Synthesis of histidinoalanine: A comparison of β-lactone and sulfamidate electrophiles

Article abstract of DOI:10.1021/jo200744d

Previous syntheses of histidinoalanine (HAL) have led to mixtures of regioisomers and/or stereoisomers. For example, opening of N-Cbz-d-serine- β-lactone (6) with Boc-l-His-OMe (5) gave a 2:1 mixture of τ- and π-regioisomers. The sulfamidate 10, derived f

Full text of DOI:10.1021/jo200744d

ARTICLE
pubs.acs.org/joc
Synthesis of Histidinoalanine: A Comparison of β-Lactone and
Sulfamidate Electrophiles
Carol M. Taylor* and Samanthi Thabrew De Silva
Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, United States
S Supporting Information
b
ABSTRACT: Previous syntheses of histidinoalanine (HAL)
have led to mixtures of regioisomers and/or stereoisomers. For
example, opening of N-Cbz-D-serine-β-lactone (6) with Boc-
L-His-OMe (5) gave a 2:1 mixture of τ- and π-regioisomers.
The sulfamidate 10, derived from N-benzyl-D-serine methyl
ester (11), was reacted with Boc-L-His-OMe (5) to give the τ-HAL derivative 17 as a single isomer in 57% yield. A similarly prepared
τ-HAL 19, bearing protecting groups that were all hydrogenolytically labile, led to the free bis-amino acid, τ-L-histidinyl-D-alanine
(τ-4), as a salt-free standard for amino acid analysis.
’ INTRODUCTION
The histidinoalanine (HAL, Figure 1) protein cross-link1 was
first isolated from a protein in 1982;2 both regioisomers have since
been identified from a number of sources. Histidinoalanines are
produced in milk products that have been heated and/or treated
with alkali;3 the nutritional consequences are unknown. These bis-
amino acids occur naturally in human tissues (connective tissue,2
dentin,4 and eye cataracts5) where levels appear to correlate with
age and disease state. Moreover, HALs have been identified in
phosphoproteins of bivalve mollusks6 where they are integral to
the process of mineralization.
Histidinoalanines presumably arise by analogy to the forma-
tion of lanthionine and lysinoalanine7 (Scheme 1) in which thiol
and ε-amino nucleophiles respectively add to dehydroalanine.
The theonellamides8 (Figure 1c) are the only example of a HAL
residue in a well-defined, small molecule. As such, these anti-
fungal agents are the imidazole analogues of the much-studied
lantibiotics, of which the best known is nisin.9
Previous syntheses of HAL, typified by that of Fujimoto
(Scheme 2),2 involved a biomimetic, conjugate addition of a
histidine to a dehydroalanine.10 This approach necessarily leads
to mixtures of regioisomers and stereoisomers. Tohdo et al.
Figure 1. (a) τ-HAL (b) π-HAL, and (c) theonellamide C (1).
addressed the stereochemical issue by employing an enantiopure
serine β-lactone as a synthon for the β-alanine cation.11 Our
pursuit of τ-histidinoalanine began with a reinvestigation of this
approach.
i
β-lactone from the PrOOCNHNHCOOiPr. Prolonged expo-
sure to silica, necessary to achieve separation, led to decomposi-
tion of the β-lactone and isolated yields of only 15ꢀ20%.
Sufficient quantities of the β-lactone were nevertheless prepared.
Gentle heating of lactone 6 with a 5-fold excess of Boc-L-His-
OMe (5) and trapping of the intermediate carboxylic acid with
trimethylsilyldiazomethane led to a mixture of the two regio-
isomers of 7 (Scheme 3). The ratio (∼2:1 in favor of the
τ-isomer) was similar to that reported by Tohdo et al.14 for the
corresponding tert-butyl esters (8).
’ RESULTS AND DISCUSSION
The histidine building block 5 was prepared according to
Abdo et al.12 D-Lactone (R)-6 was prepared according to Vederas
and co-workers;13 the procedure involves an intramolecular
Mitsunobu reaction of Cbz-Ser-OH and ideally employs DMAD,
to give 6 in 47% yield. Unfortunately, DMAD is no longer readily
available, and substitution with DIAD led to a product mixture
from which it was difficult to chromatographically separate the
Received: April 9, 2011
Published: May 25, 2011
r
2011 American Chemical Society
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|

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Scheme 1. Cross-Linking Amino Acids Derived from
Dehydroalanine
Scheme 3. Tohdo’s Approach to HALa
Scheme 2. Conjugate Addition to a Dehydroalanine
a Key HMBC correlations for compounds τ-7 and π-7 are depicted by
double-headed arrows.
