Enantioselective synthesis of N-Boc-1-naphthylglycine

Article abstract of DOI:10.1016/S0957-4166(97)00137-7

A new stereoselective synthesis of enantiomerically pure 1-naphthylglycine has been developed. The source of chirality is the catalytic Sharpless epoxidation. Regioselective and stereospecific ring-opening of the corresponding epoxy alcohol is performed e

Full text of DOI:10.1016/S0957-4166(97)00137-7

Tetrahedmn: Asymmetry, Vol 8, No. 10, pp. 1581-1586, 1997
Q
1997 Elsevier Science Ltd. All rights reserved.
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Enantioselective synthesis of N-Boc-l-naphthylglycine
Eva Medina, A n t o n VidaI-Ferran, Albert Moyano, Miquel A. Perichs* * and Antoni Riera*
Unitat de Recerca en Sfntesi Asim~trica (URSA), Departament de Qufmica Org?anica, Universitat de
Barcelona, c/Martf i Franqu~s, 1-11, 08028 Barcelona, Spain
Abstract: A new stereoselective synthesis of enantiomerically pure l-naphthylglycine
has been developed. The source of chirality is the catalytic Sharpless epoxidation.
Regioselective and stereospecific ring-opening of the corresponding epoxy alcohol is
performed either with sodium azide or with benzhydrylamine as ammonia synthetic
equivalents. Subsequent hydrogenation in the presence of (Boc)20 affords crystalline N-
Boc-3-(l-naphthyl)propane-l,2-diol which is enantiomerically enriched up to 100% ee
and oxidized to the cx-amino acid. (~) 1997 Elsevier Science Ltd
1-Naphthylglycine is a representative example of the arylglycines, an important class of non-
proteinogenic amino acids.1 Arylglycines are present in many biologically active compounds such as
cephalosporins2 or nocardicins.3 Moreover, they have potential interest as chiral building blocks or as
precursors of chiral ligands for asymmetric synthesis. To the best of our knowledge, all syntheses of
naphthylglycine described so far are either not stereoselective4 or based on a chirai auxiliary approach
in which the chiral inductor can not be recovered.5 We report here a practical catalytic enantioselective
synthesis of N-Boc- 1-naphthylglycine using a modification of our methodology based on the catalytic
Sharpless epoxidation. 6
The key intermediates in our approach to the synthesis of cx-amino acids I are N-Boc-3-amino-
1,2-diols II. These versatile chiral synthons can be prepared from epoxy alcohols III through a
regio and stereospecific ring-opening by an appropriate ammonia equivalent (Scheme 1) and we have
demonstrated their usefulness in the preparation of several biologically active compounds.7 There
are two interesting properties of these intermediates that are worth mentioning: a) they are usually
crystalline compounds thus allowing the possibility of enantioenrichment and b) the amino group has
a widely used protecting group whose presence can be also useful in the final product.
NHBoc
,O
R ~ O H
oL oo
OH
OH
II
III
Scheme 1.
In the present synthesis of N-Boc-l-naphthylglycine the starting material was the known8 allyl
alcohol 1 which was prepared in good yield from commercial aldehyde 2 according to the conventional
procedure of Wittig olefination followed by DIBAL-H reduction. The catalytic Sharpless asymmetric
epoxidation9 of allyl alcohol 1 afforded the corresponding epoxy alcohol 3 in 73% yield and with an
enantiomeric excess of 86%, as determined by HPLC (Scheme 2).
