Lasonolide A: Structural revision and total synthesis

Article abstract of DOI:10.1021/jo034930n

The proposed structure of lasonolide A was synthesized employing radical cyclization reactions of β-alkoxyacrylates for preparation of the tetrahydropyranyl units A and B, but the spectroscopic data did not match those of the natural product. Both enantio

Full text of DOI:10.1021/jo034930n

La son olid e A: Str u ctu r a l Revision a n d Tota l Syn th esis
Ho Young Song,† J ung Min J oo,† J ung Won Kang,† Dae-Shik Kim,† Cheol-Kyu J ung,†
Hyo Shin Kwak,† J in Hyun Park,† Eun Lee,*,† Chang Yong Hong,‡ ShinWu J eong,‡
Kiwan J eon,‡ and J i Hyun Park‡
School of Chemistry and Molecular Engineering, Seoul National University, Seoul 151-747, Korea, and
LG Life Science/ R&D, P.O. Box 61, Yusung, Daejon 305-380, Korea
eunlee@snu.ac.kr
Received J une 30, 2003
The proposed structure of lasonolide A was synthesized employing radical cyclization reactions of
â-alkoxyacrylates for preparation of the tetrahydropyranyl units A and B, but the spectroscopic
data did not match those of the natural product. Both enantiomers of a revised structure featuring
17E,25Z double bonds were synthesized, and the (-)-isomer was found to be the biologically active
enantiomer.
In tr od u ction
Lasonolide A is a novel cytotoxic macrolide isolated in
1994 by McConnell and co-workers at Harbor Branch
Oceanographic Institution from the shallow water Carib-
bean marine sponge Forcepia sp. It is a potent cytotoxin
with IC50 values of 40 and 2 ng/mL against the A-549
human lung carcinoma and P388 murine leukemia cell
lines, respectively. It inhibits cell adhesion in the
EL-4.IL-2 cell line with an IC50 of 19 ng/mL; however,
toxicity against this cell line is greater than 25 µg/mL.
Extensive spectroscopic investigations by the researchers
at HBOI on the natural product led to the structure 1 or
21 (Figure 1). The most characteristic features in 1/2 are
the two cis-2,6-substituted tetrahydropyran rings inte-
grated in the macrolide structure. In particular, ring A
contains a quaternary stereogenic center at C-22 with a
methyl and a hydroxymethyl substituent, and synthesis
of 1/2 calls for stereoselective assembly of a cis-2,6-
substituted tetrahydropyran unit with the attendant
quaternary center. The unique structure and the high
biological activities of lasonolide A attracted attention of
synthetic chemists, and a number of literature examples
had dealt with partial synthesis with varying degree of
success.2 We reported structural revision and the first
total synthesis of lasonolide A in 20023 and wish to
F IGURE 1. Lasonolide A: the proposed structures 1/2.
describe here a full account on the structure and syn-
thesis of this intriguing natural product.
Resu lts a n d Discu ssion
Retr osyn th etic An a lysis a n d Syn th esis of th e
P r op osed Str u ctu r es 1 a n d 2. Radical cyclization
reactions of â-alkoxyacrylates4 were to be employed in
the synthesis of both tetrahydropyran rings found in
lasonolide A structure. As the correct stereochemistry at
(4) (a) Lee, E.; Tae, J . S.; Lee, C.; Park, C. M. Tetrahedron Lett.
1993, 34, 4831-4834. (b) Lee, E.; Tae, J . S.; Chong, Y. H.; Park, Y. C.;
Yun, M.; Kim, S. Tetrahedron Lett. 1994, 35, 129-132. (c) Lee, E.; Park,
C. M. J . Chem. Soc., Chem. Commun. 1994, 293-294. (d) Lee, E.;
J eong, J .-w.; Yu, Y. Tetrahedron Lett. 1997, 38, 7765-7768. (e) Lee,
E.; Park, C. M.; Yun, J . S. J . Am. Chem. Soc. 1995, 117, 8017-8018.
(f) Lee, E.; Yoo, S.-K.; Cho, Y.-S.; Cheon, H.-S.; Chong, Y. H.
Tetrahedron Lett. 1997, 38, 7757-7758. (g) Lee, E.; Yoo, S.-K.; Choo,
H.; Song, H. Y. Tetrahedron Lett. 1998, 39, 317-318. (h) Lee, E.; Choi,
S. J . Org. Lett. 1999, 1, 1127-1128. (i) Lee, E.; Song, H. Y.; Kim, H. J .
J . Chem. Soc., Perkin Trans. 1 1999, 3395-3396. (j) Lee, E.; Kim, H.
