Stereoselective total synthesis and absolute configuration of the natural decanolides (-)-microcarpalide and (+)-lethaloxin. Identity of (+)-lethaloxin and (+)-pinolidoxin

Article abstract of DOI:10.1021/jo051353p

Convergent, stereoselective syntheses of the pharmacologically active, naturally occurring lactones (-)-microcarpalide and (+)-lethaloxin have been achieved from the commercially available, chiral reagents (R)-glycidol, (S,S)-tartaric acid, and D-ribose a

Full text of DOI:10.1021/jo051353p

Stereoselective Total Synthesis and Absolute Configuration of the
Natural Decanolides (-)-Microcarpalide and (+)-Lethaloxin.
Identity of (+)-Lethaloxin and (+)-Pinolidoxin
Jorge Garc´ıa-Fortanet,† Juan Murga,† Eva Falomir,† Miguel Carda,*,† and J. Alberto Marco*,‡
Departamento de Quı´mica Inorga´nica y Orga´nica, Universidad Jaume I, E-12071 Castello´n, Spain, and
Departamento de Quı´mica Orga´nica, Universidad de Valencia, E-46100 Burjassot, Valencia, Spain
alberto.marco@uv.es
Received June 30, 2005
Convergent, stereoselective syntheses of the pharmacologically active, naturally occurring lactones
(-)-microcarpalide and (+)-lethaloxin have been achieved from the commercially available, chiral
reagents (R)-glycidol, (S,S)-tartaric acid, and D-ribose as the starting materials. These syntheses
have further served to establish the hitherto unknown absolute configuration of (+)-lethaloxin and
to show its identity with (+)-pinolidoxin.
Introduction
The generic name decanolides encompasses a relatively
small class of naturally occurring 10-membered lactones
of polyketide origin isolated in most cases from various
species of fungi. Many of these lactones have been found
to display pharmacologically interesting features, such
as antibacterial, antitumoral, or hypolipidemic proper-
ties.1 Two of these fungal decanolides are microcarpalide
(1)2 and lethaloxin (2).3 Their structures and relative
configurations (Figure 1) were determined with the aid
of spectroscopic methods. The absolute configuration of
1, depicted in the figure, was assigned by means of the
exciton chirality method,2 whereas that of 2 still remains
unknown.
FIGURE 1. Structures of microcarpalide (1) and lethaloxin
(2).
tropical tree Ficus microcarpa L. While weakly cytotoxic
to mammalian cells, the compound acts as a strong
microfilament disrupting agent.2 These properties make
1 a potential lead structure for the development of new
anticancer drugs and, consequently, an attractive target
for synthetic studies. Lethaloxin 2 was isolated from a
strain of the fungus Mycosphaerella lethalis.3 Like 1, it
is a trihydroxy 10-membered lactone, but one of its
hydroxyl groups is esterified with sorbic acid. The first
total synthesis of 1 was published by us in 2002.5 In
subsequent years, syntheses of either the natural product
or its non-natural enantiomer were reported in the
Microcarpalide 1 was isolated from an as yet unidenti-
fied endophytic fungus4 growing on the bark of the
* To whom correspondence should be addressed. For J.A.M.: tel,
34-96-3544337; fax, 34-96-3544328.
† Universidad Jaume I.
‡ Universidad de Valencia.
(1) For a review on synthetic, biosynthetic, and pharmacological
aspects of decanolides, see: Dra¨ger, G.; Kirschning, A.; Thiericke, R.;
Zerlin, M. Nat. Prod. Rep. 1996, 13, 365-375.
(2) Ratnayake, A. S.; Yoshida, W. Y.; Mooberry, S. L.; Hemscheidt,
T. Org. Lett. 2001, 3, 3479-3481.
(4) For a review on secondary metabolites isolated from this type of
organism, see: Tan, R.-X.; Zou, W.-H. Nat. Prod. Rep. 2001, 18, 448-
459.
(5) Murga, J.; Falomir, E.; Garc´ıa-Fortanet, J.; Carda, M.; Marco,
J. A. Org. Lett. 2002, 4, 3447-3449.
(3) Arnone, A.; Assante, G.; Montorsi, M.; Nasini, G.; Ragg, E. Gazz.
Chim. Ital. 1993, 123, 71-73. The structure of lethaloxin depicted in
this paper corresponds to the enantiomer of 2.
10.1021/jo051353p CCC: $30.25 © 2005 American Chemical Society
Published on Web 10/27/2005
9822
J. Org. Chem. 2005, 70, 9822-9827

Total Synthesis of Microcarpalide and Lethaloxin
SCHEME 1. Retrosynthetic Plan for
Microcarpalide (1)
SCHEME 2. Synthesis of Microcarpalide (1)a
literature by up to six groups.6 No nominal synthesis of
2 has been published so far (see below, however). In the
following, we describe in full the details of the synthesis
of 1 and, as well, the first synthesis of 2.
a Reagents and conditions: (a) CH3(CH2)4MgBr, CuI, THF, -30
°C, 87%; (b) MOMCl, Et3N, DMAP, CH2Cl2, room temperature,
18 h, 87%; (c) TBAF, THF, 5 h, room temperature, 93%; (d)
(COCl)2, DMSO, CH2Cl2, -78 °C, 30 min, then N,N-diisopropyl-
ethylamine, 2 min at -78 °C, then room temperature; (e)
Bu3SnCH2CHdCH2, MgBr2‚Et2O, 3Å MS, CH2Cl2, 3 h at -78 °C,
then 1.5 h at -40 °C, 60% combined yield of the two last steps; (f)
9, DCC, DMAP, CH2Cl2, room temperature, 18 h, 86%; (g) 20 mol
% of catalyst Ru-I, CH2Cl2, reflux, 24 h, 67:33 E/Z mixture (see
text), 67%; (h) SMe2, BF3‚Et2O, -10 °C, 30 min, 71%; (i) (CH2SH)2,
BF3‚Et2O, CH2Cl2, 0 °C, 1 h, 66%.
