Formal Synthesis of Indolizidine and Quinolizidine Alkaloids from Vinyl Cyclic Carbonates

Article abstract of DOI:10.1002/chem.201904223

Cyclic carbonates have long been considered relatively inert molecules acting as protecting groups in complex multistep synthetic routes. This study shows that a concise, yet modular synthesis of indolizidine and quinolizidine alkaloids can be developed from vinyl-substituted cyclic carbonate (VCC) intermediates. Through a highly stereoselective palladium-catalyzed allylic alkylation reaction, these alkaloid motifs can be assembled in four synthetic and only two column purification steps. The combined results help to further advance functionalized cyclic carbonates as useful and reactive intermediates in natural product synthesis.

Full text of DOI:10.1002/chem.201904223

A Journal of
Accepted Article
Title: Formal Synthesis of Indolizidine and Quinolizidine Alkaloids from
Vinyl Cyclic Carbonates
Authors: Alex Cristofol, Christian Bohmer, and Arjan Willem Kleij
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Link to VoR: http://dx.doi.org/10.1002/chem.201904223
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COMMUNICATION
Formal Synthesis of Indolizidine and Quinolizidine Alkaloids from
Vinyl Cyclic Carbonates
Àlex Cristòfol,[a] Christian Böhmer[a] and Arjan W. Kleij*[a,b]
Abstract: Cyclic carbonates have long been considered relatively
inert molecules acting as protecting groups in complex multistep
synthetic routes. This study shows that a concise, yet modular
synthesis of indolizidine and quinolizidine alkaloids can be developed
from vinyl-substituted cyclic carbonate (VCC) intermediates. Through
a highly stereoselective palladium-catalyzed allylic alkylation reaction,
these alkaloid motifs can be assembled in four synthetic and only two
column purification steps. The combined results help to further
advance functionalized cyclic carbonates as useful and reactive
intermediates in natural product synthesis.
nucleophiles.[9] Previously, we reported on a palladium-catalyzed
allylic alkylation combining modular VCCs and nitroalkanes as
(pro)nucleophiles giving access to highly substituted, homoallylic
nitroalkanes with excellent stereocontrol (Scheme 1).[10] We
envisioned that this type of readily accessible (Z)-configured
homoallylic nitroalkanes would provide an excellent starting point
for the preparation of indolizidine and quinolizidine alkaloids
(Scheme 1) in few overall synthetic manipulations via a double
and appropriate cyclization strategy. These two families of
alkaloids comprise more than 740 compounds that have been
identified and isolated, and are naturally present in many living
organisms.[11] In particular, indolizidine and quinolizidine alkaloids
containing an aryl-substituent (such as ipalbidine, thylophorine,
antofine and cryptoleurine as well as their seco-analogues) have
received widespread attention due to their biological activities.[12]
Several synthetic approaches towards these alkaloids have been
reported,[13] but a more unifying and flexible strategy based on a
common intermediate could allow simple access to a wider array
of indolizidine and quinolizidine targets. Such a development
would be of general importance from a synthetic and medicinal
development point of view.
For many years, cyclic carbonates have been considered to be a
category of relatively inert molecules with their main application
being protecting groups of diols in synthetic organic chemistry.[1]
This has been particularly useful in polyhydroxylated molecules
such as sphingolipids or carbohydrates, and in these latter cases
the cyclic carbonate productively served to improve the selectivity
in glycosylation reactions.[2] Despite their excellent potential as
protecting groups, this intrinsic kinetic stability has hampered their
use as reactive intermediates in synthetic campaigns towards
more complex organic molecules. Therefore, the synthesis of
complex natural products using cyclic carbonates as reactive
intermediates has indeed remained scarce.[3] This is noteworthy
as their oxazolidinone (cyclic carbamate) analogues have been
extensively used in natural product synthesis as chiral
auxiliaries.[4]
Though cyclic carbonates have been seldom used in natural
product synthesis, an increasing number of research groups have
recently started to use vinyl cyclic carbonates (VCCs; see
Scheme 1) as reactive partners in transition-metal catalyzed
methodological development.[5] In particular, VCCs have been
frequently used in allylic substitution chemistry[6] and C‒H
functionalization reactions.[7] Compared to vinyl epoxides, VCCs
are not only more accessible, modular and easier to synthesize
from readily available -hydroxy ketones, but also their reactivity
has proven to be different in some cases.[8] Our group has focused
on the functionalization of vinyl cyclic carbonates in palladium-
catalyzed allylic substitution reactions with different types of
[a]
[b]
À. Cristòfol, C. Böhmer, Prof. A. W. Kleij
Institute of Chemical Research of Catalonia (ICIQ)
Barcelona Institute of Science and Technology (BIST)
Av. Països Catalans 16, 43007 Tarragona (Spain)
E-mail: akleij@iciq.es
Prof. A. W. Kleij
Catalan Institute for Research and Advanced Studies (ICREA)
Pg. Lluis Companys 23, 08010 Barcelona (Spain)
Scheme 1. Synthetic approach towards the synthesis of indolizidine and
quinolizidine alkaloids using vinyl-substituted cyclic carbonates.
