Asymmetric synthesis of (+)-negamycin

Article abstract of DOI:10.1021/jo025636i

An asymmetric synthesis of the antibiotic (+)-negamycin (1) has been achieved, starting from commercially available (5R,6S)-4-(benzyloxycarbonyl)- 5,6-diphenyl-2,3,5,6-tetrahydro-4H-1,4-oxazin-2-one (2). The synthesis involved the stabilized Wittig olefination of the lactone carbonyl group of 2 and subsequent asymmetric hydrogenation to generate the corresponding all-syn oxazine 4 with excellent diastereoselectivity. Conversion of 4 into β-alkoxy imine 7 and subsequent CeCl₃-promoted chelation-controlled allylation of 7 generated the corresponding homoallylamine 8 with good diatereoselectivity, which was readily converted into (+)-negamycin (1) in 25% overall yield over 11 steps.

Full text of DOI:10.1021/jo025636i

Asym m etr ic Syn th esis of (+)-Nega m ycin
Rajendra P. J ain and Robert M. Williams*
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523
rmw@chem.colostate.edu
Received February 20, 2002
An asymmetric synthesis of the antibiotic (+)-negamycin (1) has been achieved, starting from
commercially available (5R,6S)-4-(benzyloxycarbonyl)-5,6-diphenyl-2,3,5,6-tetrahydro-4H-1,4-oxazin-
2-one (2). The synthesis involved the stabilized Wittig olefination of the lactone carbonyl group of
2 and subsequent asymmetric hydrogenation to generate the corresponding all-syn oxazine 4 with
excellent diastereoselectivity. Conversion of 4 into â-alkoxy imine 7 and subsequent CeCl3-promoted
chelation-controlled allylation of 7 generated the corresponding homoallylamine 8 with good
diatereoselectivity, which was readily converted into (+)-negamycin (1) in 25% overall yield over
11 steps.
In tr od u ction
(+)-Negamycin (1, 2-[(3R,5R)-3,6-diamino-5-hydroxy-
hexanoyl]-1-methylhydrazinoacetic acid) (Figure 1) is an
unusual antibiotic containing a hydrazido peptide link-
age.1 First isolated in 1970 by Umezawa and co-workers2
from culture filtrates of three strains closely related to
Streptomyces purpeofuscus, the structure of negamycin
was confirmed in 1972 by total synthesis from D-galac-
turonic acid.3 Negamycin exhibits considerable inhibitory
activity toward multiple drug-resistant enteric Gram-
positive and Gram-negative bacteria and exhibits low
acute toxicity.2 Negamycin is a specific inhibitor of
protein biosynthesis with miscoding activity.4
A number of approaches have been reported in the
literature for the synthesis of racemic5 as well as optically
active6 negamycin. Most of these approaches relied on
C5 and/or C3 stereogenic centers of negamycin pre-
existing in the starting materials.6a,d-g,i-j The synthesis
F IGURE 1.
of optically active negamycin described in these literature
procedures required 8-20 steps, with yields ranging
between 7 and 32%.6 Herein, we report a concise method
for the synthesis of (+)-negamycin by employing (5R,6S)-
4-(benzyloxycarbonyl)-5,6-diphenyl-2,3,5,6-tetrahydro-
4H-1,4-oxazin-2-one (2)7 as a starting material. Our
approach involves the generation of the C5 and C3
stereogenic centers of negamycin by employing the asym-
metric hydrogenation of dihydrooxazine derivative 3 and
a chelation-controlled allylation of â-alkoxy imine 7,
respectively.
Resu lts a n d Discu ssion
* Corresponding author.
(1) Kondo, S.; Shibahara, S.; Takahashi, S.; Maeda, K.; Umezawa,
H.; Ohno, M. J . Am. Chem. Soc. 1971, 93, 6305-6306.
As shown in Scheme 1, the Wittig reaction of methyl
(triphenylphosphoranylidene)acetate with the lactone
carbonyl group8 of 2 (xylene, 210 °C, 2 h) generated the
adduct 3 in quantitative yield. This species is reasonably
presumed to arise via tautomerization of the initial
olefination product A to the thermodynamically more
stable trisubstituted olefin. The stabilized Wittig olefi-
nation of 2 has previously been studied by our group and
was applied to the asymmetric synthesis of (+)-hypusine.9
(2) Hamada, M.; Takeuchi, T.; Kondo, S.; Ikeda, Y.; Naganawa, H.;
Maeda, K.; Okami, Y.; Umezawa, H. J . Antibiot. 1970, 23, 170-171.
(3) Shibahara, S.; Kondo, S.; Maeda, K.; Umezawa, H.; Ohno, M. J .
Am. Chem. Soc. 1972, 94, 4353-4354.
(4) Uehara, Y.; Kondo, S.; Umezawa, H.; Suzukake, K.; Hori, M. J .
Antibiot. 1972, 25, 685-688 and references therein.
(5) (a) Streicher, W.; Reinshagen, H.; Turnowski, F. J . Antibiot.
