Synthesis of hydroxylated bicyclic amino acids from L-tyrosine: Octahydro-1H-indole carboxylates

Article abstract of DOI:10.1021/jo801552j

(Chemical Equation Presented) A stereoselective approach to polyhydroxylated L-Choi derivatives has been developed. The oxidative cyclization of L-tyrosine was optimized to avoid partial racemization and to allow a more efficient scale-up.

Full text of DOI:10.1021/jo801552j

(Figure 1). Parkacine (4) is an alkaloid from Amaryllis bel-
ladonna Var. blanda BrunsVigia josephinae. The significance
of these fused bicyclic scaffolds for biologically active natural
products has inspired the development of novel chemical
libraries.5
Synthesis of Hydroxylated Bicyclic Amino Acids
from L-Tyrosine: Octahydro-1H-indole
Carboxylates
Joshua G. Pierce, Dhanalakshmi Kasi, Makoto Fushimi,
Anthony Cuzzupe, and Peter Wipf*
Department of Chemistry and Center for Chemical
Methodologies & Library DeVelopment, UniVersity of
Pittsburgh, Pittsburgh, PennsylVania 15260
pwipf@pitt.edu
ReceiVed July 21, 2008
FIGURE 1. Natural products containing L-Choi-like cores.
A stereoselective approach to polyhydroxylated L-Choi
derivatives has been developed. The oxidative cyclization
of L-tyrosine was optimized to avoid partial racemization
and to allow a more efficient scale-up.
Our initial studies toward L-Choi derivatives centered around
the oxidative cyclization of L-tyrosine previously developed in
our laboratory.3o,6 Although this oxidation was originally
(3) (a) Ersmark, K.; Del Valle, J. R.; Hanessian, S. Angew. Chem., Int. Ed.
2008, 47, 1202. (b) Nie, X.; Wang, G. Tetrahedron 2008, 64, 5784. (c) Hanessian,
S.; Ersmark, K.; Wang, X.; Del Valle, J. R.; Blomberg, N.; Xue, Y.; Fjellstroem,
O. Bioorg. Med. Chem. Lett. 2007, 17, 3480. (d) Ishida, K.; Christiansen, G.;
Yoshida, W. Y.; Kurmayer, R.; Welker, M.; Valls, N.; Bonjoch, J.; Hertweck,
C.; Boerner, T.; Hemscheidt, T.; Dittmann, E. Chem. Biol. 2007, 14, 565. (e)
Hanessian, S.; Del Valle, J. R.; Xue, Y.; Blomberg, N. J. Am. Chem. Soc. 2006,
128, 10491. (f) Hanessian, S.; Tremblay, M.; Petersen, J. F. W. J. Am. Chem.
Soc. 2004, 126, 6064. (g) Ohshima, T.; Gnanadesikan, V.; Shibuguchi, T.; Fukuta,
Y.; Nemoto, T.; Shibasaki, M. J. Am. Chem. Soc. 2003, 125, 11206. (h)
Hanessian, S.; Tremblay, M. Org. Lett. 2004, 6, 4683. (i) Valls, N.; Vallribera,
M.; Font-Bardia, M.; Solans, X.; Bonjoch, J. Tetrahedron: Asymmetry 2003,
14, 1241. (j) Valls, N.; Vallribera, M.; Carmeli, S.; Bonjoch, J. Org. Lett. 2003,
5, 447. (k) Toyooka, N.; Okumura, M.; Himiyama, T.; Nakazawa, A.; Nemoto,
H. Synlett 2003, 55. (l) Hanessian, S.; Margarita, R.; Hall, A.; Johnstone, S.;
Tremblay, M.; Parlanti, L. J. Am. Chem. Soc. 2002, 124, 13342. (m) Valls, N.;
Vallribera, M.; Lopez-Canet, M.; Bonjoch, J. J. Org. Chem. 2002, 67, 4945. (n)
Valls, N.; Lopez-Canet, M.; Vallribera, M.; Bonjoch, J. Chem.sEur. J. 2001,
7, 3446. (o) Wipf, P.; Methot, J.-L. Org. Lett. 2000, 2, 4213. (p) Valls, N.;
Lopez-Canet, M.; Vallribera, M.; Bonjoch, J. J. Am. Chem. Soc. 2000, 122,
11248. (q) Bonjoch, J.; Catena, J.; Isabal, E.; Lopez-Canet, M.; Valls, N.
