Tether-controlled cycloadditions for the asymmetric synthesis of decalins: increased selectivity in acetonitrile solvent.

Article abstract of DOI:10.1021/ol0062042

[reaction: see text] The beneficial influence of cis-isopropylidene acetal tether control groups, to facilitate the asymmetric synthesis of substituted decalins by intramolecular Diels-Alder reactions, is described. Compared to trans-acetonides, these cases proceed under milder conditions to afford the cis-fused adducts from an endo transition state. An unusual acetonitrile solvent effect exerts a dramatic influence on the diastereoselectivity. This strategy leads to the chiral nonracemic bicyclo[4.4.0]decane core of diverse natural products.

Full text of DOI:10.1021/ol0062042

ORGANIC
LETTERS
2000
Vol. 2, No. 18
2793-2796
Tether-Controlled Cycloadditions for the
Asymmetric Synthesis of Decalins:
Increased Selectivity in Acetonitrile
Solvent
Alex Melekhov,† Pat Forgione, Ste´phanie Legoupy,‡ and Alex G. Fallis*
Department of Chemistry, UniVersity of Ottawa, 10 Marie Curie, Ottawa,
Ontario, Canada K1N 6N5
afallis@science.uottawa.ca
Received June 14, 2000
ABSTRACT
The beneficial influence of cis-isopropylidene acetal tether control groups, to facilitate the asymmetric synthesis of substituted decalins by
intramolecular Diels−Alder reactions, is described. Compared to trans-acetonides, these cases proceed under milder conditions to afford the
cis-fused adducts from an endo transition state. An unusual acetonitrile solvent effect exerts a dramatic influence on the diastereoselectivity.
This strategy leads to the chiral nonracemic bicyclo[4.4.0]decane core of diverse natural products.
A large number of natural products contain a decalin
(bicyclo[4.4.0]decane) ring system as an important structural
component.1 These compounds, particularly the terpenoids,
display a surprisingly wide array of biological activities that
may have medicinal potential. Figure 1 illustrates two
tory activity with potential for the treatment of arthritis, while
phytocassane E (2)3 is an antifungal agent.
Motivated in part by our interest in the synthesis of
taxoids,4 we have established that planar “tether control
groups” (aromatic rings, cis double bonds) in the side chain
have a dramatic influence on the ease of cyclization of
substituted trienes in intramolecular Diels-Alder reactions.5
Subsequent research has demonstrated that tartrate and
carbohydrate derived trans-isopropylidene acetals provide a
significant improvement.6 They limit the flexibility of the
side chain to impose a restricted geometry on the reactive
(1) Merritt, A. T.; Ley, S. V. Nat. Prod. Rep. 1992, 243.
(2) Isolation: (a) Kaise, H.; Shinohara, M.; Miyazaki, W.; Izawa, T.;
Nakano, Y.; Sugawara, M.; Sugiura, K.; Sasaki, K. J. Chem. Soc., Chem.
Commun. 1976, 726. Syntheses: (b) Corey, E.; J.; Das, J. J. Am. Chem.
Soc. 1982, 104, 5551. (c) McMurry, J.; E.; Erion, M. D. J. Am. Chem. Soc.
1985, 107, 2712. (c) Mori, K. Komatsu, M. Justus Liebigs Ann. 1988, 107.
(3) Koga, J.; Ogawa, N.; Yamauchi, T.; Kikuchi, M.; Ogasawara, N.;
Shimura, M. Phytochem. 1997, 44, 249.
(4) (a) Fallis, A. G. Acc. Chem. Res. 1999, 32, 464. (b) Fallis, A. G.
Can. J. Chem. 1999, 77, 159. (c) Fallis, A. G. Pure Appl. Chem. 1997, 69,
495.
(5) Millan, S.; Pham, T.; Lavers, J.; Fallis, A. G. Tetrahedron Lett. 1997,
38, 795.
Figure 1. Oxygenated terpenoids.
representative oxygenated examples. The fungal metabolite
K-76 (1)2 displays both anticomplement and antiinflamma-
† Present address: Albany Mocecular Research Inc., 21 Corporate Circle,
NY 12203.
‡ Present address: Laboratoire de Synthese Organique, ESA 6011,
Universite du Maine, Avenue Olivier Messiaen, 72085, Le Mans, Cedex
9, France.
(6) Wong, T.; Wilson, P. D.; Woo, S.; Fallis, A. G. Tetrahedron Lett.
1997, 38, 7045.
10.1021/ol0062042 CCC: $19.00 © 2000 American Chemical Society
Published on Web 08/18/2000

