A total synthesis of (+)-negamycin through isoxazolidine allylation

Article abstract of DOI:10.1039/c4ob00537f

The β-amino acid antibiotic (+)-negamycin has been synthesised in ten steps from epichlorohydrin via Sakurai allylation of an isoxazolidine intermediate. The key allylation reaction proceeded with complete trans-selectivity, which is attributed to electrostatic attraction between the chlorine atom and the iminium ion in the Sakurai intermediate. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ob00537f

Organic &
Biomolecular Chemistry
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A total synthesis of (+)-negamycin through
isoxazolidine allylation†
Cite this: Org. Biomol. Chem., 2014,
12, 4879
Roderick W. Bates,* Rab’iah Nisha Khanizeman, Hajime Hirao, Yu Shan Tay and
Patcharaporn Sae-Lao
Received 11th March 2014,
Accepted 20th May 2014
The β-amino acid antibiotic (+)-negamycin has been synthesised in ten steps from epichlorohydrin via
Sakurai allylation of an isoxazolidine intermediate. The key allylation reaction proceeded with complete
trans-selectivity, which is attributed to electrostatic attraction between the chlorine atom and the iminium
ion in the Sakurai intermediate.
DOI: 10.1039/c4ob00537f
www.rsc.org/obc
Negamycin 1 was isolated from the microorganism Strepto- duce the terminal amino group at a late stage. Ring opening of
myces purpeofuscus and shown to have low toxicity, but signifi- (S)-epi-chlorohydrin with vinyl magnesium bromide, catalysed
cant activity against both Gram-positive and Gram-negative by copper(I) iodide, gave the alcohol 2.5 This compound was
bacteria (Fig. 1).1
subjected to the sequence of Mitsunobu reaction with N-hydroxy-
Given the current dearth of new antibiotics, it remains a phthalimide,6 dephthaloylation and N-reprotection, which
molecule of interest. Negamycin has been the subject of a we have used extensively, to give hydroxylamine 5 in 76% yield
number of syntheses, as have its stereoisomers and analogs.2 over the three steps. A Cbz group was chosen for N-protection
A number of interesting strategies have been employed over the as we anticipated a global hydrogenation as the final step, as
years. One method to generate the required 1,3-anti relation- used by others.2c–h,m,p,q,t The N-protected hydroxylamine 5 was
ship between the hydroxyl and amino groups is to use a subjected to ozonolysis in acidic methanol. This gave the
1,3-dipolar cycloaddition to form a 3,5-trans-isoxazolidine, methoxyisoxazolidine 6a as an inconsequential mixture of dia-
followed by cleavage of the N,O-bond at a later stage. Such stereoisomers directly in 72% yield after a dimethyl sulfide
reactions often give the trans-isoxazolidine as the major work-up. To our surprise, allylation of this isoxazolidine under
stereoisomer. This method was, indeed, employed by the conditions that we reported earlier gave the Sakurai
Kibayashi.2e,h Over the years, this laboratory has reported a number product 7a as a single stereoisomer (within the limits of
of non-cycloaddition methods for the synthesis of 3,5-cis-isoxa- detection by 400 MHz NMR spectroscopy), rather than the ca.
zolidines, which give syn-1,3-aminoalcohol derivatives upon 4 : 1 mixture anticipated based upon those results.4 Sub-
N,O-bond cleavage.3 Recently we reported that the Sakurai sequent conversion to the natural product showed that this
reaction of N-carboalkoxy-3-methoxy isoxazolidines gives 3-allyl- was the desired trans isomer. We attribute the stereochemical
isoxazolidines with modest selectivity for the trans isomer.4 outcome to an electrostatic effect. The Sakurai reaction is
We considered that applying this method to the synthesis of presumed to proceed via an iminium ion intermediate 8. A
negamycin would be a useful contribution to the field.
simple aryl or alkyl substituent at C5 exerts only a modest
The 5-chloromethyl-3-methoxyisoxazolidine 6a was pre- influence as the conformation of five member rings is poorly
pared with the intention of using the chlorine atom to intro- defined. In this case, we postulate an electrostatic attraction
between the chlorine atom (δ-) and the positive iminium ion
that results in highly effective shielding of one face. Indeed,
calculation of the conformation of this iminium ion inter-
mediate indicated that the lowest energy conformation has the
chlorine atom over the isoxazolidine ring, where it would
provide efficient shielding of that face (Fig. 2).7 This confor-
Fig. 1 Negamycin.
mation was found to be 2.9 kcal mol−1 lower than the next
lowest conformation. In contrast, analogous calculation of the
Division of Chemistry and Biological Chemistry, School of Physical and
conformation of the corresponding ethyl substituted iminium
ion, placed the CH3 group of the ethyl group in free space,
away from the ring, though other conformations were within
1 kcal mol−1. This correlates with the modest selectivity
Mathematical Sciences, Nanyang Technological University, Singapore, 637371.
