Synthetic study of optically active 3-azabicyclo[3.3.0]octane-2,6,8- tricarboxylic acid

Article abstract of DOI:10.1248/cpb.51.1015

Synthesis of (1R,2S,5S,6R,8S)-3-azabicyclo[3.3.0]octane-2,6,8-tricarboxylic acid (2) from trans-4-hydroxy-L-proline (5) was attempted. A Diels-Alder reaction of 3,4-dehydroproline derivative 9 and cyclopentadiene afforded a single stereoisomer 11. The Diels-Alder adduct was smoothly converted to the hydrochloride of 2 (24) via RuO₄ oxidation. Although some racemization of the material or product was observed during the synthetic processes, the amino acid 24 proved to be optically pure.

Full text of DOI:10.1248/cpb.51.1015

August 2003
Notes
Chem. Pharm. Bull. 51(8) 1015—1020 (2003)
1015
Synthetic Study of Optically Active 3-Azabicyclo[3.3.0]octane-2,6,8-
tricarboxylic Acid
*
Yasushi ARAKAWA, Masafumi OHNISHI, Norikazu YOSHIMURA, and Shigeyuki YOSHIFUJI
Faculty of Pharmaceutical Sciences, Hokuriku University; Kanagawa-machi Ho-3, Kanazawa 920–1181, Japan.
Received April 30, 2003; accepted June 6, 2003
Synthesis of (1R,2S,5S,6R,8S)-3-azabicyclo[3.3.0]octane-2,6,8-tricarboxylic acid (2) from trans-4-hydroxy-L-
proline (5) was attempted. A Diels–Alder reaction of 3,4-dehydroproline derivative 9 and cyclopentadiene af-
forded a single stereoisomer 11. The Diels–Alder adduct was smoothly converted to the hydrochloride of 2 (24)
via RuO4 oxidation. Although some racemization of the material or product was observed during the synthetic
processes, the amino acid 24 proved to be optically pure.
Key words trans-4-hydroxy-L-proline; Diels–Alder reaction; ruthenium tetroxide; 3,4-dehydroproline; dicyclopentadiene;
(1R,2S,5S,6R,8S)-3-azabicyclo[3.3.0]octane-2,6,8-tricarboxylic acid
During our study on the synthesis of amino acids using umn (DAICEL CHIRALCEL OD), so 9 was hydrogenated to
Diels–Alder adducts,1—3) we synthesized racemic 3-azabicy- form 10a (Chart 3). Its specific rotation ([a]D18 Ϫ96.7°) was
clo[3.3.0]octane-2,6,8-tricarboxylic acid (1)1) as an analog of compared with that of 10b, which was prepared from L-pro-
kainic acid. However, considering its potential biological line ([a]D18 Ϫ101.7°),10) and the optical purity of 10a proved
uses, the optically active compound has more advantages, to be 95.1%ee.
so we wanted to synthesize optically active 3-azabicy-
Next, a Diels–Alder reaction of the dienophile 9 and cy-
clo[3.3.0]octane-2,6,8-tricarboxylic acid (2). In this paper, clopentadiene was attempted. When compound 9 was heated
we report a synthetic approach to 2 and the related chemistry. with cyclopentadiene in a bomb tube at 90 °C, almost no re-
Retro-synthesis of 2 (Chart 1) gave the Diels–Alder adduct action was observed. Additions of a Lewis acid such as
3 and then the derivative of 3,4-dehydroproline 4, which (C2H5)2O·BF3, SnCl4 or ZnI2 at Ϫ70—0 °C into the mixture
should be derived from readily available trans-4-hydroxy-L- of 9 and the cyclopentadiene were investigated, but they
proline (5). To obtain the derivative of 3,4-dehydroproline 4, resulted in the formation of polymers (when used
selective 3,4-elimination of 5 is needed,4—7) so the hydroxyl (C2H5)2O·BF3 or SnCl4) or the decomposition of 9 (when
group of 5 must be converted to a phenylseleno group, which used ZnI2). We then tried to employ dicyclopentadiene (cy-
would cause elimination similar to the Cope elimination re- clopentadiene dimer) as both reagent and solvent at a higher
action after H2O2 oxidation.4)
temperature (Chart 4). A solution of 9 in dicyclopentadiene
The starting 5 was converted to the N,C-protected form 6 was heated in a bomb tube at 150 °C (oil bath temp., inner
by usual method8) in 94% yield. Treatment of 6 with p-tolue- temp.: 135 °C), and 9 had completely disappeared after 48 h,
nesulfonyl chloride (TsCl) and 4-dimethylaminopyridine giving the desired 11a in 52% yield (after recrystallization).
(DMAP) in pyridine afforded 79) in 91% yield. Compound 7 At 170 °C (oil bath temp., inner temp.: 153 °C), 9 had disap-
was allowed to react with sodium phenylselenide,4—7) which peared after 36 h and 11b was obtained in 52% yield. Both
was generated from diphenyldiselenide and sodium borohy- products 11a, b had the same specific rotation, [a]D20 Ϫ73.2°.
dride in situ, to afford compound 8 in 97% yield. Oxidation Although four stereoisomers were expected to form in this
of 8 with 30% H2O2 in CH2Cl2 in the presence of pyridine af- reaction, only one isomer 11 was isolated; on analysis of
forded dienophile 9 in 70% yield. Estimation of the optical mass spectra (MS) for the other constituents of the reaction
purity of 9 was difficult on HPLC analysis using a chiral col- mixture, there was no peak of 297 (Mϩ), which would have
been indicative of the diastereomers. The stereochemistry of
11 was confirmed by differential nuclear Overhauser effect
1
(NOE) experiments of H-NMR; ϩNOEs were observed be-
tween a signal at d 4.58 assigned to the 3-proton and signals
at d 6.05, 6.07, 6.27, and 6.36 assigned to olefinic protons
(signals split due to the rotamers).
