Total synthesis and stereochemical revision of lagunamide A

Article abstract of DOI:10.1039/c2cc34187e

A revised configurational assignment for the marine metabolite lagunamide A is proposed and validated by total synthesis.

Full text of DOI:10.1039/c2cc34187e

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COMMUNICATION
Total synthesis and stereochemical revision of lagunamide Aw
Lu Dai,a Bo Chen,a Honghui Lei,a Zhuo Wang,b Yuqing Liu,b Zhengshuang Xu*ab and
Tao Ye*ab
Received 11th June 2012, Accepted 5th July 2012
DOI: 10.1039/c2cc34187e
A revised configurational assignment for the marine metabolite
lagunamide A is proposed and validated by total synthesis.
synthesis of lagunamide A and the resulting assignment of the
stereochemical configuration of the natural product. As outlined
retro-synthetically in Scheme 1, our synthetic approach relies
on macrolactamisation of precursor 2, which may be most
conveniently constructed by the assembly of the tetrapeptide
portion 3 and triester 4. Further retrosynthetic analysis of
fragment 4 revealed that it can be prepared from phosphonate 5,
aldehyde 6 and acid chloride 7.
Lagunamide A is a cyclodepsipeptide, recently isolated from
the marine cyanobacterium Lyngbya majuscula with the
proposed stereochemistry as shown in Scheme 1.1 Structurally,
lagunamide A comprises eleven stereogenic centers, a novel
polyketide moiety, an a-hydroxy acid unit, two L-amino acids, and
three residues of N-methyl amino acids inscribed in a 26-membered
macrocycle. Its structure was determined by extensive spectroscopic
analysis, including 2D NMR experiments. Relative and absolute
stereochemistry assignments were based on extensive NMR studies
and chiral HPLC techniques, including Marfey’s method and
modified Mosher ester analysis. Lagunamide A possessed potent
cytotoxic activities against P388 murine leukemia cell lines, with
IC50 values of 6.4 nM. We have been interested for some time in
marine secondary metabolites,2 and view their syntheses as a key
route to structural confirmation, structural modification and
subsequent activity control. Herein, we disclose the first total
The synthesis of tetrapeptide 3 commenced with the coupling
of dipeptides 8 and 9 using HATU3 as coupling reagent to afford
3 in 84% yield. Saponification of the methyl ester of 3 then
produced the corresponding acid 10 in 97% yield (Scheme 2).
The synthesis of fragment 4 began with the preparation of
phosphonate 5 (Scheme 3). Thus, D-allo-isoleucine was diazotized
to the corresponding a-acetoxycarboxylic acid4 and subsequently
converted into the t-butyl ester derivative (12)5 in 50% yield. 12
was subjected to a mild base hydrolysis giving the corresponding
a-hydroxy ester, which was then condensed with 2-(diethoxyphos-
phoryl)propanoic acid employing DIPC as the coupling reagent
to afford phosphonate 5 in 83% yield.
Our attention was turned to the preparation of aldehyde 6
and its homologation via the Horner–Wadsworth–Emmons
(HWE) olefination with phosphonate 5. Thus, treatment of
(S)-2-methylbutanal6 with the boron enolate derived from
Evans (R)-oxazolidinone 13 gave the corresponding alcohol,
which was converted into anisylidine acetal 14 in a two-step
sequence involving reductive removal of the oxazolidinone to
Scheme 2 (i) HATU, HOAt, DIPEA, DCM; (ii) LiOH, THF–H2O–
MeOH, then H+. HATU: 2-(7-Aza-1H-benzotriazole-1-yl)-1,1,3,3-
tetramethyl-uronium hexafluorophosphate; HOAT: 1-hydroxy-7-
azabenzotriazole.
Scheme 1 Retrosynthetic analysis of lagunamide A.
a Key Laboratory of Chemical Genomics, Peking University Shenzhen
Graduate School, University Town of Shenzhen, Xili,
Nanshan District, Shenzhen, 518055, China.
E-mail: bctaoye@inet.polyu.edu.hk, xuzs@pkusz.edu.cn
b Department of Applied Biology & Chemical Technology, The Hong
Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong,
China. E-mail: bctaoye@inet.polyu.edu.hk; Tel: +852 34008722
w Electronic supplementary information (ESI) available: Details of
experimental procedures and 1H NMR, 13C NMR spectra for key
intermediates. See DOI: 10.1039/c2cc34187e
Scheme 3 (i) NaNO2, H2SO4–H2O; (ii) AcCl, reflux; (iii) Boc2O, DMAP,
tBuOH; (iv) K2CO3, MeOH–H2O; (v) 2-(diethoxyphosphoryl)propanoic
acid, DIPC, Collidine, DCM. DIPC: N,N-diisopropylcarbodiimide.
c
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Chem. Commun., 2012, 48, 8697–8699 8697

