(+)-Zwittermicin A. rapid assembly of C9-C15 and a formal total synthesis

Article abstract of DOI:10.1021/jo901007v

(Chemical Equation Presented) A short, enantioselective synthesis of the C9-C15 portion of (+)-zwittermicin A is reported that exploits directional functionalization of the known hepta-2,5-diyne-1,7-diol by partial reduction of the two triple bonds follow

Full text of DOI:10.1021/jo901007v

pubs.acs.org/joc
(+)-Zwittermicin A. Rapid Assembly of C9-C15 and
a Formal Total Synthesis
Evan W. Rogers† and Tadeusz F. Molinski*,†,§
†Department of Chemistry and Biochemistry and Skaggs School of Pharmacy, and §Pharmaceutical Sciences,
University of California, San Diego, 9500 Gilman Dr., La Jolla, California 92093
tmolinski@ucsd.edu
Received May 18, 2009
A short, enantioselective synthesis of the C9-C15 portion of (+)-zwittermicin A is reported that
exploits directional functionalization of the known hepta-2,5-diyne-1,7-diol by partial reduction of
the two triple bonds followed by Sharpless asymmetric epoxidation and boron-directed double ring-
opening with sodium azide under Miyashita conditions. Subsequent desymmetrization of the
C2-symmetric diazidotetraol product converges upon (-)-3;the enantiomer of the key intermediate
of our earlier structural proof and synthesis of (-)-zwittermicin A;and constitutes a formal
synthesis of (+)-zwitttermicin A.
Introduction
that B. cereus and other bacteria that produce 1 are ubiqui-
tous in soil5 suggesting 1 may constitute a benign biocontrol
alternative to pest management strategies currently based on
synthetic pesticides. The rising interest in 1 as a “green”
biopesticide, particularly in China, has stimulated studies of
its unique biosynthesis and mechanism of action.6 Despite
appearances, the biosynthesis of (+)-1 does not originate in
carbohydrate metabolism, but from a nonribosomal peptide
synthetase/polyketide synthase pathway (NRPS/PKS) in-
volving two newly described type 1 PKS extender units:
hydroxymalonyl-acyl carrier protein (ACP) and amino-
malonyl-ACP.6 Zwittermicin A has proved difficult to iso-
late in substantial quantities due to its highly polar, charged
nature at physiological pH and sensitivity to alkaline condi-
tions.1 For example, until recently the purification of (+)-1
required application of the arcane technique of prepara-
tive paper electrophoresis,1 although purifications have
been achieved more recently by reversed phase HPLC.6e
(+)-Zwittermicin A (1) is a highly polar, water-soluble
aminopolyol antibiotic isolated from the soil-born bacter-
ium Bacillus cereus1 with significant activity against phyto-
pathogenic fungi.1,2 More importantly, 1 enhances the
activity of the endotoxin from Bacillus thuringensis, the active
ingredient in biocontrol agents used against crop pests3 and
gypsy moth Lymantria dispar,4 which annually defoliates
millions of hectares of forests in the North Eastern United
States of America. Handelsman and co-workers have shown
*To whom correspondence should be addressed. Phone: +1 858-534-
7115. Fax: +1 858-822-0368.
(1) (a) He, H.; Silo-Suh, L. A.; Handelsman, J.; Clardy, J. Tetrahedron
Lett. 1994, 35, 2499–2502. (b) Silo-Suh, L. A.; Lethbridge, B. J.; Raffel, S. J.;
He, H.; Clardy, J.; Handelsman, J. Appl. Environ. Microbiol. 1994, 60, 2023–
2030.
(2) (a) Silo-Suh, L. A.; Stabb, E. V.; Raffel, S. J.; Handelsman, J. Curr.
Microbiol. 1998, 37, 6–11. (b) Milner, L.; Stohl, E. A.; Handelsman, J.
J. Bacteriol. 1996, 178, 4266–4272. (c) Stohl, E. A.; Brady, S. F.; Clardy, J.;
Handelsman, J. J. Bacteriol. 1999, 181, 5455–5460.
(3) Liu, C.-L.; MacMullan, A. M.; Lufburrow, P. A.; Starnes, R. L;
Manker, D. C. U.S. Patent 5,976,563, Nov. 2, 1999.