As we feared that racemization would occur under the re-
Fig. 1
action conditions employed, the following experiments were
Chart 1
To whom correspondence should be addressed. e-mail: ya-arakawa@hokuriku-u.ac.jp
© 2003 Pharmaceutical Society of Japan

1016
Vol. 51, No. 8
Chart 2
Chart 5
Chart 3
Chart 6
pyridine afforded 16 in 74% yield. Dienophile 16 was sub-
jected to a Diels–Alder reaction similarly to 9, giving 17 in
51% yield. A mixture of enantiomers 11 and 17 were separa-
ble under the following condition: solvent, hexane–2-
propanol (9 : 1); flow rate, 1 ml/min; column temperature,
30 °C. The retention time for 17 was 9.2 min and that for 11
was 11.9 min. A solid of 11 obtained by separation using sil-
ica gel column chromatography from the Diels–Alder reac-
tion mixture was analyzed and proved to be 91.1%ee. After
one recrystallization of the solid from diisopropyl ether, the
optical purity increased to 99.8%ee. After two recrystalliza-
tions, 11 did not contain the enantiomer (100%ee). As such,
the once-recrystallized 11 was used as the starting material
for the following stage because of the balance between qual-
ity and quantity.
However, to avoid racemization under the Diels–Alder re-
action conditions, we next chose compound 19 as another
dienophile (Chart 5). Reduction of ester 8 by NaBH4–LiCl in
THF–EtOH11,12) followed by treatment with benzoyl chloride
(BzCl) afforded benzoate 18 in 91% yield. Oxidation of 18 in
a similar manner to that employed for the oxidation of 8 af-
forded the desired dienophile 19 in 64% yield. Compound 19
was heated with dicyclopentadiene in a bomb tube at 170 °C
for 48 h, and the Diels–Alder adduct 20 was obtained in 51%
yield. The stereochemistry of 20 was confirmed by identifi-
cation of the product 21b at the next step with 21a. Diastere-
omers of 20 were not detected by MS analysis in the other
constituents of the reaction mixture, which had Rf values on
TLC similar to that of 19.
Chart 4
attempted. A solution of 9 ([a]D18 Ϫ357.8°) in xylene was
heated in a bomb tube at 150 °C (oil bath temp.) for 48 h, and
the recovered 9 had [a]D20 Ϫ292.3°. When heated at 170 °C
(oil bath temp.) for 36 h, the recovered 9 had [a]D20 Ϫ231.5°.
When heated at 170 °C (oil bath temp.) for 48 h, the recov-
ered 9 had [a]D20 Ϫ202.4°. Despite exchanging xylene for di-
cyclopentadiene as a solvent, Diels–Alder adduct 11 ob-
tained from the reaction at 150 °C (oil bath temp.) for 48 h
had the same specific rotation [a]D20 Ϫ73.2° (after one recrys-
tallization) as that obtained from the reaction at 170 °C (oil
bath temp.) for 36 h or 48 h. Furthermore, when the obtained
11 was heated in dicyclopentadiene at these temperatures, no
formation of 9 was observed; the retro-Diels–Alder reaction
could not occur under these conditions. These results suggest
that the rate of the Diels–Alder reaction is much faster than
that of racemization of 9.
To estimate the exact optical purity of the obtained 11 and
ratio of the racemization under the reaction conditions, we
prepared the enantiomer 17 from cis-4-hydroxy-D-proline
(12) and compared the enantiomers on HPLC using a chiral
column (DAICEL CHRALCEL OD, 0.46 cm i.d.ϫ25 cm).
Compound 12 was converted to the N,C-protected form 13
by the usual method in 85% yield. Treatment of 13 with p-
TsCl in pyridine afforded 14 in 90% yield. When DMAP was
added to the reaction mixture as a catalyst similar to the con-
version of 6 to 7, the yield of the product decreased by ca.
20% with increasing by-products. Compound 14 was treated
with sodium phenylselenide to afford 15 in 97% yield. Oxi-
dation of 15 with 30% H2O2 in CH2Cl2 in the presence of
Ruthenium tetroxide (RuO4) oxidation1—3) of 11 followed
by treatment with diazomethane afforded triester 21a in 83%
yield, the specific rotation of which was [a]D20 Ϫ35.0°. In
contrast, benzoate 20 was first hydrolyzed by 1 M NaOH, oxi-
dized by ruthenium tetroxide, then treated with diazomethane
to afford 21b in 71% yield, for which the specific rotation
was [a]D20 Ϫ33.5°.
We attempted to elucidate the reason for the lower optical
purity of 21b than 21a as follows. Compound 19 was hydro-

August 2003
1017
genated in the presence of 10% Pd/C and converted to com- giving a mixture of MTPA amides in 61% yield. Mixtures of
pound 23a, for which the specific rotation was [a]D18 both epimers were separable on HPLC using a silica gel col-
Ϫ157.2°. This product was then compared with the authentic umn [JASCO Finepak SIL, hexane–AcOEt (1 : 1), retention
23b derived from L-prolinol (22),13) for which the specific ro- time; 25: 8.70 min, 26: 9.99 min] and the diastereo mixture
tation was [a]D18 Ϫ168.0°, and the optical purity of 23a from 21a proved to consist of 99% 25 and 1% 26 (or enan-
proved to be 93.6%ee. The proportion of partial racemization tiomer 27). 1H-NMR analysis also supported this result, indi-
of 23a was slightly more than that of 10a. Therefore, the cating that the signal of a proton at the 2-position of 25 exists
lower optical purity of 21b appeared to be due to the purifi- at d 4.41 and that of 26 exists at d 4.68; the component ratio
cation of 20 by recrystallization not being possible because of 25 : 26 (or enantiomer 27) was 99 : 1. The diastereomeric
20 was obtained as an oil.
purity of the MTPA amide derived from 24 reflected the opti-
Compounds 21a and 21b were heated with 6 M cal purity of (S)-(ϩ)-MTPA-Cl (98%ee). Consequently, the
HCl–AcOH at 100 °C for 24 h, respectively, and the salts of target amino acid 2 proved to be optically pure.
the target amino acid 2 (24) were quantitatively obtained by
In conclusion, while the free target 2 has not been isolated,
concentration of the aqueous solution washed with benzene. we synthesized an optically active (1R,2S,5S,6R,8S)-3-azabi-
1
The structure of 24 was confirmed by identification of H- cyclo[3.3.0]octane-2,6,8-tricarboxylic acid as hydrochloride
and 13C-NMR data with those of 1. In the case of racemic 1, 24.
the hydrochloride salt was desalted simply by being dis-
solved in water, giving the free amino acid as a crystal. In
particular, when the pH of the aqueous solution was adjusted
Experimental
Melting points were measured with a Yanagimoto micromelting point ap-
paratus and are uncorrected. Specific rotations were determined with a
to 4 with NaOH, the crystallization was quantitative. A crude
hydrochloride of 2 (from 21a) was treated similarly, but no
solid was deposited. When a crude hydrochloride of 2 (from
21b) was treated similarly, a small amount of free amino acid
was deposited, but the yield was only 6%; moreover, the
crystal did not exhibit optical rotation, consisting only of 1.