Tetrahedron: Asymmetry 1996, 7, 1899.
(4) (a) Karanjule, N. S.; Markad, S. D.; Shinde, V. S.; Dhavale, D. J. Org.
Chem. 2006, 71, 4667. (b) Fuertes, M. J.; Kaur, J.; Deb, P.; Cooperman, B. S.;
Smith, A. B., III Bioorg. Med. Chem. Lett. 2005, 15, 5146. (c) Kadlecikova, K.;
Dalla, V.; Marchalin, S.; Decroix, B.; Baran, P. Tetrahedron 2005, 61, 4743.
(d) Mmutlane, E. M.; Harris, J. M.; Padwa, A. J. Org. Chem. 2005, 70, 8055.
(e) Nie, X.; Wang, G. J. Org. Chem. 2005, 70, 8687. (f) Gravier-Pelletier, C.;
Maton, W.; Bertho, G.; Le Merrer, Y. Tetrahedron 2003, 59, 8721. (g) EI Nemr,
A. Tetrahedron 2000, 56, 8579. (h) Carretero, J. C.; Arrayas, R. G. J. Org.
Chem. 1998, 63, 2993. (i) Carretero, J. C.; Gomez Arrayas, R. J. Org. Chem.
1995, 60, 6000. (j) Burgess, K.; Henderson, I. Tetrahedron 1992, 48, 4045. (k)
Reymond, J.; Pinkerton, A. A.; Vogel, P. J. Org. Chem. 1991, 56, 2128. (l)
Doepke, W. Arch. Pharm. 1963, 296, 725.
Aeruginosin 298-A (1, Figure 1) is a serine protease inhibitor
that was isolated from a fresh water blue green algae in 1994.1
Approximately 16 members of the aeruginosin family have been
identified, and 14 share the unusual bicyclic amino acid L-Choi
[(2S,3aS,6R,7aS)-2-carboxy-6-hydroxyoctahydroindole] or a close
derivative as a common motif.2 L-Choi is an example of the
growing class of sterically encumbered proline analogues that
exert a profound influence on the secondary structure when
embedded in oligopeptide sequences. Several synthetic ap-
proaches to Choi and aeruginosin natural products have been
reported.3
Many other natural and synthetic compounds contain fused
bicyclic scaffolds that closely resemble Choi.4 The naturally
occurring di-, tri-, and tetrahydroxylated indolizidine alkaloids,
lentiginosine (an amyloglucosidase inhibitor), swainsonine (an
R-mannosidase inhibitor), and castanospermine (2, an R-glu-
cosidase inhibitor; analogue 34f) are particularly noteworthy
(1) Murakami, M.; Okita, Y.; Matsuda, H.; Okino, T.; Yamaguchi, K.
Tetrahedron Lett. 1994, 35, 3129.
(2) (a) Carroll, A. R.; Pierens, G. K.; Fechner, G.; de Leone, P.; Ngo, A.;
Simpson, M.; Hyde, E.; Hooper, J. N. A.; Bostroem, S.-L.; Musil, D.; Quinn,
R. J. J. Am. Chem. Soc. 2002, 124, 13340. (b) Ploutno, A.; Shoshan, M.; Carmeli,
S. J. Nat. Prod. 2002, 65, 973. (c) Ishida, K.; Okita, Y.; Matsuda, H.; Okino,
T.; Murakami, M. Tetrahedron 1999, 55, 10971. (d) Kodani, S.; Ishida, K.;
Murakami, M. J. Nat. Prod. 1998, 61, 1046. (e) Sandler, B.; Murakami, M.;
Clardy, J. J. Am. Chem. Soc. 1998, 120, 595. (f) Shin, H. J.; Matsuda, H.;
Murakami, M.; Yamaguchi, K. J. Org. Chem. 1997, 62, 1810. (g) Matsuda, H.;
Okino, T.; Murakami, M.; Yamaguchi, K. Tetrahedron 1996, 52, 14501. (h)
Murakami, M.; Ishida, K.; Okino, T.; Okita, Y.; Matsuda, H.; Yamaguchi, K.