J .; Kang, E. J .; Lee, I. S.; Chung, Y. K. Chirality, 2000, 12, 360-361.
(k) Lee, E.; J eong, E. J .; Kang, E. J .; Sung, L. T.; Hong, S. K. J . Am.
Chem. Soc. 2001, 123, 10131-10132. (l) Lee, E.; Choi, S. J .; Kim, H.;
Han, H. O.; Kim, Y. K.; Min, S. J .; Son, S. H.; Lim, S. M.; J ang, W. S.
Angew. Chem., Int. Ed. 2002, 41, 176-178. (m) Lee, E.; Sung, L. T.;
Hong, S. K. Bull. Kor. Chem. Soc. 2002, 23, 1189-1190. (n) Lee, E.;
Han, H. O. Tetrahedron Lett. 2002, 43, 7295-7296. (o) J eong, E. J .;
Kang, E. J .; Sung, L. T.; Hong, S. K.; Lee, E. J . Am. Chem. Soc. 2002,
124, 14655-14662. (p) For further references, see: Lee, E. In Radicals
in Organic Synthesis, Vol. 2: Applications; Renaud, P., Sibi, M. P.,
Eds.; Wiley-VCH: Weinheim, 2001; pp 303-333.
* To whom correspondence should be addressed. Tel: +82-2-880-
6646. Fax: +82-2-889-1568.
† Seoul National University.
‡ LG Life Science/R&D.
(1) Horton, P. A.; Koehn, F. E.; Longley, R. E.; McConnell, O. J . J .
Am. Chem. Soc. 1994, 116, 6015-6016.
(2) (a) Beck, H.; Hoffmann, H. M. R. Eur. J . Org. Chem. 1999,
2991-2995. (b) Nowakowski, M.; Hoffmann, H. M. R. Tetrahedron Lett.
1997, 38, 1001-1004. (c) Gurjar, M. K.; Chakrabarti, A.; Venkateswara
Rao, B.; Kumar, P. Tetrahedron Lett. 1997, 38, 6885-6888. (d) Gurjar,
M. K.; Kumar, P.; Venkateswara Rao, B. Tetrahedron Lett. 1996, 37,
8617-8620.
(3) (a) Lee, E.; Song, H. Y.; Kang, J . W.; Kim, D.-S.; J ung, C.-K.;
J oo, J . M. J . Am. Chem. Soc. 2002, 124, 384-385. (b) Lee, E.; Song, H.
Y.; J oo, J . M.; Kang, J . W.; Kim, D. S.; J ung, C. K.; Hong, C. Y.; J eong,
S.; J eon, K. Bioorg. Med. Chem. Lett. 2002, 12, 3519-3520.
10.1021/jo034930n CCC: $25.00 © 2003 American Chemical Society
Published on Web 09/17/2003
8080
J . Org. Chem. 2003, 68, 8080-8087

Lasonolide A: Structural Revision and Total Synthesis
SCHEME 1. Retr osyn th etic An a lysis of 1/2
SCHEME 2. P r ep a r a tion of 9a
a
Key: (a) BH3‚SMe2, NaBH4, THF; (b) Bu2SnO, benzene, reflux;
BnBr, TBAI, reflux; (c) MeNH(OMe)‚HCl, Me3Al, THF; (d)
H2CC(Me)MgBr, THF; (e) Et3B, NaBH4, THF-MeOH (4:1), -78
°C; (f) (p-MeO)PhCH(OMe)2, CSA, DCM; (g) DIBAL, DCM; (h)
HCCCO2Et, NMM, MeCN; (i) DDQ, DCM-H2O (20:1); (j) BrCH2Si-
Me2Cl, TEA, DMAP, benzene; (k) Bu3SnH, AIBN, benzene, reflux.
F IGURE 2. NOESY analysis of 9.
C-28 was unknown and it was deemed necessary to
prepare both of the two possible stereoisomers, the
macrolide A was considered as a pivotal intermediate in
the synthesis of the proposed structures 1/2. It was
to be prepared from the two tetrahydropyranyl frag-
ments B and G using Wittig and Stille-type reactions
(Scheme 1).
