Total synthesis of (+)-pentamethylsalvianolic acid C

Article abstract of DOI:10.1039/c3ob27478k

The total synthesis of a methylated analogue of (+)-Salvianolic acid C has been achieved. Key aspects of the synthetic route include an economical Cu(i) acetylide coupling, unique carboxyl activation conditions via microwave irradiation and a novel lipase catalysed kinetic resolution of a racemic mixture of secondary alcohol Danshensu. The preparation of this methylated analogue will not only improve the bioavailability, but also enable access to new and wider bioactivity applications for (+)-Salvianolic acid C.

Full text of DOI:10.1039/c3ob27478k

Organic &
Biomolecular Chemistry
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Total synthesis of (+)-pentamethylsalvianolic acid C†
Cite this: Org. Biomol. Chem., 2013, 11,
Benjamin L. Alford and Helmut M. Hügel*
2724
The total synthesis of a methylated analogue of (+)-Salvianolic acid C has been achieved. Key aspects of
the synthetic route include an economical Cu(I) acetylide coupling, unique carboxyl activation conditions
via microwave irradiation and a novel lipase catalysed kinetic resolution of a racemic mixture of secondary
alcohol Danshensu. The preparation of this methylated analogue will not only improve the bioavailability,
but also enable access to new and wider bioactivity applications for (+)-Salvianolic acid C.
Received 20th December 2012,
Accepted 19th February 2013
DOI: 10.1039/c3ob27478k
www.rsc.org/obc
due to their lower polarity making them more lipophilic,
specific assays for their biological function are required.12
Introduction
Danshen, the dried root and rhizome of Salvia miltiorrhiza We report here
a
short, efficient total synthesis of
Bunge, in the Lamiacea family (formerly Labiacea), is one of (+)-pentamethylsalvianolic acid C 1. The chemical synthesis
the most popular traditional Chinese medicines (TCM) that and structural variation of Danshen secondary metabolites
has been used globally to promote circulation and improve could lead to new drug candidates, thereby adding to the wide
blood flow. With a history of at least 2000 years, Danshen has variety of natural products and their analogues already esta-
been used in China for decades (since 1970) to provide thera- blished as potent pharmaceuticals.
peutic relief from stroke and angina pectoris. Other medicinal
Our retrosynthesis is outlined in Scheme 1. The key discon-
properties of this herb include antiviral, antioxidant and anti- nection between carbons C1′ and C2′′ gives the two main frag-
tumour activities.1–3 Aqueous Danshen decoctions (tea) have ments 2 and (R)-3. The benzo[b]furan core (2) was prepared via
been used for the prevention of the major causes of stroke cuprous acetylide coupling with bromoisovanillin and the
including thrombosis, atherosclerosis, oxidative damage and (R)-Danshensu component (R)-3 obtained from the racemic
cerebral ischemia–reperfusion.4,5 Investigations into the precursor via lipase catalysed kinetic resolution. The two
chemical constituents of Danshen revealed two predominant elements of the natural product were joined via esterification.
classes of secondary metabolites. A family of lipid soluble, In an attempt to access the natural polyphenol, demethylation
hydrophobic diterpenoids deemed Tanshinones and a hydro- of the five methyl ethers will be attempted given the success of
philic, polyphenolic combination of compounds consisting O’Malley et al.13 in the final stages of their total synthesis of
mainly of caffeic acid dimers, trimers or tetramers (depsides):
the Salvianolic acids. The polyphenolic acids are unique to the
Salvia genus.3 The water soluble compounds were found to be
relatively more abundant (7% (w/w) for (+)-Salvianolic acid B
in Salvia bowleyana4) and many are available.6,7
While the polyphenolic nature of Danshen water soluble
components allows access to a huge array of bioactivities
including antioxidant and anticancer activity, they are not
without their limitations. Pharmacokinetic studies on the
most abundant polyphenol, Salvianolic acid B, revealed that
the natural product has very poor bioavailability and is
metabolised to four methylated derivatives.8–11 It is currently
unknown whether these metabolites are pharmacologically
active but as they have the ability to pass through cell walls,
RMIT University, School of Applied Science [Applied Chemistry], Bowen Street,
Melbourne, Victoria, Australia. E-mail: helmut.hugel@rmit.edu.au
†Electronic supplementary information (ESI) available: Includes full experimen-
tal procedures and copies of 1H NMR and 13C NMR spectra. See DOI:
10.1039/c3ob27478k
Scheme 1 Retrosynthesis of (+)-pentamethylsalvianolic acid C.
2724 | Org. Biomol. Chem., 2013, 11, 2724–2727
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(+)-Lithospermic acid. Even though this retrosynthesis is
similar to that of Shen et al.14 in their recent total synthesis of
(+)-Salvianolic acid C, this approach offers several practical
alternatives.
Results and discussion
Synthesis of the benzo[b]furan moiety of 1 via cuprous
acetylide coupling
Ethyl bromocinnamate 4 was prepared in 74% yield from
bromoisovanillin by Knoevenagel condensation with ethyl
hydrogen malonate. As a model reaction, the Knoevenagel
condensation was attempted with 3-iodovanillin but only trace
amounts of product were observed. This is thought to be due
to the generation of HI followed by decomposition and there-
fore bromine was used as the activator in further trans-
formations. Cu(I) acetylide 5 was synthesised from aromatic
aldehyde
7
in two steps via the Bestmann–Ohira
homologation15–17 (see ESI†) followed by deprotonation in
aqueous ammonia in the presence of Cu(II)SO4 according to
Wong and co-workers.18 Coupling of 5 with 4 was then per-