1978, 31, 725. (b) Pasquet, G.; Boucherot, D.; Pilgrim, W. R.; Wright,
B. Tetrahedron Lett. 1980, 21, 931-934. (c) Pierdet, A.; Ne´de´lec, L.;
Delaroff, V.; Allais, A.; Tetrahedron 1980, 36, 1763-1772.
(6) (a)Wang, Y.-F.; Izawa, T.; Kobayashi, S.; Ohno, M. J . Am. Chem.
Soc. 1982, 104, 6465-6466. (b) Iida, H.; Kasahara, K.; Kibayashi, C.
J . Am. Chem. Soc. 1986, 108, 4647-4648. (c) Kasahara, K.; Iida, H.;
Kibayashi, C. J . Org. Chem. 1989, 54, 2225-2233. (d) Tanner, D.;
Somfai, P. Tetrahedron Lett. 1988, 29, 2373-2376. (e) De Bernardo,
S.; Tengi, J . P.; Sasso, G.; Weigele, M. Tetrahedron Lett. 1988, 29,
4077-4080. (f) Schimdt, U.; Sta¨bler, F.; Lieberknecht, A. Synthesis
1992, 482-486. (g) Maycock, C. D.; Barros, M. T.; Santos, A. G.;
Godinho, L. S. Tetrahedron Lett. 1992, 33, 4633-4636. (h) Masters, J .
J .; Hegedus, L. S. J . Org. Chem. 1993, 58, 4547-4554. (i) Socha, D.;
J urczak, M.; Chmielewski, M. Tetrahedron Lett. 1995, 36, 135-138.
(j) Davies, S. G.; Ichihara, O. Tetrahedron: Asymmetry 1996, 7, 1919-
1922.
(7) The requisite diphenyloxazine and its antipode are commercially
available from Aldrich Chemical Co.: catalog #33185-6 (CAS Registry
#105228-46-4); for the antipode, catalog #33187-2 (CAS Registry
#100516-54-9).
(8) For other examples of the stablized Wittig olefination of the
lactone carbonyl group, see: (a) Brennan, J .; Murphy, P. J . Tetrahedron
Lett. 1988, 29, 2063-2066. (b) Lakhrissi, M.; Taillefumier, C.; Chaouch,
A.; Didierjean, C.; Aubry, A.; Chapleur, Y. Tetrahedron Lett. 1998, 39,
6457-6460. (c) Sabitha, G.; Reddy, M. M.; Srinivas, D.; Yadov, J . S.
Tetrahedron Lett. 1999, 40, 165-166.
(9) J ain, R. P.; Albrecht, B. K.; DeMong, D. E. Org. Lett. 2001, 26,
4287-4289.
10.1021/jo025636i CCC: $22.00 © 2002 American Chemical Society
Published on Web 08/02/2002
J . Org. Chem. 2002, 67, 6361-6365
6361

J ain and Williams
SCHEME 1
SCHEME 3
SCHEME 2a
(dr ) 1.5-2:1). Fortunately, use of allylzinc bromide in
the presence of anhydrous cerium chloride11 (1.1 equiv)
proved to be beneficial and resulted in the formation of
8 in essentially quantitative yield and with good diaste-
reoselectivity (dr ) 4.4:1, by 1H NMR) as a nonseparable
mixture of diastereomers (Scheme 3).
The stereoselectivity of the allylation reaction is rea-
sonably assumed to be controlled by stereoelectronic as
well as steric effects. Considering the six-membered
transition state B (Scheme 3) generated because of the
chelation of CeCl3 with the â-alkoxy imine, we speculated
the axial attack of the allylzinc reagent would generate
the desired homoallylamine with anti-diastereoselectiv-
ity. The stereochemistry of the newly created stereogenic
center in the major diatereomer of 8 was confirmed by
its conversion into (+)-negamycin (1). It should be noted
that the application of catalytic CeCl3 in Zn-mediated
allylations of chiral imines has been reported in the
literature.12
As shown in Scheme 4, protection of the nitrogen atom
in the homoallylamine 8 (dr ) 4.4:1) was achieved by
using Cbz-Cl (1 M NaOH, dioxane, 0 °C, 80%), furnish-
ing the bis(Cbz) derivative 9 (dr ) 4.4:1). Oxidative
cleavage of the carbon-carbon double bond in 9 (O3,
MeOH, CH2Cl2, -78 °C; then Ph3P) and purification by
chromatography generated the aldehyde 10 in 73% yield
and as a single diastereomer. Further oxidation of
aldehyde 10 with PDC (DMF, rt) generated the desired
carboxylic acid 11 in 97% yield.
a
Reagents and conditions: (a) Cbz-Cl, Et3N, DMAP, CH2Cl2,
rt, 96%; (b) DIBALH, CH2Cl2, -78 °C, 85%; (c) BnNH2, Al2O3,
CH2Cl2, 0 °C, 98%.