SCHEME 3. Ster eoselective 6-En d o, 6-Exo
Ta n d em Ra d ica l Cycliza tion s
In the synthesis of the upper half fragment B, 6-endo-
6-exo tandem radical cyclization reactions of the (bro-
momethyl)silyloxy-substituted â-alkoxyacrylate E may
provide the bicyclic intermediate D;5 the construction of
the quaternary center and the tetrahydropyran annula-
tion may be achieved in a single step. The intermediate
E may be obtained from the protected triol F . For
preparation of the lower half fragment G, radical cycliza-
tion of the â-alkoxyacrylate I from a second triol deriva-
tive J should provide the tetrahydropyranyl interme-
diate H.
Preparation of the fragment B started with ethyl
L-malate (3), which was converted into the enone 5 via
the Weinreb amide derivative of the ester 4. Stereo-
selective reduction6 of 5 provided the syn diol, and
regioseletive reduction7 of the cyclic PMB acetal yielded
the triol derivative 6. The â-alkoxyacrylate 7 was ob-
tained from 6 via reaction with ethyl propiolate and
PMB-deprotection. Preparation and radical cyclization
reaction of the bromomethyl(dimethyl)silyl derivative 8
proceeded smoothly, and the bicyclic product 9 was
obtained as a single product (Scheme 2).
The stereochemical assignment of 9 was confirmed by
NOESY analysis (Figure 2). Clearly, the anticipated
6-endo radical cyclization proceeded efficiently, and com-
plete stereocontrol was achieved in the subsequent 6-exo
cyclization (Scheme 3).
Reduction of the ester group and reaction with pivaloyl
chloride provided the pivaloate derivative of 9, which was
converted into the diol 10 via Tamao oxidation.8 Conver-
sion to the more robust pivaloate derivative was neces-
sary prior to Tamao oxidation. Selective deprotection of
the bis(TBS) derivative of 10 yielded the primary alcohol
11. Conversion of 11 into the lower homologue 12
required selenide substitution/selenoxide elimination,
osmium tetroxide dihydroxylation/sodium periodate cleav-
age, and sodium borohydride reduction. The aldehyde
intermediate 13 was synthesized via TBS-deprotection
of 12 and subsequent acetonide protection, benzyl ether
(5) For selected examples of similar 6-endo cyclization reactions,
see: (a) Wilt, J . W. J . Am. Chem. Soc. 1981, 103, 5251-5253. (b)
Koreeda, M.; George, I. A. J . Am. Chem. Soc. 1986, 108, 8098-8100.
(6) Chen, K.-M.; Gunderson, K. G.; Hardtmann, G. E.; Prasad, K.;
Repic, O.; Shapiro, M. J . Chem. Lett. 1987, 1923-1926.
(7) Takano, S.; Akiyama, M.; Sato, S.; Ogasawara, K. Chem. Lett.
1983, 1593-1596.
(8) Tamao, K.; Kakui, T.; Akita, M.; Iwahara, T.; Kanatani, R.;
Yoshida, J .; Kumada, M. Tetrahedron 1983, 39, 983-990.
J . Org. Chem, Vol. 68, No. 21, 2003 8081

Song et al.
SCHEME 4. P r ep a r a tion of 13a
SCHEME 5. P r ep a r a tion of 19a
a
Key: (a) LiBH4, ether; (b) PivCl, DMAP, pyridine, DCM; (c)
H2O2, KF, KHCO3, THF-MeOH (1:1); (d) TBSOTf, 2,6-lutidine,
DCM; (e) CSA, MeOH, 0 °C; (f) (o-NO2)PhSeCN, Bu3P, THF; H2O2;
(g) OsO4, NMO, acetone-H2O (3:1); NaIO4; (h) NaBH4, EtOH, 0
°C; (i) HCl, MeOH; (j) Me2C(OMe)2, CSA, acetone; (k) H2, Pd(OH)2/
C, MeOH; (l) SO3‚pyridine, TEA, DMSO-DCM (1:1), 0 °C.
deprotection, and oxidation with sulfur trioxide-pyridine
complex9 (Scheme 4).
a
Key: (a) n-Bu2BOTf, TEA, DCM, 0 °C; BnOCH2CHO, -78 to
0 °C; (b) MeNH(OMe)‚HCl, Me3Al, THF; (c) H2CCHMgBr, THF;
(d) Et3B, NaBH4, THF-MeOH (2.5:1), -78 °C; (e) PhCH(OMe)2,
CSA, DCM; (f) DIBAL, DCM; (g) OsO4, NMO, acetone-H2O (3:1);
NaIO4; (h) NaBH4, EtOH, 0 °C; (i) TBSCl, imidazole, DCM, 0 °C;
(j) HCCCO2Et, NMM, MeCN; (k) HCl, MeOH, 0 °C; (l) CBr4, Ph3P,
pyridine, DCM; (m) Bu3SnH, AIBN, benzene, reflux.