Complete regioselective and stereospecific ring-opening of epoxy alcohol 3 was achieved by
treatment with sodium azide under Crotti's conditions, 1° the intermediate 3-azido-l,2-diol being
immediately reduced and in situ protected to give N-Boc-3-amino-3-naphthyl- 1,2-propanediol 4 with
* Corresponding author. Email: are@ursaqo.ub.es and mpb@ursa.qo.ub.es
1581

1582
E. MEDINA et al.
CHO
TBHP, CHzCI2,4Asieves
~ . ~ ./<~?~.O-
L-(+)-DIP (cat), Ti(OiPr)4(cat) ' ~ 5 OH
73%,86%ee
{
~
1) Ph3P=CHCOOEt
2) DIBAL-H, ether
OH
v
2
75%
1
3
Scheme 2.
an 80% overall yield. An alternative procedure that avoids the preparation of organic azides (more
suited for large scale work) involves the use of benzhydrylamine (diphenylmethylamine) as the -NH2
source. I1 The ring-opening of epoxide 3 mediated by titanium tetraisopropoxide afforded the desired
3-benzhydrylamino-3-naphthyi-1,2-propanediol 5 in 60% yield after removal of the C-2 ring-opening
product (10%). Conversion of 5 into the corresponding N-Boc derivative 4 was conveniently performed
by catalytic hydrogenation (Pearlman's catalyst) in the presence of (Boc)20 in methanol (Scheme 3).
, , ~ . ~ .,~~,~?..~., "~G~ ~
[ 80% overall yield (Two steps) 1 "'~¢O~'~"
~
~.HBOAC
OH
, - U -H-OH
3
" ?
~
O /
~
a -
~ ' ~ ...~HBL
~(~O~z~,.G
4
°0%
,-%E
°"
, , %
Scheme 3.
was, according to our expectations, a nicely crys-
N-Boc-3-amino-3-naphthyl-1,2-propanediol
4
talline compound which, on crystallization from chloroform, underwent enantiomeric enrichment
as evidenced by polarimetry. Oxidation of 4 was initially performed with RuCI3/NalO4 in
CH3CN/CCI4/HzO 12 but, somewhat surprisingly, very low yields of N-Boc-naphthylglycine were ob-
tained. This problem could be overcome by changing the oxidation conditions. When compound 4 was
submitted to oxidation with KMnO4/NalO4/Na2CO3 in dioxane/water13 N-Boc-D-naphthylglycine
was obtained in good yield. In order to check its enantiomeric excess the oxidation crude was converted
into the corresponding methyl ester which was enantiomerically pure according to HPLC analysis on
a chiral stationary phase (Chiralcel® ODR) (Scheme 4).
~ ' ~ .~HBoc
HcB°C
KMnO4, NalO4.
CH31, KHCO3
OH
OOH
"
I
~
COOMe
Na2CO3, CCI4
DMF
dioxane, water
7
6
4
73%(two steps), >99 %ee
68%
Scheme 4.

Synthesis of N-Boc- I-naphthylglycine
1583
In summary we have developed an efficient procedure for the preparation of enantiopure N-Boc-
naphthylglycine, an important non proteinogenic amino acid. The source of chirality in our approach
is a catalytic Sharpless epoxidation. Although the epoxy alcohol 3 was obtained in 85% ee, the
crystallinity of the N-Boc-3-naphthyl- 1,2-propanediol intermediate allowed enantioenrichment so the
final product could be obtained in enantiomerically pure form. Due to the broad scope of the reactions
involved, the present methodology should be of general applicability in the synthesis of arylglycines.
The use of naphthylglycine as a precursor of chiral ligands for asymmetric synthesis is under study
in our laboratories and will be described in due course.
Experimental section
General methods
Optical rotations were measured at room temperature (23°C) on a Perkin-Elmer 241 MC automatic
polarimeter (Concentration in g/100 mL). Melting points were determined on a Gallenkamp apparatus
and have not been corrected. Infrared spectra were recorded on a Perkin-Elmer 681, or on a
Nicolet 510 FT-IR instrument using NaC1 film or KBr pellet techniques. NMR spectra were acquired
on Varian XL-200 or Varian-Unity-300 instruments. I H-NMR were obtained at 200 or 300 MHz
(s=singlet, d=doublet, t=triplet, q=quartet, dt=double triplet, m=multiplet, b=broad and bd=broad
doublet). 13C-NMR were obtained at 50.3 MHz or 75.4 MHz. Carbon multiplicities have been
assigned by distortionless enhancement by polarization transfer (DEPT) experiments. Mass spectra
were recorded on a Hewlett-Packard 5890 instrument. Elemental analyses were performed by the
"Servei d'Anhlisis Elementals del CSIC de Barcelona". Chromatographic separations were carried out
using NEt3 pre-treated (2.5% v/v) SiO2 (70-230 mesh). Chromatographic analyses were performed
on a Helwett-Packard 1050 HPLC instrument equipped with Chiralcel® ODR (25 cm) column.