E-mail: roderick@ntu.edu.sg
†Electronic supplementary information (ESI) available: Copies of 1H and
13C NMR spectra and computational details. See DOI: 10.1039/c4ob00537f
This journal is © The Royal Society of Chemistry 2014
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Oxidative cleavage of alkene 7a under Sharpless’ con-
ditions9 gave carboxylic acid 9. It was found that the most
effective way to couple the acid with the known hydrazine 102k
was by formation of the acid chloride. Subsequent displace-
ment of the chloride of hydrazide 11 with azide proved trouble-
some. Attempts to achieve this using sodium azide, with or
without addition of a sodium iodide catalyst, in DMF were
fruitless, returning starting material, or, on heating, leading to
decomposition. On the other hand, use of tetra-n-butylammo-
nium azide10 in THF gave the desired azide 12, characterized
by a strong IR absorbance at 2104 cm−1. Global hydrogenation
of azide 12, involving the removal of two protecting groups,
unmasking of the amine and cleavage of the N,O-bond, then
gave (+)-negamycin 1, identical to the natural product: m.
p. 114–117 °C (Lit.:1a 110–120 °C), [α]D23 +2.5 (c 2.0, H2O) (Lit.:1a
[α]2D9 +2.5 (c 2.0, H2O)). The natural product was obtained in
93% yield after purification by ion exchange chromatography.
The overall yield is 23% over 10 steps. The completion of the
synthesis further illustrates the utility of non-cycloaddition
methods for the synthesis of N,O-heterocycles and, thence,
amino alcohols and their derivatives. The observation of the
very high stereoselectivity during the Sakurai reaction under-
lines the importance of subtle electrostatic effects upon the
conformation of reactive intermediates.
Fig. 2 Calculated lowest energy conformations of iminium ion inter-
mediates with chloromethyl and ethyl substituents.
observed with isoxazolidines with simple substituents. What-
ever the cause, the outcome was very gratifying.8 Other electro-
negative atoms would be expected to show the same behavior.
The hydroxymethyl isoxazolidine 6b was prepared in racemic
form by an analogous route starting from the TBS ether of
glycidol. The two diastereoisomers of 6b were separable by flash
chromatography (though not identified). Treatment of either
diastereoisomer with allyltrimethylsilane under the conditions
used for the conversion of 6a to 7a gave exclusively the trans-
isoxazolidine 7b. The trans stereochemistry of isoxazolidine 7b
was confirmed by conversion to 7a on treatment with thionyl
chloride and pyridine (Scheme 1).
Experimental
All reactions requiring anhydrous conditions were carried out
under a nitrogen atmosphere using oven-dried glassware
(120 °C), which was cooled under vacuum. Anhydrous tetra-
hydrofuran was distilled from sodium metal and benzophenone
Scheme 1 Negamycin synthesis.
4880 | Org. Biomol. Chem., 2014, 12, 4879–4884
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under nitrogen. Anhydrous dichloromethane was dried by distil- 1612, 1188 cm−1; MS (ESI) m/z 289 [M + Na]+; HRMS (ESI-TOF)
lation from CaH2 immediately prior to use under nitrogen. m/z [M + H]+ calculated for C13H1335ClNO3: 266.0584; found:
Anhydrous methanol was distilled from activated magnesium 266.0580; [α]2D3 +8.9 (c 0.2, CH2Cl2).
under nitrogen. All other solvents and reagents were used as
(R)-O-(1-Chloropent-4-en-2-yl)hydroxylamine (4)
received. Flash chromatography was carried out on silica gel,
230–400 mesh. 1H NMR spectra were recorded at 300, 400 or Hydrazine monohydrate (0.45 mL, 14.2 mmol) was added to a
500 MHz in CDCl3 solutions. 13C NMR spectra were recorded at solution of hydroxylamine (3) (0.59 g, 2.22 mmol) in dichloro-
the corresponding frequency on the same instruments at 75, methane (20 mL) at 0 °C. The mixture was warmed to room
100 or 125 MHz. Chemical shifts are recorded in ppm and temperature, stirred for 15 minutes, filtered through celite,
coupling constants are recorded in Hz. Optical rotations are and concentrated in vacuo to give hydroxylamine (4) (0.27 g,
given with units of 10−1 deg cm2 g−1. Rotations were measured 90%) as a colorless oil which was used without further
at a wavelength of 589 nm. Melting points are uncorrected.
purification.
