Stereodivergent synthesis of (2S,3S,4R,5R)-and (2S,3S,4R,5S)-[3,4,5- D3]proline depending on the substituent of the γ-lactam ring

Full text of DOI:10.1021/jo990866r

J . Org. Chem. 1999, 64, 9275-9278
9275
Sch em e 1a
Ster eod iver gen t Syn th esis of (2S,3S,4R,5R)-
a n d (2S,3S,4R,5S)-[3,4,5-D3]P r olin e
Dep en d in g on th e Su bstitu en t of th e
γ-La cta m Rin g
Makoto Oba,* Akiko Miyakawa, and
Kozaburo Nishiyama*
Department of Material Science and Technology,
Tokai University, 317 Nishino, Numazu,
Shizuoka 410-0395, J apan
Tsutomu Terauchi and Masatsune Kainosho
Department of Chemistry, Faculty of Science,
Tokyo Metropolitan University, 1-1 Minami-Ohsawa,
Hachioji, Tokyo 192-0397, J apan
a
Reagents and conditions: (a) D2 (5 kgf/cm2), 10% Pd/C, MeOD;
Received May 27, 1999
(b) LiEt3BH, BF3‚OEt2, 51% (two steps); (d) TBAF, 84%; (e) CrO3,
H2SO4; (f) 1 M HCl, 110 °C, then Dowex 50W-X8, 66% (two steps).
In tr od u ction
Sch em e 2a
Nowadays, a combination of a stable isotope labeling
technique and multidimensional NMR spectroscopy is the
principal method for obtaining a detailed solution struc-
ture of proteins.1,2 For more precise structure determi-
nation, stereospecific assignments for the diastereotopic
methylene protons or the methyl group are indispensable.
In view of the above background, we have recently been
engaged in preparing [3,4,5-D3]proline2 as part of our
continuing studies on the synthesis of stereoselectively
deuterated amino acids.3 Since the residue plays a
prominent role in the structure of proteins owing to its
unusual conformational preference,4 monitoring the pro-
line ring dynamics should provide information regarding
the protein interior around the residue. Our latest paper
dealt with the preparation of such prolines starting from
4-hydroxyproline;3 however, incorporation of a 13C label
into the proline framework is very difficult by this
method. This difficulty becomes a crucial drawback when
the sample is incorporated into larger biomolecules as
an analytical probe, because the selective 13C labeling is
necessary to extract NMR signals for specific residues
independently from the rest of the structure by a 2D
NMR technique such as 1H-13C HSQC. Therefore, we
herein disclose a novel protocol to access [3,4,5-D3]proline
starting from L-glutamic acid, the 13C isotopomers of
which are commercially available.
a
Reagents and conditions: (a) LiEt3BH; (b) deuteride, BF3‚OEt2;
(c) TBAF (R ) TBDPS); (d) CrO3, H2SO4; (e) 1 M HCl, 110 °C,
then Dowex 50W-X8.
1b were readily prepared from L-glutamic acid according
to the reported procedure.5 When a catalytic deuteration
of the olefin 1a was carried out using 10% palladium on
carbon in EtOD under medium pressure (5 kgf/cm2) of
deuterium gas, the corresponding 3,4-dideuterated lac-
tam 2a was obtained in 92% yield as a single diastereo-
mer, indicating a stereospecific deuterium addition oc-
curred. In this case, the H-D scrambling, which had been
encountered in the catalytic deuteration of the dehydro-
proline derivatives,3 was not observed.
To obtain the [3,4,5-D3]proline, a stereoselective reduc-
tion of the lactam carbonyl moiety with an appropriate
deuteride seems to be the most straightforward route.6
As shown in Scheme 2, a preliminary examination was
carried out using the corresponding unlabeled lactam 6a
or 6b as the starting material, and the results are
compiled in Table 1. For example, Lewis acid-promoted
reduction of the aminal 7a derived from the lactam 6a
was performed using a Bu3SnD-BF3‚OEt2 system to
afford the deuterated prolinol derivative 8. During the
course of the reaction, the TBDMS group was removed.7
After oxidation of the hydroxymethyl moiety with J ones
reagent, the deprotection procedure gave [5-D]proline in
Resu lts a n d Discu ssion
The synthetic course of [3,4,5-D3]proline is shown in
Scheme 1. The unsaturated γ-lactam derivatives 1a and
(1) Kainosho, M. Nat. Struct. Biol. 1997, 858-861.