Results and Discussion
Our retrosynthetic analysis of compound 1 (Scheme 1)
relied upon the creation of the lactone ring by means
of ring-closing metathesis (RCM).7,8 Thus, sequential
retrocleavage of the CdC and ester C-O bonds leads via
diolefin A to homoallyl alcohol B and acid C (P, P′ )
protecting groups), which can be derived in turn from (R)-
glycidol and (S,S)-tartaric acid, respectively.
The product corresponding to intermediate B, homo-
allyl alcohol 8, was prepared as described in Scheme 2.
Commercial (R)-glycidol was transformed into its known9
TPS (tert-butyldiphenylsilyl) ether 3. Epoxide opening in
3 with an n-pentylcuprate reagent10 afforded alcohol 4,
which was then protected as its MOM (methoxymethyl)11
derivative 5. Desilylation of the latter to 6 followed by
Swern oxidation under mild conditions12 afforded the
R-alkoxy aldehyde (S)-7,13 which, without purification,
was immediately allowed to react with allyl tri-n-butyl-
stannane in the presence of MgBr2‚Et2O (chelation
control conditions).14 This provided 8 in good yield and
with high stereoselectivity (the dr value was judged to
be g 98%, as the minor stereoisomer was not detected
1
by means of high-field H and 13C NMR). Intermediate
C was in the present case the known acid 9, readily
prepared from (S,S)-tartaric acid by means of a modified
literature procedure.15
Carboxylic acid 9 was then coupled with alcohol 8 in
the presence of N,N′-dicyclohexylcarbodiimide (DCC)16 to
yield the diene ester 10 (Scheme 2). This reaction set the
(6) (a) Banwell, M. G.; Loong, D. T. J. Heterocycles 2004, 62, 713-
734. (b) Ishigami, K.; Kitahara, T. Heterocycles 2004, 63, 785-790. (c)
Gurjar, M. K.; Nagaprasad, R.; Ramana, C. V.; Karmakar, S.; Moha-
patra, D. K. ARKIVOC 2005 (iii), 237-257. (d) Chavan, S. P.; Praveen,
C. Tetrahedron Lett. 2005, 46, 1939-1941. (e) Davoli, P.; Fava, R.;
Morandi, S.; Spaggiari, A.; Prati, F. Tetrahedron 2005, 61, 4427-4436.
(f) Kumar, P.; Naidu, S. V. J. Org. Chem. 2005, 70, 4207-4210.
(7) (a) Fu¨rstner, A. Angew. Chem., Int. Ed. 2000, 39, 3012-3043.
(b) Trnka, T.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18-29. (c)
Jafarpour, L.; Nolan, S. P. Adv. Organomet. Chem. 2001, 46, 181-
222. (e) Love, J. A. In Handbook of Metathesis; Grubbs, R. H., Ed.;
Wiley-VCH: Weinheim, Germany, 2003; pp 296-322. (f) Grubbs, R.
H. Tetrahedron 2004, 60, 7117-7140.
(12) (a) Mancuso, A. J.; Swern, D. Synthesis 1981, 165-185. (b)
Tidwell, T. T. Org. React. 1990, 39, 297-572. Racemization of the
aldehyde S-7 was minimized when N,N-diisopropylethylamine was
used as the base (see: Dondoni, A.; Perrone, D. Synthesis 1997, 527-
529). The ee of S-7 was judged to be g98% in an indirect way, because
no minor stereoisomers were detected by high-field NMR analysis of
crude ester 10.
(13) Both racemic and enantiomerically pure (R)-7 have been
previously synthesized using a different methodology based on arylthio-
methyl sulfoxides. See: (a) Banfi, L.; Bernardi, A.; Colombo, L.;
Gennari, C.; Scolastico, C. J. Org. Chem. 1984, 49, 3784-3790. (b)
Banfi, L.; Cabri, W.; Poli, G.; Potenza, D.; Scolastico, C. J. Org. Chem.
1987, 52, 5452-5457.
(14) For reviews on reactions with allyl tin reagents, see: (a)
Nishigaichi, Y.; Takuwa, A.; Naruta, Y.; Maruyama, K. Tetrahedron
1993, 49, 7395-7426. (b) Yamamoto, Y.; Shida, N. Adv. Detailed React.
Mech. 1994, 3, 1-44.
(15) Batty, D.; Crich, D. J. Chem. Soc., Perkin Trans. 1 1992, 3193-
3204. The reported overall yield was improved through modification
of the described procedures (see the Supporting Information).
(8) Four out of the other six syntheses of microcarpalide also make
use of this type of macrocyclization. In the remaining two syntheses,
the lactone ring is generated via macrolactonization.
(9) Guivisdalsky, P. N.; Bittman, R. J. Org. Chem. 1989, 54, 4637-
4642. For a review on glycidol and its synthetic uses, see: Hanson, R.
M. Chem. Rev. 1991, 91, 437-475.
(10) (a) Lipshutz, B. H.; Sengupta, S. Org. React. 1992, 41, 135-
631. (b) Krause, N., Ed. Modern Organocopper Chemistry; Wiley-
VCH: Weinheim, Germany, 2004.