1H NMR (400 MHz, CDCl3): δ 5.80 (1H, ddt, J = 17.4, 7.3, 3.2
Hz), 5.17 (1H, dd, J = 3.2, 1.4 Hz), 5.13 (1H, dd, J = 3.9, 1.4 Hz),
(S)-1-Chloropent-4-en-2-ol (2)
Vinyl magnesium bromide was prepared by dropwise addition 3.83–3.71 (1H, m), 3.64 (2H, dd, J = 11.4, 4.6 Hz), 2.42 (1H,
of vinyl bromide solution (10 mL of a 1M solution in THF) ddd, J = 11.5, 7.6, 1.4 Hz), 2.36 (1H, ddd, J = 11.8, 7.1, 1.4 Hz);
to a stirring suspension of magnesium turnings (0.73 g, 13C NMR (100 MHz, CDCl3): δ 134.1, 117.6, 80.4, 48.9, 38.6;
30.0 mmol) in anhydrous tetrahydrofuran (35.0 mL). The IR vmax 3306, 2982, 1043 cm−1; MS (ESI) m/z 158 [M + Na]+;
mixture was heated at reflux for 1 hour. The vinyl magnesium HRMS (ESI-TOF) m/z [M + H]+ calculated for C5H1135ClNO:
bromide was added dropwise to a cooled (−68 °C) solution of 136.0529; found: 136.0528; [α]2D1 +3.0 (c 1.0, CH2Cl2).
(S)-epichlorohydrin (0.85 mL, 10.81 mmol) containing copper(I)
(R)-Benzyl 1-chloropent-4-en-2-yloxycarbamate (5)
iodide (0.21 g, 1.08 mmol) in anhydrous diethyl ether
(45.0 mL). The mixture was stirred for 1 hour at −68 °C. The Benzyl chloroformate (1.67 mL, 12.30 mmol) was added to a
mixture was quenched with saturated aqueous ammonium solution of (4) (1.25 g, 10.30 mmol) in CH2Cl2–H2O (1 : 1)
chloride and allowed to warm to room temperature. The layers (78 mL) containing anhydrous sodium carbonate (1.31 g,
were separated and the aqueous layer was extracted with 12.30 mmol) at 0 °C. The mixture was stirred at room tempera-
diethyl ether (3 × 10 mL). The combined organic extracts were ture for 3 hours. The organic layer was separated, and the
washed with brine, dried over anhydrous magnesium sulphate, aqueous layer was extracted with CH2Cl2 (3 × 20 mL). The com-
filtered through celite and concentrated in vacuo to give bined organic layers were dried over anhydrous magnesium
alcohol (2) (1.17 g, 90%) as a colorless clear oil.
sulphate, filtered and concentrated in vacuo. The residue was
1H NMR (400 MHz, CDCl3): δ 5.82 (1H, ddt, J = 17.1, 7.1, purified by flash chromatography on silica gel eluting with
3.2 Hz), 5.17 (2H, dd, J = 13.1, 1.4 Hz), 3.90–3.85 (1H, m) 3.63 (25% EtOAc–hexane) to give hydroxylamine (5) (2.39 g, 86%) as
(1H, dd, J = 11.2, 4.1 Hz), 3.51 (1H, dd, J = 11.2, 6.8 Hz), a colorless oil.
2.39–2.31 (2H, m); 13C NMR (100 MHz, CDCl3): δ 133.2, 118.6,
70.5, 49.3, 38.6; MS (ESI) m/z 121 [M + H]+.
1H NMR (400 MHz, CDCl3): δ 7.40–7.34 (5H, m), 5.81 (1H,
ddt, J = 17.4, 7.3, 3.2 Hz), 5.18 (2H, s), 5.15 (2H, dd, J = 5.0,
1.8 Hz), 4.15–4.04 (1H, m), 3.72 (1H, dd, J = 11.9, 5.0 Hz), 3.63
(1H, dd, J = 7.5, 4.6 Hz), 2.49 (2H, ddd, J = 7.5, 3.4, 1.4 Hz);
13C NMR (100 MHz, CDCl3): δ 157.8, 135.4, 132.9, 128.7, 128.6,
The obtained data are consistent with literature values.5
(R)-2-(1-Chloropent-4-en-2-yloxy)isoindoline-1,3-dione (3)
(S)-1-Chloropent-4-en-2-ol, (2) (2.0 g, 16.60 mmol) in anhy- 128.5, 118.6, 83.9, 67.9, 43.9, 34.8; IR vmax 3083, 2982, 1707,
drous tetrahydrofuran (50 mL) was added to a mixture of 1647, 754, 698 cm−1; MS (ESI) m/z 270 [M + H]+; HRMS
N-hydroxyphthalimide (3.25 g, 19.90 mmol) and triphenylpho- (ESI-TOF) m/z [M + Na]+ calculated for C13H1635ClNO3Na:
sphine (6.53 g, 24.70 mmol). The mixture was then cooled to 292.0716; found: 292.0713; [α]2D2 +27.8 (c 0.3, CH2Cl2).