The Evans chiral imide 14 served as the starting
material for the synthesis of the lower half fragment G.
The aldol product from the reaction of the (Z)-boron
enolate of 14 and benzyloxyacetaldehyde was converted
into the hydroxy enone 15 via vinyl Grignard reaction of
the corresponding Weinreb amide.10 After stereoselective
reduction of 15, the product syn diol was converted into
the dibenzyl ether 16 via regioselective reduction of the
corresponding benzyl acetal. Osmium tetroxide dihy-
droxylation/sodium periodate cleavage followed by so-
dium borohydride reduction provided a primary alcohol,
from which the TBS ether 17 was obtained via selective
silylation. Reaction of 17 with ethyl propiolate provided
the corresponding â-alkoxyacrylate, which was converted
into the bromide 18 via TBS-deprotection and bromide
substitution. Radical cyclization of 18 proceeded unevent-
fully to give the tetrahydropyranyl intermediate 19 in
high yield (Scheme 5).
SCHEME 6. P r ep a r a tion of 25a
Benzyl deprotection via hydrogenolysis and silylation
provided the bis(TBS) ether analogue of 19, which was
converted into the corresponding aldehyde via lithium
borohydride reduction and oxidation with sulfur triox-
ide-pyridine complex. The (E)-iodovinyl derivative 20
was obtained from the aldehyde following the Takai
protocol.11 Generation of the primary hydroxy group via
selective TBS-deprotection and oxidation with sulfur
trioxide-pyridine led to the production of the corre-
sponding aldehyde in good yield, from which the (Z)-
enoate 21 was prepared following the Still procedure.12
The J ulia-J ulia reaction13 of the corresponding aldehyde
22 and the anion of the sulfone 23 provided the triene
24 in a stereoselective manner (E/Z ) 30:1). The phos-
phonium salt 25 was then obtained via the corresponding
iodide (Scheme 6).
a
Key: (a) H2, Pd/C, MeOH; (b) TBSOTf, 2,6-lutidine, DCM; (c)
LiBH4, Ether; (d) SO3‚pyridine, TEA, DMSO-DCM (1:1), 0 °C;
(e) CrCl2, CHl3, dioxane-THF (6:1); (f) CSA, MeOH; (g) SO3‚pyri-
dine, TEA, DMSO-DCM (1:1), 0 °C; (h) MeO2C(Me)CHPO(OCH2-
CF3)2, KHMDS, 18-c-6, THF, -78 °C; (i) DIBAL, DCM, -78 °C;
(j) MnO2, DCM; (k) 23, LDA, THF, -78 °C; 22, -78 °C to rt; (l)
DIBAL, DCM, -78 °C; (m) Ph3P, I2, imidazole, THF, 0 °C; (n) Ph3P,
MeCN, reflux.
(9) Parikh, J . R.; Doering, W. von E. J . Am. Chem. Soc. 1967, 89,
5505-5507.
(10) For similar reaction sequences, see: Evans, D. A.; Kaldor, S.
W.; J ones, T. K.; Clardy, J .; Stout, T. J . J . Am. Chem. Soc. 1990, 112,
7001-7031.
(11) Takai, K.; Nitta, K.; Utimoto, K. J . Am. Chem. Soc. 1986, 108,
7408-7410.
Stereochemical assignment of 25 was confirmed via
NOESY analysis of the primary alcohol derived from the
pivaloate 24 (Figure 3).
(12) Still, W. C.; Gennari, C. Tetrahedron Lett. 1983, 24, 4405-4408.
(13) Baudin, J . B.; Hareau, G.; J ulia, S. A.; Ruel, O. Tetrahedron
Lett. 1991, 32, 1175-1178.
8082 J . Org. Chem., Vol. 68, No. 21, 2003

Lasonolide A: Structural Revision and Total Synthesis
SCHEME 8. P r ep a r a tion of 37a
F IGURE 3. NOESY analysis of the alcohol from 24.
