Exploring the Chemistry of Bicyclic Isoxazolidines for the Multicomponent Synthesis of Glycomimetic Building Blocks

Article abstract of DOI:10.1021/acs.joc.6b01515

Starting from a chiral furanone, the nitrone-olefin [3 + 2] cycloaddition can be used to obtain bicyclic isoxazolidines for which we report a set of reactions to selectively modify each functional position. These synthetically versatile bicyclic isoxazolidines allowed us to obtain complex glycomimetic building blocks, like iminosugars, via multicomponent chemistry. For example, a library of 20 pipecolic acid derivatives, a recurring motif in various prescription drugs, could be obtained via a one-pot Staudinger/aza-Wittig/Ugi three-component reaction of a bicyclic isoxazolidine-derived azido-hemiacetal. Notably, specific pipecolic acids in this library were obtained via hydrolysis of an unique tricyclic imidate side product of the Ugi reaction. The azido-hemiacetal was also converted into an aza-C-glycoside iminosugar via an unprecendented one-pot Staudinger/aza-Wittig/Mannich reaction.

Full text of DOI:10.1021/acs.joc.6b01515

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Article
Exploring the chemistry of bicyclic isoxazolidines for the
multicomponent synthesis of glycomimetic building blocks
Jorin Hoogenboom, Martin Lutz, Han Zuilhof, and Tom Wennekes
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.6b01515 • Publication Date (Web): 01 Sep 2016
Downloaded from http://pubs.acs.org on September 3, 2016
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Exploring the chemistry of bicyclic isoxazolidines for the
multicomponent synthesis of glycomimetic building blocks
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Jorin Hoogenboom,† Martin Lutz,‡ Han Zuilhof,† Tom Wennekes*,†,§
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†Laboratory of Organic Chemistry, Wageningen University, Stippeneng 4, 6708 WE Wageningen, The
Netherlands
‡Bijvoet Center for Biomolecular Research, Crystal and Structural Chemistry, Utrecht University, Padualaan 8,
3584 CH Utrecht, The Netherlands
§
Department of Chemical Biology and Drug Discovery, Utrecht Institute for Pharmaceutical Sciences and
Bijvoet Center for Biomolecular Research, Utrecht University, The Netherlands
*Eꢀmail: t.wennekes@uu.nl
Abstract
Starting from a chiral furanone, the nitroneꢀolefin [3+2] cycloaddition can be used to obtain bicyclic
isoxazolidines for which we report a set of reactions to selectively modify each functional position.
These synthetically versatile bicyclic isoxazolidines allowed us to obtain complex glycomimetic
building blocks, like iminosugars, via multicomponent chemistry. For example, a library of 20
pipecolic acid derivatives ꢀ a recurring motif in various prescription drugs ꢀ could be obtained via a
oneꢀpot Staudinger/azaꢀWittig/Ugi threeꢀcomponent reaction of a bicyclic isoxazolidineꢀderived
azidoꢀhemiacetal. Notably, specific pipecolic acids in this library were obtained via hydrolysis of an
unique tricyclic imidate side product of the Ugiꢀreaction. The azidoꢀhemiacetal was also converted
into an azaꢀCꢀglycoside iminosugar via an unprecendented oneꢀpot Staudinger/azaꢀWittig/Mannich
reaction.
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Introduction
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Glycomimetics such as iminosugars and their derivatives are found in nature and display a wide
variety of biological activities. For example, the archetypical glycomimetic 1ꢀdeoxynojirimycin
(Figure 1), found in the leaves of the mulberry tree1 and certain species of bacteria,2 is a glycosidase
inhibitor. Since the report of its identification and chemical synthesis in 1967,3ꢀ5 the subsequent
decades have witnessed a vast number of studies describing the synthesis and evaluation of
biologically active glycomimetics. The value of these synthetic glycomimetics is evidenced by Nꢀ
hydroxyethylꢀ1ꢀdeoxynojirimycin (Miglitol), a clinically used drug in the treatment of type 2 diabetes6
that inhibits intestinal glucosidases, and by Nꢀbutylꢀ1ꢀdeoxynojirimycin (Miglustat), a glycosyl
transferase inhibitor used in the clinic for the treatment of Gaucher disease.7,8 Consequently,
glycomimetics hold great promise for drug discovery. Key to enabling this is the development of
synthetic methodology and novel glycomimetic building blocks to generate comprehensive and
structurally diverse libraries of glycomimetics.
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Figure 1. 1ꢀDeoxynojirimycin with analogs thereof and notable examples of pipecolic acid derivatives (in red). AMP
= 5ꢀ(adamantanꢀ1ꢀylꢀmethoxy)ꢀpentyl.
In our ongoing synthetic investigation towards novel glycomimetics we are, among others, interested
in developing azaꢀCꢀglycoside and pipecolic acidꢀbased iminosugars. For example, we have
previously reported on 1 and 2 (Figure 1), which are a more potent version of Miglustat9 and a
selective inhibitor of GBA2, respectively.10 These types of iminosugars mimic the glycan and/or the
aglycone part, and are thereby able to bind and inhibit the active site of specific carbohydrateꢀactive
enzymes. They can be constructed in several ways,11ꢀ13 for example, we have shown previously that
cyclic imines can be used in a Staudinger/azaꢀWittig/Ugi threeꢀcomponent reaction (SAWUꢀ3CR) to
obtain pipecolic acid derivatives.10 Alternatively, carbohydrate derived cyclic nitrones have also been
used to obtain azaꢀCꢀglycosides through an 1,3ꢀdipolar cycloaddition reaction.14,15 However, this
cycloaddition reaction has mainly been used to obtain bicyclic iminosugars. In contrast, the Ugi
reaction, being a multicomponent reaction, is more suited to create libraries of diverse iminosugars.
The nitroneꢀolefin [3+2] cycloaddition could, in principle, be a useful reaction towards glycomimetic
building blocks, since it can be used to install multiple neighboring stereogenic centers with high
regioꢀ and stereoselectivity in one step. The reaction has been studied extensively, however, most
advances in its application towards high regioꢀ and enantiospecific products provide Cꢀarylꢀsubstituted
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isoxazolidines that often bear Nꢀaryl groups. These products are synthetically less versatile for
elaboration towards glycomimetics.16,17
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One of the few examples of nitrones that do give rise to synthetically versatile cycloadducts are
aminoꢀacid derived nitrones 3a-b (Figure 2). These nitrones bear a deprotectable Nꢀsubstituent and
either a masked carboxylic acid18, 19 or masked aldehyde20 that enable the synthesis of synthetically
versatile isoxazolidines. In addition, the reaction has a large substrate scope, including several sugarꢀ
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derived olefins. More specifically, olefinꢀcontaining
Dꢀmannitolꢀderived furanones 4 and 5 (Figure 2)
functioned as the starting point for the current study.21ꢀ24
Figure 2. Complex glycomimetics may be obtained from bicyclic isoxazolidines, which provides many handles for
further functionalization.
The nitroneꢀolefin [3+2] cycloaddition reaction with 4 and 5 has been reported to proceed with high
regioꢀ and diastereoselectivity, and to provide a chiral bicyclic isoxazolidine in good yield. We set out
to explore the chemistry of these bicyclic isoxazolidines with the aim of creating a versatile chiral
intermediate that can be used for the synthesis of glycomimetic building blocks based on pipecolic
acid and azaꢀCꢀglycosides. Besides the earlier mentioned examples of clinically relevant iminosugars,
these building blocks also represent important functional motifs in other drugs. Pipecolic acids are a
recurring motif in, for example, Palinavir, Virginiamycin S1 and Nelfinavir (Figure 1). These drugs ꢀ
used as protease inhibitors, antibiotics or in the treatment of HIV ꢀ all contain a piperidineꢀ2ꢀ
carboxamide motif, and are typically functionalized at the 4ꢀposition with either a chiral ether
(Palinavir), ketone (Virginiamycin S1) or as part of a bicyclic system (Nelfinavir).
We here show that a versatile bicyclic isoxazolidine cycloadduct can be modified selectively at each
functional position and subsequently transformed into a diverse range of pipecolic acid derivatives via
a oneꢀpot Staudinger/azaꢀWittig/Ugi threeꢀcomponent reaction (SAWUꢀ3CR). Finally, starting from
the same isoxazolidine intermediate we synthesized an azaꢀCꢀglycoside via an unprecedented oneꢀpot
Staudinger/azaꢀWittig/Mannich (SAWM) reaction.
Results and discussion
Our initial efforts focused on the product of the nitroneꢀolefin [3+2] cycloaddition between nitrone 3a
and furanone 5, which provides known cycloadduct 7a (Scheme 1, left). This bicyclic isoxazolidine
was first reported by Ondrus et al., but has never been functionalized further.25 Ondrus and coworkers
reported that the selectivity of the cycloaddition reaction can be controlled by changing the E/Zꢀratio
of the nitrone, allowing the selective synthesis of either the synꢀ 7b or antiꢀproduct 7a. We explored if
this selectivity could be further improved and succeeded in increasing the selectivity towards the main
antiꢀproduct 7a by using toluene as the solvent. By using this solvent compound 7a was obtained in
68% yield, while providing 7b (20%) as the minor compound (Scheme 1, left). Thus far the only
reported assignment of the stereochemistry of this major diastereomer was based on NMR coupling
constants.25 We succeeded in crystallizing the closely related cycloadduct 6a (Scheme 1) – obtained
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through the reaction between furanone 4 and nitrone 3a ꢀ that enabled the unequivocal assignment of
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the relative stereochemistry by Xꢀray crystal structure determination (Scheme 1, right).
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Scheme 1. Cycloaddition reaction of furanone 4-5 with nitrone 3a-b providing cycloadducts 6-8 (left). Molecular
structure of major antiꢀcycloadduct 6a in the crystal (right). Reaction conditions: a) TBDPSCl, imidazole, DMF, −10ꢀ
20 °C, 4 h, 88%.
With the cycloaddition reaction optimized and the stereochemistry confirmed, we set out to selectively
modify the newly created functional groups in the bicyclic isoxazolidine, namely the exocyclic ester,
lactone and NꢀO bond. Initial attempts to hydrolyse the ethyl ester of cycloadduct 7a under both basic
(LiOH, ≤51% yield) and acidic conditions (HCl, 39% yield) provided the target carboxylic acid 9, but
only in mediocre yield due to degradation of the TBDPS group. Hydrolysis of the ester under neutral
conditions proved more favourable as treatment with Me3SnOH gave carboxylic acid 9 in almost
quantitative yield (Scheme 2). The carboxylic acid could then be selectively reduced towards primary
alcohol 10 in 94% over 2 steps by reducing an in situ formed mixed anhydride with sodium
borohydride.
O
OH
Bn
OH
O
O
H
H
b
O
O
N
N
Bn
O
O
10
94%
TBDPSO
TBDPSO
a
9
(2 steps)
O
R
HO
HO
TBDPSO
HO
CO2Et
Bn
CO2Et
H
H
c
d
O
N
Bn
O
N
N
Bn
78%
O
O
O
TBDPSO
TBDPSO
11
12
84%
(2 steps)
7a
R = CO2Et
8a R =
e
OMe
OMe
O
O
O
H
O
HN Cbz
OH
13
TBDPSO
Scheme 2. Selective modification of functional positions in bicyclic isoxazolidine 7a and 8a. Reaction conditions: a)
Me3SnOH, ClCH2CH2Cl, reflux, 2.5 h; b) isobutyl chloroformate, NaBH4, THF, DMF, −15 °C, 75 min., 94% over 2
steps; c) BH3ꢀSMe2, THF, 4ꢀ20 °C, 3.5 h; d) NaBH4, MeOH, −3ꢀ7 °C, 100 min., 84% over 2 steps; e) i. Raneyꢀnickel,
H2 (1 bar), THF, rt, 7 h; ii. Pd/C, Cyclohexene, THF, reflux, 3 h; iii. CbzCl, NaHCO3, THF/H2O, rt, 14 h, 45%.
We next focused on selective modification of the lactone in 7a and observed that its selective
reduction is difficult due to the presence of the NꢀO bond, which is sensitive towards reducing agents.
When employing DiBALꢀH, Lꢀselectride or NaBH4 as reducing agents, mixtures of starting material,
monoreduction towards the hemiacetal, overreduction towards the diol and additional byproducts were
observed. However, by using BH3ꢀSMe2 as the reducing agent, 7a could be selectively reduced
towards hemiacetal 11 in 78% yield, providing diol 12 (16%) as a minor product. In addition, diol 12
could be obtained in 84% yield over 2 steps by reducing crude hemiacetal 11 with NaBH4 in MeOH at
0 °C.
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Finally, we investigated cleaving the NꢀO bond in the isoxazolidineꢀring through hydrogenation.