Supporting information for this article is given via a link at the end of
the document.
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Herein, we report on a highly flexible and general synthesis of
indolizidine and quinolizidine alkaloids that takes advantage of the
modular and accessible nature of VCCs (Scheme 1). Importantly,
our strategy relies on the excellent stereoselectivity achieved in
the conversion of VCCs under palladium catalysis,[6c,6h,9] which
sets up the formal alkaloid syntheses by providing precursors with
the requisite functionalities and double bond configuration.
Moreover, by slightly adjusting the synthetic route, we were able
to access 2- and 3-substituted indolizidine alkaloids through
stereodefined homoallylic nitroalkane intermediates.
drive the overall chemoselectivity while allowing the efficient
cyclization of an amino alcohol group.[14] These unique conditions
proved to be more generally applicable and allowed to generate
OMe-ipalbidine (8a), septicine (8b), seco-antofine (8c) and
julandine (8d) in a reproducible and effective manner. Additionally,
we confirmed the structure of julandine (8d) by X-ray analysis
(Scheme 3).[15] Moreover, alkaloids 8a‒d have been previously
used to access ipalbidine and the other phenantroindolizidine and
phenantroquinolizidine alkaloids in a single step.[16]
The synthesis of VCCs 4a-c was accomplished in four steps
(Scheme 2A) starting from commercially available and
inexpensive acetophenone derivatives. First, -bromination was
carried out with N-bromosuccinimide (NBS) under acidic
conditions. Although compounds 1a-b are commercially available,
we preferred to use the freshly prepared reagents, as these
compounds tended to decompose. Subsequent hydrolysis of
crude 1a-b with aqueous sodium formate solution gave the
desired -hydroxyketones 2a-b in good yields. Compared to other
methods, this approach worked best in terms of yield and
reproducibility (see Table S1 in the Supporting Information for
more details). With -hydroxyketones 2a-b in hand, Grignard
reactions with freshly prepared organomagnesium reagents from
alkenyl bromides A and B afforded 1,2-diols 3a-c. A slight excess
of organomagnesium reagent had to be used due to
deprotonation of the free alcohol group. Therefore, purification of
3b-c (containing R2 = 3,4-dimethoxyphenyl) was needed to
remove the non-volatile 3,4-dimethoxystyrene by-product, in
contrast to gaseous propene generated from alkenyl bromide A.
Finally, facile formation of the cyclic carbonate using triphosgene
in the presence of pyridine at low temperature concluded the
preparation of VCCs 4a-c.
Once the synthesis of VCCs 4a-c was completed, the
stereoselective, catalytic formation of homoallylic nitroalkanes
was examined (Scheme 2B). Using already established
conditions for this transformation,[10] we successfully prepared
compounds 5a-d from VCCs 4a-c and nitroalkanes C-D in 46-
64% yield while maintaining excellent stereocontrol in the olefin
moiety. Importantly, the stereochemical configuration around the
double bond achieved in this step is crucial to provide the requisite
geometry for the subsequent cyclization steps affording the
1,2,3,6-tetrahydropyridine ring present in the indolizidine and
quinolizidine core.
The synthesis of the desired indolizidine and quinolizidine
alkaloids was then carried out from compounds 5a‒d, which were
first reduced to their diols using DIBAL-H at ambient temperature
(Scheme 3). The resulting nitrodiol intermediates 6a‒d were
further reduced with activated zinc under acidic conditions to
efficiently yield the polar aminodiol intermediates 7a‒d. After
screening of various conditions (see Table S2 in the Supporting
Information for more details), we found that in situ generated
SO2F2 under basic conditions smoothly enabled the formation of
the bicyclic system. We believe that the hard electrophilicity of
SO2F2 may provide suitable conditions for a charge-controlled
reaction, which may not be the case if other electrophiles are
employed. This unprecedented intramolecular double cyclization
strategy represents a useful example of the utilization of SO2F2 to
Scheme 2. Synthesis of VCCs 4a-c and their transformation into stereodefined
homoallylic nitroalkanes 5a-d. Ar
=
3,4-(OMe)2C6H3, NBS
=
N-
bromosuccinimide, p-TsOH·H2O = p-toluenesulfonic acid monohydrate, THF =
tetrahydrofuran, ND = not determined, dba = dibenzylideneacetone, DPEPhos
= bis[(2-diphenylphosphino)phenyl] ether, DBU = 1,8-diazabicyclo[5.4.0]undec-
7-ene.