SCHEME 7. P r ep a r a tion of 30a
a
Key: (a) KHMDS, THF, -78 °C; 13, -78 °C to rt; (b) CSA,
MeOH, (HOCH2)2 (s.m. 20%); (c) TBSCl, imidazole, DCM; (d) 35,
DCC, DMAP, DCM, 0 °C to rt; (e) Pd2dba3, DIPEA, NMP.
alcohol 34. Esterification reaction of 34 with the car-
boxylic acid 35 proceeded efficiently, but the stereochem-
ical integrity was compromised as the required (E)-
stannylacrylate 36 was accompanied by the corresponding
(Z)-isomer (E/Z ) 2:1). Intramolecular Stille coupling of
36 then produced the macrolide 37 without difficulty18
(Scheme 8).
a
Key: (a) cyclohexanone, BF3‚OEt2, ether, 0 °C to rt; (b)
BH3‚DMS, B(OMe)3, THF, 0 °C to rt; (c) TBSCl, imidazole, DCM;
(d) 32, NaHMDS, THF, 0 °C; (e) TBSCl, imidazole, DMAP, DCM;
(f) HF‚pyridine, pyridine, THF, 0 °C; (g) 29, DIAD, Ph3P, THF, 0
°C; (h) (NH4)6Mo7O24, H2O2, EtOH, 0 °C; (i) O3, DCM, -78 °C;
DMS, -78 °C to rt; (j) (CH3)2CHCH2CH2MgBr, THF; (k) (COCl)2,
DMSO, DIPA, DCM, -78 °C to rt; (l) Zn dust, CH2Br2, TiCl4, THF,
0 °C to rt; (m) HCl, MeOH.
The esterification/intramolecular Stille reaction strat-
egy provided the macrolide 37 from 34 in reasonable
yield, but the stereorandomization problem at the esteri-
fication step was difficult to solve. Eventually, a more
efficient transformation was achieved via the pentenoic
acid 38, a product of intermolecular Stille coupling
between 34 and 35, which yielded the macrolide 37 via
Yamaguchi lactonization.19 Low-temperature reduction
of 37 by Super-Hydride and subsequent oxidation with
sulfur trioxide-pyridine complex led to the formation of
the aldehyde 39. The Kocienski-J ulia reaction20 of the
aldehyde 39 and the anion of 30 proceeded smoothly, and
1 (28S) was obtained after careful TBS-deprotection by
hydrogen fluoride-pyridine complex. The diastereomer
2 (28R) was synthesized from the reaction of 39 and ent-
30 obtained from D-malic acid (Scheme 9). Comparison
of the 500 MHz NMR spectroscopic data of 1 and 2 with
those reported for lasonolide A revealed that neither 1
nor 2 represented the correct structure of the natural
product.
Synthesis of the side-chain fragment started with
L-malic acid (26), which was converted into the ketal 27
after selective ketal formation, borane reduction, and
TBS-protection.14 After reaction of 27 with the alcohol
32, the primary alcohol 28 was obtained via bis(TBS)-
protection and selective TBS-deprotection. Mitsunobu-
type substitution reaction of 28 with 1-phenyl-1H-
tetrazole-5-thiol (29) and selective oxidation provided the
sulfone 30.15 The alcohol 3216 was prepared from the
olefin 31 in a five-step sequence involving ozonolysis,
isopentyl Grignard addition, Swern oxidation, methyl-
enation,17 and desilylation (Scheme 7).
The coupling of fragments was initiated by the Wittig
reaction between the ylide derived from the phosphonium
salt 25 and the aldehyde 13, which generated the
required cis double bond producing the intermediate 33.
Careful acetonide deprotection of 33 and selective sily-
lation led to the formation of the corresponding secondary
Syn th esis of th e Ster eoisom er s 40 a n d 41. After
much thought, it was decided to investigate the struc-
tures 40/41, the stereoisomers of 1/2 which are enantio-
(14) Hanessian, S.; Tehim, A.; Chen, P. J . J . Org. Chem. 1993, 58,
7768-7781.
(15) Williams, D. R.; Brooks, D. A.; Berliner, M. A. J . Am. Chem.
Soc. 1999, 121, 4924-4925.
(18) Smith, A. B., III; Ott, G. R. J . Am. Chem. Soc. 1998, 120, 3935-
3948.
(19) Inanaga, J .; Hirata, K.; Saeki, H.; Katsuki, T.; Yamaguchi, M.
Bull. Chem. Soc. J pn. 1979, 52, 1989-1993.
(20) Blakemore, P. R.; Cole, W. J .; Kocienski, P. J .; Morley, A. Synlett
1998, 26-28.
(16) Fleming, I.; Patterson, I.; Pearce, A. J . Chem. Soc., Perkin
Trans. 1 1981, 256-262.
(17) Takai, K.; Hotta, Y.; Oshima, K. Tetrahedron Lett. 1978, 27,
2417-2420.
J . Org. Chem, Vol. 68, No. 21, 2003 8083

Song et al.