Scheme 4. Second Generation Retrosynthetic Analysis
separation of the two regioisomers of 7. Flash chromatography
on silica gel gave some pure τ-7 but mostly mixed fractions. We
considered other alanine β-cation synthons, and it seemed that
sulfamidates,15 as introduced by Baldwin and co-workers,16
might provide a viable solution (Scheme 4).17 Baldwin’s original
report looked at the opening of 9 with a variety of heteroatom
nucleophiles. Indeed, there have been two reports of the regio-
selective opening of sulfamidates with unsubstituted imidazole.18,19
Lubell and co-workers have used N-9-phenylfluorenyl19 and
N-fluorenylmethoxycarbonyl (Fmoc) sulfamidates.20 The groups
of Vederas21 and Halcomb22 focused on the use of p-methoxybenzyl
(PMB) protected sulfamidates to produce lanthionine and thiogly-
coside derivatives, respectively. Our early investigations indicated
that the N-benzyl derivatives were more robust than the analogous
N-PMB compounds.
Two sulfamidates were prepared, according to Scheme 5.
Attempts to convert 13 to sulfamidate 10 directly, with sulfu-
ryl chloride, were unsuccessful, presumably due to aziridine
formation.23
Our first attempt at opening sulfamidate 10 followed the
conditions of Wei and Lubell,19 employing Boc-His-OMe (5) as
the nucleophile (Scheme 6), in order to make a meaningful
comparison with the β-lactone route. After 4 h, TLC analysis
showed that all the sulfamidate 10 had been consumed, and we
isolated τ-HAL 17. Presumably, sulfamidate 10 is less reactive
than lactone 6 and thus more regioselective for Nτ. The yield of
We expected that the τ-regioisomer would be the major product
of the reaction. Nevertheless, it was important to distinguish the two
compounds unambiguously by NMR spectroscopy. Proton NMR
spectra were assigned on the basis of COSY correlations and then
the 13C NMR on the basis of HMQC and DEPT spectra. Detailed
assignments are given in the Experimental Section, and spectra are
included in the Supporting Information. The two regioisomers were
distinguished on the basis of long-range correlations in their HMBC
spectra (Scheme 3). In the case of the τ-regioisomer (τ-7), Hβ0
(δ 4.33) showed correlations to both C2 (δ 137.5) and C5
(δ 116.9) of the imidazole ring but not to C4 (δ 138.1). For the
π-isomer (π-7), Hβ0 (δ 4.33) showed correlations to both C2
(δ 138.0) and C4 (δ 126.5) of the imidazole ring, but not to C5
(δ 128ꢀ129). Sass and Marsh reported 13C chemical shifts for both
regioisomers of 4,6 isolated from bivalve molluscs. Boschin et al.
prepared and separated the two regioisomers of 4 (as mixtures
of diastereomers) and reported 13C NMR data in D2O.10b
While our compounds τ-7 and π-7 are fully protected, and our
spectra acquired in different solvents, our assignments are
consistent with the published data for the free amino acids.
While we had successfully prepared these HAL derivatives,
this route was plagued by the difficulty in preparing 6 and the
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Scheme 5. Synthesis of Sulfimidates 10 and 16
Scheme 6. Formation of τ-HAL
’ EXPERIMENTAL SECTION
General Methods. Triethylamine and pyridine were dried and
distilled from CaH2 and stored over KOH pellets. Methanol was distilled
from Mg turnings and stored over 4 Å molecular sieves. Acetonitrile,
tetrahydrofuran, and dichloromethane were dried using a solvent
purification system. The compounds were visualized by UV fluorescence
or by staining with PMA, ninhydrin, or KMnO4 stains. Proton NMR
data is reported in ppm downfield from TMS or disodium 3-trimethyl-
silyl-1-propanesulfonate (DSS) as internal standards. High resolution
mass spectra were recorded using either time-of-flight or electrospray
ionization.