(6) (a) Stabb, E. V.; Handelsman, J. Mol. Microbiol. 1998, 27, 311–322.
(b) Stohl, E. A.; Milner, J. L.; Handelsman, J. Gene 1999, 237, 403–411.
(c) Emmert, E. A.; Kilmowicz, A. K.; Thomas, M. G.; Handelsman, J. Appl.
Environ. Microbiol. 2004, 70, 104–113. (d) Chan, Y. A.; Boyne, M. T. II;
Podevels, A. M.; Klimowicz, A. K.; Handelsman, J.; Kelleher, N. L.;
Thomas, M. G. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 14349–14354.
(e) Zhao, C.; Luo, Y.; Song, C.; Liu, Z.; Chen, S.; Yu, Z.; Sun, M. Arch.
Microbiol. 2007, 187, 313–319.
(4) (a) Broderick, N. A.; Goodman, R. M.; Handelsman, J.; Raffa, K. F.
Environ. Entomol. 2003, 32, 387–391. (b) Broderick, N. A.; Goodman, R. M.;
Raffa, K. F.; Handelsman, J. Environ. Entomol. 2000, 29, 101–107.
(5) (a) Stabb, E. V.; Jacoboson, L. M.; Handelsman, J. Appl. Environ.
Microbiol. 1994, 60, 4404–4412. (b) Raffel, S. J.; Stabb, E. V.; Milner, J. L.;
Handelsman, J. Microbiology 1996, 142, 3425–3436.
7660 J. Org. Chem. 2009, 74, 7660–7664
Published on Web 09/11/2009
DOI: 10.1021/jo901007v
r
2009 American Chemical Society

Rogers and Molinski
JOCArticle
SCHEME 1. Retrosynthetic Analysis of Zwittermicin A (+)-1
SCHEME 2. Synthesis of Protected Diazidotetrol 7
Zwittermicin A [(+)-1] has not yet been synthesized. Con-
temporary interest in (+)-1 and the difficulty in obtaining
the compound from natural sources underscore an outstand-
ing need for practical syntheses of (+)-1.
We recently determined the absolute stereostructure of
(+)-1 based on amino acid configurational analysis of the
corresponding acid-hydrolysate (6 M HCl, 110 °C),7 deduc-
tive reasoning from systematic 13C NMR spectroscopic
comparisons of (+)-1 with diastereomeric models (e.g.,
L-2) constituting C9-C15 of 1,7 and the total synthesis of
the unnatural enantiomer, (-)-1.8 The differentially pro-
tected key intermediate, (+)-3, was synthesized in 17 steps
from L-serine.8 At the time, the lengthy route from the
arbitrarily chosen L-enantiomer of serine was dictated by
the need for all six stereoisomeric 2,6-diaminoheptane-
1,3,5,7-tetraols (e.g., 2) for structure elucidation of (+)-1.
As it turned out, natural (+)-1 is correlated with D-serine.
Here, we outline an improved asymmetric route to the
pseudo-C2 symmetric C9-C15 segment successfully conver-
ging upon key intermediate (-)-3, which may be employed
for the synthesis of (+)-1 by using chemistry parallel to that
we previously described for (-)-1.8
The known diepoxide 5 was prepared in two steps from 4
according to Hoffmann and co-workers12 by using a double
Sharpless asymmetric epoxidation,10 which sets the absolute
configuration depicted by using (+)-diethyl tartrate as the
auxiliary (lit. 98% ee12). Although not previously reported as
such, compound 5 was found to be crystalline and conve-
niently purified by recrystallization. Treatment of the diep-
oxide 5 with NaN3 in the presence of trimethylborate under
Miyashita conditions (B(OMe)3, DMF, 50 °C, Scheme 2)11
gave the C2 symmetric diazidotetraol 6 in 80% yield as a
mixture of isomers (10:1.1:1).
At first, the unexpectedly high water-solubility of 6 proved
troublesome for purification. After some experimentation it
was found most efficient to concentrate the crude product
until almost dry, followed by trituration (ꢀ2);first with
MeOH, then with a mixture of MeOH and CH2Cl2;and
finally recrystallization from MeOH to give 6 as a diaster-
eomerically pure crystalline solid.