components and afford chiral nonracemic adducts. In addi-
tion, they are usually available in both optical series. We
wish to report that cis-isopropylidene acetals are even better
for inducing the overlap required for facile cyclization. There
is a significant rate increase which will permit lower reaction
temperatures. In addition, it should be possible to employ
sterically encumbered dienes to generate the core nuclei of
1 and 2. These advantageous features are illustrated in
Scheme 1 with highly substituted, closely related, acetal
Scheme 2. Synthesis of Triene Precursors
Scheme 1. Comparison of cis and trans Acetal Cycloadditions
isomers. Ketone 3 cyclized to the decalin 4 at 0 °C without
the aid of Lewis acid, while the trans isomer 6 required
heating in refluxing toluene for 17 h. Thus, epimerization
of an acetal center improved the cycloaddition reaction and
led in both cases to the cis-decalin adduct. The fact that the
cis tether reaction is faster is consistent with the fact that
the transition state geometry resembles that of the cis-fused
bicyclo[4.3.0]decene, which is less strained than the trans-
acetal-trans-fused isomer.
It is usually difficult to conduct cycloadditions with an
“inside” substituent which inhibits the formation of the
required s-cis-diene conformer.7 However, the application
of this strategy to the synthesis of terpenoid skeletons requires
this inside methyl group on the diene.
To investigate the potential of a cis-acetonide unit to
circumvent this difficulty, the diol 7 was synthesized
(Scheme 2) from D-isoascorbic acid.8 Selective protection
of the primary alcohol with pivaloyl chloride was followed
by exposure to triisopropylsilyl triflate in the presence of
collidine at 0 °C to yield 8 (92%) (Scheme 2). Reduction
with diisobutylaluminum hydride (DIBAL-H) gave the
primary alcohol, and oxidation with Dess-Martin periodi-
nane afforded aldehyde 9. The requisite diene unit was
derived from the carbometalation of 2-butynol (10) with
vinylmagnesium chloride to generate magnesium chelate 11,
(7) (a) Carruthers, W. Cycloaddition Reactions in Organic Synthesis;
Pergamon: Oxford, 1990. (b) Oppolzer, W. In ComprehensiVe Organic
Synthesis; Trost, B. M., Fleming, I., Paquette, L. A., Eds.; Pergamon:
Oxford, 1991; Vol. 5, p 315. Weinreb, S. M., ref 7a, p 401. Boyes, D. L.,
ref 7a, p 451. Roush, W. R., ref 7a, p 513. Granum, K. A.; Merkel, G.;
Mulder, J. A.; Debbins, S. A.; Hsung, R. P. Tetrahedron Lett. 1998, 39,
9597.
(8) Cohen, N,; Banner, B. L.; Loprest, R. J.; Wong, F.; Rosenberger,
M.; Liu, Y.-Y.; Thom, E.; Liebman, A. A. J. Am. Chem. Soc. 1983, 105,
3661. (b) Pearson, W. H.; Lovering, F. E. J. Am. Chem. Soc. 1995, 117,
12336. (c) Pearson, W. H.; Hembre, E. J. J. Org. Chem. 1996, 61, 7217.
which was condensed with aldehyde 9.9 The stereochemistry
of secondary alcohol 13 arises from chelation-controlled
attack on the carbonyl face anti to the acetonide so the two
adjacent oxygen substituents bear a syn relationship.
(9) (a) Forgione, P.; Fallis, A. G. Tetrahedron. Lett. 2000, 41, 11. (b)
Forgione, P.; Wilson, P. D.; Fallis, A. G. Tetrahedron. Lett. 2000, 41, 17.
2794
Org. Lett., Vol. 2, No. 18, 2000