Initial hydrogenolysis attempts on substrate 7a with Pd/C or Pd(OH)2 in MeOH under atmospheric
pressure resulted in a very slow conversion. Complete hydrogenation of 7a was only observed after
several days at rt, at which point significant degradation had also occurred. Attempts to accelerate the
reaction by using transfer hydrogenation conditions (HCO2NH4, Pd/C) or staged hydrogenation
conditions, which we reported earlier for a similar compound,20 resulted in side product formation and
the target product proved to be unstable during isolation. MS analysis of the formed (side)products
indicated that a significant amount of degradation could be attributed to βꢀelimination side reactions.
We hypothesized that replacing one of the two carbonyl groups would prevent this side reaction and
this spurred the development of our recently published novel nitrone 3b bearing a acetalꢀmasked
aldehyde.20 Cycloaddition of this nitrone with furanone 5 provided cycloadduct 8a in 63% yield.
Cycloadduct 8a was then subjected to a staged hydrogenation with Raneyꢀnickel, followed by a
transfer hydrogenation with Pd/C in cyclohexene, to produce the amine that was protected in situ as a
Cbzꢀcarbamate to provide compound 13 in 45% yield over the three succesive transformations.
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Encouraged by the versatile synthetic scope, our next aim was to synthesize a small library of
functionalized glycomimetic building blocks from a common precursor derived from this bicyclic
isoxazolidine. We envisioned that commercially available furanone 4 could be used to obtain azidoꢀ
hemiacetal 17 in three to four steps (Scheme 3) that in turn could be used in the SAWUꢀ3CR for the
synthesis of a small library of pipecolic acidꢀbased iminosugars.26
We initially focused on synthesising azidoꢀcycloadduct 16 via azidoꢀfuranone 15 (Scheme 3), but it
proved impossible to produce intermediate 15, since conversion of 4 into 15 via a Mitsunobu reaction
led to degradation. A twoꢀstage reaction towards 15 via tosylate 14 was also unsuccessful. Compound
16 was however successfully prepared in 72% yield by installing the azide after the cycloaddition
using a Mitsunobu reaction on the previously obtained cycloadduct 6a. However, this reaction proved
less reliable at larger scales, but we could obtain compound 16 reliably at a 19 gram scale in 89% yield
by first converting compound 6a to its mesylate, immediately followed by substitution with NaN3. The
lactone in 16 could then be selectively reduced to the target azidoꢀhemiacetal 17 using BH3ꢀSMe2.
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Scheme 3. Synthesis of azidoꢀhemiacetal 17, precursor for the SAWUꢀ3CR. Reaction conditions: a) TsCl, pyridine,
DCM, −15ꢀ5 °C, 1 h, 70%; b) nitrone 3a, toluene, reflux, 4.5 h, 55% (6a) & 31% (6b); c) (PhO)2P(O)N3, Diisopropyl
azodicarboxylate, PPh3, THF, −20ꢀ20 °C, 1.5 h, 72%; d) i. MsCl, Et3N, DCM, 0 °C, 50 min. ii. NaN3, DMF, 60 °C, 90
min., 89% over 2 steps; e) BH3ꢀSMe2, THF, 4ꢀ20 °C, 7 h, 34%; f) PMe3, THF, EtOH, 4 °C, 3 h; g) (AcO)3BHNa, THF,
4 °C, 1.5 h, 66% over 2 steps.
With the key intermediate (17) for the SAWUꢀ3CR now in hand, the feasibility of this reaction was
investigated by first attempting a tandem oneꢀpot Staudinger/azaꢀWittigꢀreaction. Hence, exposing
compound 17 to PMe3 gave cyclic imine 18 that could be directly converted to iminosugar 19 by
reduction with NaBH(OAc)3 in the same pot. Encouraged by these results, the complete SAWUꢀ3CR
sequence was performed on compound 17 with tertꢀbutylisocyanide and acetic acid to give pipecolic
acid 20a as the major product (Scheme 4).
Scheme 4. The SAWUꢀ3CR with azidoꢀhemiacetal 17. Reaction conditions: a) i. PMe3, EtOH, 4 °C, 3 h; ii. Acetic
acid (1.25 equiv.), tbutylisocyanide (1.25 equiv.), EtOH/TFE, 0ꢀ20 °C, 16 h, 60%, ratio 20a:21a = 51:49.
However, in addition to the expected Ugiꢀproduct 20a another product was also isolated in a
considerable yield. Detailed analysis revealed this product to be compound 21a that contains to the
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best of our knowledge an unprecedented tricyclic imidate (Scheme 4). We propose that 21a results
from an intramolecular side reaction during the Ugiꢀreaction. The different products obeserved in the
Ugiꢀ3CR of cyclic imine 18 result from the initial synꢀ or antiꢀattack of the isocyanide after imine
protonation (Scheme 5). Compound 20a is formed by an antiꢀattack by tertꢀbutylisocyanide relative to
the free alcohol. The resulting nitrilium ion intermediate II then undergoes the expected carboxylate
attack and rearrangement seen in Ugi reactions.
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Scheme 5. Reaction mechanism explaining the formation of antiꢀproduct 20a and tricyclic imidate 21a.
However, when the reaction proceeds via a synꢀattack of the tertꢀbutylisocyanide, the resulting
nitrilium ion (IV) can also be attacked intramolecularly via a pseudoꢀboat conformation by the free
alcohol instead of a carboxylate. Such a pseudoꢀboat conformation places the hydroxy group of IV in
close proximity of the electrophilic carbon atom of the nitrilium ion, enabling the intramolecular
attack. The required pseudoꢀboat conformation for the formation of cyclic imidate 21a is actually the
most stable conformation of the sixꢀmembered ring as determined by M11/6ꢀ311+g(d,p) density
functional calculations (see supporting information for details and 3D images of this structure). We
propose this is because of electrostatic stabilizing interactions between the lone pairs of the alcohol O
atom and the partially positively charged C atom of the nitrilium group. In comparison, the chair
conformations that have either the alcohol or the nitrilium moiety axial are several kcal/mol higher in
energy. The resulting tricyclic compound 21a is surprisingly stable, allowing purification by column
chromatography and complete characterization.
We serendipitously discovered that this side product (21a) hydrolyzed selectively over the course of
several months towards amide 22, the complementrary pipecolic acid with respect to 20a (Scheme 5).
We initially attempted to reproduce this process in a more practical timeꢀframe using acid catalysis,
which is typically used to convert imidates to the corresponding amides.27ꢀ29 However, no conversion
was observed at lower temperatures (20ꢀ25 °C), but the incomplete conversion to product 22 and a
similar compound was observed at higher temperatures. We reasoned that the imidate of compound
21a is first converted to an ethyl ester, similar to other published observations,30ꢀ32 followed by the
formation of the amide (Scheme 6). Indeed, by MS analysis the intermediate diꢀester 23 was observed
in situ, which was eventually converted to either the product 22, or as we presume, the amide resulting
from amideꢀbond formation with the original ester of compound 21a, since the intermediate contains
two ethyl esters that are probably equally reactive. Attempting the hydrolysis under basic conditions
(NaOH), only led to degradation. However, when the hydrolysis of 21a was performed under neutral
conditions in a THF/water mixture at 90 °C in a closed vessel, product 22 was isolated in 70% yield.
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Scheme 6. Acid hydrolysis of imidate 21a leads to mixtures, while hydrolysis under neutral conditions in THF/H2O
provides amide 22 in good yield.
These efficient conditions to obtain compound 22 from 21a made it possible to obtain both pipecolic
acid stereoisomers via the SAWUꢀ3CR. Notably, the SAWUꢀ3CR products 20 and 21 display very
different retention times during chromatography, which makes purification relatively straightforward.
Next, the scope of the SAWUꢀ3CR was investigated with azidoꢀhemiacetal 17. To this end, we
selected a diverse set of acids and isocyanides and subjected them to the SAWUꢀ3CR with 17 (Table
1). We initially used a large excess of acid and isocyanide (3 equiv.; entry 1), but observed that the
reaction actually benefited from using the acid and isocyanide in only a minor excess (1.25 equiv.;
entry 2). These results were obtained by using acetic acid as the carboxylic acid, but when employing
different acids similar or better yields were obtained (entries 3ꢀ5), indicating that the reaction tolerates
a variety of acids.
Table 1. Substrate scope of the SAWU-3CRa with hemiacetal 17.
entry acid
1
isocyanide
Product yieldb ratio
20a/21a 53%c 56:44
2
3
4
5
20a/21a 60% 51:49
20b/21a 65% 46:54
20c/21a 76% 36:64
20d/21a 68% 48:52
6
20e/21e 54% 53:47
7a
7b
20f/21f
28% 1:0d
79% 42:58
(TFE)
8
20g/21g 30% 56:44
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9a
9b
20h/21h 20% 82:18
74% 33:67
(TFE)
10a
10b
20i/21i
13% 1:0d
59% 31:69
(TFE)
11
20j/21i
16% 1:0d
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a Reaction conditions: a) i. PMe3, EtOH, 4 °C, 3h; ii. Acid (1.25 equiv.), isocyanide (1.25 equiv.), EtOH/TFE, 0ꢀ20 °C, 16 h.
b Isolated yield. c 3.00 equiv. of tbutylisocyanide and acetic acid were used. d NMR showed only traceꢀamounts of the imidate
when using EtOH as solvent. TFE = 2,2,2ꢀtrifluoroethanol
However, when primary, secondary and aromatic isocyanides were used we observed a significant
drop in the reaction yields (entry 6ꢀ11). While the corresponding pipecolic acid derivatives 20e-j could
still be isolated in all cases, the tricyclic imidates 21e-j were obtained in reduced yields or sometimes
only observed as trace amounts in the reaction mixtures. It has been reported that unwanted side
reactions during Ugi reactions can be suppressed by performing the reaction in the less nucleophilic
solvent 2,2,2ꢀtrifluoroethanol (TFE) rather than in methanol.33ꢀ36 The use of TFE as the solvent indeed
resulted in significantly increased yields for the three selected reactions with a primary (entry 9b),
secondary (entry 7b) and an even more challenging aromatic isocyanide (entry 10b), to 74, 79 and 59
% yield, respectively. Notably, the tricylclic imidates (21f, 21h, 21i) were now isolated as the major
compounds. The observed effect of reaction conditions and components on the initial synꢀ or antiꢀ
attack of the isocyanides and resulting diastereoselectivity of the Ugiꢀ3CR with cyclic imines has been
reported before.37,38 The complex multistep reaction mechanism and intermediates involved in the Ugiꢀ
3CR, however, prevent us from explaining the observed differences in the ratio of 20 and 21 when
different solvents, carboxylic acids of isocyanides are employed. Remarkably, in a single case (entry
10b), a stable formamidine was isolated in 25% yield that probably resulted from attack of a second 4ꢀ
methoxyisocyanide on the corresponding imidate 21i, followed by a rearrangement (see supporting
information for details).
As shown above, imidate 21 can be conveniently hydrolyzed to provide the synꢀproduct as a free
amine, which provides a handle for further modification. Likewise, facile modification of the antiꢀ
product is also possible, since the 4ꢀpentenoic handle – which is tolerated in the SAWUꢀ3CR reaction
– can be removed in almost quantitative yield (Scheme 7).
Scheme 7. Facile deprotection of antiꢀproduct 20f to provide iminosugar 24.
While the synthesis of both diastereomers via the SAWUꢀ3CR is ideal for obtaining a diverse library
of glycomimetic building blocks, we also wanted to explore the possibility of increasing the selectivity
towards one of the SAWUꢀ3CR products. We reasoned that the reaction could be made more selective
towards the antiꢀproduct by introducing a bulky protecting group on the free hydroxyl group thus
preventing synꢀattack of an isocyanide. To this end, a oneꢀpot procedure was developed in which the
hydroxyl group that results from the Staudinger/azaꢀWittig reaction was protected in situ as a TES
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ether. The crude silylꢀprotected imine was then subjected to an Ugi reaction to selectively provide only
antiꢀproducts in 53% as a mixture of 26a and partially TESꢀdeprotected 20a (Scheme 8). Antiꢀproduct
20a could also be obtained as the sole product in 41% by incorporating the TESꢀdeprotection step in
situ.
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Scheme 8. In situ TESꢀprotection of the cyclic imine results in selective antiꢀproduct formation.
Finally, we investigated if our Staudinger/azaꢀWittig derived cyclic imine (18) could also be a suitable
electrophile in other imineꢀmediated reactions, analogously to a recent paper describing the
Staudinger/azaꢀWittig/Grignard reaction.39 To that end, we choose to investigate the Petasis and
Mannich reaction, which both have not yet been used in conjunction with a Staudinger/azaꢀWittig
generated imine. While the Petasis reaction did not result in any conversion to the target product in our
hands (see supporting information), we were able to modify imine 18 via a Mannich reaction. In situ
treatment of 18 with malonic acid provided azaꢀCꢀglycoside 27 as a pure isomer in 62% yield after
crystallization (Scheme 9).