Results and Discussion
Conceptually, synthesis of 1 takes advantage of the nom-
inal C2 axial symmetry embodied within 3 and bidirectional
assembly from known diyne 4 that constitutes the intact
C9-C15 carbon skeleton (Scheme 1).9
The route exploits asymmetric reagent control over the
achiral starting material, 4, with an intact C9-C15 carbon
skeleton, and capitalizes upon highly enantioselective double
Sharpless epoxidation10 and a double regioselective Miya-
shita-type opening of epoxide rings in 5 by 2 equiv of
NaN3, which directs the nitrogen groups to C2 and C6.11
One disadvantage of this route, which only became apparent
during the course of investigations, was unexpectedly high
water-solubility of a key intermediate that precluded stan-
dard extractive workup by solvent-partitioning, and neces-
sitated a different isolation-purification strategy (vide infra).
It was desirable to desymmetrize 6 by mono-O-protection
at the primary OH group with a reductively labile protecting
group that could be later cleaved, simultaneously, with
hydrogenolysis of the azide groups to reveal the NH2 groups.
Selective O-benzylation of the primary OH groups would
satisfy these criteria. Successful desymmetrization of diols
with Ag2O/BnBr has been reported,13 but in our hands these
(7) Rogers, E. W.; Molinski, T. F. Org. Lett. 2007, 9, 437–440.
(8) Rogers, E. W.; Dalisay, T. F.; Molinski, T. F. Angew. Chem., Int. Ed.
2008, 47, 8086–8089.
(9) Cessac, J. W.; Rao, P. N.; Kim, H. K. Amino Acids 1994, 6, 97–105.
(10) Katsuki, T.; Sharpless, K. B. J. Am. Chem. Soc. 1980, 102, 5974–
5976.
(11) (a) Hayakawa, H.; Okada, N.; Miyazawa, M.; Miyashita, M. Tetra-
hedron Lett. 1999, 40, 4589–4592. (b) Sasaki, M.; Tanino, K.; Hirai, A.;
Miyashita, M. Org. Lett. 2003, 5, 1789–1791.
(12) Hoffmann, R. W.; Kahrs, B. C.; Schiffer, J.; Fleischhauer, J. J. Chem.
Soc., Perkin Trans. 2 1996, 2407–2414.
ꢀ
(13) Bouzide, A.; Sauve, G. Tetrahedron Lett. 1997, 38, 5945–5948.
J. Org. Chem. Vol. 74, No. 20, 2009 7661

Rogers and Molinski
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TABLE 1. Optimization of Desymmetrization of 6 by Monotritylation
gave slightly better conversion of 7, and the highest yield of 9
(73%, entry 6) while minimizing equilibration to the internal
acetonide 10.15
entry no.
TrCl (equiv)
temp (°C)
time (h)
yields 7:8a (%)
1
2
3
1.0
0.8
0.8
50
23
60
4
17
69
54:19
54:17
69:14
After protection of the secondary hydroxyl group in 9
as the MOM ether 12 (90% yield),16 simultaneous removal
of the O-trityl protecting group and reduction of the two
N3 groups was effected by hydrogenation (Pd-C, H2, 7 atm,
TFA/CF3CH2OH) to give the TFA double salt of diamine
13,17 which was converted without purification to the bis-N,
N-dibenzylamine (-)-3 (BnBr, K2CO3, CH3CN, 47%, over
two steps).18 The product (-)-3 was identical in all respects
to (+)-3 except for optical rotation, which was opposite in
sign ((-)-3, [R] -25.2 (c 2.8 CHCl3) {lit.8 (+)-3, [R]21D +28.8
(c 2.01, CHCl3)}). Since we had previously demonstrated
transformation of (+)-3 to (-)-ent-zwittermicin A [(-)-1],8
the foregoing constitutes a formal synthesis of the natural
product, (+)-1.
aIsolated yields.
SCHEME 3. Synthesis of (-)-3
Some comments on the synthesis of (-)-3 are in order. The
target is rapidly assembled from the achiral diyne 4 with
introduction of asymmetry in the form of known diepoxide 5
obtained through a conventional Sharpless epoxidation.
