dipartimento Scienze Chimiche, Universitaꢀdi Padova, v. Marzolo 1, 35131 Padova, Italy, and Department
of Chemistry, Brown University, Providence, Rhode Island 02912
§
curci@ba.iccom.cnr.it
Received May 4, 2010
With use of methyl(trifluoromethyl)dioxirane (TFDO), the oxidation of some tripeptide esters protected
at the N-terminus with carbamate or amide groups could be achieved efficiently under mild conditions
with no loss of configuration at the chiral centers. Expanding on preliminary investigations, it is found
that, while peptides protected with amide groups (PG=Ac-, Tfa-, Piv-) undergo exclusive hydroxylation
at the side chain, their analogues bearing a carbamate group (PG=Cbz-, Moc-, Boc-, TcBoc-) give
competitive and/or concurrent hydroxylation at the terminal N-H moiety. Valuable nitro derivatives are
also formed as a result of oxidative deprotection of the carbamate group with excess dioxirane. A
rationale is proposed to explain the dependence of the selectivity upon the nature of the protecting group.
SCHEME 1. Common Dioxiranes in the Isolated Form
Introduction
Over the past two decades, the dimethyldioxirane (DDO)
(1a)1 and its trifluoro analogue 1b (TFDO)2 (Scheme 1) have
fruitfully been employed to accomplish the selective oxy-
functionalization of natural targets such as steroids, vita-
mine D3 derivatives, and terpenes.3 Among the many useful
dioxirane oxidations, a transformation that counts among
the highlights is the oxygenation of simple, “unactivated”
C-H bonds; high tertiary vs secondary selectivities (Rst from
15 to over 250) can be routinely achieved.2,3a,4
Some N-protected derivatives of R-amino esters5 and
peptides6 have also been examined; in this case also, it was
shown that dioxiranes offer advantages over classical oxida-
tion methods, affording the selective oxyfunctionalization of
tethered alkyl side chains. High regioselectivity for O-insertion
into the γ-CH bond of leucine (Leu) residues with respect to
the weaker R-CH bonds was observed with no appreciable loss
of configuration at the chiral centers.5,6
Interestingly, we reported that using the powerful TFDO
(1b) in the oxidation of Boc protected amino esters Boc-
L-Val-OCH3 and Boc-L-Leu-OCH3;concurrent with the
oxyfunctionalization at the side chain;the hydroxylation
of the protected NH moiety takes place; removal of the
(1) (a) Murray, R. W.; Jeyaraman, R. J. Org. Chem. 1985, 50, 2847.
(b) Cassidei, L.; Fiorentino, M.; Mello, R.; Sciacovelli, O.; Curci, R. J. Org.
Chem. 1987, 52, 699.
(2) Mello, R.; Fiorentino, M.; Sciacovelli, O.; Curci, R. J. Org. Chem.
1988, 53, 3890.
(3) For recent dioxirane reviews, see: (a) Curci, R.; D’Accolti, L.; Fusco,
C. Acc. Chem. Res. 2006, 39, 1. (b) Bach, R. D. In The Chemistry of Peroxides;
Patai, S., Ed.; Wiley: New York, 2006; Vol. 2, Chapter 1. (c) Adam, W.; Zhao,
C.-G.; Kavitha, J. Organic Reactions; Wiley: Hoboken, NJ, 2007, Vol. 69,
pp 1-346. See also references cited therein.
(4) Mello, R.; Fiorentino, M.; Fusco, C.; Curci, R. J. Am. Chem. Soc.
1989, 111, 6749.
(5) Detomaso, A.; Curci, R. Tetrahedron Lett. 2001, 42, 755.
(6) (a) Rella, M. R.; Williard, P. G. J. Org. Chem. 2007, 72, 525. (b)
Saladino, R.; Mezzetti, M.; Mincione, E.; Torrini, I.; Paglialunga Paradisi,
M.; Mastropietro, G. J. Org. Chem. 1999, 64, 8468.
4812 J. Org. Chem. 2010, 75, 4812–4816
Published on Web 06/18/2010
DOI: 10.1021/jo100855h
r
2010 American Chemical Society

Annese et al.