Tetrahedron Lett. 1995, 36, 2785.
(5) Doi, T.; Hoshina, Y.; Mogi, H.; Yamada, Y.; Takahashi, T. J. Comb.
Chem. 2006, 8, 571.
(6) For initial reaction development, see: (a) Wipf, P.; Kim, Y. Tetrahedron
Lett. 1992, 33, 5477. For applications in total synthesis, see: (b) Wipf, P.; Spencer,
S. R. J. Am. Chem. Soc. 2005, 127, 225, and references therein.
10.1021/jo801552j CCC: $40.75
Published on Web 09/04/2008
2008 American Chemical Society
J. Org. Chem. 2008, 73, 7807–7810 7807

SCHEME 1. Oxidative Cyclization of L-Tyrosine
SCHEME 2. Synthesis of Polyhydroxylated Hydroindoles
TABLE 1. Optimization of Methanolysis Procedure To Produce 7
entry
conditions
ratioa
ee (%)a
1
2
3
4
5
6
7
8
Na2CO3 (1 equiv), rt, 3 h
NaHCO3 (2 equiv), rt, 14 h
NaHCO3 (2 equiv), MWI, 80 °C, 20 min 7:6, 6:1
Na2HPO4 (1 equiv), rt, 27 h
NaOAc (1 equiv), rt, 12 h
Li2CO3 (1 equiv), rt, 16 h
Cs2CO3 (1 equiv), rt, 5 min
i-Pr2NEt (1 equiv), rt, 19 h
DMAP (1 equiv), rt, 20 min
NaOMe (1 equiv), -78 °C, 80 min
NaOMe (1 equiv), -25 °C, 14 h
3 M KOH/H2O, -20 °C, 10 min
7 only
7 only
53
67
12, 2
diastereomers was achieved by formation of their hydrochloride
salts in methanol, followed by selective precipitation to generate
12 as a single diastereomer in 47% yield based on 10.
The configuration at the C4 and C5 positions of 11 was
8:6, 3.6:1 93, 86
8:6, 4.6:1 92, 89
7 only
7 only
7 only
50
51
58
1
initially assigned based on H NMR analysis; specifically, the
hydrogen atoms were assigned by COSY and HMQC, and the
relative configuration was determined in 1H NMR double
resonance experiments from the coupling constants J4,5 ) 3.5
Hz and J5,6 ) 2.4 Hz for 11 (Figure 2). Among the C6-R-
hydroxy isomers 13 and 11, only 11 provided a good agreement
between the dihedral angles of the minimized structure and these
coupling constants.8 This assignment was further confirmed by
an X-ray analysis of the N-alkylated derivative 14. In the solid
state, this compound adopted a conformation closely analogous
to the calculated structure of 11 shown in Figure 2, with dihedral
angles τ HC6-C5H ) -63.3° and τ HC5-C4H ) 53.7°. It is
interesting to note that the osmate in the dihydroxylation step
favors an approach from the concave R-face in spite of the syn-
orientation to the equatorial secondary alcohol.9 This result
demonstrates the role of the tertiary alcohol at the bridgehead
position in controlling the anti-approach of osmium tetraoxide
to the allylic double bond.
The synthesis of the C6-epimer 15 began with a reduction of
enone 7 with L-Selectride at -78 °C to give the axial alcohol
in 77% yield and in >95:5 selectivity when the reagent was
added by syringe pump (Scheme 3).3o,6,10 Subsequent dihy-
droxylation of the diol gave 16 as a single diastereomer by 1H
NMR analysis. Tetrol 16 was subsequently deprotected to
provide hydroindole 17.