(11) Greene, T. W.; Wuts, P. G. M., Protective Groups in Organic
Synthesis, 3rd ed.; Wiley: New York, 1999; pp 27-33.
J. Org. Chem, Vol. 70, No. 24, 2005 9823

Garc´ıa-Fortanet et al.
stage for the crucial RCM,7 which was successful with
ruthenium catalyst Ru-I. Thus, a 0.001 M solution of 10
and 20 mol % of Ru-I was heated at reflux for 24 h in
dry, degassed CH2Cl2. This provided a 2:1 E/Z mixture
of macrocyclic lactones 11, from which the E isomer
(depicted) was isolated by means of column chromatog-
raphy on silica gel. It is worth mentioning here that the
use of the second-generation ruthenium catalyst Ru-II7c
gave rise almost exclusively to Z-11. Similar differences
in behavior between these two catalyst types have
previously been observed by other groups during the
synthesis of decanolides.17,18 These authors attributed the
different stereochemical outcomes to the higher activity
of the imidazolylidene-substituted catalyst, which was
able to isomerize the CdC bond of the RCM product. In
consequence, the E/Z ratio was no longer kinetically con-
trolled but was, rather, the result of a chemical equilib-
rium. This caused a marked enhancement in the per-
centage of the Z isomer, which in their molecules was
shown to be the thermodynamically more stable one. In
our compounds, the same explanation is supported by
theoretical calculations on lactone 11, which show that
the Z isomer is more stable that the E isomer by about
2 kcal/mol.19
Selective removal of the MOM group in 11 was feasible
under mild conditions20 and furnished acetonide 12, the
properties of which (NMR, MS) were identical with those
reported.2 Preparation of the target molecule 1 was
finally achieved by one-pot removal of all protecting
groups in compound 11.21 The physical and spectral
properties of synthetic 1 turned out to be identical with
those reported for the natural compound. As reported by
Hemscheidt and co-workers for the natural product,2 the
NMR spectra of synthetic 1 at room temperature revealed
the presence of two slowly interconverting conformers in
an approximate (3-3.5):1 ratio.
FIGURE 2. Structures of herbarumin I (13), herbarumin
II (14) and two stereoisomers (15, 16) of lethaloxin (pinoli-
doxin).
co-workers17a disclosed the synthesis of the structurally
related decanolides herbarumins I (13) and II (14) (Figure
2) and cleared by synthetic methods the stereochemical
puzzle around pinolidoxin, a further decanolide of the
same structural type. This compound, however, was
represented in that paper with the structure and absolute
configuration 2. Furthermore, these authors depicted the
structure and relative configuration of lethaloxin as 15,
which is not the previously reported structure.3,22 Adding
more matter to this confusion, the structure of lethaloxin
was depicted as 16 in a later publication of Ley and co-
workers, who described there their own synthesis of 14.23
These inconsistencies led us to undertake our own
synthesis of lethaloxin. An added element of interest in
the present case is given by the interesting pharmaco-
logical properties of members of this compound class.1
In addition, lactones of this type have also been found
to exhibit marked phytotoxicity, which has been re-
lated to its inhibitory activity on phenylalanine ammonia
lyase, a key plant enzyme involved in the biosynthesis
of phenylpropanoids.24 This feature renders these lac-
tones promising lead structures for the search of novel
herbicides. Furthermore, lactones 13 and 14 have re-
cently been found to inhibit the activation of the calm-
odulin-dependent enzyme cAMP phosphodiesterase, a
property which might endow them with agrochemical
and medicinal interest.25 As shown in the following, we
have found that the structure and absolute configura-
tion of natural (+)-lethaloxin is represented by the
structural formula 2 and is therefore identical with
natural (+)-pinolidoxin.26
The isolation and structure elucidation of lethaloxin
was reported in 1993, and its relative configuration was
proposed to be 2.3 The compound was not mentioned
again in the literature until 2002, when Fu¨rstner and
(16) (a) Mikołajczyk, M.; Kiełbasin´ski, P. Tetrahedron 1981, 37,
233-284. (b) Mulzer, J. In Comprehensive Organic Synthesis; Trost,
B. M., Fleming, I., Winterfeldt, E., Eds.; Pergamon Press: Oxford, U.K.,
1991; Vol. 6, pp 323-380.
(17) (a) Fu¨rstner, A.; Radkowski, K.; Wirtz, C.; Goddard, R.; Leh-
mann, C. W.; Mynott, R. J. Am. Chem. Soc. 2002, 124, 7061-7069.
These authors used a ruthenium catalyst structurally similar to A but
having an indenylidene group instead of the benzylidene moiety. The
other catalyst was close to B but with an additional CdC bond in the
imidazole ring. (b) Liu, D.; Kozmin, S. A. Org. Lett. 2002, 4, 3005-
3007. (c) For the creation of medium-sized and large rings via RCM,
see pertinent citations in ref 17a and: Prunet, J. Angew. Chem., Int.
Ed. 2003, 42, 2826-2830.
(18) Responsible for this and related nonmetathetic processes may
be ruthenium hydride intermediates, the formation of which has
recently been demonstrated in the case of Ru-II: Hong, S. H.; Day,
M. W.; Grubbs, R. H. J. Am. Chem. Soc. 2004, 126, 7414-7415. It is
also worth mentioning that, although more rarely, the catalyst Ru-I
has also been found to induce E-Z isomerizations: Kalesse, M.;
Quitschalle, M.; Claus, M.; Gerlach, K.; Pahl, A.; Meyer, H. H. Eur. J.
Org. Chem. 1999, 2817-2823.