(2) Wuthrich, K. NMR of Proteins and Nucleic Acids; J ohn Wiley &
Sons: New York, 1986. Roberts, G. C. K. NMR of Macromolecules. A
Practical Approach; Oxford University Press: Oxford, 1993.
(3) Oba, M.; Terauchi, T.; Hashimoto, J .; Tanaka, T.; Nishiyama,
K. Tetrahedron Lett. 1997, 38, 5515-5518. Oba, M.; Terauchi, T.;
Miyakawa, A.; Nishiyama, K. Tetrahedron: Asymmetry 1999, 10, 937-
945.
(4) Nishiyama, K.; Oba, M.; Ueno, R.; Morita, A.; Nakamura, Y.;
Kainosho, M. J . Labeled Compds. Radiopharm. 1994, 34, 831-837.
Oba, M.; Ueno, R.; Fukuoka, M.; Kainosho, M.; Nishiyama, K. J . Chem.
Soc., Perkin Trans. 1 1995, 1603-1609. Oba, M.; Nakajima, S.;
Nishiyama, K. J . Chem. Soc., Chem. Commun. 1996, 1875-1876. Oba,
M.; Terauchi, T.; Miyakawa, A.; Kamo, H.; Nishiyama, K. Tetrahedron
Lett. 1998, 39, 1595-1598. Oba, M.; Terauchi, T.; Owari, Y.; Imai, Y.;
Motoyama, I.; Nishiyama, K. J . Chem. Soc., Perkin Trans. 1 1998,
1275-1281.
(5) Rose, G. D.; Gierasch, L. M.; Smith, J . A. Adv. Protein Chem.
1985, 37, 1-109.
(6) Ohfune, T.; Tomita, M. J . Am. Chem. Soc. 1982, 104, 3511-3513.
Woo, K.-C.; J ones, K. Tetrahedron Lett. 1991, 32, 6949-6952.
(7) Lactam reduction using LiEt3BH followed by Et3SiH-BF3‚OEt2
system was reported. See: Pedregal, C.; Ezquerra, J .; Escribano, A.;
Carreno, M. C.; Garcia Ruano, J . L. Tetrahedron Lett. 1994, 35, 2053-
2056.
10.1021/jo990866r CCC: $18.00 © 1999 American Chemical Society
Published on Web 11/18/1999

9276 J . Org. Chem., Vol. 64, No. 25, 1999
Notes
Ta ble 1. Ster eoselective Red u ction of th e Am id o
Ca r bon yl Moiety of γ-La cta m 6
9
run
lactam
deuteride
yield, %a
5R:5Sb
1
2
3
4
5
6a
6a
6a
6a
6b
Bu3SnD
Et3SiD
Ph3SiD
(TMS)3SiD
(TMS)3SiD
57
38
41
42
43
81:19
87:13
92:8
97:3
99:1
a
b
Isolated yield based on the γ-lactam derivative. Determined
by 1H NMR analysis.
57% yield based on the starting lactam 6a , and the
stereoselectivity of the deuterium substitution at the C-4
1
was determined by H NMR integration to be 5R:5S )
81:19 (run 1). This stereochemical outcome can be at-
tributed to a preferential delivery of the deuterium atom
anti to the resident siloxymethylene group in the inter-
mediate iminium ion produced from the aminal 7a .
When deuteriosilanes were employed as a deuterium
source instead of Bu3SnD, a remarkable improvement of
the stereoselectivity in the deuterium addition was
observed (runs 2-4), especially in the case of tris-
(trimethylsilyl)deuteriosilane (TMS3SiD, 5R:5S ) 97:3).
The advantage of changing the reducing agent from
stannane to silane cannot be explained at the present
stage; however, steric interaction between the silane and
the siloxymethylene group is obviously responsible for
the observed anti-selective deuteration. In fact, on re-
placing the TBDMS protecting group by the more bulky
tert-butyldiphenylsilyl (TBDPS) group, the ratio of 5R:
5S rose to 99:1 (run 5). Consequently, the reaction
conditions shown in run 5 were adopted for the reduction
of the amido carbonyl moiety of the deuterated lactam
2b, giving the [3,4,5-D3]prolinol derivative 3 in 51% yield.