0 °C and diisopropyl azodicarboxylate (5.8 mL, 29.70 mmol) in
(5R)-2-Benzyloxycarbonyl-5-chloromethyl-
3-methoxyisoxazolidine (6a)
anhydrous tetrahydrofuran (5 mL) was added dropwise with
the temperature maintained at 0 °C until complete addition.
The mixture was warmed to room temperature and stirred Amberlyst-15 (66.0 mg, 0.48 mmol) was added to a solution of
overnight. The solvent was removed in vacuo. The residue was (5) (2.56 g, 9.50 mmol) in methanol (52.0 mL) at −78 °C. The
purified by dry-loading flash chromatography on silica gel temperature was maintained at −78 °C while a mixture of
eluting with (15% EtOAc–hexane) to give hydroxylamine (3) ozone and oxygen was bubbled through the solution until a
(4.30 g, 98%) as a colorless solid. Mp = 60–71 °C; 1H NMR blue color was observed. Dimethyl sulfide (8.40 mL) was added
(400 MHz, CDCl3): δ 7.90–7.76 (4H, m), 5.96 (1H, ddt, J = 17.4, to the solution and it was stirred overnight, then filtered and
7.3, 3.2 Hz), 5.23 (1H, dd, J = 16.7, 1.4 Hz), 5.21 (1H, dd, J = concentrated in vacuo. Water was added, and the mixture was
11.7, 1.8 Hz), 4.47–4.44 (1H, m), 3.78 (2H, dd, J = 11.9, 4.6 Hz), extracted with ethyl acetate (3 × 30 mL). The combined organic
2.73 (1H, ddd, J = 14.7, 6.4, 6.0 Hz), 2.63 (1H, ddd, J = 16.1, extracts were washed with brine, dried over anhydrous mag-
11.2, 6.8 Hz); 13C NMR (100 MHz, CDCl3): δ 163.7, 134.6, nesium sulphate, filtered through celite and concentrated
131.8, 128.8, 123.6, 118.9, 86.0, 43.4, 34.9; IR vmax 1790, 1732, in vacuo to give isoxazolidine (6a) (1.95 g, 72%) as a colorless
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clear oil and as an inconsequential 1 : 1 mixture of
diastereomers.
1H NMR (400 MHz, CDCl3): δ 7.32–7.41 (5H, m), 5.26 (1H,
d, J = 12.1 Hz), 5.20 (1H, d, J = 12.1 Hz), 4.75–4.68 (1H, m),
1H NMR (400 MHz, CDCl3): δ 7.40–7.31 (5H, m), 5.47 (1H, 4.54–4.48 (1H, m), 3.56 (1H, dd, J = 11.7, 5.3 Hz), 3.33 (1H, dd,
dd, J = 6.3, 1.6 Hz), 5.27–5.17 (3H, m), 4.68–4.59 (1H, m), J = 11.2, 7.5 Hz) 2.90 (1H, dd, J = 16.9, 5.5 Hz), 2.60 (1H, dd, J =
4.40–4.33 (1H, m), 3.71 (1H, dd, J = 11.4, 5.5 Hz), 3.64 (1H, dd, 16.7, 8.5 Hz), 2.48 (1H, ddd, J = 17.6, 8.2, 4.6 Hz), 2.33 (1H,
J = 11.4, 6.8 Hz), 2.58 (1H, ddd, J = 14.8, 6.4, 1.4 Hz), 2.46 (1H, ddd, J = 17.6, 8.5, 4.1 Hz); 13C NMR (100 MHz, CDCl3): δ 174.9,
ddd, J = 13.6, 7.3, 1.8 Hz), 2.36 (1H, ddd, J = 14.7, 7.1, 1.4 Hz), 157.4, 135.6, 128.8, 128.7, 128.5, 80.1, 68.6, 55.9, 44.4, 39.1,
2.22 (1H, ddd, J = 13.6, 6.7, 1.8 Hz); 13C NMR (100 MHz, 37.7; IR vmax 2961, 1732, 1714, 1454, 1423, 752, 698 cm−1
;
CDCl3): δ 157.0, 145.7, 135.6, 128.8, 128.7, 128.5, 128.3, MS (ESI) m/z 336 [M + Na]+; HRMS (ESI-TOF) m/z [M + Na]+
91.2, 90.3, 80.8, 56.2, 56.0, 44.8, 44.7, 43.6, 39.3, 39.1; calculated for C14H1635ClNO5Na: 336.0615; found: 336.0612;
IR vmax 3034, 2955, 1454, 1741, 7752, 698 cm−1; MS (ESI) m/z [α]D22 +13.1 (c 2.0, CH2Cl2).