(E)-3-(1-Naphthyl)-2-propen-l-ol,
1
To a solution of ethyl triphenylphosphoranylacetate (32.75 g, 94 mmol) in dichloromethane (100
mL) l-naphthaldehyde (13.28 g, 85 mmol) in dichloromethane (50 mL) was added. The mixture was
stirred at reflux for 90 minutes. The solvent was removed in vacuo and the residual oil was extracted
with hexanes (10×25 mL). The hexane solution was concentrated yielding 25.1 g of ethyl 3-(1-
naphthyl)acrylate (slightly impurified with triphenylphosphine oxide) as an oil. This oil was solved
in diethyl ether (100 mL) and cooled to 0°C. To this solution, 1 M DIBALH (170 mL, 170 mmol) in
hexanes was added. After 2 hours of stirring at room temperature, the reaction mixture was diluted
with diethyl ether (200 mL), cooled to 0°C and quenched with a careful addition of brine (200 mL).
Then, 4 M HCI (200 mL) was added dropwise. The aqueous layer was extracted with diethyl ether
(3× 150 mL) and the combined organic phases were washed with brine, dried (sodium sulfate) and
evaporated. The residue was chromatographed eluting with hexane/ethyl acetate mixtures yielding
117 g of 1 (75%) as an oil that was distilled before its use in the Sharpless epoxidation (172-175;
lit. 209-210°C/18 torr8). IR (film) v 3334, 3047, 2863, 1653 cm -1. 1H-NMR (200 MHz, CDCI3) 8
2.05 (broad s, 2H), 4.39 (dd, J=l.4, 5.4 Hz, 2H), 6.35 (dt, J=5.4, 15.8 Hz, IH), 7.21-8.12 (m, 8H)
ppm. 13C-NMR (50 MHz, CDC13) 8 63.7 (CH2), 123.7 (CH), 123.8 (CH), 125.5 (CH), 125.7 (CH),
126.0 (CH), 127.9 (CH), 128.0 (CH), 128.4 (CH), 131.0 (CH), 131.7 (C), 133.5 (C), 134.3 (C) ppm.
MS (El) m/e 218 (M÷, 43%), 173 (100%).
(2S,3S )-3-(1-Naphthyl)-2,3-epoxy propanol. 3
Into a 500 mL flask were introduced dry powdered 4 ,~ molecular sieves (1.35 g) and anhydrous
dichloromethane (180 mL) under nitrogen. After cooling to - 2 0 ° C (CO2/CC14 bath) the following
reagents were introduced sequentially via cannula under stirring: L-(+)-diisopropyltartrate (0.44 g,
1.89 mmol) in dichloromethane (20 mL), titanium tetraisopropoxide (0.38 mL, 1.29 mmol) and 5.4 M
solution of ten-butyl hydroperoxide in isooctane (9.3 mL, 50.2 mmol). The mixture was stirred 1 hour
at - 2 0 ° C and a solution of (E)-3-naphthyl-2-propen-l-ol 1 (6.02 g, 32.66 mmol) (previously distilled