After removal of the TBDPS group by tetrabutylammo-
nium fluoride (TBAF), oxidation and deprotection pro-
cedures afforded the [3,4,5-D3]proline 5 in 66% yield. The
optical purity was checked by HPLC analysis using a
chiral stationary phase column to be 98% ee (L-form). The
1
F igu r e 1. 400 MHz H NMR spectra of (2S,3S,4R,5R)-[3,4,5-
D3]proline (5; top), (2S,3S,4R,5S)-[3,4,5-D3]proline (14; middle),
and unlabeled proline (bottom) in D2O.
Sch em e 3a
1
400 MHz H NMR spectrum of the proline 5 is depicted
in Figure 1 (top). Coupled with the fact that the signal
assigned to the 3S-proton completely disappeared, the
relative intensity ratio for the 5R- and 5S-proton signals
is 99:1, which indicates a stereospecific formation of the
(2S,3S,4R,5R)-isomer.
To prepare the corresponding (5S)-isomer, changing the
deuterium source in the reduction step seems to be the
most convenient. We actually performed the reduction
of the lactam 2b using LiEt3BD; however, the efficiency
of the deuterium incorporation was unsatisfactory. The
source of the hydrogen could not be specified.
On the other hand, our previous paper dealt with the
preparation of (2S,3S,4R,5S)-[3,4,5-D3]proline in which
the 5S configuration was established on the basis of the
syn-selective deuteration of the aminal derived from
pyroglutamate derivatives, even though the selectivity
was moderate (5R:5S ) 10:90).3 Therefore, we next
carried out the interconversion of a siloxymethylene
group into the tert-butyl ester prior to the reduction of
the amido carbonyl moiety with optimization of the
reaction conditions for the selective deuteration.
As shown in Scheme 3, deprotection of the silyl ether
2b and subsequent oxidation of the alcohol 10 with RuO4
gave the [3,4-D2]pyroglutamic acid derivative 11 in
quantitative yield. The tert-butyl ester 12 was obtained
a
Reagents and conditions: (a) TsOH, MeOH, 99%; (b) RuO2,
NaIO4 quant; (c) Me2NCH(OCH2CMe3)2, t-BuOH, 50%; (d) LiEt3BH,
then TsOH, MeOH, 80% (two steps); (e) Bu3SnD, BF3‚OEt2, in
toluene; (f) 1 M HCl, 110 °C, then Dowex 50W-X8, 96% (two steps).
in 50% yield by reacting the acid 11 with dimethylform-
amide di-tert-butyl acetal prepared in situ from the
corresponding dineopentyl acetal with tert-butyl alcohol.8
The amido function was reduced at this stage. Thus, the
[3,4-D2]pyroglutamate 12 was treated with LiEt3BH,
followed by methanol in the presence of TsOH to afford
5-methoxy[3,4-D2]prolinate 13 in 80% yield. After optim-
ization of the reaction conditions for selective reduction
of the aminal 13, highly syn-selective deuteration (5R:
5S ) 3:97) was achieved by employment of a Bu3SnD-
BF3‚OEt2 system in toluene. This 1,3-stereocontrol can
be attributed to the known Cieplak-type stereoelectronic
effect9 observed in a few Lewis acid-catalyzed addition
reactions, although most of the reported syn additions
(8) Kelly D. R.; Roberts S. M.; Newton R. F. Synth. Commun. 1979,
9, 295-299.
(9) Widmer U. Synthesis 1983, 135-136.

Notes
J . Org. Chem., Vol. 64, No. 25, 1999 9277
149.57, 174.28; MS (EI) m/z 382 [(M - OBut)+]; HRMS m/z
382.1820 [(M - OBut)+, calcd for C22H24D2NO3Si 382.1808].
(2S)-N-ter t-Bu toxyca r bon yl-2-ter t-bu tyld ip h en ylsiloxy-
m eth yl[3,4,5-D3]p yr r olid in e (3b). To a solution of 2-[3,4-D2]-
pyrrolidone 2b (2.06 g, 4.37 mmol) in THF (45 mL) was added
a solution of 1 M LiEt3BH in THF (5.2 mL, 5.2 mmol) at -78 °C
under an Ar atmosphere, and the reaction mixture was stirred
for 0.5 h. Then, the reaction was quenched with saturated
aqueous NaHCO3 (9 mL) at -78 °C, and the mixture was
warmed to 0 °C. After addition of 30% H2O2 (2 mL), the mixture
was stirred for an additional 20 min and concentrated to dryness.