309 [M + Na]+; HRMS (ESI-TOF) m/z [M + Na]+ calculated
for C13H1635ClNO4Na: 308.0666; found: 308.0667; [α]2D2 −10.4
(3R,5R)-Benzyl 3-(2-(2-(2-(benzyloxy)-2-oxoethyl)-
2-methylhydrazinyl)-2-oxoethyl)-5-(chloromethyl)isoxazolidine-
2-carboxylate (11)
(c 8.8, CH2Cl2).
Dimethylformamide (0.50 mL) was added to a solution of acid
(9) (1.52 g, 4.84 mmol) and oxalyl chloride (0.741 mL,
8.65 mmol) in dichloromethane (15 mL). The mixture was
stirred for 0.5 h. The volatiles were removed in vacuo and the
residue was taken up in dichloromethane (83 mL). Hydrazine
(10) (1.12 g, 5.60 mmol) followed by pyridine (0.700 mL,
8.65 mmol) were added and the mixture was stirred for 24 h.
The volatiles were removed in vacuo and the residue was puri-
fied by flash chromatography on silica gel eluting with (30%
MeOH–EtOAc) to give hydrazide (11) (1.65 g, 70%) as a dark
brown oil.
(3S,5R)-3-Allyl-2-benzyloxycarbonyl-5-chloromethylisoxazolidine
(7a)
Boron trifluoride diethyl etherate (1.78 mL, 14.2 mmol) was
added dropwise to a solution of (6a) (1.56 g, 5.46 mmol) and
allyltrimethylsilane (2.61 mL, 16.4 mmol) in anhydrous
dichloromethane (80 mL) at −70 °C. The mixture was stirred at
−40 °C for 24 h. The mixture was quenched with triethylamine
(2.0 mL) at −40 °C, then allowed to warm to room temperature.
Water was added, and the mixture was extracted with dichloro-
methane (3 × 20 mL). The combined organic extracts were
washed with brine, dried over anhydrous sodium sulphate, fil-
tered and concentrated in vacuo. The residue was purified by
flash chromatography on silica gel eluting with (3–4% EtOAc–
hexane) to give isoxazolidine (7a) (1.42 g, 88%) as a clear
yellow oil.
1H NMR (400 MHz, CDCl3): δ 7.38–7.35 (10H, m), 5.20–5.14
(4H, m), 4.74–4.68 (1H, m), 4.49–4.47 (1H, m), 3.73 (1H, d, J =
18.0 Hz), 3.68 (1H, d, J = 18.0 Hz), 3.52 (1H, dd, J = 12.1,
5.7 Hz), 3.32 (1H, dd, J = 11.4, 8.2 Hz), 2.74 (3H, s), 2.47 (1H,
dd, J = 14.3, 6.4 Hz), 2.32 (1H, dd, J = 14.3, 6.4 Hz), 2.39 (2H,
m); 13C NMR (100 MHz, CDCl3): δ 170.5, 167.8, 157.4, 135.8,
135.6, 135.4, 128.9, 128.8, 128.7, 128.6, 80.3, 68.5, 66.7, 57.6,
44.5, 39.8, 37.2; IR vmax 2955, 1714, 1668, 1385 cm−1; MS (ESI)
m/z 512 [M + Na]+; HRMS (ESI-TOF) m/z [M + Na]+ calculated
for C24H2835ClN3O6Na: 512.1564; found: 512.1560; [α]2D2 +42.5
(c 11.3, CH2Cl2).