Critical for the success of this rapid target assembly is the
regioselective introduction of the two nitrogens by strategic
use of a double epoxide opening of 5 with NaN3 under
Miyashita conditions. The regioselectivity of the Miyashita
conditions generally favors a rare endo mode ring-opening at
C2 through a metal-chelated intermediate.11b,19 The isomer
ratio obtained by azide attack at C2 or C3 of the epoxyalco-
hol is substrate dependent; C2 attack gives 2-azido-1,3-diols
with regioselectivity that increases with substitution at C4 by
bulky groups, but particularly oxy substituents (e.g., BnO,
silyloxy). In our earlier synthesis of stereoisomers of 2 by a
linear approach involving opening of differentially protected
mono-epoxyalcohols,7 we found stereoisomers behaved dif-
ferently toward NaN3-B(OMe)3. The highest ratio of C-2 to
C-3 azide attack was found for an unwanted syn-3,5 isomer
(9:1), but the anti-3,5 isomer required for the natural relative
configuration of 1 gave a lower ratio (7:3).7 In the case of
diepoxide 5, with an anti-3,5 configuration, the regioselec-
tivity of highly directed double-epoxide ring-opening (∼10:1,
azide attack at C2 and C6) appears to be amplified by two
factors: the presence of the second epoxide ring in 5 and
enhanced preference of azide attack at C2 in the intermediate
alkoxy azide formed from the first ring-opening. In the latter
case, with an unencumbered C5 substituent (1,2-oxirane),
the directing effect is less likely to be simply steric in nature,
but may be dependent on secondary dipole interactions.
Since this reaction is useful for installation of vicinal N,O-
functional groups in asymmetric synthesis of other biologi-
cally important aminoalkanols (e.g., phytosphingosines20),
the substitution patterns that determine regioselectivity of
epoxyalcohol ring-opening under Miyashita conditions are
deserving of further study.
conditions were unsatisfactory with 6 and led to inseparable
mixtures of products. It was found that more efficient mono-
O-protection of the C1 hydroxyl (Scheme 2) could be
achieved with a trityl group.
A survey (Table 1) of reaction conditions found that
optimized yields of monoprotected diazidotetraol 7 (69%
yield) were obtained with 0.8 equiv of chlorotriphenylme-
thane in pyridine (entry 3).14 The undesired doubly tritylated
side product 8 was recycled to 7 upon treatment with acid
(CF3COOH, MeOH, 48% yield).
Protection of the 1,3-diol OH groups in triol 7 was achie-
ved by conversion to an external acetonide 9 (Scheme 3).
Optimization of the ratio of terminal and internal acetonides
required a fine balance of reaction temperature, limited
reagent equivalencies, acid catalyst, and time (Table 2).
Reaction of 7 with excess 1,1-dimethoxypropane and acet-
one at 50 °C (entry 1) led to complete loss of starting
material, but gave an unacceptable mixture of 9, the internal
acetonide 10, and the monomethyl mixed acetal 11. Treat-
ment of 7 with 2.5 equiv of 2-methoxypropene at 0 °C to
room temperature returned only starting material; however,
a higher temperature (50 °C) and use of 2.0 equiv of reagent
gave good conversion and a high ratio of the desired com-
pound (9:10:11 = 64:8:14, entry 7). Optimal conditions were
found with slightly more 2-methoxypropene (2.5 equiv) that
(16) (a) Zhai, H.; Liu, P.; Luo, S.; Fang, F.; Zhao, M. Org. Lett. 2002, 4,
ꢀ
4385–4386. (b) Narasaka, K.; Sakakura, T.; Uchimaru, T.; Guedin-Vuong,
D. J. Am. Chem. Soc. 1984, 106, 2954–2961.
(17) Mirrington, R. N.; Schmalzl, K. J. J. Org. Chem. 1972, 37, 2877–
2881.
(18) Yamazaki, N.; Kibayashi, C. J. Am. Chem. Soc. 1989, 111, 1396–
1408.
(19) In the absence of B(OMe)3, the predominant mode of ring opening
gives 3-azido-1,2-diols as the major products.
(20) Howell, A. R.; Ndakala, A. J. Curr. Org. Chem. 2002, 6, 365–391.
(14) Greene, T. W.; Wuts, P. G. M. Protective Groups in Organic Synth-
esis, 3rd ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 1999.
(15) (a) Evans, M. E.; Parrish, F. W.; Long, L., Jr. Carbohydr. Res. 1967,
3, 453–462. (b) Grieco, P. A.; Yokoyama, Y.; Withers, G. P.; Okuniewicz, F.