NR-tert-Butoxycarbonyl-L-histidine Methyl Ester (5).12
Triethylamine (11.48 mL, 8.28 g, 82.6 mmol, 2.0 equiv) and
(Boc)2O (21.74 g, 94.98 mmol, 2.3 equiv) were added to a suspension
of HCl
this reaction depended heavily on the purity of the sulfamidate
reaction partner; once recrystallized according to Pilkington and
Wallis,24 this gave reproducible results. Alternative conditions for
hydrolysis of the intermediate aminosulfamic acid, using propa-
nethiol and a Lewis acid, according to Kim and So,18 led to more
complex product mixtures and lower yields of 17.
3 L-His OMe (10.0 g, 41.3 mmol, 1.0 equiv) in methanol
3
(150 mL). The reaction mixture was stirred for 2 h at rt and monitored
by TLC until no starting material was detected. The reaction mixture
was concentrated and then partitioned between ethyl acetate (500 mL)
and water (500 mL). The combined organic layers were dried over
anhydrous MgSO4, filtered, and concentrated. The residue was sus-
pended in methanol (100 mL), and K2CO3 (571 mg, 4.13 mmol, 0.1
equiv) was added. The mixture was heated at 67 °C for 4 h. When all of
the starting material was consumed, the mixture was concentrated and
partitioned between ethyl acetate (500 mL) and water (500 mL). The
combined organic layers were washed with brine (50 mL), dried over
anhydrous MgSO4, filtered, and concentrated. The residue was pur-
ified by flash chromatography on silica gel, eluting with 2:1
EtOAcꢀheaxanesf100% EtOAcf95:5 EtOAcꢀMeOH, affording 5
as a colorless solid. (6.766 g, 61%); Rf 0.27 (9:1 CH2Cl2ꢀMeOH);
While the regioselective formation of τ-HAL 17 was a pleasing
result, this compound has limited utility. The removal of
all protecting groups, to produce an amino acid standard for
the identification of τ-HAL in peptide/protein hydrolysates,
would require several steps. In this regard, we prepared sulfa-
midate 16 (Scheme 5) and reacted it with commercially avail-
able Cbz-L-His-OBn (18) to give 19, again as a single compound
(Scheme 6). The yields of this reaction ranged from 20 to 57%; the
significant fluctuations are attributed to an inability to purify
sulfamidate 16 by recrystallization. Hydrogenolytic cleavage
of all protecting groups gave salt-free τ-HAL (4). Matsunaga
et al. reported NOEs between H5 of the imidazole and CR
and Cβ of both residues, but H2 of the imidazole showed
NOEs only to HR0 and Hβ0 of the “Ala unit.”8a We observed
analogous cross-peaks in the NOESY spectrum, affording
evidence for the τ-regioisomer.
[R]D ꢀ9.70 (c 1.00, CHCl3). 1H NMR (CD3OD, 400 MHz) δ 1.40
25
(s, 9H), 2.94 (dd, J = 14.7, 8.5 Hz, 1H) 3.06 (dd, J = 14.7, 5.4 Hz, 1H),
3.67 (s, 3H), 4.38 (dd, J = 8.5, 5.4 Hz, 1H), 6.85 (s, 1H), 7.5 (s, 1H);
13C NMR (CDCl3, 100 MHz) δ 26.7, 28.3, 50.6, 53.3, 54.5, 78.6, 134.3,
155.8,172.0; HRMS (þTOF) calcd for C12H20N3O4 (M þ H)þ
270.1448, obsd 270.1450.
N-(Benzyloxycarbonyl)-D-serine-β-lactone (6).13. Diisopropyl
azodicarboxylate (1.87 mL, 1.80 mg, 8.36 mmol, 1.0 equiv) was added
dropwise, over 10 min, to a solution of triphenylphosphine (2.19 g,
8.36 mmol, 1.0 equiv) in THF (60 mL) at ꢀ78 °C. A solution of Cbz-D-Ser-
OH (2.0 g, 8.36 mmol, 1.0 equiv) in THF (13 mL) was added dropwise to
the mixture over 30 min. The mixture was stirred at ꢀ78 °C for 20 min, the
cooling bath removed, and the mixture slowly warmed, with stirring, to rt
over 2.5 h. The mixture was concentrated, and the residual pale yellow syrup
was purified by flash chromatography on silica gel, eluting with 4:1
hexanesꢀEtOAc, to give (R)-6 as a colorless solid (300 mg, 17%).