(19) Theoretical calculations were first performed at the semiem-
pirical level (AM1) and gave a difference in energy contents of 2 kcal/
mol between both stereoisomers. When the calculations were made
with ab initio methods (HF/3-21G), the difference turned out to be 1.9
kcal/mol.
Our approach to the lethaloxin framework (Scheme 3;
P, P′ ) protecting groups) was based on the same concept
used in the microcarpalide synthesis and relied again on
the formation of the lactone ring via RCM. Thus, retro-
cleavage of the CdC bond and of the sorboyl ester moiety
(22) To the best of our knowledge, no structural revision of lethaloxin
has appeared in the literature since the initial report of 1993.
(23) D´ıez, E.; Dixon, D. J.; Ley, S. V.; Polara, A.; Rodrı´guez, F. Helv.
Chim. Acta 2003, 86, 3717-3729. In this paper, the stereostructure
of pinolidoxin has also been erroneously depicted.
(24) (a) Vurro, M.; Ellis, B. E. Plant Sci. 1997, 126, 29-38. (b)
Evidente, A.; Capasso, R.; Andolfi, A.; Vurro, M.; Zonno, M.-C. Nat.
Toxins 1998, 6, 183-188. (c) Zonno, M.-C.; Vurro, M. Weed Res. 1999,
39, 15-20.
(25) Rivero-Cruz, J. F.; Mac´ıas, M.; Cerda-Garc´ıa-Rojas, C.; Mata,
R. J. Nat. Prod. 2003, 66, 511-514.
(26) For a synthesis of the non-natural enantiomer of pinolidoxin,
see ref 17b.
(20) Naito, H.; Kawahara, E.; Maruta, K.; Maeda, M.; Sasaki, S. J.
Org. Chem. 1995, 60, 4419-4427. See also: Fuji, K.; Kawabata, T.;
Fujita, E. Chem. Pharm. Bull. 1980, 28, 3662-3664.
(21) Sinha, S. C.; Keinan, E. J. Org. Chem. 1997, 62, 377-386.
9824 J. Org. Chem., Vol. 70, No. 24, 2005

Total Synthesis of Microcarpalide and Lethaloxin
SCHEME 3. Retrosynthetic Plan for Lethaloxin
(2)
SCHEME 4. Synthesis of (+)-Lethaloxin (2)a
gives diolefin D. Further cleavage of the ester function
generates alcohol E and acid F. Since the former has the
same absolute configuration in its triol fragment as
D-ribose, this commercially available sugar was chosen
as the chiral precursor. Acid F can be obtained from (R)-
glycidol in a way similar to that followed in the synthesis
of microcarpalide (Scheme 2).
The specific details of the synthesis are depicted in
Scheme 4. D-Ribose was converted in two steps into diol
17 as described.27 Diol 17 was then dehydrated to epoxide
18 by means of the Mitsunobu reaction.28 Epoxide ring
opening in 18 was performed with an ethylcuprate
reagent9 and yielded alcohol 19 in 48% overall yield from
D-ribose acetonide.29 Furthermore, epoxide 3 was treated
as described30 with an allylcuprate reagent to afford
alcohol 20, which was then protected as its MOM
derivative 21. Desilylation and sequential two-step oxi-
dation furnished acid 23,31 which was coupled with
alcohol 19 using the Yamaguchi procedure.32 This pro-
vided ester 24, which was subjected to RCM in the
presence of catalyst Ru-I. As in the synthesis of micro-
carpalide, an E/Z mixture of cyclic olefins 25 was formed
in 74% overall yield (ratio of geometric isomers 76:24,
with the E isomer predominating). Both stereoisomers
were separated by column chromatography and identified
by their spectral features.33,34 Compound E-25 was
subjected to selective deprotection20 of the MOM group
to yield hydroxy lactone E-26,35 which was then esterified
with sorbic acid using again the Yamaguchi procedure.
The resulting ester 2735 was subjected to cleavage of the
acetonide moiety21 to provide the dihydroxy lactone 2,
a Reagents and conditions: (a) PPh3, diisopropyl azodicarboxy-
late, CHCl3, reflux, overnight; (b) EtMgBr, CuI, THF, -30 °C, 55%
overall from 17; (c) ref 30; (d) MOMCl, N,N-diisopropylethylamine,
CH2Cl2, room temperature, 18 h, 88%; (e) TBAF, THF, 5 h, room
temperature, 95%; (f) (COCl)2, DMSO, CH2Cl2, -78 °C, 30 min,
then N,N-diisopropylethylamine, 2 min at -78 °C, then 0 °C; (g)
NaClO2, NaH2PO4, aqueous tBuOH; (h) 2,4,6-trichlorobenzoyl
chloride, N,N-diisopropylethylamine, DMAP, THF, room temper-
ature, overnight, 96%; (i) 20% PhCHdRuCl2(PCy3)2, CH2Cl2, 6 h,
∆, 74%, 76:24 E/Z mixture (see text); (j) SMe2, BF3‚Et2O, -10 °C,
30 min, 69%; (k) sorbic acid, 2,4,6-trichlorobenzoyl chloride, N,N-
diisopropylethylamine, DMAP, THF, room temperature, overnight,
67%; (l) (CH2SH)2, BF3‚Et2O, CH2Cl2, 0 °C, 1 h, 81%. (m) Ac2O,
pyridine, room temperature, 77%.
(27) Moon, H. R.; Choi, W. J.; Kim, H. O.; Jeong, L. S. Tetrahedron:
Asymmetry 2002, 13, 1189-1193.