Application of compound 3 (as the corresponding ethyl
ester) as a key precursor in the synthesis of (R)-(-)-
carnitine has also been reported from this laboratory.10
Hydrogenation of 3 with PdCl2 (20 mol %, 115 psi of
H2, MeOH, 2 equiv of concd HCl, rt, 24 h) resulted in
the formation of the desired all-syn-substituted oxazine
4 in essentially quantitative yield and with at least a 94:6
diastereomeric ratio (by 1H NMR). The relative and
absolute stereochemistry of the newly created stereogenic
center in the major diastereomer of 4 was determined
1
as “R” by H NMR NOE measurements that revealed a
syn-relationship of the protons at C2, C5, and C6 of the
oxazine ring. The high degree of asymmetric induction
in the reduction step can be readily explained by adsorp-
tion of the substrate on the catalyst surface and subse-
quent hydrogenation of the double bond from the steri-
cally less-hindered face of the molecule.
Protection of the secondary amine in 4 was achieved
by treatment with Cbz-Cl (Et3N, DMAP, CH2Cl2, rt) to
obtain the desired N-Cbz-morpholine 5 in 96% yield
(Scheme 2). Reduction of the ester group in 5 was
achieved by using DIBALH (CH2Cl2, -78 °C) to obtain
the corresponding aldehyde 6 in 85% yield. Conversion
of aldehyde 6 to the â-alkoxy imine 7 was achieved in
excellent yield by treatment with benzylamine in the
presence of alumina.
Chelation-controlled allylation of imine 7 was studied
by using allyl-metal reagents in the presence of various
Lewis acids. Thus, treatment of 7 with allyltrimethylsi-
lane or allyltributyltin in the presence of a Lewis acid
(TiCl4, BF3‚Et2O, or AlCl3; -78 to 0 °C) resulted in the
formation of the desired homoallylamine 8, however, in
low yields (30-45%) and with poor diastereoselectivity
Condensation of 11 with the PTSA salt of benzyl (1-
methylhydrazino)acetate6h was performed by using 1-ethyl-
3-(3-dimethylaminopropyl)carbodiimide hydrochloride
(Et3N, HOBT, CH2Cl2, rt) to obtain the coupling product
12 in 80% yield. Hydrogenolysis of 12 (40 psi of H2, 10%
Pd/C, MeOH, H2O, AcOH, 75 °C) produced the acetate
salt of (+)-negamycin, which on purification by ion-
exchange chromatography (Amberlite CG-50, NH4+ form)
25
afforded (+)-negamycin (1)6 in 75% yield ([R]D ) +1.9
(c 1, H2O); lit.6h [R]D ) +1.7 (c 0.6, H2O); lit.6b [R]D
)
20
+2.3 (c 4.07, H2O)). The spectroscopic data for this
substance matched those reported in the literature for
(+)-negamycin.
Con clu sion
In conclusion, we have demonstrated a concise, asym-
metric, and stereocontrolled method for the synthesis of
(11) Takeda, N.; Imamoto, T. Org. Synth 1999, 76, 228-238.
(12) Basile, T.; Bocoum, A.; Savoia, D.; Umani-Ronchi, A. J . Org.
Chem. 1994, 59, 7766-7773.
(10) J ain, R. P.; Williams, R. M. Tetrahedron Lett. 2001, 42, 4437-
4440.
6362 J . Org. Chem., Vol. 67, No. 18, 2002

Asymmetric Synthesis of (+)-Negamycin
SCHEME 4a
mmol, 1 equiv) in MeOH (140 mL) were added HCl (10 M,
1.25 mL, 12.5 mmol, 2 equiv) and PdCl2 (0.22 g, 1.25 mmol,
0.2 equiv), and the mixture was stirred under 115 psi of H2
and at ambient temperature for 24 h. The catalyst was
removed by filtration through Celite, and the pad of Celite was
washed with hot MeOH. The combined filtrates were concen-
trated under reduced pressure to obtain 2.17 g (99%) of crude
4 (dr ) 94:6 by 1H NMR) as a yellow amorphous powder which
was pure by NMR. [R]D ) +49.7 (c 1, MeOH); 1H NMR (300
25
MHz, CD3OD, 300 K) δ 7.63-7.60 (m, 2H), 7.34-7.09 (m, 8H),
5.38 (d, 1H, J ) 3.3 Hz), 4.99 (d, 1H, J ) 3.3 Hz), 4.63-4.54
(m, 1H), 3.77 (s, 3H), 3.43-3.29 (m, 2H), 2.95 (d, 2H, J ) 6.3
Hz); minor diastereomer (visible peaks) δ 5.61 (d, J ) 3.6 Hz),
3.69 (s); 13C NMR (100 MHz, CD3OD, 300 K) δ 171.9, 137.9,
132.8, 132.4, 130.7, 129.8, 129.3, 128.7, 126.5, 79.0, 72.7, 58.5,
52.7, 41.6, 38.6; IR (KBr) 3000-2500 (br), 1738, 1604, 1583
cm-1. HRMS (FAB+): calcd for C19H22NO3 (m/z), 312.1599;
found (m/z), 312.1599.
