Synthetic studies of pseurotin A: Preparation of an advanced lactam aldehyde intermediate

Article abstract of DOI:10.1039/b512701g

An efficient synthesis of the lactam core of pseurotin A has been accomplished. Key features of this synthesis include a tandem oxidation-cyclization to form the lactam from an acetylenic amide precursor. Although coupling of a lactam aldehyde with an app

Full text of DOI:10.1039/b512701g

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A R T I C L E
Synthetic studies of pseurotin A: preparation of an advanced lactam
aldehyde intermediate
Judith M. Mitchell and Nathaniel S. Finney*†
Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla,
CA, 92093-0358, USA. E-mail: finney@oci.unizh.ch
Received 7th September 2005, Accepted 23rd September 2005
First published as an Advance Article on the web 1st November 2005
An efficient synthesis of the lactam core of pseurotin A has been accomplished. Key features of this synthesis include
a tandem oxidation–cyclization to form the lactam from an acetylenic amide precursor. Although coupling of a
lactam aldehyde with an appropriate side chain was not effective, it is anticipated that incorporating a partial side
chain at an earlier stage should permit completion of the total synthesis of pseurotin A.
possess as a common structural feature a unique and highly
functionalized heterospirocyclic framework containing an N-
acyl hemiaminal and three contiguous stereogenic centers, one
Introduction
Chitin-poly-b-(1,4)-GlcNAc is an essential component of the
cell wall of virtually all infectious fungi.1 As this polymer is
of them quaternary. Pseurotin A and related natural products
absent in humans, the enzyme responsible for chitin biosynthesis
are thus both challenging and important synthetic targets.
(chitin synthase, CS) is an appealing therapeutic target. Despite
this, little progress has been made in the development of chitin
synthase inhibitors. The most extensively studied naturally
Tamm et al. have previously reported efforts directed to the
synthesis of pseurotin A,11 and recently Tadano et al. and
Hayashi et al. published the first total syntheses of pseurotin
occurring inhibitors are the polyoxins and nikkomycins,2 which
A.12–14 We report here our synthetic studies on pseurotin A
share as a common structural feature a ribosyl amino acid core.
(developed concurrently with Tadano’s and Hayashi’s work)15
While these are among the most potent known inhibitors of
CS, they have proven ineffective in vivo. A handful of other
natural products are also known to be competitive inhibitors
of CS. Among the most potent is pseurotin A (Fig. 1),
which culminate in the synthesis of a highly functionalized
lactam precursor to pseurotin A.
which is notably distinct in structure from the polyoxins and
Results and discussion
nikkomycins, and we undertook the synthesis of pseurotin A as
Retrosynthesis and synthetic overview
part of our broader program in chitin synthase inhibition.3
Retrosynthetic analysis (Scheme 1) was based on the precedent
for the formation of 2H-furan-3-ones from acyclic precursors
(e.g., B→A).11c Unraveling of the left-hand ring required a b-
diketone (B) as a precursor, which was anticipated to arise from
the corresponding b-hydroxyketone, in turn derived from an
aldol-type coupling of an aldehyde-bearing lactam (C) and an
ethyl ketone (D). This disconnection is distinct from those previ-
ously reported,11–13 all of which relied on an ethyl-ketone bearing-
lactam or lactone being coupled with an aldehyde (Scheme 2).
In principle this disconnection allows for a more convergent
synthesis of pseurotin A, although in practice it proved to
be a liability (vide infra). Lactam C was anticipated to arise
from an unprecedented intramolecular cyclization of a primary
amide onto an a-dicarbonyl (E) generated in situ via oxidation
of an alkyne (F). The required alkyne-containing amide (F)
could potentially be prepared via the desymmetrization of an
equivalent of tetraol G (rather than drawing from the chiral
pool), which was expected to derive from a simple alkene
precursor, H.
Fig. 1 Pseurotin A and related natural products. (Asterisks denote
stereocenters of unknown configuration.)
Pseurotin A was first isolated from cultures of Pseudeurotium
ovalis ssp. in 1976 and its structure was elucidated by Tamm
et al. in 1981.4 Pseurotin A, 8-O-desmethylpseurotin A and
three structurally related molecules, azaspirene, and synerazol
and FD-839, have subsequently been isolated from cultures
of Aspergillus fumigatus ssp. It was discovered in 1993 that
pseurotin A is a competitive inhibitor of chitin synthase,5 as
well as an apomorphine antagonist,6 and in 1996 it was found
to induce the proliferation of nerve cells.7 Azaspirene has since
been shown to inhibit angiogenesis,8 while synerazol and FD-
839 exhibit antifungal antibiotic activity;9,10 FD-839 is also
reported to inhibit growth in leukemia cell cultures. In addition
to this remarkable range of biological activities, these molecules
Synthesis of the lactam core
Synthesis commenced with conversion of commercially avail-
able 1-chloro-2-chloromethyl-2-propene to the corresponding
bisbenzoate (H→1, Scheme 3). Dihydroxylation under standard
conditions afforded diol 2, which was subsequently protected
as the 4-phenylcyclohexylidene acetal (3), obtained as an
inseparable and essentially indistinguishable pair of achiral
diastereomers. (This acetal was chosen for its UV activity,
in order to facilitate HPLC analysis and chromatographic
isolation.) Benzoate cleavage produced diol 4, again as an
indistinguishable mixture of diastereomers.
Direct silylation of 4 provided the corresponding TBS ether in
modest yield (42%; 83% based on recovered starting material).
† Present address: Organisch-chemisches Institut, Universita¨t Zu¨rich,
Winterthurerstrasse 190, 8057 Zu¨rich.
4 2 7 4
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 4 2 7 4 – 4 2 8 1
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
©

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of either enantiomer of the monoacyl derivative of 4.16 In
principle this allows for the enantioselective preparation of 5,
although this has not been further developed. With convenient
differentiation of diol 4 established, installation of the pheny-
lacetylene fragment was undertaken.
Swern oxidation of alcohol 5 (Scheme 4) provided aldehyde 6
and set the stage for diastereoselective phenylacetylide addition.
All attempts led to the formation of 7 as a separable mixture
of diastereomers at the new stereocenter, slightly favoring the
undesired diastereomer.17 The diastereoselectivity varied from
1 : 1.5 to 1 : 2, depending on the solvent (THF vs. Et2O,
e.g.), counterion (Li+ vs. Na+, e.g.) and presence of additive
(TMEDA, EtAlCl2, DMPU, etc.). Aldehyde 6 failed to react
under Carreira’s conditions for asymmetric acetylide addition.18
Scheme 4 Synthesis of propargylic alcohol 7.
Scheme 1 Restrosynthesis and synthesis of Pseurotin A.
Enantioselective reduction of the corresponding propargylic
ketone (8) was also explored. Among the numerous potentially
selective reductions studied were those employing (ipc)2BH and
chiral oxazaborolidine reagents.19,20 The selectivity with these
and other chiral reducing agents was not significantly better
than that of the acetylide addition, although the oxidation–
reduction sequence did allow efficient recycling of the undesired
diastereomer 7b. In the end, the diastereomers were separated
chromatographically, allowing access to 7a in diastereomerically
pure form. (Prior to determination of the relative configuration,
7a and 7b were each carried forward separately; only details for
the transformations of 7a are presented here.)
Scheme 2 The conceptual disconnection from other syntheses.