Scheme 9. Oneꢀpot reaction of the Staudinger/azaꢀWittig/Mannich (SAWM) reaction to give compound 27.
Conclusion
In summary, the nitroneꢀolefin [3+2] cycloaddition reaction can be used to give highly functionalized
bicylic isozazolidine cycloadducts in good yield and stereoselectivity. These cycloadducts are
synthetically highly versatile and can be selectively modified at each functional position, which allows
for the synthesis of a wide variety of glycomimetic building blocks. In this way it is possible to make a
library of 20 pipecolic acid derivatives and an azaꢀCꢀglycoside by converting the bicyclic
isozazolidine into an azidoꢀhemiacetal, and using this in a oneꢀpot Staudinger/azaꢀWittig/Ugi threeꢀ
component reaction (SAWUꢀ3CR) or an unprecedented Staudinger/azaꢀWittig/Mannich reaction
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reaction. The SAWUꢀ3CR on this azidoꢀhemiacetal also produced an unprecedented tricyclic imidate
that can be converted into corresponding pipecolic acidꢀbased glycomimetic compounds.
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Experimental Section
9
General information and methods
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All moisture sensitive reactions were carried out under an argon atmosphere, using ovenꢀdried glassware, unless otherwise stated.
Dichloromethane (CH2Cl2, >99.8%) and toluene (>99.8%) were purified over aluminum oxide under argon using a solvent
purification system. Reagents were obtained from commercial sources and used without further purification unless stated otherwise.
RaneyꢀNickel was bought from commercial sources (W.R. Grace adn Co. Raney®; 2800, as a slurry in H2O), which was washed with
anhydrous THF three times before use. Palladium on carbon (Pd/C) was bought from commercial sources (10 wt. % loading, matrix
activated carbon support). Analytical TLC was performed using prepared plates of silica gel (60 Fꢀ254 on aluminium) or aluminum
oxide (60Å, Fꢀ254 on aluminum) and then, according to the functional groups present on the molecules, revealed with UV light or
using staining reagents: ninhydrin (1.5% in nꢀBuOH with 3% AcOH) for amines or basic solution of KMnO4 (1.0% in H2O) for
general staining. silica gel 60 (70–230 mesh) or aluminium oxide (0.05ꢀ0.15 mm particle size, neutral, Brockmann Activity grade I)
1
was used for flash chromatography. H NMR were recorded at 400 MHz and 500 MHz. 13C NMR spectra were recorded at 100
MHz. Chemical shifts are reported in parts per million (ppm), calibrated on the residual peak of the solvent, whose values are
referred to tetramethylsilane (TMS, δTMS = 0) as the internal standard or the signal of the deuterated solvent. 13C NMR spectra were
performed with proton decoupling. Where indicated, NMR peak assignments were made using COSY and HSQC experiments.
Electrospray ionization (ESI) mass analyses were performed on a mass spectrometer with a linear ion trap mass analyser, while high
resolution ESI mass analyses were recorded on a Orbitrap highꢀresolution mass spectrometer. Infrared analyses were performed on a
FTꢀIR spectrometer. Optical rotations were measured on a polarimeter (Sodium Dꢀline, λ = 589 nm).
Ethyl (3aR,6R,6aS)-2-benzyl-6-(((tert-butyldiphenylsilyl)oxy)methyl)-4-oxohexahydrofuro[3,4-d]isoxazole-3-carboxylate (7)
A solution of nitrone 3a (0.333 gr, 1.61 mmol, 1.14 equiv.) in toluene (9 mL) was heated to 40 °C for 2 hours, followed by the
addition of furanone 2 (0.498 gr, 1.41 mmol, 1.0 equiv.) and additional toluene (3 mL). The reaction mixture was heated to reflux for
4 hours and then conc. in vacuo. The residue was purified via column chromatography (5ꢀ15% EtOAc in petroleum ether 40ꢀ60)
afforded 7a as a yellow oil (563 mg, 68%) RF = 0.52 (8:2; PE:EtOAc) and 7b (154 mg, 20%) as a yellow oil. RF = 0.44 (8:2;
PE:EtOAc). NMR signals of the major adduct 7a anti:a 1H NMR (400 MHz, Chloroformꢀd) δ 7.68 – 7.55 (m, 4H), 7.51 – 7.23 (m,
11H), 4.87 (d, J = 6.3 Hz, 1H), 4.56 (d, J = 2.1 Hz, 1H), 4.26 (q, J = 7.2, 2H), 4.20 (s, 1H), 4.14 – 4.06 (m, 2H), 3.95 (s, 1H), 3.90
(dd, J = 11.6, 2.4 Hz, 1H), 3.73 (dd, J = 11.6, 1.9 Hz, 1H), 2.05 (s, 1H), 1.31 (t, J = 7.2 Hz, 3H), 1.04 (s, 9H). NMR signals of the
major adduct 7b syn:a 1H NMR (400 MHz, Chloroformꢀd) δ 7.66 – 7.54 (m, 4H), 7.48 – 7.27 (m, 11H), 4.91 (dd, J = 7.6, 1.7 Hz,
1H), 4.56 (q, J = 2.1 Hz, 1H), 4.35 – 4.24 (m, 3H), 3.92 – 3.76 (m, 3H), 3.76 – 3.66 (m, 2H), 1.34 (t, J = 7.1 Hz, 3H), 1.02 (s, 9H).
aNMR signals in accordance with NMR spectra by Ondrus et al.25
(3S,3aR,6R,6aS)-2-benzyl-6-(((tert-butyldiphenylsilyl)oxy)methyl)-3-(hydroxymethyl)tetrahydrofuro[3,4-d]isoxazol-4(2H)-
one (10)
To a solution of ester 7a (0.443 gr, 0.736 mmol, 1.0 equiv.) in 1,2ꢀdichloroethane (11 mL) was added Me3SnOH (0.540 gr, 2.99
mmol, 4.06 equiv.). The resulting suspension was heated to reflux for 2.5 h and then concentrated under a N2 stream and then
dissolved in EtOAc (50 mL). The organic layer was washed with 1M aq. HCl (5 mL, 2×), brine (5 mL), dried (MgSO4) and then
conc. in vacuo, affording crude compound 9, which was used without further purification. A small sample of crude 9 was purified by
flash column chromatography (5% MeOH in DCM) to obtain an analytically pure sample. RF = 0.43 (10% MeOH in DCM). 1H
NMR (400 MHz, Chloroformꢀd) δ 7.62 (ddd, J = 8.0, 2.6, 1.5 Hz, 4H), 7.49 – 7.30 (m, 11H), 4.83 (d, J = 6.5 Hz, 1H), 4.74 – 4.59
(m, 1H), 4.26 – 4.15 (m, 3H), 4.09 (d, J = 13.1 Hz, 1H), 3.92 (dd, J = 11.6, 2.6 Hz, 1H), 3.76 (dd, J = 11.6, 1.9 Hz, 1H), 1.04 (s, 9H).
HRMS calcd for C30H32O6NSi − H+ [M−H+]: 530.2004 Found 530.1997. Crude 9 was dissolved in dry THF (18 mL) and added to a
flameꢀdried 3ꢀneck roundꢀbottom flask and cooled to −15 °C. To this solution was added isobutyl chloroformate (1.07 gr, 7.82
mmol, 10.0 equiv.), followed by the dropwise addition of a suspension of NaBH4 (0.297 gr, 7.85 mmol, 10.0 equiv.) in DMF (7.0
mL) over 10 minutes. The resulting reaction mixture was stirred at −15 °C for 75 min. and then quenched with 1M aq. HCl (25 mL).
The reaction mixture was extracted with EtOAc (75 mL, 25 mL, 2×). The combined organic layers were washed with brine (25 mL),
dried (MgSO4) and conc. in vacuo. The residue was purified by flash column chromatography (20ꢀ30% EtOAc in petroleum ether
40ꢀ60), affording compound 10 (385 mg, 94% over 2 steps) as a yellow oil, which crystallized upon standing. RF = 0.32 (7:3;
PE:EtOAc). mp = 109−110 °C. 1H NMR (400 MHz, Chloroformꢀd) δ 7.67 – 7.58 (m, 4H), 7.50 – 7.37 (m, 6H), 7.37 – 7.27 (m, 5H),
4.73 (d, J = 6.5 Hz, 1H), 4.59 (t, J = 2.2 Hz, 1H), 4.12 – 4.00 (m, 2H), 3.91 (dd, J = 11.5, 2.5 Hz, 1H), 3.74 (dd, J = 11.6, 1.9 Hz,
1H), 3.72 – 3.62 (m, 3H), 3.44 (q, J = 3.8 Hz, 1H), 2.17 (s, 1H), 1.04 (s, 9H). 13C NMR (101 MHz, CDCl3) δ 177.5, 136.5, 135.7,
135.6, 132.5, 132.0, 130.3, 130.2, 129.0, 128.7, 128.1, 128.1, 128.0, 83.0, 81.0, 70.6, 64.0, 62.2, 61.6, 52.8, 26.9, 19.2. FTꢀIR (neat):
ν = 3282, 2931, 2859, 1775, 1427 cmꢀ1. HRMS calcd for C30H35O5NSi + Na+ [M+Na+]: 540.2177 Found 540.2162. [α]D20 = −32.0 (c
= 0.40, CHCl3).
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Ethyl
carboxylate
(3S,3aR,6R,6aS)-2-benzyl-6-(((tert-butyldiphenylsilyl)oxy)methyl)-4-hydroxyhexahydrofuro[3,4-d]isoxazole-3-
(11)
and
ethyl
(3S,4S,5S)-2-benzyl-5-((R)-2-((tert-butyldiphenylsilyl)oxy)-1-hydroxyethyl)-4-
(hydroxymethyl)isoxazolidine-3-carboxylate (12).
To a roundꢀbottom flask containing cycloadduct 7a (0.404 g, 0.722 mmol, 1.0 equiv.) at 4 °C was added a cold (4 °C) 2M solution of
BH3ꢀSMe2 in THF (4.00 ml, 8.00 mmol, 11.1 equiv.). The mixture was allowed to warm to room temperature and stirred for 3.5
hours and quenched by carefully adding MeOH (0.30 ml). The mixture was concentrated in vacuo to afford the crude as a mixture of
compound 11 and 12 (0.392 g) as a colourless oil, which was generally used to obtain compound 12 without further purification.
Optionally, the crude could be purified by flash column chromatography (30ꢀ50% EtOAc in petroleum ether 40ꢀ60), affording an
anomeric mixture of hemiacetal 11 (320 mg, 78%) as a colourless oil (HRMS calcd for C32H39O6NSi + Na+ [M+Na+]: 584.2439
Found 584.2429) RF = 0.57 (7:3; PE:EtOAc) and diol 12 (60 mg, 16%) as a colourless oil.
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Hemiacetal 11 (0.100 g, 162 mmol, 1.0 equiv.) was dissolved in MeOH (3.0 ml) and cooled to −2 °C, NaBH4 (13.5 mg, 0.357
mmol, 2.0 equiv.) was added and the resulting solution was then stirred at −3ꢀ7°C for 1 h. Additional NaBH4 (3.4 mg, 0.5 equiv.)
was added and the reaction mixture was stirred for an additional 40 min., followed by the portionꢀwise addition of 1M aq. HCl (10
mL). The reaction mixture was extracted with EtOAc (10 mL, 3×) and the combined organic layers were washed with sat. aq.
NaHCO3 solution (10 ml) and brine (10 ml). Then it was dried over MgSO4 and concentrated in vacuo. The resulting oil was
purified by flash column chromatography (30ꢀ50% EtOAc in petroleum ether 40ꢀ60), affording diol 12 (87.2 mg, 87%) as a
colourless oil. RF = 0.27 (7:3; PE:EtOAc). 1H NMR (400 MHz, Chloroformꢀd) δ 7.64 (ddt, J = 9.8, 6.8, 1.5 Hz, 4H), 7.50 – 7.30 (m,
6H), 7.24 (s, 5H), 4.18 (dd, J = 9.3, 7.1 Hz, 1H), 4.09 (q, J = 7.1 Hz, 2H), 3.99 (d, J = 13.2 Hz, 1H), 3.96 – 3.87 (m, 3H), 3.83 (dd, J
= 10.5, 3.2 Hz, 1H), 3.76 – 3.63 (m, 3H), 3.28 (d, J = 4.6 Hz, 1H), 3.24 – 3.13 (m, 1H), 3.10 (d, J = 7.7 Hz, 1H), 1.22 (t, J = 7.2 Hz,
3H), 1.06 (s, 9H). 13C NMR (101 MHz, CDCl3) δ 170.1, 135.8, 135.7, 135.6, 133.0, 132.7, 130.1, 130.1, 129.7, 128.3, 128.0, 127.7,
78.3, 69.8, 65.5, 62.3, 61.6, 60.6, 52.1, 27.0, 19.4, 14.2. FTꢀIR (neat): ν = 3385, 2931, 2857, 1738, 1472, 1428 cmꢀ1. HRMS calcd for
C32H41O6NSi + H+ [M+H+]: 564.2776 Found 564.2766. [α]D20 = −22.7 (c = 1.00, CHCl3).