The residue was extracted with ether. The ethereal solution was
washed with H2O and dried over MgSO4. Evaporation of the
solvent gave a crude 5-hydroxy[3,4-D2]prolinol derivative (2.08
g) in quantitative yield which was used in the next step without
purification.
were based on 1,2-stereocontrol directed by an adjacent
OTBDMS group.11,12 It was also pointed out in the
foregoing works that reaction with a tin reagent in a
toluene solution was the most efficient;11 however, an
explanation of the effects of the reagents and solvents
awaits theoretical verification.
The obtained [3,4,5-D3]prolinate, without purification,
was submitted to the deprotection procedures to give
[3,4,5-D3]proline 14 in 96% yield with 92% ee (C-2). The
1
400 MHz H NMR spectrum of the proline 14 is shown
in Figure 1 (middle). The complete reversal of the relative
intensities of the 5R- and 5S-proton signals also indicates
the selective formation of the (2S,3S,4R,5S)-isomer.
In conclusion, we have achieved a stereodivergent
synthesis of (2S,3S,4R,5R)- and (2S,3S,4R,5S)-[3,4,5-D3]-
proline starting from L-glutamic acid. The synthesis was
based on the catalytic deuteration of the olefin 1 and the
stereoselective reduction of the amido function under
steric or stereoelectronic control depending on the sub-
stituent of the γ-lactam ring. Incorporation of a 13C label
into the deuterated proline framework has now become
feasible using the corresponding 13C-labeled L-glutamic
acid as the starting material. Furthermore, the NMR
spectra of selectively labeled proteins produced by incor-
porating such a proline will provide useful structural
information. Studies that address these issues will be
published in due course.
To a solution of the 5-hydroxy[3,4-D2]prolinol (694 mg, 3.50
mmol) in CH2Cl2 (15 mL) was added tris(trimethylsilyl)deute-
riosilane (870 mg, 3.50 mmol) and trifluoroborane etherate (454
mg, 3.20 mmol) in two portions at intervals of 0.5 h at -78 °C
under an argon atmosphere, and the resulting solution was
stirred for 2 h. Then the reaction mixture was quenched with
saturated aqueous NaHCO3 (7.5 mL), and the organic layer was
dried over MgSO4 and evaporated. After removal of the solvent,
the residue was chromatographed on silica gel. Elution with a
mixture of hexane and ethyl acetate (96:4) afforded oily [3,4,5-
D3]prolinol 3b (350 mg, 51%): 1H NMR (CDCl3) δ 1.04 (s, 9 H),
1.33 and 1.45 (2 s, 9 H), 1.77 and 1.88 (2 m, 1 H), 2.01 and 2.10
(2 m, 1 H), 3.32 (m, 1 H), 3.51 and 3.77 (2 m, 1 H), 3.71 (m, 1
H), 3.84 and 3.95 (2 m, 1 H), 7.38 (m, 6 H), 7.65 (m, 4 H); 13C
NMR (CDCl3) δ 19.28, 22.65 (br), 26.88, 27.84 (br), 28.51, 46.55
(br), 58.28, 64.67 (br), 79.03 (br), 127.66 (4 C), 129.60 (4 C),
133.70, 133.82, 135.58, 135.60, 154.53; MS (EI) m/z 442 (M+);
HRMS m/z 442.2703 (M+, calcd for C26H34D3NO3Si 442.2731).
(2S)-N-ter t-Bu toxyca r bon yl-2-h yd r oxym eth yl[3,4,5-D3]-
p yr r olid in e (4). To a solution of compound 3b (325 mg, 0.735
mmol) in THF (5 mL) was added 1 M Bu4NF in THF (1.6 mL,
1.60 mmol), and the resulting solution was stirred at room
temperature for 1.5 h. After removal of the solvent, the residue
was chromatographed on silica gel. Elution with a mixture of
hexane and ethyl acetate (70:30) afforded oily [3,4,5-D3]prolinol
4 (126 mg, 84%). The structure was confirmed by comparing its
spectral data with those of the commercially available unlabeled
Exp er im en ta l Section
1
Gen er a l P r oced u r es. H and 13C NMR spectra were meas-
ured at 400 and 100 MHz, respectively. All chemical shifts are
reported as δ values (ppm) relative to residual chloroform (δH
7.26), sodium 3-(trimethylsilyl)[2,2,3,3-D4]propionate (δH 0.00),
or the central peak of deuteriochloroform (δC 77.0). High-
resolution mass spectra (EI) were obtained at an ionization
potential of 70 eV. Optical purity was determined on a HPLC
system equipped with a chiral column and 2 mM CuSO4 solution
as an eluent. All other reagents were of commercial grade and
used as supplied.