J.; Wand, C.-L. J. J. Org. Chem. 1978, 43, 4178–4182. (c) Fnton, E.; Gelas, J.;
Horton, D. J. Chem. Soc., Chem. Commun. 1980, 21–22.
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Rogers and Molinski
JOCArticle
TABLE 2. Conversion of Diol 7 to External Acetonides 9 and 11 and Internal Acetonide 10 in DMF
yields of 9,
10, 11 (%)b
entry no.
reagent
equiv
catalyst
temp (°C)
time (h)
recd 7 (%)
1
2
3
4
5
6
7
a
Me2C(OMe)2-acetonea
CH2dC(OMe)CH3c
CH2dC(OMe)CH3
CH2dC(OMe)CH3
CH2dC(OMe)CH3
CH2dC(OMe)CH3
CH2dC(OMe)CH3
xs
2.5
2.5
2.0
2.0
2.5
2.0
d
PPTS
PPTS
TsOHc
TsOH
CSA
50
4
36
28
2
20
4
24
0
0
0
100
100
0 to 23
0 to 23
0 to 23
0 to 23
50
f
0
-
d,f
47: 18: 0
73: 17: 0
64: 8: 14
-
e,f
PPTS
PPTS
-
f
50
4
-
b
c
e
f
˚
1:1, no DMF. Isolated yields. Mol sieves 4 A. Partial loss of Tr group. Some remaining 7. Not determined
In conclusion, (-)-3 was synthesized in 10 steps from
commercially available materials. This represents a substan-
tially shorter route (seven fewer steps) than our previous
synthesis of (þ)-3 from L-serine. The rapid assembly of the
C9-C15 portion of (þ)-1 was made possible through three
efficient bidirectional functionalizations, starting with a
symmetrical diyne, with stereo- and regiocontrol of N and
O addition reactions. This formal synthesis of zwittermicin A
[(þ)-1] validates a route for procurement of quantities of the
natural product for ongoing investigations of its biological
properties.
Method 2. Under an atmosphere of nitrogen, a solution of
diol 8 (38 mg, 52 μmol) in MeOH was adjusted to pH 2 with TFA
and stirred at room temperature for 10 h. The mixture was
quenched with triethylamine (0.5 mL) and concentrated under
reduced pressure. Separation of the mixture by flash chroma-
tography (silica, 1:1 EtOAc-hexane, then 1:4 MeOH-CH2Cl2)
provided 7 (12.2 mg, 48%) and recovered 8 (7.6 mg, 20%) as
viscous oils along with some 6.
7: IR (neat) ν 3349, 3086, 3058, 3032, 2928, 2883, 2094, 1658,
1595, 1489, 1448, 1317, 1264, 1218, 1072, 1031, 900, 855, 747
cm-1; [R]21 -21.6 (c 6.25, CHCl3); 1H NMR (500 MHz,
D
CDCl3) δ 7.47-7.44 (m, 6H), 7.34-7.30 (m, 6H), 7.25 (tt, J =
7.2, 1.2 Hz, 3H), 4.00-3.93 (m, 2H), 3.81, (m, 2H), 3.49-3.44
(m, 2H), 3.40-3.35 (m, 2H), 1.59 (m, 2H); 13C NMR (125 MHz,
CDCl3) δ 143.4 (C), 128.7 (CH), 128.2 (CH), 127.5 (CH), 87.8
(C), 69.0 (CH), 68.9 (CH), 66.4 (CH), 65.5 (CH), 63.7 (CH2),
62.5 (CH2), 35.4 (CH2); HRESIMS m/z 511.2067 [M þ Na]þ,
calcd for C26H28N6O4Na1 511.2064.
Experimental Section
General Experimental Information. Details of general proce-
dures can be found in the Supporting Information.