Benzyl
((S)-1-((3R,4S,5R)-5-(((tert-butyldiphenylsilyl)oxy)methyl)-4-hydroxy-2-oxotetrahydrofuran-3-yl)-2,2-
dimethoxyethyl)carbamate (13)
A solution of cycloadduct 8a (0.124 gr, 0.221 mmol, 1.00 equiv.) in dry THF (1.0 mL) was added to a roundꢀbottom flask containing
Raneyꢀnickel (0.400 gr). The reaction mixture was placed under and hydrogen atmosphere (1 bar; balloon) and stirred vigorously for
5 h. TLC indicated incomplete conversion at this point so the reaction mixture was transferred to a roundꢀbottom flask containing
fresh Raneyꢀnickel (0.500 gram) and stirred under an hydrogen atmosphere (1 bar; balloon) for an additional 2 hours. The reaction
mixture was then placed under an argon atmosphere (1 bar) and Pd/C (0.100 gram) was added, followed by cyclohexene (2.5 mL).
The resulting mixture was heated to reflux for 3 h and then cooled to 4 °C. Finally, H2O (0.6 mL), NaHCO3 (74 mg, 0.883 mmol,
4.00 equiv.) and CbzCl (0.094 mL, 0.662 mmol, 3.00 equiv.) were added sequentially and the resulting reaction mixture was stirred
for 14 h, while allowing the mixture to warm to room temperature. The reaction mixture was filtered over Celite, the celite was
subsequently washed with THF and the combined filtrate conc. in vacuo. The resulting residue was purified by flash column
chromatography (20ꢀ30% EtOAc in petroleum ether 40ꢀ60), affording compound 13 (60.5 mg, 45%) as a yellow oil. RF = 0.23 (7:3;
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PE:EtOAc). H NMR (400 MHz, Chloroformꢀd) δ 7.73 – 7.55 (m, 4H), 7.50 – 7.29 (m, 11H), 5.68 (d, J = 8.4 Hz, 1H), 5.25 – 5.03
(m, 3H), 4.69 (d, J = 2.5 Hz, 1H), 4.48 (dd, J = 5.3, 2.5 Hz, 1H), 4.43 (t, J = 2.5 Hz, 1H), 4.30 (td, J = 8.5, 2.4 Hz, 1H), 3.89 (dd, J =
11.8, 2.9 Hz, 1H), 3.77 (dd, J = 11.7, 2.1 Hz, 1H), 3.50 (s, 3H), 3.47 (s, 3H), 3.45 – 3.39 (m, 1H), 1.02 (s, 9H). 13C NMR (101 MHz,
CDCl3) δ 175.2, 157.8, 136.0, 135.8, 135.6, 132.6, 132.0, 130.2, 128.7, 128.5, 128.3, 128.0, 103.5, 85.3, 71.8, 67.6, 63.9, 57.0, 56.9,
50.5, 47.9, 26.8, 19.2. HRMS calcd for C33H41O8NSi + Na+ [M+Na+]: 630.2494 Found 630.2483.
(S)-(5-oxo-2,5-dihydrofuran-2-yl)methyl 4-methylbenzenesulfonate (14).
To a solution of furanone 4 (215.6 mg, 1.890 mmol, 1.0 equiv.) in dry DCM (1.0 mL) at −15 °C was added pyridine (0.377 mL, 4.67
mmol, 2.5 equiv.), followed by the portionꢀwise addition of TsCl (0.540 gr, 2.83 mmol, 1.5 equiv.). The reaction mixture was stirred
at −15 °C and allowed to warm to 5 °C over 1 hour. The reaction mixture was diluted with DCM (30 mL) and washed with 1M aq.
HCl (10 mL, 3×), sat. aq. NaHCO3 (10 mL), brine (10 mL). The organic phase was dried (MgSO4) and then conc. in vacuo. The
resulting residue was purified by flash column chromatography (0ꢀ100% MeOH in DCM), affording the product 14 (355 mg, 70%)
as a colorless oil. 1H NMR (400 MHz, Chloroformꢀd) δ 7.81 – 7.73 (m, 2H), 7.44 (dd, J = 5.8, 1.6 Hz, 1H), 7.37 (d, J = 8.0 Hz, 2H),
6.21 (dd, J = 5.8, 2.1 Hz, 1H), 5.19 (tt, J = 4.8, 1.9 Hz, 1H), 4.29 – 4.17 (m, 2H), 2.46 (s, 3H). 13C NMR (101 MHz, CDCl3) δ 171.7,
151.8, 145.7, 132.2, 130.2, 128.1, 123.9, 79.8, 67.5, 21.8. FTꢀIR (neat): ν = 3100, 2956, 2927, 1756, 1598, 1494, 1452, 1400 cmꢀ1.
HRMS calcd for C12H12O5S + Na+ [M+Na+]: 291.0298. Found 291.0290. [α]D20 = −46.5 (c = 1.00, CHCl3).
Ethyl (3aR,6R,6aS)-2-benzyl-6-(hydroxymethyl)-4-oxohexahydrofuro[3,4-d]isoxazole-3-carboxylate (6a) Nitrone 3a (4.83 gr,
23.3 mmol, 1.09 equiv.) was added to a solution of furanone 4 (2.44 mg, 21.4 mmol, 1.00 equiv.) in toluene (10 mL) at 45 °C. The
reaction mixture was heated to reflux for 4.5 hours and then conc. in vacuo. The residue was purified via column chromatography
(30ꢀ40% EtOAc in heptane) to afford 6a (80.5 mg, 55%) as a yellow oil (that crystallized upon standing) and 6b (25.1 mg, 31%) as a
yellow solid. NMR signals of the major adduct 6a anti:a RF = 0.27 (1:1; PE:EtOAc). mp = 83−84 °C. 1H NMR (400 MHz,
Chloroformꢀd) δ 7.44 – 7.04 (m, 5H), 4.73 (d, J = 6.4 Hz, 1H), 4.50 (d, J = 2.5 Hz, 1H), 4.15 (q, J = 7.1 Hz, 2H), 4.05 (q, J = 13.8
Hz, 2H), 3.95 – 3.75 (m, 3H), 3.67 (d, J = 12.3 Hz, 1H), 2.57 (s, 1H), 1.23 (t, J = 7.2 Hz, 3H). NMR signals of the minor adduct 6b
syn:a RF = 0.22 (1:1; PE:EtOAc). mp = 144−145 °C. 1H NMR (400 MHz, Chloroformꢀd) δ 7.38 – 7.26 (m, 5H), 4.86 (dd, J = 7.7, 2.2
Hz, 1H), 4.59 (q, J = 2.4 Hz, 1H), 4.32 – 4.19 (m, 3H), 3.94 (ddd, J = 12.4, 5.1, 2.5 Hz, 1H), 3.84 (d, J = 13.9 Hz, 1H), 3.80 – 3.70
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(m, 2H), 3.67 (d, J = 7.6 Hz, 1H), 1.82 (dd, J = 6.8, 5.2 Hz, 1H), 1.32 (t, J = 7.1 Hz, 3H). [α]D20 = +115.6 (c = 1.00, CHCl3). a NMR
signals in accordance with NMR spectra by Ondrus et al.25
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Ethyl (3S,3aR,6R,6aS)-6-(azidomethyl)-2-benzyl-4-oxohexahydrofuro[3,4-d]isoxazole-3-carboxylate (16)
Method A:
To a solution of cycloadduct 6a (31.0 mg, 0.0965 mmol, 1.00 equiv.) in dry THF (0.5 mL) at −20 °C was added PPh3 (50.6 mg,
0.193 mmol, 2.00 equiv.), followed by the dropwise addition of diisopropyl azodicarboxylate (38.0 ꢁL, 0.193 mmol, 2.00 equiv.)
and (PhO)2P(O)N3 (41.5 ꢁL, 0.193 mmol, 2.00 equiv.). The resulting reaction mixture was stirred at −20 to −15 °C for 30 min. and
then allowed to warm to room temperature and stirred for 1 hour. H2O (10 mL) and EtOAc (10 mL) were then added to the reaction
mixture and the biphasic system was separated. The water layer was extracted with EtOAc (5 mL, 2×) and the combined organic
layers were washed with brine (5 mL), dried (MgSO4) and then conc. in vacuo. The residue was purified by flash column
chromatography (2ꢀ5% MeOH in DCM), affording compound 16 (24.0 mg, 72%) as a yellow oil. RF = 0.44 (7:3; PE:EtOAc).
Method B:
To a solution of cycloadduct 6a (16.79 gr, 52.26 mmol, 1.00 equiv.) in DCM (86 mL) at 0 °C was added Et3N (15.35 mL, 110 mmol,
2.10 equiv.), followed by the dropwise addition of MsCl (15.35 mL, 110 mmol, 2.10 equiv.). The resulting reaction mixture was
stirred at 4 °C for 50 min.. Sat. aq. NaHCO3 (400 mL) was added to the reaction mixture, followed by DCM (1 L). The resulting
biphasic system was separated and the water layer was extracted with DCM (2×400 mL, 100 mL) and the combined organic layers
were dried (MgSO4) and then conc. in vacuo, affording the crude mesylated intermediate, which was dissolved in DMF (86 mL).
NaN3 (14.0 gr, 215 mmol, 4.00 equiv.) was added the reaction mixture was heated to 60 °C for 90 min. Additional DMF (50 mL)
was added, followed by H2O (400 mL). The reaction mixture was extracted with Et2O (1 L, 400 mL, 2×200 mL) and the combined
organic layers were washed with 5% aq. LiCl (200 mL, 2×), dried (MgSO4) and conc. in vacuo. The residue was purified by flash
column chromatography (10ꢀ25^EtOAc in petroleum ether 40ꢀ60), affording compound 16 (17.67 gr, 89% over 2 steps) as an orange
oil. RF = 0.44 (7:3; PE:EtOAc). 1H NMR (400 MHz, Chloroformꢀd) δ 7.38 – 7.24 (m, 5H), 4.66 (d, J = 6.5 Hz, 1H), 4.60 (t, J = 3.1
Hz, 1H), 4.25 (q, J = 7.2 Hz, 2H), 4.18 – 4.03 (m, 2H), 4.03 – 3.87 (m, 2H), 3.71 (dd, J = 13.3, 3.3 Hz, 1H), 3.56 (dd, J = 13.3, 3.0
Hz, 1H), 1.32 (t, J = 7.1 Hz, 3H). 13C NMR (101 MHz, CDCl3) δ 175.3, 167.9, 136.2, 128.8, 128.5, 127.8, 81.5, 79.8, 69.4, 62.1,
60.3, 52.7, 52.5, 14.2. FTꢀIR (neat): ν = 2983, 2108, 1779, 1734, 1606, 1497, 1455 cmꢀ1. HRMS calcd for C16H18O5N4 + H+ [M+H+]:
347.1350. Found 347.1344 [α]D20 = +32.5 (c = 1.00, CHCl3).
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Ethyl (3S,3aR,6R,6aS)-6-(azidomethyl)-2-benzyl-4-hydroxyhexahydrofuro[3,4-d]isoxazole-3-carboxylate (17).