JOCArticle
SCHEME 2. Oxidation of Boc-L-Leu-OMe with TFDO (1b)
CHART 1
Boc protecting group with TFA affords the corresponding
N-hydroxyamino methyl esters in high yield. An example is
shown in Scheme 2.5 With use of the less powerful DDO (1a)
instead of 1b, these oxidations are rather sluggish, requiring
long reaction times to afford just the hydroxylation at the
side chain in low yield.6b
Subsequent extension of these studies to the TFDO oxida-
tion of Boc-protected di- and tripeptide esters bearing alkyl
side chains showed that hydroxylation of the terminal N-H
can be achieved efficiently, with no appreciable loss of con-
figuration at the chiral centers.6a We also found that, while
N-Boc peptides undergo oxyfunctionalization at the terminal
N-H preferentially, the corresponding N-acetyl (Ac) peptides
experience exclusive hydroxylation at the side chain (CH3)2-
C-H.5,6a The chemoselectivity observed appeared puzzling, so
that we decided to expand our investigations aiming to shed
light into this peculiar effect of the protecting group (PG).
Results and Discussion
We began by examining the oxidation of the model tri-
peptide methyl ester D-Leu-L-Ala-L-Ala-OCH3 (2) (Chart 1)
in order to gain insight into the effect of changing the PG
from Boc- (2a) and Ac- (2b) to Tfa (trifluoroacetyl)- (2c) and
Piv (pivaloyl)- (2d) on product distribution. The results are
shown in Table 1; the reactions were carried out under the
mild conditions reported therein.
The oxidation procedure merely involved the addition of
an aliquot of standard dioxirane solution (0.5-1.0 M)2 to an
acetone solution of the tripeptide on a 75-100 mg scale.
Reaction mixtures were separated by preparative TLC
(silica gel, eluent Et2O/hexane/acetone) and products identi-
fied by 1H and 13C NMR, as well as by HRMS. For instance,
the mass spectra of monohydroxylation products 3 and 4
were significant; with respect to the parent ion peak (M)
of starting materials 2a-g, these showed a mass increase of
m/z þ16 consistent with the incorporation of a single oxygen
atom, whereas the bis-hydroxylation derivatives 5 displayed
a mass increase at m/z þ32.
TABLE 1. Oxidation of Boc-, Ac-, Tfa-, and Piv- Derivatives 2a-d with
TFDO (1b)a
entry
1
substrate
PG
ox/subb
product
yield (%)c
2a
Boc
2.4
3a
5a
4b
4c
4d
47
28
62
66
68
2
3
4
2b
2c
2d
Ac
Tfa
Piv
4
5
4
aAll reactions were routinely run in acetone at 0 °C, reaction time
4-6 h, substrate conversion 80-97% (based on the amount of recovered
starting material). bMolar ratio of dioxirane oxidant to substrate.
cIsolated yield, based on the amount of starting material reacted.
Results in Table 1 show that just the carbamate Boc-
protecting group allows hydroxylation at the terminal NH,
yielding mainly 3a (PG = Boc). The latter is accompanied by
the consecutive overoxidation product 5a, in lower yield.
Instead, the other PGs examined yield side chain oxidation
products only (4b-d), in spite of the varying electronic and
steric effects. This change in selectivity prompted us to
examine the TFDO oxidation of a few substrates presenting
a carbamate PG other than Boc, i.e. 2e-g as well as 77 and 98
(Chart 2). The results are collected in Table 2. Similar to Boc-
protected 2a (Table 1), it is seen that oxidation of its TcBoc
The actual site of oxidation could be established by NMR
spectroscopy. In particular, the absence of a NH resonance
in the 1H NMR spectra of products 3 and 5 was significant; in
the proton spectra of 4 and 5, the two methyl resonance
doublets present for substrates 2 were replaced by two lower
field singlets, as a result of O-insertion into the (CH3)2C-H
bond. With respect to the starting material, in the {1H}13C
NMR spectra of 3 and 5 quite telling was a lower field shift
of the resonance pertaining to carbon adjacent to the PG-
N(OH)- group, and the appearance of a resonance at ca. 70.5
ppm (C-OH) for products 4 and 5.
(7) Toniolo, C.; Pantano, M.; Formaggio, F.; Crisma, M.; Bonora,
G. M.; Aubry, A.; Bayeul, D.; Dautant, A.; Boesten, W. H. J.; Schoemaker,
H. E.; Kamphuis, J. Int. J. Biol. Macromol. 1994, 16, 7.
(8) Barrett, A. G. M.; Pilipauskas, D. J. Org. Chem. 1990, 55, 5170.
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Annese et al.
