Stereoselective synthesis of (+)-loline alkaloid skeleton

Article abstract of DOI:10.1021/jo502493e

The loline alkaloids present a compact polycyclic pyrrolizidine skeleton and contain a strained five-membered ethereal bridge, structural features that have proven challenging for synthetic chemists to incorporate since the discovery of this natural product family more than 100 years ago. These alkaloids are produced by mutualistic fungal symbionts (endophytes) living on certain species of pasture grasses and protect the host plant from insect herbivory. The asymmetric total synthesis of loline alkaloids is reported and extends our first-generation (racemic) synthesis of this alkaloid family. Key to the synthesis is a diastereoselective tethered aminohydroxylation of a homoallylic carbamate function and a Petasis Borono-Mannich addition.

Full text of DOI:10.1021/jo502493e

Article
pubs.acs.org/joc
Stereoselective Synthesis of (+)-Loline Alkaloid Skeleton
Kelsey E. Miller, Anthony J. Wright, Margaret K. Olesen, M. Todd Hovey, and Jonathan R. Scheerer*
Department of Chemistry, The College of William & Mary, P.O. Box 8795, Williamsburg, Virginia 23187, United States
S
* Supporting Information
ABSTRACT: The loline alkaloids present a compact
polycyclic pyrrolizidine skeleton and contain a strained five-
membered ethereal bridge, structural features that have proven
challenging for synthetic chemists to incorporate since the
discovery of this natural product family more than 100 years
ago. These alkaloids are produced by mutualistic fungal
symbionts (endophytes) living on certain species of pasture grasses and protect the host plant from insect herbivory. The
asymmetric total synthesis of loline alkaloids is reported and extends our first-generation (racemic) synthesis of this alkaloid
family. Key to the synthesis is a diastereoselective tethered aminohydroxylation of a homoallylic carbamate function and a Petasis
Borono-Mannich addition.
the largest number of grass−endophyte associations, although
the insect feeding deterrent activity of 6 appears modest against
most insect species.7 The loline alkaloids are produced in the
greatest abundance of the four protective alkaloid families. In
some cases, loline alkaloid content has been observed at more
than 10 mg/g of dry endophyte-infected plant material, an
amount far in excess of fungal hyphae mass.8 The loline alkaloids
have potent insecticidal activity and antifeedant effects
(comparable to nicotine) and have been evaluated against
several important commercial insect pest species.9 Loline (1) and
the roughly 20 related congeners10 bearing different substitution
at the C1-exo amine (see representative examples 2−5) are
possibly the most intriguing family due to a rich history, a
compact and strained polycyclic structure, and remarkable
biological activity, as well as the sense that there is still much
left to discover about this alkaloid class.11
The chemical history of loline alkaloids began with the
isolation of norloline (3), originally named temuline, from
Lolium temulentum in the late 1800s.12 The loline alkaloids have
since been isolated from a variety of grass species (or more
specifically, symbiota) and, in at least one case, found in morning
glory.13 Although the fungal endophyte of L. temulentum was
identified as early as 1904,14 the protective alkaloids were not
explicitly linked to the endophytes until the 1980s.15,11 The
biosynthesis of loline alkaloids has also received significant
attention. An informed biosynthetic pathway has emerged
through recent efforts at the genomic and biochemical levels
and is complemented by extensive isotope precursor feeding
studies.16,11
INTRODUCTION
■
Four alkaloidslolines, peramine, ergots, and indole diter-
penescan be produced by mutualistic fungal symbionts
(endophytes) living on certain species of pasture grasses
(Poaceae; e.g., fescues, ryes).1 These bioactive alkaloids protect
the host plant from invertebrate and vertebrate herbivores. The
gene clusters that code for the biosynthesis of all four alkaloids
have been determined, which has recently enabled facile
determination of the alkaloid profile (by PCR) in most known
grass−fungal associations (symbiota).2 Broad chemotypic
variation is observed, and symbionts have been identified that
produce between 0 and 3 of these alkaloid families; none have yet
been identified that code for production of all four alkaloids.3
The mutualistic endophytes are ascribed to either of the closely
related genera Epichloae or Neotyphodium. Endophytes from the
latter lack the ability to reproduce sexually and are entirely
dependent on systemic infection of most tissues, including the
seed, in order to facilitate transmission to the next generation.
This intriguing evolutionary story of symbiosis has not gone
unnoticed, and there are several reviews that encompass the
topic.4
Of the four natural product families that can arise from grass−
endophyte mutualistic associations, ergot alkaloids are the most
well-studied owing to their abundant production from a different
paradigm, that of the parasitic fungi Claviceps purpurea, which
infects several cereal grains.5 Ergot alkaloids such as ergovaline 8
(Figure 1) are believed to function primarily as vertebrate feeding
deterrents, but their activity against invertebrate herbivores is
known and may be presently underappreciated. The complete
roles of the other alkaloid families (lolines, peramine, indole
diterpenes) are less well understood. Indole diterpenes (e.g.,
lolitrem B, Figure 1) are the assumed causative agents of ryegrass
staggers, a type of livestock toxicosis.6 This disorder can develop
when ruminants forage on endophyte-infected grass that
produces indole diterpene alkaloids. Lolines (e.g., 1−5) and
peramine (6) are primarily active against insects and show few
negative effects on mammalian herbivores. Peramine is present in
Over the past several decades, the loline skeleton has been
constructed only a handful of times. The dearth of synthetic
routes is perhaps owed to the congested assemblage of structural
features of the loline alkaloids and their undesirable physical
properties. The tricyclic ring system features a strained ethereal
Received: October 30, 2014
© XXXX American Chemical Society
A
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Figure 1. Protective alkaloids from mutualistic grass−fungal endophyte associations.
Scheme 1. Overview of Loline Alkaloid Synthesis and Proposed Revisions
bridge, four contiguous stereogenic centers, and two basic
nitrogen atoms. The volatile and basic pyrrolizidine core can
prove difficult to handle and readily absorbs CO2 from the
atmosphere to form a zwitterionic carbamate.17 Two un-
successful attempts18 precede the first synthesis of ( )-1 by
Tufariello in 1986.19 The first asymmetric synthesis followed in
2000 by Blakemore and White.20 In 2011, both our laboratory21
and that of Trauner22 reported, respectively, on the synthesis of
( )- and (+)-loline alkaloids. This article describes the
completion of our second-generation synthesis and construction
of the (+)-loline core.
stoichiometric oxidant). Stereocontrol in the aminohydroxyla-
tion event can be rationalized based on nonbonding interactions
that minimize allylic (A1,3) strain (see 3D depiction of 9). The Z-
configuration in 9 is essential for stereocontrol and effective
gearing of the amino substituent above the Re face of the alkene.
The completion of the synthesis from the aminohydroxylation
product 10 required only four operations to deliver the loline
alkaloid skeleton 15. After activation of both hydroxyl functions
as mesylates and the secondary carbamate as the Cbz mixed
imide, the intermediate 11 was subjected to Cs2CO3 in methanol.
