Synthesis of the landomycinone skeleton

Article abstract of DOI:10.1021/jo1018377

The synthesis of the highly functionalized tetracyclic skeleton of landomycinone (2), the aglycon of landomycins, was performed using two pivotal steps relying on metal-catalyzed reactions. They are (1) a [2 + 2 + 2] cycloaddition of alkynes promoted by W

Full text of DOI:10.1021/jo1018377

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Synthesis of the Landomycinone Skeleton
Xavier Bugaut, Xavier Guinchard, and Emmanuel Roulland*
CNRS, Institut de Chimie des Substances Naturelles (ICSN), UPR 2301, Avenue de la Terrasse,
91198 Gif-sur-Yvette, France
emmanuel.roulland@icsn.cnrs-gif.fr
Received September 16, 2010
The synthesis of the highly functionalized tetracyclic skeleton of landomycinone (2), the aglycon of
landomycins, was performed using two pivotal steps relying on metal-catalyzed reactions. They are
(1) a [2 þ 2 þ 2] cycloaddition of alkynes promoted by Wilkinson’s catalyst to build rings B and C
concomitantly and (2) a ring-closing metathesis followed by aromatization to build ring D.
1. Introduction
isolated in lower amounts from the same fermentation experi-
ments. Landomycins D and E were isolated from another
actinobacteria named Streptomyces globisporum (strain 1912).2
Landomycin A (1) (Figure 1) was isolated in 1990 and
identified as the main bioactive compound extracted from a
broth of the actinobacteria Streptomyces cyanogenus (strain
S136, DSM 5087).1 The aglycon core of this glycosidic anti-
biotic is an angular tetracyclic decaketide named landomyci-
none (2), which is bonded to the hexasaccharidic chain at a
phenolic position. Three related compounds featuring altered
oligosaccharidic side chains, named landomycins B-D, were
(1) (a) Henkel, T.; Rohr, J.; Beale, J. M.; Schwenen, L. J. Antibiot. 1990,
43, 492–503. (b) For the revision of the position of the oligosaccharidic chain,
see: Weber, S.; Zolke, C.; Rohr, J.; Beale, J. M. J. Org. Chem. 1994, 59, 4211–
4214.
(2) Krohn, K.; Rohr, J. Top. Curr. Chem. 1997, 188, 127–195. and references
cited therein.
(3) Rohr, J.; Thiericke, R. Nat. Prod. Rep. 1992, 9, 103–137.
(4) (a) Westrich, L.; Domann, S.; Faust, B.; Bedford, D.; Hopwood, D.,
A.; Bechthold, A. FEMS Microbiol. Lett. 1999, 170, 381–387. (b) Rebets, Y.;
FIGURE 1. Landomycin A (1).
ꢀ
Ostash, B.; Luzhetskyy, A.; Hoffmeister, D.; Brana, A.; Mendez, C.; Salas,
J. A.; Bechthold, A.; Fedorenko, V. FEMS Microbiol. Lett. 2003, 222, 149–
153. (c) Ostash, B.; Rix, U.; Rix, L. L. R.; Liu, T.; Lombo, F.; Luzhetskyy, A.;
All landomycins contain the same aglycon, but the length
of their oligosaccharidic side chain (two to six sugars) is
variable. They belong to the large family of angucycline anti-
biotics, this name reflecting the angular geometry of their
aglycon skeleton.2,3 A large number of genetic engineering
studies were conducted in order to understand the biosynth-
esis of landomycins and resulted in the production of many
new related compounds possessing structural modifications
on both the aglycon and/or the oligosaccharide.4
~
ꢁ
Gromyko, O.; Wang, C.; Brana, A. F.; Mendez, C.; Salas, J. A.; Fedorenko,
V.; Rohr, J. Chem. Biol. 2004, 11, 547–555. (d) Zhu, L.; Ostash, B.; Rix, U.;
Nur-e-Alam, M.; Mayers, A.; Luzhetskyy, A.; Mendez, C.; Salas, J. A.;
Bechthold, A.; Fedorenko, V.; Rohr, J. J. Org. Chem. 2005, 70, 631–638. (e)
€
Luzhetskyy, A.; Zhu, L.; Gibson, M.; Fedoryshyn, M.; Durr, C.; Hofmann,
C.; Hoffmeister, D.; Ostash, B.; Mattingly, C.; Adams, V.; Fedorenko, V.;
Rohr, J.; Bechthold, A. ChemBioChem 2005, 6, 675–678. (f) Luzhetskyy, A.;
€
Taguchi, T.; Fedoryshyn, M.; Durr, C.; Wohlert, S.-E.; Novikov, V.;
Bechthold, A. ChemBioChem 2005, 6, 1406–1410. (g) Zhu, L.; Luzhetskyy,
A.; Luzhetska, M.; Mattingly, C.; Adams, V.; Bechthold, A.; Rohr, J.
ChemBioChem 2007, 8, 83–88. (h) Krauth, C.; Fedoryshyn, M.; Schleberger,
C.; Luzhetskyy, A.; Bechthold, A. Chem. Biol. 2009, 16, 28–35.
All landomycins, as well as landomycinone (2), are en-
dowed with diverse biological activities, but their marked
8190 J. Org. Chem. 2010, 75, 8190–8198
Published on Web 10/28/2010
DOI: 10.1021/jo1018377
r
2010 American Chemical Society

Bugaut et al.
JOCArticle
antitumor properties are of particular interest.5 Landomycin A
(1) is the most cytostatic member of the family and exhibits
activity on cells comparable with clinically used anticancer
agents such as bleomycin, cisplatin, doxorubicin, or paclitaxel.5
One must notice that even multidrug-resistant cell lines remain
sensitive to these compounds.7 Studies of its mode of action
demonstrated that landomycin A (1) inhibits DNA synthesis
and cell cycle progression at the G1/S phase.6 Landomycin E
has the same effect on DNA synthesis but without any
modification of cell cycle7 and seems to induce cell death via
depolarization of the membrane of mitochondria.
SCHEME 1. Retrosynthetic plan for the synthesis of 2
While the syntheses of some of the oligosaccharidic chains
or of the landomycinone (2) have been reported,8 to date there
is no total synthesis of any member of this family. The first
study of the aglycon synthesis consisted of the biomimetic
preparation of a putative biosynthetic intermediate via a base-
triggered double cyclization to form rings A and B.9 In 2004,
Roush et al. reported the only synthesis of landomycinone
(2).10 The key step of this approach was a moderately efficient
SCHEME 2. Functionalization of Ring A
€
Dotz benzannulation requiring a stoichiometric amount of
organochromium reagent.
Herein, we wish to discuss our results on route to land-
omycinone (2) that led to the assembly of the skeleton of the
target using transition-metal-catalyzed reactions as key
steps.
2. Results and Discussion
2.1. Retrosynthetic Plan. Due to its unique and densely
hydroxylated polycyclic structure, landomycinone (2) is a
difficult target. The most obvious challenge of its synthesis is
the formation of the angular ring B which bears an asym-
metric alcohol function particularly prone to elimination.
We intended to design a strategy based on metal-catalyzed
reactions (Scheme 1). The pivotal step, a [2 þ 2 þ 2] cyclo-
addition of alkynes, would allow the simultaneous con-
struction of both rings B and C.11,12 We planned other key
(5) Ostash, B.; Korynevska, A.; Stoika, R.; Fedorenko, V. Mini-Rev.
Med. Chem. 2009, 9, 1040–1051.
(6) Crow, R. T.; Rosenbaum, B.; Smith, R., III; Guo, Y.; Ramos, K. S.;
Sulikowski, G. A. Bioorg. Med. Chem. Lett. 1999, 9, 1663–1666.
(7) Korynevska, A.; Heffeter, P.; Matselyukh, B.; Elbling, L.; Micksche,
M.; Stoika, R.; Berger, W. Biochem. Pharmacol. 2007, 74, 1713–1726.
(8) For total syntheses of the hexasaccharide of landomycin A, see:
(a) Guo, Y.; Sulikowski, G. A. J. Am. Chem. Soc. 1998, 120, 1392–1397.