(28) (a) Mitsunobu, O. Synthesis 1981, 1-28. (b) Hughes, D. L. Org.
React. 1992, 42, 335-656. (c) Valentine, D. H., Jr.; Hillhouse, J. H.
Synthesis 2003, 317-334.
(29) Alcohol 19 has also been prepared by Fu¨rstner and co-
workers17a from the highly expensive D-ribonolactone.
(30) Dixon, D. J.; Ley, S. V.; Reynolds, D. J. Chem. Eur. J. 2002, 8,
1621-1636.
which proved identical with natural (+)-lethaloxin in its
optical rotation and spectral features3 (see the Supporting
Information). These physical and spectral data were also
identical with those reported for both natural36 and
synthetic17a (+)-pinolidoxin. This is also the case for the
(31) The enantiomer of acid 23 has been reported by Fu¨rstner and
co-workers.17a
(32) Inanaga, J.; Hirata, K.; Saeki, H.; Katsuki, T.; Yamaguchi, M.
Bull. Chem. Soc. Jpn. 1979, 52, 1989-1993.
J. Org. Chem, Vol. 70, No. 24, 2005 9825

Garc´ıa-Fortanet et al.
Lactones 11. Diolefin 10 (330 mg, 0.8 mmol) was dissolved
in dry, degassed CH2Cl2 (20 mL) and added dropwise within
1 h to a refluxing solution of ruthenium catalyst Ru-I (131
mg, 0.16 mmol) in dry, degassed CH2Cl2 (780 mL). The reaction
mixture was heated at reflux until consumption of the starting
material (20-24 h, TLC monitoring). Solvent removal in vacuo
and column chromatography of the residue on silica gel
(hexanes-EtOAc, 9:1) first furnished (E)-11 (139 mg, 45%) and
then (Z)-11 (67 mg, 22%). When the same reaction was
performed in the presence of ruthenium catalyst Ru-II (reac-
tion time 1 h), lactone (Z)-11 was obtained as the sole
stereoisomer in 73% yield. Physical and spectral data of (E)-
11: oil; [R]D ) -18.1° (c 0.6; CHCl3); IR νmax 1733 (lactone
CdO) cm-1; 1H NMR (500 MHz) δ 5.75 (1H, m), 5.34 (1H, dd,
J ) 15.5, 9 Hz), 4.92 (1H, m), 4.70 (1H, d, J ) 7 Hz), 4.68 (1H,
d, J ) 7 Hz), 3.93 (1H, t, J ) 9 Hz), 3.70-3.60 (2H, m), 3.40
(3H, s), 2.70-2.60 (1H, m), 2.54 (1H, dt, J ) 13.3, 4.5 Hz),
2.44 (1H, m), 2.40-2.30 (2H, m), 2.10 (1H, m), 2.00 (1H, m),
corresponding diacetates. These two natural decanolides
therefore are identical.
Concluding Remarks
In summary, convergent, stereoselective syntheses of
the pharmacologically active lactones 1 and 2 have been
achieved from the commercially available, chiral reagents
(R)-glycidol, (S,S)-tartaric acid, and D-ribose as the
starting materials. Since both enantiomers of these chiral
starting materials are available, our synthesis is adapt-
able to the preparation of not only the natural products
themselves but also of enantiomers, diastereoisomers,
and further analogues thereof. All of these derivatives
will be useful for future pharmacological structure-
activity relationships. In addition, our syntheses have
served to establish the absolute configuration of natural
(+)-lethaloxin and to show its identity with (+)-pinoli-
doxin.
1.41 (6H, s), 1.40-1.20 (9H, br m), 0.88 (3H, t, J ) 7 Hz); 13
C
NMR (125 MHz) δ 171.8, 108.8 (C), 130.2, 129.4, 84.4, 79.8,
79.3, 73.6 (CH), 96.5, 34.3, 31.8, 30.8, 30.5, 29.4, 25.5, 25.4,
22.6 (CH2), 56.1, 27.2, 27.0, 14.1 (CH3); HR CIMS m/z (relative
intensity) 385.2600 (M + H+, 8), 369 (25), 353 (11), 327 (42),
295 (84), 265 (80), 220 (25), 157 (100), calcd for C21H37O6
385.2590. For the graphical NMR spectra of (E)-11, see the
Supporting Information in ref 5. For the physical and spectral
data of (Z)-11, see the Supporting Information of this paper.
Lactone 12. Lactone (E)-11 (25 mg, 0.065 mmol) was
dissolved in dimethyl sulfide (2 mL), and the solution was
cooled to -10 °C and treated with BF3‚Et2O (82 µL, 0.65
mmol). The reaction mixture was stirred at the same temper-
ature for 30 min. Workup (CH2Cl2) and column chromatogra-
phy on silica gel (hexanes-EtOAc, 1:1) yielded 12 (16 mg,
71%): oil; [R]D ) -27.3° (c 0.4; CHCl3); IR νmax 3270 (br, OH),
1719 (lactone CdO) cm-1; NMR data identical with those
reported;2 HR EIMS m/z (relative intensity) 340.2256 (M+, 2),
325 (M+-Me, 26), 238 (22), 180 (100), 123 (82), 110 (84), 85
(56), 70 (58), calcd for C19H32O5 340.2249. For the graphical
NMR spectra of 12, see the Supporting Information in ref 5.
Experimental Section
General Features. These are described in detail in the
Supporting Information.
