Synthesis of (R,R)- and (S,S)-Norphos

Article abstract of DOI:10.1055/s-2008-1032133

Oxidation of (E)-1,2-bis(diphenylphosphanyl)ethene with hydrogen peroxide gave (E)-1,2-bis(diphenylphosphoryl)ethene. Diels-Alder reaction of (E)-1,2-bis(diphenylphosphoryl)ethene with cyclopentadiene yielded 2,3-bis(diphenylphosphoryl)bicyclo[2.2.1]hept-5-ene (NorphosO). rac-NorphosO was resolved with O,O-dibenzoyltartaric acid. (R,R)- and (S,S)-NorphosO were reduced with trichlorosilane to give 2,3-bis(diphenylphosphanyl)bicyclo[2.2.1] hept-5-ene [(R,R)- and (S,S)-Norphos]. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2008-1032133

PAPER
405
Synthesis of (R,R)- and (S,S)-Norphos
Synthesis of
(
R
,
R
)-
e
and(
S
,
S
)-
N
n
orphos ri Brunner,* Manfred Muschiol, Manfred Zabel1
Institut für Anorganische Chemie, Universität Regensburg, Universitätsstr. 31, 93040 Regensburg, Germany
Received 17 September 2007; revised 13 November 2007
Dedicated to Prof. Michael P. Doyle on the occasion of his 65th birthday
O
Abstract: Oxidation of (E)-1,2-bis(diphenylphosphanyl)ethene
PPh2
PPh2
Cl
with hydrogen peroxide gave (E)-1,2-bis(diphenylphosphor-
yl)ethene. Diels–Alder reaction of (E)-1,2-bis(diphenylphosphor-
yl)ethene with cyclopentadiene yielded 2,3-bis(diphenyl-
phosphoryl)bicyclo[2.2.1]hept-5-ene (NorphosO). rac-NorphosO
was resolved with O,O-dibenzoyltartaric acid. (R,R)- and (S,S)-Nor-
phosO were reduced with trichlorosilane to give 2,3-bis(diphen-
ylphosphanyl)bicyclo[2.2.1]hept-5-ene [(R,R)- and (S,S)-Norphos].
H2O2
KPPh2
Ph2P
Ph2P
O
Cl
1
2
Key words: (R,R)- and (S,S)-Norphos, (R,R)- and (S,S)-2,3-
bis(diphenylphosphanyl)bicyclo[2.2.1]hept-5-ene, enantioselective
catalysis, ligand
O
Ph2P
O
PPh2
PPh2
(S,S)-3
(R,R)-3
+
Ph2P
O
O
(R,R)- and (S,S)-Norphos, (R,R)- and (S,S)-2,3-bis(diphe-
nylphosphanyl)bicyclo[2.2.1]hept-5-ene, are among the
optically active bisphosphane ligands most frequently
used in enantioselective catalysis.2 The ligands are com-
mercially available from Strem Chemicals.3 The synthesis
of (R,R)- and (S,S)-Norphos was described in a prelimi-
nary communication4 and in a patent.5 Here, the synthesis
of (R,R)- and (S,S)-Norphos is given in detail with im-
provements compared to the preliminary communication
and patent. Norphos has been hydrogenated at the double
bond of the norbornene skeleton to give 2,3-bis(diphe-
1. (–)-DBTA
2. SiHCl3
1. (+)-DBTA
2. SiHCl3
Ph2P
Ph2P
PPh2
PPh2
(S,S)-4
(R,R)-4
Scheme 1 Synthesis of (R,R)- and (S,S)-Norphos
rac-3, the less soluble diastereomer (S,S)-3/(–)-DBTA
formed polymer chains connected by strong C(O)–O–
H···O=P bonds.17
nylphosphanyl)bicyclo[2.2.1]heptane (re-Norphos).6,7
A
chromatographic resolution of rac-Norphos has been de-
scribed.8
The combination of rac-3, dissolved in chloroform, with
one equivalent of (–)-O,O-dibenzoyltartaric acid, dis-
solved in ethyl acetate, gave a precipitate, in which (R,R)-
3/(–)-DBTA was enriched to ~75–80%. The success of
the resolution, here and in the following steps, was
checked by measuring the optical rotations. Recrystalliza-
tion of the enriched (R,R)-3/(–)-DBTA from methanol af-
forded diastereomerically pure (R,R)-3/(–)-DBTA, from
which (–)-O,O-dibenzoyltartaric acid was removed. At
this stage the enantiomeric purity of (R,R)-3 could be
checked by NMR. On addition of (+)-10-camphorsulfonic
acid the 31P NMR signals of the enantiomers of 3 separat-
ed in deuterochloroform solution. Precipitation of (R,R)-
3/(–)-DBTA from chloroform–ethyl acetate and recrystal-
lization of the enriched (R,R)-3/(–)-DBTA from methanol
(vide infra), was more efficient than repeated precipitation
from ethanol, as described in the preliminary communica-
tion.4
The Norphos synthesis, shown in Scheme 1, starting from
(E)-1,2-bis(diphenylphosphanyl)ethene (1),9 which was
oxidized to (E)-1,2-bis(diphenylphosphoryl)ethene (2)9
because 1 does not function as a dienophile in the Diels–
Alder reaction with cyclopentadiene. For the synthesis of
Norphos, (E)-1,2-bis(diphenylphosphoryl)ethene (2) was
required in large amounts, hence its preparation from the
simple chemicals potassium diphenylphosphide, (E)-1,2-
dichloroethene, and hydrogen peroxide is included here.