Rf 0.41 (1:1 HexꢀEtOAc); mp 125ꢀ128 °C, lit.13b mp 133ꢀ134 °C;
’ CONCLUSIONS
In summary, we report experimental details and characteriza-
tion data for the reaction of Boc-His-OMe (5) with a serine
β-lactone (6) to produce two regioisomers of HAL, as commu-
nicated previously by Tohdo et al.11 We produced a single
regioisomer, τ-HAL, by invoking a cyclic sulfamidate as the
electrophile. Moreover, we have produced the free bis-amino
acid 4 that can be used as an authentic standard to identify this
cross-link in other peptides and proteins. To maximize the
potential of a τ-HAL building block for peptide synthesis, efforts
are ongoing in our laboratory to produce an orthogonally
protected τ-HAL for complex molecule synthesis, including the
theonellamides.
29
22
[R]D þ23.6 (c 1.00, CH3CN), lit.13b [R]D þ26.5 (c 1, MeCN), lit.13d
26
[R]D þ34.2 (c 0.5, MeCN). 1H NMR (acetone-d6, 400 MHz) δ 4.42ꢀ
4.48 (m, 2H), 5.11 (br s, 2H), 5.28 (dd, J = 6.7, 5.0 Hz, 1H), 5.30 (dd, J =
6.7, 5.0 Hz, 1H), 7.30ꢀ7.38 (m, 5H); 13C NMR (acetone-d6, 100 MHz) δ
60.4, 66.4, 67.4, 128.5, 128.7, 128.8, 129.2, 137.3, 156.3, 170.3; HRMS
(þTOF) calcd for C11H12NO4 (MHþ) 222.0766, obsd 222.0764.
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Dimethyl NR-tert-Butoxycarbonyl-NR0-carbobenzyloxy-
τ-L-histidino-D-alaninate (S,R)-τ-7 and (S,R)-π-7. Boc-His-OMe
(5) (3.0 g, 11 mmol, 5 equiv) was added to a solution of N-Cbz-D-serine-
β-lactone (6) (490 mg, 2.26 mmol, 1 equiv) in MeCN (30 mL). The
mixture was stirred at 40 °C for 4 h, cooled, and concentrated. Analysis of
the crude product mixture by 1H NMR, and integration of the imidazole
H5signals in each isomer, estimates theτ:π ratio at 3.7:1.0 The residue was
subjected to flash chromatography on silica gel, eluting with 2% MeOH in
CH2Cl2 to afford some pure (S,R)-τ-7 (83 mg), many fractions that
contained both isomers (150 mg), and a trace of (S,R)-π-7 (6 mg) for a
total yield of 213 mg (63%).
solution of Bn-D-Ser-OMe (13) (3.268 g, 15.6 mmol, 1.0 equiv) in
CH2Cl2 (45 mL) at ꢀ78 °C. Thionyl chloride (1.92 mL, 2.23 g, 18.7
mmol, 1.2 equiv) was added dropwise over 10 min, and the solution was
stirred at ꢀ78 °C for 45 min and then allowed to warm to rt over 1 h.
Duringthis time, there was some precipitate formed. The reaction mixture
was quenched by the addition of 1% aq HCl (90 mL). The aqueous layer
was extracted with CH2Cl2 (3 ꢁ 150 mL), and the combined organic
layers were washed with brine (300 mL), dried over MgSO4, filtered, and
concentrated. The orange residue was dissolved in MeCN (160 mL), and
the solution was cooled to ꢀ5 °C. Ruthenium(III) chloride trihydrate
(41% Ru, 193 mg, 0.8 mmol, 0.05 equiv) was added, followed by NaIO4
(4.01 g, 18.7 mmol, 1.2 equiv) and H2O (160 mL). The black reaction
mixture was stirred for 20 min at ꢀ5 °C and a further 20 min at rt. The
mixture was partitioned between CH2Cl2 (600 mL) and saturated
NaHCO3 (250 mL). The aqueous layer was extracted with CH2Cl2
(2 ꢁ 150 mL), and the combined blue-green-black organic layers were
washed with brine (150 mL), dried over MgSO4, filtered, and concen-