JOCArticle
CHART 2
(3e,g and 4e,g), but now the valuable nitro derivative 6 is also
formed (entries 1 and 3).
Furthermore, the terminal nitro compound 8 largely pre-
vails as the major product in the oxidation of R-CH3
branched peptide derivative 7 (entry 4).9 Similarly, for the
oxidation of Cbz-protected R-amino ester 9 (entry 5) we
observed that formation of 1110 (the 4-butanolide derived
from preliminary side chain hydroxylation; cf. Scheme 2)5,6
is accompanied by the production of nitro derivative 1011 in
some 20% yield.
For the transformations above, it seems safe to assume
that the terminal nitro functionality would result from
oxidative deprotection of the carbamate group. It might be
envisaged that;preceded or followed by hydroxylation at
the terminal N-H;this takes place along with dioxirane
oxidation at the “activated” benzylic C-H bond (a process
which is well documented),12 release of PhCO2H, and facile
decarboxylation of the resulting, labile carbamic acid inter-
mediate HO2C-N(OH)-.
In agreement with this view, we observe that the conver-
sion of 9 into nitro derivative 10 and of substrate 2g into 6 are
both accompanied by the production of benzoic acid in
equimolar amounts.
Clearly, a key step here consists of the final oxidative
conversion of the freed hydroxyamino moiety into a nitro
group. This step could mimic in part the sequence envisaged
for the dioxirane oxidation of amines with excess dioxirane,
i.e., RNH2 f RNH;OH f RNdO f RNO2.13 Hence in
the controlled reaction of amino sugars with stoichiometric
DDO (1a) at -40 °C, the oxidation of the -NH2 moieties
could be tuned to stop at the hydroxylamine stage.14 The
oxidation of secondary amines to hydroxylamines is readily
achieved with a single dioxirane equivalent.13c It is known
that amides resist oxidation by dioxiranes, although the
corresponding amines are readily oxidized. Actually, this
transformation is often employed to protect amines.3b
In view of the above findings, a few words are in order
concerning the still unsettled mechanistic dilemma concern-
ing how the N-hydroxylations actually take place.3 We
consider it is unlikely that, at odds with what was established
for the oxygenation of unactivated C-H bonds,3 this could
occur by direct dioxirane O-insertion into the N-H moiety.
TABLE 2. OxidationofSubstratesHaving the Terminal NH Functional-
ity Protected with Carbamate Groups with TFDO (1b) a
entry
1
substrate
ox/subb
PG
products
yield (%)c
2e
2.4
Moc
3ed
4e
6
28e
26
13
57
21e
15e
44
24
60
20
40
2
3
2f
5
5
TcBoc
Cbz
4f
5f f
3gd
4g
6g
2g
4
5
7
9
6
5
Cbz
Cbz
8g
10g
11
aAll reactions were routinely run in acetone at 0 °C, reaction time
2-4 h, substrate conversion 65- 80% (based on the amount of recove-
red starting material). bMolar ratio of dioxirane oxidant to substrate.
cIsolated yield, based on the amount of starting material reacted.
dObtained in mixture with the unreacted starting material. eAs estimated
by integration of the 1H NMR signals relative to the leucine R-CH’s
of the compounds in the mixture. fObtained in a mixture with 4f. gThe
formation of the nitro derivatives 6, 8, and 10 is accompanied by the
production of benzoic acid in equimolar amount.
(9) It is worthy of note that the analogous peptide 2g, lacking a Me group
at the C R to the terminal N-H, affords mainly side chain hydroxylation. The
varying selectivity might be due to subtle changes in the preferred conforma-
tion adopted by peptide 7 in undergoing dioxirane attack, owing to limited
flexibility resulting from branching at the R-carbon. ( Toniolo, C.; Crisma,
M.; Formaggio, F.; Peggion, C. Biopolymers 2001, 60, 396 and references
cited therein).
(10) Altman, J.; Moshberg, R.; Ben-Ishai, D. Tetrahedron Lett. 1975, 16,
3737.
(11) (a) Rozen, S.; Bar-Haim, A.; Mishani, E. J. Org. Chem. 1994, 59,
1208. (b) Laloo, D.; Mahanti, M. K. J. Chem. Soc., Dalton Trans. 2 1990, 311.
(c) Rawalay, S. S.; Shechter, H. J. Org. Chem. 1967, 32, 3129.
(12) (a) Murray, R. W.; Jeyaraman, R.; Mohan, L. J. Am. Chem. Soc.