9
8:6, 2.3:1 97, 93
10
11
12
8:6, 9:1
7 only
7 only
99, 99
87
97
a Determined by HPLC analysis of crude reaction mixtures using a
Chiracel AD-H column; individual yields were not determined. MWI:
microwave irradiation.
performed in methanol and subsequently nitromethane, a 3:1
mixture of acetonitrile:isopropanol has proven more successful
upon scale-up (Scheme 1). This optimized procedure led to
spirocycle 6 in 42% yield and >99% ee. Subsequent metha-
nolysis of the spirocycle was performed according to a literature
procedure.6 While at first not immediately apparent, we came
to realize that the conversion of the spirocycle 6 to hydroxy-
hydroindole 7 occurred in a mediocre 53% ee (Table 1, entry
1). This partial racemization is likely to occur at the lactone
stage prior to methanolysis. We screened a variety of bases and
measured product ratios and enantiomeric excess by chiral
HPLC analysis. Stronger bases and/or extended reaction times
gave products with further deteriorated enantiomeric purity,
while weak bases promoted only spirocycle opening to generate
hydroxydienone 8. Gratifyingly, lowering the reaction temper-
ature and using NaOMe had a beneficial effect, providing 7 in
87% ee (Table 1, entry 11). Addition of water and switching
the base to KOH not only improved the ee further to 97%, but
also decreased the reaction time to 10 min and increased the
yield to 80%.7 Utilizing this protocol, 7 could be prepared in
overall 34% yield and 97% ee from L-tyrosine on a 50 g scale.
After identifying a stereoselective protocol to the desired
hydroindole 7, our focus turned toward the synthesis of
hydroxylated L-Choi derivatives. Realizing the potential to
generate a number of stereoisomers from the common inter-
mediate 7, we first subjected the enone to NaBH4 at 0 °C to
provide in 73% yield the axial alcohol 9, which upon dihy-
droxylation led to tetrol 10 in 1.4:1 facial selectivity and 78%
yield (Scheme 2).3o,6 The Cbz group was removed by hydro-
genolysis to yield a mixture of amines (11), and separation of
In contrast to 11, the differentiation of the two possible
diastereomers of 17 by coupling constant analysis was not
possible since the two energy minimized structures had very
similar dihedral angles. Fortunately, sulfonylation of 17 provided
the crystalline material 18 which enabled the unambiguous
(8) Conformational analysis of tetrahydroxy hydroindole scaffolds was
performed with MOE 2005.06/MMFF94 force field/10 000 step stochastic
conformational search. Only the lowest-energy conformations and the corre-
sponding dihedral angles are shown for each diastereomer.
(9) In general, the double bond in allylic alcohols is dihydroxylated on the
face opposite to the hydroxyl group, mainly driven by repulsive steric
interactions: (a) Cha, J. K.; Christ, W. J.; Kishi, Y. Tetrahedron 1984, 40, 2247.
(b) Cha, J. K.; No-Soo, K. Chem. ReV. 1995, 95, 1761. See also: (c) Donohoe,
T. J. Synlett 2002, 1223. However, there is very limited precedence on the facial
selectivity of dihydroxylation of anti-1,4-cyclohexenediols; for the cis-1,4-diol
configuration the preferred stereoselectivity is also anti: (d) Carless, H. A. J.;
Busia, K.; Oak, O. Z. Synlett 1993, 672.
(10) For selectivities of reducing compounds analogous to 7 with NaBH4,
Dibal-H, and L-Selectride, see: Hanessian, S.; Guillemette, S.; Ersmark, K. Chimia
2007, 61, 361.
(7) Quenching with aqueous HCl at-20 °C was essential for high ee’s.
7808 J. Org. Chem. Vol. 73, No. 19, 2008

SCHEME 4. Reductive Amination of Polyhydroxylated
L-Choi Scaffolds
FIGURE 2. Conformational analysis of 13 and 11 and stereoview of
the X-ray structure of 14.
SCHEME 3. Synthesis of Polyhydroxylated Hydroindoles
and Stereoview of the X-ray Structure of 18
tions to both intermediates 12 and 17, we produced tertiary
amines bearing thiophene (19), cyclohexyl (14), cyclopropyl
(20), and pyridine (21) hetero- and carbocyclic ring functionality
(Scheme 4). These conversions illustrate the broad substituent
diversity that can be introduced with this transformation.