2R,5R,6S-2-(Meth oxycar bon ylm eth yl)-5,6-diph en ylm or -
p h olin e-4-ca r boxylic Acid Ben zyl Ester (5). To a solution
of 4 (2 g, 5.76 mmol, 1 equiv) in anhydrous CH2Cl2 (30 mL)
were added Et3N (3.22 mL, 23.10 mmol, 4 equiv) and DMAP
(0.071 g, 0.576 mmol, 0.1 equiv). To this stirred mixture was
added dropwise Cbz-Cl (1.24 mL, 8.68 mmol, 1.5 equiv) over
a period of 15 min, and the mixture was stirred at ambient
temperature for 5.5 h. The reaction mixture was concentrated
under reduced pressure and purified by flash chromatography
on silica gel (8:2 petroleum ether/ethyl acetate) to furnish 2.46
a
Reagents and conditions: (a) Cbz-Cl, 1 M NaOH, dioxane,
0-5 °C, 80%; (b) O3, CH2Cl2, MeOH, -78 °C, 73%; (c) PDC, DMF,
rt, 97%; (d) benzyl (1-methylhydrazino)acetate-PTSA salt, 1-ethyl-
3-(3-dimethylaminopropyl)carbodiimide hydrochloride, Et3N, HOBT,
CH2Cl2, rt, 80%; (e) 40 psi of H2, 10% Pd/C (25 mol %), MeOH,
H2O, AcOH, 75 °C, 75%.
25
g (96%) of 5 as a clear colorless gum. [R]D ) -62.8 (c 1,
CHCl3); 1H NMR (400 MHz, DMSO-d6, 393 K) δ 7.37-7.06 (m,
15H), 5.38 (d, 1H, J ) 3.2 Hz), 5.20 (d, 1H, J ) 12.8 Hz), 5.14
(d, 1H, J ) 12.8 Hz), 5.13 (d, 1H, J ) 3.2 Hz), 4.27-4.20 (m,
1H), 4.06 (dd, 1H, J ) 3.2, 13.2 Hz), 3.69 (s, 3H), 3.18 (dd, 1H,
J ) 11.2, 13.2 Hz), 2.85 (dd, 1H, J ) 5.6, 15.6 Hz), 2.79 (dd,
1H, J ) 6.8, 15.6 Hz); 13C NMR (100 MHz, CDCl3, 300 K)
(mixture of rotamers) δ 170.6, 170.5, 155.1, 155.0, 138.1, 138.0,
136.4, 136.3, 136.2, 130.1, 129.7, 128.4, 128.1, 128.0, 127.9,
127.8, 127.7, 127.3, 126.9, 125.6, 125.4, 79.9, 79.8, 73.4, 73.2,
67.6, 67.4, 57.7, 56.7, 51.9, 43.2, 42.9, 38.6, 38.4; IR (CHCl3)
(+)-negamycin by using commercially available (5R,6S)-
4-(benzyloxycarbonyl)-5,6-diphenyl-2,3,5,6-tetrahydro-
4H-1,4-oxazin-2-one (2)7 as a starting material. Current
efforts are focused on extending the stabilized Wittig
homologation on the oxazinones to other amino acid-
derived natural products and alkaloids.
1740, 1699, 1604, 1585 cm-1. HRMS (FAB+): calcd for C27H28
NO5 (m/z), 446.1967; found (m/z), 446.1971.
-
Exp er im en ta l Section
2R,5R,6S-2-(2-Oxoeth yl)-5,6-diph en ylm or ph olin e-4-car -
boxylic Acid Ben zyl Ester (6). To a solution of 5 (1.9 g, 4.26
mmol, 1 equiv) in anhydrous CH2Cl2 (40 mL) was added at
-78 °C DIBALH (1 M solution in toluene, 8.54 mL, 8.54 mmol,
2 equiv) dropwise over a period of 30 min. The reaction mixture
was further stirred at -78 °C for an additional 2 h, after which
it was quenched by adding 50 mL of water. The reaction
mixture was filtered through a pad of Celite, dried (Na2SO4),
and concentrated under reduced pressure to obtain the crude
6, which on purification by flash chromatography on silica gel
(85:15 petroleum ether/ethyl acetate) furnished 1.63 g (85%)
of 6 as a white amorphous solid. [R]D25 ) -70.4 (c 1.7, CHCl3);
1H NMR (300 MHz, DMSO-d6, 393 K) δ 9.83 (t, 1H, J ) 1.5
Hz), 7.35-7.06 (m, 15H), 5.38 (d, 1H, J ) 3.6 Hz), 5.21-5.11
(m, 3H), 4.44-4.35 (m, 1H), 4.05 (dd, 1H, J ) 3.3, 13.5 Hz),
3.15 (dd, 1H, J ) 11.4, 13.5 Hz), 2.89-2.86 (m, 2H); 13C NMR
(75 MHz, CDCl3, 300 K) (mixture of rotamers) δ 198.9, 198.8,
154.7, 154.6, 137.6, 137.5, 136.0, 135.9, 129.7, 129.3, 128.2,
127.8, 127.6, 127.5, 127.1, 126.7, 125.2, 125.1, 79.7, 71.9, 71.7,
67.4, 67.3, 57.5, 56.6, 46.6, 46.5, 43.1, 42.9; IR (CHCl3) 1725,
1698, 1604, 1585 cm-1. HRMS (FAB+): calcd for C26H26NO4
(m/z), 416.1861; found (m/z), 416.1870.