The methanolysis of 11 was not regioselective and afforded
products of both endocyclic (desired) and exocyclic imide
cleavage. Under the basic reaction conditions, the desired
endocyclic cleavage intermediate 12 underwent a subsequent
selective 5-exo etherification to afford the bicycle 13. Removal of
the pyrrolidine carbamate protection (TFA) and subsequent
liberation of the nucleophilic amine (NEt3) led to N-C3
cyclization and delivered the loline alkaloid skeleton 15.
RESULTS AND DISCUSSION
■
An overview of our first-generation synthesis toward the loline
alkaloid skeleton is depicted (Scheme 1). In the key operation, a
diastereoselective tethered aminohydroxylation (TA) was
performed on the homoallylic carbamate 9 using the original
reaction conditions reported by Donohoe and co-workers (t-
BuOCl, NaOH, K2OsO4).23 The resulting product 10 was
obtained as a single diastereomer in good yield (68%
accompanied by 17% recovered starting material). This reaction
serves as one of the only examples of an efficient tethered
aminohydroxylation of a homoallylic substrate that employs the
original reaction conditions (using hypochlorite as the
Although we were successful in our approach, we required
quantities of loline alkaloids and related derivatives for biological
and biosynthetic investigation that would not be readily
accessible given the synthetic route. Revisions were necessary
and we aimed for a more concise synthesis and that would
provide enantioenriched product. Our first-generation synthesis
B
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of the loline alkaloids is efficient (4−5 steps) forward from the
TA reaction; however, precursor 9 required 12 steps to assemble.
A primary goal of the second-generation synthesis was to retain
the TA reaction, but access a synthetic equivalent of 9 in fewer
steps. We wanted to also preserve several elements of the
endgame strategy, in particular the sequence of ring formation
where etherification precedes pyrrolizidine formation. In this
way, very mild reaction conditions can be employed for the
etherification (23 °C, MeOH, Cs2CO3). All other syntheses
prepare the C2−C7 ethereal bond as the final ring construction,
which requires forcing conditions and is often complicated by
undesired elimination products.19,20,22
Our second-generation synthesis planned to intercept
substrates that could take advantage of the more advanced
procedures for the TA reaction, namely, the use of a N-
pentafluorobenzyloxy substituted carbamate (see intermediate
18), which affords greater yield of product, permits lower catalyst
loading, and avoids the use of (and problems associated with) t-
butyl hypochlorite as a stoichiometric oxidant.24 Lastly, we
anticipated that, by using the Petasis borono-Mannich addition,
we could access the necessary cis-configured pyrrolidine
substrate 17 in short order.
pyrrolidinone 22 was promoted with HMDS with catalytic
TMSCl.25 Selective N-desilylation of 22 could be conveniently
accomplished by addition of ethanol prior to concentration of the
reaction mixture. The resulting lactam 23 was deprotonated
(0.95 equiv LiHMDS, −78 °C), and the derived lithium amide
was captured with CbzCl. Aqueous workup with 1 M HCl
removed the TMS silyl ether and revealed the hydroxyl residue in
24. This reaction sequence leading from 21 to 24 was performed
in two reaction vessels, did not require chromatography, and was
easily performed on multigram scale. A single recrystallization
step afforded 24 in 82% yield over the two operations. Reduction
of the imide in 24 (NaBH4, MeOH, 0 °C) preceded
diastereoselective Petasis borono-Mannich (PBM) addition.26
The PBM product 26 is produced as a single diastereomer in
good yield (67−82% yield over 2 steps) using methylpentanediol
boronate 25, an air- and chromatographically stable boronate not
previously recognized as competent in PBM reactions of this
type with N-acyliminium ions.27 Addition of the vinyl residue to
C8 through direction of the adjacent C7-hydroxyl is consistent
with other PBM reactions and likely occurs through an
intermediate resembling 27.
Two tasks were required to elaborate the Petasis product 26
into a valid TA precursor: (1) The C7-hydroxyl required
conversion to an appropriate carbamate functionality, and (2) 26
is lacking one carbon (C3, loline numbering) from the natural
product skeleton. This carbon needed to extend the alkene
terminus (C2) to create a Z-disubstituted alkene. While a
modified PBM reaction designed to directly incorporate a Z-
configured boronate bearing necessary C3 allylic oxygenation
(effectively protected to withstand the acidic reaction con-
ditions) was a potential solution, the relative dearth of
expeditious methods for construction of such a substrate led us
away from this approach. Rather, we turned to a more classical
two-step sequence that intercepted lactone 29, a substrate that
closely resembled an intermediate from our first-generation
synthesis of the loline alkaloids. Toward this end, the C7-
hydroxyl was converted to the α,β-unsaturated ester 28 with
acryloyl chloride. Conversion of 28 to the lactone 29 by ring-
closing metathesis (RCM) required experimental optimization
(see table, Scheme 3). Attempted RCM with Grubbs first-
generation catalyst (A) afforded only starting material. We were
able to observe some product with Grubbs second-generation
Our second-generation synthesis of the loline alkaloid skeleton
started from (S)-4-amino-2-hydroxybutanoic acid (21), a readily
available chiral pool reagent (Scheme 2). Condensation of 21 to
Scheme 2. Construction of Pyrrolidine Core via Petasis
Borono-Mannich Addition
Scheme 3. Preparation of Cbz-Protected Aminohydroxylation Precursors
C
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catalyst (B) at high catalyst loading (3 additions of 4 mol %; 12
mol % total) in toluene at 80 °C, albeit with relatively low
conversion (ca. 50%, entry 2). The Hoveyda−Grubbs catalyst C
showed improved turnover. Although no reaction was detected
with C in CH2Cl2 at 40 °C (entry 3), at more elevated
temperatures (toluene 80 °C), greater conversion to product was
observed. Further reaction optimization revealed that (CH2Cl)2
was a superior solvent for this metathesis. In practice, addition of
catalyst C (5 mol %) to a preheated solution of 28 in (CH2Cl)2 at
reflux led to 100% conversion and reliably afforded a 90%
isolated yield of lactone 29.
Scheme 5. Tethered Aminohydroxylation Reactions
Hydrolysis of the lactone in 29 with aqueous LiOH gave the
derived carboxylate and alkylation with MeI afforded the Z-α,β-
unsaturated ester 30. Attempts to convert the lactone 29 directly
to ester 30 (NaOMe in MeOH, or K2CO3 in MeOH, or NEt3 in
MeOH) led to undesired products resulting from hetero-
conjugate addition or epimerization at the γ-position. The
unsaturated ester 30 was reasonably slow to relactonize and
permitted conversion of the hydroxyl moiety into a carbamate
functional group. In preparation for the advanced procedure for
the TA reaction, the N-pentafluorobenzyloxy substituted
carbamate 32 was prepared via the intermediate N-hydroxy
carbamate 31. From lactone 29, the pentafluorobenzyloxy
carbamate 32 was prepared in 4 steps (2 chromatographic
separations) in 57% overall yield. The primary carbamate could
be prepared in a 3-step sequence using trichloroacetyl isocyanate,
followed by hydrolysis of the intermediate imide to give 33 in
70% yield from lactone 29.