1986, 108, 2470. (b) Mello, R.; Cassidei, L.; Fiorentino, M.; Fusco, C.; Curci,
R. Tetrahedron Lett. 1990, 31, 3067. (c) Kuck, D.; Schuster, A. Z. Natur-
forsch. 1991, 46B, 1223. (d) Marples, B. A.; Muxworthy, J. P.; Baggaley,
K. H. Synlett 1992, 646.
(13) (a) Murray, R. W.; Jeyaraman, R.; Mohan, L. Tetrahedron Lett.
1986, 27, 2335. (b) Murray, R. W.; Rajadhyaksha, S. N.; Mohan, L. J. Org.
Chem. 1989, 54, 5783. (c) Crandall, J. K.; Reix, T. J. Org. Chem. 1992, 57,
6759. (d) Synthesis of nitro-substituted cyclopropanes and spiropentanes via
oxidation of the corresponding amino derivatives: Volkova, Y. A.; Ivanova,
O. A.; Budynina, E. M.; Revunov, E. V.; Averina, E. B. Tetrahedron Lett.
2009, 50, 2793.
(2,2,2-trichloro-tert-butyloxycarbonyl) counterpart (entry 2,
Table 2) gives rise to N-H hydroxylation product 5f besides
side chain O-insertion (4f) under the given conditions. For
the sake of simplicity, among the several results in our hands,
we have chosen to report in Tables 1 and 2 just those oxidant/
substrate ratios that produce the best isolated yields of the
given products, with sizable substrate conversion during a
reasonable reaction time.
The oxidation of Moc (methyloxycarbonyl) derivative 2e
and Cbz derivative 2g also displays terminal N-H hydro-
xylation along with O-insertion at the side chain (CH3)2C-H
(14) Wittman, M. D.; Halcomb, R. L.; Danishefsky, S. S. J. Org. Chem.
1990, 55, 1981.
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Annese et al.
JOCArticle
Conclusions
Be the mechanistic details as they may, results herein
suggest that the powerful dioxirane TFDO (1b) should be
the oxidant of choice for the synthesis of unnatural peptides or
amino acids, presenting selective oxidative modifications at
the protected N-H element and/or at the side chain C-H
moiety as those of the D-Leu N-terminal residue in the
substrates examined. This finding is valuable if one recalls
that the products usually are ammonia (or ammonium salts)
and nitrogen-free residues when amino acids or peptides are
directly oxidized with common oxidants.11 Then, our method
should bring a major flexibility in the design of novel bioactive
peptide analogues. In particular, the efficient production of
N-OH derivatives 3a, 3e, 3g, and 5 is notable because the
synthesis of N-hydroxypeptides represents a challenging goal;
in fact, these are key intermediates in metabolic pathways and
can be found in human and animal tumors.16
In addition, the TFDO oxidations that yield the terminal
nitro derivatives 6, 8, and 10 represent no small feat. Indeed,
the family of such nitro compounds stand for the starting
point for several useful synthetic applications.17 It is also
remarkable that, presumably because of a nonradical oxida-
tion mechanism, the reactions reported herein occur with
complete retention of configuration. This stereochemical
outcome is observed with all products reported, with the
obvious exception of 6 and 10, in which case the chirality at
the proximal R-CH was not retained due to the considerable
acidity of the hydrogen adjacent to the nitro group.11
FIGURE 1. Energy barrier to a TS featuring a quasipyramidal
nitrogen lone pair receiving dioxirane electrophilic attack.
Rather, it might result from previous electrophilic oxidation
at the lone pair on nitrogen, followed by the rapid conversion
of the N-oxide intermediate into a hydroxylamino derivative,
i.e., R(H)(H)N: f R(H)(H)Nþ-O- f R(H)N-OH .
If this is the case, for the transformation at hand it remains
unclear why only carbamate protected peptide esters display
N-H hydroxylation, while those presenting the amide PG
do not. An answer might be found considering first that,
with respect to the amine moiety in the amide group, the
availability of the nitrogen lone pair for dioxirane electro-
philic attack is widely diminished because of delocalization
into the carbonyl over the π system. Thus, in an amide group
-NH-C(:O)-, for an approximately planar three-atom fra-
mework the rotational barrier to twist the C-N linkage is
increased by more than 10 kcal/mol as compared to ordinary
amines.15a
Second, one might recall that several investigations15 have
shown that the barrier to rotation about partial C-N double
bonds in amides and carbamates (urethanes) varies mark-
edly. Actually, in a carbamate group -O-C(:O)-NH-, two
resonances (nN f π*C-O and nO f π*C-O) compete with
each other for the delocalization onto the same π*C-O
orbital, thus lowering the activation energy to rotation about
partial C-N double bonds (Figure 1) by ca. 2-4 kcal mol-1
with respect to that of comparable amides (Eq = 12.4 to 14.3
kcal mol-1).15 Thus, as sketched in Figure 1, dioxirane attack
should be facilitated.