1584
E. MEDINA et al.
and stored 24 h with 4 ,~ molecular sieves) in dichioromethane ( 10 mL) was added. After 8 h of stirring
at the same temperature the reaction was quenched by addition of 10% NaOH solution saturated with
NaCI (2.2 mL) and diethyl ether (27 mL). The mixture was allowed to warm to 10°C, and anhydrous
MgSO4 (2.61 g) and Celite® (0.33 g) were added. After 15 min stirring at room temperature the
mixture was filtered through a short Celite® pad. The solvents were evaporated in vacuo and the
excess of ten-butyl hydroperoxide removed by azeotropic distillation with toluene (3x 125 mL). The
crude was then chromatographed eluting with hexanes/ethyl acetate mixtures yielding 3.70 g of 3
(73% yield). The enantiomeric excess was determined to be 86% ee by HPLC (Chiralcel® ODR, 0.5
mL/min NaC104 aq. 0.5 M/MeOH 1/9; tR (2R,3R) 13.1 min; tl~ (2S,3S) 19.0 min). [et]D +50.3 (c 1,
CHC13). IR (film) v 3417, 3058, 2925, 2867, 1598, 1510, 1452, 1084 cm -1. IH-NMR (300 MHz,
CDC13) ~ 2.24 (broad s, 1H), 3.21 (m, 1H,), 3.97 (ddd, J=13.5, 5.7, 3.6 Hz, 1H), 4.15 (ddd, J=13.5,
1.0, 1.0 Hz, IH), 4.57 (d, J=2.4 Hz, 1H), 7.44-8.09 (m, 7H) ppm. 13C-NMR (75 MHz, CDC13) 6 53.8
(CH), 61.6 (CH) 61.6 (CH2), 122.2 (CH), 122.7 (CH), 125.4 (CH), 125.9 (CH), 126.4 (CH), 128.2
(CH), 128.7 (CH), 131.1 (C), 132.7 (C), 133.2 (C) ppm. MS (CI-NH3) m/e: 201 (M+I +, 1%), 218
(M+I8+, 100%).
(2R, 3R)-3-Diphenylmethylam ino-3-( l-naphthyl)- 1,2-propanediol, 5
To a solution of (2S,3S)-3-(l-naphthyl)-2,3-epoxypropanol 3 (0.72 g, 3.6 mmol) in dichloromethane
(30 mL) were added via cannula benzhydrilamine (0.95 mL, 5.43 mmol) in dichloromethane (7 mL)
and titanium tetraisopropoxide (1.65 mL, 5.43 mmol) in dichloromethane (7 mL). After 3 hours of
stirring at room temperature the reaction was quenched with 10% NaOH solution saturated with
NaC1. The mixture was stirred 15 hours, filtered through a short Celite® pad and washed throughly
with dichloromethane. The aqueous layer was extracted with dichloromethane (3x20 mL) and the
combined organic phases were dried (MgSO4) and evaporated. The crude was chromatographed
eluting with hexanes/ethyl acetate mixtures to afford 0.82 g of 5 (60%) and 0.14 g of (2R,3S)-2-
diphenylmethylamino-3-(1-naphthyl)- 1,3-propanediol (10.5%). 5 m.p. 55-57°C. [a]D -- 11.2 (c 1,
CHCI3). IR (KBr) v 3409, 3060, 2950, 1597, 1510, 1493, 1028 cm -1. 1H-NMR (200 MHz, CDC13)
~i 2.89 (broad, 3H), 3.4-3.7 (m, 2H), 3.9 (b, IH), 4.6 (b, IH), 4.65 (s, 1H), 7.1-8.1 (m, 7H) ppm.
13C-NMR (50 MHz, CDCI3) ~i 58.0 (CH), 63.7 (CH), 64.6 (CH2), 73.1 (CH), 122.7 (CH), 125.5
(CH), 125.7 (CH), 126.1 (CH), 126.9 (CH), 127,1 (CH), 127.3 (CH), 127.8 (CH), 128.1 (CH), 128.5
(CH), 128.6 (CH), 128.8 (CH), 142.1 (C), 144.0 (C) ppm. MS (CI-NH3) m/e: 384 (M+I +, 100%),
322 ( M - 6 1 +, 1%). Anal. Calcd for C26H2502N: C, 81.43; H 6.57; N, 3.65. Found: C, 81.43; H, 6.59;
N, 3.45.