(Scheme 4, eq 1). This substrate (9) underwent efficient TA
reaction to afford 10 (Scheme 5).
A similar reduction was planned for the Cbz-protected
unsaturated ester 33 (Scheme 4, eq 2); however, reduction
under the same conditions (Dibal, −78 °C) proved more
complicated. Under these conditions, consumption of 33 was
observed (as evident by TLC), but following aqueous workup,
only a small amount of the allylic alcohol 34 was apparent (5%
yield). The bulk of the reaction mixture contained predom-
inantly aldehyde-derived products. On the basis of this
observation, we reasoned that reduction was incomplete and
the tetrahedral intermediate derived from N-Cbz-protected 33
was considerably more stable than the corresponding tetrahedral
intermediate derived from N-Boc-protected 35. Increasing the
reaction duration (up to 8 h) or reaction temperature did not
noticeably encourage collapse of the tetrahedral intermediate
and, at temperatures above −40 °C, the Cbz-carbamate became
reactive toward the reductant. A brief survey of other hydride
sources did not provide an efficient nor selective reduction.
Alternative hydride sources (LiBH4; LiAlH4; LiBEt3H) all
afforded significant quantities of the saturated alcohol 36 in
addition to the desired allylic alcohol 34 (see table, entries 2−4,
Scheme 4). Fortunately, reduction with Dibal (4 equiv) in the
presence of BF3·OEt2 (3 equiv) at −78 °C (see table, entry 5,
Scheme 4) afforded the desired alcohol 34 as the only product in
56% isolated yield (unoptimized).
Although 33 is a potential substrate for aminohydroxylation,
we knew from our earlier work with the related Boc-protected
precursor 35 (Scheme 4) that TA reaction would be
Scheme 4. Reduction of Z-α,β-Unsaturated Ester
With a serviceable method to prepare 34 confirmed, we
attempted the key aminohydroxylation. As with the aforemen-
tioned reduction, the Cbz protection appeared to significantly
alter the reactivity of this substrate as compared to the Boc-
protected derivative. While TA reaction with Boc-protected 9
proceeded well (Scheme 5, eq 4), the analogous reaction with
Cbz-protected 34 (eq 5) afforded none of the desired
aminohydroxylation product.
Fortunately, the advanced procedures for the TA reaction with
the N-pentafluorobenzyloxy functionalized carbamate 32 cleanly
achieved the desired transformation, and product 38 was
observed as a single diastereomer in excellent yield (Scheme 5,
eq 6).
unsuccessful. Attempted TA reaction with 35 (t-BuOCl,
NaOH, K2OsO4) gave 37 as the only product, a result of
intramolecular heteroconjugate addition of the carbamate to the
unsaturated ester (Scheme 5, eq 3). The undesired conjugate
addition pathway in 35 was avoided by tempering the
electrophilic nature of the alkene. Reduction of the ester in 35
(Dibal, −78 °C) proceeded cleanly and gave allylic alcohol 9
D
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From the TA product 38, five operations were required to
tripartite relationship. Results from these ongoing efforts will
construct the loline alkaloid core (Scheme 6). The correct
be reported in due course.
Scheme 6. Completion of Loline Alkaloid Skeleton
EXPERIMENTAL SECTION
■
Experimental conditions and spectral data were published previously for
compounds 9−15.21
Benzyl (S)-3-Hydroxy-2-oxopyrrolidine-1-carboxylate (24).
Chlorotrimethylsilane (0.270 mL, 2.1 mmol, 0.05 equiv) was added to
a mixture of (S)-4-amino-2-hydroxybutanoic acid 1 (5.00g, 42.0 mmol),
xylene (100 mL), and HMDS (61.5 mL, 294 mmol, 7.0 equiv) at rt. The
reaction mixture was heated to reflux for 12 h, cooled to rt, and diluted
with absolute ethanol (200 mL). The solvents were removed under
reduced pressure to afford lactam 23 (7.30 g, quant recovery) as a tan
solid, which was used without further purification. Spectral data for
lactam 23 match published data.26 A portion of this material, (S)-3-
((trimethylsilyl)oxy)pyrrolidin-2-one (23) (3.27 g, 19.6 mmol), was
dissolved in THF (75 mL) at −78 °C, and LiHMDS (1.0 M soln in
THF, 18.6 mmol, 0.95 equiv) was added dropwise over 5 min. After
stirring for 0.5 h at −78 °C, CbzCl (3.50 g, 20.56 mmol, 1.05 equiv) was
added to the reaction dropwise over 5 min. The solution was warmed to
23 °C over 1 h and quenched with 1.0 M aqueous HCl (30 mL). The
reaction mixture was poured into a separatory funnel and extracted with
ethyl acetate (3 × 30 mL). The organic layers were combined, washed
with brine (2 × 30 mL), dried (Na2SO4), filtered, and concentrated in
vacuo. The resulting white powder was purified by recrystallization
(EtOAc/hexanes) to afford the desired product 24 (3.66 g, 82% yield) as
a white powder: mp 99.8−100.7 °C: TLC (60% EtOAc in hexanes), Rf:
0.70 (UV, CAM); [α]2D5 = −63.9 (c 1.94, CH2Cl2); IR (film) 3448, 3085,
3028, 2989, 2879, 1778, 1689, 1385, 1282, 1227 cm−1. Spectra of 24 are
oxidation state at C3 was installed by reduction of the ester in 38
with LiBH4. The resulting intermediate diol was primed as the
bis-mesylate, and the carbamate was activated as the mixed imide
39. Imide 39 underwent selective cleavage of the t-butoxy
carbamate (with Cs2CO3 in MeOH) and subsequent 5-exo
etherification to give the bicyclic core 40 as the only observed
product. The exclusive selectivity for endocyclic carbamate
cleavage with imide 39 is notable. Carbamate cleavage with the
related mixed imide (possessing benzyl substituent) provided
both endo- and exocyclic cleavage products (see intermediate 11,
Scheme 1). Removal of the Cbz-group in 40 by hydrogenolysis
with Perlman’s catalyst revealed the nucleophilic secondary
amine, which underwent spontaneous N-C3 cyclization to
establish the pyrrolizidine core and loline tricyclic framework.