(2R,3S,5S,6R)-2,6-Diazidoheptane-1,3,5,7-tetraol (6). Under
an atmosphere of nitrogen, trimethylborate (1.56 mL, 1.43 g,
13.7 mmol) was added to a solution of 5 (550 mg, 3.43 mmol) in
anhydrous DMF (17.2 mL). The solution was stirred for 30 min
at room temperature then NaN3 (893 mg, 13.7 mmol) was added
and the reaction was heated to 40 °C and stirred for 4 h then
heated to 50 °C for a further 4 h. The reaction was cooled to
room temperature and quenched by addition of a saturated
solution of NaHCO3 (50 mL) and the solution was stirred a
further 1 h. The mixture was concentrated to dryness under
reduced pressure then treated with methanol (200 mL) and the
solids were removed by filtration. The solution was again
concentrated, followed by addition of a mixed solvent (3:2
MeOH/CH2Cl2, 200 mL) before concentration of the filtrate
under reduced pressure. Purification of the crude product by
flash chromatography (silica, gradient 1:19 to 3:2 MeOH-
CH2Cl2) followed by reversed phase chromatography (20 g of
C18-silica, 1:19 MeOH-H2O) provided 6 (672 mg, 80%, isomer
ratio 10:1.1:1 by NMR analysis) as a white solid. Recrystalliza-
tion of the solid from methanol gave pure 6 (393 mg): mp 132 °C;
IR (neat) ν 3201, 2950, 2919, 2871, 2137, 2097, 1445, 1405, 1320,
1267, 1137, 1078, 1064, 1029, 1006, 910, 862 cm-1; [R]21D þ5.3
(c 2.13, CH3OH); 1H NMR (500 MHz, CD3OD) δ 3.87 (m, 2H),
3.81 (dd, J = 11.6, 3.7 Hz, 2H), 3.60 (dd, J = 11.6, 8.1 Hz, 2H),
3.45 (m, 2H), 1.59 (dd, J = 7.8, 5.2 Hz, 2H); 13C NMR (125
MHz, CD3OD) δ 70.4 (CH), 68.5 (CH), 63.0 (CH2), 37.0
(CH2); HRMS m/z 245.1004 [M - H]-, calcd for C7H13N6O4
245.1004.
(2S,3R)-3-Azido-1-((4S,5R)-5-azido-2,2-dimethyl-1,3-dioxan-
4-yl)-4-(trityloxy)butan-2-ol (9). Under an atmosphere of nitro-
gen 2-methoxypropene (3.6 μL, 19 μmol) was added to a stirred
solution of triol 7 (9.2 mg, 19 μmol) and PPTS (0.4 mg, 2 μmol)
in DMF (100 μL) at room temperature. The mixture was heated
to 50 °C and stirred for 4 h, cooled to room temperature, and
quenched with saturated aqueous NaHCO3 (3 mL). The mixture
was extracted with ethyl acetate (3 ꢀ 4 mL) and combined
organic extracts washed with brine (3 mL), dried over Na2SO4,
and concentrated under reduced pressure. Separation of the
mixture by flash chromatography (silica, 1:9 EtOAc-hexane)
provided 9 (7.3 mg, 73%) and 10 (1.7 mg, 17%) as viscous oils. 9:
[R]22D -29.0 (c 6.56, CHCl3); IR (neat) ν 3465, 3058, 2993, 2923,
2877, 2101, 1596, 1489, 1448, 1380, 1264, 1200, 1159, 1070, 980,
898, 821, 747, 701 cm-1 1H NMR (500 MHz, CDCl3)
;
δ 7.49-7.44 (m, 6H), 7.35-7.30 (m, 6H), 7.26 (tt, J = 7.4,
1.2 Hz, 3H), 3.96 (dd, J = 11.7, 5.5 Hz, 1H), 3.94-3.84 (m,
2H), 3.69 (dd, J = 11.5, 10.0 Hz, 1H), 3.49 (m, 1H), 3.45 (dd,
J = 10.0, 4.0 Hz, 1H), 3.35 (dd, J = 10.0, 6.9 Hz, 1H), 3.24 (ddd,
J = 10.0, 10.0, 5.8 Hz, 1H), 2.60 (d, J = 7.5 Hz, OH), 1.78 (ddd,
J = 14.3, 9.5, 2.6 Hz, 1H), 1.58 (ddd, J = 14.3, 8.6, 2.3 Hz, 1H),
1.43 (s, 3H), 1.35 (s, 3H); 13C NMR (125 MHz, CDCl3) δ 143.6
(C), 128.7 (CH), 128.1 (CH), 127.4 (CH), 99.3 (C), 87.6 (C), 69.7
(CH), 68.0 (CH), 65.9 (CH), 63.6 (CH2), 62.4 (CH2), 58.0 (CH),
35.4 (CH2), 28.8 (CH3), 19.2 (CH3); HRESIMS m/z 551.2372
[M þ Na]þ, calcd for C29H32N6O4Na1 551.2377.