1
compound. H NMR (CDCl3) δ 1.47 (s, 9 H), 1.52 and 1.80 (2 m,
2 H), 3.29 (d, J ) 7 Hz, 1 H), 3.58 (dd, J ) 11 and 8 Hz, 1 H),
3.63 (dd, J ) 11 and 3 Hz, 1 H), 3.95 (br, 1 H); 13C NMR (CDCl3)
δ 23.29 (t, J ) 19 Hz), 28.16 (t, J ) 20 Hz), 28.38, 46.97 (t, J )
20 Hz), 59.93, 67.22, 79.94, 156.85; MS (EI) m/z 204 (M+); HRMS
m/z 173.1387 [(M - CH2OH)+, calcd for C9H13D3O2N 173.1369].
(2S,3S,4R,5R)-[3,4,5-D3]P r olin e (5). To a solution of [3,4,5-
D3]prolinol 4 (126 mg, 0.608 mmol) in acetone (10 mL) was added
J ones reagent, prepared from CrO3 (243 mg, 2.43 mmol), sulfuric
acid (0.2 mL), and H2O (0.5 mL), and the resultant suspension
was stirred at room temperature for 0.5 h. Then the reaction
mixture was quenched by 2-propanol (0.2 mL), and the insoluble
materials were filtered off. The organic layer was separated,
diluted with ethyl acetate (100 mL), washed with H2O (20 mL
× 5) and brine (20 mL), dried over MgSO4, and evaporated.
Deprotection of the resultant crude N-tert-butoxycarbonyl[3,4,5-
D3]proline was carried out in 1 M HCl (15 mL) at 110 °C for 3
h followed by treatment with Dowex 50W-X8 to give [3,4,5-D3]-
proline 5 (47 mg, 66%) as a colorless solid, mp 207-213 °C dec.
(5S)-N-ter t-Bu toxyca r bon yl-5-ter t-bu tyld im eth ylsiloxy-
m eth yl-2-[3,4-D2]p yr r olid on e (2a ). A mixture of olefin 1a
(2.46 g, 7.51 mmol) and 10% Pd/C (491 mg) in MeOD (10 mL)
was stirred at room temperature for 1 h under medium pressure
(5 kgf/cm2) of deuterium gas. After removal of the catalyst using
a Celite pad, evaporation of the solvent gave 2-[3,4-D2]pyrrolidone
2a (2.27 g, 92%) as an oil. The structure was confirmed by
comparing its spectral data with those of the known unlabeled
compound.6 1H NMR (CDCl3) δ 0.02 (s, 3 H), 0.03 (s, 3 H), 0.86
(s, 9 H), 1 52 (s, 9 H), 1.98 (d, J ) 9 Hz, 1 H), 2.66 (d, J ) 10 Hz,
1 H), 3.67 (dd, J ) 2 and 10 Hz, 1 H), 3.90 (dd, J ) 4 and 10 Hz,
1 H), 4.15 (br, 1 H); HRMS m/z 216.1156 [(M - OTBDMS)+,
calcd for C10H14D2O4N 216.1205].