trated. Flash chromatography on silica gel, eluting with 2:1 hexa-
nesꢀEtOAc, afforded 10 as a colorless oil (3.125 g, 74% over two
steps). The material solidified on storage in the refrigerator and could
29
(S,R)-τ-7 Rf 0.52 (9:1 CH2Cl2ꢀMeOH); [R]D ꢀ23.3 (c 1.00,
CHCl3). 1H NMR (CDCl3, 400 MHz) δ 1.43 (s, 9H, Boc tBu), 2.95 (dd,
J = 14.5, 4.5 Hz, 1H, Hβa His), 3.04 (dd, J = 14.5, 4.5 Hz, 1H, Hβb His),
3.68 (s, 3H, COOCH3 His), 3.78 (s, 3H, COOCH3 Ala), 4.33 (br s, 2H,
Hβ0 Ala), 4.52ꢀ4.54 (m, 1H, HR His), 4.60 (m, 1H, HR0 Ala), 5.14
(s, 2H, CH2Ph), 5.59 (d, J = 6.0 Hz, 1H, NH His), 5.88 (d, J = 8.1 Hz,
1H, NH Ala) 6.56 (s, 1H, H5 His), 7.23 (s, 1H, H2 His), 7.33ꢀ7.40 (m,
5H, Cbz Ph); 13C NMR (CDCl3, 100 MHz) δ 28.3 (Boc tBu), 30.2 (Cβ
His), 47.9 (Cβ0 Ala), 52.1 (COOCH3 Ala), 53.1 (COOCH3 His), 53.5
(CR His), 54.9 (CR0 Ala), 67.4 (Cbz CH2), 79.6 (Boc CMe3), 116.9
(C5, His), 128.2 (Cbz CH), 128.4 (Cbz CH), 128.6 (Cbz CH), 135.8
(Cbz 4 °C), 137.5, (C2 His) 138.1 (C4, His), 155.6 (Boc, Cbz, 2 ꢁ
CdO), 169.4 (COOMe Ala), 172.5 (COOMe, His); HRMS (þTOF)
calcd for C24H33N4O8 (MHþ) 505.2292, obsd 505.2289.
29
be recrystallized from diethyl ether. Rf 0.44 (1:1 HexꢀEtOAc); [R]D
þ1.7 (c 1.00, CHCl3); 1H NMR (CDCl3, 400 MHz) δ 3.72 (s, 3H), 4.07
(dd, J = 7.4, 4.6 Hz, 1H), 4.50 (d, J = 2.2 Hz, 2H), 4.60 (dd, J = 9.0, 4.0 Hz,
1H), 4.66 (dd, J = 9.0, 4.6 Hz, 1H), 7.33ꢀ7.42 (m, 5H); 13C NMR
(CDCl3, 100 MHz) δ 50.3, 53.1, 58.1, 67.4, 128.6, 128.8, 129.1, 133.5,
168.2; HRMS (ꢀTOF) calcd for C11H12NO5S (M ꢀ H)þ 270.0441,
obsd 270.0439.
26
(S,R)-π-7. Rf 0.41 (9:1 CH2Cl2ꢀMeOH); [R]D þ5.2 (c 0.3,
1
CHCl3). H NMR (CD3OD, 400 MHz) δ 1.40 (s, 9H), 2.92ꢀ3.00
Dimethyl NR-tert-Butoxycarbonyl-NR0-benzyl-τ-L-histidino-
D-alaninate (17). A solution of sulfamidate 10 (102 mg, 0.38 mmol,
1 equiv) and Boc-His-OMe (5) (312 mg, 1.16 mmol, 3 equiv) in anhydrous
DME (10 mL) was stirred for 5 h at 80 °C. During this time, the reaction
mixture became cloudy, and by the end, a yellow oil separated out from the
solvent. The mixture was cooled to rt, and then a solution of 1 M KH2PO4
(25 mL) was added to quench the reaction (by hydrolysis of the sulfamic
acid) and stirred vigorously for 2 h. More water (10 mL) was added, the
mixture was extracted with EtOAc (3 ꢁ 25 mL), and the combined organic
layers were dried over MgSO4, filtered, and concentrated. The residue was
purified by flash chromatography on silica gel, eluting with 4% MeOH in
CH2Cl2, to give 17 as a colorless solid (77 mg, 57%). Rf 0.50 (9:1
(m, 1H), 3.11ꢀ3.17 (m, 1H), 3.71 (s, 3H), 3.75 (s, 3H), 4.26 (dd, J =
14.2, 9.2 Hz, 1H), 4.41ꢀ4.51 (m, 2H), 4.55ꢀ4.58 (m, 1H), 5.09 (s, 2H),
6.75 (s, 1H), 7.29ꢀ7.34 (m, 5H), 7.52 (s, 1H). On the time scale of the
13C NMR experiment, two species were observed in an approximately
2:1 ratio. Where distinct signals were observed for the minor species,
these are indicated in parentheses. 13C NMR (CD3OD, 100 MHz) δ
25.0 (25.1), 26.7, 44.1 (44.2), 50.9, 51.2, 52.3 (52.2), 53.8 (53.7), 65.9,
78.8, 125.8, 126.8, 127.1 127.2, 127.5, 136.0, 137.4 (137.5), 155.8, 156.2,
169.3, 171.4; HRMS (þTOF) calcd for C24H33N4O8 (MHþ) 505.2292,
obsd 505.2289.