(b) Roush, W. R.; Bennett, C. E. J. Am. Chem. Soc. 2000, 122, 6124–6125. (c)
Yu, B.; Wang, P. Org. Lett. 2002, 4, 1919–1922. For a total synthesis of the
trisaccharide of landomycin E, see: (d) Zhou, M.; O’Doherty, G. A. Org.
Lett. 2008, 10, 2283–2286. For the combinatorial synthesis of a library
of related hexasaccharides, see: (e) Tanaka, H.; Yamaguchi, S.; Yoshizawa,
A.; Takagi, M.; Shin-ya, K.; Takahashi, T. Chem. Asian J. 2010, 5, 1407–
1424.
(9) Krohn, K.; Frese, P.; Freund, C. Tetrahedron 2000, 56, 1193–1196.
(10) (a) Roush, W. R.; Neitz, R. J. J. Org. Chem. 2004, 69, 4906–4912.
(b) Discrepancies between analytical data of “natural” and “synthetic”
landomycinone induced Roush et al. to propose a revised structure for the
aglycon. Nevertheless, further purification of the biosynthetic product solved
this problem and confirmed structure 2. See ref 4d.
transformations comprising a ring-closing metathesis (RCM)
to create aromatic ring D13 and a challenging Pd(0)-catalyzed
cross-coupling at a sterically hindered o,o0-disubstituted
position of ring A.
2.2. Preparation of the Diynes. Our synthesis started from
allyl ether 4 obtained from the commercially available2-meth-
oxy-4-methylphenol3 (Scheme2).14 Claisen rearrangement of
allyl ether 4 was carried out in a microwave oven, using N-
methylpyrrolidinone (NMP) as the solvent to improve mi-
crowave absorption.15 The resulting phenol 5 was then
converted into the corresponding triflate 6 in quantitative
yield.
In the next stage, we focused our attention on the Pd(0)-cata-
lyzed cross-coupling of 6 to introduce a (trimethylsilyl)ethynyl
(11) For reviews on [2 þ2þ2] cycloadditions of alkynes, see: (a) Kotha,
S.; Brahmachary, E.; Lahiri, K. Eur. J. Org. Chem. 2005, 4741–4767.
(b) Chopade, P. R.; Louie, J. Adv. Synth. Catal. 2006, 348, 2307–2327.
(c) Agenet, N.; Buisine, O.; Slowinski, F.; Gandon, V.; Aubert, C.; Malacria,
M. Org. React. 2007, 68, 1–302. (d) Galan, B. R.; Rovis, T. Angew. Chem.,
Int. Ed. 2009, 48, 2830–2834.
(13) For reviews on the synthesis of aromatic rings by ring-closing
metathesis, see: (a) Donohoe, T. J.; Orr, A. J.; Bingham, M. Angew. Chem.,
Int. Ed. 2006, 45, 2664–2670. (b) van Otterlo, W. A. L.; de Koning, C. B.
Chem. Rev. 2009, 109, 3743–3782.
(12) For the use of Co-catalyzed [2 þ2þ2] cycloadditions for the synthesis
of angucyclines with an aromatic angular ring, see: (a) Kalogerakis, A.;
Groth, U. Org. Lett. 2003, 5, 843–844. (b) Kalogerakis, A.; Groth, U. Synlett
2003, 1886–1888. (c) Kesenheimer, C.; Groth, U. Org. Lett. 2006, 8, 2507–
2510. (d) Kesenheimer, C.; Kalogerakis, A.; Meissner, A.; Groth, U.
Chem.;Eur. J. 2010, 16, 8805–8821.
(14) Srikrishna, A.; Satyanarayana, G. Tetrahedron 2006, 62, 2892–2900.
(15) (a) This reaction has been effected by prolonged heating in a sealed
tube (see ref 14) but proved inconvenient to reproduce. (b) For a previous
report on the benefitial effect of microwaves and a polar solvent in Claisen
rearrangements, see: Davis, C. J.; Hurst, T. E.; Jacob, A. M.; Moody, C. J.
J. Org. Chem. 2005, 70, 4414–4422.
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JOCArticle
SCHEME 3. Preparation of Several Diynes for the [2 þ 2 þ 2] Cycloaddition
moiety. Unfortunately, classic Sonogashira conditions16 did
not lead to the desired product 7, probably because of steric
hindrance of the triflate function of 6. These fruitless results
prompted us to turn our attention toward a Suzuki-Miyaura
cross-coupling involving potassium (trimethylsilyl)ethynyltri-
fluoroborate as the nucleophilic coupling partner.17 To per-
form the cross-coupling efficiently and without loss of the
trimethylsilyl group, an adaptation of the conditions described
by Molander et al. was required.17 Thus, the use of Buchwald’s
ligand RuPhos18 along with a milder base (K3PO4 instead of
Cs2CO3) led to the isolation of alkyne 7 in high yield. A selective
osmium-catalyzed oxidation of the allyl group afforded then
diol 8, which served as precursor for the preparation of several
diynes.
SCHEME 4. Attempts of Intermolecular [2 þ 2 þ 2] Cycload-
dition
For this purpose, an oxidative cleavage was performed
affording the fragile aldehyde 9 (Scheme 3, route A). Con-
densation with lithium (trimethylsilyl)acetylide 10a furnished a
propargylic alcohol, which was directly protected as the corre-
sponding silyl ether 11a. Afterward, diyne 12a was easily
obtained by removal of the acetylenic TMS protective groups
under basic conditions. Alternatively, Grignard reagents 10b
and 10c could be used in a similar sequence, enabling the syn-
thesis of diynes 11b and 11c in good yields (Scheme 3, Route B).
Subsequent treatment of compound 11c with NaOH yielding
alcohol 12c by the selective cleavage of the tert-butyldiphenyl-
silyl (TBDPS) protective group orthogonally to the TBS.19
Finally, a similar strategy yielded diyne 11d from diol 13
(Scheme 3, route C).
11a-d or 12c were attempted using reaction partners such
as acetylene (14a), but-2-yne-1,4-diol (14b) or dimethyl
acetylenedicarboxylate (14c) (Scheme 4, eq 1). Several classic
catalytic systems11 (RhCl(PPh3)3,20 Rh(cod)2BF4-H8-
24
BINAP,21 Cp*Ru(cod)Cl,22 [Ir(cod)Cl]2-ddpe,23 Co2(CO)8
)
were screened but did not afford tricyclic compounds in syntheti-
cally useful yields (<20%). The use of sterically hindered
substituted diynes may account for these disappointing
results. This hypothesis was confirmed by the successful
Wilkinson’s complex-catalyzed cycloaddition of the far less
hindered bis-terminal diyne 12a with but-2-yn-1,4-diol 14b,
2.3. Intermolecular [2 þ 2 þ 2] Cycloaddition. Many inter-
molecular [2 þ 2 þ 2] cycloadditions of substituted diynes
ꢁ
(16) Chinchilla, R.; Najera, C. Chem. Rev. 2007, 107, 874–922.
(17) (a) Molander, G. A.; Katona, B. W.; Machrouhi, F. J. Org. Chem.
2002, 67, 8416–8423. For recent reviews on the use of these stable and highly
convenient organoboron derivatives, see: (b) Molander, G. A.; Ellis, N. Acc.
^
Chem. Res. 2007, 40, 275–286. (c) Darses, S.; Genet, J.-P. Chem. Rev. 2008,
108, 288–325. (d) Molander, G. A.; Canturk, B. Angew. Chem., Int. Ed. 2009,
48, 9240–9261.
(20) Grigg, R.; Scott, R.; Stevenson, P. J. Chem. Soc., Perkin Trans. 1
1988, 1357–1364.
(18) (a) Milne, J. E.; Buchwald, S. L. J. Am. Chem. Soc. 2004, 126, 13028–
13032. (b) Molander, G. A.; Brown, A. R. J. Org. Chem. 2006, 71, 9681–9686.