As it turned out that the deposited crystal of the amino acid
was a racemate, crude 2 (from 21a) was purified as a hy-
drochloride; reprecipitation of the hydrochloride from water–
acetone gave 24 as a white powder in 83% yield.
JASCO DIP-370 polarimeter. NMR spectra, except for the amino acids,
were recorded in chloroform-d (CDCl3) on a GSX-400 spectrometer using
tetramethylsilane as an internal standard. For the amino acids, analysis was
performed in 2 M deuterium chloride (DCl) using 1,4-dioxane as an internal
standard (d: 3.7 for 1H-NMR and d: 67.4 for 13C-NMR). Infrared (IR) spec-
tra were recorded on a Hitachi 270-30 spectrophotometer. Mass spectra
(MS) were obtained with a JEOL JMS-DX300 instrument. TLC was per-
formed on Silica gel 60 F254 plates (0.25 mm, Merck). Column chromatogra-
phy was performed on silica gel (Kieselgel 60, 70—230 mesh, Merck) or
alumina (Aluminium oxide 90, 70—230 mesh, Merck). Flash chromatogra-
phy was performed on silica gel (Silica Gel 60, 230—400 mesh, Nacalai
Tesque).
To evaluate the optical purity of the target amino acid,
racemic 1 was esterified with SOCl2–MeOH, followed by
treatment with (S)-(ϩ)-a-methoxy-a-trifluoromethylphenyl-
acetyl chloride [(S)-(ϩ)-MTPA-Cl, 98%ee],14,15) and con-
verted into a comparable mixture of both epimers 25 and 26
in 62% yield. Optically active 24 was treated similarly to 1,
Fig. 2. NOE Relationships of Diels–Alder Adduct 11
Chart 7
Fig. 3
Chart 8
Fig. 4

1018
Methyl (2S,4R)-1-Benzoyl-4-hydroxypyrrolidine-2-carboxylate (6)
Vol. 51, No. 8
1754 (CϭO), 1646 (CϭO). MS m/z: 231 (Mϩ).
Thionyl chloride (21.8 ml) was added dropwise to MeOH (500 ml) at
Ϫ10 °C. After 30 min, trans-4-hydroxy-L-proline (5, 26.2 g, 0.20 mol) was
added to the solution, and the whole was stirred at room temperature
overnight. The reaction mixture was concentrated under reduced pressure,
and MeOH (300 ml) was added to the residue, after which the solution was
concentrated under reduced pressure to give a white solid. The solid was dis-
solved in 1,4-dioxane (400 ml), and the solution was cooled in an ice bath.
After aqueous NaHCO3 (50.4 g/450 ml) was added in several portions, ben-
zoyl chloride (25.8 ml, 0.22 mol) was added dropwise with vigorous stirring
at 0 °C and the stirring was continued for 4 h at room temperature. The reac-
tion mixture was concentrated to a half volume under reduced pressure. The
white precipitate was filtered off and washed with water (200 ml). The fil-
trate was saturated with NaCl, and the whole was extracted with CHCl3
(400 mlϫ2). The organic layer was washed with brine (120 ml), dried over
anhydrous Na2SO4, and concentrated under reduced pressure to give a white
solid. A mixture of the white precipitate and the white solid was recrystal-
lized from benzene to give compound 6 (47.0 g, 94%) as colorless prisms,
mp 143—145 °C, (lit.8) mp 141 °C), [a]D21 Ϫ144.1° (cϭ1.0, CHCl3), [lit.8)
[a]D25 Ϫ142.4° (cϭ1.6, CHCl3)].
Methyl (2S,4R)-1-Benzoyl-4-(p-toluenesulfonyloxy)pyrrolidine-2-car-
boxylate (7) A solution of p-toluenesulfonyl chloride (23.0 g, 0.12 mol) in
pyridine (120 ml) was added dropwise to a solution of 6 (25.0 g, 0.10 mol)
and DMAP (20 mg) in pyridine (80 ml) at 0 °C, and the mixture was stirred
at room temperature for 6 d. The mixture was then concentrated under re-
duced pressure, and water was added to the residue at 0 °C. The whole was
extracted with AcOEt (400 ml). The organic layer was washed with 5% HCl
(30 mlϫ3) and water (40 mlϫ3), dried over anhydrous Na2SO4, and concen-
trated under reduced pressure to give a white solid. Recrystallization from
AcOEt–hexane gave compound 7 (36.8 g, 91%) as colorless needles, mp
113—115 °C, (lit.9) mp 113—114.5 °C), [a]D20 Ϫ76.7° (cϭ1.0, CHCl3), [lit.9)
[a]D26 Ϫ61.8° (cϭ1.14, EtOH)]. 1H-NMR16) (CDCl3) d: 2.14—2.68 (5H, m),
3.33—3.89 (5H, m), 4.57 (0.2H, br t), 4.80 (0.8H, t, Jϭ8.2 Hz), 5.10 (1H,
br), 7.30—7.70 (9H, m). 13C-NMR16) (CDCl3) d: 21.68 (q), 35.43 (t), 52.57
(q), 55.05 (t), 57.41 (d), 79.14 (d), 127.44 (d), 127.61 (d), 128.37 (d), 130.07
Estimation of the Optical Purity of 9 by Converting to 10a Com-
pound 9 (302 mg) was dissolved in MeOH (20 ml) and hydrogenated in the
presence of PtO2 (40 mg) under a pressure of 2 atm for 12 h. The catalyst
was filtered off, and the filtrate was concentrated under reduced pressure to
give 10a (294 mg, 97%) as a white solid, mp 87.5—89 °C, [a]D18 Ϫ96.7°
(cϭ1.0, CHCl3). Authentic 10b was prepared from L-proline by the reported
procedure,10) mp 89—89.5 °C, (lit. mp 89—89.5 °C), [a]D18 Ϫ101.7° (cϭ1.0,
CHCl3). Comparison of the specific rotations indicated the optical purity of
9 was 95.1%ee.
Partial Racemization of 9 at Higher Temperature A solution of 9
(200 mg) in xylene (5 ml) was heated in a bomb tube at 150 °C or 170 °C
for 36—48 h. The mixture was concentrated under reduced pressure, and
the residue was subjected to column chromatography on silica gel
[hexane–AcOEt (4 : 1)] to recover partially racemized 9, which was then an-
alyzed by optical rotation.