2R,5R,6S-2-(2-Ben zylim in oeth yl)-5,6-d ip h en ylm or p h o-
lin e-4-ca r boxylic Acid Ben zyl Ester (7). To a solution of 6
(1.5 g, 3.61 mmol, 1 equiv) in anhydrous CH2Cl2 (15 mL) was
added at 0 °C neutral Al2O3 (3 g), followed by dropwise addition
of benzylamine (394 µL, 3.61 mmol, 1 equiv). The reaction
mixture was further stirred at 0 °C for an additional 30 min,
after which it was filtered through a pad of Celite and
concentrated under reduced pressure to obtain 1.79 g (98%)
of crude 7 as a white amorphous solid which was pure by NMR.
Gen er a l Met h od s. All reactions requiring anhydrous
conditions were performed under a positive pressure of argon
using oven-dried glassware (120 °C) that was cooled under
argon. THF was distilled from sodium benzophenone ketyl,
and dichloromethane was distilled from CaH2. Column chro-
matography was performed on Merck silica gel Kieselgel 60
(230-400 mesh).
5R,6S-2-Met h oxyca r b on ylm et h yl-5,6-d ip h en yl-5,6-d i-
h yd r o-[1,4]oxa zin e-4-ca r boxylic Acid Ben zyl Ester (3).
The mixture of 2 (2.23 g, 5.76 mmol, 1 equiv) and methyl
(triphenylphosphoranylidene)acetate (4.80 g, 14.4 mmol, 2.5
equiv) in xylene (15 mL) was heated to 210 °C in a preheated
oil bath for 2 h in a sealed tube. The mixture was then cooled
to ambient temperature, and the solvent was removed under
reduced pressure to obtain the crude product, which on
purification by flash chromatography on silica gel (7:3 petro-
leum ether/ethyl acetate) furnished 2.55 g (100%) of 3 as a
25
1
pale yellow gum. [R]D ) +202.8 (c 2, CHCl3); H NMR (400
MHz, DMSO-d6, 393 K) δ 7.29-6.95 (m, 15H), 6.70 (s, 1H),
5.34 (d, 1H, J ) 3.2 Hz), 5.32 (d, 1H, J ) 3.2 Hz), 5.17 (d, 1H,
J ) 12.8 Hz), 5.11 (d, 1H, J ) 12.8 Hz), 3.68 (s, 3H), 3.39 (d,
1H, J ) 16.4 Hz), 3.31 (d, 1H, J ) 16.4 Hz); 13C NMR (75 MHz,
CDCl3, 300 K) (mixture of rotamers) δ 169.8, 151.3, 136.2,
136.1, 135.9, 135.6, 135.5, 134.1, 132.8, 128.2, 128.1, 128.0,
127.5, 127.4, 127.3, 127.1, 126.2, 126.0, 105.7, 104.9, 78.5, 78.3,
67.7, 67.3, 60.1, 58.9, 51.8, 37.5; IR (CHCl3) 1743, 1705, 1605,
1586 cm-1. HRMS (FAB+): calcd for C27H25NO5 (m/z), 443.1732;
found (m/z), 443.1729.
2R,5R,6S-2-(Meth oxycar bon ylm eth yl)-5,6-diph en ylm or -
p h olin e Hyd r och lor id e (4). To a solution of 3 (2.77 g, 6.25
J . Org. Chem, Vol. 67, No. 18, 2002 6363

J ain and Williams
[R]D ) -41.3 (c 1, CHCl3); 1H NMR (400 MHz, DMSO-d6,
the solution was refluxed in residual methanol for 30 min.
Removal of solvent under reduced pressure afforded crude 10,
which on purification by chromatography on silica gel (8:2
petroleum ether/ethyl acetate) furnished 1.03 g (73%) of 10
25
393 K) δ 8.01 (t, J ) 4.8 Hz, 1H), 7.34-7.09 (m, 20H), 5.37 (d,
1H, J ) 3.2 Hz), 5.18 (d, 1H, J ) 12.4 Hz), 5.13 (d, 1H, J )
12.4 Hz), 5.10 (d, 1H, J ) 3.2 Hz), 4.60 (s, 2H), 4.26-4.21 (m,
1H), 4.07 (dd, 1H, J ) 3.2, 13.2 Hz), 3.14 (dd, 1H, J ) 11.2,
13.2 Hz), 2.75 (t, 2H, J ) 5.6 Hz); 13C NMR (75 MHz, CDCl3,
300 K) (mixture of rotamers) δ 162.0, 161.9, 155.1, 155.0, 138.7,
138.2, 138.1, 136.4, 136.2, 130.0, 129.7, 128.4, 128.0, 127.8,
127.2, 126.9, 125.4, 79.9, 79.8, 74.4, 67.6, 67.4, 67.3, 65.1, 57.9,
57.8, 56.9, 56.8, 43.4, 43.3, 39.8, 39.5; IR (CHCl3) 1697, 1604,
1584 cm-1. HRMS (FAB+): calcd for C33H33N2O3 (m/z),
505.2491; found (m/z), 505.2499.