N-Benzyl-D-serine Methyl Ester (13). Dry methanol (30 mL)
was added to flame-dried, powdered 4 Å molecular sieves (3 g) in a
round-bottomed flask. To this suspension were added, sequentially,
27
1
CH2Cl2ꢀMeOH); [R]D þ56 (c 1.00, CHCl3). H NMR (CDCl3,
400 MHz) δ 1.43 (s, 9H), 2.99 (dd, J = 7.3, 5.0 Hz, 1H), 3.07 (dd, J = 7.3,
5.0 Hz, 1H), 3.49ꢀ3.53 (m, 1H), 3.66 (d, J = 10.9 Hz, 1 H), 3.68 (s, 3H),
3.72 (s, 3H), 3.83 (d, J = 13.3 Hz, 1H), 4.08 (d, J = 5.6 Hz, 1H), 4.09 (d, J =
5.6 Hz, 1H), 4.53 (d, J = 4.8 Hz, 1H), 5.82 (br d, J = 8.1 Hz, 1H), 5.89 (brd,
J = 8.1 Hz, 1H), 6.67 (d, J = 6.3 Hz, 1H), 7.23ꢀ7.35 (m, 5 H), 7.39 (s, 1H);
13C NMR (CDCl3, 100 MHz) δ 28.3, 30.1, 49.4, 52.0, 52.1, 52.2, 52.4, 53.5,
60.9, 61.0, 79.5, 117.1, 127.4, 128.1, 128.5, 137.5, 138.9, 155.6, 172.5, 172.6;
HRMS (þTOF) calcd for C23H33N4O6 (MHþ) 461.2394, obsd 461.2404.
Benzyl 4R,3-Benzyl-2-oxo-1,2,3-oxathiazolidine-4-carboxylate
(16). N-Benzyl-D-serine Benzyl Ester (14). Flame-dried 4 Å molecular sieves
3 D-Ser-OMe (11) (3.0 g, 19.3 mmol, 1 equiv), triethylamine
HCl
(2.69 mL, 1.95 g, 19.3 mmol, 1 equiv) and benzaldehyde (1.95 mL,
2.05 g, 19.3 mmol, 1 equiv). The mixture was stirred for 16 h at rt, filtered
through Celite, washed well with methanol, and concentrated. The acid-
labile imine could not be visualized by TLC, so the success of the
1
reaction was gauged by H NMR: aldehyde starting material δ 10.03;
imine product δ 8.43. Provided there was negligible aldehyde, the
material was carried directly into the reduction reaction. The oily yellow
imine was redissolved in MeOH (30 mL) and treated portion-wise with
NaBH4 (729 mg, 19.3 mmol, 1 equiv) at 0 °C. After 4 h at 0 °C, water
(10 mL) and EtOAc (10 mL) were added. The organic layer was washed
with brine, dried over MgSO4, filtered, and concentrated to give a yellow
oil (3.268 g, 81%). This compound was typically utilized without further
purification. However, for the purposes of characterization, flash chro-
matography on silica gel, eluting with 100% EtOAc, provided 13 as a pale
(1.5 g) were added to a solution of HCl D-Ser-OBn (12) (1.0 g, 4.37 mmol,
3
1.0 equiv) in anhydrous MeOH (10 mL) at rt under N2. Triethylamine
(618 μL, 450 mg, 4.37 mmol, 1.0 equiv) and benzaldehyde (436 μL, 458 mg,
4.37 mmol, 1.0 equiv) were added sequentially. The mixture was stirred for
24 h at RT, filtered through Celite, washing well with methanol, and
concentrated. The acid-labile imine could not be visualized by TLC, so the
success of the reaction was gauged by 1H NMR: aldehyde starting material
δ9.86; imine product δ8.21. Provided there was negligible aldehyde, the
material was carried directly into the reduction reaction. The oily yellow
product was redissolved in MeOH (10 mL) and treated portion-wise with
NaBH4 (0.242 mg, 6.43 mmol, 1.0 equiv) at 0 °C. After 4 h, H2O and EtOAc
(25 mL each) were added, and the organic layer was separated, washed with
brine, dried over MgSO4, filtered, and concentrated to give Bn-D-Ser-OBn
(14) that was utilized without further purification.