1H NMR (400 MHz, CDCl3): δ 7.41–7.31 (5H, m), 5.78 (1H,
ddt, J = 12.1, 6.9, 3.7 Hz), 5.23 (1H, d, J = 12.4 Hz), 5.16 (1H, d,
J = 12.4 Hz), 4.48–4.45 (1H, m), 4.38–4.36 (1H, m), 3.54 (1H,
dd, J = 11.2, 5.5 Hz), 3.29 (1H, dd, J = 11.4, 7.8 Hz), 2.47 (2H,
ddd, J = 16.9, 14.2, 6.8 Hz), 2.34–2.36 (2H, m); 13C NMR
(100 MHz, CDCl3): δ 157.7, 135.9, 133.8, 128.7, 128.6, 128.5,
118.4, 80.2, 68.2, 58.7, 44.5, 39.1, 36.7; IR vmax 3066, 3032,
2954, 1703, 1641, 993, 918, 754, 698 cm−1; MS (ESI) m/z 297
[M + H]+; HRMS (ESI-TOF) m/z [M + Na]+ calculated for
C15H1835ClNO3Na: 318.0873; found: 318.0878; [α]2D2 +93.5
(c 3.5, CH2Cl2).
Benzyl 2-((3R,5R)-5-azidomethyl-2-(benzyloxycarbonyl)-
isoxazolidinyl)acetate (12)
n-Bu4N+N3 (0.0820 g, 0.330 mmol) was added to a stirred
−
solution of chloride (11) (0.080 g, 0.163 mmol) in tetrahydro-
furan (5 mL) at 25 °C. The mixture was heated at reflux for
24 h. The solvent was removed in vacuo and the residue was
purified by flash chromatography on silica gel eluting with
2-((3R,5R)-2-Benzyloxycarbonyl-5-(chloromethyl)isoxazolidinyl)-
acetic acid (9)
Ruthenium trichloride (0.9 mg, 0.00440 mmol) was added to (10% MeOH–EtOAc) to give azide (12), (0.067 g, 83%) as a dark
a solution of isoxazolidine (7a) (0.060 g, 0.20 mmol), and brown oil.
sodium metaperiodate (0.175 g, 0.82 mmol) in chloroform
1H NMR (400 MHz, CDCl3): δ 7.39–7.33 (10H, m), 5.26–5.14
(0.40 mL), acetonitrile (0.40 mL), water (0.60 mL). The mixture (4H, m), 4.70–4.65 (1H, m), 4.41–4.17 (1H, m), 3.73 (1H, d, J =
was stirred vigorously for 2 h at room temperature. Dichloro- 18.0 Hz), 3.68 (1H, d, J = 18.0 Hz), 3.32 (1H, dd, J = 7.1, 4.6 Hz),
methane (2 mL) was added and the phases were separated. 3.15 (1H, dd, J = 8.7, 4.6 Hz), 2.75 (3H, s), 2.45 (1H, dd, J =
The aqueous layer was extracted with dichloromethane 14.2, 6.4 Hz), 2.32 (1H, dd, J = 14.4, 6.6 Hz), 1.64 (2H, m); 13C
(3 × 10 mL). The combined organic extracts were washed with NMR (100 MHz, CDCl3): δ 174.8, 169, 155, 157.4, 135.6, 135.4,
brine, dried over anhydrous magnesium sulphate, filtered and 135.2, 128.9, 128.8, 128.7, 128.6, 80.0, 68.6, 62.6, 55.9, 48.9,
concentrated in vacuo to give carboxylic acid (9) (0.06 g, 96%) 44.4, 39.1, 37.7; IR vmax 3242, 3065, 3034, 2957, 2104, 1745,
as a bright orange-yellow oil.
1693, 1497, 745, 698; MS (ESI) m/z 519 [M + Na]+; HRMS
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(ESI-TOF) m/z [M + Na]+ calculated for C24H2835ClN6O6Na: 117.9, 8.2, 64.0, 34.6, 25.6, 18.1, −5.7; IR vmax 3078, 2922,
519.1968; found: 519.1968; [α]2D2 +23.3 (c 2.5, CH2Cl2).
1738, 1462, 1188 cm−1; MS (ESI) m/z 284 [M + Na]+; HRMS
(ESI-TOF) m/z [M
Na]+ calculated for C19H27NO4SiNa:
+
(+)-Negamycin (1)
384.1607; found: 384.1608.