(19) For other examples of such selective deprotections of TBDPS-ethers
in the presence of TBS-ethers in natural product synthesis, see: (a) Kabat,
M. M.; Lange, M.; Wovkulich, P. M.; Uskokovic, M. R. Tetrahedron Lett.
1992, 33, 7701–7704. (b) Hatakeyama, S.; Irie, H.; Shintani, T.; Noguchi, Y.;
Yamada, H.; Nishizawa, M. Tetrahedron 1994, 50, 13369–13376. (c) Cook,
C.; Guinchard, X.; Liron, F.; Roulland, E. Org. Lett. 2010, 12, 744–747.
(21) Yamamoto, Y.; Arakawa, T.; Ogawa, R.; Itoh, K. J. Am. Chem. Soc.
2003, 125, 12143–12160.
(22) Tanaka, K.; Toyoda, K.; Wada, A.; Shirasaka, K.; Hirano, M.
Chem.;Eur. J. 2005, 11, 1145–1156.
(23) Takeuchi, R.; Tanaka, S.; Nakaya, Y. Tetrahedron Lett. 2001, 42,
2991–2994.
(24) Gandon, V.; Leca, D.; Aechtner, T.; Vollhardt, K. P. C.; Malacria,
M.; Aubert, C. Org. Lett. 2004, 6, 3405–3407.
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SCHEME 5. Construction of the Tether and Intramolecular
[2 þ 2 þ 2] Cycloaddition
(Scheme 5).34 The Mitsunobu reaction turned out to be the
best method for the coupling of alcohol 12c with carboxylic
acid 17.35 Nonetheless, a thorough optimization of the condi-
tions was necessary. The first trials were indeed very promising
(isolated yield often reaching 80%) but poorly reproducible
(yield sometimes dropped to 10-20%). Finally, we found that
the order of addition of the various reagents was instrumental.
Thus, the reproducibility issue was solved by premixing an
excess of di-tert-butyl azodicarboxylate (DTBAD)36 with PPh3
in THF at 0 °C,37 followed by the addition of 2 equiv of
carboxylic acid with respect to the phosphine,38 and then the
addition of the alcohol. This procedure furnished ester 18 in a
reproducible 97% yield.
With triyne 18 in hand, we searched for efficient [2 þ 2 þ 2]
cycloaddition conditions. Delightfully, the reaction of 18 with
the Wilkinson’s catalyst (5 mol %, rt, 48 h) led to a full conver-
sion furnishing tetracyclic product 19 in an 82% isolated yield,
demonstrating the dramatic effect of the tether.39 In refluxing
CH2Cl2, the reaction time was shortened in half and the yield
improved to 93%. One notices that the double bond present in
the substrate 18 was inert to these conditions. Moreover, the
latter conditions are sufficiently mild to avoid the troublesome
aromatization of ring B by elimination of the oxygen at C6.40
2.5. Formation of Ring D by Ring-Closing Metathesis. Ring D
was envisaged to arise by RCM followed by aromatization
(Table 1). For this purpose, a second alkene function had to
be introduced. Thus, selective addition41 of vinyllithium to the
lactone function of 19 led to the acid sensitive dienic hemiketal 20
as an inseparable mixture of four diastereoisomers.42 Formation
of aromatic compounds by RCM has been a very active research
field over the past few years,13 and it has been shown that newly
formed rings bearing an alcohol or a ketone function could be
readily aromatized by either elimination of water43 or by tauto-
merization.44 However, the corresponding transformation of a
affording tricyclic product 15 in a 78% yield (Scheme 4, eq 2).
Unfortunately, the C ring of product 15 does not bear the
functionalities at positions C7 or C12 that would serve as
precursor for the naphthoquinone motif of 2.25
2.4. Intramolecular [2 þ 2 þ 2] Cycloaddition. The results
presented above confirm that the [2 þ 2 þ 2] cycloaddition of
substituted alkynes is difficult. Unfortunately, our strategy
definitely required the cycloaddition of such substituted alkynes
which is the only way to introduce oxidizable functions as pre-
cursors of the quinonic ring C. A popular solution of the problem
is to render the [2 þ 2 þ 2] cycloaddition all-intermolecular by
means of a cleavable tether. In such an approach, the negative
effect of steric hindrance would be diminished by a more
favorable entropy of the system. Moreover, regioselectivity
would be ensured. Since the seminal examples by Nishiyama26
and Stork,27 the use of tethers,28 in particular silicon-containing
ones,29 has become a classical synthetic method. In the case of
[2þ2þ2] cyclodditions, a comparable strategy has already been
applied with silicon-30 and boron-containing31 linkers. For our
part, we chose to explore the interesting potential offered by the
ester tether.32 Esters, indeed, possess all required properties of
an efficient tether: (i) many efficient methods are available for
their installation, (ii) [2 þ 2þ2] cycloadditions of 1,6-diynes tied
together by an ester bond are generally very efficient,33 and (iii)
the resulting lactone is a function with a rich reactivity.
(34) (a) Ratnayake, A. S.; Hemscheidt, T. Org. Lett. 2002, 4, 4667–4669.
(b) Young, D. G. J.; Burlison, J. A.; Peters, U. J. Org. Chem. 2003, 68, 3494–
3497.
(35) (a) For seminal work, see: Mitsunobu, O.; Yamada, M. Bull. Chem.
Soc. Jpn. 1967, 40, 2380–2382. (b) For a recent review, see: Swamy, K. C. K.;
Kumar, N. N. B.; Balaraman, E.; Kumar, K. V. P. P. Chem. Rev. 2009, 109,
2551–2651.
(36) Pelletier, J. C.; Kincaid, S. Tetrahedron Lett. 2000, 41, 797–800.
(37) For a detailed discussion on the mechanism of Mitsunobu reaction
and on the impact of the order of introduction of the reactants, see: Hughes,
D. L. Org. React. 1992, 42, 335–656.
(38) It was shown that using a second equivalent of carboxylic acid could
prevent nonproductive nucleophilic attack of the carboxylate: Hughes, D. L.;
Reamer, R. A.; Bergan, J. J.; Grabowski, E. J. J. J. Am. Chem. Soc. 1988, 110,
6487–6491.
(39) Tethers built from diynoic acids and monalkyols were also pre-
pared. They gave significantly worse results, [2 þ 2 þ 2] cycloaddition
products being obtained in yield lower than 25%, along with many
degradation products.
(40) For an example of a [2 þ 2 þ 2] cycloaddition accompanied by
aromatization through elimination of TBDMSOH, see ref 12.
(41) Cavicchioli, S.; Savoia, D.; Trombini, C.; Umani-Ronchi, A. J. Org.
Chem. 1984, 49, 1246–1251, and references therein
To build our ester tether, carboxylic acid 17 was prepared
in four steps from acrolein via known terminal alkyne 16
(25) Attempted metalation on ring C of product 15 was unsuccesful.
(26) Nishiyama, H.; Kitajima, T.; Matsumoto, M.; Itoh, K. J. Org. Chem.
1984, 49, 2298–2300.
(27) Stork, G.; Kahn, M. J. Am. Chem. Soc. 1985, 107, 500–501.
(28) Gauthier, D. R.; Zandi, K. S.; Shea, K. J. Tetrahedron 1998, 54,
2289–2338.
(29) (a) Bols, M.; Skrydstrup, T. Chem. Rev. 1995, 95, 1253–1277.
(b) Fensterbank, L.; Malacria, M.; Sieburth, S. M. N. Synthesis 1997
813–854.
(30) (a) Chouraqui, G.; Petit, M.; Aubert, C.; Malacria, M. Org. Lett.
2004, 6, 1519–1521. (b) Chouraqui, G.; Petit, M.; Phansavath, P.; Aubert, C.;
Malacria, M. Eur. J. Org. Chem. 2006, 1413–1421.
(42) (a) Compound 20 and all its derivatives are extremely acid-sensitive
and tend to degrade via aromatization of ring B. More details concerning
their handling are described in the Supporting Information. (b) These four
diastereoisomers are a priori valuable, since the RCM-aromatization se-
quence should destroy two asymmetric centers.
(43) Selected examples: (a) Huang, K.-S.; Wang, E.-C. Tetrahedron Lett.