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afford the corresponding diol followed by treatment of the
resultant diol with anisaldehyde dimethyl acetal in the presence
of a catalytic amount of PPTS. DIBAL reductive cleavage of
the anisylidine acetal7 14 to the primary alcohol followed by
Dess–Martin oxidation afforded the corresponding aldehyde,
which was then subjected to a diastereoselective allylation
using Keck’s protocol8 to give homoallylic alcohols 15 and
15a in 94% combined yield and anti–syn ratio 7 : 3. The
stereochemistry of 15 was confirmed by examining the spectral
properties of acetonide 16, which was derived from removal of the
PMB protecting group in 15 with DDQ, and re-protection of the
resulting diol with 2,2-dimethoxypropane to afford acetonide 16
in 88% yield over two steps. The acetonide methyl groups (23.8,
25.3) (d in ppm, vide infra) and ketal carbon (100.4) of 16 exhibited
13C NMR chemical shifts characteristic of anti-1,3-diol acetonides
(23.6–25.6 and 100.2–101.0) and distinct from those observed for
syn-1,3-diol acetonides (18.6–19.9/29.8–30.2, and 98.0–99.3).9
Protection of the homoallylic alcohol 15 as its TES ether, followed
by oxidative cleavage of the double bond led to aldehyde 6, which
was then homologated by Horner–Wadsworth–Emmons conden-
sation to give a,b-unsaturated ester 17 in 80% yield (over three
steps). To our surprise, cleavage of the PMB ether in the presence of
the TES group proved to be problematic. Thus, treatment of 17 with
DDQ, even under buffered conditions, resulted in partial cleavage
of the TES group to give rise to diol 18 (32%) along with the
desired product (19, 64%). We were delighted to find that the
significant differences in the steric environment of the 1,3-diol (18)
permitted selective re-protection of the less sterically demanding
hydroxyl. In the event, treatment of diol 18 with TESOTf and
2,6-lutidine at À78 1C afforded the desired mono-protected diol
19 in 78% yield. Esterification of sterically demanding hydroxyl in
19 was anticipated to be challenging. Gratifyingly, treatment of 19
with Fmoc-N-Me–Ala–Cl in the presence of DMAP in toluene
afforded triester 4 in 65% yield.
Scheme 4 (i) Bu2BOTf, DIPEA, DCM, 0 1C; then S-2-methylbutanal,
À78 1C; (ii) NaBH4, Et2O–MeOH; (iii) anisaldehyde dimethyl acetal, PPTS,
DCM; (iv) DIBAL-H, DCM, À78 to À10 1C; (v) (a) Dess–Martin period-
inane, NaHCO3, DCM; (b) allyltributylstannane, BF3ÁOEt2, DCM, À78 1C;
(vi) DDQ, phosphate buffer; (vii) DMP, PTSA; (viii) TESOTf, 2,6-lutidine,
DCM, À78 1C; (ix) OsO4, NaIO4, 2,6-lutidine, dioxane–H2O; (x) 5, DIPEA,
LiCl, MeCN; (xi) DDQ, phosphate buffer; (xii) TESOTf (1.0 eq.),
2,6-lutidine, DCM; (xiii) Fmoc-N-Me-Ala-Cl, DMAP, toluene, 60 1C.
The final assembly of lagunamide A fragments is illustrated in
Scheme 5. Thus, treatment of 4 with diethylamine effected the
removal of the Fmoc protecting group to give the corresponding
free amine, which underwent a HATU-mediated coupling reaction
with acid 10 to provide precursor 2 in 82% yield. Simultaneous
deprotection of the t-butyl ester, TES ether and Boc-protecting
groups afforded the desired amino acid which was immediately
activated by HATU to afford lagunamide A 1 in 71% overall yield.
In parallel, homoallylic alcohol 15a was also transformed into the
respective diastereoisomer of lagunamide A (1a). Unfortunately,
neither 1H nor 13C NMR spectra for both 1 and 1a were identical
with those of the natural product.
Scheme 5 (i) Et2NH, MeCN; (ii) 10, HATU, HOAT, collidine,
DMF; (iii) TFA, DCM; (iv) HATU, HOAT, collidine, DMF.
acid-catalyzed deprotection of the acetonide functionality
produced diol 18, which is identical to the material obtained
by the approach illustrated in Scheme 4.
Having established that the published structure was incorrect we
wished to verify the correct structure for lagunamide A. The actual
structure of lagunamide A appeared to be epimeric to the proposed
structure at one or more stereocenters in the ring system.
Lagunamide A shares several structural features with the