Consistent with this view is our finding that, when an
effective electron-withdrawing group (R) is present in the
carbamate PG, the diminished electron density on oxygen
hinders the competition with the nitrogen for conjugation
to the carbonyl, so that the rotational barrier to the transi-
tion state increases;15 as a result, the process of N-oxidation
becomes less favorable. This seems to be the case for the
TcBoc- substrate 2f. In this instance, the presence of the
electron-withdrawing group (R = Cl3C-(CH3)2C-) signifi-
cantly reduces the amount of the N-hydroxylation product
5f with respect to that of side chain oxidation 4f (entry 2,
Table 2).
Experimental Section
Starting Materials. Boc-tripeptide 2a was obtained following
coupling procedures in solution and then used for the synthesis of
substrates 2b-g upon deprotection (TFA or dry HCl/MeOH)18
and reprotection of the free terminal amino group as appropriate.
Tripeptide 7 was synthesized according to a reported method;7
Cbz-Leu-OCH3 (9) was obtained starting with the corresponding
commercial acid upon reaction with CH3I.19 These substrates
presented purity >95% (HPLC and/or 1H NMR).
N-tert-Butyloxycarbonyl-D-leucyl-L-alanyl-L-alanine methyl ester
(2a): mp 78-80 °C; [R]D -32.4 (c 0.96, CH3OH); HRMS-ESI
(M þ Hþ) calcd for C18H34N3O6 388.2448, found 388.2396.
þ
N-Acetyl-D-leucyl-L-alanyl-L-alanine methyl ester (2b): mp 188-
189 °C; [R]D -34.8 (c 0.93, CH3OH); HRMS-ESI (M þ Hþ)
calcd for C15H28N3O5þ 330.2029, found 330.2023. N-Trifluoro-
acetyl-D-leucyl-L-alanyl-L-alanine methyl ester (2c): mp 172-
173 °C; [R]D -32.6 (c 1.07, CH3OH); HRMS-ESI (M þ Hþ)
þ
calcd for C15H25F3N3O5 384.1746, found 384.1846. N-Tri-
methylacetyl-D-leucyl-L-alanyl-L-alanine methyl ester (2d): mp
110-113 °C; [R]D -37.7 (c 0.96, CH3OH); HRMS-ESI (M þ
Hþ) calcd for C18H34N3O5 372.2498, found 372.2531. N-
þ
Methyloxycarbonyl-D-leucyl-L-alanyl-L-alanine methyl ester (2e):
mp 160-162 °C; [R]D -18.3 (c 0.7, CH3OH); HRMS-ESI
þ
It is apparent that further careful work is in order to
put our intriguing mechanistic hypothesis to the test. In any
case, in view of the limited electron density at nitrogen of
the -O-C(:O)-NH- moieties, it is perhaps not surprising
that a potent oxidant such as TFDO should be needed for
N-hydroxylation.
(MþNa) calcd for C15H27N3NaO6 368.1798, found 368.1787.
N-(2,2,2-Trichloro-tert-butyloxycarbonyl)-D-leucyl-L-alanyl-L-ala-
nine methyl ester (2f): mp 81-83 °C; [R]D -23.6 (c 3.4, CH3OH);
(16) (a) Yanagisawa, A.; Takeshita, S.; Izumi, Y.; Yoshida, K. J. Am.
Chem. Soc. 2010, 132, 5328. (b) Merino, P.; Tejero, T. Angew.Chem., Int. Ed.
2004, 43, 2995 and references cited therein.
(17) (a) Eyer, M.; Seebach, D. J. Am. Chem. Soc. 1985, 107, 3601. (b)
Ram, S.; Ehrenkaufer, R. E. Synthesis 1986, 133. (c) Gogte, V. N.; Natu,
A. A.; Pore, V. S. Synth. Commun. 1987, 17, 1421.
(18) For instance, see: Shendage, D. M.; Froehlich, R.; Haufe, G. Org.