In conclusion, we have developed an improved protocol for
our tyrosine oxidation-methanolysis methodology that not only
provides a higher yield but also prevents the potential for
deterioration of enantiomeric excess that had previously gone
unnoticed. The conversion of this hydroindole scaffold to ring-
and side-chain-functionalized L-Choi derivatives was achieved
in a stereoselective and high-yielding sequence, and the relative
configuration of all derivatives was confirmed through NMR
and X-ray analyses. The biological evaluation of this class of
compounds is currently in progress.
Experimental Section
Oxidative Spirocyclization of Tyrosine. (S)-Benzyl 2,8-dioxo-
1-oxaspiro[4.5]deca-6,9-dien-3-ylcarbamate (6). To a solution of
PhI(OAc)2 (39.1 g, 119 mmol) in MeCN/i-PrOH (4:1, 130 mL) was
added a solution of Cbz-tyrosine (25.0 g, 79.3 mmol) in MeCN/i-
PrOH (4:1, 150 mL) dropwise over 1 h. The reaction mixture was
stirred for an additional 2 h, quenched with saturated aq NaHCO3 (500
mL), and extracted with EtOAc (2 × 500 mL). The combined organic
layers were washed with NaHCO3 (3 × 500 mL) and brine, dried
(MgSO4), and concentrated. A heterogeneous mixture of the crude
residue in 80% EtOAc/hexanes (400 mL) was filtered though a pad
of SiO2. The resulting solution was concentrated and purified by
chromatography on SiO2 (45% EtOAc/hexanes to 60% EtOAc/
hexanes). The orange solid was recrystallized from hot EtOAc/hexanes
(at -20 °C overnight after being dissolved in boiling solvent) to yield
9.57 g (39%) of spirocycle 6 as colorless needles: [R]D -26.0 (c 1.08,
CH2Cl2); IR (CH2Cl2) 3060, 2956, 1789, 1710, 1674, 1635, 1523, 1198
cm-1; 1H NMR δ 7.53-7.20 (m, 5 H), 6.86 (br d, 2 H, J ) 7.5 Hz),
6.43-6.20 (m, 2 H), 5.59 (br s, 1 H), 5.14 (br s, 2 H), 4.70-4.54 (m,
1 H), 2.74 (dd, 1 H, J ) 11.1, 10.2 Hz), 2.49 (app t, 1 H, J ) 12.3
Hz); 13C NMR δ 184.2, 173.8, 156.1, 146.3, 144.4, 135.8, 129.8, 129.2,
128.8, 128.6, 128.3, 67.7, 50.5, 38.0; EIMS m/z 313 (M+, 15), 269
(15), 226 (15), 107 (95), 91 (100); HRMS (EI) m/z calcd for
C17H15NO5 313.0950, found 313.0940.
assignment of the relative configuration through X-ray analysis.
On the basis of our previous result, it was not surprising that
the approach of the osmium reagent occurred opposite to the
cis-1,4-diol functionality, exclusively from the R-face of alkene
15.