29
yellow oil (897 mg, 67%); Rf 0.62 (9:1 CH2Cl2ꢀMeOH); [R]D þ1.22
(c 1.00, MeOH). 1H NMR (CD3OD, 400 MHz) δ 3.38 (app. t, J = 5.0
Hz, 1 H), 3.68 (d, J = 12.8 Hz, 1H), 3.72 (s, 3H), 3.74 (d, J = 5.0 Hz, 1H),
3.75 (d, J = 5.0 Hz, 1H), 3.82 (d, J = 12.8 Hz, 1H), 7.22ꢀ7.35 (m, 5H);
13C NMR (CD3OD, 100 MHz) δ 50.4, 50.7, 61.4, 61.7, 126.3, 127.5,
127.6, 138.5, 172.8; HRMS (þTOF) calcd for C11H16NO3 (MHþ)
210.1124, obsd 210.1126.
Methyl 4R,3-Benzyl-2-oxo-1,2,3-oxathiazolidine-4-carbox-
ylate (10). Pyridine (6.32 mL, 6.18 g, 78 mmol, 5 equiv) was added to a
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The residue was dissolved in CH2Cl2 (24 mL). Pyridine (1.75 mL,
1.71 g, 21.6 mmol, 5.0 equiv) was added and the reaction mixture cooled
to ꢀ78 °C. Thionyl chloride (375 μL, 615 mg, 5.18 mmol, 1.2 equiv)
was added dropwise over 30 min, and the solution was stirred at ꢀ78 °C
for 45 min and then allowed to warm to rt over 1 h. The reaction mixture
was quenched by the addition of 1% HCl (38 mL). The aqueous layer
was extracted with CH2Cl2 (150 mL), and the combined organic layers
were washed with saturated NaHCO3 (38 mL), dried over MgSO4,
filtered, and concentrated to give a yellow residue.
’ ASSOCIATED CONTENT
Supporting Information. 1H and 13C NMR spectra for
S
b
all compounds, along with 1Hꢀ H COSY, HMQC, and HMBC
for τ-7 and π-7. This material is available free of charge via the
Internet at http://pubs.acs.org.
1
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: cmtaylor@lsu.edu.
The residue was dissolved in MeCN (75 mL) and cooled to ꢀ5 °C.
Ruthenium(III) chloride hydrate (35ꢀ40% Ru, ∼45 mg, 0.21 mmol, 0.05
equiv), NaIO4 (1.11 g, 5.18 mmol, 1.2 equiv), and H2O (75 mL) were
then added sequentially, and the reaction mixture was stirred for 20 min at
ꢀ5 °C and a further 20 min at rt. The mixture was partitioned between
CH2Cl2 (300 mL) and saturated NaHCO3 (75 mL). The aqueous layer
was extractedwith CH2Cl2 (2 ꢁ 75 mL), and the combined organic layers
were washed with brine (75 mL), dried over MgSO4, filtered, and
concentrated. The residue was purified by flash chromatography on silica
gel, eluting with 3:1 hexanesꢀEtOAc, to afford 16 as a colorless solid (643
’ ACKNOWLEDGMENT
The authors thank the National Science Foundation
(CH-0809143) for support of this work. We thank Dr. Dale Treleavan
and Dr. Thomas Weldeghiorghis of the LSU NMR facility for
their expertise in acquiring 2D data. We thank Dr. John Wallis
(Nottingham Trent University) for information on the purifica-
tion of compound 10 and Drs. Soon Mog So (University of
Toronto) and Moon Kim (Seoul National University) for details
about the reaction of 10 with imidazole. In connection with an
earlier approach to HAL, C.M.T. thanks the Marsden Fund
(03-MAU-032-PSE), administered by the Royal Society of New
Zealand, for their financial support, and Julia Strautmann and
Weihua Wang for contributions.