SCHEME 9. P r ep a r a tion of 1/2a
SCHEME 10. P r ep a r a tion of 45a
a
Key: (a) 35, Pd2dba3, DIPEA, NMP; (b) 2,4,6-Cl3PhCOCl, TEA,
THF; DMAP, benzene, reflux; (c) LiEt3BH, THF, -78 °C (s.m.
19%); (d) SO3‚pyridine, TEA, DMSO-DCM (1:1), 0 °C; (e) 30,
KHMDS, DME, -55 °C; 39, -55 °C to rt; (f) HF‚pyridine, THF, 0
°C to rt; (g) ent-30, KHMDS, DME, -55 °C; 39, -55 °C to rt.
a
Key: (a) KHMDS, THF, -78 °C; 13, -78 °C to rt; (b)
CSA, MeOH, (HOCH2)2 (s.m. 18%); (c) TBSCl, imidazole, DCM;
(d) 35, Pd2dba3, DIPEA, NMP; (e) 2,4,6-Cl3PhCOCl, TEA, THF;
DMAP, benzene, reflux.
SCHEME 11. P r ep a r a tion of 40/41a
F IGURE 4. Stereoisomers 40/41.
meric in ring B, as candidates for the natural product.
As the two tetrahydropyran units in lasonolide A were
connected through stereochemically innocuous diene
and triene bridges, it would have been difficult to dis-
tinguish the structures 40/41 from 1/2 by NMR analysis
(Figure 4).
For the synthesis of 40/41, the ylide from the phos-
phonium salt ent-25 (prepared from ent-14) was allowed
to react with the aldehyde 13, and the coupled product
42 was obtained in high yield. Acetonide-deprotection of
42 and TBS-protection of the primary hydroxyl group
produced the secondary alcohol intermediate 43, which
was then converted into the pentenoic acid 44 via
intermolecular Stille coupling with 35. Yamaguchi lac-
tonization reaction of 44 afforded a new macrolide 45 in
an acceptable yield (Scheme 10).
a
Key: (a) LiEt3BH, THF, -78 °C; (b) SO3‚pyridine, TEA,
DMSO-DCM (1:1), 0 °C; (c) 30, KHMDS, DME, -55 °C; 46, -55
°C to rt; (d) HF‚pyridine, THF, 0 °C to rt; (e) ent-30, KHMDS,
DME, -55 °C; 46, -55 °C to rt.
The macrolide aldehyde 46 was prepared from 45
following the Super-Hydride reduction/sulfur trioxide-
pyridine oxidation protocol. Synthesis of 40 and 41 was
then duly accomplished via Kocienski-J ulia reaction of
the aldehyde 46 and the anion of either 30 or ent-30
(Scheme 11). From comparison of the NMR spectra of 40/
41 and the natural product, it was clear that neither 40
nor 41 represented the correct structure.
Syn th esis of th e (17E)-Isom er s. One of the most
striking features in the NMR spectra of 1/2 and 40/41
was the chemical shift values of H-19 centered around δ
4.7. The chemical shift value of H-19 for the natural
F IGURE 5. Chemical shift values of the geometric isomers
47/48.
product was reported to be δ 4.30, and it was apparent
that H-19 signals of the synthetic samples 1/2 and 40/
41 are located approximately 0.4 ppm downfield from the
correct value. This was happily reminiscent of the situ-
ation encountered in the ambruticin synthesis4l (Figure
5). Two geometric isomers 47 and 48 were obtained in
the crucial olefination reaction en route to ambruti-
cin: the chemical shift values for H-7 in 47 and 48 were
8084 J . Org. Chem., Vol. 68, No. 21, 2003

Lasonolide A: Structural Revision and Total Synthesis
SCHEME 13. P r ep a r a tion of 57a
F IGURE 6. (17E)-Isomers 49/50.
SCHEME 12. P r ep a r a tion of 53a
a
a
Key: (a) LiHMDS, DMF-HMPA (4:1), -35 °C; 13, -35 °C to
Key: (a) LiHMDS, THF, -78 °C; 22, -78 °C to rt; (b) CSA,
rt; (b) TBAF, THF; (c) 29, DIAD, Ph3P, THF, 0 °C; (d) (NH4)6Mo7O24
H2O2, EtOH, 0 °C to rt.
,
MeOH, (HOCH2)2 (s.m. 18%); (c) TBSCl, imidazole, DCM; (d) 35,
DIC, DMAP, DCM; (e) Pd2dba3, DIPEA, NMP.