2001, 42, 6155–6157. (b) Yoshida, K.; Kawagoe, F.; Iwadate, N.; Takahashi, H.;
Imamoto, T. Chem. Asian J. 2006, 1, 611–613. (c) Takahashi, H.; Yoshida, K.;
Yanagisawa, A. J. Org. Chem. 2009, 74, 3632–3640.
(44) (a) For a pioneering example, see: van Otterlo, W. A. L.; Ngidi, E. L.;
Coyanis, E. M.; de Koning, C. B. Tetrahedron Lett. 2003, 44, 311–313.
(b) For a leading reference, see: Yoshida, K.; Imamoto, T. J. Am. Chem. Soc.
2005, 127, 10470–10471.
(31) Yamamoto, Y.; Ishii, J.-i.; Nishiyama, H.; Itoh, K. J. Am. Chem.
Soc. 2004, 126, 3712–3713.
(32) For the use and valorization of a temporary ester link in a transan-
nular Diels-Alder reaction, see: Suzuki, T.; Usui, K.; Miyake, Y.;
Namikoshi, M.; Nakada, M. Org. Lett. 2004, 6, 553–556.
(33) For a very efficient example in total synthesis, see: Witulski, B.;
Zimmermann, A.; Gowans, N. D. Chem. Commun. 2002, 2984–2985.
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TABLE 1. Formation of Ring D by RCM
might acidify the reaction medium which enhances the forma-
tion of 23 by acid-catalyzed dehydration of intermediate 24.
Thus, buffering the reaction medium should diminish the for-
mation of 23 in favor of 22. Even though Grubbs’ catalyst 21 is
known to be sensitive to basic conditions,49 we undertook a
screening of weak non-nucleophilic bases. The addition of
NaHCO3 (entry 6) or Proton Sponge [1,8-bis(dimethylamino)-
naphthalene] (entry 7) led to degradation products. We even-
tually found that 2,6-lutidine (20 mol %) (entry 8)50 was mild
enough to preserve the catalytic activity, while buffering the
reaction medium efficiently, as we reproducibly isolated 22 in
54% yield and 23 in only 13%. Adding more 2,6-lutidine (entry
9) had no further positive impact. This last cyclization step
allowed us to obtain the tetracyclic framework of landomyci-
none 2with a remarkably high global yield of 19% over 15 steps.
Despite the success of the metathesis step described above,
we chose to explore the possibility of forcing the opening of
the five-membered lactol function of compound 20 prior to
RCM (Scheme 6), in order to avoid the formation of com-
pound 23. We first tried to open the lactol ring by reduction,
but the Luche conditions (NaBH4, CeCl3 7H2O, MeOH,
3
0 °C)51 or the use of LiAlH4 (in THF or TBME, from room
temperature to reflux) did not afford diol 25, and extensive
degradation of the starting material was the sole result. Deg-
radation was also observed when we tried to form dicarbonyl
26 under oxidative conditions using IBX52 or Dess-Martin
periodinane53 or under Oppenauer conditions [Al(O-i-Pr)3,
excess acetone, toluene, reflux].54 Surprisingly, further at-
tempts using MnO2 or PCC55 resulted only in the formation
of lactone 19 by formal loss of the vinyl function. It has
already been reported several times that the exposure of a lactol
to silylating conditions might allow its opening by silylation of
the masked alcohol.56 Pleasingly, in the presence of TESCl and
imidazole, lactol 20 rapidly and efficiently yielded R,β-unsatu-
rated ketone 27. This new diene 27 was, however, reluctant to
RCM, even under forced conditions (Grubbs II or Hoveyda-
Grubbs II catalysts, toluene, 80 °C, 12 h), as only a trace
amount of naphthol 28 could be detected in the reaction
mixture, along with some starting material and degradation
products. Even the addition of Ti(O-i-Pr)4, known to prevent
deleterious catalyst complexation to carbonyl groups, was
unsuccessful.48
entry
additive
resultsa
1
2
3
4
5
6
7
8
9
no additive
22 (33%) þ 23 (28%)
only 23c
no reaction
2,6-dichlorobenzoquinone (10 mol %)b only 23 (77%)
B-chlorocatecholborane (10 mol %)b
Ti(Oi-Pr)4 (1.5 equiv)b
H2O (1 equiv)b
22 (19%) þ 23 (27%)
NaHCO3 (2 equiv)
degradation
degradation
Proton Sponge (20 mol %)d
2,6-lutidine (20 mol %)
2,6-lutidine (1.1 equiv)e
22 (54%) þ 23 (13%)
22 (43%) þ 23 (25%)
aIsolated yields starting from lactone 19. b5 mol % of catalyst 21 was
used. cProduct 23 was identified in the 1H NMR of the reaction crude but
not isolated. d1,8-Bis(dimethylamino)naphthalene. e2 h.
vinyl hemiketal, such as 20, seems unprecedented. Reaction of
diene 20 with Grubbs’ second-generation catalyst 21, followed
by in situ protection of the unstable phenol as a methyl ether,
afforded a mixture of cyclized compounds 22 and 23 in an almost
1:1 ratio (entry 1).
It is very likely that both 22 and 23 derive from common
cyclized intermediate 24, the aromatization occurring either
by dehydration or by hemiketal opening followed by tauto-
merization (Table 1, scheme). The use of the Hoveyda-Grubbs
second-generation catalyst did not improve these results. We
then turned our attention toward the use of additives, which are
common in RCM reactions. The addition of 2,6-dichloroben-
zoquinone (entry 2)45 or B-chlorocatecholborane (entry 3)46 led
to unwanted 23 exclusively, in up to 77% yield,47 whereas the
presence of Ti(O-i-Pr)4 (entry 4) totally inhibited the RCM.48
We thought that the addition of 1 equiv of water would disfavor
the dehydration, but it had a negative impact on the outcome of
the reaction (entry 5). We then realized that the desired phenol
2.6. Study of the Functionalization of Ring C. The next
challenge was to cleave the benzylic alcohol function on ring
C of compound 22, which is a remnant of the tether, while
installing an oxidizable function as a quinone precursor. We
first envisioned removing this fragment using oxidative con-
ditions that would provide an oxygen atom at position C7
(Scheme 7). For that purpose, the benzylic alcohol 22 was
oxidized into the corresponding aldehyde 29 using a large
(49) Dinger, M. B.; Mol, J. C. Eur. J. Inorg. Chem. 2003, 2827–2833.
(50) A single example of the use of 2,6-di(tert-butyl)pyridine as additive in
an enyne metathesis had already been reported: Yoshida, K.; Shishikura, Y.;
Takahashi, H.; Imamoto, T. Org. Lett. 2008, 10, 2777–2780.
(51) Luche, J.-L. J. Am. Chem. Soc. 1978, 100, 2226–2227.
(52) Wirth, T. Angew. Chem., Int. Ed. 2001, 40, 2812–2814.
(53) Dess, D. B.; Martin, J. C. J. Org. Chem. 1983, 48, 4155–4156.
(54) de Graauw, C. F.; Peters, J. A.; van Bekkum, H.; Huskens, J.
Synthesis 1994, 1007–1017.
(55) Corey, E. J.; Suggs, J. W. Tetrahedron Lett. 1975, 16, 2647–2650.
(56) For selected examples in natural product synthesis, see: (a) Magnus,
P.; Giles, M.; Bonnert, R.; Kim, C. S.; McQuire, L.; Merritt, A.; Vicker, N.
J. Am. Chem. Soc. 1992, 114, 4403–4405. (b) White, J. D.; Wang, G.;
Quaranta, L. Org. Lett. 2003, 5, 4983–4986.
(45) This additive is usually employed to avoid double-bond migration in
alkene metathesis: Hong, S. H.; Sanders, D. P.; Lee, C. W.; Grubbs, R. H.
J. Am. Chem. Soc. 2005, 127, 17160–17161.
(46) Vedrenne, E.; Dupont, H.; Oualef, S.; Elkaım, L.; Grimaud, L.
¨
Synlett 2005, 670–672.