as a white amorphous solid. [R]D ) -17.0 (c 1.4, CHCl3); 1H
25
NMR (300 MHz, DMSO-d6, 393 K) δ 9.51 (t, 1H, J ) 1.8 Hz),
7.37-7.07 (m, 25H), 5.30 (d, 1H, J ) 3.6 Hz), 5.17 (d, 1H, J )
12.6 Hz), 5.15 (d, 1H, J ) 12.3 Hz), 5.12 (d, 1H, J ) 12.6 Hz),
5.06 (d, 1H, J ) 12.3 Hz), 4.86 (d, 1H, J ) 3.6 Hz), 4.65-4.57
(m, 1H), 4.54 (d, 1H, J ) 15.6 Hz), 4.40 (d, 1H, J ) 15.6), 3.71-
3.60 (m, 2H), 2.91 (dd, 1H, J ) 11.1, 13.2 Hz), 2.81 (dd, 2H, J
) 1.8, 6.9 Hz), 2.11-2.04 (m, 1H), 1.94-1.84 (m, 1H); 13C NMR
(100 MHz, CDCl3, 300 K) (mixture of rotamers) δ 199.9, 199.4,
156.6, 155.1, 154.9, 154.8, 138.1, 138.0, 137.7, 137.5, 136.3,
136.1, 129.8, 129.5, 128.3, 128.0, 127.9, 127.7, 127.6, 127.5,
127.4, 127.1, 126.9, 126.8, 125.5, 125.3, 79.4, 73.7, 73.3, 72.9,
67.4, 67.3, 67.0, 57.9, 57.1, 52.2, 50.8, 48.9, 48.6, 47.8, 43.5,
2R,2′S,5R,6S-2-(2′-Ben zylam in open t-4′-en yl)-5,6-diph en -
ylm or p h olin e-4-ca r boxylic Acid Ben zyl Ester (8). To a
solution of 7 (1.82 g, 3.61 mmol, 1 equiv) in anhydrous THF
(20 mL) was added at -40 °C a 1 M suspension of anhydrous
CeCl3 in THF (4 mL, 4 mmol, 1.1 equiv) dropwise over a period
of 10 min. The mixture was stirred at -40 °C for 4 h. To the
resulting milky suspension was added a solution of allylzinc
bromide in THF (prepared by dropwise addition of allyl
bromide (1.29 mL, 14.5 mmol, 4 equiv) to a vigorously stirred
suspension of Zn (0.975 g, 14.5 mmol, 4 equiv) in anhydrous
THF (15 mL) over a period of 30 min in a 25 °C water bath
and by stirring the mixture for an additional 3 h) over a period
of 15 min. The reaction mixture was further stirred at -40 °C
for an additional 30 min, after which it was quenched by
adding 50 mL of water and 50 mL of 2 M HCl. The mixture
was warmed to rt and diluted with ethyl acetate (200 mL).
The organic layer was separated and washed with water (50
mL), 1 M NaOH (50 mL), water (50 mL), and brine (50 mL).
The organic layer was dried (Na2SO4) and concentrated to
obtain 1.9 g (96%) of crude 8 (dr ) 4.4:1) as a clear colorless
43.3, 36.8, 35.8; IR (CHCl3) 1721, 1696, 1604, 1584 cm-1
.
HRMS (FAB+): calcd for C43H43N2O6 (m/z), 683.3121; found
(m/z), 683.3134.
2R,2′R,5R,6S-2-(2′-(Ben zylb en zyloxyca r b on yla m in o)-
3′-ca r b oxyp r op yl)-5,6-d ip h en ylm or p h olin e-4-ca r b oxyl-
ic Acid Ben zyl Ester (11). To the solution of 10 (0.85 g, 1.24
mmol, 1 equiv) in anhydrous DMF (4 mL) was added PDC
(1.632 g, 4.34 mmol, 3.5 equiv), and the mixture was stirred
at ambient temperature for 12 h. The solvent was removed
under reduced pressure, and the residue was purified by flash
chromatography on silica gel (ethyl acetate) to furnish 0.85 g
25
(97%) of 11 as a white amorphous solid. [R]D ) -28.0 (c 1,
CHCl3); 1H NMR (400 MHz, DMSO-d6, 373 K) δ 11.80 (br, 1H),
7.39-7.07 (m, 25H), 5.31 (br, 1H), 5.18 (d, 1H, J ) 12.8 Hz),
5.15 (d, 1H, J ) 12.8 Hz), 5.13 (d, 1H, J ) 12.8 Hz), 5.07 (d,
1H, J ) 12.8 Hz), 4.81 (d, 1H, J ) 3.6 Hz), 4.56-4.10 (m, 1H),
4.52 (d, 1H, J ) 16.0 Hz), 4.45 (d, 1H, J ) 16.0 Hz), 3.66-
3.57 (m, 2H), 2.88 (dd, 1H, J ) 11.2, 12.8 Hz), 2.67 (d, 2H, J
) 7.2 Hz), 2.10-1.98 (m, 1H), 1.93-1.86 (m, 1H); 13C NMR
(100 MHz, CDCl3, 300 K) (mixture of rotamers) δ 176.7, 176.1,
156.8, 155.0, 138.2, 138.1, 137.6, 136.3, 136.1, 129.9, 129.5,
128.3, 128.0, 127.9, 127.7, 127.6, 127.3, 127.1, 126.8, 126.7,
125.5, 125.3, 79.3, 73.7, 73.3, 73.0, 67.5, 67.4, 67.0, 57.9, 57.0,
53.4, 52.9, 51.5, 43.6, 43.3, 39.1, 38.0, 36.8, 36.5, 35.8; IR
(CHCl3) 1729, 1698, 1604, 1585 cm-1. HRMS (FAB+): calcd
for C43H43N2O7 (m/z), 699.3070; found (m/z), 699.3050.