26-membered cyclodepsipeptides aurilide A–C12 and kuloke-
kahilide-213 (also isolated from Lyngbya majuscula). However,
the assignment of C39 stereochemistry for lagunamide A
(Scheme 1) appears unusual on the basis of the anticipated
common bacterial biogenesis of these related marine natural
products. We therefore hypothesized that the C39 stereocenter of
originally proposed lagunamide A structure may require revision.
Thus, we turned to synthesis of the C39-epimers (R-configuration)
of the originally proposed structure of lagunamide A (1b, 1c). This
was readily achieved by following the same synthetic procedure as
In order to verify whether the synthesis would lead to the
compromise of the stereochemical integrity of diol 18, an alter-
native approach has been designed and executed (Scheme 6). Thus,
protection of the known homoallylic alcohol 2010 as its TES ether,
followed by oxidative cleavage of the olefin afforded the corre-
sponding aldehyde, which was then converted into homoallylic
alcohol 21 by reagent-controlled anti crotylation.11 Desilylation of
21 was carried out with HCl in methanol, and the corresponding
1,3-diol was re-protected as the acetonide 22 in 89% yield over
two steps. Hydrogenation of the alkene resulted in concomitant
cleavage of the benzyl ether and oxidation of the resultant alcohol
provided aldehyde 23 in 92% yield. Horner–Wadsworth–
Emmons olefination of 23 with phosphonate 5 followed by
c
8698 Chem. Commun., 2012, 48, 8697–8699
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diastereomers 1d and 1e (Fig. 1), where the original L-allo-isoleucine
was replaced with L-isoleucine. These syntheses proceeded
smoothly under the previous conditions, and the L-isoleucine was
readily incorporated into the synthesis to produce 1d and 1e. Upon
completion, it was clear from comparing 13C NMR data (Fig. 2)
that the true configuration of natural lagunamide A is 1e. The
HRMS, and optical rotation of synthetic 1e were also identical to
those of the natural product.14
Scheme 6 (i) TESOTf, 2,6-lutidine, DCM, À78 1C; (ii) OsO4, NaIO4, 2,6-
lutidine, dioxane–H2O; (iii) E-2-butene, KOBut, n-BuLi, À78 1C, (À)-Ipc2-
BOMe, BF3ÁOEt2; then Et3N, H2O2; (iv) HCl, MeOH; (iv) DMP, PPTS,
60 1C; (vi) H2, Pd/C, MeOH; (vii) Dess–Martin periodinane, NaHCO3,
DCM; (viii) 5, DIPEA, LiCl, MeCN; (ix) PTSA, MeOH.
In summary, the first total synthesis of lagunamide A was
completed, leading to a revision of the reported stereochemistry.
We acknowledge financial support from the Hong Kong
Research Grants Council (Projects: PolyU 5040/10P; PolyU
5037/11P; and PolyU 5020/12P) and The Hong Kong Poly-
technic University (PolyU 5636/08M; PolyU 5634/09M); Fong
Shu Fook Tong Foundation and Joyce M. Kuok Foundation;
The National Science Foundation of China (21072007 and
21133002); The Shenzhen Bureau of Science, Technology and
Information (JC200903160367A, JC201005260-102A
JC201005260220A), and GDSFC (10151805704000005).
&
Notes and references
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toluene, 80 1C, 86%; (iv) E-2-butene, KOBut, n-BuLi, À78 1C,
(+)-Ipc2BOMe, BF3ÁOEt2, then Et3N, H2O2.
Fig. 1 Structures of lagunamides A 1d and 1e.
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Fig. 2 Differences in 13C NMR shifts between natural lagunamide A
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for 1, by employing homoallylic alcohols 21b and 25 as key
building blocks that were readily prepared from the common
intermediate 20 as outlined in Scheme 7. On examining the
NMR spectra of 1b and 1c, we noticed that the data of 1b matched
very closely the reported values of the natural lagunamide A.
However, there were still some discrepancies in chemical shifts in
the C4–C9 region. On the basis of these chemical shift variations,
we hypothesized that the stereochemistry of C39 may have been
(R), but there was at least one further incorrect assignment made
in the original isolation paper. Hence, we elected to synthesize
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 8697–8699 8699