(47) This efficient preparation of compound 23 prompted us to study its a
posteriori opening, in order to convert it into a valuable intermediate.
Nonetheless, its treatment with an oxidizing reagent, or with a nucleophile
in the presence of a Lewis acid, were unsuccessful.
€
(48) Furstner, A.; Langemann, K. J. Am. Chem. Soc. 1997, 119, 9130–
9136.
8194 J. Org. Chem. Vol. 75, No. 23, 2010

Bugaut et al.
JOCArticle
SCHEME 6. Opening of the Five-Membered Ring of Lactol 20 before Alkene Metathesis
SCHEME 7. Removal of the Tether
excess of commercially available activated MnO2.57 Trials
employing Dess-Martin,53 Corey-Schmidt,58 and Swern59
conditions only afforded complex mixtures. Various Dakin
oxidation conditions to transform aldehyde 29 into phenol 30
were tried (H2O2-K2CO3, H2O2-PhSeSePh,60 m-CPBA,61
AcOOH, Oxone62), but none of them afforded the expected
product.
Searching for an alternative cleavage of this C-C bond,
we turned our attention toward Curtius rearrangement.63 To
test this option, aldehyde 29 was oxidized using Lindgren-
Kraus-Pinnick reaction conditions,64 giving the corre-
sponding carboxylic acid 31 in 62% yield. We then used
(PhO)2PON3 to transform 31 into the acyl azide 32.65
However, in the presence of triethylamine, either in DMF
or in dichloromethane, this reagent afforded complex
mixtures that did not contain acyl azide 32 but led in low
yield to the pentacyclic γ-lactone 33, along with isocyanate
34, the desired Curtius rearrangement product. The in-
stability of the intermediate 32 is unusual; its room-tem-
perature rearrangement is likely initiated by the release of
steric constraints through expulsion of a molecule of N2.
The unexpected formation of γ-lactone 33 might be due to
trapping of the neighboring acylium cation resulting from
the loss of N3- from acyl azide 32 by the OTBS group at C6
or from the direct (PhO)2PON3 activation of the carboxylic
acid function of 31. Despite the low observed yields for this
last step, we have validated our strategy, as we successfully
removed the tether used for the rhodium-catalyzed [2 þ2þ2]
cycloaddition while installing a nitrogen atom as a precursor
of the quinonic ring C. By modifying the nature of the
protective group at C6, we should obtain better yields for
this step. We envision finishing the total synthesis of land-
omycinone (2) by performing the hydrolysis of isocyanate
34 affording then aniline 35 that would deliver the targeted
quinone 36 by oxidation by Fremy’s salt (Scheme 7).66 At
the end game, the methyl protecting groups on the phenol
functions would be removed employing Lewis acids such as
(57) Purchased from Sigma-Aldrich and used shortly after opening the
container. “Older” MnO2 proved not reactive enough, whereas highly
activated MnO2 produced from KMnO4 and MnCl2 4H2O (Fatiadi, A. J.
3
Synthesis 1978, 65–104) yielded a complex mixture
(58) Corey, E. J.; Schmidt, G. Tetrahedron Lett. 1979, 20, 399–402.
(59) Omura, K.; Swern, D. Tetrahedron 1978, 34, 1651–1660.
(60) Syper, L. Synthesis 1989, 167–172.
(61) Franck, R. W.; Gupta, R. B. J. Org. Chem. 1985, 50, 4632–4635.
(62) Travis, B. R.; Sivakumar, M.; Hollist, G. O.; Borhan, B. Org. Lett.
2003, 5, 1031–1034.
67
68
BCl3 or MgI2 in the mild conditions usually used for
(63) Scriven, E. F. V.; Turnbull, K. Chem. Rev. 1988, 88, 297–368.
(64) (a) Lindgren, B. O.; Nilson, T. Acta Chem. Scand. 1973, 27, 888–890.
(b) Kraus, G. A.; Taschner, M. J. J. Org. Chem. 1980, 45, 1175–1176. (c) Bal,
B. S.; Childers, W. E., Jr.; Pinnick, H. W. Tetrahedron 1981, 37, 2091–2096.
(65) Shioiri, T.; Ninomiya, K.; Yamada, S. J. Am. Chem. Soc. 1972, 94,
6203–6205.
(66) Zimmer, H.; Lankin, D. C.; Horgan, S. W. Chem. Rev. 1971, 71,
229–246.
(67) Nagaoka, H.; Schmid, G.; Iio, H.; Kishi, Y. Tetrahedron Lett. 1981,
22, 899–902.
(68) Yamaguchi, S.; Nedachi, M.; Yokoyama, H.; Hirai, Y. Tetrahedron
Lett. 1999, 40, 7363–7365.
J. Org. Chem. Vol. 75, No. 23, 2010 8195

Bugaut et al.
JOCArticle
OMe neighboring carbonyl functions, and the TBS would
be removed under acidic conditions as already described by
Roush.10a
56.0 (CH3), 39.0 (CH2), 22.1 (CH3), 0.3 (3 ꢀ CH3); LRMS
(ESI) m/z 281 [M þ Na]þ, 313 [M þ MeOH þ Na]þ; HRMS
(ESI) m/z calcd for C16H22OSi [M þ Na]þ 281.1338, found
281.1350.
[10-(tert-Butyldimethylsilyloxy)-5-methoxy-7-methyl-9,10-di-
hydrophenanthrene-2,3-diyl]dimethanol (15). A solution of diyne
12a (54.9 mg, 0.167 mmol, 1 equiv) in degassed EtOH (3 mL)
was added to RhCl(PPh3)3 (7.7 mg, 8.4 μmol, 5 mol %) and but-
2-yne-1,4-diol 14b (71.9 mg, 0.836 mmol, 5 equiv). The reaction
mixture was refluxed for 16 h, diluted with water (10 mL), and
extracted with CH2Cl2 (3 ꢀ 10 mL). The combined organic
phases were washed with water (10 mL), dried over Na2SO4, and
filtered, and the solvents were removed under reduced pressure.
Purification by flash chromatography (heptane/EtOAc, 1:1) af-
forded tricyclic diol 15 (54.0 mg, 0.130 mmol, 78%) as a colorless
3. Conclusion
The densely functionalized and highly unsaturated tetra-
cyclic skeleton of landomycinone (2) has been efficiently
synthesized using three metal-catalyzed pivotal steps. First,
finely tuned conditions were designed to perform an efficient
Suzuki-Miyaura cross-coupling between potassium (trime-
thylsilyl)ethynyltrifluoroborate and the sterically hindered
aromatic triflate 6. Second, a remarkably efficient Rh(I)-
catalyzed [2 þ2 þ2] cycloaddition allowed the concomitant
formation of rings B and C, delivering lactone 19. This key
intermediate has been synthesized in twelve steps and in a
50% overall yield. Third, a RCM under specially designed
buffered conditions has secured the formation of aromatic
ring D. Finally we have validated our strategy, as the tether
used for the Rh(I)-catalyzed [2 þ 2 þ 2] cycloaddition was
replaced by an oxidizable function that prefigures quinonic
ring C. Further improvements and developments based on
this promising strategy are currently under investigation and
will be reported in due course.
amorphous solid: Rf 0.20 (heptane/EtOAc, 1:1); IR (neat) ν (cm-1
)
3330, 2928, 2855, 1611, 1580, 1461, 1318, 1255, 1170, 1093, 1056,
1
1006, 934, 904, 868, 835, 776; H NMR (500 MHz, CDCl3) δH
(ppm) 8.25 (s, 1H), 7.54 (s, 1H), 6.73 (s, 1H), 6.71 (s, 1H),
4.85-4.73 (m, 5H), 3.89 (s, 3H), 2.92-2.77 (m, 4H), 2.36 (s,
3H), 0.99 (s, 9H), 0.16 (s, 3H), 0.14 (s, 3H); 13C NMR (75.5
MHz, CDCl3) δC (ppm) 156.9 (Cq), 141.5 (Cq), 138.7 (Cq), 137.84
(Cq), 137.75 (Cq), 137.2 (Cq), 132.1 (Cq), 129.8 (CH), 125.3 (CH),
122.2 (CH), 119.7 (Cq), 111.6 (CH), 69.6 (CH), 64.9 (CH2), 64.8
(CH2), 55.7 (CH3), 40.0 (CH2), 26.1 (3 ꢀ CH3), 21.7 (CH3), 18.5
(Cq), -4.3 (CH3), -4.7 (CH3); LRMS (ESI) m/z 437 [M þ Na]þ,
851 [2M þ Na]þ; HRMS (ESI) m/z calcd for C24H34O4Si
[M þ Na]þ 437.2124, found 437.2115.