To a roundꢀbottom flask containing lactone 15 (9.28 gr, 26.8 mmol, 1.0 equiv.) at 4 °C was added a 4 °C a 2M solution of BH3ꢀSMe2
in THF (110 mL, 220 mmol, 8.2 equiv.). The resulting solution was stirred at 4 °C for 15 minutes and then allowed to warm to room
temperature and stirred for 7 hours. The reaction mixture was cooled to 4 °C and quenched by the portionꢀwise addition of MeOH
(200 mL). The reaction mixture was then conc. in vacuo and then purified by flash column chromatography (25ꢀ50 EtOAc in
heptane) affording an anomeric mixture of compound 17 (3.16 gr, 34%) as a white solid. RF = 0.58 (1:1; PE:EtOAc). mp = 96−98
°C. 1H NMR (400 MHz, Chloroformꢀd) δ 7.41 – 7.22 (m, 5H), 5.57 (d, J = 5.3 Hz, 1H), 4.63 (dd, J = 7.1, 1.0 Hz, 1H), 4.32 (dd, J =
7.3, 4.8 Hz, 1H), 4.29 – 4.13 (m, 3H), 3.96 (d, J = 13.7 Hz, 1H), 3.58 (dd, J = 12.7, 7.4 Hz, 1H), 3.49 – 3.32 (m, 4H), 1.29 (t, J = 7.1
Hz, 3H). 13C NMR (101 MHz, CDCl3) δ 169.0, 136.3, 136.2, 129.1, 128.9, 128.6, 128.4, 128.4, 127.9, 127.8, 103.4, 98.1, 83.9, 83.8,
83.4, 80.5, 70.9, 66.1, 61.8, 61.6, 61.0, 60.7, 55.6, 53.9, 52.5, 14.3, 14.2. FTꢀIR (neat): ν = 3436, 2981, 2936, 2099, 1734, 1497,
1455 cmꢀ1
HRMS calcd for C16H20O5N4 + H+ [M+H+]: 349.1506. Found 349.1496. [α]D20 = −54.8 (c = 1.00, CHCl3).
Ethyl (3aR,7R,7aS)-2-benzyl-7-hydroxyoctahydroisoxazolo[4,5-c]pyridine-3-carboxylate (19).
To a solution of azidoꢀaldehyde 17 (0.106 gr, 0.304 mmol, 1 equiv.) in dry EtOH (1.6 mL) at 0 °C was added a solution of
trimethylphosphine (1M in THF, 0.610 mL, 2 equiv.). The reaction mixture was stirred at 0 °C for 3 hours, then concentrated and
subsequently coꢀevaporated with dry toluene (3×). The residue was dissolved in dry THF (1.6 mL) and cooled to 0 °C. (AcO)3BHNa
(0.184 gr, 0.868 mmol, 2.86 equiv.) was added and the resulting reaction mixture was stirred for 1.5 h. The reaction was then
quenched with sat. aq. NaHCO3 (20 mL) and then extracted with EtOAc (20 ml, 2×10 mL). The combined organic layers were
washed with brine (10 mL), dried (MgSO4) and conc. in vacuo. The water layer was then additionally extracted with DCM (10 mL,
5×) and the combined organic layers were dried (MgSO4) and conc. in vacuo. The residues from the EtOAcꢀ and the DCMꢀ
extractions were both purified by flash column chromatography (5ꢀ10% MeOH in DCM) to afford compound 19 (61.1 mg, 66%) as
white crystals. RF = 0.14 (10% MeOH in DCM). mp = 120−122 °C. 1H NMR (400 MHz, Methanolꢀd4) δ 7.47 – 7.22 (m, 5H), 4.36 –
4.25 (m, 2H), 4.21 (d, J = 13.2 Hz, 1H), 4.06 (q, J = 7.1 Hz, 2H), 3.82 (ddd, J = 10.0, 4.8, 3.8 Hz, 1H), 3.41 (d, J = 3.1 Hz, 1H), 2.81
(dd, J = 12.7, 5.7 Hz, 2H), 2.77 – 2.63 (m, 2H), 2.38 (dd, J = 12.7, 9.5 Hz, 1H), 1.16 (t, J = 7.2 Hz, 3H). 13C NMR (101 MHz,
MeOD) δ 172.3, 137.0, 131.2, 129.3, 128.8, 78.4, 70.2, 67.6, 63.5, 62.3, 48.1, 47.6, 46.2, 14.4. FTꢀIR (neat): ν = 3308, 2847, 1734,
1498 ,1454, 1414 cmꢀ1. HRMS calcd for C16H23O4N2 + H+ [M+H+]: 307.1652. Found 307.1644. [α]D20 = −43.0 (c = 1.00, CHCl3).
General procedure A & B
To a roundbottom flask containing a 0.18M solution of azidoꢀaldehyde 17 (1 equiv.) in dry EtOH at 0 °C was added a solution of
trimethylphosphine (1M in THF, 2 equiv.). The reaction mixture was stirred at 0 °C for 3 hours, then concentrated and
subsequenently coꢀevaporated with dry toluene (3×). The crude cyclic imine was dissolved dry EtOH (0.3M) (procedure A) or dry
2,2,2ꢀtrifluoroethanol (0.3M) (procedure B), divided in the appropriate amount of portions and cooled to 0 °C. Next, the appropriate
carboxylic acid (1.25 equiv.) and isocyanide (1.25 equiv.) were successfully added dropwise and the resulting reaction mixture was
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stirred for 20 hours while allowing the mixture to warm to room temperature. Saturated NaHCO3 was added to the mixture was
extracted with EtOAc (3×). The combined organic layers were washed with sat. NaHCO3 and brine, dried (MgSO4) and conc. in
vacuo. The SAWUꢀ3ꢀCR products were purified by silica gel flash column chromatography.
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Ethyl
(3S,3aR,4R,7R,7aS)-5-acetyl-2-benzyl-4-(tert-butylcarbamoyl)-7-hydroxyoctahydroisoxazolo[4,5-c]pyridine-3-
carboxylate 20a and ethyl (3S,3aR,4S,7R,7aS,Z)-2-benzyl-9-(tert-butylimino)octahydro-7,4-(epoxymethano)isoxazolo[4,5-
c]pyridine-3-carboxylate 21a
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According to general procedure A the crude was obtained and purified via silicagel column chromatography (50ꢀ100% EtOAc in
heptane then 0ꢀ100% EtOH in EtOAc) affording both 20a (45.7 mg, 31%) and 21a (40.3 mg, 29%) as light yellow oils. NMR signals
and other experimental data of compound 20a: RF = 0.41 (100% EtOAc) 1H NMR (400 MHz, Chloroformꢀd) δ 7.39 – 7.23 (m, 5H),
6.41 (s, 1H), 5.05 (d, J = 1.4 Hz, 1H), 4.56 (dd, J = 9.3, 3.6 Hz, 1H), 4.27 (dd, J = 13.9, 9.1 Hz, 1H), 4.17 (qd, J = 7.1, 1.6 Hz, 2H),
4.08 – 3.98 (m, 2H), 3.64 (td, J = 9.4, 1.4 Hz, 1H), 3.36 – 3.20 (m, 2H), 3.18 (s, 1H), 2.53 – 2.44 (m, 1H), 2.05 (s, 3H), 1.31 – 1.21
(s, 12H, tBu & COOCH2CH3). 13C NMR (101 MHz, CDCl3) δ 172.8, 169.7, 168.7, 134.9, 130.1, 128.4, 128.0, 73.2, 68.1, 64.5, 61.9,
59.9, 52.5, 51.3, 45.2, 45.1, 28.7, 21.7, 14.2. FTꢀIR (neat): ν = 3333, 2968, 1746, 1685, 1641, 1538, 1455, 1406 cmꢀ1. HRMS calcd
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for C23H33N3O6+H+ [M+H+]: 448.2442. Found. 448.2425. [α]D = +51.2 (c = 1.00, CHCl3). NMR signals and other experimental
data of compound 21a: RF = 0.42 (100% EtOH) 1H NMR (400 MHz, Chloroformꢀd) δ 7.44 – 7.36 (m, 2H), 7.39 – 7.30 (m, 2H), 7.35
– 7.24 (m, 1H), 4.55 (ddd, J = 4.3, 2.3, 0.9 Hz, 1H), 4.34 (d, J = 13.6 Hz, 1H), 4.29 – 4.17 (m, 2H), 4.21 – 4.10 (m, 1H), 3.81 (d, J =
13.6 Hz, 1H), 3.52 (d, J = 3.1 Hz, 1H), 3.39 – 3.23 (m, 3H), 2.88 (dd, J = 12.1, 0.9 Hz, 1H), 2.22 – 2.02 (m, 1H), 1.36 (s, 9H), 1.29
(t, J = 7.1 Hz, 3H). 13C NMR (101 MHz, CDCl3) δ 169.1, 155.2, 136.9, 129.1, 128.4, 127.6, 75.0, 75.0, 70.3, 69.4, 61.9, 61.7, 53.4,
52.4, 51.1, 42.8, 30.0, 14.3. FTꢀIR (neat): ν = 3378, 3034, 2966, 1741, 1684, 1516, 1477, 1455 cmꢀ1. HRMS calcd for
C21H29N3O4+H+ [M+H+]: 388.2231. Found. 388.2223. [α]D20 = −32.4 (c = 1.00, CHCl3).
Ethyl (3S,3aR,4R,7R,7aS)-2-benzyl-4-(tert-butylcarbamoyl)-7-hydroxy-5-(pent-4-enoyl)octahydroisoxazolo[4,5-c]pyridine-3-
carboxylate 20b and ethyl (3S,3aR,4S,7R,7aS,Z)-2-benzyl-9-(tert-butylimino)octahydro-7,4-(epoxymethano)isoxazolo[4,5-
c]pyridine-3-carboxylate 21a.
According to general procedure A the crude was obtained and purified via column chromatography (50ꢀ100% EtOAc in heptane
then 0ꢀ100% EtOH in EtOAc) affording both 20b (41.4 mg, 30%) and 21a (39.2 mg, 35%) as yellow oils. NMR signals and other
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experimental data of compound 20b: RF = 0.76 (100% EtOAc). H NMR (400 MHz, Chloroformꢀd) δ 7.39 – 7.27 (m, 5H), 6.43 (s,
1H), 5.93 – 5.78 (m, 1H), 5.14 – 4.98 (m, 3H), 4.57 (dd, J = 9.3, 3.6 Hz, 1H), 4.30 (d, J = 14.1 Hz, 1H), 4.18 (qd, J = 7.2, 2.4 Hz,
2H), 4.11 – 3.98 (m, 2H), 3.70 – 3.61 (m, 1H), 3.40 – 3.22 (m, 2H), 3.17 (s, 1H), 2.56 – 2.27 (m, 5H), 1.31 – 1.24 (m, 12H). 13C
NMR (101 MHz, CDCl3) δ 174.6, 169.8, 168.6, 137.0, 134.8, 130.1, 128.4, 128.0, 115.8, 73.3, 68.1, 64.6, 61.9, 59.9, 52.7, 51.3,
45.1, 44.7, 32.4, 28.9, 28.7, 14.2. FTꢀIR (neat): ν = 3338, 2976, 1737, 1677, 1635, 1537, 1498, 1454, 1415 cmꢀ1. HRMS calcd for
C26H37N3O6+H+ [M+H+]: 488.2755. Found. 488.2741. [α]D20 = +66.5 (c = 1.00, CHCl3). NMR signals and other experimental data of
compound 21a: RF = 0.42 (100% EtOH) 1H NMR (400 MHz, Chloroformꢀd) δ 7.44 – 7.36 (m, 2H), 7.39 – 7.30 (m, 2H), 7.35 – 7.24
(m, 1H), 4.55 (ddd, J = 4.3, 2.3, 0.9 Hz, 1H), 4.34 (d, J = 13.6 Hz, 1H), 4.29 – 4.17 (m, 2H), 4.21 – 4.10 (m, 1H), 3.81 (d, J = 13.6
Hz, 1H), 3.52 (d, J = 3.1 Hz, 1H), 3.39 – 3.23 (m, 3H), 2.88 (dd, J = 12.1, 0.9 Hz, 1H), 2.22 – 2.02 (m, 1H), 1.36 (s, 9H), 1.29 (t, J =
7.1 Hz, 3H). 13C NMR (101 MHz, CDCl3) δ 169.1, 155.2, 136.9, 129.1, 128.4, 127.6, 75.0, 75.0, 70.3, 69.4, 61.9, 61.7, 53.4, 52.4,
51.1, 42.8, 30.0, 14.3. FTꢀIR (neat): ν = 3378, 3034, 2966, 1741, 1684, 1516, 1477, 1455 cmꢀ1. HRMS calcd for C21H29N3O4+H+
[M+H+]: 388.2231. Found. 388.2223. [α]D20 = −32.4 (c = 1.00, CHCl3).
Ethyl
c]pyridine-3-carboxylate
(epoxymethano)isoxazolo[4,5-c]pyridine-3-carboxylate 21a.