( 2R,3R)-3-(tert-Butoxycarbonylamino)-3-(1-naphthyl)- l,2-propanediol, 4
Method A (from 5)
A suspension of (2R,3R)-3-diphenylmethylamino-3-(l-naphthyl)-1,2 propanediol 5 (0.25 g, 0.652
mmol), di-tert-butyl dicarbonate (0.192 g, 0.848 mmol) and Pd(OH)2/C (15%, 37 mg) in MeOH (0.8
mL) (two drops of ethyl acetate were added to increase the solubility of 5) was hydrogenated at room
temperature and atmospheric pressure until no starting material could be observed by TLC (approx.
2 days). The reaction mixture was filtered and the solvents evaporated in vacuo. The residue was
chromatographed eluting with hexanes/ethyi acetate mixtures yielding 0.18 g of 4 (87%).
Method B (from 3)
To a solution of (2S,3S)-3-(l-naphthyl)-2,3-epoxypropanol 3 (1 g, 5 mmol) in CH3CN (25 mL),
were added 13.1 g of lithium perchlorate. After 10 minutes stirring, sodium azide (1.62 g, 24.8 mmol)
was added and the mixture was heated at 65°C for 24 hours under nitrogen. The reaction was treated
with water (300 mL) and extracted with diethyl ether (3x 190 mL). The combined organic phases were
dried (MgSO4) and evaporated. The residue, composed by 1.2 g of (2R,3R)-3-azido-3-(1-naphthyl)- 1,2-
propanediol was immediately solved in ethyl acetate (11 mL). This solution was added to a suspension

1585
Synthesis of N-Boc-1-naphthylglycine
of di-tert-butyl dicarbonate (1.55 g, 6.9 mmol) and Pd/C (10%, 0.121 g) in ethyl acetate (2 mL) and
hydrogenated at atmospheric pressure until no starting material could be observed by TLC (approx.
24 hours). The reaction mixture was filtered and the solvents evaporated in vacuo. The residue was
chromatographed eluting with hexanes/ethyl acetate mixtures yielding 1.27 g of 4 (80% yield) as a
white solid that was recrystallized from diethyl ether, m.p. 108-109°C. [ot]D --16.0 (c 1, CHCI3). IR
(KBr) v: 3401, 2977, 1686, 1507, 1393, 1368, 1067 cm -1. IH-NMR (200 MHz, CDC13) 6 1.44 (s,
9H), 2.6 (d, J=8 Hz, 1H), 3.4-4.2 (m, 4H), 5.15 (bd, IH), 5.6 (t, J=8.5 Hz, 1H), 7.2-8.1 (m, 7H) ppm.
13C-NMR (50 MHz, CDCI3) ~i 28.8 (CH3), 52.4 (CH), 63.5 (CH2), 73.3 (CH), 81.1 (C), 123.4 (CH),
124.4 (CH), 125.8 (CH), 126.3 (CH), 127,1 (CH), 129.1 (CH), 129.4 (CH), 131.9 (C), 134.1 (C), 136
(C), 157.9 (C) ppm. MS (CI-NH3) m/e: 318 (M+l +, 32%), 335 (M+18+, 100%), 352 (M+35+, 3%).
Anal. Calcd for: (C18H2304N): C, 68.11; H, 7.30; N, 4.41. Found: C, 68.04; H, 7.39; N, 4.34.