The Diels–Alder reaction of 2 and cyclopentadiene was
carried out at 160 °C in toluene in an autoclave. Although
2 was almost insoluble in this solvent, yields of rac-2,3-
bis(diphenylphosphoryl)bicyclo[2.2.1]hept-5-ene10 (rac-
3, rac-NorphosO) of 95% were achieved.
rac-NorphosO (rac-3) was resolved with O,O-dibenzoyl-
tartaric acid (DBTA). Acids have been used several times
for the resolution of phosphane oxides.4,11–16 In the case of
The enantiomerically pure (R,R)-3 was reduced with
trichlorosilane in an autoclave at 100 °C (inert atmo-
sphere). An IR test showed that the PO bands at 1183 and
1116 cm–1 were absent in the product. The resulting crude
SYNTHESIS 2008, No. 3, pp 0405–0408
Advanced online publication: 01.11.2008
DOI: 10.1055/s-2008-1032133; Art ID: Z22307SS
x
x
.
x
x
.2
0
0
8
© Georg Thieme Verlag Stuttgart · New York

406
H. Brunner et al.
PAPER
h. Dioxane and other volatiles were removed and the residue was
extracted with Et2O in a Soxhlet; yield: 111 g (83%).
(R,R)-4 was dissolved in hot ethanol (N2 atmosphere). Ad-
dition of water yielded enantiomerically pure (R,R)-4 in
crystalline form. This purification procedure was superior
to a recrystallization from heptane, which gave higher
yields but did not remove impurities efficiently.
(E)-1,2-Bis(diphenylphosphoryl)ethene (2)
Compound 1 (111 g, 0.295 mol) was dissolved in acetone (1.5 L) in
air and oxidized with 30% H2O2 (133 mL) in H2O (1.2 L), which
was added while stirring in portions of 30–50 mL. The temperature
was maintained between 40–50 °C; stirring at this temperature was
continued for 2–3 h. The soln was concentrated to remove acetone
and cooled in an ice bath for 10 h. The precipitate was filtered,
washed with H2O to remove excess H2O2, and dried; yield: 112 g
(93%).
Enriched material from mother liquors of recrystalliza-
tions at different stages of the synthesis can be collected
and re-introduced into the resolution process.
An X-ray crystal analysis was carried out with crystals of
(S,S)-Norphos [(S,S)-4], which established the S,S-config-
uration. Figure 1 shows an Ortep plot. Interestingly, two
phenyl rings of the vicinal diphenylphosphanyl groups
form a p-stack with a midpoint distance of 3.75 Å (dihe-
dral angle Cipso–Pendo–Pexo–Cipso –32.35°). The phosphorus
atoms, 4.18 Å apart from each other, bind metal atoms in
five-membered chelate rings,18–23 which are stiffened by
the norbornene skeleton. This stiffening protects the che-
late rings from flipping between d- and l-conforma-
tions,18,24 an important point in enantioselective catalysis.
