4496
J . Org. Chem. 1996, 61, 4496-4497
Sch em e 1
Syn th esis of Tr icyclic F u r a n s by a Novel
Ta n d em Rea ction : In tr a m olecu la r [3 + 2]-
Heter o-An n u la tion of Allylsila n es w ith
Cyclic 1,3-Dik eton es1
Dieter Schinzer* and Gerhard Panke
Institut fu¨r Organische Chemie der Technischen
Universita¨t Braunschweig, Hagenring 30,
D-38106 Braunschweig, Germany
Received December 14, 1995
Silicon-containing compounds have found widespread
application in natural product synthesis.2 In this paper,
we wish to report a novel tandem reaction utilizing a
silicon-terminated cyclization of allylsilanes with cyclic
1,3-diketones.3 A tricyclic furan system is formed in a
stereoselective way probably via an intramolecular [3 +
2]-hetero-annulation reaction.4 Compounds of type 3 are
useful precursors for D-E-F subunits of the complex
triterpenes of the hopane type (Scheme 1).5
Triterpenoid 1 of the hopane type has been isolated
from leaflets of Cyathea spinulosa (“hego” in J apanese),
a tall evergreen tree fern (more than 4 m high) with large
leaves (2 m long), distributed in the subtropical and
tropical areas of J apan, Taiwan, the Philippines, China,
Burma, India, and Nepal.6
Sch em e 2
As part of an ongoing program dealing with diastereo-
selective cyclizations using allyl- and propargylsilanes,
we are interested in developing a new annulation strat-
egy for heterocycles.7-9 1,3-Diketones of types 10, 11, 12,
and 14 can be synthesized by simple alkylation reactions
of 6 and the requisite silanes 7, 8, and 9 in DMF as
solvent and with NaH as base (Scheme 2).10
1,3-Diketones of type 10, 11, 12, and 14 are useful
precursors for cyclization studies since they are prochiral,
which can be used for asymmetric cyclizations.11
The following cyclizations were carried out using Et2-
AlCl as the Lewis acid catalyst. As shown in Scheme
3,12 compound 10 (SiR3 ) TMS, R1 ) H) gave hydrin-
danones 15 (R1 ) H, R2 ) H) and 16 (R1 ) H, R2 ) H) in
a ratio of 3:1 (47% yield). Compound 11 generated 17 in
57% yield. In addition, compound 18 was obtained in
12% yield and subsequently transformed into 17 by the
use of TBAF. Finally, compounds 12 and 14 cyclized to
form 19 and 20 in 71% and 74% yield.
13
14
Another noteworthy aspect is the use of branched
allylsilane side chains (R1) Me). To our surprise, in
addition to the expected bicyclic cyclization product 16
(47%, R1 ) Me, R2 ) H), a second tricyclic reaction
product 21 (SiR3 ) TMS, R1 ) Me) was isolated in 18%
yield using Et2AlCl as the Lewis acid promoter. Both
compounds were isolated as single diastereomers.12 The
product ratio was dependent on the Lewis acid catalyst
and the reaction conditions used. By employment of Me3-
Al the product ratio was changed, and 35% of 21 (SiR3 )
TMS, R1 ) Me) and 34% of 15 (R1 ) Me, R2 ) TMS) and
16 (R1 ) Me, R2) TMS)13 as a ratio of 1:4 were isolated
(Scheme 4).
* Author to whom correspondence should be addressed.
(1) Dedicated to Clayton H. Heathcock on the occasion of his 60th
birthday.
(2) Langkopf, E.; Schinzer, D. Chem. Rev. 1995, 95, 1375.
(3) Molander, G. A.; Andrews, S. W. Tetrahedron 1988, 44, 3869.
(4) Masse, C. E.; Panek, J . S. Chem. Rev. 1995, 95, 1293.
(5) Ageta, H.; Arai, Y. Phytochemistry 1983, 22, 1801.
(6) Arai, Y.; Koide, N.; Ohki, F.; Ageta, H.; Yang, L.-L.; Yen, K.-Y.
Chem. Pharm. Bull. 1994, 42, 228.
(7) Schinzer, D. Synthesis 1988, 263.
(8) Schinzer, D.; Bo, Y. Angew. Chem. 1991, 103, 727; Angew. Chem.,
Int. Ed. Engl. 1991, 30, 687.
In order to improve the yield of tricyclic compounds
21, the size of the silyl group should be increased.14
Accordingly, we decided to synthesize several bulky
(9) Schinzer, D.; Langkopf, E. Synlett 1994, 375.
(10) (a) Dauben, W. G.; Hart, D. J . J . Org. Chem. 1977, 42, 3787.
(b) Schore, N. E.; Najdi, S. D. J . Org. Chem. 1987, 52, 5296.
(11) (a) Panke, G. Ph.D. Thesis, Technical University of Braunsch-
weig, 1996. (b) Selectivities in Lewis Acid Promoted Reactions; Schinzer,
D., Ed.; Kluwer: Dordrecht, 1989.
(12) All new compounds gave spectral and analytical spectrometric
data consistent with the assigned structures. Compounds 19 and 22
have also been synthesized by an independent route by Mori: Ki-
noshita, A.; Mori, M. Chem. Lett. 1994, 8, 1475.
(13) J ones, P. G. Unpublished results.
(14) (a) Kno¨lker, H.-J .; Foitzik, N.; Goesmann, H.; Graf, R. Angew.
Chem., Int. Ed. Engl. 1993, 32, 1081. (b) Kno¨lker, H.-J .; Graf, R.
Tetrahedron Lett. 1993, 34, 4765. (c) Kno¨lker, H.-J .; Foitzik, N.; Graf,
R.; Pannek, J .-B.; J ones, P. G. Tetrahedron 1993, 49, 9955. (d) Kno¨lker,
H.-J .; Graf, R. Synlett 1994, 131.
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