The resulting product, N-Boc norloline (20), was identical to
material previously prepared by Trauner and co-workers.22
Conversion of 20 into two loline natural products has been
accomplished, and because the interconversion of several loline
congeners is known, the synthesis of 20 represents a formal total
synthesis of many of the loline alkaloids in this natural product
family.10,22 Our second-generation synthesis of the loline core is
characterized by several highly diastereo- and regioselective
reactions. The tethered aminohydroxylation was the reaction of
greatest strategic importance to the synthesis. This synthesis
demonstrates the ability to use the TA reaction to deliver the
nitrogen and oxygen functionalities with excellent stereo- and
regiocontrol. Additionally, the TA reaction offers a direct route to
rapidly construct the four contiguous stereogenic centers in the
molecule, arguably one of the more intricate features of the loline
skeleton. The unsuccessful aminohydroxylation reactions high-
lighted in Scheme 5 (eqs 3 and 5) offer additional fodder as to the
capricious nature of the original TA reaction conditions that
employ t-BuOCl to generate in situ the reactive N-chloro-
carbamate.24 The successful TA reactions (eqs 4 and 6) provide
another valuable demonstration that this reaction sequence can
be applied in complex contexts.28 In particular, the successful
transformation of 32 to 38 (eq 6) illustrates the important
advance Donohoe and co-workers have achieved by extension of
this chemistry to include the N-pentafluorobenzyloxy carbamate
substrates.
1
complicated by imide rotamers. H NMR (400 MHz, CDCl3) δ 7.40
(m, 5H), 5.29 (s, 2H), 4.38 (m, 1H), 3.89 (m, 1H), 3.60−3.53 (td, J1 =
6.6 Hz, J2 = 10.5 Hz, 1H), 2.48−2.42 (m, 1H), 2.00−1.94 (m, 1H); 13C
NMR (100 MHz, CDCl3) δ 174.4, 151.1, 134.9, 128.6, 128.5, 128.2,
77.2, 70.4, 68.3, 42.1, 27.0; HRMS (ES+): Exact mass calcd for
C12H13NO4Na+[M + Na]+, 258.0737. Found 258.0734.
Benzyl (2S,3S)-3-Hydroxy-2-vinylpyrrolidine-1-carboxylate
(26). To a solution of imide 24 (0.670 g, 2.85 mmol) in MeOH (20
mL) at 0 °C was added NaBH4 (55 mg, 1.43 mmol, 0.5 equiv) in one
portion. After stirring for 0.5 h at 0 °C, the reaction was quenched with
sat. NaHCO3 (20 mL) and the mixture was concentrated to remove the
bulk of MeOH. The mixture was transferred to a separatory funnel and
extracted with EtOAc (3 × 20 mL). The organic portions were
combined, dried (Na2SO4), filtered, and concentrated in vacuo. The
desired reduction product (0.675 g, 2.84 mmol, 99% yield) was obtained
as a white powder and used directly in the subsequent reaction without
further purification: TLC (60% EtOAc in hexanes), Rf: 0.40 (UV,
CAM). Spectral data for benzyl (3S)-2,3-dihydroxypyrrolidine-1-
carboxylate match published data.26 To a solution of benzyl (3S)-2,3-
dihydroxypyrrolidine-1-carboxylate (0.675 g, 2.85 mmol) and vinyl
boronate 25 (0.482 g, 3.13 mmol, 1.1 equiv) in CH2Cl2 (20 mL) at −78
°C was added dropwise BF3·Et2O (1.40 mL, 11.4 mmol, 4 equiv). The
solution was warmed to 0 °C for 2 h and stirred at room temperature for
an additional 3 h. The reaction was quenched with sat. NaHCO3 (20
mL), and the mixture was transferred to a separatory funnel. The organic
layer was removed, and the aqueous layer was extracted with chloroform
(3 × 5 mL). The organic fractions were combined and washed with
brine (50 mL), dried (Na2SO4), and concentrated in vacuo. The
resulting yellow oil was purified by flash column chromatography on
silica gel (elution: 20 → 80 EtOAc in hexanes) to afford 26 (506 mg,
72% yield over the two steps) as a clear oil: TLC (60% EtOAc in
hexanes), Rf. 0.40 (UV, CAM); [α]2D5 = −1.09 (c 0.93, CH2Cl2); IR
(film) 3419, 3083, 3072, 3033, 2978, 2951, 2894, 2361, 1956, 1698,
1592, 1540, 1480, 1448, 1357, 1257, 1213 cm−1. The spectra of 26 are
complicated by carbamate rotamers. 1H NMR (400 MHz, CDCl3) 7.33
(m, 5H), 5.81 (m, 1H), 5.22 (m, 2H), 5.09 (m, 2H), 4.35 (m, 2H), 3.56
(m, 2H), 2.23 (1H), 2.06 (m, 1H), 1.88 (m, 1H); 13C (100 MHz,
CDCl3) δ 136.6, 133.9, 133.5, 128.4, 128.3, 127.8, 118.2, 117.9, 72.3,
71.8, 66.7, 62.9, 62.3, 43.7, 31.6 30.8; HRMS (ES+): Exact mass calcd for
C14H17NO3Na+ [M + Na]+, 270.1100. Found 270.1099.
The described synthesis route can deliver a sufficient quantity
of loline alkaloids in order to begin to address questions of
biological, biosynthetic, and pharmacological importance as well
as to deconvolute the remarkable plant−fungus−herbivore
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Benzyl (2S,3S)-3-(Acryloyloxy)-2-vinylpyrrolidine-1-carbox-
ylate (28). Homoallylic alcohol 26 (357 mg, 1.45 mmol) was added
to a flame-dried flask. After flushing the vessel with N2, the substrate was
dissolved in CH2Cl2 (6 mL) and iPr2NEt (1.26 mL, 7.23 mmol) and
DMAP (12 mg, 0.072 mmol) were added and the reaction flask was
cooled to −78 °C. In a separate flame-dried pear-shaped flask, acryloyl
chloride (0.36 mL, 4.35 mmol) was diluted with CH2Cl2 (3 mL). The
acryloyl chloride solution was added dropwise over 10 min via cannula.
After stirring for 1 h at −78 °C, the reaction was warmed to rt for 0.5 h
and then quenched with 1 M HCl (10 mL). The mixture was transferred
to a separatory funnel, and the organic layer was removed. The aqueous
portion was extracted with CH2Cl2 (3 × 10 mL). The organic portions
were combined, washed with NaHCO3 (20 mL), dried (Na2SO4),
filtered, and concentrated in vacuo. The resulting residue was purified by
flash chromatography on silica gel (elution: 10% → 45% EtOAc in
hexane) to afford 28 (394 mg, 90% yield) as a pale yellow oil: TLC (40%
EtOAc in Hexanes), Rf: 0.50 (UV, CAM); [α]2D5 = −37.9 (c 1.19,
CH2Cl2); IR (film) 3066, 3033, 2985, 2955, 2892, 2361, 2339, 1723,
1703, 1635, 1406, 1355, 1296, 1267, 1190, 1129, 1106, 1069, 1052 cm−1.