(5S)-N-ter t-Bu toxyca r bon yl-5-ter t-bu tyld ip h en ylsiloxy-
m eth yl-2-[3,4-D2]p yr r olid on e (2b). Using the procedure de-
scribed for the synthesis of compound 2a , 2-[3,4-D2]prrolidone
2b (2.06 g) was obtained as an oil from the olefin 1b (1.97 g,
4.37 mmol) in quantitative yield. The structure was confirmed
by comparing its spectral data with those of the known unlabeled
compound.6 1H NMR (CDCl3 δ 1.04 (s, 9 H), 1.43 (s, 9 H), 2.08
(d, J ) 10 Hz, 1 H), 2.76 (d, J ) 10 Hz, 1 H), 3.69 (dd, J ) 2 and
11 Hz, 1 H), 3.89 (dd, 4 and 11 Hz, 1 H), 4.2 (br, 1 H), 7.41 (m,
6 H), 7.62 (m, 4 H); 13C NMR (CDCl3) δ 18.87, 20.36 (t, J ) 20
Hz), 26.56, 27.73, 31.54 (J ) 20 Hz), 58.39, 64.77, 82.20, 127.54
and 127.56, 129.57 (4 C), 140.98 and 141.33, 135.20 and 135.24,
The spectral data were identical with those reported by us.3 1
H
NMR (D2O) δ 1.97 (dd, J ) 8 and 7 Hz, 1 H), 2.05 (dd, J ) 7 and
7 Hz, 1 H), 3.32 (d, J ) 7 Hz, 0.01 H), 3.41 (d, J ) 7 Hz, 0.99 H),
4.13 (d, J ) 7 Hz, 1 H).
ter t-Bu tyl (2S,3S,4R)-N-ter t-Bu toxycar bon yl[3,4-D2]pyr o-
glu ta m a te (12). To a solution of compound 2a (1.97 g, 6.02
mmol) in MeOH (60 mL) was added p-toluenesulfonic acid (103
mg, 0.6 mmol), and the resulting solution was stirred at room
temperature overnight. After removal of the solvent, the residue
was extracted with ethyl acetate. The organic layer was washed
with saturated aqueous NaHCO3 and dried over MgSO4. Evapo-
ration of the solvent afforded oily 5-hydroxymethyl[3,4-D2]-
pyrrolidone 10 (1.30 g, 99%), which was used in the next step
(10) Cieplak, A. S. J . Am. Chem. Soc. 1981, 103, 4540-4552.
(11) Bernardi, A.; Micheli, F.; Potenza, D.; Scolastico, C.; Villa, R.
Tetrahedron Lett. 1990, 31, 4949-4952. Ryu, Y.; Kim, G. J . Org. Chem.
1995, 60, 103-108.
(12) Thaning, M.; Winstrand, L.-G. J . Org. Chem. 1990, 55, 1406-
1408; J eroncic, L. O.; Cabal, M. P.; Danishefsky, S. J .; Shulte, G. M.
J . Org. Chem. 1991, 56, 387-395.
1
without purification. H NMR (CDCl3) δ 1.55 (s, 9 H), 1.94 (dd,
J ) 2 and 10 Hz, 1 H), 2.31 (br t, J ) 6 Hz, 1 H), 2.66 (d, J ) 10

9278 J . Org. Chem., Vol. 64, No. 25, 1999
Notes
Hz, 0.99 H), 3.74 (ddd, J ) 4, 6 and 11 Hz, 1 H), 3.86 (ddd, J )
4, 6 and 11 Hz, 1 H), 4.23 (dt, J ) 2 and 4 Hz, 1 H).
dried over MgSO4. After removal of the solvent, the crude tert-
butyl 5-hydroxyprolinate was directly treated with MeOH (10
mL) in the presence of p-toluenesulfonic acid (17.1 mg, 0.0996
mmol) for 1 h. The concentrated reaction mixture was diluted
with ethyl acetate (50 mL). The organic layer was washed three
times with saturated aqueous NaHCO3 (10 mL) and dried over
MgSO4. After removal of the solvent, the residue was chromato-
graphed on silica gel. Elution with a mixture of hexane and ethyl
acetate (96:4) afforded oily 5-methoxy[3,4-D2]prolinate 13 (256
mg, 80%) as unseparable diastereoisomers. The spectral data
were identical with those reported by us.3 1H NMR (CDCl3) δ
1.437, 1.440, 1.45, 1.46, 1.48, and 1.49 (6 s, 18 H), 1.84-2.11
(m, 2 H), 3.34, 3.39, 3.40, and 3.43 (4 s, 3 H), 4.14-4.20 (m, 1
H), 5.12-5.29 (m, 1 H).