Methyl (1R,2S,3S,6R,7S)-4-Benzoyl-4-azatricyclo[5.2.1.02,6]dec-8-ene-
3-carboxylate (11) A mixture of compound 9 (3.35 g, 14.5 mmol) and di-
cyclopentadiene (20 ml) was heated in a bomb tube at 150 °C (oil bath
temp., inner temp.: 135 °C) for 48 h or at 170 °C (oil bath temp., inner temp.:
153 °C) for 36 h. The reaction mixture was subjected to column chromatog-
raphy on silica gel (benzene, then AcOEt), and the fractions eluted with
AcOEt were collected. The residue after concentration was subjected to flash
chromatography [hexane–AcOEt (5 : 1)], and the resultant white solid was
recrystallized from i-Pr2O, giving 11 (2.24 g, 52%) as colorless prisms, mp
1
118—120 °C, [a]D20 Ϫ73.2° (cϭ1.0, CHCl3). H-NMR16) (CDCl3) d: 1.35—
1.63 (3H, m), 2.82—3.15 (4H, m), 3.64—3.85 (4H, m), 4.58 (1H, d,
Jϭ2.6 Hz), 6.00—6.10 (1H, m), 6.27 and 6.36 (1H, dd, Jϭ5.5, 2.9 Hz),
7.22—7.45 (5H, m). 13C-NMR16) (CDCl3) d: 42.75 (d), 45.10 (d), 46.89 (d),
47.13 (d), 47.28 (d), 47.89 (d), 48.66 (d), 51.24 (t), 51.47 (t), 51.53 (t), 51.81
(t), 52.32 (q), 60.91 (d), 64.62 (d), 126.02 (d), 126.84 (d), 128.26 (d), 128.49
(d), 129.60 (d), 129.81 (d), 133.85 (d), 134.70 (d), 134.81 (d), 135.78 (d),
136.85 (s), 137.31 (s), 169.22 (s), 169.88 (s), 172.94 (s), 173.06 (s).
KBr
IR n cmϪ1: 1748 (CϭO), 1634 (CϭO). MS m/z: 297 (Mϩ). Anal. Calcd
max
for C18H19NO3: C, 72.71; H, 6.44; N, 4.71. Found: C, 72.72; H, 6.40; N,
4.75. HPLC analysis using a chiral column [DAICEL CHRALCEL OD,
0.46 cm I.D.ϫ25 cm, solvent: hexane–2-propanol (9 : 1), sample injection:
1 ml (1.00 g/l CHCl3 solution), flow rate 1 ml/min, column temp.: 30 °C]
showed the ratio of the enantiomers was 99.9 : 0.01. The crystals were used
in the next step, but could be further purified by one more recrystallization
to give optically pure 11, mp 119—121 °C, [a]D20 Ϫ73.3° (cϭ1.0, CHCl3).
Methyl (2R,4R)-1-Benzoyl-4-hydroxypyrrolidine-2-carboxylate (13)
Thionyl chloride (1.70 ml) was added dropwise to MeOH (38.5 ml) at
Ϫ10 °C. After 30 min, cis-4-hydroxy-D-proline (12, 1.96 g, 14.9 mmol) was
added to the solution and the whole was stirred at room temperature
overnight. The reaction mixture was concentrated under reduced pressure,
and MeOH (23.0 ml) was added to the residue, after which the solution was
concentrated under reduced pressure to give a white solid. The solid was dis-
solved in 1,4-dioxane (31.0 ml), and the solution was cooled in an ice bath.
After aqueous NaHCO3 (3.86 g/35.0 ml) was added in several portions, ben-
zoyl chloride (1.97 ml, 16.8 mmol) was added dropwise with vigorous stir-
ring at 0 °C and vigorous stirring was continued for 4 h at room temperature.
The reaction mixture was concentrated to a half volume under reduced pres-
sure. The white precipitate was filtered off and washed with water (40 ml).
The filtrate was saturated with NaCl, and the whole was extracted with
CHCl3 (40 mlϫ2). The organic layer was washed with brine (40 ml), dried
over anhydrous Na2SO4, and concentrated under reduced pressure to give a
white solid. A mixture of the white precipitate and the white solid (3.69 g)
was recrystallized from benzene to give compound 13 (3.17 g, 85%) as col-
orless prisms, mp 100.5—101.5 °C, [a]D21 Ϫ25.4° (cϭ1.0, CHCl3). 1H-
NMR16) (CDCl3) d: 1.98—2.44 (2H, m), 3.57—3.92 (6H, m), 4.33—4.68
(2H, m), 7.28—7.53 (5H, m). 13C-NMR16) (CDCl3) d: 36.89 (t), 39.63 (t),
(d), 130.69 (d), 133.38 (s), 135.00 (s), 145.35 (s), 169.70 (s), 171.98 (s).
KBr
IR n cmϪ1: 1742 (CϭO), 1628 (CϭO). MS m/z: 403 (Mϩ).
max
Methyl (2S,4S)-1-Benzoyl-4-phenylselenopyrrolidine-2-carboxylate (8)
Under argon atmosphere, NaBH4 (2.04 g, 54 mmol) was slowly added to a
solution of diphenyl diselenide (8.43 g, 27 mmol) in THF–MeOH
(50 ml–50 ml), and the mixture was stirred until NaBH4 was completely dis-
solved. A solution of 7 (17.2 g, 43 mmol) in THF (50 ml) was added to the
pale yellow solution, and the whole was refluxed for 2 h. The mixture was
then concentrated under reduced pressure, and the residue was dissolved in
AcOEt (300 ml). The solution was washed with water (200 mlϫ3), dried
over anhydrous Na2SO4, and concentrated under reduced pressure. The re-
sulting yellow oil was subjected to column chromatography on silica gel
(benzene, then AcOEt), and the fractions eluted with AcOEt were collected.