equiv), and the reaction was allowed to warm to rt overnight. The
reaction mixture was transferred to a separatory funnel and partitioned
between brine (5 mL) and CHCl3 (5 mL). The organic layer was
removed, the aqueous layer was extracted with CHCl3 (4 × 5 mL), and
the organic layers were combined, washed with brine (40 mL), dried
(Na2SO4), and concentrated in vacuo. The resulting oil was purified by
flash column chromatography (elution: 40% → 80% EtOAc in hexanes)
to afford 33 (243 mg, 70% yield over 3 steps) as a white solid: TLC (60%
EtOAc in hexanes), Rf: 0.30 (UV, CAM); [α]2D5 = +81.9 (c 1.67,
CH2Cl2); IR (film): 3399, 2954, 2885, 1715, 1689, 1606, 1415, 1348,
1198, 1172, 1105, 1043 cm−1. The spectra of 33 are complicated by
carbamate rotamers. 1H NMR (400 MHz, CDCl3) δ 7.33 (m, 5H), 6.17
(m, 1H, 5.87 (m, 1H, J = 10.9 Hz), 5.54 (s, 2H), 5.10 (m, 4H), 3.67 (m,
4H), 3.56 (m, 1 H), 2.09 (m, 2H); 13C NMR (100 MHz, CDCl3) δ
165.9, 155.7, 154.9, 146.4, 136.4, 128.2, 127.9, 127.7, 127.6, 120.2, 77.3,
76.2, 75.7, 66.8, 59.5, 58.4, 51.3, 51.2, 45.0, 44.7, 31.2, 30.6 HRMS
(ES+): Exact mass calcd for C17H20N2O6Na+ [M + Na]+, 371.1214.
Found 371.1215.
Benzyl (2S,3S)-3-(Carbamoyloxy)-2-((Z)-3-hydroxyprop-1-
en-1-yl)pyrrolidine-1-carboxylate (34). Unsaturated ester 33 (187
mg, 0.54 mmol) was dissolved in CH2Cl2 (7.2 mL). After cooling to −78
°C, BF3·OEt2 (0.23 mL, 1.86 mmol, 3.5 equiv) was introduced over 5
min and stirred at −78 °C for an additional 5 min. A solution of Dibal-H
(0.5 M in CH2Cl2, 4.32 mL, 2.16 mmol) was added dropwise over 10
min. After 0.5 h, the reaction was quenched with EtOAc (1 mL) and
stirred for 5 min. The reaction was warmed to rt and diluted with conc.
HCl (5 mL) and stirred for 5 min to dissolve aluminum salts. The
mixture was transferred to a separatory funnel with EtOAc (10 mL), and
the organic layer was removed. The aqueous portion was extracted with
EtOAc (3 × 10 mL). The combined organic layers were washed with sat.
aqueous NaHCO3 (10 mL) and brine (10 mL), dried (Na2SO4), filtered,
and concentrated in vacuo. The resulting residue was purified by flash
chromatography on silica gel (elution: 55% → 100% EtOAc in hexane)
to afford 34 (96 mg, 56% yield) as a white solid: TLC (80% EtOAc in
Hexanes), Rf: 0.25 (UV, CAM); [α]2D5 = −42.2 (c 1.06, CH2Cl2); IR
(film) 3816, 3406, 3213, 2953, 1696, 1421, 1344, 1207, 1086 cm−1. The
1
The spectra of 28 are complicated by carbamate rotamers. H NMR
(400 MHz, CDCl3) δ 7.32 (m, 5H), 6.42−6.38 (d, J = 17.2, 1H), 6.13−
6.06 (dd, J1 = 17.2 Hz, J2 = 10.2 Hz, 1H), 5.85−5.82 (d, J = 10.2 Hz, 1H),
5.68 (br. s., 1H), 5.25−5.08 (m, 5H), 4.67−4.63 (t, J = 6.6 Hz, 1H),
3.58−3.47 (m, 2H), 2.24−2.17 and 2.07−1.98 (m, 2H); 13C NMR (100
MHz, CDCl3) δ 165.1, 136.5, 132.5, 131.4, 127.9, 117.4, 73.4, 66.8, 60.6,
43.1, 28.2; HRMS (ES+): Exact mass calcd for C17H19NO4Na+ [M +
Na]+, 324.1206. Found 324.1204.
Benzyl (3aS,7aS)-5-Oxo-3,3a,5,7a-tetrahydropyrano[3,2-b]-
pyrrole-1(2H)-carboxylate (29). Compound 28 (294 mg, 0.98
mmol) was added to a flame-dried two-neck flask, fitted with a reflux
condenser and flushed with N2. Dichloroethane (19.7 mL) was added,
and the reaction mixture was heated to reflux 77 °C (bath temp. 85 °C)
for 10 min. Hoveyda−Grubbs second-generation catalyst was added (45
mg, 0.068 mmol) in one potion. After stirring at reflux for 15 h under N2,
the reaction mixture was cooled to rt and concentrated in vacuo. The
resulting residue was purified by flash chromatography on silica gel
(elution: 15% → 70% EtOAc in hexane) to afford 29 (242 mg, 90%
yield) as a brown oil: TLC (60% EtOAc in Hexanes), Rf: 0.40 (UV,
CAM); [α]2D5 = +228 (c 0.19, CH2Cl2); IR (film) 3063, 2955, 2892,
2361, 2339, 1729, 1700, 1555, 1418, 1358, 1333, 1249, 1207, 1109, 1047
1
spectra of 34 are complicated by carbamate rotamers. H NMR (400
MHz, CDCl3) δ 7.34 (m, 5H), 5.96 (m, 1H), 5.74 (m, 1H), 5.43−5.29
(m, 1H), 5.12−5.05 (m, 4H), 4.34 (m, 1H), 4.05 and 3.74 (m, 2H), 2.37
(br. s), 2.19−2.16 and 2.06−2.04 (m, 2H); 13C NMR (100 MHz,
CDCl3) δ 156.2, 154.6, 136.2, 132.7, 128.8, 128.5, 127.8, 127.5, 125.7,
74.2, 73.2, 67.1, 55.3, 43.3, 29.2; HRMS (ES+): Exact mass calcd for
C16H20N2O5Na+ [M + Na]+, 343.1264. Found 343.1266.
1
cm−1. The spectra of 29 are complicated by carbamate rotamers. H
NMR (400 MHz, CDCl3) δ 7.36 (m, 5H), 7.23−7.19 and 6.91−6.87
(dd, J1 = 10.2 Hz, J2 = 4.8 Hz, 1H), 6.07−6.01 (t, J = 10.2 Hz, 1H), 5.21−
5.06 (m, 3H), 4.32 (s, 1H), 3.72−3.67 and 3.63−3.56 (m, 2H), 2.28−
2.18 (m, 2H); 13C NMR (100 MHz, CDCl3) δ 161.8, 154.5, 142.3,
136.1, 128.4, 127.9, 120.9, 79.2, 67.3, 51.3, 44.6, 31.2; HRMS (ES+):
Exact mass calcd for C15H15NO4Na+ [M + Na]+, 296.0893. Found
296.0894.