Ester 10. Acid 9 (1.5 mmol in crude form, see the prep-
aration in the Supporting Information), alcohol 8 (230 mg,
1 mmol), and DMAP (6 mg, 0.05 mmol) were dissolved in
dry CH2Cl2 (6 mL) and treated with a solution of DCC (310
mg, 1.5 mmol) in dry CH2Cl2 (6 mL). The reaction mixture
was then stirred at room temperature for 18 h, diluted with
CH2Cl2 (10 mL), and filtered to eliminate the solid N,N′-
dicyclohexylurea. Solvent removal gave a residue, which was
dissolved in Et2O (40 mL) and worked up. Solvent removal
and column chromatography on silica gel (hexanes-EtOAc,
9:1) provided ester 10 (355 mg, 86% based on 8): oil; [R]D
)
1
+4.2° (c 1; CHCl3); IR νmax 1738 (ester CdO) cm-1; H NMR
(500 MHz) δ 5.80-5.70 (2H, m), 5.36 (1H, br d, J ) 17.2 Hz),
5.24 (1H, br d, J ) 10.4 Hz), 5.10-5.00 (3H, m), 4.69 (1H, d,
J ) 7 Hz), 4.67 (1H, d, J ) 7 Hz), 3.98 (1H, t, J ) 7.8 Hz),
3.69 (1H, dt, J ) 8.3, 3.5 Hz), 3.58 (1H, m), 3.39 (3H, s), 2.55-
2.40 (3H, m), 2.33 (1H, m), 1.95 (1H, m), 1.82 (1H, m), 1.50
(2H, m), 1.40 (3H, s), 1.38 (3H, s), 1.40-1.25 (8H, br m), 0.87
(3H, t, J ) 7 Hz); 13C NMR (125 MHz) δ 172.5, 108.8 (C), 135.1,
133.9, 82.4, 79.5, 78.0, 73.7 (CH), 119.0, 117.7, 96.7, 34.7, 31.7,
30.7, 30.5, 29.4, 26.9, 25.3, 22.6 (CH2), 55.9, 27.2, 26.9, 14.0
(CH3); HR EIMS m/z (relative intensity) 412.2834 (M+, 1), 397
(11), 229 (58), 125 (100), 98 (64), calcd for C23H40O6 412.2825.
For the graphical NMR spectra of 10, see the Supporting
Information in ref 5.
Microcarpalide (1). An ice-cooled solution of lactone (E)-
11 (77 mg, 0.2 mmol) in dry CH2Cl2 (5 mL) was treated with
ethanedithiol (65 µL, 0.8 mmol) and BF3‚Et2O (50 µL, 0.4
mmol). The reaction mixture was stirred at 0 °C for 1 h.
Workup (EtOAc) and column chromatography on silica gel
(hexanes-EtOAc, 1:1) afforded 1 (40 mg, 66%): oil; [R]D
)
-20.2° (c 0.4; MeOH), lit. [R]D ) -22° (c 0.67; MeOH); IR νmax
3400 (br, OH), 1724 (lactone CdO) cm-1; NMR data identical
with those reported.2 For the graphical NMR spectra of
synthetic 1, see the Supporting Information in ref 5.
Ester 24. Acid 23 (2 mmol in crude form; see the prepara-
tion in the Supporting Information), alcohol 19 (200 mg, 1
mmol), N,N-diisopropylethylamine (435 µL, 2.5 mmol), and
DMAP (6 mg, 0.05 mmol) were dissolved in dry THF (35 mL)
and treated with 2,4,6-trichlorobenzoyl chloride (312 µL, 2
mmol). The reaction mixture was then stirred overnight at
room temperature. Workup (Et2O) and column chromatogra-
phy on silica gel (hexanes-EtOAc, 7:3) furnished ester 24 (342
mg, 96% based on 19): oil; [R]D ) -24.6° (c 2.1, CHCl3); IR
(33) After selective cleavage of the MOM group in olefin Z-25, the
crystalline hydroxy lactone Z-26 was formed. The structure and
absolute configuration of Z-26 were confirmed by means of an X-ray
diffraction analysis. Crystallographic data (excluding structure factors)
have been deposited at the Cambridge Crystallographic Data Center
as Supporting Information with reference CCDC-271535. Copies of the
data can be obtained, free of charge, on application to the CCDC, 12
Union Road, Cambridge CB2 1EZ, U.K. (fax, +44(0)-1223-336033;
e-mail, deposit@ccdc.cam.ac.uk).
νmax 1752 (ester CdO) cm-1 1H NMR δ 5.85-5.75 (2H, m),
;
5.35 (1H, br d, J ) 17.3 Hz), 5.22 (1H, br d, J ) 10.3 Hz), 5.04
(1H, dq, J ) 17, 1.5 Hz), 5.00 (1H, overlapped m), 4.98 (1H,
dt, J ) 7.6, 3.5 Hz), 4.67 (1H, d, J ) 6.8 Hz), 4.63 (1H, d, J )
6.8 Hz), 4.60 (1H, br t, J ) 6.5 Hz), 4.19 (1H, dd, J ) 7.6, 6.5
Hz), 4.08 (1H, dd, J ) 7.8, 4.7 Hz), 3.39 (3H, s), 2.20 (2H, m),
1.90-1.80 (2H, m), 1.70-1.60 (2H, m), 1.47 (3H, s), 1.36 (3H,
s), 1.40-1.25 (2H, m), 0.91 (3H, t, J ) 7.3 Hz); 13C NMR δ
171.5, 108.8 (C), 137.3, 133.1, 78.8, 78.1, 74.4, 72.3 (CH), 118.7,
115.5, 96.0, 33.4, 32.1, 29.4, 17.8 (CH2), 56.0, 27.6, 25.3, 14.0
(CH3); HR EIMS m/z (relative intensity) 341.1940 (M+ - Me,
12), 127 (34), 98 (100), calcd for C19H32O6 - Me 341.1964.