With the phenylacetylene fragment in place it was necessary
to address the conversion of the required amide precursor to
the oxidative cyclization. Alcohol 7a was first protected as the
benzyl ether, and the silyl ether was then removed by treatment
with fluoride (Scheme 5). Oxidation of alcohol 10 followed by
treatment with PyBOP and ammonium hydroxide then provided
amide 13 in good yield.21,22
Scheme 3 Synthesis of the fully differentiated tetraol 5.
Oversilylation was surprisingly facile, and it proved simplest,
operationally, to run the reaction to partial conversion and
separate the desired monosilylated product from remaining
starting material. Differentiation of the diol could also be
effected in 73% overall yield by sequential monoacylation (Ac2O,
pyridine, quant.), silylation (TBSOTf, NEt3, CH2Cl2, 99%) and
deacylation (CH3O−Na+, CH3OH, 73%). Preliminary studies
on the enantioselective desymmetrization of 4 by enzymatic
acylation have identified conditions for the selective preparation
Scheme 5 Synthesis of alkyne 5.
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 4 2 7 4 – 4 2 8 1
4 2 7 5

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This set the stage for the tandem oxidation–cyclization to form
the lactam core of pseurotin A. Oxidation of 13 with potassium
permanganate under neutral conditions afforded the desired
five-membered lactam 15 in excellent raw yield (Scheme 6).23
The intermediate a-dicarbonyl (14) was not observed, nor
was the alternative six-membered lactam. While lactam 15
proved extremely difficult to purify, treatment of crude 15 with
acidic methanol introduced the necessary methyl ether with
concomitant acetal cleavage to yield diol 16. This transformation
proceeded in variable yield, although no byproducts could be
isolated. We believe that this is due to the degradation of the
a-diketone intermediate regenerated under acidic conditions,
consistent with the instability of 15 to silica gel chromatography.
Notably, 16 was formed as a single diastereomer possessing
the natural configuration at the acetal stereocenter,24 and
this represents one of only three times that the lactam core
of pseurotin A has been prepared in fully functionalized
form.12,13
Fig. 2 X-Ray crystal structure of epi-18 (H atoms omitted for clarity).
The requisite coupling partner, ethyl ketone 23 was readily
prepared from the known aldehyde 21, which can be prepared
in five steps from D-glucose.11e Grignard reaction of ethylmag-
nesium bromide and 21 produced alcohol 22, followed by Swern
oxidation to afford 23 in good yield (Scheme 8).
Scheme 8 Preparation of the ‘left half’ ethyl ketone.
Elaboration of aldehyde lactam
With both components in hand, the crucial aldol condensation
of aldehyde 20 and ketone 23 was undertaken (Scheme 9).
However, the fragments failed to couple as anticipated. While the
instability of the aldehyde component was initially considered
as the source of the problem, it was soon determined that ketone
23 could not be induced to couple with any substrate: treatment
of ketone 23 with base under a variety of conditions led to
decomposition, even in the presence of simple electrophiles such
as benzaldehyde.
Scheme
6
Formation and cyclization of the key a-diketone
intermediate.
Conversion of diol 16 to the corresponding aldehyde was the
final obstacle before coupling with the appropriate ketone could
be attempted. Oxidation in the presence of the unprotected
tertiary hydroxyl proved problematic, and a series of protect-
ing group manipulations were required. Monoacetate 17 was
prepared in good yield by treatment of 16 with Ac2O and NEt3
(Scheme 7). Incubation of 17 with excess Ac2O and DMSO
then produced methylthiomethyl (MTM) ether 18. Subsequent
acetate cleavage afforded the desired alcohol 19, which was
oxidized to the corresponding aldehyde 20.
Scheme 9 Coupling—or lack thereof—between ketone 23 and alde-
hyde 20.
In light of this result, it was decided to modify the lactam core
so that coupling to aldehyde 21 (Scheme 8) could be attempted.
Successful aldol condensations between aldehyde 21 and various
sterically-hindered ethyl ketones were reported in the early
synthetic studies of pseurotin A,11d,e providing ample precedent
for the desired reaction. (This approach has since been used in
both reported total syntheses.)12,13 Thus, the only modification
our original synthetic route required was the addition of an
ethyl fragment to aldehyde 20 with subsequent oxidation to the
corresponding ketone (Scheme 10). However, treatment with
ethyllithium or ethylmagnesium bromide led only to recovered
starting material, even at elevated temperatures. A variety of
other nucleophiles were screened (lithium acetylide, e.g.), but in
no case was addition to 20 observed. While there is no clear
explanation for the intransigence of this aldehyde, it was clear
Scheme 7 Synthesis of aldehyde 4.
At this stage, X-ray crystallographic analysis of epi-18,
prepared from 7b via the same synthetic route as that described
for 18 from 7a, provided the long-awaited determination that
18, and thus acetylide adduct 7a, possessed the correct relative
configuration at the secondary stereogenic center (Fig. 2).
Scheme 10 Inadequacy of aldehyde 20 as an electrophile.
4 2 7 6
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 4 2 7 4 – 4 2 8 1

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that an alternate strategy involving earlier extension of the side
chain would be required for the preparation of pseurotin A.
7.52 (t, J = 7.5 Hz, 2H), 7.37 (t, J = 7.5 Hz, 4H), 4.50 (s,
4H), 3.76 (s, 2H). 13C NMR (100 MHz, CDCl3), d: 166.5, 133.1,
129.5, 129.1, 128.2, 73.5, 65.2, 63.5. FTIR (neat), cm−1: 3455(m),
3067(w), 2960(w), 1731(s), 1607(w), 1458(m), 1277(s), 1112(s).
HRMS (MALDI-FTMS), m/z: Calcd for C18H18O6 (M + Na+):
353.0996. Found: 353.0987. TLC (5% CH3OH–CH2Cl2), Rf:
0.21 (UV, KMnO4).
Conclusion
We have described here our first-generation synthetic approach
to the chitin synthase inhibitor, pseurotin A. This target is
important not only in that it is a competitive inhibitor of this
important fungal enzyme, but also in that it is structurally
distinct from the polyoxins and nikkomycins, which have proven
ineffective in treating human fungal infections. Notable elements
of our approach include the enzymatic desymmetrization of a
meso intermediate to set the quaternary stereocenter, and the
oxidation of an acyclic amide–alkyne to generate an a-diketone
which undergoes diastereoselective in situ cyclization to form the
lactam core of pseurotin A. Advanced aldehyde intermediates
based on this lactam failed to couple with even the simplest
carbanions, delineating the need for a new synthetic strategy.
The details of an alternate route, in which the side chain is
incorporated earlier in the synthesis, will be described in a
subsequent manuscript.
1,3-Dibenzoyl-(2-hydroxymethyl)-1,2,3-propanetriol, 4-phenyl-
cyclohexylidene acetal (3). Phenylcyclohexanone (7.9 g,
45 mmol 1.1 equiv.) and p-toluenesulfonic acid (1.6 g, 8.4 mmol,
0.2 equiv) were added to a solution of 2 (14 g, 41 mmol,
1 equiv.) in benzene (500 mL) in a flask equipped with a
Dean Stark apparatus and heated to reflux. After 12 hours
the reaction was cooled to room temperature and diluted with
EtOAc (500 mL), then washed with H2O (1 × 500 mL), saturated
aqueous NaHCO3 (1 × 500 mL), and saturated aqueous NaCl
(1 × 500 mL). The organic phase was dried over Na2SO4 and
concentrated under reduced pressure to afford a brown oil.
Purification of the residue by flash column chromatography (1%
CH3OH–CH2Cl2) yielded 3 (16 g, 33 mmol, 81%) as a brown oil.