Benzyl (2S,3S)-3-(Carbamoyloxy)-2-(3-hydroxypropyl)-
pyrrolidine-1-carboxylate (36). TLC (60% EtOAc in Hexanes),
Rf: 0.30 (UV, CAM); [α]D25 = +30.7 (c 1.5, CH2Cl2); IR (film) 3337,
2942, 1692, 1611, 1422, 1344, 1199, 1086, 1052 cm−1. The spectra are
1
complicated by carbamate rotamers. H NMR (400 MHz, CDCl3) δ
7.33 (m, 5H), 5.10 (m, 2H), 4.98 (m, 1H), 3.68 (m, 2H), 3.45 (m, 2H),
2.12 (m, 2H), 2.01 (m, 2H), 1.85 (m, 1H), 1.68 (m, 2H), 1.58 (m, 2H);
13C NMR (100 MHz, CDCl3) δ 156.1, 155.3, 136.6, 128.5, 128.0, 73.6,
Benzyl (2S,3S)-3-(Carbamoyloxy)-2-((Z)-3-methoxy-3-oxo-
prop-1-en-1-yl)pyrrolidine-1-carboxylate (33). To a solution of
lactone 29 (283 mg, 1.03 mmol) in THF (5.3 mL) and H2O (1.7 mL)
was added LiOH·H2O (54 mg, 1.29 mmol, 1.25 equiv) at rt. After
stirring for 1 h, the reaction mixture was transferred to a separatory
funnel and partitioned between 0.2 M HCl (10 mL) and EtOAc (5 mL).
The organic layer was removed, and the aqueous layer was extracted
with additional EtOAc (4 × 5 mL). The organic fractions were
combined and washed with brine (40 mL), dried (Na2SO4), and
concentrated in vacuo. The resulting oil was dissolved in DMF (5 mL) at
rt, and K2CO3 (171 mg, 1.24 mmol, 1.2 equiv) and MeI (0.64 mL, 10.3
mmol, 10.0 equiv) were added. After stirring for 2 h, the reaction mixture
was transferred to a separatory funnel and diluted with a brine and 1.0 M
HCl solution (10 mL, 10:1 brine:HCl) and extracted with CHCl3 (5
mL). The organic layer was removed, and the aqueous layer was
extracted with CHCl3 (4 × 5 mL). The organic layers were combined,
washed with brine (40 mL), dried (Na2SO4), and concentrated in vacuo.
The resulting oil was dissolved in CH2Cl2 (5 mL) and cooled to 0 °C.
Trichloroacetyl isocyanate (0.182 mL, 1.55 mmol. 1.5 equiv) was added,
and the reaction was stirred for 30 min and concentrated in vacuo. The
residue was dissolved in MeOH (4.0 mL) and H2O (1.0 mL) and cooled
to 0 °C. To this solution was added NaHCO3 (173 mg, 2.06 mmol, 2
66.9, 62.7, 51.6, 43.5, 29.9, 28.1, 25.0; HRMS (ES+): Exact mass calcd for
C16H22N2O5Na+ [M + Na]+, 345.1421. Found 345.1420.
Benzyl (2S,3S)-3-((Hydroxycarbamoyl)oxy)-2-((Z)-3-me-
thoxy-3-oxoprop-1-en-1-yl)pyrrolidine-1-carboxylate (31). To
a solution of lactone 29 (1.50 g, 5.50 mmol) in THF (15 mL) and H2O
(5 mL) was added LiOH·H2O (280 mg, 6.8 mmol, 1.25 equiv) at rt.
After stirring for 1 h, the reaction mixture was transferred to a separatory
funnel and partitioned between 0.2 M HCl (30 mL) and EtOAc (20
mL). The organic layer was removed, and the aqueous layer was
extracted with additional EtOAc (4 × 10 mL). The organic portions
were combined and washed with brine (40 mL), dried (Na2SO4), and
concentrated in vacuo. The resulting oil was dissolved in DMF (5 mL) at
rt, and K2CO3 (0.85 g, 6.6 mmol, 1.2 equiv) and MeI (1.25 mL, 20
mmol) were added. After stirring for 1 h, the reaction mixture was
transferred to a separatory funnel and diluted with a brine (20 mL) and
1.0 M HCl solution (6 mL) and extracted with CHCl3 (10 mL). The
organic layer was removed, and the aqueous layer was extracted with
CHCl3 (4 × 10 mL). The organic layers were combined, dried
(Na2SO4), and concentrated in vacuo. The resulting oil was dissolved in
F
DOI: 10.1021/jo502493e
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Article
pyridine (12 mL), and CDI (1.78 g, 11 mmol) was added in one portion.
After stirring for 12 h at rt, the reaction was cooled to 0 °C and H2NOH·
HCl (1.50 g, 21.4 mmol) was added, and the reaction was allowed to
warm slowly to rt over 5 h. The reaction mixture was diluted with 0.5 M
HCl (50 mL), transferred to a separatory funnel, and extracted with
EtOAc (4 × 20 mL). The combined organic portions were washed with
brine (40 mL), dried (Na2SO4), and concentrated in vacuo. The
resulting residue was purified by flash chromatography on silica gel
(elution: 30% → 100% EtOAc in hexane) to afford the desired N-
hydroxy carbamate 31 (1.22 g, 61% over 3 steps) as a colorless oil: TLC
(60% EtOAc in Hexanes), Rf: 0.20 (UV, CAM); [α]2D5 = +71.3 (c 1.03,
CH2Cl2); IR (film) 3303, 3066, 3032, 2993, 2954, 2898, 1714, 1617,
1539, 1455, 1357, 1255, 1199, 1110, 1034, 996, 918, 816, 765, 733, 699,
667 cm−1. The spectra of 31 are complicated by carbamate rotamers. 1H
NMR (400 MHz, CDCl3) δ 7.75−7.71 (d, J = 18.8 Hz, 1H), 7.55 (br.s,
1H), 7.34 (m, 5H), 6.13−6.05 (m, 1H), 5.88−5.78 (d, J = 10.4 Hz, 1H),
5.58−5.52 (m, 2H), 5.11−5.08 (m, 2H), 3.67−3.49 (m, 5H), 2.04 (m,
2H); 13C NMR (100 MHz, CDCl3) δ 166.3, 166.1, 157.7, 155.0, 146.1,
136.1, 135.9, 128.3, 128.2, 127.8, 127.6, 120.4, 67.0, 58.5, 58.5, 51.3,
45.0, 44.6, 31.0, 30.5. Exact mass calcd for C17H20N2O7Na+ [M + Na]+,
387.1163 Found 387.1162.
(5 mL) and cooled to 0 °C. A solution of LiBH4 (3 M in THF, 0.50 mL,
1.5 mmol) was introduced via syringe. After stirring for 20 min, the
reaction was diluted with sat. aq. NH4Cl (10 mL) and brine (10 mL) and
extracted with EtOAc (10 × 10 mL). The combined organic portions
were dried (Na2SO4), filtered, and concentrated in vacuo to give 160 mg
of a colorless oil. This residue was purified by flash chromatography on
silica gel (elution: 0 → 15% MeOH in CHCl3) to afford the desired diol
compound (155 mg, 91% yield) as a colorless oil: TLC (5% MeOH in
EtOAc), Rf: 0.25 (UV, CAM); [α]2D5 = +74.6 (c 2.00, CH2Cl2); IR(film)
3392, 2954, 2926, 2895, 1695, 1423, 1356, 1201, 1114, 1062, 971, 907
1
cm−1. The spectra are complicated by carbamate rotamers. H NMR
(400 MHz, CDCl3) δ 7.33 (m, 5H) 7.00−6.85 (br. s, 1H), 5.16 (d, J =
12.6 Hz, 1H), 5.07 (m, 1H), 5.05 (d, J = 12.6, 1H), 4.79 (m, 1H), 4.23
(m, 3H), 3.77 (m, 3H), 3.46 (m, 1H), 2.18 (m, 1H), 1.98 (m, 1H); 13C
NMR (100 MHz, CDCl3) δ 155.8, 154.1, 135.9, 128.6, 128.2, 127.9,
79.0, 73.4, 67.5, 62.8, 53.9, 52.0, 45.1, 32.0; Exact mass calcd for
C16H20N2O6Na+ [M + Na]+, 359.1214. Found 359.1213.