10: 1H NMR (500 MHz, CDCl3) δ 7.47-7.44 (m, 6H),
7.34-7.29 (m, 6H), 7.25 (tt, J = 7.2, 1.2 Hz, 3H), 3.98 (dt,
J = 8.9, 6.3 Hz, 1H), 3.91 (dt, J = 9.5, 6.0 Hz, 1H), 3.74 (br m,
1H), 3.67 (br m, 1H), 3.55 (m, 1H), 3.47 (m, 1H), 3.27 (dd, J=
10.0, 6.6 Hz, 1H), 3.22 (dd, J = 10.0, 4.8 Hz, 1H), 1.99 (br s,
OH), 1.79 (m, 1H), 1.70 (m, 1H), 1.29 (s, 3H), 1.27 (s, 3H);
LRESIMS m/z 551.20 [M þ Na]þ, calcd for C29H32N6O4Na1
551.2377.
(2R,3S,5S,6R)-2,6-Diazido-7-(trityloxy)heptane-1,3,5-triol
(7). Method 1. Under an atmosphere of nitrogen chlorotriphe-
nylmethane (104 mg, 374 μmol) was added to a stirred solution
of tetraol 6 (115 mg, 467 μmol) in pyridine (2.3 mL) at room
temperature. The mixture was heated to 60 °C and stirred for
5 h. The mixture was then concentrated under reduced pressure
and separated by flash chromatography (silica, 25 to 50% ethyl
acetate in hexane then 20% methanol in dichloromethane) to
provide 7 (125 mg, 69%) and 8 (39 mg, 14%) as viscous oils
along with recovered 6.
(4S,5R)-5-Azido-4-((2S,3R)-3-azido-2-(methoxymethoxy)-
4-(trityloxy)butyl)-2,2-dimethyl-1,3-dioxane (12). Chloromethyl
methyl ether (115 μL, 1.51 mmol) was added to a stirred
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solution of alcohol 9 (80.0 mg, 151 μmol) and Hunig’s base (500
μL, 3.03 mmol) in dichloromethane (200 μL) at 0 °C. The
mixture was warmed to room temperature and stirred for 38 h
then quenched by addition of water (5 mL). The mixture was
extracted with ethyl ether (4 ꢀ 5 mL) and combined extracts
washed with brine (5 mL), dried over Na2SO4, and concentrated
under reduced pressure. Purification of the mixture by flash
chromatography (silica, 1:9 EtOAc-hexane) provided 12 (75.2
mg, 90%) as a viscous oil: IR (neat) ν 3058, 2992, 2935, 2886,
2100, 1596, 1490, 1448, 1371, 1264, 1221, 1197, 1154, 1076, 1030,
981, 918, 808, 763, 747, 702 cm-1; [R]21D -33.6 (c 4.55, CHCl3);
1H NMR (500 MHz, CDCl3) δ 7.47-7.44 (m, 6H), 7.34-7.30
(m, 6H), 7.25 (tt, J = 7.3, 1.2 Hz, 3H), 4.63 (d, J = 6.8 Hz, 1H),
4.56 (d, J = 6.8 Hz, 1H), 3.96 (dd, J = 11.5, 5.4 Hz, 1H), 3.87 (m,
1H), 3.81 (m, 1H), 3.75-3.65 (m, 2H), 3.36 (s, 3H), 3.26 (dd, J =
10.0, 7.7 Hz, 1H), 3.15 (dd, J = 10.0, 5.2 Hz, 1H), 3.12 (dd, J =
9.7, 5.4 Hz, 1H), 1.92 (ddd, J = 14.0, 10.0, 2.0 Hz, 1H), 1.40 (s,
3H), 1.30 (s, 3H), 1.28 (m, 1H); 13C NMR (125 MHz, CDCl3) δ
143.7 (C), 128.8 (CH), 128.0 (CH), 127.3 (CH), 99.0 (C), 97.6
(CH2), 87.4 (C), 75.0 (CH), 68.5 (CH), 65.8 (CH), 63.3 (CH2),
62.5 (CH2), 58.8 (CH), 56.1 (CH3), 34.4 (CH2), 28.8 (CH3), 19.3
(CH3); HRESIMS m/z 595.2629 [M þ Na]þ, calcd for
C31H36N6O5Na1 595.2639.