SCHEME 14. P r ep a r a tion of 49/50a
δ 3.78 and 4.18, respectively. In other words, the H-7
chemical shift value of the cis isomer 48 was 0.4 ppm
downfield from that of the trans isomer 47.
Based on the above analysis, candidates with a trans
double bond at C-17,18 appeared quite promising, and
it was decided to investigate structures 49/50, which are
(17E)-isomers of 1/2 (Figure 6).
For synthesis of 49/50, Kocienski-J ulia reaction21 of
the sulfone 51 and the aldehyde 13 led to the trans olefin
52. The sulfone 53 was then obtained after TBDPS-
deprotection, Mitsunobu-type substitution with 29, and
selective oxidation (Scheme 12).
a
Key: (a) LiEt3BH, THF, -78 °C (s.m. 17%); (b) SO3‚pyridine,
A second trans olefination reaction between the sulfone
53 and the aldehyde 22 proceeded smoothly to yield the
coupled product 54. The secondary alcohol 55 was
obtained via acetonide-deprotection of 54 and subsequent
TBS-protection of the primary hydroxy group. Esterifi-
cation of 55 with the acid 35 led to the preparation of
the trans-â-stannylacrylate 56. Removal of the undesired
cis-â-stannylacrylate was difficult and the product mix-
ture was directly subjected to intramolecular Stille
coupling reaction, which provided the macrolactone 57
(Scheme 13).
The pivaloyl macrolide 57 was converted into the
aldehyde 58 upon reductive removal of the pivaloate
group and oxidation with sulfur trioxide-pyridine com-
plex. The Kocienski-J ulia reaction of 58 with the sulfone
30 and careful TBS-deprotection yielded the target
compound 49. The epimeric product 50 was obtained
likewise from ent-30 (Scheme 14). The NMR spectra of
49/50 were found to be quite similar to the spectra
recorded for the natural product: indeed, the signals for
H-19 were found around δ 4.3 as we expected. But
TEA, DMSO-DCM (1:1), 0 °C; (c) 30, KHMDS, DME, -78 °C; 58,
-78 °C to rt; (d) HF‚pyridine, pyridine, THF; (e) ent-30, KHMDS,
DME, -78 °C; 58, -78 °C to rt.
discrepancies persisted in the NMR spectra, particularly
in the region for vinylic protons.
Syn th esis of th e (17E,25Z)-Isom er s: Str u ctu r e 60
Is (-)-La son olid e A. At this point, the most likely spot
left for further modification was easily deduced: the
geometry of the double bond at C-25,26. It was decided
to investigate 59/60, which are (17E,25Z)-isomers of 1/2
(Figure 7).
The phosphonium salt 61 was prepared from the
alcohol 28 via iodide substitution, and Wittig reaction
between the corresponding ylide and the aldehyde 58 led
to the product 59 after subsequent TBS-deprotection.
Using the enantiomeric phosphonium salt ent-61, the
epimeric product 60 was also prepared (Scheme 15).
(21) Liu, P.; J acobsen, E. N. J . Am. Chem. Soc. 2001, 123, 10772-
10773.
J . Org. Chem, Vol. 68, No. 21, 2003 8085

Song et al.
SCHEME 16. P r ep a r a tion of 66a
F IGURE 7. (17E,25Z)-Isomers 59/60.
SCHEME 15. P r ep a r a tion of 59/60a
a
Key: (a) Ph3P, I2, THF; (b) Ph3P, MeCN, reflux; (c) 61,
KHMDS, THF, -78 °C; 58, -78 °C to rt; (d) HF‚pyridine, pyridine,
THF, 0 °C; (e) ent-61, KHMDS, THF, -78 °C; 58, -78 °C to rt.
a
Key: (a) LiHMDS, THF, -78 °C; ent-22, -78 °C to rt.; (b) CSA,
MeOH, (HOCH2)2; (c) TBSCl, imidazole, DCM; (d) 35, DIC, DMAP,
DCM; (e) Pd2dba3, DIPEA, Ph2PO2NBu4, DMF.
SCHEME 17. P r ep a r a tion of 62a
F IGURE 8. Stereoisomer 62.
Comparison of the 500 MHz NMR spectra of 59/60 and
the natural product revealed that 60 represented the
correct structure of lasonolide A.
a
Key: (a) LiEt3BH, THF, -78 °C (s.m. 30%); (b) SO3‚pyridine,
TEA, DMSO-DCM (1:1), 0 °C; (c) ent-61, KHMDS, THF, -78 °C;
67, -78 °C to rt; (d) HF‚pyridine, pyridine, THF.