3: 1H NMR (400 MHz, CDCl3), d: 8.09 (d, J = 7.0 Hz, 4H), 7.58
(t, J = 7.0 Hz, 2H), 7.46 (t, J = 7.0 Hz, 4H), 7.33–7.19 (m, 5H),
4.62–4.52 (m, 4H), 4.21–4.17 (m, 2H), 2.57 (m, 1H), 2.02–1.74
(m, 8H). 13C NMR (100 MHz, CDCl3), d: 165.7, 146.0, 145.9,
133.0, 129.5, 129.4, 129.36, 129.31, 128.3, 128.2, 128.13, 128.10,
126.7, 126.6, 125.89, 125.86, 111.0, 110.7, 80.2, 79.8, 68.2, 67.9,
65.3, 65.2, 43.2, 42.9, 36.3, 36.0, 31.5, 31.1. FTIR (neat), cm−1:
3067(w), 3034(w), 2944(m), 2861(w), 1724(s), 1607(m), 1451(m),
1268(s), 1107(s), 711(s). HRMS (MALDI-FTMS), m/z: Calcd
for C30H30O6 (M + Na+): 509.1934. Found: 509.1915. TLC (20%
EtOAc–hexanes), Rf: 0.40 (UV, DNP).
Experimental
General
All reactions were carried out in oven or flame dried glassware
under an atmosphere of nitrogen in dry solvent, except where
noted. THF and CH2Cl2 were dried by passage through an
activated column of alumina, and pyridine and acetonitrile
were distilled from CaH2. All other reagents were used as
obtained, unless otherwise stated. Thin layer chromatography
was performed on silica gel 60 (F254, 250 nm, EM Science) plates
and visualized with UV light or stained with KMnO4, dini-
trophenylhydrazine (DNP), or phosphomolybdic acid (PMA).
Flash column chromatography was performed using silica gel
(Selecto Scientific, 32–63 nm) or reverse phase (EM Science,
(2-Hydroxymethyl)-1,2,3-propanetriol, 4-phenylcyclohexyli-
dene acetal (4). Sodium methoxide (7.50 g, 139 mmol, 4.4
equiv.) was added to a solution of 3 (5.3 g, 31.5 mmol,
1 equiv.) in THF (500 mL) under N2. After 12 hours the
reaction was quenched with saturated aqueous NaCl (100 mL)
and extracted with EtOAc (3 × 400 mL), then dried over
Na2SO4 and concentrated under reduced pressure. The residue
was purified by flash column chromatography (1% CH3OH–
CH2Cl2) to afford 4 (7.0 g, 25.3 mmol, 80%) as a white solid.
1
silica gel 60, RP-18) as indicated. H-NMR data are reported
in ppm relative to the solvent (CHCl3 at 7.26 ppm); coupling
constants have been rounded to the nearest 0.5 Hz. Proton
decoupled 13C-NMR spectra are reported in ppm relative to
solvent as internal standard (CDCl3 at 77.0 ppm).
1
4: H NMR (400 MHz, CDCl3), d: 7.19–7.33 (m, 5H), 3.98 (s,
1,3-Dibenzoyl-(2-methylidene)-1,3-propanediol (1). Sodium
benzoate (15.7 g, 109 mmol, 2.5 equiv.) was added to methallyl
dichloride (5.00 mL, 43.2 mmol, 1 equiv.) in DMF (80 mL),
and the solution was heated to 80 ◦C under N2. After 12 hours
the reaction was cooled to room temperature, then quenched
slowly with saturated aqueous NH4Cl (200 mL). The reaction
was further diluted with H2O (200 mL), then extracted with Et2O
(3 × 250 mL). The combined ethereal extracts were washed with
H2O (2 × 200 mL). The organic phase was dried over Na2SO4
and concentrated under reduced pressure to yield 1 (12.8 g,
2H), 3.74 (d, J = 12.0 Hz, 1H), 3.70 (d, J = 12.0 Hz, 1H),
3.30 (br s, 1H), 2.54–2.60 (m, 1H), 1.71–1.91 (m, 8H). 13C
NMR (100 MHz, CDCl3), d: 145.8, 128.1, 126.5, 125.9, 109.8,
82.8, 67.8, 64.0, 43.0, 36.4, 31.1. FTIR (neat), cm−1: 3418(s),
2936(s), 2874(m), 1500(w), 1450(w), 1118(s), 1083(m), 1043(s).
HRMS (MALDI-FTMS), m/z: Calcd for C16H22O4 (M + Na+):
301.1410. Found: 301.1401. TLC (5% CH3OH–CH2Cl2), Rf:
0.34 (UV, DNP).
1-O-tert-Butyldimethylsilyl-(2-hydroxymethyl)-1,2,3-propane-
triol, 4-phenylcyclohexylidene acetal (5). TBSCl (0.71 g,
4.71 mmol, 1.15 equiv.) was added to a solution of diol 4
(1.23 g, 4.42 mmol, 1 equiv.) and imidazole (0.36 g, 5.29 mmol,
1.30 equiv.) in THF (100 mL) at 0 ◦C. The reaction was allowed
to warm to room temperature over 12 hours while stirring
under N2. The reaction was then diluted with EtOAc (150 mL)
and washed with saturated aqueous NaCl (2 × 200 mL). The
organic phase was dried over Na2SO4 and concentrated under
reduced pressure to afford a brown oil. Purification by flash
column chromatography (10% CH3OH–CH2Cl2) yielded 5
(0.72 g, 1.85 mmol, 41%) as a yellow oil, as well as unreacted
diol 4 (0.52 g, 1.87 mmol, 42%). 5: 1H NMR (400 MHz,
CDCl3), d: 7.33–7.19 (m, 5H), 4.06 (d, J = 9.0 Hz, 1H), 3.94 (d,
J = 9.0 Hz, 1H), 3.74–3.62 (m, 4H), 2.58–2.53 (m, 1H), 2.33
(br s, 1H), 1.97–1.70 (m, 8H). 13C NMR (100 MHz, CDCl3),
d: 146.1, 128.1, 126.6, 125.9, 109.8, 82.3, 67.9, 65.3, 64.7,
43.1, 36.6, 36.0, 31.6, 31.5, 25.9, 18.2, −5.39, −5.42. FTIR
(neat), cm−1: 3480(m), 2951(s), 2858(s), 1607(w), 1475(m),
1
43.2 mmol, ≥99%) as a brown oil. 1: H NMR (400 MHz,
CDCl3), d: 8.05 (d, J = 7.5 Hz, 4H), 7.56 (t, J = 7.5 Hz, 2H), 7.43
(t, J = 7.5 Hz, 4H), 5.46 (s, 2H), 4.97 (s, 4H). FTIR (neat), cm−1:
3067(w), 2952(w), 1723(s), 1602(m), 1452(m), 1274(s), 1110(s).
TLC (20% EtOAc–hexanes), Rf: 0.46 (UV, anisaldehyde).
1,3-Dibenzoyl-(2-hydroxymethyl)-1,2,3-propanetriol (2). To
a solution of 1 (12.8 g, 43.2 mmol, 1 equiv.) in 1 : 1 H2O–tBuOH
(800 mL) were added K3Fe(CN)6 (43.2 g, 131 mmol, 3.0 equiv.),
K2CO3 (18.0 g, 130.2 mmol, 3.0 equiv.), and K2OsO4·2H2O
(0.83 g, 2.3 mmol, <0.1 equiv.). The biphasic reaction was
degassed three times by evacuating and backfilling with N2, then
stirred at room temperature under N2. After 18 hours Na2S2O4
(150 g) was added and the reaction was stirred for another hour.