After establishing a facile approach to hydroxylated L-Choi
derivatives, we pursued a reductive amination as a way to install
diverse functionality onto the secondary amine. To this end, a
number of conditions were screened to achieve high yields in
the amination. MP-cyanoborohydride resin11 was found to be
the most effective reducing agent, which we attributed to its
greater stability in methanol. Applying these optimized condi-
(11) Le Bourdonnec, B.; Goodman, A. J.; Graczyk, T. M.; Belanger, S.; Seida,
P. R.; DeHaven, R. N.; Dolle, R. E. J. Med. Chem. 2006, 49, 7290.
J. Org. Chem. Vol. 73, No. 19, 2008 7809

Indoline Scaffold. (2S,3aR,7aR)-1-Benzyl 2-methyl 3a-hy-
droxy-6-oxo-3,3a,7,7a-tetrahydro-1H-indole-1,2(2H,6H)-dicar-
boxylate (7). To a solution of KOH (3.0 M in H2O, 200 mL) and
MeOH (200 mL) at -20 °C was added a solution of 6 (6.48 g,
20.7 mmol) in 150 mL of MeOH in one portion. The reaction
mixture was allowed to stir at -20 °C for 20 min during which
time the solution became brown in color. The mixture was quenched
by addition of 10% HCl (100 mL), extracted with EtOAc (3 ×
150 mL), washed with brine, dried (MgSO4), filtered, and concen-
trated. The crude residue was purified by chromatography on SiO2
(60% EtOAc/hexames to 80% EtOAc/hexanes) to yield 5.68 g
(80%) of methyl ester 7 as a colorless waxy solid: [R]D -130 (c
1.0, CH2Cl2); IR (CH2Cl2) 3034, 2953, 1756, 1686, 1415, 1351,
H), 3.87-3.82 (m, 1.5 H), 3.71 (s, 1.5 H), 3.56 (s, 1.5 H), 2.92
(app ddd, 1 H, J ) 13.8, 10.8, 1.2 Hz), 2.37-2.29 (m, 0.5 H),
2.25-2.18 (m, 0.5 H), 2.15 (app dd, 1 H, J ) 14.4, 0.6 Hz), 1.63
(ddd, 0.5 H, J ) 13.8, 10.8, 2.4 Hz), 1.59 (ddd, 0.5 H, J ) 13.8,
10.8, 2.4 Hz); 13C NMR (methanol-d4, 150 MHz, rotamers) δ 175.3,
174.9, 156.6, 156.0, 138.0, 137.8, 129.5, 129.4, 129.1, 129.0, 129.0,
128.9, 82.3, 81.3, 73.6, 70.6, 70.5, 68.2, 68.1, 63.6, 63.4, 58.8, 58.7,
52.8, 52.7, 38.5, 37.4, 32.1, 31.3; ESIMS m/z 404 ([M + Na]+,
100), 365 (15); HRMS (ESI) m/z calcd for C18H23NO8Na (M +
Na) 404.1321, found 404.1305.
Typical Procedure for Reductive Amination. (2S,3aS,4S,5S,6R,7aR)-
Methyl 3a,4,5,6-tetrahydroxy-1-(pyridin-3-ylmethyl) octahydro-1H-
indole-2-carboxylate (21). To a solution of 17 (50.0 mg, 0.202 mmol)
in MeOH (2 mL) were added acetic acid (57.9 µL, 1.01 mmol, 5
equiv), pyridine-3-carboxaldehyde (29.1 µL, 0.303 mmol, 1.5
equiv), and MP-cyanoborohydride resin (2.34 mmol/g, 2.5 equiv,
216 mg, 0.506 mmol). The reaction mixture was stirred at rt for
48 h, filtered, neutralized with 2 M NH3 in MeOH, concentrated in
vacuo, and purified by chromatography on SiO2 (short plug, 5%
MeOH/EtOAc to 10% MeOH/EtOAc) to yield 51.0 mg (75%) of
21 as a colorless oil: [R]D -32.6 (c 0.95, CH2Cl2); IR (CH2Cl2)
1
1124 cm-1; H NMR (DMSO-d6, 380 K) δ 7.43-7.26 (m, 5 H),
6.76 (d, 1 H, J ) 10.2 Hz), 5.90 (d, 1 H, J ) 10.5 Hz), 5.40 (br s,
1 H), 5.18-5.03 (m, 2 H), 4.50 (dd, 1 H, J ) 9.3, 3.0 Hz), 4.24
(dd, 1 H, J ) 9.3, 5.7 Hz), 3.61 (s, 3 H), 2.93 (dd, 1 H, J ) 15.9,
5.4 Hz), 2.62 (dd, 1 H, J ) 16.2, 9.6 Hz), 2.58 (dd, 1 H, J ) 13.2,
9.3 Hz), 2.29 (ddd, 1 H, J ) 13.2, 3.0, 0.6 Hz); 13C NMR (DMSO-
d6, 380 K) δ 195.4, 170.9, 153.0, 148.8, 136.1, 127.7, 127.6, 127.2,
126.9, 126.9, 73.9, 65.8, 63.7, 58.0, 51.0; ESI-MS m/z 368 ([M +
Na]+, 50), 302 (10); HRMS (ESI) m/z calcd for C18H19NO6Na (M
+ Na) 368.1110, found 368.1121.