(3S,3aR,4R,7R,7aS)-2-benzyl-4-(tert-butylcarbamoyl)-7-hydroxy-5-(2,2,2-trifluoroacetyl)octahydroisoxazolo[4,5-
20c and ethyl (3S,3aR,4S,7R,7aS,Z)-2-benzyl-9-(tert-butylimino)octahydro-7,4-
According to general procedure A the crude was obtained and purified via column chromatography (30ꢀ50% EtOAc in heptane then
0ꢀ40% EtOH in EtOAc) affording impure 20c and pure 21a (70 mg, 49%) as a crystalline solid. Impure 20c was further purified via
column chromatography (20% acetone in toluene) affording 20c (50.5 mg, 27%) as a colorless oil. NMR signals and other
experimental data of compound 20c: RF = 0.47 (1:1; PE:EtOAc). 1H NMR (400 MHz, Chloroformꢀd) δ 7.36 – 7.28 (m, 5H), 6.09 (s,
1H), 4.98 (d, J = 1.5 Hz, 1H), 4.56 (dd, J = 9.1, 3.9 Hz, 1H), 4.32 (d, J = 14.2 Hz, 1H), 4.21 (qd, J = 7.2, 6.5, 1.3 Hz, 2H), 4.12 –
3.98 (m, 2H), 3.67 – 3.58 (m, 2H), 3.36 (s, 1H), 2.25 (s, 1H), 1.35 – 1.27 (m, 12H). 13C NMR (101 MHz, CDCl3) δ 168.5, 168.0,
159.6, 159.2, 158.8, 158.4, 135.1, 129.7, 129.4, 128.6, 128.1, 120.6, 117.7, 114.9, 112.0, 72.9, 68.4, 63.6, 62.2, 60.4, 54.3, 51.9,
45.1, 44.1, 28.7, 14.2.
FTꢀIR (neat): ν = 3368, 2975, 1731, 1673, 1526, 1455 cmꢀ1. HRMS calcd for C23H30F3N3O6+H+ [M+H+]: 502.2159. Found.
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502.2143. [α]D = +32.7 (c = 0.93, CHCl3). NMR signals and other experimental data of compound 21a: RF = 0.42 (100% EtOH).
1H NMR (400 MHz, Chloroformꢀd) δ 7.44 – 7.36 (m, 2H), 7.39 – 7.30 (m, 2H), 7.35 – 7.24 (m, 1H), 4.55 (ddd, J = 4.3, 2.3, 0.9 Hz,
1H), 4.34 (d, J = 13.6 Hz, 1H), 4.29 – 4.17 (m, 2H), 4.21 – 4.10 (m, 1H), 3.81 (d, J = 13.6 Hz, 1H), 3.52 (d, J = 3.1 Hz, 1H), 3.39 –
3.23 (m, 3H), 2.88 (dd, J = 12.1, 0.9 Hz, 1H), 2.22 – 2.02 (m, 1H), 1.36 (s, 9H), 1.29 (t, J = 7.1 Hz, 3H). 13C NMR (101 MHz,
CDCl3) δ 169.1, 155.2, 136.9, 129.1, 128.4, 127.6, 75.0, 75.0, 70.3, 69.4, 61.9, 61.7, 53.4, 52.4, 51.1, 42.8, 30.0, 14.3. FTꢀIR (neat):
ν = 3378, 3034, 2966, 1741, 1684, 1516, 1477, 1455 cmꢀ1. HRMS calcd for C21H29N3O4+H+ [M+H+]: 388.2231. Found. 388.2223.
[α]D20 = −32.4 (c = 1.00, CHCl3).
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Ethyl
(3S,3aR,4R,7R,7aS)-5-benzoyl-2-benzyl-4-(tert-butylcarbamoyl)-7-hydroxyoctahydroisoxazolo[4,5-c]pyridine-3-
carboxylate 20d and ethyl (3S,3aR,4S,7R,7aS,Z)-2-benzyl-9-(tert-butylimino)octahydro-7,4-(epoxymethano)isoxazolo[4,5-
c]pyridine-3-carboxylate 21a.
According to general procedure A the crude was obtained and purified via column chromatography (50ꢀ100% EtOAc in heptane
then 0ꢀ100% EtOH in EtOAc) affording 20d (56.4, 33%) as a white turbid oil and 21a (46.4 mg, 35%) as a yellow oil. NMR signals
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and other experimental data of compound 20d: RF = 0.30 (1:1; PE:EtOAc). H NMR (400 MHz, Chloroformꢀd) δ 7.49 – 7.30 (m,
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10H), 6.85 (s, 1H), 5.23 (s, 1H), 4.58 (dd, J = 9.1, 4.0 Hz, 1H), 4.37 (d, J = 14.0 Hz, 1H), 4.21 (q, J = 7.2 Hz, 2H), 4.11 (d, J = 14.0
Hz, 1H), 3.93 – 3.83 (m, 1H), 3.70 (t, J = 9.5 Hz, 1H), 3.63 – 3.43 (m, 2H), 3.11 (d, J = 12.9 Hz, 1H), 2.61 (s, 1H), 1.33 (s, 9H), 1.31
– 1.26 (m, 3H). 13C NMR (101 MHz, CDCl3) δ 173.7, 169.5, 168.6, 135.0, 134.7, 130.5, 130.0, 128.5, 127.9, 127.7, 73.7, 67.6, 64.4,
61.9, 60.0, 53.4, 51.3, 46.7, 44.6, 28.8, 14.2. FTꢀIR (neat): ν = 3304, 2933, 1735, 1681, 1621, 1511, 1414 cmꢀ1. HRMS calcd for
C28H35N3O6+H+ [M+H+]: 510.2599. Found. 510.2587. [α]D20 = +71.9 (c = 1.00, CHCl3).
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Ethyl
(3S,3aR,4R,7R,7aS)-2-benzyl-4-(butylcarbamoyl)-7-hydroxy-5-(pent-4-enoyl)octahydroisoxazolo[4,5-c]pyridine-3-
carboxylate 20e and ethyl (3S,3aR,4S,7R,7aS,Z)-2-benzyl-9-(butylimino)octahydro-7,4-(epoxymethano)isoxazolo[4,5-
c]pyridine-3-carboxylate 21e.
According to general procedure A the crude was obtained and purified via column chromatography (60ꢀ100% EtOAc in heptane
then 0ꢀ100% EtOH in EtOAc) affording both 20e (47.5 mg, 28%) and 21e (26%)a as yellow oils. NMR signals and other
experimental data of compound 20e: RF = 0.44 (75% EtOAc in PE). 1H NMR (400 MHz, Chloroformꢀd) δ 7.41 – 7.21 (m, 5H), 6.60
– 6.45 (m, 1H), 5.85 (ddt, J = 16.1, 10.7, 6.0 Hz, 1H), 5.19 – 4.96 (m, 3H), 4.58 (dd, J = 9.3, 3.6 Hz, 1H), 4.31 (d, J = 14.1 Hz, 1H),
4.17 (qt, J = 7.4, 3.7 Hz, 2H), 4.14 – 3.97 (m, 2H), 3.69 (t, J = 9.3 Hz, 1H), 3.37 – 3.23 (m, 2H), 3.16 (q, J = 6.7 Hz, 3H), 2.48 – 2.24
(m, 5H), 1.42 (p, J = 7.2 Hz, 2H), 1.35 – 1.20 (m, 5H), 0.89 (t, J = 7.3 Hz, 3H). 13C NMR (101 MHz, CDCl3) δ 174.6, 170.6, 168.6,
137.1, 134.9, 130.1, 128.4, 128.0, 115.7, 73.2, 68.2, 64.6, 61.9, 59.9, 52.0, 45.3, 44.7, 39.4, 32.5, 31.6, 28.9, 20.1, 14.2, 13.8. FTꢀIR
(neat): ν = 3340, 3034, 2959, 2931, 2873, 1735, 1636, 1529, 1497, 1454, 1415 cmꢀ1. HRMS calcd for C26H38N3O6+H+ [M+H+]:
488.2755. Found. 488.2745
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[α]D = +51.2 (c = 0.60, CHCl3). NMR signals and other experimental data of compound 21ea. RF = 0.27 (100% EtOH). HRMS
calcd for C21H29N3O4+H+ [M+H+]: 388.2231. Found. 388.2214. The minor product could not be completely purified, but a
a
relatively pure fraction was obtained for structure determination by NMR.
Ethyl (3S,3aR,4R,7R,7aS)-2-benzyl-4-(cyclohexylcarbamoyl)-7-hydroxy-5-(pent-4-enoyl)octahydroisoxazolo[4,5-c]pyridine-3-
carboxylate 20f and ethyl (3S,3aR,4S,7R,7aS,Z)-2-benzyl-9-(cyclohexylimino)octahydro-7,4-(epoxymethano)isoxazolo[4,5-
c]pyridine-3-carboxylate 21f.
According to general procedure B the crude was obtained and purified via column chromatography (50ꢀ100% EtOAc in heptane then
0ꢀ40% EtOH in EtOAc) affording 20f (50.6 mg, 34%) as a light yellow oil and 21f (55.5 mg, 46%) as a yellow oil. NMR signals and
other experimental data of compound 20f: RF = 0.54 (75% EtOAc in PE). 1H NMR (400 MHz, Chloroformꢀd) δ 7.37 – 7.27 (m, 5H),
6.43 (d, J = 8.1 Hz, 1H), 5.85 (dddd, J = 16.2, 10.7, 5.8, 3.8 Hz, 1H), 5.18 – 4.99 (m, 3H), 4.57 (dd, J = 9.3, 3.6 Hz, 1H), 4.31 (d, J =
14.0 Hz, 1H), 4.22 – 4.13 (m, 2H), 4.12 – 3.98 (m, 2H), 3.73 – 3.58 (m, 2H), 3.40 – 3.25 (m, 2H), 3.20 (s, 1H), 2.39 (ddt, J = 16.7,
13.2, 8.5 Hz, 5H), 1.89 – 1.80 (m, 1H), 1.80 – 1.72 (m, 1H), 1.70 – 1.52 (m, 4H), 1.38 – 1.22 (m, 6H), 1.22 – 1.03 (m, 3H). 13C
NMR (101 MHz, CDCl3) δ 174.6, 169.6, 168.7, 137.0, 135.0, 130.0, 128.4, 128.0, 115.8, 73.3, 68.2, 64.6, 61.9, 60.0, 52.1, 48.4,
45.2, 44.7, 32.9, 32.7, 32.5, 28.9, 25.6, 24.7, 14.2. FTꢀIR (neat): ν = 3338, 2932, 2856, 1735, 1636, 1525, 1452, 1416 cmꢀ1. HRMS
calcd for C28H39N3O6+H+ [M+H+]: 514.2912. Found. 514.2898. [α]D = +42.9 (c = 1.00, CHCl3). NMR signals and other
experimental data of compound 21f: RF = 0.47 (100% EtOH). H NMR (400 MHz, Chloroformꢀd) δ 7.30 (m, 5H), 4.55 – 4.43 (m,
1H), 4.27 (d, J = 13.8 Hz, 1H), 4.24 – 4.05 (m, 4H), 3.83 (d, J = 13.8 Hz, 1H), 3.65 – 3.54 (m, 1H), 3.54 – 3.49 (m, 1H), 3.37 – 3.20
(m, 3H), 2.86 (d, J = 12.1 Hz, 1H), 1.82 – 1.70 (m, 4H), 1.63 (d, J = 12.7 Hz, 1H), 1.36 – 1.18 (m, 8H). 13C NMR (101 MHz, CDCl3)
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δ 169.0, 155.9, 136.5, 129.2, 128.3, 127.6, 75.1, 70.5, 68.8, 61.6, 61.3, 53.4, 51.8, 51.2, 42.9, 34.0, 33.7, 25.9, 25.2, 25.1, 14.2. FTꢀ
IR (neat): ν = 2928, 2853, 1738, 1689, 1451 cmꢀ1. HRMS calcd for C23H31N3O4+H+ [M+H+]: 414.2387. Found. 414.2391. [α]D
=
20
−24.1 (c = 1.00, CHCl3).
Ethyl
(3S,3aR,4R,7R,7aS)-2-benzyl-7-hydroxy-5-(pent-4-enoyl)-4-(((S)-1-phenylethyl)carbamoyl)octahydroisoxazolo[4,5-
20g and ethyl (3S,3aR,4S,7R,7aS,Z)-2-benzyl-9-(((S)-1-phenylethyl)imino)octahydro-7,4-
c]pyridine-3-carboxylate
(epoxymethano)isoxazolo[4,5-c]pyridine-3-carboxylate 21g.