After evaporation of the solvent, the residue was recrystallized from
AcOEt–i-Pr2O, giving compound 8 (16.0 g, 97%) as colorless needles, mp
77.5—78.5 °C, [a]D19 Ϫ85.7° (cϭ1.0, CHCl3). 1H-NMR16) (CDCl3) d: 1.93—
2.43 (1H, m), 2.55—3.06 (1H, m), 3.48—4.36 (6H, m), 4.41 (0.2H, br), 4.69
(0.8H, t, Jϭ8.4 Hz), 7.20—8.10 (10H, m). 13C-NMR16) (CDCl3) d: 36.75 (t),
37.48 (d), 52.41 (q), 56.22 (t), 58.97 (d), 126.73 (d), 127.37 (s), 127.57 (d),
128.32 (d), 129.34 (d), 130.65 (d), 134.94 (d), 135.38 (s), 169.17 (s), 171.75
KBr
(s). IR n cmϪ1: 1750 (CϭO), 1640 (CϭO). MS m/z: 389 (Mϩ). Anal.
max
Calcd for C19H19NO3Se: C, 58.77; H, 4.93; N, 3.61. Found: C, 58.81; H,
4.97; N, 3.60.
Methyl (2S)-1-Benzoylpyrrolidin-3-ene-2-carboxylate (9) Thirty-per-
cent aqueous H2O2 solution (28 ml, 0.25 mol) was added dropwise to a solu-
tion of 8 (19.4 g, 0.05 mol) and pyridine (8.0 ml, 0.10 mol) in CH2Cl2
(100 ml) at 0 °C, and the whole was then vigorously stirred at room tempera-
ture for 2 h. CH2Cl2 (100 ml) was added to the reaction mixture, washed
with 5% HCl (100 mlϫ2), sat. NaHCO3 (100 ml), and water (100 mlϫ3).
The organic layer was dried over anhydrous Na2SO4, and concentrated under
reduced pressure. The residual orange oil was subjected to column chro-
matography on silica gel [hexane–AcOEt (4 : 1)] to give 9 (8.10 g, 70%) as a
pale yellow oil, [a]D18 Ϫ357.8° (cϭ1.1, CHCl3). 1H-NMR16) (CDCl3) d: 3.49
(0.2H, s), 3.79 (0.8H, s), 4.16—4.54 (2H, m), 5.11 (0.2H, br s), 5.47 (0.8H,
m), 5.73 (0.2H, br), 5.88 (0.8H, ddd, Jϭ6.2, 1.8, 1.8 Hz), 5.94 (0.8H, dd,
Jϭ2.2, 1.8 Hz), 6.10 (0.2H, br), 7.39—7.59 (5H, m). 13C-NMR16) (CDCl3)
d: 52.37 (q), 52.44 (q), 53.86 (t), 55.90 (t), 66.42 (d), 68.12 (d), 124.61 (d),
52.57 (q), 52.84 (q), 58.15 (t), 58.20 (d), 69.26 (d), 70.99 (d), 127.29 (d),
KBr
128.38 (d), 130.48 (d), 135.52 (s), 170.07 (s), 174.62 (s). IR n cmϪ1: 3324
max
(OH), 1766 (CϭO), 1610 (CϭO). MS m/z: 249 (Mϩ). Anal. Calcd for
C13H15NO4: C, 62.64; H, 6.07; N, 5.62. Found: C, 62.93; H, 6.10; N, 5.57.
Methyl (2R,4R)-1-Benzoyl-4-(p-toluenesulfonyloxy)pyrrolidine-2-car-
boxylate (14) A solution of p-toluenesulfonyl chloride (2.07 g, 10.9 mmol)
in pyridine (14 ml) was added dropwise to a solution of 13 (2.25 g,
9.03 mmol) in pyridine (9 ml) at 0 °C, and the mixture was stirred at room
temperature for 4 d. The mixture was then concentrated under reduced pres-
sure, water (20 ml) was added to the residue at 0 °C, and the whole was
stirred at room temperature for 0.5 h, then extracted with AcOEt (40 ml).
The organic layer was washed with 5% HCl (20 mlϫ3) and water
124.86 (d), 126.52 (d), 127.02 (d), 128.40 (d), 128.67 (d), 129.31 (d), 129.93
neat
(d), 130.24 (d), 135.93 (s), 136.38 (s), 169.63 (s), 170.14 (s). IR n cmϪ1
:
max

August 2003
1019
H, 4.96; N, 3.00.
(20 mlϫ5), dried over anhydrous Na2SO4, and concentrated under reduced
pressure. The residue was subjected to column chromatography on silica gel
[hexane–AcOEt (2 : 1)] to give 14 (3.27 g, 90%) as a white powder. Recrys-
tallization from benzene–hexane gave colorless needles, mp 100—101.5 °C,
(2S)-(1-Benzoyl-3-pyrrolin-2-yl)methyl Benzoate (19) Thirty-percent
aqueous H2O2 solution (12 ml) was added dropwise to a solution of 18
(9.29 g, 20 mmol) and pyridine (2.0 ml) in CH2Cl2 (100 ml) at 0 °C, and the
whole was then vigorously stirred at room temperature for 5 h. The reaction
mixture was washed with 5% HCl (50 mlϫ2), sat. NaHCO3 (50 ml), and
water (50 mlϫ3). The organic layer was dried over anhydrous Na2SO4, and
concentrated under reduced pressure. The residual orange oil was subjected
to column chromatography on silica gel [hexane–AcOEt (5 : 1)] to give 19
(3.92 g, 64%) as a pale yellow oil, [a]D18 Ϫ393.0° (cϭ1.0, CHCl3). 1H-NMR
(CDCl3) d: 4.03—4.13 (1H, m), 4.29—4.45 (2H, m), 4.69 (1H, dd, Jϭ11.2,
4.0 Hz), 4.75 (1H, dd, Jϭ11.2, 2.4 Hz), 5.42 (1H, br), 5.77—5.96 (2H, m),
7.30—8.09 (10H, m). 13C-NMR (CDCl3) d: 56.59 (t), 63.62 (d), 64.10 (t),
1
[a]D25 ϩ59.3° (cϭ1.0, CHCl3). H-NMR16) (CDCl3) d: 2.33—2.49 (5H, m),
3.63—5.00 (7H, m), 7.27—7.76 (9H, m). 13C-NMR16) (CDCl3) d: 21.69 (q),
34.90 (t), 52.61 (q), 54.10 (t), 56.85 (d), 126.78 (d), 127.17 (d), 127.76 (d),
128.48 (d), 130.02 (d), 130.56 (d), 133.38 (s), 135.34 (s), 145.34 (s), 169.70
KBr
(s), 170.98 (s). IR n cmϪ1: 1748 (CϭO), 1622 (CϭO). MS m/z: 403
max
(Mϩ). Anal. Calcd for C20H21NO6S: C, 59.54; H, 5.25; N, 3.47. Found: C,
59.43; H, 5.25; N, 3.64.