Typical Procedure for the Dihydroxylation of 1,4-Dihydroxy-
2-cyclohexenes. (2S,3aS,4S,5S,6R,7aR)-1-Benzyl 2-methyl 3a,4,5,6-
tetrahydroxyhexahydro-1H-indole-1,2(2H,3H)-dicarboxylate (16).
To a solution of 15 (1.00 g, 2.88 mmol in THF (25 mL)) and water
(2.5 mL) were added methanesulfonamide (307 mg, 3.17 mmol,
1.1 equiv), NMO-H2O (1.19 g, 8.64 mmol, 3.0 equiv), and osmium
tetraoxide (0.3 M in toluene, 747 mg, 0.288 mmol, 0.960 mL),
and the reaction mixture was stirred at rt for 48 h. After this time,
additional osmium tetraoxide (0.3 M in toluene, 747 mg, 0.288
mmol, 0.960 mL) was added and the mixture was stirred for an
additional 72 h. The solution was quenched with Na2SO3, diluted
with H2O and EtOAc, extracted with EtOAc (4×), washed with
brine, dried (MgSO4), and concentrated. The crude residue was
purified by chromatography on SiO2 (EtOAc to 5% MeOH/EtOAc)
to yield 810 mg (74%) of tetraol 16 as a colorless waxy solid: [R]D
-33.4 (c 0.44, MeOH); IR (CH2Cl2) 3410, 3063, 2952, 2901, 1686,
1419, 1216, 1056 cm-1; 1H NMR (methanol-d4, 600 MHz, rotamers)
δ 7.42-7.26 (m, 5 H), 5.18 (d, 0.5 H, J ) 12.6 Hz), 5.15 (d, 0.5
H, J ) 12.6 Hz), 5.14 (d, 0.5 H, J ) 12.6 Hz), 4.99 (d, 0.5 H, J )
12.6 Hz), 4.41 (app ddd, 1 H, J ) 19.8, 10.2, 1.2 Hz), 4.12-4.04
(m, 1 H), 3.96 (app dd, 1 H, J ) 16.8, 3.6 Hz), 3.92-3.87 (m, 0.5
3319, 2949, 1730, 1432, 1365, 1211, 1063 cm-1 1H NMR
;
(methanol-d4) δ 8.40 (br s, 1 H), 8.30 (d, 1 H, J ) 3.9 Hz), 7.78
(d, 1 H, J ) 7.5 Hz), 7.27 (dd, 1 H, J ) 7.5, 4.8 Hz), 3.92-3.64
(m, 5 H), 3.52-3.40 (m, 1 H), 3.44 (s, 3 H), 3.30-3.21 (m, 1 H),
2.87 (dd, 1 H, J ) 13.8, 10.5 Hz), 1.80-1.61 (m, 3 H); 13C NMR
(methanol-d4) δ 177.4, 150.5, 149.0, 139.1, 136.7, 125.3, 81.8, 74.6,
74.2, 69.7, 66.0, 62.6, 52.5, 51.1, 39.6, 27.5; ESI-MS m/z 339 ([M
+ H]+, 10), 321 (100), 261 (30), 246 (40); HRMS (ESI) m/z calcd
for C16H22N2O6 (M + H) 339.1556, found 339.1544.
Acknowledgment. This work has been supported by the NIH/
NIGMS CMLD program (GM067082) and, in part, by R01-
AI33506. J.G.P. would like to thank the Mellon Foundation for
a predoctoral fellowship. We thank Dr. Steve Geib (University
of Pittsburgh) for X-ray crystallographic analyses.
Supporting Information Available: Experimental proce-
dures for compounds 9,10, 12, 14, 15, and 17-20, copies of
1H and 13C NMR spectra for 6, 7, 9, 10, 12, and 14-21, and
CIF files for 14 and 18. This material is available free of charge
via the Internet at http://pubs.acs.org.
JO801552J
7810 J. Org. Chem. Vol. 73, No. 19, 2008