Syn th esis of th e Ster eoisom er 62. The correct
structure of lasonolide A was finally identified, but the
discrepancies encountered during the course of the
investigation prompted us to consider the structure 62,
the stereoisomer of 60 which is enantiomeric in ring B
(Figure 8). It was realized that comparison of the NMR
spectra of 60, 62, and the natural product would remove
any uncertainty concerning the true structure of lasono-
lide A.
The Kocienski-J ulia reaction of the sulfone 53 and the
aldehyde ent-22 afforded the intermediate 63, which was
converted into the ester 65 via the alcohol 64. Intra-
molecular Stille coupling reaction of 65 produced a new
macrolide 66 (Scheme 16).
reconfirming the identity of the natural product structure
as 60.
Syn th esis of (+)-La son olid e A a n d Biologica l
Assa y: (-)-La son olid e A Is th e Biologica lly Mor e
Active En a n tiom er . The sample of 60 synthesized
above featured 500 MHz NMR spectra identical to
those of the natural product (it showed sharper hydroxyl
proton signals and did not contain upfield signals from
impurities) but exhibited the specific rotation [R]20
D
-24.1 (c 0.055, CDCl3), which was opposite to the
reported value [R]D +24.4 (c 0.045, CDCl3)1 for the natural
product. Assuming the literature value of the specific
rotation of the natural product was correct, the structure
of natural lasonolide A was determined to be ent-60.
Synthesis of ent-60 was then carried out following the
established procedure but using enantiomeric starting
materials. The specific rotation of the sample of ent-60
The macrolide aldehyde 67 was obtained from 66 via
reduction/oxidation protocol, and Wittig reaction of 67,
and the ylide from ent-61 provided the product 62 after
TBS-deprotection (Scheme 17). The NMR spectra of 62
were clearly different from those of the natural product
8086 J . Org. Chem., Vol. 68, No. 21, 2003

Lasonolide A: Structural Revision and Total Synthesis
F IGURE 9. (+)-Lasonolide A, ent-60.
F IGURE 10. (-)-Lasonolide A, 60, the biologically active
enantiomer.
TABLE 1. GI50 (µM) Va lu es of La son olid e A a n d Rela ted
Com p ou n d s
Con clu sion
sample
A549
HCT-116
NCI-H460
1
49
50
59
60
ent-60
62
>10
3.2
2
5
0.1
0.04
0.009
<0.003
3
5
In the present studies, the two tetrahydropyranyl
fragments of lasonolide A were prepared stereoselectively
via radical cyclization reactions of â-alkoxyacrylates; in
particular, excellent stereocontrol was achieved in the
introduction of the quaternary center at C-22 via 6-endo-
6-exo tandem radical cyclization reactions. The full
structure of natural lasonolide A was determined un-
equivocally as 60, which is the (17E,25Z)-isomer of the
structure originally proposed.
0.04
0.02
<0.003
<0.003
2
0.05
0.02
6
>10
>10
>10
prepared matched the value reported for the natural
product (Figure 9).
The samples synthesized were subjected to biological
assay (Table 1). Surprisingly, it was found that the (-)-
enantiomer 60 was the most potent compound tested. On
the contrary, the (+)-enantiomer ent-60 was found to
possess much lower activities. Accordingly, we were
forced to conclude that the active (and natural) enan-
tiomer of lasonolide A is the (-)-enantiomer 60 (Figure
10) and the original report on the optical rotation data
for natural lasonolide A is in error.
Inspection of the data in Table 1 reveals that the (R)-
configuration at C-28 enhances the activity. The cis
double bond at C-25,26 is important, and the trans double
bond at C-17,18 is essential, for the compound 1 was
almost devoid of activity. The stereoisomer 62 was
completely devoid of activity.
Ack n ow led gm en t. We thank the Ministry of Sci-
ence and Technology, Republic of Korea, and Korea
Institute of Science and Technology Evaluation and
Planning for a National Research Laboratory grant
(1999). Brain Korea 21 graduate fellowship grants to
H.Y.S., C.-K.J ., H.S.K., and J .H.P. are gratefully ac-
knowledged.
Su p p or tin g In for m a tion Ava ila ble: Full experimental
procedures and spectral data for intermediates and 1H NMR
spectra of 1, 2, 40, 41, 49, 50, 59, 60, 62, and natural lasonolide
A and a collection of 13C NMR spectra. This material is
available free of charge via the Internet at http://pubs.acs.org.
J O034930N
J . Org. Chem, Vol. 68, No. 21, 2003 8087