Lett. 2004, 6, 3675.
(19) Garner, P.; Park, J. M. Org. Synth. 1992, 70, 18.
(15) (a) Pontes, R. M.; Basso, E. A.; dos Santos, F. P. J. Org. Chem. 2007,
72, 1901. (b) Modarresi-Alam, A. R.; Najafi, P.; Rostamizadeh, M.; Keykha,
H.; Bijanzadeh, H. R.; Kleinpeter, E. J. Org. Chem. 2007, 72, 2208. (c)
Yamagami, C.; Takao, N.; Takeuchi, Y. Aust. J. Chem. 1986, 39, 457 and
references cited therein.
J. Org. Chem. Vol. 75, No. 14, 2010 4815

Annese et al.
JOCArticle
HRMS-ESI (M þ Naþ) calcd for C18H30Cl3N3NaO6þ 512.1098,
found 512.1095. N-Benzyloxycarbonyl-D-leucyl-L-alanyl-L-alanine
methyl ester (2g): mp 157-158 °C; [R]D -27 (c 1.33, CH3OH);
N-Methyloxycarbonyl-N-hydroxy-D-leucyl-L-alanyl-L-alanine
methyl ester (3e): obtained in mixture (ca. 1:2, by 1H NMR) with
starting material 2e; HRMS-ESI (M þ Naþ) calcd for C15H27-
N3NaO7þ 384.1747, found 384.1727.
N-Methyloxycarbonyl-γ-hydroxy-D-leucyl-L-alanyl-L-alanine
methyl ester (4e): mp 153-156 °C; [R]D -19.1 (c 0.4, CH3OH);
HRMS-ESI (M þ Hþ) calcd for C15H28N3O7þ 362.1927, found
362.1910.
N-(2,2,2-Trichloro-tert-butyloxycarbonyl)-γ-hydroxy-D-leucyl-
L-alanyl-L-alanine methyl ester (4f): mp 67-69 °C; [R]D -20.3
(c 0.5, CH3OH); HRMS-ESI (M þ Naþ) calcd for C18H30Cl3-
N3NaO7þ 528.1047, found 528.1027.
HRMS-FAB (M þ Naþ) calcd for C21H31N3NaO6 444.2111,
þ
found 444.2120. N-Benzyloxycarbonyl-r-methyl-D-leucyl-L-ala-
nyl-L-alanine methyl ester (7):7 mp 101-102 °C; [R]D -38.1 (c
0.5, CH3OH). N-Benzyloxycarbonyl-L-leucine methyl ester (9):8
oil; [R]D -29 (c 1.8, CH3OH) [lit.8 -28 (c 2.1, CH3OH)]; physical
constants and spectral data in agreement with literature.8
The following procedure is representative of the TFDO
oxidations of substrates 2a-g, 7, and 9:
Oxidation of N-tert-Butyloxycarbonyl-D-leucyl-L-alanyl-L-
alanine Methyl Ester (2a) with 1b. Solutions of 0.8-1.0 M
methyl(trifluoromethyl)dioxirane (1b) were made available ado-
pting procedures, equipment, and precautions already reported
in detail.2 To a stirred solution of 2a (76.5 mg, 0.197 mmol) in
acetone (2 mL) at 0 °C was added in one portion a standardized
cold solution of TFDO (1b) in 1,1,1- trifluoropropanone (TFP)
(0.55 M, 0.85 mL, 0.473 mmol).20 The reaction progress was
monitored by TLC (silica gel, Et2O/n-hexane 3:1) and by
following the dioxirane decay (iodometry).2 Upon reaction
completion (ca. 4 h), the solvent was removed in vacuo and
the reaction mixture was separated by preparative TLC (silica
gel, Et2O/n-hexane 3:1); besides unreacted starting material
(2.3 mg, 5.94 μmol, conversion 97%), pure products 3a (36.4
mg, 0.090 mmol) and 5a (22.3 mg, 0.053 mmol) could be isolated
respectively in 47% and 28% yield (based on the amount of
starting material reacted).
N-(2,2,2-Trichloro-tert-butyloxycarbonyl)-N,γ-dihydroxy-D-
leucyl-L-alanyl-L-alanine methyl ester (5f): obtained in mixture
(ca. 1:1, by 1H NMR) with product 4f; HRMS-ESI (M þ Naþ)
calcd for C18H30Cl3N3NaO8þ 544.0996, found 544.1224.