The face-exposed/edge-exposed arrangement of the phe-
nyl rings in these chelate systems leads to unexpected
similarities in the CD spectra of image/mirror-image re-
lated compounds.18,25–27
rac-2,3-Bis(diphenylphosphoryl)bicyclo[2.2.1]hept-5-ene (rac-
3)
The Diels–Alder reaction of 2 (10 g, 0.023 mol) and distilled cyclo-
pentadiene (40 mL) (obtained by cracking dicyclopentadiene in hot
decalin) in anhyd toluene (150 mL) was carried out in an autoclave
(300 mL size) at 160 °C for 2–3 h with stirring. After cooling the
volatiles (toluene, dicyclopentadiene) were removed. Then petro-
leum ether (~1 L) was added in which rac-3 was insoluble. Filtra-
tion and washing with petroleum ether yielded almost quantitatively
rac-3; yield: 11 g (95%); mp >230 °C.
IR (KBr): 1183 (s), 1116 cm–1 (s, P=O).
1H NMR (400 MHz, CDCl3): d = 7.85–6.95 (m, 20 H, HPh), 6.28 (m,
1 H, =CH), 5.75 (m, 1 H, =CH), 3.80–3.70 [m, 1 H, CH(PPh2)],
3.28–3.20 [m, 1 H, CH(PPh2)], 2.98 (m, 1 H, CH), 2.90–2.85 (m, 1
H, CH), 2.20–2.16 (m, 1 H, CH), 1.28–1.20 (m, 1 H, CH).
1
13C NMR (100.6 MHz, CDCl3): d = 38.5 (d, JP,C = 75.0 Hz, C3),
1
2
39.2 (dd, JP,C = 66.4 Hz, JP,C = 1.3 Hz, C2), 46.9 (d,
2JP,C = 4.0 Hz, C4), 47.2 (dd, JP,C = 2.2 Hz, JP,C = 0.9 Hz, C1),
48.1 (d, JP,C = 11.2 Hz, C7), 128.09 (d, JP,C = 11.7 Hz, m-CPh),
128.10 (d, JP,C = 11.7 Hz, m-CPh), 128.3 (d, JP,C = 11.7 Hz, m-
2
3
3
3
3
3
3
2
CPh), 128.6 (d, JP,C = 11.2 Hz, m-CPh), 130.1 (d, JP,C = 8.5 Hz, o-
CPh), 130.53 (d, 2JP,C = 9.0 Hz, o-CPh), 130.55 (d, 2JP,C = 8.5 Hz, o-
2
4
CPh), 130.6 (d, JP,C = 8.5 Hz, o-CPh), 130.7 (d, JP,C = 2.7 Hz, p-
CPh), 131.0 (d, JP,C = 3.1 Hz, p-CPh), 131.2 (d, 4JP,C = 2.7 Hz, 2 p-
4
CPh), 130.7 (d, 4JP,C = 2.7 Hz, p-CPh), 132.2 (d, 2JP,C = 96.0 Hz, Cq of
Ph), 133.7 (d, 1JP,C = 97.40 Hz, Cq of Ph), 132.2 (d, 1JP,C = 97.8 Hz,
1
Cq of Ph), 134.6 (d, JP,C = 97.4 Hz, Cq of Ph), 135.3 (d,
3JP,C = 4.5 Hz, C6), 137.9 (d, 3JP,C = 12.1 Hz, C5).
31P NMR (162 MHz, CDCl3): d = 32.5, 29.3 (d, 3JPP = 9.5 Hz).
MS (EI, 70 eV): m/z (%) = 494.1 (13, M), 429.0 (8), 325.1 (3),
294.1 (18), 293.1 (100), 279.1 (3), 227.1 (14), 202.1 (7), 201.1 (36),
183.0 (9).
Figure 1 Ortep view of the structure of (S,S)-Norphos [(S,S)-4].
Ellipsoids are drawn at the 50% probability level.28,29
Anal. Calcd for C31H28O2P2 (494.5): C, 75.30; H, 5.60. Found: C,
75.25; H, 5.85.
Melting points (not corrected): Büchi SMP 20. IR: Beckman IR
4240. 1H/13C/31P NMR: Bruker Avance 400 (400/100.6/162 MHz,
T = 27 °C), TMS as internal standard and H3PO4 as external stan-
dard. MS: Finnigan MAT 95 (EI, 70 eV). Optical rotation: Perkin-
Elmer Polarimeter 241. X-ray analysis: STOE-IPDS.