25
gum which was pure by NMR. [R]D ) -26.2 (c 1, CHCl3); 1H
NMR (300 MHz, DMSO-d6, 393 K) δ 8.08-8.00 (m, 1H), 7.33-
7.05 (m, 20H), 5.98-5.85 (m, 1H), 5.36 (d, 1H, J ) 3.3 Hz),
5.18 (d, 1H, J ) 12.6 Hz), 5.12 (d, 1H, J ) 12.6 Hz), 5.10-
5.02 (m, 2H), 4.53 (br, 1H), 4.10-4.01 (m, 1H), 3.96 (dd, 1H, J
) 3.6, 13.2 Hz), 3.84 (d, 1H, J ) 13.2 Hz), 3.06 (d, 1H, J )
13.2 Hz), 3.96 (dd, 1H, J ) 11.1, 13.2 Hz), 3.04-2.94 (m, 1H),
2.40-2.24 (m, 2H), 1.91-1.82 (m, 1H), 1.73-1.64 (m, 1H); IR
(CHCl3) 2925, 1697, 1639, 1604, 1585 cm-1. HRMS (FAB+):
calcd for C36H39N2O3 (m/z), 547.2960; found (m/z), 547.2965.
2R,2′S,5R,6S-2-(2′-(Ben zylb en zyloxyca r b on yla m in o)-
p en t-4′-en yl)-5,6-d ip h en ylm or p h olin e-4-ca r boxylic Acid
Ben zyl Ester (9). To a solution of 8 (1.9 g, 3.47 mmol, 1 equiv)
in dioxane (50 mL) was added 1 M NaOH (10 mL, 10 mmol),
and the mixture was cooled in an ice-water bath (0-5 °C).
To this mixture was added Cbz-Cl (991 µL, 6.94 mmol, 2
equiv) dropwise, and the reaction mixture was further stirred
at the same temperature for 12 h, after which it was diluted
with ethyl acetate (100 mL). The organic layer was separated,
washed with water (50 mL) followed by brine (50 mL), dried
(Na2SO4), and concentrated to afford the crude product, which
on purfication by flash chromatography on silica gel (8:2
petroleum ether/ethyl acetate) furnished 1.9 g (80%) of 9 as a
clear colorless gum. [R]D25 ) -13.2 (c 1, CHCl3); 1H NMR (400
MHz, DMSO-d6, 393 K) δ 7.34-7.03 (m, 25H), 5.68-5.58 (m,
1H), 5.27 (d, 1H, J ) 3.6 Hz), 5.14-4.87 (m, 6H), 4.81 (d, 1H,
J ) 3.6 Hz), 4.48 (d, 1H, J ) 16.0 Hz), 4.31 (d, 1H, J ) 16.0
Hz), 4.22-4.12 (m, 1H), 3.64-3.54 (m, 2H), 2.88-2.82 (m, 1H),
2.43-2.30 (m, 2H), 1.99-1.90 (m, 1H), 1.82-1.75 (m, 1H); IR
(CHCl3) 1697, 1604, 1584 cm-1. HRMS (FAB+): calcd for
Syn th esis of 12. To a mixture of 11 (0.85 g, 1.21 mmol, 1
equiv), benzyl (1-methylhydrazino)acetate-PTSA salt (0.886
g, 2.42 mmol, 2 equiv), and HOBT (0.327 g, 2.42 mmol, 2 equiv)
in anhydrous CH2Cl2 (10 mL) was added Et3N (1.7 mL, 12.1
mmol, 10 equiv) which was followed by dropwise addition of a
solution of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hy-
drochloride (0.464 g, 2.42 mmol, 2 equiv) in anhydrous
CH2Cl2 (20 mL). The reaction mixture was stirred at ambient
temperature for 12 h. Solvent was removed under reduced
pressure, and the residue was dissolved in ethyl acetate (200
mL) and washed with 1 M HCl (5 × 50 mL), water (1 × 50
mL), 1 M NaOH (3 × 50 mL), water (1 × 50 mL), and brine (1
× 50 mL). The organic layer was dried (Na2SO4) and concen-
trated to obtain the crude 12, which on purification by flash
chromatography on silica gel (1:1 petroleum ether/ethyl ac-
etate) furnished 0.857 g (80%) of 12 as a white amorphous
solid. [R]D25 ) -23.6 (c 1, MeOH); 1H NMR (400 MHz, DMSO-
d6, 393 K) δ 8.69 (br, 1H), 7.38-7.06 (m, 30H), 5.31 (d, 1H, J
) 3.2 Hz), 5.18 (d, 1H, J ) 12.4 Hz), 5.15 (d, 1H, J ) 12.4 Hz),
5.14 (d, 1H, J ) 12.4 Hz), 5.07 (d, 1H, J ) 12.4 Hz), 5.14 (s,