Benzyl (4R,4aS,7aS)-4-((R)-1,2-Bis((methylsulfonyl)oxy)-
ethyl)-2-oxohexahydropyrrolo[2,3-e][1,3]oxazine-5(2H)-
carboxylate. The starting diol (55 mg, 0.16 mmol) was dissolved in
pyridine (1.5 mL), and MsCl (40 μL, 0.49 mmol) was added via syringe.
After stirring at rt for 1.25 h, the reaction mixture was transferred to a
separatory funnel and partitioned between CHCl3 (10 mL) and H2O/
Brine (1:1, 10 mL). The organic portion was removed, and the aqueous
portion was extracted with additional CHCl3 (3 × 10 mL). The
combined organic portions were dried (Na2SO4), filtered, and
concentrated in vacuo. The resulting residue was purified by flash
chromatography on silica gel (elution: 75 → 100% EtOAc in hexanes) to
afford the desired bismesylate (63 mg, 80% yield) as a colorless oil: TLC
(EtOAc), Rf: 0.40 (UV, CAM); [α]D = +52.0 (c 0.45, CH2Cl2); IR (film)
3366, 3268, 3032, 2939, 1707, 1422, 1358, 1175, 1117, 919 cm−1. The
Benzyl (2S,3S)-2-((Z)-3-Methoxy-3-oxoprop-1-en-1-yl)-3-
((((7,7,7,7,7-pentafluoro-7λ8-hepta-2,4,6-triynoyl)oxy)-
carbamoyl)oxy)pyrrolidine-1-carboxylate (32). To N-hydroxy
carbamate 31 (1.07 g, 2.94 mmol) in CH2Cl2 (35 mL) at 0 °C was
added NEt3 (0.45 mL, 3.2 mmol, 1.1 equiv), followed by pentafluoro-
benzoyl chloride (0.42 mL, 3.0 mmol). The reaction was stirred at 0 °C
for 15 min, diluted with sat. aq. NH4Cl (30 mL), and transferred to a
separatory funnel. The organic portion was removed, and the aqueous
portion was extracted with additional CH2Cl2 (2 × 20 mL). The
combined organic fractions were washed with sat. aq. NaHCO3 (30
mL), dried (Na2SO4), and concentrated in vacuo. The resulting residue
was purified by flash chromatography on silica gel (elution: 20 → 80%
EtOAc in hexanes) to afford the desired compound (32, 1.54 g, 94%
yield) as a colorless oil: TLC (60% EtOAc in hexanes), Rf: 0.60 (UV,
CAM): [α]2D5 = +66.1 (c 1.00, CHCl3); IR (film) 3197, 2953, 2903, 1923,
1866, 1789, 1760, 1701, 1653, 1576, 1503, 1416, 1359, 1326, 1255,
1184, 1105, 998, 912, 818, 755, 697 cm−1. The spectra of 32 are
1
spectra are complicated by carbamate rotamers. H NMR (400 MHz,
CDCl3) δ 7.35 (m, 5H), 7.11 and 6.93 (m, 1H), 5.13 (m, 3H), 4.47 (m,
3H), 4.12 (m, 1H), 3.80 (m, 1H), 3.47 (m, 1H), 3.12 (m, 5H), 2.21 and
2.04 (m, 2H); 13C NMR (100 MHz, CDCl3) δ 155.4, 152.9, 135.9,
128.7, 128.6, 128.2, 127.9, 78.6, 78.5, 68.0, 67.4, 67.3, 53.8, 50.7, 49.4,
39.8, 37.6, 32.2; Exact mass calcd for C18H24N2O10S2Na+ [M + Na]+,
515.0766. Found 515.0763.
1
complicated by carbamate rotamers. H NMR (400 MHz, CDCl3) δ
5-Benzyl 3-(tert-Butyl)(4R,4aS,7aS)-4-((R)-1,2-bis((methyl-
sulfonyl)oxy)ethyl)-2-oxotetrahydropyrrolo[2,3-e][1,3]-
oxazine-3,5(2H,4H)-dicarboxylate (39). The bismesylated carba-
mate (57 mg, 0.12 mmol) was dissolved in THF (1.2 mL), and Boc2O
(40 μL, 0.18 mmol) and DMAP (10 mg, 0.08 mmol) were added
successively. After stirring at rt for 1 h, the reaction mixture was diluted
with sat. aq. NH4Cl (10 mL) and extracted with EtOAc (3 × 10 mL).
The combined organic portions were washed with brine, dried
(Na2SO4), filtered, and concentrated in vacuo. The resulting product
39 (70 mg, 98% yield) was obtained as a colorless oil. This material was
used directly without purification: TLC (EtOAc), Rf: 0.75 (UV, CAM);
[α]2D5 = +96.6 (c 0.85, CH2Cl2); IR (film) 2985, 2941, 2890, 1798, 1704,
1417, 1371, 1180, 1123, 972, 929 cm−1. The spectra of 39 are
8.78 (br.s, 1H), 7.50−7.18 (m, 5H), 6.15−6.03 (dd, 1H), 5.94−5.83
(dd, J = 10.9 Hz, 1H), 5.74 (s, 1H), 5.63−5.58 (d, 1H), 5.14−5.07 (m,
2H), 3.83−3.14 (m, 5H), 2.35−2.04 (m, 2H); 13C NMR (100 MHz,
CDCl3) δ 166.0, 158.2, 155.0, 154.7, 145.7, 128.5, 128.3, 128.0, 127.7,
120.9, 104.7, 78.3, 67.2, 58.8, 51.4, 45.2, 31.1, 30.7. Exact mass calcd for
C24H19F5N2O8Na+ [M + Na]+, 581.0954 Found 581.0952.