Resolution of rac-3 with O,O-Dibenzoyltartaric Acid
Enrichment of (R,R)-3/(–)-DBTA and (S,S)-3
Two solns were made: rac-3 (50 g, 0.10 mol) was dissolved in
CHCl3 (650 mL) and (–)-O,O-dibenzoyltartaric acid monohydrate
[(–)-DBTA, 38 g, 0.10 mol] was dissolved in EtOAc (1.7 L). Both
solns were combined at r.t. and stirred; precipitation began after 5–
10 min. The mixture was stirred for 1 h, the slightly brown precipi-
tate was filtered and washed with petroleum ether (bp 40–60 °C);
yield: 40 g. This contained (R,R)-3/(–)-DBTA enriched to ~75–
80%; [a]D –77 to –79.5 (c 1, 99% EtOH).
(E)-1,2-Bis(diphenylphosphanyl)ethene (1)
KPPh2 soln was formed as follows: under N2, K (50 g, 1.28 mol)
was added to anhyd dioxane (1.3 L) (pretreated with KOH, refluxed
over K, and subsequently distilled under N2). Then Ph2PCl (125
mL, 0.7 mol) was added through a dropping funnel over 2–3 h. Sim-
ilar to Grignard reactions large amounts of Ph2PCl should only be
added after the reaction had started. The mixture was stirred under
reflux for 10 h.
The mother liquor and the petroleum ether washings of the precipi-
tate were combined; evaporation of the volatiles gave a residue en-
riched in (S,S)-3/(–)-DBTA to ~80% (48 g). The residue was
dissolved in CHCl3 (400 mL) and shaken with aq KOH [300 mL,
containing KOH (15 g)] to remove (–)-DBTA. The CHCl3 soln was
washed with aq KOH [200 mL, containing KOH (5 g)] and then
with H2O (200 mL). The soln was dried (Na2SO4) and thus the
After cooling, (E)-1,2-dichloroethene (68 mL, 0.88 mol) was added
to the KPPh2 soln through a dropping funnel over 1–2 h keeping the
temperature between 0 and 5 °C. Stirring was continued at r.t. for 10
Synthesis 2008, No. 3, 405–408 © Thieme Stuttgart · New York

PAPER
Synthesis of (R,R)- and (S,S)-Norphos
407
cloudy CHCl3 soln became clear. Evaporation of the solvent and
drying gave 3 enriched with (S,S)-3; yield: 27.5 g (99%). This sam-
ple was saved for resolution with (+)-O,O-dibenzoyltartaric acid.
troleum ether (bp 40–60 °C), and drying gave crystallized (S,S)- or
(R,R)-4; yield: 3.8 g (75%); mp 120 °C.
(S,S)-4: [a]D +47 to +48 (c 1, CHCl3).
(R,R)-4: [a]D –46 to –47 (c 1, CHCl3).
Diastereomerically Pure (R,R)-3/(–)-DBTA
1H NMR (400 MHz, CDCl3): d = 7.58–7.10 (m, 20 H, HPh), 6.29 (m,
1 H, =CH), 6.08 (m, 1 H, =CH), 2.96–2.88 [m, 1 H, CH(PPh2)], 2.79
(m, 2 H, CH), 2.29–2.25 [m, 1 H, CH(PPh2)], 1.10–1.08 (m, 1 H,
CH2), 0.94–0.92 (m, 1 H, CH2).
Enriched (R,R)-3/(–)-DBTA (40 g) {[a]D –77 to –79.5 (c 1, 99%
EtOH)} was dissolved in hot MeOH (350 mL); if the resulting soln
was cloudy, it was filtered. Cooling to r.t. and then cooling in an ice
bath for 15 min gave diastereomerically pure (R,R)-3/(–)-DBTA,
which was washed with petroleum ether (bp 40–60 °C) and dried;
yield 26 g; [a]D –88 (c 1, 99% EtOH).
31P NMR (162 MHz, CDCl3): d = 0.1, –2.0 (3JPP = 2.9 Hz).
MS (EI, 70 eV): m/z (%) = 462.3 (2, M), 396.2 (5), 319.1 (1), 287.2
(4), 278.2 (21), 277.2 (100), 262.2 (8), 231.3 (3), 186.1 (3), 185.1
(18), 184.1 (9), 183.1 (66).