To obtain a preliminary indication of the cumulative effects
of the cis-acetate and the methyl substituent on the diene,
the hydroxyl groups in 13 were reacted simultaneously with
chloromethyl methyl ether in the presence of diethylisopro-
pylamine to provide the bis-methoxymethyloxy ether 14. The
silyl group was removed with tetrabutylammonium fluoride
to afford the allylic alcohol 16. Oxidation of this secondary
alcohol 16 with Dess-Martin periodinane in dichloro-
methane generated decatriene ketone 17 in situ, which
cyclized spontaneously to 23 at room temperature (21 °C)
over 60 h or more efficiently in 4 h at 40 °C during the
oxidation reaction (87%) (Scheme 3). The mild conditions
Table 1. Solvent Effects: Cycloaddition Diastereoselectivitiy
Ratios
Scheme 3. Cycloadditions of Substituted Dienes
However, the most intriguing result is reflected in the data
for entry 5, in which the ratio of 24/27 increased to 23:1
with acetonitrile as the solvent at 40 °C. In comparison, entry
2 in chloroform under similar conditions afforded a 5.3:1
ratio. Generally, the rates and stereoselectivities of Diels-
Alder reactions are immune to significant solvent effects.12
Experiments conducted in water, in which substantial rate
enhancements are observed, are an exception.13 Normally
the diastereofacial selectivity is not affected in aqueous
conditions unless the components are partially water-
soluble.14 In these examples the adduct ratios are influenced
by the hydrogen-bond donating ability of the solvent.
It seems likely that this “acetonitrile effect” is a conse-
quence of the preferred association of the solvent dipole with
the oxygen atoms on the top face of the molecule. This
interaction may resemble the type of charge-transfer and
π-π interactions encountered in aromatic systems.
If hydrogen bonding is important, then a free hydroxyl
on the underside of the molecule should reduce the diaster-
eomeric ratio. To examine this possibility and provide an
additional, differentially functionalized adduct, alcohol 15
was treated with tetrabutylammonium fluoride and oxidized
with manganese dioxide (Scheme 2). The expected ketone
20 was not isolated but rather hemiacetal 19 was formed
preferentially. In refluxing chloroform the lactol and keto
forms are in equilibrium and the cis diastereomer 25 was
generated in a ratio of 5:1. This value increased to 11:1 in
for this cycloaddition of a sterically encumbered system are
particularly expedient.
For natural product syntheses, differentiation of the
hydroxyl groups is required. Consequently iododiene 1210
was prepared from 10 via 11, and the derived lithium salt
condensed with 9 to afford alcohol 15 in which the
R-hydroxy diastereomer predominated.11
Protection of the secondary alcohol as its acetate, removal
of the silyl ether, and Dess-Martin periodinane oxidation
afforded the highly substituted cycloaddition precursor 18.
In this instance the ketone could be isolated, but as
summarized in Table 1 (entry 1) at room temperature (21
°C) it cyclized slowly over 2 weeks to the decalin. Treatment
of the dienes with Lewis acid did not facilitate cycloaddition
but led to decomposition. As expected, higher temperatures
reduced the level of selectivity.
(12) (a) Sauer, J.; Sustmann, R. Angew. Chem., Int. Ed. Engl. 1980, 19,
779. (b) Reichardt, C. SolVents and SolVent Effects in Organic Chemistry;
VCH: Cambridge, 1990.
(13) (a) Rideout, D.; C.; Breslow, R. J. Am. Chem. Soc. 1980, 103, 7817.
(b) Engberts, J. B. F. N. Pure Appl. Chem. 1995, 67, 823. (c) Meijer, A.;
Sijbreu, O.; Engberts, J. B. F. N. J. Org. Chem. 1998, 63, 8989.
(14) Lubineau, A.; Auge´, J.; Lubin, N. J. Chem. Soc., Perkin Trans. 1
1990, 3011. (b) Cativiela, C.; Garcia, J. I.; Mayoral, J. A.; Royo, A. J.;
Salvatella, L. J. Phys. Org. Chem. 1992, 5, 230.
(10) Wong, T.; Tjepkema, M. W.; Audrain, H.; Wilson, P. D.; Fallis, A.
G. Tetrahedron Lett. 1996, 37, 755.
(11) Tjepkema, M. W.; Wilson, P. D.; Audrain, H.; Fallis, A. G. Can. J.
Chem. 1997, 75, 1543.
Org. Lett., Vol. 2, No. 18, 2000
2795