The reaction was diluted with saturated aqueous NH4Cl
(500 mL), then extracted with EtOAc (3 × 500 mL). The organic
phase was dried over Na2SO4 and concentrated under reduced
pressure to yield 2 (12.1 g, 36.9 mmol, 85%) as a yellow oil.
1
2: H NMR (400 MHz, CDCl3), d: 8.00 (d, J = 7.5 Hz, 4H),
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 4 2 7 4 – 4 2 8 1
4 2 7 7

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1252(m), 1093(s), 839(s). HRMS (MALDI-FTMS), m/z: Calcd
for C22H36O4Si (M + Na+): 415.2275. Found: 415.2255. TLC
(5% CH3OH–CH2Cl2), Rf: 0.46 (UV, DNP).
4.93 (d, J = 12.0 Hz, 1H), 4.65 (d, J = 12.0 Hz, 1H), 4.57
(s, 1H), 4.17–4.09 (m, 2H), 3.94 (d, J = 10.0 Hz, 1H), 3.73
(d, J = 10.0 Hz, 1H), 2.59–2.50 (m, 1H), 2.22–1.65 (m, 8H),
0.90 (s, 9H), 0.11–0.07 (m, 6H). 13C NMR (100 MHz, CDCl3),
d: 146.6, 146.5, 137.7, 137.5, 131.64, 131.58, 128.32, 128.29,
128.16, 128.13, 128.0, 127.9, 127.5, 126.8, 126.7, 125.8, 122.6,
110.6, 110.3, 87.5, 87.3, 85.8, 85.7, 85.3, 84.6, 71.5, 71.3,
70.7, 70.4, 67.9, 67.4, 63.2, 63.1, 43.5, 43.3, 36.6, 36.3, 35.9,
31.8, 31.71, 31.67, 31.4, 26.0, 22.8, 18.4, 14.3, −5.25, −5.28.
FTIR (neat), cm−1: 3062(w), 3029(m), 2950(s), 2929(s), 2857(s),
1615(w), 1492(m), 1374(w), 1254(m), 1090(s), 837(s). HRMS
(MALDI-FTMS), m/z: Calcd for C37H46O4Si (M + Na+):
605.3057. Found: 605.3046. TLC (10% EtOAc–hexanes), Rf:
0.52 (UV, DNP).
1-O-tert-Butyldimethylsilyl-(2-hydroxymethyl)-1,2-dihydroxy-
propan-3-al, 4-phenylcyclohexylidene acetal (6). To a −78 ◦C
solution of oxalyl chloride (0.32 mL, 3.68 mmol, 2.03 equiv.)
in CH2Cl2 (12 mL) was added DMSO (0.33 mL, 4.65 mmol,
2.57 equiv.) under N2. After ten minutes alcohol 5 (0.70 g,
1.81 mmol, 1 equiv.) was added in CH2Cl2 (8 mL) and the
mixture stirred for 45 minutes. NEt3 (0.80 mL, 5.74 mmol) was
then added, and the reaction was warmed to room temperature.
After two hours the reaction was diluted with CH2Cl2 (50 mL)
and washed with saturated aqueous NH4Cl (1 × 50 mL) and
saturated aqueous NaCl (1 × 50 mL). The organic phase was
dried over Na2SO4 and concentrated under reduced pressure
to afford aldehyde 6 (0.65 g, 1.57 mmol, 87%) as a yellow
3-O-Benzyl-(2-hydroxymethyl)-1,2,3-trihydroxy-5-phenylpent-4-
yne, 4-phenyl-cyclohexylidene acetal (10). Tetrabutylammo-
nium fluoride (5.1 mL, 5.1 mmol, 1.0 M in THF, 1.5 equiv.) was
added to a solution of silyl ether 9 (2.0 g, 3.4 mmol, 1 equiv.)
in THF (100 mL). After 12 hours the reaction was diluted
with EtOAc (100 mL) and washed with saturated aqueous
NaCl (3 × 150 mL). The organic phase was dried over Na2SO4
and concentrated under reduced pressure. Purification by
flash column chromatography (10% EtOAc–hexanes) afforded
1
oil. 6: H NMR (400 MHz, CDCl3), d: 9.78 (d, J = 4.0 Hz,
1H), 7.35–7.20 (m, 5H), 4.29–4.26 (m, 1H), 4.06–3.99 (m,
1H), 3.91–3.83 (m, 1H), 2.61–2.60 (m, 1H), 1.73–2.07 (m, 8H),
0.95–0.94 (m, 9H), 0.15–0.13 (m, 6H). 13C NMR (100 MHz,
CDCl3), d: 201.9, 146.1, 146.0, 128.18, 128.16, 126.64, 126.60,
125.93, 111.45, 111.22, 87.12, 86.7, 66.9, 66.8, 64.2, 63.9, 43.3,
43.0, 36.04, 36.03, 35.9, 35.5, 31.34, 31.30, 31.2, 25.84, 25.80,
18.34, 18.31, −5.34, −5.36, −5.40, −5.42. FTIR (neat), cm−1:
3034(w), 2944(s), 2861(s), 2713(w), 1736(s), 1607(w), 1254(s),
1094(s), 839(s). HRMS (MALDI-FTMS), m/z: Calcd for
C22H34O4Si (M + Na+): 413.2118. Found: 413.2119. TLC (20%
EtOAc–hexanes), Rf: 0.63 (UV, DNP).
1
alcohol 10 (1.1 g, 2.5 mmol, 75%). 10: H NMR (400 MHz,
CDCl3), d: 7.22–7.54 (m, 15H), 4.97 (d, J = 11.5 Hz, 1H), 4.65
(d, J = 11.5 Hz, 1H), 4.56 (s, 1H), 4.24 (d, J = 9.5 Hz, 1H),
4.13 (d, J = 9.5 Hz, 1H), 3.98–3.92 (m, 2H), 2.57–2.60 (m, 1H),
2.20–1.72 (m, 9H). 13C NMR (100 MHz, CDCl3), d: 146.20,
146.18, 137.11, 137.08, 131.7, 131.6, 128.51, 128.48, 128.3,
128.1, 127.9, 127.8, 126.7, 126.6, 125.9, 122.2, 122.1, 110.8,
110.5, 87.9, 87.7, 84.9, 84.8, 84.0, 83.4, 71.9, 71.7, 71.4, 71.3,
68.0, 67.8, 63.7, 63.6, 43.3, 43.1, 36.6, 36.3, 36.1, 31.60, 31.57,
31.3. FTIR (neat), cm−1: 3497(m), 3029(m), 2933(s), 2863(s),
2242(w), 1599(w), 1491(s), 1453(m), 1093(s), 1070(s), 936(m).
HRMS (EI), m/z: Calcd for C31H32O4 (M+): 468.2301. Found:
468.2311. TLC (20% EtOAc–hexanes), Rf:0.38 (UV, DNP).
1-O-tert-Butyldimethylsilyl-(2-hydroxymethyl)-1,2,3-trihy-
droxy-5-phenylpent-4-yne, 4-phenylcyclohexylidene acetal (7).