stirred solution of amine 13 (<6.6 mg, 23.5 μmol) and K2CO3
(195 mg, 1.41 mmol) in anhydrous acetonitrile (550 μL) at
room temperature. The mixture was stirred for 4.5 d then
quenched by addition of water (5 mL). The mixture was ex-
tracted with ethyl acetate (4 ꢀ 4 mL) and combined extracts
washed with brine (5 mL), dried over Na2SO4, and concentrated
under reduced pressure. Purification of the mixture by
flash chromatography (silica, step gradient of 1:9 to 1:3
EtOAc-hexane) provided (-)-3 (7.0 mg, 47%) as an amorphous
solid: IR (neat) ν 3476, 3065, 3030, 2995, 2943, 2882, 2812, 1597,
1492, 1457, 1379, 1265, 1221, 1151, 1108, 1029, 977, 916, 758,
706 cm-1; [R]21 -25.2 (c 2.80, CHCl3) {lit.8 for (þ)-3 [R]21
D
D
þ28.8 (CHCl3, c 2.01)}); 1H NMR (500 MHz, CDCl3)
δ 7.40-7.23 (m, 20H), 4.77 (d, J = 6.4 Hz, 1H), 4.68 (d, J =
6.4 Hz, 1H), 4.15 (m, 1H), 4.02 (t, J = 9.6 Hz, 1H), 4.00-3.88
(m, 6H), 3.84 (d, J = 13.6 Hz, 2H), 3.70 (d, J = 13.6 Hz, 2H),
3.59 (d, J = 14.0 Hz, 2H), 3.40 (s, 3H), 3.31 (br s, 1H), 2.78-2.70
(m, 2H), 2.14 (dd, J = 13.6, 9.6 Hz, 1H), 1.90 (ddd, J = 14.8,
10.8, 2.4 Hz, 1H), 1.36 (s, 3H), 1.33 (s, 3H); 13C NMR (125 MHz,
CDCl3) δ 140.0 (C), 139.7 (C), 129.2 (CH), 128.8 (CH), 128.5
(CH), 128.4 (CH), 127.3 (CH), 127.1 (CH), 98.9 (CH), 98.7 (C),
76.2 (CH), 67.0 (CH), 62.6 (CH), 58.5 (CH2), 58.0 (CH), 57.9
(CH2), 56.5 (CH3), 54.9 (CH2), 54.8 (CH2), 38.7 (CH2), 27.9
(CH3), 20.9 (CH3); HRMS m/z 639.3791 [M þ H]þ, calcd for
C40H51N2O5 639.3793.
(2R,3S)-2-Amino-4-((4S,5R)-5-amino-2,2-dimethyl-1,3-diox-
an-4-yl)-3-(methoxymethoxy)butan-1-ol (13). Ten percent Pd/C
(6.3 mg, 5.9 μmol, 25 mol % Pd) was added to a solution of 12
(13.1 mg, 23.5 μmol) in dry trifluoroethanol (1.5 mL) and the
mixture was agitated under H2 (7 atm) for 17 h in a Parr shaker.
The mixture was adjusted to pH 4 with TFA, placed under H2
(7 atm), and agitated for a further 4.5 h on a Parr shaker.
Filtration of the mixture through a 0.45 μm syringe filter and
concentration under reduced pressure provided crude 13, which
was used without further purification.
Acknowledgment. HRMS measurements were provided
by R. New (UC Riverside) and Y. X. Su (UC San Diego).
Financial support for this work was provided by the Na-
tional Institutes of Health (RO1 AI039987).
Supporting Information Available: Description of general
1
and experimental features and H and 13C NMR spectra for
(2R,3S)-2-(Dibenzylamino)-4-((4S,5R)-5-(dibenzylamino)-2,2-
dimethyl-1,3-dioxan-4-yl)-3-(methoxymethoxy)butan-1-ol ((-)-3).
Benzyl bromide (56.3 μL, 471 μmol) was added dropwise to a
compounds (-)-3, 6, 7, 9, and 10. This material is available free
of charge via the Internet at http://pubs.acs.org.
7664 J. Org. Chem. Vol. 74, No. 20, 2009