N-Boc-D-1-Naphthylglycine, 6
A mixture of (2R,3R)-3-(tert-butoxycarbonylamino)-3-(l-naphthyl)-1,2-propanediol 4 (0.45 g, 1.41
mmol), Na2CO3 (75 mg, 0.7 mmol), NalO4 (1.2 g, 5.65 mmol), KMnO4 (45 mg, 0.28 mmol) in
dioxane (3 mL) and water (!.3 mL) was vigorously stirred at room temperature until no starting
material could be observed by TLC (approx. 10 hours). Ethyl acetate (30 mL) was added and the
mixture was acidified by addition of 2 M HC1. The organic phase was washed with brine, dried and
evaporated yielding 0.4 g of an oil that was disgregated in hexane (95%). The crude can be purified by
chromatography (SiO2, CH2CI2/methanol 0-1%) yielding 0.27 g (64% yield) of pure 6. An alternative
non-chromatographic procedure can also be used. The crude was solved in NaHCO3 solution and the
aqueous phase was washed with ethyl acetate, acidified and extracted again with ethyl acetate. The
organic phase was then dried and evaporated yielding a solid that was recrystallized from hexane/ether
to yield 0.29 g (68% yield) of 6. m.p. 182-183° [et]D -147.7 (c 1, MeOH). IR (KBr) v 3299, 2979,
1723, 1686, 1396, 1369, 1163 cm -l. IH-NMR (300 MHz, CDC13/MeOD, 55°C) 8 1.6 (s, 9H), 4.7
(b, 2H), 6.1 (s, 1H), 7.2-8.1 (m, 7H) ppm. 13C-NMR (75 MHz, CDCI3/MeOD, 55°C) 6 28.8 (CH3),
56.1 (CH), 81.0 (C), 124.5 (CH), 126.3 (CH), 126.5 (CH), 126.9 (CH), 127.6 (CH), 128.6 (CH), 130.0
(CH), 132.6, (C), 134.7 (C), 135.5 (C), 174.6 (C), ppm. MS (CI-NH3) re~e: 302 (M+I ÷, 3%), 319
(M+I8+, 100%).
N-Boc-D-1-Naphthylglycine methyl ester, 7
Compound 4 (0.12 g, 0.37 mmol) was oxidized under the previously described conditions. The
crude reaction was solved in dimethylformamide (0.77 mL) and KHCO3 (93 mg, 0.93 mmol) was
added. The flask was purged with nitrogen and methyl iodide (0.062 mL, 0.74 mmol) was added via
syringe. The mixture was stirred 16 h at room temperature and quenched by addition of water (2 mL).
The product was extracted with 1:1 benzene/ethyl acetate mixtures (3 ×3mL). The combined organic
phases were washed with water, 5% Na2SO3 and brine, dried (MgSO4) and evaporated. The crude
was chromatographed eluting with hexane/ethyl acetate mixtures yielding 87 mg of 7 (73% from 4,
two steps). Starting from crystalline 4 only the R enantiomer could be detected by HPLC analysis
(>99% ee) (Chiralcel® ODR, NaCIO4 aq. 0.5 M/MeOH 25/75) (tR (S)-7, 23.34 min, tR(R)-7, 26.26
min). [0t]D --144 (c 1, CHC13). IR (KBr) v: 3378, 3060, 2979, 1750, 1713, 1497, 1165 cm -1. 1H-
NMR (300 MHz, CDCi3) ~ 1.4 (s, 9H), 3.7 (s, 3H), 5.5 (b, IH), 6.1 (d, J=9 Hz, 1H), 7.2-8.1 (m, 7H)
ppm. 13C-NMR (50 MHz, CDC13) ~ 28.2 (CH3), 52.6 (CH3), 54.6 (CH), 80.3 (C), 123.2 (CH), 125.2
(CH), 125.3 (CH), 125.9 (CH), 126.8 (CH), 128.8 (CH), 129.2 (CH), 130.9 (C), 133.3 (C), 133.9 (C),
155.1 (C), 171.8 (C) ppm. MS (CI-NH3) m/e: 316 (M+I ÷, 9%), 333 (M+18+, 100%).
Acknowledgements
Financial support from CIRIT-CICYT (QFN93-4407) from DGICYT (PB95-0265) and from CIRIT
(GRQ93-1083) is gratefully acknowledged. A.V.-E is grateful to Ministerio de Educaci6n y Ciencia
for a post-doctoral fellowship.

1586
E. MEDINA et al.
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(Received in UK 5 March 1997)