Lithium phenylacetylide (1.80 mL, 1.80 mmol, 1.00 M in
THF, 1.08 equiv.) was added to a solution of aldehyde 7
(0.65 g, 1.66 mmol, 1 equiv.) in THF (15 mL) under N2. After
three hours the reaction was diluted with EtOAc (50 mL)
and washed with saturated aqueous NaCl (2 × 50 mL). The
organic phase was dried over Na2SO4 and concentrated under
reduced pressure to afford a viscous brown oil. Purification by
flash column chromatography (2% EtOAc–hexanes) yielded
the two propargylic alcohol diastereomers of alcohol 7 (7a,
3-O-Benzyl-(2-hydroxymethyl)-2,3-dihydroxy-5-phenylpent-4-
ynal, 4-phenyl-cyclohexylidene acetal (11). To a −78 ◦C
solution of oxalyl chloride (0.42 mL, 4.83 mmol, 2.00 equiv.)
in CH2Cl2 (20 mL) was added DMSO (0.43 mL, 6.06 mmol,
2.51 equiv.) under N2. After ten minutes alcohol 10 (1.13 g,
2.41 mmol, 1 equiv.) was added in CH2Cl2 (10 mL) and
the mixture stirred for one hour. NEt3 (1.0 mL, 7.17 mmol,
2.98 equiv.) was then added, and the reaction was warmed to
room temperature. After two hours the reaction was diluted
with CH2Cl2 (100 mL) and washed with saturated aqueous
NaCl (3 × 100 mL). The organic phase was dried over Na2SO4
and concentrated under reduced pressure to afford aldehyde 11
1
0.18 g, 0.37 mmol, 30%; 7b, 0.49 g, 1.00 mmol, 60%). 7a: H
NMR (400 MHz, CDCl3), d: 7.20–7.34 (m, 10H), 4.70 (d, J =
9.0 Hz, 1H), 4.23 (d, J = 9.0 Hz, 1H), 4.14 (d, J = 9.0 Hz,
1H), 4.10 (d, J = 9.5 Hz, 1H), 3.66 (d, J = 9.5 Hz, 1H), 3.19
(d, J = 9.0 Hz, 1H), 2.50–2.59 (m, 1H), 1.70–2.12 (m, 8H),
0.92 (s, 9H), 0.13 (s, 6H). 13C NMR (100 MHz, CDCl3), d:
146.2, 146.1, 131.6, 131.5, 128.3, 128.2, 128.10, 128.07, 126.7,
126.6, 125.9, 122.5, 110.6, 110.3, 87.6, 87.5, 85.9, 85.8, 83.7,
82.9, 69.0, 68.7, 66.2, 65.8, 65.3, 64.9, 43.3, 43.1, 36.7, 35.9,
35.5, 31.6, 31.3, 31.2, 25.9, 18.3, −5.4. FTIR (neat), cm−1:
3447(m), 3034(m), 2953(s), 2930(s), 2858(s), 1747(w),
1615(w), 1491(m), 1254(s), 1096(s), 838(s). HRMS (MALDI-
FTMS), m/z: Calcd for C30H40O4Si (M + Na+): 515.2588.
Found: 515.2586. TLC (5% CH3OH–CH2Cl2), Rf: 0.52
(UV, DNP).
1
(1.04 g, 2.23 mmol, 93%). 11: H NMR (400 MHz, CDCl3),
d: 9.87 (d, J = 7.5 Hz, 1H), 7.49–7.18 (m, 15H), 4.94–4.89
(m, 1H), 4.66–4.59 (m, 1H), 4.34–4.28 (m, 1H), 4.13–4.07
(m, 1H), 2.61–2.55 (m, 1H), 2.28–1.65 (m, 8H). 13C NMR
(100 MHz, CDCl3), d: 202.0, 201.4, 146.1, 146.0, 131.8, 131.7,
128.9, 128.34, 128.29, 128.26, 128.22, 128.17, 128.0, 126.8,
126.7, 126.0, 121.8, 112.6, 112.3, 89.1, 88.4, 87.9, 83.2, 83.1,
71.3, 71.2, 70.9, 70.7, 68.1, 67.7, 43.4, 43.1, 36.3, 35.7, 35.2,
31.6, 31.4, 31.3, 31.2, 29.8, 24.2. FTIR (neat), cm−1: 3076(w),
3043(w), 2931(s), 2869(m), 2226(w), 1736(s), 1491(m), 1450(m),
1089(s), 1071(s). HRMS (MALDI-FTICR), m/z: Calcd for
C31H30O4 (M + Na+): 489.2036. Found: 489.2039. TLC (20%
EtOAc–hexanes), Rf: 0.50 (UV, DNP).
1-tert-Butyldimethylsilyloxy-2-benzyl-(2-hydroxymethyl)-1,2,3-
trihydroxy-5-phenylpent-4-yne, 4-phenylcyclohexylidene acetal
(9). Benzyl bromide (0.45 mL, 3.76 mmol) was added to a
solution of tetrabutylammonium iodide (0.35 g, 0.95 mmol,
1.11 equiv.), sodium hydride (0.33 g, 8.25 mmol, 60% dispersion
in oil, 2.43 equiv.), and alcohol 7a (1.67 g, 3.39 mmol, 1 equiv.)
in THF (50 mL) under N2. After five hours the reaction was
diluted with EtOAc (100 mL) and washed with saturated
aqueous NaCl (3 × 150 mL). The organic phase was dried
over Na2SO4 and concentrated under reduced pressure to
afford benzyl ether 9 (1.96 g, 3.36 mmol, 99%) as a yellow
3-O-Benzyl-(2-hydroxymethyl)-2,3-dihydroxy-5-phenylpent-4-
ynoic acid, 4-phenylcyclohexylidene acetal (12). To a solution
of aldehyde 11 (1.04 g, 2.23 mmol, 1 equiv.) in tBuOH (50 mL)
and H2O (16 mL) were added 2-methyl-2-butene (5.00 mL,
10.0 mmol, 2.0 M in THF, 4.93 equiv.), NaH2PO4 (0.38 g,
2.75 mmol, 1.23 equiv.), and sodium chlorite (0.79 g, 6.99 mmol,
1
oil. 9: H NMR (400 MHz, CDCl3), d: 7.46–7.19 (m, 15H),
4 2 7 8
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 4 2 7 4 – 4 2 8 1

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3.14 equiv.). After three hours, the reaction was diluted with
EtOAc (150 mL) and washed with saturated aqueous NaCl
(3 × 100 mL). The organic phase was dried over Na2SO4
and concentrated under reduced pressure to yield acid 12
(1.06 g, 2.20 mmol, 99%) as a foamy white solid. 12: 1H NMR
(400 MHz, CDCl3), d: 7.48–7.18 (m, 15H), 4.94–4.90 (m, 1H),
4.74–4.65 (m, 2H), 4.49–4.44 (m, 1H), 4.26–4.22 (m, 1H),
2.54–2.36 (m, 1H), 2.39–1.61 (m, 8H). 13C NMR (100 MHz,
CDCl3), d: 173.4, 145.8, 136.5, 136.2, 131.72, 131.68, 128.9,
128.3, 128.23, 128.20, 127.94, 127.88, 126.8, 126.6, 126.0, 121.7,
113.6, 113.2, 88.7, 86.8, 86.3, 83.0, 71.5, 71.2, 69.6, 43.3, 43.0,
35.6, 35.2, 34.9, 31.5, 31.4, 30.8, 29.8. FTIR (neat), cm−1:
3431(w), 3059(w), 3034(m), 2929(s), 2864(m), 2234(w), 1728(s),
1607(m), 1491(m), 1452(m), 1087(s), 1070(s). HRMS (EI), m/z:
Calcd for C31H30O5 (M+): 482.2093. Found: 482.2097. TLC
(10% CH3OH–CH2Cl2), Rf: 0.50 (UV, DNP).