4. Experimental Section
4-(tert-Butyldimethylsilyloxy)-5-(2-ethynyl-3-methoxy-5-met-
hylphenyl)pent-2-ynyl 4-(tert-butyldimethylsilyloxy)hex-5-en-2-
ynoate (18). Di-tert-butyl diazodicarboxylate (463 mg, 2.01
mmol, 1.8 equiv) and PPh3 (468 mg, 1.79 mmol, 1.6 equiv) were
placed in a 50-mL flask at 0°C. THF (3 mL) was added,
affording a yellow solution that was allowed to reach room
temperature and stirred for 30 min, resulting in an off-white
suspension. The reaction medium was cooled to 0°C before
dropwise addition of a solution of carboxylic acid 17 (858 mg,
3.57 mmol, 3.2 equiv) in THF (4 mL);the reaction mixture
became clear again. Then, 15 min later, a solution of alcohol
12c (400 mg, 1.12 mmol, 1 equiv) in THF (4 mL) was added
dropwise, and the reaction mixture was stirred at 0 °C for 30 min.
This mixture was then diluted with an saturated aqueous solution
of NaHCO3 (25 mL) and extracted with EtOAc (3ꢀ25 mL). The
combined organic phases were washed with water (25 mL) and
brine (25 mL), dried over Na2SO4, and filtered, and the solvents
were removed under reduced pressure. Filtration through a short
pad of silica gel (elution with heptane/EtOAc, 4:1) followed by
purification by flash chromatography (heptane/EtOAc, 30:1 to
25:1) afforded pure ester 18 (629 mg, 1.08 mmol, 97%) as a
colorless amorphous solid: Rf 0.43 (heptane/EtOAc, 4:1); IR
(neat) ν (cm-1) 3310, 2954, 2929, 2856, 2237, 2102, 1723, 1608,
2-Allyl-6-methoxy-4-methylphenol (5). A solution of allyl
ether 4 (5.94 g, 33.3 mmol, 1 equiv) in NMP (12 mL) was
irradiated in a microwave oven (T = 220 °C, t = 15 min). The
reaction mixture was then diluted with water (75 mL) and extracted
with Et2O (3 ꢀ 75 mL). The combined organic phases were washed
with water (75 mL) and brine (75 mL), dried over Na2SO4, and
filtered, and the solvents were removed under reduced pressure.
Purification by flash chromatography (heptane/EtOAc, 20:1-
14:1) afforded phenol 5 (4.87 g, 27.3 mmol, 80%) as a pale yellow
oil. The spectroscopic and analytical data were in accordance with
those reported in the literature:14 Rf 0.14 (heptane/toluene, 1:1); IR
(neat) ν (cm-1) 3528, 3076, 3003, 2915, 2842, 1606, 1495, 1425,
1362, 1291, 1232, 1148, 1073, 907, 831; 1H NMR (500 MHz,
CDCl3) δH (ppm) 6.561 (s, 1H), 6.555 (s, 1H), 6.00 (ddt, J=16.8,
10.1, and 6.4 Hz, 1H), 5.50 (s, 1H), 5.11-5.01 (m, 2H), 3.86 (s, 3H),
3.37 (d, J = 6.4 Hz, 2H), 2.26 (s, 3H); 13C NMR (75.5 MHz,
CDCl3) δC (ppm) 146.3 (Cq), 141.2 (Cq), 137.0 (CH), 128.9 (Cq),
125.6 (Cq), 122.6 (CH), 115.5 (CH2), 109.8 (CH), 56.1 (CH3), 34.0
(CH2), 21.2 (CH3); LRMS(ESI) m/z 201 [M þ Na]þ; HRMS (ESI)
m/z calcd for C11H14O2 [M þ Na]þ 201.0891, found 201.0877.
[(2-Allyl-6-methoxy-4-methylphenyl)ethynyl]trimethylsilane
(7). Pd(OAc)2 (12.9 mg, 57.5 μmol, 3 mol %), RuPhos (53.7 mg,
0.115 mmol, 6 mol %), potassium (trimethylsilyl)ethynyltri-
fluoroborate (587 mg, 2.88 mmol, 1.5 equiv), and K3PO4 (1.63
g, 7.67 mmol, 4 equiv) were placed in a 25-mL flask equipped
with a condenser. A solution of triflate 6 (595 mg, 1.92 mmol, 1
equiv) in THF (5.7 mL) and degassed water (570 μL) were
added, and the reaction mixture was refluxed for 4 h. Filtration
on a short pad of silica gel (elution with Et2O) followed by
purification by flash chromatography (heptane/toluene, 8:2-6:4)
afforded alkyne 7 (440 mg, 1.70 mmol, 89%) as a yellow oil: Rf 0.35
(heptane/EtOAc, 10:1); IR (neat) v (cm-1) 2958, 2837, 2150, 1607,
1568, 1462, 1318, 1249, 1154, 1078, 860, 840, 759; 1H NMR (500
MHz, CDCl3) δH (ppm) 6.63 (s, 1H), 6.53 (s, 1H), 5.97 (ddd, J =
17.1, 10.1, and 6.7 Hz, 1H), 5.10 (dd, J= 17.1 and 1.2 Hz, 1H), 5.04
(dd, J = 10.1 and 1.2 Hz, 1H), 3.85 (s, 3H), 3.50 (d, J = 6.7 Hz,
2H), 2.31 (s, 3H), 0.26 (s, 9H); 13C NMR (75.5 MHz, CDCl3) δC
(ppm) 160.7 (Cq), 144.6 (Cq), 139.9 (Cq), 136.7 (CH), 122.0 (CH),
115.9 (CH2), 109.4 (CH), 109.1 (Cq), 102.4 (Cq), 100.1 (Cq),
1
1570, 1462, 1362, 1315, 1229, 1144, 1087, 941, 836, 778; H
NMR (500 MHz, CDCl3) δH (ppm) 6.71 (s, 1H,), 6.59 (s, 1H),
5.89 (ddd, J=16.8, 10.1, and 4.9 Hz, 1H), 5.44 (d, Jtrans = 16.8
Hz, 1H), 5.23 (d, Jcis =10.1 Hz, 1H), 5.00 (d, J = 4.9 Hz, 1H),
4.77 (s, 2H), 4.68 (dd, J=8.2 and 5.8 Hz, 1H), 3.88 (s, 3H), 3.49
(s, 1H), 3.14 (dd, J=12.8 and 5.8 Hz, 1H), 3.07 (dd, J = 12.8
and 8.2 Hz, 1H), 2.33 (s, 3H), 0.92 (s, 9H), 0.82 (s, 9H), 0.16
(s, 3H), 0.14 (s, 3H), -0.02 (s, 3H), -0.10 (s, 3H); 13C NMR
(75.5 MHz, CDCl3) δC (ppm) 161.0 (Cq), 152.7 (Cq), 141.5
(Cq), 139.8 (Cq), 135.8 (CH), 124.9 (CH), 116.6 (CH2), 109.9
(CH), 108.2 (Cq), 89.2 (Cq), 87.5 (Cq), 85.0 (CH), 78.7 (Cq),
77.5 (Cq), 76.3 (Cq), 63.6 (CH), 62.8 (CH), 56.1 (CH3), 53.8
(CH2), 43.5 (CH2), 25.84 (3 CH3), 25.80 (3 ꢀ CH3), 22.0 (CH3),
18.4 (Cq), 18.3 (Cq), -4.5 (CH3), -4.8 (CH3), 4.9 (CH3), -5.2
(CH3); LRMS (ESI) m/z 603 [M þ Na]þ; HRMS (ESI) m/z
calcd for C33H48O5Si2 [M þ Na]þ 603.2947, found 603.2938.