Methyl
(2R,4S)-1-Benzoyl-4-phenylselenopyrrolidine-2-carboxylate
(15) Under argon atmosphere, NaBH4 (0.38 g, 10 mmol) was slowly added
to a solution of diphenyl diselenide (1.52 g, 4.87 mmol) in THF–MeOH
(9.0 ml–9.0 ml), and the mixture was stirred until NaBH4 was completely
dissolved. A solution of 14 (3.13 g, 7.76 mmol) in THF (9.0 ml) was added
to the pale yellow solution, and the whole was refluxed for 2 h. The solution
was concentrated under reduced pressure, and the residue was dissolved in
AcOEt (100 ml). The solution was then washed with water (40 mlϫ3), dried
over anhydrous Na2SO4, and concentrated under reduced pressure. The re-
sulting yellow oil was subjected to column chromatography on silica gel
(benzene, then AcOEt) to give compound 15 (2.92 g, 97%) as a colorless oil,
126.78 (d), 127.11 (d), 127.40 (d), 128.43 (d), 128.63 (d), 129.57 (d), 130.19
neat
max
(s), 130.19 (d), 133.13 (d), 136.55 (s), 166.30 (s), 170.34 (s). IR n cmϪ1
:
1724 (CϭO), 1644 (CϭO), 1624 (CϭC). MS m/z: 307 (Mϩ).
(1R,2S,3S,6R,7S)-(4-Benzoyl-4-azatricyclo[5.2.1.02,6]dec-8-en-3-
yl)methyl Benzoate (20) A mixture of compound 19 (2.50 g, 8.13 mmol)
and dicyclopentadiene (15 ml) was heated in a bomb tube at 170 °C for 48 h.
The reaction mixture was subjected to column chromatography on silica gel
(benzene, then AcOEt), and the fractions eluted with AcOEt were collected.
The residue after concentration was subjected to flash chromatography
[hexane–Et2O (3 : 1)] to give 20 (1.55 g, 51%) as a pale yellow oil,
[a]D21 Ϫ84.3° (cϭ1.0, CHCl3). 1H-NMR16) (CDCl3) d: 1.35 (1H, d,
Jϭ8.4 Hz), 1.48 (1H, d, Jϭ8.4 Hz), 3.60—3.83 (1H, m), 4.15 (0.3H, m),
4.48 (2H, d, Jϭ4.4 Hz), 4.55 (0.7H, m), 5.95 (1H, dd, Jϭ5.7, 2.9 Hz), 6.31
(1H, dd, Jϭ5.5, 2.9 Hz), 7.27—8.07 (10H, m). 13C-NMR16) (CDCl3) d:
45.43 (d), 46.81 (d), 46.93 (d), 47.01 (d), 51.38 (t), 52.31 (t), 57.73 (d),
66.29 (t), 126.81 (d), 128.23 (d), 128.52 (d), 129.60 (d), 129.69 (d), 130.04
1
[a]D25 ϩ37.9° (cϭ1.0, CHCl3). H-NMR16) (CDCl3) d: 2.36—2.42 (2H, m),
3.50—3.57 (2H, m), 3.77—4.01 (4H, m), 4.79—4.82 (1H, m), 7.22—7.59
(10H, m). 13C-NMR16) (CDCl3) d: 36.13 (t), 36.76 (t), 38.31 (d), 52.41 (q),
55.80 (t), 58.62 (d), 58.99 (d), 126.59 (s), 127.31 (d), 127.55 (d), 128.31 (d),
128.38 (d), 129.31 (d), 130.34 (d), 134.96 (d), 135.22 (d), 135.69 (s), 169.64
neat
(s), 172.37 (s). IR n cmϪ1: 1750 (CϭO), 1642 (CϭO). MS m/z: 389
max
(Mϩ).
(s), 133.08 (d), 134.35 (d), 134.91 (d), 137.37 (s), 166.56 (s), 169.25 (s).
Methyl (2R)-1-Benzoylpyrrolidin-3-ene-2-carboxylate (16) Thirty-
percent aqueous H2O2 solution (4.0 ml) was added dropwise to a solution of
15 (2.73 g, 7.03 mmol) and pyridine (1.12 ml) in CH2Cl2 (15 ml) at 0 °C, and
the whole was then vigorously stirred at room temperature for 2 h. CH2Cl2
(15 ml) was added to the reaction mixture, then washed with 5% HCl
(15 mlϫ2), sat. NaHCO3 (15 ml), and water (15 mlϫ3). The organic layer
was dried over anhydrous Na2SO4, and concentrated under reduced pressure.
The residual orange oil was subjected to column chromatography on silica
gel [hexane–AcOEt (4 : 1)] to give 16 (1.21 g, 74%) as a pale yellow oil,
[a]D25 ϩ338.0° (cϭ1.0, CHCl3). 1H-NMR, 13C-NMR, IR, and MS data com-
pletely coincided with those of the enantiomer 9.
neat
IR n cmϪ1: 1724 (CϭO), 1634 (CϭO). MS m/z: 373 (Mϩ).
max
Trimethyl
(1R,2S,5S,6R,8S)-3-Benzoyl-3-azabicyclo[3.3.0]octane-
2,6,8-tricarboxylate (21a) Derived from Compound 11 A solution of 11
(1.19 g, 4.00 mmol) in AcOEt (30 ml), RuO2·xH2O (10 mg), and a 10%
NaIO4 aqueous solution (34 ml) were mixed and then vigorously stirred at
0 °C for 12 h. The AcOEt layer was separated, and the aqueous layer was ex-
tracted with AcOEt (50 mlϫ6). Isopropyl alcohol (10 ml) was added to the
combined AcOEt layers and the solution was left to stand for 1 h. The pre-
cipitated RuO2 was filtered off and the solution was dried over anhydrous
Na2SO4, then concentrated under reduced pressure. The residue was dis-
solved in MeOH (10 ml) and treated with diazomethane. The solution was
concentrated, and the residual white solid was recrystallized from
AcOEt–hexane to give 21a (1.29 g, 83%) as colorless needles, mp 81.5—
82.5 °C, [a]D20 Ϫ35.0° (cϭ1.0, CHCl3). 1H-NMR16) (CDCl3) d: 2.14 (1H,
ddd, Jϭ12.7, 6.3, 6.3 Hz), 2.46 (1H, ddd, Jϭ12.7, 12.7, 12.7 Hz), 2.94—
3.13 (4H, m), 3.41—3.82 (11H, m), 4.25 and 4.71 (1H, each br s), 7.29—
7.50 (5H, m). 13C-NMR16) (CDCl3) d: 29.24 (t), 45.20 (d), 45.95 (d), 46.42
(d), 49.10 (d), 51.34 (t), 51.82 (q), 52.14 (q), 52.50 (q), 60.90 (d), 127.16
Methyl (1S,2R,3R,6S,7R)-4-Benzoyl-4-azatricyclo[5.2.1.02,6]dec-8-ene-
3-carboxylate (17) A mixture of compound 16 (1.16 g, 5.02 mmol) and di-
cyclopentadiene (7 ml) was heated in a bomb tube at 150—170 °C for 48 h.