formed using the optimised conditions of Scammells et al.19
to form the 2-arylbenzo[b]furan core 6 in 51% yield after
recrystallization from EtOAc (Scheme 2). The diyne by-product
(12% yield) from the homocoupling of 5 was removed by FCC.
The ethyl ester of 6 was then hydrolysed with LiOH in aqueous
THF (83% yield) to form the benzo[b]furan scaffold 2 in a total
of four linear steps with an overall yield of 22% from
isovanillin.
Scheme 3 Preparation of (R)-3 and determination of ee.
Preparation of (S) and (R)-Danshensu via Darzens glycidic
ester synthesis followed by kinetic resolution
hexane (Scheme 3). This was then hydrogenated at atmos-
pheric pressure in the presence of 10% Pd/C to form α-hydroxy
ester rac-9 with an excellent yield of 85% after FCC.22 The
racemic mixture was then separated by kinetic lipase catalysed
resolution. Using the lipase from Burkholderia cepacia, as rec-
ommended by Adam et al.23 the optimum conditions were 10
equivalents of an acyl donor (vinyl acetate), up to 1 weight
equivalent of lipase, performing the operation at 50 °C for
48 hours. After removing the acetylated product (10) from the
crude mixture by FCC and repeating the resolution, (R)-3 was
obtained in 47% yield. Both fractions of 10 were then deacety-
lated under methanolysis conditions (K2CO3, MeOH) to afford
(S)-3 in 69% yield. For the determination of enantiomeric
excess (ee) the two optically active α-hydroxy esters were esteri-
fied with (1S)-(+)-10-camphorsulfonyl chloride 11 (Scheme 3)
and the resulting diastereomers (12) were analysed via 1H
NMR (CDCl3) spectroscopy.24 The ee for both Danshensu enan-
tiomers was determined to be 96% (see ESI†). This is an
improvement over the resolution performed by Eicher et al.25
as the ee for the Danshensu motif has been increased by more
than 10%. The identity of (R,S) and (S,S) diastereomic CH3
singlets was confirmed by preparing a (+)-Danshensu standard
from commercial (+)-Rosmarinic acid (according to O’Malley
Epoxide trans-8 was prepared by Darzens condensation of
aldehyde 7 with methyl chloroacetate.20,21 When the reaction
was performed with NaOMe as the base, epoxide trans-8 was
obtained in 49% yield after recrystallisation from EtOAc–
1
et al.13) and comparing the H NMR spectra obtained for the
diastereomer formed between this and 11 and rac-9 and 11.
Scheme 2 Synthesis of 2-arylbenzo[b]furan 2.
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Esterification of the benzo[b]furan scaffold with
(R)-Danshensu
Deprotection of methyl (+)-pentamethylsalvianolate C (14)
The methyl ester in 14 could be selectively hydrolysed with
At first the Steglich protocol was employed to esterify the two Me3SnOH to form carboxylic acid 1 in 71% yield (Scheme 4).
building blocks as has been done in related syntheses.13,26,27 Other ester hydrolysis techniques including LiOH in aqueous
However, when this approach was attempted only the activated THF and Ba(OH)2 in MeOH were not selective for the methyl
carboxylic acid was formed and a large amount of unreacted ester as has been reported by Nicolaou et al.31 An attempt was
alcohol remained. When DIC (N,N′-diisopropylcarbodiimide) made to demethylate the remaining methyl ethers using the
was used trace amounts of desired ester 14 were observed with conditions described by O’Malley et al.13 Unfortunately only
the major product being the activated carboxylic acid inter- complete decomposition was observed and an alternative pro-
mediate and unreacted alcohol.28 When acid chloride 13 was cedure was required. Using the BBr3 demethylation conditions
prepared the esterification with (R)-3 proceeded readily in of Lingham et al.32 with the knowledge of McOmie et al.33 four
refluxing CH2Cl2. This approach was similar to that done out of five methyl ethers could be removed. Attempts were
by Bogucki and Charlton29 in their total synthesis of made to isolate the mono-methylated product via prep-TLC
(S)-(−)-Rosmarinic acid. To achieve quantitative yields of 13 but due to the incredibly low yield, complete characterisation
the activation was performed using microwave irradiation with could not be carried out. In future work, the use of alternative
tetrachloroethylene (C2Cl4) as the solvent. Freshly prepared 13 protective groups is recommended.
was then treated with (R)-3 (2 : 1) in refluxing CH2Cl2 in the
presence of Et3N to afford methyl (+)-pentamethylsalvianolate
C 14 in 71% yield (Scheme 4). Surprisingly during the reaction
an anhydride by-product (see ESI†) was formed and isolated
Conclusions
(+)-Pentamethylsalvianolic acid C (1) has been prepared in a
total of six linear steps with an overall yield of 11% from iso-
vanillin. Highlights of the synthetic route include an economical
Cu(I)-acetylide coupling with ethyl bromocinnamate 4 to form
the 2-arylbenzo[b]furan skeleton 6, lipase mediated resolution
of rac-9, novel acid chloride formation and selective hydrolysis
of the methyl ester in 14 with Me3SnOH. The use of inexpen-
sive, stable reagents has made this synthesis and potentially
related syntheses scalable. Biological testing of analogue 1 is
currently underway and will be presented in due course.
from the CH2Cl2 solution by precipitation from the reaction
mixture in 20% yield. It is known that this by-product did not
form during the microwave activation as the entire sample of
the acid chloride was soluble in CH2Cl2 at room temperature.
The structure of 14 was confirmed by comparison of the 1H
NMR data obtained for methyl (+)-pentamethylsalvianolate
C from the original isolation (see ESI†).30
Acknowledgements
Thanks are due to RMIT University – School of Applied
Sciences for financial support and the opportunity to under-
take postgraduate studies and to Sally Duck (Monash Univer-
sity) for preparing the high resolution mass spectra.
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