According to general procedure A the crude was obtained and purified via column chromatography (50ꢀ100% EtOAc in heptane
then 0ꢀ100% EtOH in EtOAc) affording both 20g (30.1 mg, 17%) and 21ga (19.2 mg, 13%) as yellow oils. NMR signals and other
experimental data of compound 20g: RF = 0.17 (1:1; PE:EtOAc). H NMR (400 MHz, Chloroformꢀd) δ 7.39 – 7.18 (m, 9H), 7.15
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(dd, J = 7.2, 1.9 Hz, 2H), 6.94 (d, J = 8.2 Hz, 1H), 5.89 – 5.73 (m, 1H), 5.20 (d, J = 1.4 Hz, 1H), 5.10 – 4.90 (m, 3H), 4.53 (dd, J =
9.2, 3.6 Hz, 1H), 4.29 (d, J = 14.0 Hz, 1H), 4.24 – 4.13 (m, 2H), 4.02 (d, J = 14.0 Hz, 1H), 3.97 – 3.86 (m, 1H), 3.75 – 3.59 (m, 1H),
3.29 – 3.09 (m, 3H), 2.46 – 2.26 (m, 5H), 1.42 (d, J = 7.0 Hz, 3H), 1.28 (t, J = 7.1 Hz, 4H). 13C NMR (101 MHz, CDCl3) δ 174.8,
169.7, 168.6, 143.5, 136.9, 134.9, 130.0, 128.8, 128.4, 127.9, 127.4, 125.7, 115.8, 73.2, 68.2, 64.3, 61.9, 59.9, 52.0, 49.2, 45.0, 44.6,
32.5, 28.8, 22.4, 14.2.
FTꢀIR (neat): ν = 3340, 2959, 2931, 2873, 1735, 1636, 1529, 1497, 1454, 1415 cmꢀ1. HRMS calcd for C30H37N3O6+Na+ [M+Na+]:
558.2575. Found. 558.2567. [α]D = +64.5 (c = 1.00, CHCl3). NMR signals and other experimental data of compound 21ga: RF =
20
0.37 (100% EtOH). HRMS calcd for C25H29N3O4+H+ [M+H+]: 436.2231. Found. 436.2211. The minor product could not be
a
completely purified, but a relatively pure fraction was obtained for structure determination by NMR.
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Ethyl
(3S,3aR,4R,7R,7aS)-2-benzyl-4-(benzylcarbamoyl)-7-hydroxy-5-(pent-4-enoyl)octahydroisoxazolo[4,5-c]pyridine-3-
carboxylate 20h and ethyl (3S,3aR,4S,7R,7aS,Z)-2-benzyl-9-(benzylimino)octahydro-7,4-(epoxymethano)isoxazolo[4,5-
c]pyridine-3-carboxylate 21h.
According to general procedure B the crude was obtained and purified via column chromatography (50ꢀ100% EtOAc in heptane then
0ꢀ100% EtOH in EtOAc) affording both 20h (37.1 mg, 24%) and 21h (61.4 mg, 50%) as yellow oils. NMR signals and other
experimental data of compound 20h: RF = 0.59 (75% EtOAc in PE). 1H NMR (400 MHz, Chloroformꢀd) δ 7.41 – 7.21 (m, 8H), 7.21
– 7.12 (m, 2H), 6.94 (t, J = 6.0 Hz, 1H), 5.21 (d, J = 1.4 Hz, 1H), 5.06 – 4.90 (m, 2H), 4.57 (dd, J = 9.2, 3.6 Hz, 1H), 4.45 – 4.24 (m,
3H), 4.24 – 4.13 (m, 2H), 4.13 – 3.98 (m, 2H), 3.72 (td, J = 9.3, 1.4 Hz, 1H), 3.30 (q, J = 5.2, 4.3 Hz, 2H), 3.18 (s, 1H), 2.53 (s, 1H),
2.48 – 2.21 (m, 4H), 1.26 (t, J = 7.1 Hz, 3H). 13C NMR (101 MHz, CDCl3) δ 174.7, 170.6, 168.6, 138.0, 136.9, 134.9, 130.1, 128.8,
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128.4, 127.9, 127.6, 127.4, 115.7, 73.2, 68.2, 64.5, 61.9, 59.8, 52.0, 45.2, 44.6, 43.6, 32.4, 28.8, 14.2. FTꢀIR (neat): ν = 3339, 2980,
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2929, 1734, 1634, 1524, 1497, 1454, 1416 cmꢀ1. HRMS calcd for C29H35N3O6+Na+ [M+Na+]: 544.2418. Found. 544.2411. [α]D
=
+37.3 (c = 1.00, CHCl3). NMR signals and other experimental data of compound 21h: RF = 0.38 (100% EtOH). 1H NMR (400 MHz,
Chloroformꢀd) δ 7.43 – 7.20 (m, 10H), 4.62 – 4.54 (m, 1H), 4.50 – 4.34 (m, 2H), 4.27 – 4.07 (m, 4H), 3.71 (d, J = 13.9 Hz, 1H), 3.61
(d, J = 3.3 Hz, 1H), 3.34 – 3.23 (m, 2H), 3.19 (d, J = 8.3 Hz, 1H), 2.89 (d, J = 12.1 Hz, 1H), 1.25 (t, J = 7.1 Hz, 3H). 13C NMR (101
MHz, CDCl3) δ 169.0, 157.9, 140.7, 136.3, 129.4, 128.5, 128.3, 128.3, 127.6, 126.7, 75.0, 70.9, 68.4, 61.6, 60.9, 51.8, 51.2, 49.4,
43.0, 14.2. FTꢀIR (neat): ν = 3030, 2981, 2873, 1737, 1688, 1604, 1496, 1454 cmꢀ1. HRMS calcd for C24H27N3O4+H+ [M+H+]:
422.2074. Found. 422.2073. [α]D20 = −49.5 (c = 0.80, CHCl3).
Ethyl (3S,3aR,4R,7R,7aS)-5-benzoyl-2-benzyl-7-hydroxy-4-((4-methoxyphenyl)carbamoyl)octahydroisoxazolo[4,5-c]pyridine-
3-carboxylate
(epoxymethano)isoxazolo[4,5-c]pyridine-3-carboxylate
(20i)
and
ethyl
(3S,3aR,4S,7R,7aS,Z)-2-benzyl-9-((4-methoxyphenyl)imino)octahydro-7,4-
(21i) and ethyl (3S,3aR,4S,7R,7aS,E)-2-benzyl-9-((4-
methoxyphenyl)imino)-5-((E)-((4-methoxyphenyl)imino)methyl)octahydro-7,4-(epoxymethano)isoxazolo[4,5-c]pyridine-3-
carboxylate (S1).
According to general procedure B the crude was obtained and purified via column chromatography (50ꢀ100% EtOAc in heptane then
0ꢀ40% EtOH in EtOAc) affording both 20i (29.0 mg, 18%), 21i (52.6 mg, 41%) and S1 (42.9 mg, 25%) as yellow oils. NMR signals
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and other experimental data of compound 20i: RF = 0.65 (75% EtOAc in PE). H NMR (400 MHz, Chloroformꢀd) δ 8.62 (s, 1H,
NH), 7.43 – 7.28 (m, 7H), 6.88 – 6.76 (m, 2H), 5.93 – 5.75 (m, 1H), 5.26 (d, J = 1.4 Hz, 1H), 5.16 – 4.95 (m, 2H), 4.65 (dd, J = 9.3,
3.6 Hz, 1H), 4.33 (d, J = 14.0 Hz, 1H), 4.19 (qd, J = 7.1, 1.6 Hz, 3H), 4.05 (d, J = 14.0 Hz, 1H), 3.83 – 3.68 (m, 4H), 3.41 – 3.14 (m,
3H), 2.51 – 2.22 (m, 5H), 1.28 (t, J = 7.1 Hz, 3H). 13C NMR (101 MHz, CDCl3) δ 175.3, 168.6, 168.5, 156.6, 136.9, 134.8, 130.8,
130.2, 128.4, 128.0, 121.5, 115.9, 114.3, 73.0, 68.2, 64.6, 62.0, 59.9, 55.6, 53.0, 45.1, 44.7, 32.5, 28.8, 14.2. FTꢀIR (neat): ν = 3308,
2935, 1738, 1683, 1634, 1512, 1415 cmꢀ1. HRMS calcd for C29H35N3O7+Na+ [M+Na+]: 560.2367. Found: 560.2360. [α]D20 = +64.2 (c
= 0.50, CHCl3). NMR signals and experimental data of the major adduct 21i: RF = 0.51 (100% EtOH). 1H NMR (400 MHz,
Chloroformꢀd) δ 7.38 (d, J = 6.7 Hz, 2H), 7.34 – 7.21 (m, 4H), 7.13 (d, J = 8.8 Hz, 2H), 6.86 (d, J = 8.8 Hz, 2H), 4.61 – 4.54 (m,
1H), 4.37 – 4.26 (m, 2H), 4.26 – 4.12 (m, 2H), 3.90 (d, J = 13.8 Hz, 1H), 3.81 (s, 3H), 3.72 (d, J = 3.1 Hz, 1H), 3.50 – 3.34 (m, 2H),
3.29 (dd, J = 12.2, 4.3 Hz, 1H), 2.92 (d, J = 12.1 Hz, 1H), 1.29 (t, J = 7.1 Hz, 3H). 13C NMR (101 MHz, CDCl3) δ 169.0, 157.0,
156.5, 138.4, 136.4, 129.3, 128.4, 127.7, 124.6, 114.0, 75.1, 71.1, 68.9, 61.8, 61.5, 55.6, 52.2, 51.4, 42.8, 14.3. FTꢀIR (neat): ν =
20
2935, 2836, 1737, 1679, 1606, 1504, 1455 cmꢀ1. HRMS calcd for C24H28N3O5+H+ [M+H+]: 438.2023. Found: 438.2023. [α]D
=
+135.6 (c = 0.50, CHCl3). NMR signals and other experimental data of compound S1: RF = 0.53 (75% EtOAc in PE). 1H NMR (500
MHz, Chloroformꢀd) δ 7.72 (s, 1H), 7.37 (d, J = 7.4 Hz, 2H), 7.29 (dt, J = 11.6, 6.3 Hz, 3H), 7.21 – 7.12 (m, 2H), 6.96 – 6.89 (m,
2H), 6.90 – 6.82 (m, 4H), 4.87 – 4.73 (m, 1H), 4.41 (d, J = 8.0 Hz, 1H), 4.40 – 4.31 (m, 2H), 4.29 – 4.18 (m, 2H), 3.97 – 3.86 (m,
2H), 3.82 (s, 3H), 3.79 (s, 3H), 3.56 – 3.43 (m, 3H), 1.31 (t, J = 7.1 Hz, 3H). 13C NMR (101 MHz, CDCl3) δ 168.5, 157.0, 156.4,
152.7, 149.7, 144.1, 137.5, 136.1, 129.3, 128.4, 127.8, 124.9, 121.9, 114.6, 114.1, 75.1, 71.6, 68.1, 62.1, 61.4, 56.4, 55.6, 55.6, 51.7,
44.3, 14.3. FTꢀIR (neat): ν = 2934, 2835, 1736, 1689, 1625, 1577, 1505, 1464, 1426, 1408 cmꢀ1. HRMS calcd for C32H35N4O6+H+
[M+H+]: 571.2551. Found. 571.2548. [α]D20 = −79.1 (c = 1.00, CHCl3).
Ethyl
(3S,3aR,4R,7R,7aS)-2-benzyl-7-hydroxy-4-(((4-methoxyphenyl)carbamoyl)carbamoyl)-5-
20j and ethyl (3S,3aR,4S,7R,7aS,Z)-2-benzyl-9-((4-
phenyloctahydroisoxazolo[4,5-c]pyridine-3-carboxylate
methoxyphenyl)imino)octahydro-7,4-(epoxymethano)isoxazolo[4,5-c]pyridine-3-carboxylate 21i.
According to general procedure A the crude was obtained and purified via column chromatography (50ꢀ100% EtOAc in heptane
then 0ꢀ100% EtOH in EtOAc) affording both 20j (26.5 mg, 16%) as a yellow oil, while 21i was only formed in trace amounts under
1
these conditions. NMR signals and other experimental data of compound 20j: RF = 0.21 (1:1; PE:EtOAc). H NMR (400 MHz,
Chloroformꢀd) δ 9.10 (s, 1H), 7.52 – 7.33 (m, 12H), 6.86 (d, J = 8.8 Hz, 2H), 5.44 (s, 1H), 4.68 (dd, J = 9.1, 4.0 Hz, 1H), 4.43 (d, J =
14.1 Hz, 1H), 4.25 (q, J = 6.8 Hz, 2H), 4.16 (d, J = 14.1 Hz, 1H), 4.02 – 3.91 (m, 1H), 3.85 – 3.76 (m, 4H), 3.70 – 3.47 (m, 2H), 3.13
(dd, J = 12.9, 3.1 Hz, 1H), 2.39 (s, 1H), 1.31 (t, J = 7.2 Hz, 3H). 13C NMR (101 MHz, CDCl3) δ 174.4, 168.6, 168.2, 156.7, 134.9,
134.2, 131.0, 130.9, 130.2, 128.7, 128.6, 128.2, 128.1, 121.6, 114.4, 73.5, 67.4, 64.5, 62.2, 60.0, 55.7, 53.8, 46.9, 44.7, 14.3. FTꢀIR
(neat): ν = 3304, 3064, 2933, 2837, 1735, 1681, 1621, 1601, 1577, 1511, 1454, 1414 cmꢀ1. HRMS calcd for C31H33N3O7+Na+
[M+Na+]: 582.2211. Found. 582.2189. [α]D20 = +44.8 (c = 0.50, CHCl3).