(34) As in the case of microcarpalide, the catalyst Ru-II provided
only the undesired Z-25. Again, the preferential formation of the more
stable Z isomer is due to thermodynamic control of the RCM process
by the catalyst Ru-II. It is worth mentioning, however, that, with
appropriately allocated substituents in the 10-membered ring, the E
isomer may become the more stable from the thermodynamic point of
view and thus favored in the RCM process, even with catalyst
Ru-II.23
(35) Lactones E-26 and 27 correspond to compounds 24 and 45 in
ref 17a, but the physical and spectral data of the last two compounds
are not given in the Supporting Information of that paper. The physical
and spectral data of the enantiomer of 45 (named 43 there) are given,
however.
(36) Evidente, A.; Lanzetta, R.; Capasso, R.; Vurro, M.; Bottalico,
A. Phytochemistry 1993, 34, 999-1003.
Lactones 25. Diolefin 24 (285 mg, 0.8 mmol) was dissolved
in dry, degassed CH2Cl2 (20 mL) and added dropwise within
9826 J. Org. Chem., Vol. 70, No. 24, 2005

Total Synthesis of Microcarpalide and Lethaloxin
1 h to a refluxing solution of ruthenium catalyst Ru-I (131
mg, 0.16 mmol) in dry, degassed CH2Cl2 (780 mL). The reaction
mixture was heated at reflux until consumption of the starting
material (4-6 h, TLC monitoring). Solvent removal in vacuo
and column chromatography of the residue on silica gel
(hexanes-EtOAc, 9:1) provided first E-25 (150 mg, 57%) and
then Z-25 (47 mg, 18%). When the same reaction was
performed in the presence of ruthenium catalyst Ru-II (reac-
tion time, 2 h), lactone Z-25 was obtained as the sole stereo-
isomer in 72% yield. Physical and spectral data of (E)-25: oil;
trichlorobenzoyl chloride (80 µL, ca. 0.5 mmol). The reaction
mixture was then stirred overnight at room temperature.
Workup (Et2O) and column chromatography on silica gel
(hexanes-EtOAc, 9:1) furnished lactone 27 (63 mg, 67%, 86%
based on recovered starting material), together with unreacted
(E)-26 (16 mg): oil; [R]D ) +145.5° (c 1, CHCl3), lit.17a for the
enantiomer [R]D ) -158.9° (c 0.54, CH2Cl2); IR νmax 1721 (br,
lactone and ester CdO) cm-1; the NMR spectra, which show
the presence of two conformers, are essentially identical with
those reported;17a HR EIMS m/z (relative intensity) 378.2061
(M+, 2), 320 (11), 283 (18), 235 (11), 110 (66), 83 (77), 95 (100),
calcd for C21H30O6 378.2042.
[R]D ) +30.2° (c 1, CHCl3); IR νmax 1730 (lactone CdO) cm-1
.
The 1H NMR spectrum shows the presence of two slowly
interconverting conformers in an approximate 2.5 to 3:1
relation, which give rise to two sets of partly overlapped signals
(the visible signals of the minor conformer with relative proton
intensities are given in italics). The 13C NMR spectrum also
shows two sets of signals (signals of the minor conformer are
given in italics): 1H NMR δ 5.84 (1H, dd, J ) 15.6, 3.3 Hz),
5.63 (1H, m), 5.49 (1H, br dd, J ) 16.5, 8 Hz), 4.98 (1H, ddd,
J ) 10, 9, 3 Hz), 4.79 (1H, br t, J ) 9 Hz), 4.73 (1H, br t, J )
7.3 Hz), 4.68 (2H, s), 4.66 (1H, m), 4.29 (1H, br d, J ) 5.5 Hz),
4.17 (2H, m), 4.00 (1H, dd, J ) 10, 4.6 Hz), 3.39 (3H, s), 3.33
(3H, s), 2.40 (1H, m), 2.30 (1H, m), 2.20 (1H, m), 2.20-2.05
(2H, m), 2.00 (1H, m), 1.80 (2H, m), 1.65 (1H, m), 1.51 (3H, s),
1.44 (3H, s), 1.36 (3H, s), 1.33 (3H, s), 1.35-1.25 (2H, m), 0.91
(3H, t, J ) 7 Hz); 13C NMR δ 174.4, 172.4, 109.1, 108.9 (C),
131.7, 128.6, 128.0, 123.2, 78.2, 78.1, 78.0, 76.2, 75.7, 73.3, 71.4
(CH), 95.9, 95.7, 34.2, 33.8, 31.9, 30.3, 27.2, 26.0, 18.1, 17.8
(CH2), 55.8, 55.7, 28.5, 27.9, 26.2, 25.2, 13.9, 13.8 (CH3); HR
EIMS m/z 328.1843 (M+, 1), 283 (18), 171 (66), 111 (100), calcd
for C17H28O6 328.1885. For the physical and spectral data of
Z-25, see the Supporting Information of this paper.
Lethaloxin (2). An ice-cooled solution of lactone 27 (57 mg,
0.15 mmol) in dry CH2Cl2 (4 mL) was treated with ethanedi-
thiol (25 µL, 0.3 mmol) and BF3‚Et2O (20 µL, ca. 0.15
mmol). The reaction mixture was stirred at 0 °C for 1 h.