The reaction mixture was subjected to column chromatography on silica gel
(benzene, then AcOEt), and the AcOEt eluate was concentrated under re-
duced pressure. The residue was subjected to flash chromatography
[hexane–AcOEt (5 : 1)], and the resultant white solid was recrystallized from
i-Pr2O, giving 17 (0.76 g, 51%) as colorless prisms, mp 118—120 °C,
1
[a]D25 ϩ73.1° (cϭ1.0, CHCl3). H-NMR, 13C-NMR, IR, and MS data com-
(d), 128.26 (d), 130.10 (d), 135.99 (s), 169.46 (s), 172.13 (s), 172.30 (s) ,
KBr
172.43 (s). IR n cmϪ1: 1744 (CϭO), 1630 (CϭO). MS m/z: 389 (Mϩ).
pletely coincided with those of the enantiomer 11.
max
Anal. Calcd for C20H23NO7: C, 61.69; H, 5.95; N, 3.60. Found: C, 61.74; H,
(2S,4S)-(1-Benzoyl-4-phenylselenopyrrolidin-2-yl)methyl
Benzoate
5.93; N, 3.54.
Trimethyl
(18) Under argon atmosphere, LiCl (4.66 g, 110 mmol) and NaBH4
(4.16 g, 110 mmol) were added to a solution of compound 8 (21.3 g,
54.9 mmol) in THF (90 ml), and EtOH (160 ml) was dropwise added to the
mixture with stirring at room temperature. The stirring was continued
overnight. Water (180 ml) was added, and the whole was extracted with
CH2Cl2 (100 mlϫ3). The organic layer was dried over anhydrous Na2SO4
and concentrated under reduced pressure. The residual yellow oil was dis-
solved in anhydrous pyridine (110 ml) and cooled in an ice bath. Benzoyl
chloride (8.6 g, 61 mmol) was added dropwise, and the mixture was stirred
overnight. The mixture was then concentrated under reduced pressure, and
the residue was dissolved in CH2Cl2 (400 ml). The solution was washed with
2% HCl (100 mlϫ2), sat. NaHCO3 (10 ml), and water (100 mlϫ3), dried
over anhydrous Na2SO4, and concentrated under reduced pressure. The
residue was subjected to column chromatography on alumina (AcOEt), and
the resulting solid was recrystallized from AcOEt–hexane to give 18 (23.2 g,
91%) as colorless prisms, mp 107—108 °C, [a]D21 Ϫ74.3° (cϭ1.0, CHCl3).
1H-NMR16) (CDCl3) d: 2.03—2.11 (1H, m), 2.59—2.64 (1H, m), 3.38—
3.58 (2H, m), 3.78—3.93 (1H, m), 4.46—4.78 (3H, m), 7.19—8.06 (15H,
m). 13C-NMR (CDCl3) d: 35.19 (t), 36.93 (d), 56.15 (d), 57.13 (t), 64.44 (t),
127.09 (d), 127.61 (s), 128.25 (d), 128.37 (d), 128.48 (d), 129.22 (d), 129.61
(1R,2S,5S,6R,8S)-3-Benzoyl-3-azabicyclo[3.3.0]octane-
2,6,8-tricarboxylate (21b) Derived from Compound 20 1 M NaOH
(5.0 ml) was added dropwise to a solution of 20 (1.87 g, 5.01 mmol) in
MeOH (30 ml), and the mixture was stirred at 0 °C for 4 h. Water (100 ml)
was added, and the whole was extracted with AcOEt (200 mlϫ3). The or-
ganic layer was washed with water (100 ml), dried over anhydrous Na2SO4,
and concentrated under reduced pressure. The residual pale yellow oil was
dissolved in AcOEt (50 ml). The solution, RuO2·xH2O (30 mg), and a 10%
NaIO4 solution (112 ml) were mixed and then vigorously stirred at 0 °C for
8 h. The AcOEt layer was separated and the aqueous layer was extracted
with AcOEt (200 mlϫ5). Isopropyl alcohol (15 ml) was added to the com-
bined AcOEt layers, and the solution was left to stand for 1 h. The precipi-
tated RuO2 was filtered off, and the solution was dried over anhydrous
Na2SO4, then concentrated under reduced pressure. The residue was dis-
solved in MeOH (15 ml) and treated with diazomethane. The solution was
concentrated, and the residual colorless oil was subjected to flash chro-
matography [hexane–AcOEt (2 : 1)] to give a white solid. Recrystallization
from AcOEt–hexane gave 21b (1.38 g, 71%) as colorless needles, mp 81—
82.5 °C, [a]D20 Ϫ33.5° (cϭ1.0, CHCl3). 1H-NMR, 13C-NMR, IR, and MS
data completely coincided with those of 21a.
(d), 129.92 (s), 130.57 (d), 133.06 (d), 135.11 (d), 136.05 (s), 166.23 (s),
KBr
169.85 (s). IR n cmϪ1: 1726 (CϭO), 1636 (CϭO). MS m/z: 465 (Mϩ).