8.32 (d, J = 7.5 Hz, 2H), 7.61 (t, J = 7.5 Hz, 1H), 7.44–7.18
(m, 8H), 4.97 (d, J = 11.5 Hz, 1H), 4.40 (s, 1H), 4.35 (d, J =
11.5 Hz, 1H), 4.04–3.87 (m, 2H), 3.66 (d, J = 10.5 Hz, 1H), 3.18
(s, 3H). 13C NMR (100 MHz, CD3OD), d: 196.6, 175.7, 134.9,
131.7, 129.5, 129.3, 129.2, 129.1, 129.0, 128.8, 85.6, 82.0, 75.0,
64.8, 63.6, 52.0. FTIR (neat), cm−1: 3316(s), 2935(w), 1725(s),
1683(m), 1467(w), 1115(m). HRMS (MALDI-FTICR), m/z:
Calcd for C20H21NO6 (M + Na+): 394.1261. Found: 394.1262.
TLC (10% CH3OH–CH2Cl2), Rf: 0.35 (UV, DNP).
2-Benzoyl-2-O-methyl-3-O-benzyl-2,3,4-trihydroxy-4-acetoxy-
methylpyrrolidin-5-one (17). Acetic anhydride (0.34 mL,
3.6 mmol, 1.41 equiv.) was added to a solution of lactam diol
16 (0.95 g, 2.55 mmol, 1 equiv.), NEt3 (0.43 mL, 3.08 mmol,
1.21 equiv.), and DMAP (0.12 g, 1.02 mmol, 0.40 equiv.) in THF
(20 mL). After 12 hours the reaction was diluted with EtOAc
(100 mL) and washed with saturated NH4Cl (3 × 75 mL). The
organic phase was dried over Na2SO4 and concentrated under
reduced pressure. Purification by flash column chromatography
(2% CH3OH–CH2Cl2) yielded acetate 17 (0.71 g, 1.72 mmol,
68%). 17: 1H NMR (400 MHz, CDCl3), d: 8.25 (d, J = 7.5 Hz,
2H), 7.64 (t, J = 7.5 Hz, 1H), 7.51 (t, J = 7.5 Hz, 2H), 7.33
(br s, 1H), 7.19–7.13 (m, 3H), 6.72 (d, J = 6.8 Hz, 2H), 4.75
(d, J = 12.5 Hz, 1H), 4.50 (s, 1H), 4.48 (d, J = 10.0 Hz, 1H),
4.19 (d, J = 12.5 Hz, 1H), 3.83 (d, J = 10.0 Hz, 1H), 3.66
(br s, 1H), 3.30 (s, 3H), 2.12 (s, 3H). 13C NMR (100 MHz,
CDCl3), d: 191.8, 173.4, 170.6, 135.6, 134.0, 133.9, 129.4, 128.7,
128.1, 127.9, 127.7, 95.6, 85.9, 78.6, 74.5, 62.9, 51.4, 21.0. FTIR
(neat), cm−1: 3319(m), 2927(w), 2853(w), 1732(s), 1693(m),
1451(w), 1376(w), 1234(m), 1100(m). HRMS (EI), m/z: Calcd
for C22H23NO7 (M+): 413.1468. Found: 413.1442. TLC (5%
CH3OH–CH2Cl2), Rf: 0.27 (UV, DNP).
3-O-Benzyl-(2-hydroxymethyl)-2,3-dihydroxy-5-phenylpent-4-
ynamide, 4-phenylcyclohexylidene acetal (13). To a solution
of acid 12 (1.07 g, 2.22 mmol, 1 equiv.) in THF (50 mL) were
added NEt3 (0.70 mL, 5.02 mmol, 2.26 equiv.) and PyBOP
(1.38 g, 2.60 mmol, 1.17 equiv.). After ten minutes 28% NH3
in H2O (1.40 mL, 11.9 mmol, 5.36 equiv.) was added and
the mixture was allowed to stir for 12 hours. The reaction
was then diluted with EtOAc (100 mL) and washed with
saturated aqueous NaCl (3 × 100 mL). The organic phase was
dried over Na2SO4 and concentrated under reduced pressure.
Purification of the residue by flash column chromatography
(1% CH3OH–CH2Cl2) afforded amide 13 (0.80 g, 1.66 mmol,
75%) as a foamy white solid. 13: 1H NMR (400 MHz, CDCl3),
d: 7.57–7.23 (m, 15H), 6.99 (br s, 1H), 6.94 (br s, 1H), 5.02–4.98
(m, 1H), 4.89 (s, 1H), 4.76 (d, J = 12.0 Hz, 1H), 4.54–4.48 (m,
1H), 4.29 (d, J = 9.2 Hz, 1H), 2.70–2.60 (m, 1H), 2.50–1.71 (m,
8H). 13C NMR (100 MHz, CDCl3), d: 174.8, 145.7, 145.6, 136.7,
136.5, 131.44, 131.40, 128.4, 128.05, 128.02, 127.98, 127.81,
127.77, 127.51, 127.49, 126.5, 126.4, 125.8, 122.8, 121.8, 112.6,
112.3, 87.9, 87.8, 86.9, 86.3, 84.0, 83.9, 71.7, 71.5, 71.34, 71.28,
69.8, 69.6, 43.0, 42.8, 35.4, 35.3, 35.2, 35.0, 31.3, 30.6. FTIR
(neat), cm−1: 3478(s), 3356(m), 3029(m), 2936(s), 2865(m),
2250(m), 1693(s), 1573(m), 1492(m), 1452(m), 1088(s). HRMS
(MALDI-FTICR), m/z: Calcd for C31H31NO4 (M + Na+):
504.2145. Found: 504.2126. TLC (2% CH3OH–CH2Cl2), Rf:
0.32 (UV, DNP).
2-Benzoyl-2-O-methyl-3-O-benzyl-2,3,4-trihydroxy-4-acetoxy-
methylpyrrolidin-5-one, methylthiomethyl ether (18). Acetic
anhydride (9.20 mL, 97.5 mmol, 57.4 equiv.) and DMSO
(9.20 mL, 130 mmol, 76.2 equiv.) were combined with acetate
17 (0.70 g, 1.70 mmol, 1 equiv.) and stirred under N2. After
40 hours the reaction was diluted with CH2Cl2 (100 mL) and
washed with saturated aqueous NaCl (2 × 100 mL) and H2O
(1 × 100 mL). The organic phase was dried over Na2SO4 and
concentrated under reduced pressure to yield MTM ether 18
(0.70 g, 1.50 mmol, 90%). 18: 1H NMR (400 MHz, CDCl3), d:
8.29 (d, J = 8.0 Hz, 2H), 7.64 (t, J = 8.0 Hz, 1H), 7.51 (t, J =
8.0 Hz, 2H), 7.19–7.11 (m, 4H), 6.65 (d, J = 7.0 Hz, 2H), 5.10
(d, J = 13.0 Hz, 1H), 4.87 (d, J = 10.8 Hz, 1H), 4.77 (d, J =
10.8 Hz, 1H), 4.52–4.50 (m, 2H), 4.01 (d, J = 13.0 Hz, 1H), 4.67
2-Benzoyl-3-O-benzyl-2,3,4-trihydroxy-4-hydroxymethylpyrro-
lidin-5-one (15). Magnesium sulfate (1.50 g, 12.5 mmol,
2.41 equiv.) and sodium bicarbonate (0.28 g, 3.29 mmol, 0.64
equiv.) were added to a solution of amide 13 (2.49 g, 5.17 mmol,
1 equiv.) in acetone (190 mL) and H2O (110 mL). Potassium
permanganate (3.52 g, 22.3 mmol, 4.31 equiv.) was then added
and the reaction was allowed to stir for one hour. Sodium
nitrite (3.50 g, 50.7 mmol, 9.81 equiv.) and 10% H2SO4 (30 mL)
were then added to quench the reaction. EtOAc (200 mL)
was added and the reaction washed with saturated aqueous
NaCl (2 × 250 mL). The organic phase was dried over Na2SO4
and concentrated under reduced pressure to afford lactam 15
(2.53 g, 4.93 mmol, 95% crude) which was used in the next
reaction without purification.