Workup (EtOAc) and column chromatography on silica gel
(hexanes-EtOAc, 1:1) afforded 2 (41 mg, 81%): oil; [R]D
) +126.7° (c 0.25, CHCl3), lit.3 for natural (+)-lethaloxin
[R]D ) +129° (c 0.2, CHCl3), lit.36 for natural (+)-pinoli-
doxin, [R]D ) +142.9° (c 0.31, CHCl3), lit.17a for synthetic
(+)-pinolidoxin, [R]D ) +143.2° (c 0.25, CHCl3), lit.17a for
synthetic (-)-pinolidoxin, [R]D ) -114.4° (c 0.54, CH2Cl2), lit.17b
for synthetic (-)-pinolidoxin, [R]D )-140° (c 0.2, CHCl3); IR
νmax 3480 (br, OH), 1719 (br, lactone and ester CdO) cm-1
;
the NMR spectra, which show the presence of only one
conformer, are essentially identical with those of natural
lethaloxin (NMR spectra of the natural sample were available)
and, as well, with those of either natural or synthetic pinoli-
doxin (see Tables 1 and 2 in the Supporting Information); HR
EIMS m/z (relative intensity) 338.1745 (M+, 1), 253 (2), 141
(8), 95 (100), 67 (22), calcd for C18H26O6 338.1729.
Lactone (E)-26. Lactone (E)-25 (132 mg, 0.4 mmol) was
dissolved in dimethyl sulfide (12 mL), cooled to -10 °C, and
treated with BF3‚Et2O (0.5 mL, ca. 4 mmol). The reaction
mixture was stirred at the same temperature for 30 min.
Workup (CH2Cl2) and column chromatography on silica gel
(hexanes-EtOAc, 7:3) yielded E-26 (78 mg, 68%): oil; [R]D
+81.4° (c 1.8, CHCl3); IR νmax 3480 (br, OH), 1728 (lactone
CdO) cm-1. The 1H NMR spectrum shows the presence of two
slowly interconverting conformers in an approximate 2.5 to
3:1 relation, which give rise to two sets of partly overlapped
signals (the visible signals of the minor conformer with relative
proton intensities are given in italics). The 13C NMR spectrum
also shows two sets of signals (signals for the minor conformer
are given in italics): 1H NMR δ 5.79 (1H, dd, J ) 15.7, 3.3
Hz), 5.74 (1H, overlapped m), 5.66 (1H, br ddd, J ) 15.7, 11.5,
4.2 Hz), 5.53 (1H, br dd, J ) 16.3, 8.4 Hz), 5.03 (1H, ddd, J )
10, 9, 2.7 Hz), 4.70 (2H, m), 4.64 (1H, br s), 4.39 (1H, br d, J
) 5.5 Hz), 4.30 (1H, m), 4.20 (1H, dd, J ) 9.8, 6.6 Hz), 3.98
(1H, dd, J ) 10, 4.7 Hz), 2.50-2.45 (1H, m), 2.40 (1H, br s,
OH), 2.20-2.10 (2H, m), 2.00 (1H, m), 1.80 (1H, m), 1.65 (1H,
m), 1.60-1.50 (1H, m), 1.54 (3H, s), 1.44 (3H, s), 1.36 (3H, s),
1.40-1.30 (2H, m), 1.33 (3H, s), 0.91 (3H, t, J ) 7.2 Hz); 13C
NMR δ 174.3, 109.2, 109.0 (C), 131.1, 129.5, 127.8, 124.6, 78.1,
77.9, 77.8, 76.0, 71.9, 70.3, 70.2 (CH), 34.2, 33.9, 32.4, 32.2,
27.0, 18.2, 17.9 (CH2), 28.5, 27.8, 26.2, 25.1, 25.0, 13.8 (CH3);
HR EIMS m/z (relative intensity) 284.1693 (M+, 14), 269 (36),
241 (34), 115 (100), calcd for C15H24O5 284.1623.
Acknowledgment. Financial support has been
granted by the Spanish Ministry of Education and
Science (Project BQU2002-00468) and by the AVCyT
(Projects GRUPOS03/180 and GV05/52). J.M. thanks
the Spanish Ministry of Education and Science for a
contract of the Ramo´n y Cajal program. J.G.-F. thanks
the Spanish Ministry of Education and Science for a
predoctoral fellowship (FPU program). We further thank
Prof. G. Nasini, from the Dipartimento di Chimica,
Materiali ed Ingegneria Chimica “Giulio Natta”, Po-
litecnico di Milano, Milano, Italy, for sending us NMR
spectra of natural lethaloxin.
Supporting Information Available: Text giving a de-
scription of general spectroscopic features and several experi-
mental procedures, tables giving physical and spectral data
of synthetic compounds 4-6, 8, Z-11, 21, 22, Z-25, and Z-26,
a comparison of 1H and 13C NMR data of 2 with those of
natural lethaloxin and both natural and synthetic pinolidoxin,
1
and a comparison of H NMR data of the diacetate of 2 (28)
with those of the diacetates of natural lethaloxin and natural
pinolidoxin, figures giving graphical NMR spectra of com-
pounds 2, 4-6, 8, 10, E-11, Z-11, 12, 21, 22, and 24-28, and
a CIF file giving crystallographic data of compound (Z)-26. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Lactone 27. Lactone E-26 (71 mg, 0.25 mmol), N,N-
diisopropylethylamine (110 µL, ca. 0.6 mmol), DMAP (6 mg,
0.05 mmol), and sorbic acid (56 mg, 0.5 mmol) were sequen-
tially dissolved in dry THF (10 mL) and treated with 2,4,6-
JO051353P
J. Org. Chem, Vol. 70, No. 24, 2005 9827