Enantiomerically Pure (R,R)-3
Diastereomerically pure (R,R)-3/(–)-DBTA (26 g) {[a]D –88 (c 1,
99% EtOH)} was dissolved in CHCl3 (200 mL) and shaken with aq
KOH [150 mL, containing KOH (9 g)] to remove (–)-DBTA . The
CHCl3 soln was washed with aq KOH [100 mL, containing KOH (3
g)], with H2O (100 mL), and dried (Na2SO4). Evaporation of the sol-
vent gave of enantiomerically pure (R,R)-3; yield: 14.5 g (99%);
[a]D –59 (c 1, CHCl3).
Anal. Calcd for C31H28P2 (462.5): C, 80.50; H, 6.06. Found: C,
80.52; H, 6.30.
References
(1) X-ray crystal structure analysis.
The enantiomeric purity of (R,R)-3 [or (S,S)-3] can be checked by
31P NMR. In a soln of rac-3 (5 mg) and D-(+)-10-camphorsulfonic
acid (13 mg) in CDCl3 (0.8 mL), the 31P NMR signals of the enan-
tiomers of 3 separate.
(2) Brunner, H.; Zettlmeier, W. Handbook of Enantioselective
Catalysis with Transition Metal Compounds; VCH:
Weinheim, 1993.
(3) Strem Chemicals Catalog, 2006, 188.
(4) Brunner, H.; Pieronzcyk, W. Angew. Chem., Int. Ed. Engl.
1979, 18, 620; Angew. Chem. 1979, 91, 655.
(5) Brunner, H.; Pieronzcyk, W. DE 2908358, 1980; Chem.
Abstr. 1981, 94, 103556f.
Enrichment of (S,S)-3/(+)-DBTA
Two solns were made: enriched (S,S)-3 (27.5 g, 0.56 mol) was dis-
solved in CHCl3 (400 mL) and (+)-O,O-dibenzoyltartaric acid
monohydrate [(+)-DBTA, 21 g, 0.055 mol] was dissolved in EtOAc
(940 mL). Both solns were combined at r.t. and stirred; precipitation
began immediately. The mixture was stirred for 1 h, the precipitate
was filtered and washed with petroleum ether (bp 40–60 °C); yield:
33 g; [a]D +80 to +81.5 (c 1, 99% EtOH).
(6) Kyba, E. P.; Davis, R. E.; Juri, P. N.; Shirley, K. R. Inorg.
Chem. 1981, 20, 3616.
(7) Brunner, H.; Vitulli, G.; Porzio, W.; Zocchi, M. Inorg. Chim.
Acta 1985, 96, 67.
(8) Benincori, T.; Cirilli, R.; Ferretti, R.; La Torre, F.; Piccolo,
O.; Zanitti, L. Chromatographia 2002, 1/2, 25.
(9) Aguiar, A. M.; Daigle, D. J. Am. Chem. Soc. 1964, 86, 2299.
(10) Nesterova, N. P.; Medved, T. Y.; Polikarpov, Y. M.;
Kabachnik, M. I. Izv. Akad. Nauk SSSR, Ser. Khim. 1974, 10,
2210.
(11) Meisenheimer, J.; Lichtenstadt, L. Chem. Ber. 1911, 44, 356.
(12) Meisenheimer, J.; Casper, J.; Höring, M.; Lauter, W.;
Lichtenstadt, L.; Samuel, W. Justus Liebigs Ann. Chem.
1926, 449, 213.
(13) Kreuzfeld, H. J.; Doebler, C. Z. Chem. 1984, 24, 405.
(14) Minami, T.; Okada, Y.; Nomura, R.; Hirota, S.; Nagahara,
Y.; Fukujama, K. Chem. Lett. 1986, 613.
(15) Takaya, H.; Mashima, K.; Koyano, K.; Yagi, M.;
Kumobayashi, H.; Taketomi, T.; Akutagawa, S.; Noyori, R.
J. Org. Chem. 1986, 51, 629.
Diastereomerically Pure (S,S)-3/(+)-DBTA
Enriched (S,S)-3/(+)-DBTA (33 g) {[a]D +80 to +81.5 (c 1, 99%
EtOH)} was dissolved in hot MeOH (300 mL). Cooling to r.t. and
then cooling in an ice bath for 15 min gave diastereomerically pure
(S,S)-3/(+)-DBTA, which was washed with petroleum ether (bp 40–
60 °C) and dried; yield: 26 g; [a]D +89 (c 1, 99% EtOH).