(2S,3S,4R,5S)-[3,4,5-D3]P r olin e (14). To a solution of 5-meth-
oxy[3,4-D2]prolinate 13 (78.0 mg, 0.257 mmol) in toluene (10 mL)
were added Bu3SnD (178 mg, 0.616 mmol) and trifluoroborane
etherate (84 mg, 0.566 mmol) in two portions at intervals of 0.5
h at -78 °C under an argon atmosphere, and the resulting
solution was stirred for 2 h. Then the reaction mixture was
quenched with saturated aqueous NaHCO3 (5 mL), and the
organic layer was dried over MgSO4 and evaporated. The
deprotection of the resultant crude [3,4,5-D3]prolinate was
carried out in 1 M HCl (15 mL) at 110 °C for 3 h followed by a
treatment with Dowex 50W-X8 to give [3,4,5-D3]proline 14 (29.0
mg, 96%) as a colorless solid, mp 223-225 °C dec. The spectral
data were identical with those reported by us.3 1H NMR (D2O)
δ 1.97 (dd, J ) 7 and 7 Hz, 1 H), 2.05 (dd, J ) 7 and 7 Hz, 1 H),
3.32 (d, J ) 7 Hz, 0.97 H), 3.41 (d, J ) 7 Hz, 0.03 H), 4.13 (d, J
) 7 Hz, 1 H).
To a suspension of sodium metaperiodate (3.34 g, 15.6 mmol)
and RuCl3‚nH2O (110 mg) in H2O (9 mL) was added a solution
of compound 10 (340 mg, 1.56 mmol) in acetone (15 mL). The
resulting two-phase mixture was vigorously stirred at room
temperature for 1 h. The layers were separated, to the organic
phase was added 2-propanol (4.6 mL), and the mixture was
stirred for 1 h. After removal of the precipitated RuO2 using a
Celite pad, the filtrate was extracted with chloroform, concen-
trated, and dried over MgSO4. Evaporation of the solvent gave
[3,4-D2]pyroglutamic acid 11 (362 mg) in quantitative yield as
a colorless solid: mp 117-120 °C; 1H NMR (CDCl3) δ 1.50 (s, 9
H), 2.11 (dd, J ) 9 and 3 Hz, 1 H), 4.64 (d, J ) 3 Hz, 1 H), 6.10
(br, 1 H).
To a refluxing solution of the crude [3,4-D2]pyroglutamic acid
11 (195 mg, 0.84 mmol) in benzene (5 mL) was added a mixture
of N,N-dimethylformamide dineopentyl acetal (352 mg, 1.51
mmol) and tert-butyl alcohol (186 mg, 2.52 mmol), and the
reaction mixture was stirred for 0.5 h. Then the cooled reaction
mixture was diluted with ethyl acetate (30 mL), washed with
saturated aqueous NaHCO3 and brine, and dried over MgSO4.
After removal of the solvent, the residue was chromatographed
on silica gel. Elution with a mixture of hexane and ethyl acetate
(90:10) afforded oily tert-butyl [3,4-D2]pyroglutamate 12 (120 mg,
50%). The spectral data were identical with those reported by
us.3 1H NMR (CDCl3) δ 1.47 (s, 9 H), 1.49 (s, 9 H), 1.96 (dd, J )
10 and 2 Hz, 1 H), 2.57 (d, J ) 10 Hz, 1 H), 4.46 (d, J ) 2 Hz,
1 H); 13C NMR (CDCl3) δ 21.19 (t, J ) 21 Hz), 27.85 (6 C), 30.64
(t, J ) 21 Hz), 59.47, 82.10, 83.09, 149.37, 170.32, 173.11.
ter t-Bu tyl (2S,3S,4R,5RS)-N-ter t-Bu toxycar bon yl-5-m eth -
oxy[3,4-D2]p r olin a te (13). To a solution of tert-butyl [3,4-D2]-
pyroglutamate 12 (303 mg, 1.06 mmol) in THF (10 mL) was
added a solution of 1 M LiEt3BH in THF (1.27 mL, 1.27 mmol)
at -78 °C under an Ar atmosphere, and the reaction mixture
was stirred for 0.5 h. Then, the reaction was quenched with
saturated aqueous NaHCO3 (5 mL) at -78 °C, and the mixture
was warmed to 0 °C. After addition of 30% H2O2 (1 mL), the
mixture was stirred for an additional 30 min and concentrated.
The residue was extracted with ether, washed with H2O, and
Ack n ow led gm en t. This work has been supported
by CREST (Core Research for Evolutional Science and
Technology) of the J apan Science and Technology
Corporation (J ST) and a Grant-in-Aid for Scientific
Research (No. 09740480) from the Ministry of Educa-
tion, Science, Sports, and Culture of J apan.
J O990866R