A solution of azide (12) (24.1 mg, 0.0485 mmol) in a
2 : 1 mixture of methanol and 5% aqueous acetic acid
(2.28 mL) containing 10% palladium on charcoal (4 mg) in a
( )-O-((1-t-Butyldimethylsilyloxy)pent-4-en-2-yloxy)-hydroxylamine
(4b)
Fisher-Porter tube was placed under H2 (40 psi) and stirred at 1H NMR (400 MHz, CDCl3): δ 5.82 (1H, m), 5.35 (2H, brs), 5.06
room temperature for 24 h. The mixture was then filtered (2H, m), 3.68–3.58 (3H, m), 2.30–2.27 (2H, m), 0.89 (9H, s),
through a wet celite pad washing with water. The volatiles were 0.05 (3H, s); 13C NMR (100 MHz, CDCl3): δ 134.9, 116.8, 84.0,
removed in vacuo and the residue was purified by ion-exchange 63.7, 34.5, 25.9, 18.3, −5.4; IR vmax 3318, 3250, 2954, 2928,
chromatography on amberlite CG-50 (NH4 resin), eluting 1641, 1256, 837 cm−1; MS (ESI) m/z 254 [M + Na]+; HRMS
+
with water followed by 0.5% NH4OH (aq.). Ninhydrin-active (ESI-TOF) m/z [M + H]+ calculated for C11H25NO2Si: 232.1733;
fractions were collected and concentrated in vacuo to give found: 232.1733.
(+)-negamycin (1) (11.3 mg, 94%) as colorless powder.
( )-Benzyl 1-(t-butyldimethylsilyloxy)pent-4-en-2-yloxycarbamate
Mp = 114–117 °C (lit. mp: 110–120 °C);1a 1H NMR
(5b)
(400 MHz, D2O): δ 3.84–3.79 (1H, m), 3.27–3.22 (1H, m), 3.23
(2H, s), 2.85 (1H, dd, J = 13.2, 3.5 Hz), 2.70 (1H, dd, J = 13.2, 1H NMR (400 MHz, CDCl3): δ 7.75 (1H, brs), 7.35–7.37 (5H, m),
9.0 Hz), 2.46 (3H, s), 2.40 (1H, dd, J = 14.6, 6.4 Hz), 2.15 (1H, 5.87 (1H, ddt, J = 10.3, 7.0, 5.9 Hz), 5.16 (2H, s), 5.12 (1H, dd,
dd, J = 14.5, 7.3 Hz), 1.48–1.37 (2H, m); 13C NMR (100 MHz, J = 3.3, 1.6 Hz), 5.08 (1H, dd, J = 3.3, 1.6 Hz), 3.88 (1H, m), 3.73
D2O): δ 177.1, 170.9, 65.6, 61.0, 45.2, 44.9, 43.8, 41.2, 39.6; [α]2D3 (2H, m), 2.38–2.33 (2H, ddd, J = 6.2, 2.7, 1.4 Hz), 0.90 (9H, s),
+2.5 (c 2.0, H2O) lit: [α]2D9 +2.5 (c 2.0, H2O).1a
0.07 (6H, s); 13C NMR (100 MHz, CDCl3): δ 157.4, 135.7, 134.0,
128.5, 128.3, 128.2, 117.2, 85.3, 67.3, 64.0, 34.2, 25.8, 18.1,
−5.5; IR vmax 3271, 3069, 2954, 1732, 1454, 1254, 837,
Tetrabutylammonium azide11
A solution of NaN3 (1.00 g, 15.4 mmol) in water (2.3 mL) was 777 cm−1; MS (ESI) m/z 388 [M + Na]+; HRMS (ESI-TOF) m/z
added to tetrabutylammonium hydroxide (40% aqueous solu- [M + Na]+ calculated for C19H31NO4SiNa: 388.1920; found:
tion, 2.00 g, 7.69 mmol). Dichloromethane (12 mL) was added, 388.1926.
and the organic layer was separated. The aqueous layer was
extracted with dichloromethane (3 × 4 mL). The combined
organic layers were dried over anhydrous magnesium sulphate,
( )-2-Benzyloxycarbonyl-5-hydroxymethyl-3-methoxyisoxazolidine
(6b)
filtered and concentrated to give the salt as a white crystalline
solid (2.10 g, 96%).
Less polar diastereomer. 1H NMR (400 MHz, CDCl3):
δ 7.39–7.35 (5H, m), 5.42 (1H, dd, J = 5.9, 1.8 Hz), 5.25 (1H, d,
J = 12.2 Hz), 5.19 (1H, d, J = 12.2 Hz), 4.55–4.49 (1H, m),
3.69–3.57 (1H, m) 3.37 (3H, s), 3.29–3.20 (1H, m), 2.55 (1H,
brs), 2.34–2.20 (2H, m); 13C NMR (100 MHz, CDCl3): δ 157.7,
IR vmax 2014 cm−1
.
Synthesis of the hydroxy analogs
In all cases, the procedures used were the same as for the 135.6, 135.4, 128.7, 128.4, 91.3, 81.5, 68.7, 61.9, 55.9, 36.3.
chloro compounds. All compounds in the hydroxy series are
racemic.