Ethyl (3S,3aR,4S,7R,7aS)-2-benzyl-4-(tert-butylcarbamoyl)-7-hydroxyoctahydroisoxazolo[4,5-c]pyridine-3-carboxylate (22).
To a solution of imidate 21a (38.0 mg, 0.098 mmol) in tetrahydrofuranꢀd8 (2.0 mL) was added D2O (2.0 mL). The resulting solution
was heated in a sealed vessel in an oil bath to 80 °C for 4 h and then heated at 90 °C for 3 days. The reaction mixture was then
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concentrated in vacuo concentrated and coꢀevaporated with dry toluene to remove excess water. The residue was purified by flash
column chromatography (30ꢀ50% acetone in toluene), affording amide 22 (28.0 mg, 70%) as a colorless oil. RF = 0.18 (1:1;
toluene:acetone). 1H NMR (400 MHz, Chloroformꢀd) δ 7.30 – 7.18 (m, 5H), 6.64 (s, 1H), 4.23 (dd, J = 7.0, 4.2 Hz, 1H), 4.15 – 4.03
(m, 3H), 3.98 (d, J = 13.8 Hz, 1H), 3.77 – 3.68 (m, 1H), 3.61 – 3.52 (m, 2H), 3.37 – 3.29 (m, 1H), 2.98 (dd, J = 14.2, 3.6 Hz, 1H),
2.51 (dd, J = 14.2, 2.1 Hz, 1H), 1.25 (s, 9H), 1.18 (t, J = 7.2 Hz, 3H). 13C NMR (101 MHz, CDCl3) δ 169.6, 169.4, 135.8, 129.2,
128.6, 127.9, 76.9, 66.1, 65.3, 61.4, 58.9, 58.7, 51.0, 49.2, 45.8, 28.7, 14.1. FTꢀIR (neat): ν = 3378, 3297, 3034, 2966, 2931, 2874,
1741, 1684, 1515, 1477, 1455 cmꢀ1. HRMS calcd for C21H32N3O5+H+ [M+H+]: 406.2336. Found. 406.2334. [α]D20 = −72.9 (c = 1.00,
CHCl3).
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Ethyl (3S,3aR,4R,7R,7aS)-2-benzyl-4-(cyclohexylcarbamoyl)-7-hydroxyoctahydroisoxazolo[4,5-c]pyridine-3-carboxylate 24
To a solution of compound 20f (37.0 mg, 0.072 mmol, 1.0 equiv.) in a THF/H2O mixture (3:1, 1.4 mL) was added I2 (54.9 mg, 0.22
mmol, 3.0 equiv.). The reaction mixture was stirred for 20 min and then quenched by the addition of 1M aq. Na2S2O3 (4 mL) and
stirred for 30 min. The reaction mixture was poured into a mixture of 1M aq. Na2S2O3/sat. aq. NaCl (1/1, v/v, 10 mL) and then
extracted with EtOAc (4×), dried (MgSO4) concentrated in vacuo, affording a yellow oil. The residue was purified by flash column
chromatography (0ꢀ20% EtOH in EtOAc) providing compound 24 (29.9 mg, 94%) as a yellow oil. RF = 0.44 (20% EtOH in EtOAc).
1H NMR (400 MHz, Chloroformꢀd) δ 7.48 – 7.24 (m, 5H), 7.13 (s, 1H), 4.37 (d, J = 13.8 Hz, 1H), 4.33 – 4.20 (m, 2H), 4.18 – 4.06
(m, 2H), 3.98 – 3.80 (m, 2H), 3.79 – 3.64 (m, 2H), 3.64 – 3.51 (m, 1H), 3.32 – 3.19 (m, 1H), 3.00 – 2.86 (m, 2H), 1.94 – 1.78 (m,
2H), 1.78 – 1.65 (m, 2H), 1.65 – 1.52 (m, 1H), 1.40 – 1.10 (m, 8H). 13C NMR (101 MHz, CDCl3) δ 169.8, 169.4, 135.9, 130.0,
128.6, 128.0, 75.4, 69.3, 64.6, 61.9, 61.7, 56.6, 48.6, 46.3, 45.0, 32.9, 25.5, 24.9, 14.2. FTꢀIR (neat): ν = 3233, 2931, 2855, 1737,
1654, 1539, 1452 cmꢀ1. HRMS calcd for C23H33N3O5+H+ [M+H+]: 432.2493. Found. 432.2483. [α]D20 = −42.5 (c = 0.40, CHCl3).
2-((3S,3aR,4S,7R,7aS)-2-benzyl-3-(ethoxycarbonyl)-7-hydroxyoctahydroisoxazolo[4,5-c]pyridin-4-yl)acetic acid (27).
To a roundbottom flask containing a solution of azidoꢀaldehyde 17 (0.106 gr, 0.287 mmol, 1 equiv.) in dry EtOH (1.6 mL) at 0 °C
was added a solution of trimethylphosphine (1M in THF, 0.575 mL, 2 equiv.). The reaction mixture was stirred at 0 °C for 3 hours
and then concentrated and coꢀevaporated with dry toluene (3×). The residue was dissolved in a 2:1 mixture of dry Et2O/THF (3.0
mL) and malonic acid (38.5 mg, 0.370 mmol, 1.29 equiv.) was added. The resulting reaction mixture was stirred for 19 h, which
resulted in the formation of a sticky oil. The oil was dissolved by the addition of dry EtOH (1.5 mL), providing a yellow solution that
quickly formed a white precipitate. This reaction mixture was allowed to stir for another 2 hours, followed by the addition MeOH (5
mL). The resulting solution was conc. in vacuo, affording a yellow foam, which purified by trituration with hot THF and subsequent
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trituration in hot MeOH, affording compound 27 (66.0 mg, 62%) as an offꢀwhite solid. RF = 0.10 (30% MeOH in DCM). H NMR
(400 MHz, Methanolꢀd4) δ 7.45 – 7.24 (m, 5H), 4.33 – 4.20 (m, 3H), 4.20 – 4.01 (m, 3H), 3.56 (d, J = 2.6 Hz, 1H), 3.22 (dd, J =
12.1, 4.5 Hz, 1H), 3.00 (dd, J = 12.1, 10.4 Hz, 1H), 2.95 – 2.76 (m, 2H), 2.50 (dd, J = 17.3, 3.3 Hz, 1H), 2.29 (dd, J = 17.3, 7.9 Hz,
1H), 1.21 (t, J = 7.1 Hz, 3H). 13C NMR (101 MHz, DMSOꢀd6) δ 172.8, 170.4, 136.9, 129.1, 128.0, 127.2, 77.1, 69.5, 66.0, 62.3,
60.5, 53.1, 50.9, 46.1, 37.4, 13.9. FTꢀIR (neat): ν = 2925, 2869, 1736, 1640, 1557, 1498, 1456, 1420 cmꢀ1. HRMS calcd for
C18H24N2O6+H+ [M+H+]: 365.1707. Found. 365.1695. [α]D20 = −44.8 (c = 0.50, CHCl3).
Modified SAWU-3CR procedure with in situ alcohol-protection
To a roundbottom containing a solution of azidoꢀaldehyde 17 (0.104 gr, 0.299 mmol, 1 equiv.) in dry EtOH (1.6 mL) at 0 °C was
added a solution of trimethylphosphine (1M in THF, 0.600 mL, 2 equiv.). The reaction mixture was stirred at 0 °C for 2 hours and
then concentrated and coꢀevaporated with dry toluene (3×). The residue was dissolved in dry THF (2.5 mL) and imidazole (23.5 mg,
0.345 mmol, 1.15 equiv.) was added. The resulting reaction mixture was cooled to 0 °C, followed by the dropwise addition of TESCl
(58.0 ꢁL, 0.345 mmol, 1.15 equiv.). The reaction mixture was stirred for 45 minutes at 0 °C and then another 50 minutes at room
temperature. The mixture was then cooled to 0 °C, followed by the addition of acetic acid (86.0 ꢁL, 1.50 mmol, 5.0 equiv.) and
tbutyl isocyanide (170 ꢁL, 1.50 mmol, 5.00 equiv.). The resulting mixture was allowed to warm to rt and stirred for 3 days. The
reaction was quenched by the addition of sat. aq. NaHCO3 (5 mL) and EtOAc (20 mL). The biphasic system was separated and the
water layer was extracted with EtOAc (10 mL, 2×). The combined organic layers were washed with sat. aq. NaHCO3 (10 mL), brine
(10 mL), dried (MgSO4) and then conc. in vacuo. The residue was purified by flash column chromatography (20ꢀ100% EtOAc in
heptane), providing compound 26a (61.6 mg, 36%) and compound 20a (23.5 mg, 18%) as yellow oils. NMR signals and other
experimental data of compound 26a: RF = 0.28 (7:3; PE:EtOAc). 1H NMR (400 MHz, Chloroformꢀd) δ 7.42 – 7.20 (m, 5H), 6.36 (s,
1H), 4.95 (s, 1H), 4.44 (dd, J = 9.1, 2.9 Hz, 1H), 4.28 – 4.18 (m, 2H), 4.17 – 4.02 (m, 3H), 3.65 (t, J = 9.2 Hz, 1H), 3.34 (t, J = 10.7
Hz, 1H), 3.19 (dd, J = 10.4, 5.6 Hz, 1H), 3.02 – 2.83 (m, 1H), 2.02 (s, 3H), 1.30 – 1.19 (m, 12H), 0.93 (t, J = 7.9 Hz, 9H), 0.59 (q, J
= 8.0 Hz, 6H). 13C NMR (101 MHz, CDCl3) δ 172.8, 170.0, 169.1, 135.0, 130.4, 128.1, 127.7, 73.8, 68.9, 65.3, 61.7, 60.1, 52.6,
51.2, 46.3, 45.6, 28.6, 21.6, 14.2, 6.8, 4.8. FTꢀIR (neat): ν = 3326, 2958, 2877, 1738, 1681, 1644, 1532, 1455, 1411 cmꢀ1. HRMS
calcd for C29H48N3O6Si+H+ [M+H+]: 562.3307. Found. 562.3281.
Modified SAWU-3CR procedure with in situ alcohol-protection & deprotection
To a roundbottom containing a solution of azidoꢀaldehyde 17 (0.104 gr, 0.299 mmol, 1 equiv.) in dry EtOH (1.6 mL) at 0 °C was
added a solution of trimethylphosphine (1M in THF, 0.598 mL, 2 equiv.). The reaction mixture was stirred at 0 °C for 2 hours and
then concentrated and coꢀevaporated with dry toluene (3×). The residue was dissolved in dry THF (2.5 mL) and imidazole (24.0 mg,
0.353 mmol, 1.15 equiv.) was added. The resulting reaction mixture was cooled to 0 °C, followed by the dropwise addition of TESCl
(60.0 ꢁL, 0.357 mmol, 1.19 equiv.). The reaction mixture was stirred for 55 minutes at 0 °C and then another 30 minutes at room
temperature. The mixture was then cooled to 0 °C, followed by the addition of tbutyl isocyanide (170 ꢁL, 1.50 mmol, 5.00 equiv.)
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and acetic acid (86.0 ꢁL, 1.50 mmol, 5.0 equiv.). The resulting mixture was allowed to warm to rt and stirred for 17 h. Finally, H2O
(0.25 mL) and 1M HCl in Et2O (1.5 mL) were added and the solution was stirred at rt for 2 h, followed by the addition of sat. aq.
NaHCO3 (5 mL) and EtOAc (20 mL). The biphasic system was separated and the water layer was extracted with EtOAc (10 mL,
2×). The combined organic layers were washed with sat. aq. NaHCO3 (8 mL), brine (8 mL), dried (MgSO4) and then conc. in vacuo.
The residue was purified by flash column chromatography (50ꢀ100% EtOAc in heptane), providing compound 20a (65.7 mg, 41%)
as a yellow oil.
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Acknowledgments
The authors thank dr. Jorge Escorihuela for computational assistance and the Netherlands Foundation
for Scientific Research (NWO) for funding this research via a ChemThem: Chemical Biology grant
(728.011.105), a VENI grant (722.011.006) to TW and financing the Xꢀray diffractometer.
Supporting information
Xꢀray crystallographic data for compound 6a (CIF) and
Characterization data and copies of 1H and 13C NMR spectra for all compounds. (PDF)
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