(d, J = 10.0 Hz, 1H), 3.26 (s, 1H), 2.24 (s, 3H), 2.10 (s, 3H). 13
C
NMR (100 MHz, CDCl3), d: 191.8, 171.4, 169.8, 135.7, 133.9,
133.8, 129.3, 128.7, 128.1, 127.8, 127.5, 95.5, 86.6, 82.7, 74.6,
70.8, 58.6, 50.9, 21.0, 14.6. FTIR (neat), cm−1: 2925(w), 1744(s),
1721(s), 1693(m), 1449(w), 1231(m), 1100(m), 1033(m). HRMS
(EI), m/z: Calcd for C24H27NO7S (M + H+): 474.1586. Found:
474.1568. TLC (5% CH3OH–CH2Cl2), Rf: 0.73 (UV, DNP).
Crystal structure determination of compound epi-18
Crystal data. C24H27NO7S, 473.53, triclinic, a = 8.9592(7), b =
˚
9.2569(6), c = 15.3512(11) A, cell angle a = 79.4720(10), cell
angle b = 83.1290(10), cell angle c = 69.3510(10)◦, U =
3
¯
˚
1169.19(15) A , T = 218(2) K, space group P1, Z = 2, l =
0.183 mm−1, 7140 reflections measured, 4097 independent
reflections (Rint = 0.0196) which were used in all calculations.
The final wR(F2) was 0.1614.‡
2-Benzoyl-2-O-methyl-3-O-benzyl-2,3,4-trihydroxy-4-hydroxy-
methylpyrrolidin-5-one (16). Concentrated hydrochloric acid
(2.5 mL) was added to a solution of crude acetal 15 (2.53 g,
4.93 mmol, 1 equiv.) in ordinary CH3OH (80 mL) and the
mixture heated to reflux. After 12 hours the reaction was cooled
to room temperature and diluted with EtOAc (200 mL), then
washed with saturated aqueous NaCl (3 × 200 mL). The organic
phase was dried over Na2SO4 and concentrated under reduced
pressure. Purification by flash column chromatography (5%
CH3OH–CH2Cl2) afforded lactam diol 16 (0.95 g, 2.56 mmol,
2-Benzoyl-2-O-methyl-3-O-benzyl-2,3,4-trihydroxy-4-hydroxy-
methylpyrrolidin-5-one, methylthiomethyl ether (19). Sodium
methoxide (0.16 g, 2.96 mmol, 5.92 equiv.) was added to a solu-
tion of acetate 18 (0.24 g, 0.50 mmol, 1 equiv.) in THF (6 mL).
‡ CCDC reference number 283232. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/b512701g.
1
52%) as a white solid. 16: H NMR (400 MHz, CD3OD), d:
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 4 2 7 4 – 4 2 8 1
4 2 7 9

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After 12 hours the reaction was diluted with EtOAc (50 mL)
and washed with saturated aqueous NaCl (3 × 50 mL). The
organic phase was dried over Na2SO4 and concentrated under
reduced pressure to yield alcohol 19 (0.15 g, 0.33 mmol, 69%)
as a white solid. 19: 1H NMR (400 MHz, CDCl3), d: 8.27 (d, J
= 7.5 Hz, 2H), 7.63 (t, J = 7.5 Hz, 1H), 7.51 (t, J = 7.5 Hz,
2H), 7.35–7.10 (m, 4H), 6.69 (d, J = 7.0 Hz, 2H), 5.07 (d, J =
11.5 Hz, 1H), 4.88 (d, J = 11.5 Hz, 1H), 4.72 (d, J = 9.5 Hz,
1H), 4.49 (s, 1H), 4.16 (dd, J = 3.0 Hz, J = 13.0 Hz, 1H), 3.87
(dd, J = 11.0 Hz, J = 13.0 Hz, 1H), 3.75 (d, J = 9.5 Hz, 1H),
3.25 (s, 3H), 2.78 (dd, J = 3.0 Hz, J = 11.0 Hz, 1H), 2.34 (s,
3H). 13C NMR (100 MHz, CDCl3), d: 191.9, 172.4, 135.9, 134.0,
133.7, 129.2, 128.7, 128.0, 127.9, 127.8, 95.4, 86.4, 84.3, 74.8,
70.3, 57.8, 51.0, 14.8. FTIR (neat), cm−1: 3439(m), 2956(m),
2923(m), 2853(m), 1716(s), 1693(s), 1458(w), 1097(s). HRMS
(MALDI-FTICR), m/z: Calcd for C22H25NO6S (M + Na+):
454.1295. Found: 454.1292. TLC (5% CH3OH–CH2Cl2), Rf:
0.35 (UV, DNP).
6H). 13C NMR (100 MHz, CDCl3), d: 209.5, 138.2, 122.1, 102.2,
83.5, 75.8, 33.4, 21.5, 19.6, 13.9, 6.8. FTIR (neat), cm−1: 2971(m),
2938(m), 2879(m), 1719(s), 1460(m), 1408(m), 1146(s), 1094(m).
HRMS (CI), m/z: Calcd for C11H19O3 (M + H+): 199.1334.
Found: 199.1341. TLC (5% CH3OH–CH2Cl2), Rf: 0.65 (UV,
DNP).
Acknowledgements
We thank the ARCS Foundation for a graduate Fellowship
(JMM), the NIH (GM60875) for direct support and the NIH
and NSF for infrastructure support (GM62116, GM61894,
CHE-9709183).
References and notes
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69; (c) R. A. Merz, M. Horsch, L. E. Nyhlen and D. M. Rast, EXS,
1999, 87, 9–37; (d) E. Cabib, Adv. Enzymol. Relat. Areas Mol. Biol.,
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2 D. Zhang and M. J. Miller, Curr. Pharm. Des., 1999, 5, 73–99.
3 (a) A. R. Yeager and N. S. Finney, Bioorg. Med. Chem., 2004, 12,
6451–6460; (b) R. Chang, P. Moquist and N. S. Finney, Carbohydr.
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Finney, Org. Biomol. Chem., 2003, 1, 39–41.
4 (a) P. Bloch, C. Tamm, P. Bollinger, T. J. Petcher and H. P. Weber,
Helv. Chim. Acta, 1976, 59, 133–137; (b) P. Bloch and C. Tamm, Helv.
Chim. Acta, 1981, 64, 304–315.
5 J. Wenke, H. Anke and O. Sterner, Biosci., Biotechnol., Biochem.,
1993, 57, 961–964.