More polar diastereomer. 1H NMR (400 MHz, CDCl3):
δ: 7.38–7.35 (5H, m), 5.46–5.41 (1H, m), 5.24 (1H, d, J =
12.4 Hz), 5.20 (1H, d, J = 12.4 Hz), 4.23 (1H, m), 3.67 (1H, m),
3.87 (1H, m), 3.40 (3H, s), 2.47 (1H, m), 2.23 (1H, ddd, J = 14.5,
( )-1-(t-Butyldimethylsilyloxy)pent-4-en-2-ol (2b)
1H NMR (400 MHz, CDCl3): δ 5.84 (1H, ddt, J = 17.4, 10.1, 7.7, 2.2 Hz), 3.40 (1H, brs), 2.08 (1H, brs); 13C NMR (100 MHz,
7.2 Hz), 5.07–5.14 (2H, m), 3.70 (1H, m), 3.63 (1H, dd, J = 10.1, CDCl3): δ 157.0, 135.4, 128.7, 128.6, 128.2, 91.3, 82.1, 68.3,
3.7 Hz), 3.46 (1H, dd, J = 10.1, 6.9 Hz), 2.40 (1H, d, J = 3.6 Hz), 62.0, 56.0, 36.5; HRMS (ESI-TOF) m/z [M + Na]+ calculated for
2.24 (2H, m), 0.90 (9H, s), 0.07 (6H, s); 13C NMR (100 MHz, C13H17NO5Na: 290.1004; found: 290.1005; MS (ESI) m/z 290
CDCl3): δ 134.4, 117.3, 71.1, 66.5, 37.6, 25.7, 25.6, 18.2, −5.4; [M + Na]+.
IR vmax 3423, 3076, 2954, 2928, 1641, 1258, 837, 777 cm−1
;
( )-3-Allyl-2-benzyloxycarbonyl-5-(hydroxymethyl)isoxazolidine
(7b)
1H NMR (400 MHz, C6D6): δ 7.32–7.38 (5H, m), 5.59 (1H, ddt,
J = 17.1, 10.4, 6.9 Hz), 4.90 (2H, s), 4.86 (2H, m), 4.04 (1H, m),
3.87 (1H, m), 3.17 (1H, m), 2.93 (2H, ddd, J = 12.8, 7.6, 4.9 Hz),
2.16 (1H, m), 1.89 (1H, m), 1.38 (1H, m), 1.29 (1H, m);
MS (ESI) m/z 217 [M + H]+; HRMS (ESI-TOF) m/z [M + H]+ calcu-
lated for C11H24O2Si: 217.1624; found: 217.1621.
The obtained data are consistent with literature values.11
( )-2-((1-t-Butyldimethylsilyloxy)pent-4-en-2-yloxy)isoindoline-
1,3-dione (3b)
1H NMR (400 MHz, CDCl3): δ 7.79–7.82 (2H, m), 7.70–7.74 (2H, 13C NMR (100 MHz, CDCl3): δ 159.0, 135.6, 134.1, 128.7, 128.8,
m), 5.93 (1H, m), 5.14 (2H, m), 4.38 (1H, m), 3.90 (1H, m), 3.78 128.7, 128.5, 118.2, 81.7, 68.5, 62.1, 59.6, 39.4, 34.3; MS (ESI)
(1H, m), 2.52 (2H, m), 0.76 (9H, s), −0.03 (3H, s), −0.06 (3H, s); m/z 300 [M + Na]+; HRMS (ESI-TOF) m/z [M + H]+ calculated
13C NMR (100 MHz, CDCl3): δ 163.8, 134.2, 133.2, 129.1, 123.3, for C15H19NO4: 278.1392; found: 278.1387.
This journal is © The Royal Society of Chemistry 2014
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Paper
Organic & Biomolecular Chemistry
( )-Benzyl 3-allyl-5-chloromethyl(isoxazolidine-2-carboxylate) (11)
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drous dichloromethane (5 mL) containing pyridine (1.14 μL,
0.0141 mmol) at 0 °C. The mixture was warmed to room temp-
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lowed by ice water. The mixture was extracted with
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were washed with sodium bicarbonate, dried over anhydrous
magnesium sulphate, filtered and concentrated in vacuo. The
residue was purified by flash chromatography on silica gel
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(6.10 mg, 58%) as a colorless clear oil, identical to the material
prepared earlier.
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8 Carreira has recently revived a proposal that chlorine sub-
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Acknowledgements
Financial support from Nanyang Technological University is
gratefully acknowledged.
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4884 | Org. Biomol. Chem., 2014, 12, 4879–4884
This journal is © The Royal Society of Chemistry 2014