Benzyl (4R,4aS,7aS)-4-((R)-1-Hydroxy-2-methoxy-2-oxo-
ethyl)-2-oxohexahydropyrrolo[2,3-e][1,3]oxazine-5(2H)-
carboxylate (38). Carbamate 32 (76 mg, 0.136 mmol) was dissolved in
t-BuOH/water solution (3:1, 2.0 mL). In a separate vessel, a solution of
K2OsO4·H2O (1.3 mg, 2.5 mol %) in water (0.5 mL) was added
dropwise over 10 min. After stirring at rt under N2 for 1.5 h, the reaction
was quenched with addition of sodium sulfite (30 mg, 200 mg/mmol)
and stirred for an additional 0.5 h. The solvent was azeotropically
removed with toluene and chloroform and concentrated in vacuo. The
resulting residue was purified by flash chromatography on silica gel
(elution: 0% → 10% MeOH in CHCl3) to afford the desired
aminohydroxylation product 38 (46 mg, 93% yield) as a colorless oil:
TLC (5% MeOH in CHCl3), Rf: 0.33 (UV, CAM); [α]2D5 = +44 (c 1.63,
CHCl3); IR (film) 3326, 3017, 2954, 2907, 1744, 1696, 1536, 1414,
1355, 1212, 1112, 759 cm−1. The spectra of 38 are complicated by
carbamate rotamers. 1H NMR (400 MHz, CDCl3) δ 7.35 (m, 5H), 6.93
and 6.77 (m, 1H), 5.13−5.06 (m, 3H), 4.77 (m, 1H), 4.47 and 4.29 (m,
2H), 4.02 (m, 1H), 3.83−3.72 (m, 3H), 3.53 and 3.48−3.41 (m, 2H),
2.20−2.16 and 1.99−1.96 (m, 2H); 13C NMR (100 MHz, CDCl3) δ
172.0, 155.5, 154.1, 136.0, 128.5, 128.2, 127.9, 79.2, 77.2, 72.8, 67.2,
53.8, 52.8, 44.7, 31.9. Exact mass calcd for C17H20N2O7Na+ [M + Na]+,
387.1163. Found 387.1162.
1
complicated by carbamate rotamers. H NMR (400 MHz, CDCl3) δ
7.33 (m, 5H), 5.30 (m, 2H), 5.13 (m, 2H), 5.00 (m, 1H), 4.47 (m, 2H),
3.12 (m, 6H), 2.26 (m, 1H), 2.16 (m, 1H), 1.49 (s, 9H); 13C NMR (100
MHz, CDCl3) δ 154.5, 151.1, 147.6, 128.7, 128.6, 128.5, 128.4, 128.3,
128.1, 68.0, 67.5, 66.9, 55.9, 55.4, 54.2, 45.1, 44.7, 38.9, 38.8, 37.8, 37.7,
32.8, 27.7; Exact mass calcd for C23H32N2O12S2Na+ [M + Na]+,
615.1289. Found 615.1287.
Benzyl (2S,3R,3aS,6aS)-3-((tert-Butoxycarbonyl)amino)-2-
(((methylsulfonyl)oxy)methyl)hexahydro-4H-furo[3,2-b]-
pyrrole-4-carboxylate (40). The mixed imide 39 (20 mg, 0.034
mmol) was dissolved in MeOH (0.65 mL), and Cs2CO3 (11 mg, 0.034
mmol) was added in one portion. The reaction was stirred at rt for 2.5 h
and concentrated to remove the bulk of MeOH. The residue was
transferred to a separatory funnel and partitioned between CHCl3 (10
mL) and H2O/sat. aq. NaHCO3 (1:1, 10 mL). The organic portion was
removed, and the aqueous portion was extracted with additional CHCl3
(2 × 5 mL). The combined organic portions were dried (Na2SO4),
filtered, and concentrated in vacuo to yield an amorphous solid (19 mg).
This residue was dissolved in MeOH (1.0 mL), and Pd(OH)2 (20 wt %
Benzyl (4R,4aS,7aS)-4-((R)-1,2-Dihydroxyethyl)-2-oxohexa-
hydropyrrolo[2,3-e][1,3]oxazine-5(2H)-carboxylate. Amino-
hydroxylation product 38 (185 mg, 0.51 mmol) was dissolved in THF
G
DOI: 10.1021/jo502493e
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The Journal of Organic Chemistry
Article
on carbon, 11 mg) was added. The reaction vessel was flushed for 10 min
with H2 gas. The exit line was removed, and the reaction was stirred
under an atmosphere of H2 for 1.5 h (when the TLC indicated
consumption of starting material). The reaction vessel was flushed with
N2, and the mixture was filtered through Celite. The filter pad was
washed with aqueous 10% Na2CO3 (5 mL) and CHCl3 (10 mL). The
filtrate was transferred to a separatory funnel, and the organic portion
was removed. The aqueous portion was extracted with additional CHCl3
(3 × 5 mL). The combined organic portions were dried (Na2SO4),
filtered, and concentrated in vacuo. The resulting residue was purified by
flash chromatography on silica gel (elution: 0 → 5% MeOH in CHCl3)
to afford N-boc-norloline 20 (6 mg, 72% yield over 2 steps) as an
amorphous solid. 1H and 13C NMR spectral data for 20 match
previously prepared material17 (see the Supporting Information): [α]2D5
= +35.8 (c 0.5, CHCl3); lit. +38.7 (c 0.35).
Agbidye, J.; Blythe, L. L.; Craig, A. M. Vet. Hum. Toxicol. 2001, 43, 140−
146.
(7) (a) Rowan, D. D.; Hunt, M. B.; Gaynor, D. L. J. Chem. Soc., Chem.
Commun. 1986, 935−936. (b) Rowan, D. D. Agric., Ecosyst. Environ.
1993, 44, 103−122.
(8) Zhang, D.-X.; Nagabhyru, P.; Schardl, C. L. Plant Physiol. 2009,
150, 1072−1082.
(9) Riedell, W. E.; Kieckhefer, R. E.; Petroski, R. J.; Powell, R. G. J.
Entomol. Sci. 1991, 26, 122−129.
(10) Petroski, R. J.; Yates, S. G.; Weisleder, D.; Powell, R. G. J. Nat.
Prod. 1989, 52, 810−817.
(11) For a review of loline alkaloids, see: Schardl, C. L.; Grossman, R.
B.; Nagabhyru, P.; Faulkner, J. R.; Mallik, U. P. Phytochemistry 2007, 68,
980−996.
(12) First isolation: Hofmeister, F. Arch. Exp. Pathol. Pharmakol. 1892,
30, 203−230.
(13) Whether the loline alkaloids are produced by the morning glory
plant or its associated endophyte has not yet been established: Tofern,
B.; Kaloga, M.; Witte, L.; Hartmann, T.; Eich, E. Phytochemistry 1999,
51, 1177−1180.
ASSOCIATED CONTENT
■
S
* Supporting Information
Spectroscopic data (1H NMR and 13C NMR) for all new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
(14) Freeman, E. M. Philos. Trans. R. Soc. London, Ser. B 1904, 196, 1−
27.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: jrscheerer@wm.edu (J.R.S.).
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Jeffress Memorial Trust. K.E.M.
was supported in part by funding from a Howard Hughes
Medical Institute Undergraduate Science Education Grant to
The College of William & Mary.
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DOI: 10.1021/jo502493e
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