8196 J. Org. Chem. Vol. 75, No. 23, 2010

Bugaut et al.
JOCArticle
4-(tert-Butyldimethylsilyloxy)-11-(1-(tert-butyldimethylsil-
yloxy)allyl)-9-methoxy-7-methyl-4,5-dihydrophenanthro[1,2-c]-
furan-1(3H)-one (19). A solution of RhCl(PPh3)3 (30.7 mg, 33.1
μmol, 5 mol %) in CH2Cl2 (2 mL) was added to a solution of
triyne 18 (385 mg, 0.663 mmol, 1 equiv) in CH2Cl2 (5 mL). The
reaction mixture was refluxed for 24 h and then filtered through
a short pad of silica gel (elution with heptane/EtOAc, 10:1).
Purification by flash chromatography (heptane/EtOAc,
30:1-25:1) afforded a mixture of both diastereoisomers of
the tetracyclic lactone 19 (360 mg, 0.620 mmol, 93%) as a
white solid. The two diastereoisomers were separated by
0.30 (heptane/EtOAc, 4:1); IR (neat) ν (cm-1) 3500, 2928, 2855,
1612, 1575, 1461, 1360, 1304, 1251, 1171, 1113, 1003, 970, 934,
834, 776; 1H NMR (500 MHz, C6D6) δH (ppm) 9.63 (s, 1H), 6.79
(d, J=8.5 Hz, 1H), 6.66 (s, 1H), 6.59 (s, 1H), 6.36 (d, J=8.5 Hz,
1H), 5.96 (dd, J=3.1 and 2.4 Hz, 1H), 5.53 (dd, J=12.1 and 4.6
Hz, 1H), 5.44 (dd, J = 12.1 and 11.0 Hz, 1H), 3.51 (s, 3H), 3.33
(dd, J=11.0 and 4.6 Hz, 1H), 3.26 (s, 3H), 3.03 (dd, J=15.6 and
3.1 Hz, 1H), 2.87 (dd, J=15.6 and 2.4 Hz, 1H), 2.23 (s, 3H), 1.15
(s, 9H), 0.80 (s, 9H), 0.32 (s, 3H), 0.20 (s, 3H), 0.08 (s, 3H), 0.05 (s,
3H); 13C NMR (75.5 MHz, C6D6) δC (ppm) 157.7 (Cq), 151.0
(Cq), 147.0 (Cq), 138.0 (Cq), 137.1 (Cq), 135.8 (Cq), 130.5 (Cq),
129.9 (Cq), 125.1 (Cq),124.0 (CH), 123.4 (CH), 121.4 (Cq), 117.1
(Cq), 112.0 (CH), 111.8 (CH), 106.0 (CH), 65.1 (CH), 60.3 (CH2),
55.7 (CH3), 55.5 (CH3), 39.3 (CH2), 26.2 (3 ꢀ CH3), 26.0
(3 ꢀ CH3), 21.5 (CH3), 18.7 (Cq), 18.3 (Cq), -3.5 (CH3), -3.8
(CH3), -4.2 (CH3), -4.3 (CH3); LRMS (ESI) m/z 445 [M -
TBSOH - H2O þ H]þ, 617 [M þ Na]þ, 649 [MþMeOH þ Na]þ;
HRMS (ESI) m/z calcd for C34H50O5Si2 [M þ Na]þ 617.3095,
found 617.3104; mp 155-156 °C (EtOAc/heptane).
[7-Methoxy-9-methyl-11,12-dihydro-1H-tetrapheno[8,7-bc]-
furan-5,12-diyl)bis(oxy)]bis(tert-butyldimethylsilane) (23). 2,6-
Dichlorobenzoquinone (2.4 mg, 13.4 μmol, 10 mol %) was
added to a solution of diene 20 (81.5 mg, 0.134 mmol, 1 equiv)
in CH2Cl2 (3.5 mL). A solution of Grubbs II catalyst (5.7 mg,
6.7 μmol, 5 mol %) in CH2Cl2 (3 mL) was added, and the
reaction mixture was refluxed for 1 h. Filtration through a
short pad of neutralized silica gel (elution with heptane/
EtOAc, 10:1) followed by purification by flash chromatogra-
phy (neutralized silica gel; heptane/EtOAc, 50:1) afforded
cyclic ether 23 (57.7 mg, 0.103 mmol, 77%) as a pale yellow
foam: Rf 0.57 (heptane/EtOAc, 4:1); IR (neat) ν (cm-1) 2953,
2929, 2856, 1610, 1574, 1474, 1428, 1371, 1316, 1255, 1102,
1042, 1007, 979, 912, 856, 837, 777, 702; 1H NMR (500 MHz,
C6D6) δH (ppm) 9.28 (s, 1H), 6.94 (d, J=7.6 Hz, 1H), 6.78 (d,
J=7.6 Hz, 1H), 6.72 (s, 1H), 6.69 (s, 1H), 6.26 (d, J=15.0 Hz,
1H), 5.96 (d, J = 15.0 Hz, 1H), 4.98 (dd, J = 11.6 and 4.0 Hz,
1H), 3.67 (s, 3H), 3.53 (dd, J=13.7 and 11.6 Hz, 1H), 2.93 (dd,
J=13.7 and 4.0 Hz, 1H), 2.30 (s, 3H), 1.30 (s, 9H), 1.01 (s, 9H),
0.41 (s, 3H), 0.34 (s, 3H), 0.12 (s, 3H), 0.05 (s, 3H); 13C NMR
(75.5 MHz, C6D6) δC (ppm) 157.6 (Cq), 156.9 (Cq), 143.7 (Cq),
138.3 (Cq), 137.4 (Cq), 133.7 (Cq), 132.7 (Cq), 131.5 (Cq),
130.0 (Cq), 125.9 (Cq), 122.3 (CH), 122.0 (Cq), 121.0 (CH),
116.1 (CH), 112.2 (CH), 100.0 (CH), 79.0 (CH2), 71.3 (CH),
55.4 (CH3), 41.2 (CH2), 26.24 (3 ꢀ CH3), 26.19 (3 ꢀ CH3), 21.5
(CH3), 18.7 (Cq), 18.3 (Cq), -4.0 (CH3), -4.1 (CH3), -4.3
(CH3), -4.7 (CH3); LRMS (ESI) m/z 585 [M þ Na]þ; HRMS
(ESI) m/z calcd for C33H46O4Si2 [M þ Na]þ 585.2832, found
585.2847.