2-Benzoyl-2-O-methyl-3-O-benzyl-2,3,4-trihydroxy-4-carbal-
dehydopyrrolidin-5-one, methylthiomethyl ether (20). To a
−78 ◦C solution of oxalyl chloride (0.03 mL, 0.13 mmol,
1.10 equiv.) in CH2Cl2 (0.50 mL) was added DMSO (0.03 mL,
0.35 mmol, 2.70 equiv.) under N2. After ten minutes alcohol 19
(0.06 g, 0.13 mmol, 1 equiv.) was added in CH2Cl2 (1 mL) and
the mixture stirred for one hour. NEt3 (0.06 mL, 0.43 mmol,
3.31 equiv.) was then added, and the reaction was warmed to
room temperature. After two hours the reaction was diluted
with CH2Cl2 (30 mL) and washed with saturated aqueous NaCl
(3 × 30 mL). The organic phase was dried over Na2SO4 and
concentrated under reduced pressure to afford aldehyde 20
(0.05 g, 0.12 mmol, 96%). 20: 1H NMR (400 MHz, CDCl3), d:
9.68 (s, 1H), 8.15 (d, J = 7.5 Hz, 2H), 7.56–6.89 (m, 9H), 5.46
(d, J = 5.5 Hz, 2H), 4.76 (s, 1H), 4.52 (d, J = 11.0 Hz, 1H),
4.35 (d, J = 11.0 Hz, 1H), 3.28 (s, 3H), 2.31 (s, 3H). MS (ESI),
m/z: Calcd for C22H23NO6S (M + Na+): 452. Found: 452. TLC
(5% CH3OH–CH2Cl2), Rf: 0.30 (UV, DNP).
6 J. Wink, S. Grabey, M. Gareis, R. Thierecke, R. Kirsch, Eur. Pat.
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7 D. Komagata, S. Fujita, N. Yamashita, S. Saito and T. Morino,
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Su and C. Tamm, Helv. Chim. Acta, 1995, 78, 1278–1290; (c) X.
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68.
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62, 161–166.
13 Y. Hayashi, M. Shoji, S. Yamaguchi, T. Mukaiyama, J. Yamaguchi,
H. Kakeya and H. Osada, Org.Lett., 2003, 5, 2287–2290.
14 For recent total syntheses of azaspirene and synerazol, see ref. 12a
and: (a) Y. Hayashi, M. Shoji, J. Yamaguchi, K. Sato, S. Yamaguchi,
T. Mukaiyama, K. Sakai, Y. Asami, H. Kakeya and H. Osada, J. Am.
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Mukaiyama, H. Gotoh, S. Yamaguchi, M. Nakata, H. Kakeya and
H. Osada, J. Org. Chem., 2005, 70, 5643–5654.
(Z)-(3R,4S)-2,3,4-trihydroxynon-5-ene (22). Ethylmagne-
sium bromide (8.30 mL, 8.30 mmol, 1.0 M in THF, 1.52 equiv.)
was added to a solution of aldehyde 21 (0.93 g, 5.46 mmol,
1 equiv.) in THF (20 mL) under N2. After three hours the
reaction was diluted with EtOAc (50 mL) and washed with
saturated aqueous NaCl (3 × 50 mL). The organic phase was
dried over Na2SO4 and concentrated under reduced pressure to
yield alcohol 22 (0.89 g, 4.44 mmol, 82%) as a yellow oil. 22: 1H
NMR (400 MHz, CDCl3), d: 5.71–5.54 (m, 2H), 5.08–5.04 (m,
1H), 4.82 (t, J = 8.5 Hz, 1H), 3.92–3.81 (m, 1H), 3.47–3.41 (m,
1H), 2.19–2.01 (m, 2H), 1.49–1.39 (m, 5H), 1.03–0.95 (m, 6H).
13C NMR (100 MHz, CDCl3), d: 136.6, 124.2, 100.7, 80.7, 74.3,
71.0, 27.2, 21.2, 19.9, 14.2, 10.1. FTIR (neat), cm−1: 3488(m),
2965(s), 2936(s), 2878(s), 1699(w), 1479(m), 1409(m), 1145(s),
1097(s), 905(m). TLC (5% CH3OH–CH2Cl2), Rf: 0.68 (UV,
DNP).
(Z)-(3R,4S)-3,4-dihydroxynon-5-en-2-one (23). To a −78 ◦C
solution of oxalyl chloride (0.58 mL, 6.67 mmol, 1.50 equiv.)
in CH2Cl2 (14 mL) was added DMSO (0.63 mL, 8.88 mmol,
2.00 equiv.) under N2. After ten minutes alcohol 22 (0.89 g,
4.44 mmol, 1 equiv.) was added in CH2Cl2 (7 mL) and the
mixture stirred for one hour. NEt3 (1.90 mL, 13.63 mmol,
3.07 equiv.) was then added, and the reaction was warmed to
room temperature. After two hours the reaction was diluted with
CH2Cl2 (50 mL) and washed with saturated aqueous NH4Cl (1 ×
50 mL) and saturated aqueous NaCl (1 × 50 mL). The organic
phase was dried over Na2SO4 and concentrated under reduced
pressure to afford ketone 23 (0.85 g, 4.29 mmol, 96%) as a pale
15 (a) J. M. Mitchell, PhD Dissertation, University of California, San
Diego, 2003; (b) J. M. Mitchell, N. S. Finney, Abstracts of Papers,
221st ACS National Meeting, 2001, ORGN-575.
16 Enzymatic desymmetrization studies of 4 were carried out with
the ChiroScreenTM-TE enzyme screening kit (Altus Biologics, Inc.,
Cambridge, MA). Acylation reactions were carried out with a
suspension of enzyme in diisopropyl ether, using vinyl acetate as
the acyl donor. Acylation with Mucor javanicus lipase produced the
monoacylated derivative of 4 in 76% yield and >99% ee. Acylation
with Candida antarctica “B” lipase afforded the opposite enantiomer
of the monoacylated derivative in 91% yield and >90% ee. The
absolute configurations of the enzymatic acylation products were not
determined. Enantiomeric excess was determined by chiral HPLC
analysis of crude reaction mixtures on a Chiralcel-OD analytical
column (Daicel Chemical Industries, Ltd., Tokyo, Japan), using 5%
isopropanol–hexanes as the mobile phase.
1
yellow oil. 23: H NMR (400 MHz, CDCl3), d: 5.62 (dt, J =
11.0 Hz, J = 7.5 Hz, 1H), 5.15–5.08 (m, 2H), 4.96 (t, J = 6.8 Hz,
1H), 4.36 (d, J = 8.0 Hz, 1H), 2.64–2.54 (m, 1H), 2.42–2.32 (m,
1H), 2.14–1.99 (m, 2H), 1.51 (d, J = 5.0 Hz, 3H), 0.99–0.95 (m,
4 2 8 0
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 4 2 7 4 – 4 2 8 1

View Article Online
17 The relative configuration was established via X-ray crystallographic
analysis of epi-18. See Fig. 2.
22 J. Coste, D. Le-Nguyen and B. Castro, Tetrahedron Lett., 1990, 31,
205–208.
23 N. S. Srinivasan and D. G. Lee, J. Org. Chem., 1979, 44, 1574. Hayashi
et al. rely on a similar oxidative cyclization of an N-Boc protected
amide for formation of the lactam. See ref. 13.
24 The formation of a single diastereomer at the methoxy-bearing
center presumably reflects a strong thermodynamic preference for
this isomer. While the origin of this preference is not obvious, it is con-
sistent with the configuration observed for 8-O-desmethylpseurotin
A, which has a hydroxyl group in place of the methoxy substituent
and would thus quickly equilibrate to a mixture of isomers in the
absence of a strong thermodynamic bias.
18 (a) D. E. Frantz, R. Fassler, C. S. Tomooka and E. M. Carreira, Acc.
Chem. Res., 2000, 33, 373; (b) D. E. Frantz, R. Fassler and E. M.
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Anand and E. M. Carreira, Org. Lett., 2001, 3, 3017; (d) D. Boyall,
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21 B. O. Lindgren and T. Nilsson, Acta Chem. Scand., 1973, 27, 888–890.
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 4 2 7 4 – 4 2 8 1
4 2 8 1