N-Benzyloxycarbonyl-N-hydroxy-D-leucyl-L-alanyl-L-alanine
1
methyl ester (3g): obtained in mixture (ca. 1:1, by H NMR)
with starting material 2g; HRMS-ESI (M þ Hþ) calcd for
C21H32N3O7þ 438.2240, found 438.2337.
N-Benzyloxycarbonyl-γ-hydroxy-D-leucyl-L-alanyl-L-alanine
methyl ester (4g): mp 133-136 °C; [R]D -17.0 (c 0.8, CH3OH);
HRMS-FAB (M þ Naþ) calcd for C21H31N3NaO7þ 460.2060,
found 460.2074.
N-(4-Methyl-2-nitropentanoyl)-L-alanyl-L-alanine methyl ester
(6): obtained as a 1:1 mixture (1H NMR) of 2R- and 2S-diastereo-
isomers; HRMS-FAB (M þ Naþ) calcd for C13H23N3NaO6
þ
340.1485, found 340.1470.
N-tert-Butyloxycarbonyl-N-hydroxy-D-leucyl-L-alanyl-L-alanine
methyl ester (3a): mp 78-80 °C; [R]D -24.8 (c 2.5, CH3OH);
N-[(S)-2,4-Dimethyl-2-nitropentanoyl]-L-alanyl-L-alanine methyl
ester (8):mp 78-81 °C; [R]D -17.6 (c0.75, CH3OH); HRMS-FAB
(M þ Naþ) calcd for C14H25N3NaO6þ 354.1641, found 354.1648.
Methyl 4-methyl-2-nitropentanoate (10)10 (oil) and (S)-2-(benzy-
loxycarbonylamino)-4,4-dimethyl-4-butanolide (11):9 mp 91-
92 °C [lit.9 mp 94 °C]; [R]D -4.0 (c 3.0, CHCl3); spectral data
in agreement with literature.
HRMS-FAB (M þ Naþ) calcd for C18H33N3NaO7 426.2216,
þ
found 426.2391.
N-tert-Butyloxycarbonyl-N,γ-dihydroxy-D-leucyl-L-alanyl-L-
alanine methyl ester (5a): mp 52-54 °C; [R]D -23.1 (c 2.0,
þ
CH3OH); HRMS-FAB (M þ Naþ) calcd for C18H33N3NaO8
442.2165, found 442.2156.
N-Acetyl-γ-hydroxy-D-leucyl-L-alanyl-L-alanine methyl ester
(4b): mp 141-143 °C; [R]D -17.6 (c 1.1, CH3OH); HRMS-
Acknowledgment. Thanks are due to the Ministry of Edu-
cation of Italy (MIUR, grant PRIN 2008), to the National
Research Council (CNR, Rome, Italy), the NIH (GM-35982),
and the NSF (CHE-0718275) for financial support. One of us
(C.A.) is grateful to Brown University for generous hospitality
during a period spent at the Chemistry Department as Visiting
Scientist. Thanks are due to Dr. R. Mello (University of
Valencia, Spain) and to Dr. Tun-Li Shen (Brown University)
for performing some of the HRMS analyses.
FAB (M þ Naþ) calcd for C15H27N3NaO6 368.1798, found
þ
368.1792.
N-Trifluoroacetyl-γ-hydroxy-D-leucyl-L-alanyl-L-alanine methyl
ester (4c): mp 142-143 °C; [R]D -34.6 (c 1.8, CH3OH); HRMS-
FAB (M þ Naþ) calcd for C15H24F3N3NaO6 422.1515, found
þ
422.1525.
N-Trimethylacetyl-γ-hydroxy-D-leucyl-L-alanyl-L-alanine methyl
ester (4d): mp 152-154 °C; [R]D -56.3 (c 1.2,þCH3OH); HRMS-
FAB (M þ Naþ) calcd for C18H33N3NaO6 410.2267, found
410.2260.
Supporting Information Available: General experimental
details, supplemental 1H and 13C NMR characterization data of
oxidation products and of new starting materials, and sample
HPLC runs. This material is available free of charge via the
Internet at http://pubs.acs.org.
(20) As for alternative solvents to acetone, the choice is limited to solvents
which resist oxidation by the powerful dioxirane oxidant. In practice, options
are confined to using chlorinated solvents (CH2Cl2, CHCl3, CCl4), as well as
MeCN.
4816 J. Org. Chem. Vol. 75, No. 14, 2010