Enantiomerically Pure (S,S)-3
Diastereomerically pure (S,S)-3/(+)-DBTA (26 g) {[a]D +89 (c 1,
99% EtOH)} was dissolved in CHCl3 (200 mL) and shaken with aq
KOH [150 mL, containing KOH (9 g)] to remove (+)-DBTA . The
CHCl3 soln was washed with aq KOH [100 mL, containing KOH (3
g)] and H2O (100 mL) and dried (Na2SO4). Evaporation of the sol-
vent gave enantiomerically pure (S,S)-3; yield: 16 g (99%); [a]D +60
(c 1, CHCl3).
(16) Brunner, H.; Zettlmeier, W. Bull. Soc. Chim. Belg. 1991,
100, 247.
(17) Brunner, H.; Pieronzcyk, W.; Schönhammer, B.; Streng, K.;
Bernal, I.; Korp, J. Chem. Ber. 1981, 114, 1137.
(18) Brunner, H.; Rahman, A. F. M. M. J. Organomet. Chem.
1981, 214, 373.
(S,S)- and (R,R)-2,3-Bis(diphenylphosphanyl)bicy-
clo[2.2.1]hept-5-ene [(+)- and (–)-Norphos, (S,S)- and (R,R)-4]
Under N2, (S,S)- or (R,R)-3 (9 g, 0.018 mol), SiHCl3 (15 g, 0.11
mol), and anhyd toluene (150 mL) were placed in a 300-mL auto-
clave. The autoclave was heated to 100 °C. After cooling the vola-
tiles were removed including excess SiHCl3. The residue was
dissolved in toluene (50 mL). At 0 °C a soln of NaOH (25 g) in N2-
saturated H2O (75 mL) was added in portions. After shaking the tol-
uene phase was passed through basic alumina (3–4 cm). The extrac-
tion process was repeated. Removal of the toluene gave a residue
which was suspended in petroleum ether (bp 40–60 °C) (30 mL),
filtered, washed with petroleum ether, and dried to give crude (S,S)-
or (R,R)-4 (5.5 g, 65%).
(19) Brunner, H.; Rahman, A. F. M. M.; Bernal, I. Inorg. Chim.
Acta 1984, 83, L93.
(20) Brunner, H.; Eder, R. Z. Naturforsch., B: Chem. Sci. 1990,
456, 579.
(21) Nishiyama, H.; Brunner, H.; Jones, P. G. J. Organomet.
Chem. 1991, 405, 247.
(22) Brunner, H.; Grau, I.; Zabel, M. Organometallics 2004, 23,
3788 (5652).
(23) Brunner, H.; Muschiol, M.; Tsuno, T.; Zabel, M.
Organometallics, submitted for publication.
(24) Brunner, H.; Winter, A.; Breu, J. J. Organomet. Chem. 1998,
553, 285.
Under N2, (S,S)- or (R,R)-4 (5.5 g) was dissolved in hot 99% EtOH
(310 mL), then H2O (50 mL) was added. After cooling to r.t. and
then cooling in an ice bath for 15 min, filtration, washing with pe-
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H. Brunner et al.
PAPER
(25) Consiglio, G.; Morandini, F.; Ciani, G.; Sironi, A.;
Kretschmer, M. J. Am. Chem. Soc. 1983, 105, 1391.
(26) Morandini, F.; Consiglio, G.; Straub, B.; Ciani, G.; Sironi,
A. J. Chem. Soc., Dalton Trans. 1983, 2293.
(27) Morandini, F.; Consiglio, G.; Lucchini, V. Organometallics
1985, 4, 1202.
(29) Crystallographic data (excluding structure factors) for the
structure of (+)-(S,S)-Norphos have been deposited with the
Cambridge Crystallographic Data Centre. Copies of the data
can be obtained, free of charge, on application to CCDC, 12
Union Road, Cambridge CB2, UK [fax: +44 (1223)336033
or e-mail: deposit@ccdc.cam.ac.uk].
(28) The CCDC deposition number for (+)-(S,S)-Norphos is
659276. Unit cell parameters: a = 5.91520 (10), b = 14.2274
(2), c = 28.3338 (4), space group P212121.
Synthesis 2008, No. 3, 405–408 © Thieme Stuttgart · New York