Next, we investigated the introduction of substituents on the
cyclopentane ring of the indolizidine core to provide access to
novel alkaloid scaffolds. Since previous methods for such alkaloid
synthesis start from proline or related pyrrolidine compounds
which are difficult to functionalize,[12b] we envisioned a potential
divergent approach that would allow to acommodate substituents
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at positions 2 and 3 of the indolizidine core in an efficient and step-
economical manner. Therefore, a straightforward reduction of the
NO2 group in compound 5a with activated zinc under acidic
conditions gave clean access to aminoester intermediate E
(Scheme 4).
intermediate 11) with a diastereomeric ratio of 8.5:1. Subsequent
reduction of the lactam with LiAlH4 provided indolizidine alkaloid
12 in good yield. On the other hand, activation of the lactam with
triflic anhydride at -78 ºC using 2,6-di-tert-butyl-4-methylpyridine
(DTBMP) as a bulky base, and subsequent double alkylation with
excess MeMgBr afforded the 3,3-dimethyl indolizidine derivative
13.[19]
Scheme 3. Synthesis of indolizidine and quinolizidine alkaloids. DIBAL-H =
diisobutylaluminum hydride, DIPEA = N,N-diisopropylethylamine, DMF = N,N-
dimethylformamide.
We expected that intermediate E would spontaneously cyclize
to lactam product 9, but the conversion after 16 h in refluxing
MeOH was surprisingly low. The desired cyclization could be
accelerated using a catalytic amount of triazabicyclodecene
(TBD). Hereafter, a Mitsunobu reaction gave access to the
cyclohexene ring of the indolizidine motif. Unfortunately, standard
Mitsunobu conditions using DEAD and PPh3 led to a complex
mixture. However, using the less frequently used combination of
tetramethylazodicarboxamide (TMAD) and tributylphosphine
[P(n-Bu)3] in THF afforded the desired bicyclic lactam 10 in 84%
yield (see Table S3 in the Supporting Information for more
details).[17] We further confirmed the structural assignation of the
key bicyclic lactam intermediate 10 by X-ray analysis (Scheme
4).[18] From this intermediate, it was possible to introduce
substituents at the 2- and 3-position of the indolizidine motif in a
straightforward fashion. First, by taking advantage of the
nucleophilic nature of the generated lithium enolate, it was
possible to introduce a benzyl substituent at the 2-position (cf.,
Scheme 4. Synthesis of substituted indolizidine derivatives. TBD
triazabicyclodecene, TMAD = tetramethylazodicarboxamide, LDA = lithium
=
diisopropylamide, Tf2O
methylpyridine.
= triflic anhydride, DTBMP = 2,6-di-tert-butyl-4-
In conclusion, we have developed a flexible and general
synthetic route for the synthesis of indolizidine and quinolizidine
alkaloids from readily available vinyl-substituted cyclic carbonates
(VCCs). A palladium-catalyzed allylic alkylation reaction of VCC
building blocks provides useful stereodefined homoallylic
nitroalkane intermediates,[20] which can be conveniently
transformed into indolizidine or quinolizidine alkaloids in three
additional synthetic and one purification step(s). Moreover, this
synthetic route allows a divergent approach towards the synthesis
of a wider variety of 2- and 3-substituted indolizidine derivatives
through the intermediacy of a bicyclic lactam. This work thus
further exemplifies that suitably substituted cyclic carbonates can
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Acknowledgements
We thank the Cerca program/Generalitat de Catalunya, ICREA,
MINECO (CTQ2017-88920-P), and AGAUR (2017-SGR-232) for
support. C.B. thanks the Erasmus+ fellowship program. E.C.
Escudero-Adán and M. Martínez Belmonte are acknowledged for
the X-ray analyses.
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Keywords: alkaloids • indolizidine • palladium • quinolizidine •
vinyl cyclic carbonate
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Raising alkaloids: A flexible
synthesis of indolizidine and
quinolizidine alkaloids is
Àlex Cristòfol, Christian Böhmer
and Arjan W. Kleij*
facilitated by stereoselective
conversions of vinyl cyclic
carbonates under Pd catalysis
furnishing key stereodefined
homoallylic nitroalkane
Page No. – Page No.
intermediates. The vinyl cyclic
carbonate substrates are
showcased as modular and
accessible reagents useful in
natural product synthesis.
Formal Synthesis of Indolizidine
and Quinolizidine Alkaloids from
Vinyl Cyclic Carbonates
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