Estimation of the Optical Purity of 19 by Converting to 23a Com-
pound 19 (300 mg) was dissolved in MeOH (20 ml) and hydrogenated in the
max
Anal. Calcd for C25H23NO3Se: C, 64.65; H, 4.99; N, 3.02. Found: C, 64.69;

1020
Vol. 51, No. 8
62%). The mixture was used in 1H-NMR and HPLC analyses. Second, com-
pound 24 (27.9 mg, 0.10 mmol) was similarly treated to give a mixture of 25
and 26 (or its enantiomer 27) in 61% yield. Analyses on HPLC were done
under the condition [JASCO Finepak SIL, 0.46 cm i.d.ϫ25 cm, solvent:
hexane–AcOEt (1 : 1), sample injection: 20 ml (1.00 g/l CHCl3 solution),
flow rate 1 ml/min, column temp.: 40 °C] and showed that the ratio of the
enantiomers derived from 24 was 99 : 1. (retention time; 25: 8.70 min, 26:
9.99 min). 1H-NMR analyses were carried out in CDCl3 solutions by the
comparison of integrated intensities of d 4.41 (25) and d 4.68 (26), and these
results supported the HPLC results.
presence of 10% Pd/C (50 mg) under a pressure of 3 atm for 6 h. The catalyst
was filtered off, and the filtrate was concentrated under reduced pressure to
give 23a (295 mg, 98%) as a colorless oil, [a]D18 Ϫ157.2° (cϭ1.0, CHCl3).
Authentic 23b was prepared from 22 by the reported procedure,13)
[a]D18 Ϫ168.0° (cϭ1.0, CHCl3), [lit. [a]D21 Ϫ154.0° (CHCl3)]. Comparison of
the specific rotations indicated that the optical purity of 19 was 93.6%ee.
(1R,2S,5S,6R,8S)-3-Azabicyclo[3.3.0]octane-2,6,8-tricarboxylic Acid
Hydrochloride (24) Compound 21a (778 mg, 2.00 mmol) was heated in
AcOH (40 ml) and 6 M HCl (40 ml) at 100 °C for 24 h. The reaction mixture
was concentrated under reduced pressure, and water (50 ml) was added to
the residue. The whole was washed with benzene (50 mlϫ3) to remove ben-
zoic acid, and the aqueous layer was filtered. The filtrate was concentrated
under reduced pressure to give 24 (553 mg, 99%) as a white solid, which was
reprecipitated from water–acetone to give a white powder (465 mg, 83%),
Acknowledgement This work was supported in part by the Special Re-
search Fund of Hokuriku University.
1
mp 213 °C (dec.), [a]D20 ϩ38.3° (cϭ1.0, 2 M HCl). H- and 13C-NMR data
References and Notes
1) Yasuda M., Saito S., Arakawa Y., Yoshifuji S., Chem. Pharm. Bull., 43,
1318—1324 (1995).
2) Arakawa Y., Yasuda M., Ohnishi M., Yoshifuji S., Chem. Pharm. Bull.,
45, 255—259 (1997).
3) Arakawa Y., Goto T., Kawase K., Yoshifuji S., Chem. Pharm. Bull., 46,
674—680 (1998).
4) Sharpless K. B., Lauer R. F., J. Am. Chem. Soc., 95, 2697—2699
(1973).
5) Dormoy J.-R., Synthesis, 1982, 753—756 (1982).
6) Rüeger H., Benn M. H., Can. J. Chem., 60, 2918—2920 (1982).
7) Goli D. M., Cheesman B. V., Hassan M. E., Lodaya R., Slama J. T.,
Carbohydr. Res., 259, 219—241 (1994).
8) Baldwin J. E., Bamford S. J., Fryer A. M., Rudolph M. P. W., Wood M.
E., Tetrahedron, 53, 5233—5254 (1997).
9) Portoghese P. S., Turcotte J. G., Tetrahedron, 27, 961—967 (1971).
10) Johnson L. F., Robertson A. V., Simpson W. R. J., Witkop B., Aust. J.
Chem., 19, 115—134 (1966).
11) Hamada Y., Shioiri T., Tetrahedron Lett., 23, 1193—1196 (1982).
12) Hamada Y., Shioiri T., Chem. Pharm. Bull., 30, 1921—1924 (1982).
13) Gordon J. J., Devlin J. P., East A. J., Ollis W. D., Sutherland I. O.,
Wright D. E., Ninet L., J. Chem. Soc., Perkin Trans. 1, 1975, 819—825
(1975).
14) Dale J. A., Dull D. L., Mosher H. S., J. Org. Chem., 34, 2543—2549
(1969).
15) Dale J. A., Mosher H. S., J. Am. Chem. Soc., 95, 512—519 (1973).
16) Some signals split or broaden due to the rotamers or the conformers.
KBr
completely coincided with those of racemic 1.1) IR n cmϪ1: 3168, 1740.
max
MS (FAB) m/z: 244 (MϩϪCl). Anal. Calcd for C10H14ClNO6: C, 42.95; H,
5.05; N, 5.01. Found: C, 42.84; H, 4.99; N, 4.97.
Compound 21b was treated similarly to 21a and afforded similar results
before recrystallization. However, when a white solid obtained from 21b be-
fore recrystallization was dissolved in a minimum amount of water and the
solution was adjusted to pH 4 with NaOH, a 6% yield of free 1 was crystal-
lized as colorless prisms, mp 284 °C (dec.), [a]D20 0° (cϭ1.0, 2 M HCl). In the
case of 21a, no solid precipitated from the pH 4 solution.
Estimation of the Optical Purity of 24 First, for an authentic sample,
racemic 1 was converted to MTPA amide. Thionyl chloride (33 ml) was
added to MeOH (5.0 ml) at Ϫ10 °C. After 30 min, compound 1 (24.3 mg,
0.10 mmol) was added to the solution, and the whole was stirred at room
temperature overnight. The reaction mixture was concentrated under re-
duced pressure, and MeOH (5.0 ml) was added to the residue, after which
the solution was concentrated under reduced pressure to give a white solid
(33.0 mg). This product was dissolved in anhydrous pyridine (5.0 ml), and
the solution was cooled in an ice bath. (S)-(ϩ)-MTPA-Cl (Aldrich, 98%ee,
27.8 mg, 0.11 mmol) and DMAP (1.2 mg) were added with vigorous stirring
at 0 °C, and the stirring was continued for 24 h at room temperature. The re-
action mixture was concentrated under reduced pressure. Water (10 ml) was
added, and the whole was extracted with benzene (20 ml). The benzene layer
was washed with cold 5% HCl (10 mlϫ2), sat. NaHCO3 (10 ml), and water
(10 mlϫ3). The organic layer was dried over anhydrous Na2SO4 and concen-
trated under reduced pressure. The residue was subjected to column chro-
matography on silica gel (AcOEt) to give a mixture of 25 and 26 (31.0 mg,