29
mg; 40% over three steps). Rf 0.57 (2:1 HexꢀEtOAc); [R]D þ54.7
(c 1.00, CHCl3). 1H NMR (CDCl3, 400 MHz) δ 4.05 (dd, J = 7.4, 4.5 Hz,
1H), 4.41 (d, J = 14.3 Hz, 1H), 4.46 (d, J = 14.3 Hz, 1H), 4.55 (dd, J = 8.9,
7.5 Hz, 1H), 4.62 (dd, J = 8.9, 4.5 Hz, 1H), 5.10 (d, J = 12.1 Hz, 1H), 5.13
(d, J = 12.1 Hz, 1H), 7.28ꢀ7.38 (m, 10H); 13C NMR (CDCl3, 100 MHz)
δ 50.2, 58.2, 65.0, 67.3, 67.9, 126. 8, 127.4, 128.4, 128.5, 128.61, 128.68,
128.98, 133.4, 134.4, 167.5; HRMS (þTOF) calcd for C17H17NO5NaS
(M þ Na)þ 370.0719, obsd 370.0725.
Dibenzyl NR-tert-Carbobenzyloxy-NR0-benzyl-τ-L-histidi-
no-D-alaninate 19. Cbz-His-OBn (18) (2.11 g, 5.55 mmol, 3 equiv)
was added to a solution of sulfamidate 16 (643 mg, 1.85 mmol, 1 equiv)
in anhydrous DME (52 mL) and stirred for 5 h at 80 °C. The mixture
was cooled to rt, and then a solution of 1 M KH2PO4 (50 mL) was added
to quench the reaction (by hydrolysis of the sulfamic acid). The mixture
was extracted with EtOAc (3 ꢁ 100 mL), and the combined organic
layers were washed with brine (30 mL), dried over MgSO4, filtered, and
concentrated. The residue was purified by flash chromatography on
silica gel, eluting with 2:1 EtOAcꢀhexanes, to afford 19 as a colorless
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29
solid (637 mg, 57%). Rf 0.61 (9:1 CH2Cl2ꢀMeOH); [R]D þ10.2
(c 1.00, CHCl3). 1H NMR (CDCl3, 400 MHz) δ 2.07 (br s, 1H), 2.95
(dd, J = 14.6, 3.1 Hz, 1H), 3.03 (dd, J = 14.6, 3.1 Hz, 1H), 3.44 (dd, J =
11.5, 5.6 Hz, 1H), 3.58 (d, J = 13.2 Hz, 1H), 3.75 (d, J = 13.2 Hz, 1 H),
3.88ꢀ3.99 (m, 2H), 4.61ꢀ4.64 (m, 1H), 5.02ꢀ5.16 (m, 6H), 6.26
(d, J = 8.9 Hz, 1 H), 6.41ꢀ6.43 (m, 1H), 7.15ꢀ7.36 (m, 21H) ; 13C
NMR (CDCl3, 100 MHz) δ 29.6, 49.0, 51.8, 51.9, 53.9, 60.6, 60.7,
66.6(2C), 67.2, 116.9, 117.1, 127.2, 127.8, 127.9, 128.0, 128.17, 128.24,
128.26, 128.32, 128.6, 134.8, 135.6, 136.4, 136.9, 137.4, 138.7, 156.1,
171.3, 171.8; HRMS (þTOF) calcd for C38H39N4O6 (MHþ) 647.2864,
obsd 647.2870.
τ-L-Histidino-D-alanine (4). To a solution of protected histidi-
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29
[R]D þ12.6 (c 1.00, 1.0 N HCl), lit.8a for a 1.2:1.0 mixture of
23
diastereomers: [R]D þ9.8 (c 0.07, 1.0 N HCl). 1H NMR (D2O, 400
MHz) δ 3.16ꢀ3.29 (m, 2H, His Hβ), 3.97 (app. t, J = 5.5 Hz, 1H, His
HR), 4.19 (app. t, J = 5.0 Hz, 1H, Ala HR0), 4.65 (d, J = 5.0, 2H, Ala
Hβ0), 7.33 (s, 1H, H5), 8.44 (s, 1H, H2); 13C NMR (D2O, 100 MHz) δ
29.3 (Cβ His), 50.9 (Cβ Ala), 56.4 (CR His), 56.9 (CR0 Ala), 122.8 (C5
His), 133.8 (C4 His), 139.6 (C2 His), 173.1, 175.5; HRMS (þTOF)
calcd for C9H15N4O4 (MHþ) 243.1091, obsd 243.1087.
5707
dx.doi.org/10.1021/jo200744d |J. Org. Chem. 2011, 76, 5703–5708

The Journal of Organic Chemistry
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