2H), 4.78 (d, 1H, J ) 3.2 Hz), 4.49 (d, 1H, J ) 12.0), 4.45 (d,
1H, J ) 12.0), 4.54-4.42 (m, 1H), 3.66-3.56 (m, 4H), 2.86 (t,
1H, J ) 11.6 Hz), 2.62 (br, 5H), 2.05-1.99 (m, 1H), 1.93-1.82
(m, 1H); 13C NMR (100 MHz, CDCl3, 300 K) (mixture of
rotamers) δ 174.1, 173.6, 170.1, 169.4, 168.7, 168.3, 156.8,
156.6, 155.1, 154.9, 138.2, 138.1, 137.6, 136.4, 136.3, 136.2,
134.9, 129.8, 129.5, 128.4, 128.3, 128.2, 128.1, 127.9, 127.8,
127.6, 127.5, 127.2, 127.0, 126.6, 126.5, 125.5, 125.4, 125.3,
79.2, 79.0, 73.8, 73.6, 73.2, 72.9, 72.7, 67.3, 67.2, 66.9, 66.7,
C
44H45N2O5 (m/z), 681.3328; found (m/z), 681.3341.
2R,2′R,5R,6S-2-(2′-(Ben zylb en zyloxyca r b on yla m in o)-
4′-oxobu tyl)-5,6-d ip h en ylm or p h olin e-4-ca r boxylic Acid
Ben zyl Ester (10). To a solution of 9 (1.4 g, 2 mmol, 1 equiv)
in anhydrous MeOH (40 mL) and anhydrous CH2Cl2 (20 mL)
was bubbled O3 at -78 °C until the solution turned blue. At
this point, Ar gas was bubbled through the solution for 5 min
to remove excess O3. Ph3P (0.682 g, 2.6 mmol, 1.3 equiv) was
added, and the mixture was warmed to and stirred at ambient
temperature overnight. CH2Cl2 was distilled off at 1 atm, and
6364 J . Org. Chem., Vol. 67, No. 18, 2002

Asymmetric Synthesis of (+)-Negamycin
66.4, 58.4, 57.8, 57.7, 57.4, 57.0, 56.9, 54.5, 53.5, 52.7, 44.7,
43.6, 43.5, 43.3, 43.2, 39.8, 38.5, 36.8, 36.4, 35.7; IR (CHCl3)
3325, 1737, 1694, 1605, 1585 cm-1. HRMS (FAB+): calcd for
9.0, 12.9 Hz), 2.45 (s, 3H), 2.20 (d, 2H, J ) 6.3 Hz), 1.46-1.39
(m, 2H); 13C NMR (100 MHz, D2O, 300 K) δ 177.2, 170.9, 65.6,
60.9, 45.2, 45.0, 43.9, 41.1, 39.5; IR (KBr) 3500-2500, 1662,
1602, 1450, 1401, 1315, 1131 cm-1. HRMS (FAB+): calcd for
C9H21N4O4 (m/z), 249.1562; found (m/z), 249.1565.
C
53H55N4O8 (m/z), 875.4019; found (m/z), 875.4002.
(+)-Nega m ycin (1). To a solution of 12 (0.2 g, 0.22 mmol,
1 equiv) in MeOH (24 mL) were added AcOH (1.2 g, 20 mmol)
and water (6 mL). To this clear solution was added in portions
10% Pd/C (0.06 g, 0.056 mmol, 0.25 equiv), and the mixture
was hydrogenated under 40 psi of H2 and at 75 °C for 4-4.5
h. The reaction mixture was filtered through a pad of Celite
and concentrated under reduced pressure. The residue was
dissolved in water and purified on Amberlite CG-50 resin
(NH4+ form), eluting with 1.5% aq NH4OH. The eluents were
concentrated under reduced pressure to furnish 0.043 g (75%)
of 1 as a white powder. [R]D25 ) +1.9 (c 1, H2O); 1H NMR (300
MHz, D2O, 300 K) δ 3.86-3.78 (m, 1H), 3.30-3.18 (m, 1H),
3.21 (s, 2H), 2.87 (dd, 1H, J ) 3.3, 12.9 Hz), 2.70 (dd, 1H, J )
Ack n ow led gm en t. This work was supported by the
National Science Foundation (Grant CHE 9731947). We
thank Prof. Louis S. Hegedus (Department of Chemis-
try, Colorado State University) for providing the 1H and
13C NMR spectra of authentic negamycin.
Su p p or tin g In for m a tion Ava ila ble: 1H and/or 13C spec-
tra of all new compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
J O025636I
J . Org. Chem, Vol. 67, No. 18, 2002 6365