preparative HPLC (L 16 mm; 12 mL min-1; heptane/EtOAc,
3
30:1) affording two samples from a 20 mg aliquot. Diastereoi-
somer A (9.0 mg) was obtained as a white solid and diastereoi-
somer B (11.4 mg) as a colorless amorphous solid. Their absolute
configuration was not determined. Diastereoisomer A: Rf 0.65
(heptane/EtOAc, 10:1, 2 elutions); IR (neat) ν (cm-1) 2953,
2928, 2855, 1751, 1606, 1593, 1462, 1348, 1316, 1253, 1210,
1095, 1053, 1027, 1004, 912, 853, 834, 777, 681; 1H NMR (500
MHz, CDCl3) δH (ppm) 8.57 (s, 1H), 6.75 (s, 1H), 6.69 (s, 1H),
6.49 (d, J=5.2 Hz, 1H), 5.93 (ddd, J=17.1, 10.4, and 5.2 Hz,
1H), 5.66 (d, J = 15.9 Hz, 1H), 5.40 (d, Jtrans = 17.1 Hz, 1H),
5.37 (d, J=15.9 Hz, 1H), 4.98 (m, 2H), 3.89 (s, 3H), 2.89 (dd,
J=14.0 and 11.9 Hz, 1H), 2.82 (dd, J=14.0 and 4.6 Hz, 1H),
2.37 (s, 3H), 0.97 (s, 18H), 0.20 (s, 3H), 0.17 (s, 3H), 0.14 (s,
3H), 0.10 (s, 3H); 13C NMR (75.5 MHz, CDCl3) δC (ppm)
171.0 (Cq), 157.4 (Cq), 143.6 (Cq), 142.2 (Cq), 141.2 (CH),
140.2 (Cq), 138.0 (Cq), 137.3 (Cq), 133.7 (Cq), 127.0 (CH),
121.8 (CH), 119.6 (Cq), 119.2 (Cq), 113.3 (CH2), 111.9 (CH),
70.9 (CH2), 70.8 (CH), 69.0 (CH), 55.8 (CH3), 40.2 (CH2),
26.2 (3 ꢀ CH3), 26.1 (3 ꢀ CH3), 21.8 (CH3), 18.6 (Cq),
18.3 (Cq), -3.7 (CH3), -4.4 (CH3), -4.5 (CH3), -4.7 (CH3);
LRMS (ESI) m/z 603 [M þ Na]þ; HRMS (ESI) m/z calcd for
C
33H48O5Si2 [M þ Na]þ 603.2938, found 603.2935. Diaster-
eoisomer B: Rf 0.58 (heptane/EtOAc, 10:1, 2 elutions); IR
(neat) ν (cm-1) 2953, 2928, 2856, 1753, 1606, 1594, 1461, 1348,
1317, 1253, 1211, 1096, 1055, 1029, 1004, 913, 855, 835, 777,
680; 1H NMR (500 MHz, CDCl3) δH (ppm) 8.67 (s, 1H), 6.76
(s, 1H), 6.71 (s, 1H), 6.37 (d, J = 5.2 Hz, 1H), 6.06 (ddd, J =
17.1, 10.4, and 5.2 Hz, 1H), 5.65 (d, J=15.6 Hz, 1H), 5.47 (d,
Jtrans =17.1 Hz, 1H), 5.38 (d, J=15.6 Hz, 1H), 5.04 (d, Jcis
=
10.4 Hz, 1H), 4.96 (dd, J=11.9 and 4.6 Hz, 1H), 3.89 (s, 3H),
2.95 (dd, J=14.3 and 11.9 Hz, 1H), 2.85 (dd, J=14.3 and 4.6
Hz, 1H), 2.38 (s, 3H), 0.96 (s, 9H), 0.92 (s, 9H), 0.18 (s, 3H),
0.14 (s, 3H), 0.10 (s, 3H), -0.02 (s, 3H); 13C NMR (75.5 MHz,
CDCl3) δC (ppm) 171.0 (Cq), 157.4 (Cq), 143.5 (Cq), 142.3
(Cq), 141.2 (CH), 140.2 (Cq), 138.0 (Cq), 137.3 (Cq), 133.4
(Cq), 126.9 (CH), 122.0 (CH), 119.7 (Cq), 119.2 (Cq), 113.2
(CH2), 111.9 (CH), 70.9 (CH2), 70.5 (CH), 69.1 (CH), 55.7
(CH3), 40.1 (CH2), 26.2 (3 CH3), 26.0 (3 CH3), 21.8 (CH3), 18.4
(Cq), 18.3 (Cq), -3.8 (CH3), -4.4 (CH3), -4.6 (CH3), -4.7
(CH3); LRMS (ESI) m/z 603 [M þ Na]þ; HRMS (ESI) m/z
calcd for C33H48O5Si2 [M þ Na]þ 603.2938, found 603.2927.
[6,11-Bis(tert-butyldimethylsilyloxy)-1,8-dimethoxy-3-methyl-
5,6-dihydrotetraphen-7-yl]methanol (22). 2,6-Lutidine (103 μL of
a freshly prepared 1.0 M solution in CH2Cl2, 0.103 mmol, 20
mol %) was added to a solution of diene 20 (314 mg, 0.516
mmol, 1 equiv) in CH2Cl2 (10 mL). A solution of Grubbs II
catalyst (13.2 mg, 15.5 μmol, 3 mol %) in CH2Cl2 (5 mL) was
added, and the reaction mixture was refluxed for 1 h. K2CO3
(357 mg, 2.58 mmol, 5 equiv), acetone (10 mL), and Me2SO4
(98 μL, 1.03 mmol, 2 equiv) were successively added, and the
reflux was maintained during 10 h. Filtration through a short
pad of neutralized silica gel (elution with heptane/EtOAc, 4:1)
followed by purification by flash chromatography (neutralized
silica gel; heptane/EtOAc, 50:1-4:1) afforded benzylic alcohol
22 (167 mg, 0.281 mmol, 54%) as a light brown solid. Cyclic ether
23 (37.8 mg, 67.2 μmol, 13%) was also isolated as a byproduct: Rf
6,11-Bis(tert-butyldimethylsilyloxy)-7-isocyanato-1,8-dimethoxy-
3-methyl-5,6-dihydrotetraphene (34). Et3N (15 μL, 0.104 mmol, 3
equiv) and (PhO)2PON3 (16 μL, 69.2 μmol, 2 equiv) were
successively added to a solution of carboxylic acid 31 (24.8 mg,
34.6 μmol, 1 equiv) in CH2Cl2 (700 μL), and the reaction mixture
was stirred at room temperature for 1 h. Filtration on neutralized
silica (elution with CH2Cl2) and evaporation of the solvent at
room temperature, followed by purification by flash chromatog-
raphy (neutralized silica; Pent/Et2O, 20:1-10:1) afforded isocya-
nate 34 (4.0 mg, 6.6 μmol, 19%) as a colorless amorphous solid.
Lactone 33 was also isolated from the reaction mixture: Rf 0.54
(heptane/EtOAc, 4:1); IR (neat) ν (cm-1) 2928, 2855, 2284
(NdCdO), 1611, 1593, 1461, 1333, 1316, 1288, 1259, 1104,
1
1070, 970, 933, 901, 835, 777; H NMR (500 MHz, C6D6) δH
(ppm) 9.46 (s, 1H), 6.72 (d, J=8.2 Hz, 1H), 6.70 (s, 1H), 6.55 (s,
1H), 6.29 (d, J=8.2 Hz, 1H), 5.67 (dd, J=2.7 and 2.1 Hz, 1H),
3.54 (s, 3H), 3.49 (s, 3H), 2.96 (dd, J = 15.3 and 2.7 Hz, 1H),
2.74 (dd, J = 15.3 and 2.1 Hz, 1H), 2.20 (s, 3H), 1.13 (s, 9H),
0.79 (s, 9H), 0.29 (s, 3H), 0.19 (s, 3H), 0.18 (s, 3H), 0.02 (s, 3H);
13C NMR (125.8 MHz, C6D6) δC (ppm) 157.7 (Cq), 150.8 (Cq),
J. Org. Chem. Vol. 75, No. 23, 2010 8197

Bugaut et al.
JOCArticle
146.1 (Cq), 138.5 (Cq), 137.3 (Cq), 132.8 (Cq), 130.8 (Cq),
130.4 (Cq), 125.8 (Cq), 125.2 (Cq), 123.3 (CH), 121.6 (CH),
120.2 (Cq), 113.0 (CH), 111.6 (CH), 105.4 (CH), 64.6 (CH),
55.5 (CH3), 55.1 (CH3), 39.3 (CH2), 26.1 (3 ꢀ CH3), 25.9 (3 ꢀ
CH3), 21.6 (CH3), 18.6 (Cq), 18.4 (Cq), -3.9 (CH3), -4.2 (2 *
CH3), -4.5 (CH3); one carbon signal was not visualized (prob-
ably hidden by the solvent signal); LRMS (ESI) m/z 628
[M þ Na]þ; HRMS (ESI) m/z calcd for C34H47NO5Si2 [MþNa]þ
628.2891, found 628.2895.
Acknowledgment. This work has been financially sup-
ported by the Institut de Chimie des Substances Naturelles
(ICSN) and the Centre National pour la Recherche Scienti-
fique (CNRS).
Supporting Information Available: Experimental proce-
dures and characterization data for all new compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
8198 J. Org. Chem. Vol. 75, No. 23, 2010