acetonitrile at 70 °C (95% yield). This implies that the nitrile
is not hydrogen bonded to the free hydroxyl. Instead, in these
cases the acetonitrile solvent influences the facial preference
and improves the stereoselectivity of these cyclizations.15 It
is possible that this phenomenon may be observed in other
oxygen-rich systems in the presence of acetonitrile.
Finally, additional evidence, which demonstrated the
distinct synthetic advantage of cis compared to trans ac-
etonides as control elements in intramolecular Diels-Alder
precursors, was provided by 22 (Scheme 3). Compound 17
cyclized at 21 °C (above) and yet 22, its C5 acetal epimer,
required 24 h in a sealed tube at 220 °C to generate the
corresponding adduct 26 in 72% yield.
In conclusion, we have established the advantage of cis-
acetonides as tether control groups for the synthesis of
decalins from highly substituted decadienones. Epimerization
studies of these adducts, to prepare the requisite trans isomers
for selected targets, are in progress. This protocol is
particularly useful for difficult substitution patterns. The
stereoselectivity is enhanced when acetonitrile is the solvent,
as a consequence of its association with the oxygen-rich keto-
acetal dienophile. This strategy will afford rapid entry to
highly oxygenated multicyclic skeletons.
There are four possible transition state geometries for these
decatrienones; Figure 2 illustrates the two most important
Figure 2. Cycloaddition endo transition state conformations.
Acknowledgment. We are grateful to the Natural Sci-
ences and Engineering Research Council of Canada for
financial support of this research. P.F. thanks Torcan
Chemical Ltd. and the Government of Ontario for an OGSST
Fellowship. We thank the referees for their suggestions.
endo conformations. Previous investigations have established
that cis-fused boatlike transition states are usually favored
in related systems.16 As illustrated, the boatlike orientation
is clearly preferred compared to the competing chairlike
conformation, due to the minimization of nonbonded interac-
tions in the boatlike arrangement.
Supporting Information Available: Experimental pro-
cedures and spectroscopic data for compounds 3-6, 17-
18, and 22-26. This material is available free of charge via
the Internet at http://pubs.acs.org.
(15) Preparation of a diene which lacked this hydroxyl was also of interest
for comparison with the examples in Scheme 2 and because it is absent in
several skeletons of interest. The tosylate and the epoxide below were
synthesized independently from the alcohol 7, but various organometallic-
based coupling procedures failed.
OL0062042
(16) For an excellent discussion of related intramolecular transition states,
see: (a) Coe, J. W.; Roush, W. R. J. Org. Chem. 1989, 54, 915. (b) Roush,
W. R.; Koyama, K.; Curtin, M. L.; Moriarty, K. J. J. Am. Chem. Soc. 1996,
118, 7502. (c) Roush, W. R. Stereochemical and Synthetic Studies of the
Intramolecular Diels-Alder Reaction. AdV. Cycloaddition 1990, 2, 91. For
recent, related examples of decalin lactones, see: (d) Kim, P.; Nantz, M.
H.; Kurth, M. J.; Olmstead, M. M. Org. Lett. 2000, 2, 1831. (e) Jung, M.
E.; Huang, A.; Johnson, T. W. Org. Lett. 2000, 2, 1835.
2796
Org. Lett., Vol. 2, No. 18, 2000