Fully stereocontrolled total syntheses of (-)-cylindricine C and (-)-2-epicylindricine C: A departure in sulfonamide chemistry

Article abstract of DOI:10.1002/anie.200460178

A common intermediate was used in the total syntheses of (-)-cylindricine C and (-)-2-epicylindricine C (see picture) in a process that involves the initial oxidative spirocyclization of a phenolic sulfonamide. The remainder of the molecules is constructe

Full text of DOI:10.1002/anie.200460178

Communications
hitherto problematic transformation may be carried out
through the oxidation of sulfonamide derivatives of primary
amines with PhI(OAc)2 ((diacetoxyiodo)benzene, DIB) in
excellent yield.[1] Further studies have revealed that the
sulfonamide unit may function not only as a modulator of the
reactivity of the primary amino group to allowan otherwise
“impossible” transformation, but also (and especially so) as a
useful implement for the subsequent elaboration of spiro-
cycles 2 (Z = SO2R) into more-complex synthetic goals.
Herein, we describe the fully stereocontrolled total syntheses
of (ꢀ)-cylindricine C (3) and its stereoisomer (ꢀ)-2-epicylin-
dricine C (4), which has not yet been observed as a natural
product, from a common precursor 14, to illustrate some
possibilities inherent to the combination of oxidative spiro-
cyclization technology with such sulfonamide-based trans-
formations.
Natural Products Synthesis
Fully Stereocontrolled Total Syntheses of
(ꢀ)-Cylindricine C and (ꢀ)-2-Epicylindricine C:
A Departure in Sulfonamide Chemistry**
Cylindricines are structurally unique alkaloids produced
by the ascidian, Clavelina cylindrica.[2] Their unusual archi-
tecture and (moderate) cytotoxic activity have elicited sub-
stantial interest in the synthetic arena.[3,4] Research in this
domain has shown that the series of 2-epi derivatives may
result from a Michael-[5] or Mannich-type[3e] cyclization of
suitable precursors. A synthesis capable of providing either 3
or 4 from a common intermediate, such as 14 in the present
case, is clearly not subject to any stereochemical uncertainty.
Commercial d-homotyrosine[6] was elaborated into the
sulfonamide 5 in a conventional fashion.[7] Subsequent
oxidation of 5 by DIB in hexafluoro-2-propanol induced
cyclization into 6 in excellent yield. The primary OH group
Sylvain Canesi, Denis Bouchu, and Marco A. Ciufolini*
The oxidative spirocyclization of phenolic primary amines
(1!2; Z = H) holds considerable potential in the chemical
syntheses of spirocyclic natural products and pharmaceuti-
cally interesting molecules. We recently disclosed that this
was then protected as
a bulky tert-butyldiphenylsilyl
(TBDPS) ether prior to the desymmetrization of the locally
symmetrical dienone in 7 through the regioselective base-
promoted 1,4-addition of the anion of the sulfonamide group.
It is apparent that the methyl group of the mesylamide is
poised to become the carbonyl group of 3 and 4. Conversion
of 7 into 8 occurred with satisfactory diastereoselectivity
(d.r. = 7:1) upon treatment of 7 with KHMDS at ꢀ1008C.[8]
The two regioisomers thus produced were not separable at
this point, but the minor isomer was readily removed at the
stage of compound 10, which resulted upon treatment of 8
with PhSH and BF3·OEt2 to form 9 diastereoselectively,[9]
followed by the desulfurization of the latter with Raney Ni[10]
(Scheme 1).
[*] S. Canesi, Dr. D. Bouchu, Prof. Dr. M. A. Ciufolini
Laboratoire de Synth›se et MØthodologie Organiques, CNRS UMR
5181
UniversitØ Claude Bernard Lyon 1 and Ecole SupØrieure de Chimie
Physique Electronique de Lyon
43, Bd. du 11 Novembre 1918, 69622 Villeurbanne cedex (France)
Fax (+33)472-432-963
E-mail: ciufi@cpe.fr
[**] We thank the MENRT (doctoral fellowship to S.C.), the CNRS, and
the RØgion Rhône-Alpes for support of our research. We are grateful
to Ms. Laurence Rousset for the mass spectral data, to Professor
Monique Perrin for the X-ray crystallographic studies, and to Prof.
Adrian J. Blackman for 1H NMR spectra. M.A.C. is the recipient of a
Merck & Co. Academic Development Award.
The importance of the sulfonamide function in the
construction of the third and final ring of the molecule
becomes fully apparent at this juncture. Deprotonation of 10
with tBuLi[11] and capture of the anion with racemic octene
oxide activated by BF3·OEt2[12] resulted in the formation of 11,
Supportinginformation for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200460178
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Angewandte
Chemie
the nitrogen atom. We thus envision that the selective
delivery of the copper(i) agent to the Re face of the double
bond occurs from a complex such as 17. Similar directed
organocopper additions are documented.[16]
Scheme 1. a) PhI(OAc)2, (CF3)2CHOH, room temperature; b) tBuPh2-
SiCl, imidazole, DMF, room temperature, 82% over two steps;
c) KHMDS, THF, ꢀ1008C, 89% (d.r.=7:1); d) PhSH, BF3·OEt2 (cat.),
CH2Cl2, 08C, 77%; e) Raney Ni, EtOH/THF, 77%. DMF=N,N-di-
methylformamide, KHMDS=potassium hexamethyldisilazide
Intermediate 14 is then readily and diastereoselectively
elaborated into either cylindricine C or into 2-epicylindrici-
ne C (Scheme 2). Compound (ꢀ)-4 was prepared through an
initial one-pot, sequential intramolecular reductive amination
reaction (AcOH, NaBH3CN)[17] and borane oxidation process
(H2O2, NaOH). This resulted in the highly stereoselective[15]
formation of 15. A rationale for this elevated diastereoselec-
tivity is proposed as follows: A MM + force field study
suggests that the configuration of the borylated stereogenic
carbon center defines 18 as the most stable conformer of the
presumed iminium ion intermediate. Reduction of 18 under
stereoelectronic control (axial delivery of hydride)[18] yields
15. Dess–Martin oxidation[13] of 15 and deprotection led to
4.[19]
which was then subjected to a Dess–Martin oxidation[13] to
give 12. A one-pot sequence, which involved sequential b-
elimination and Miyaura borylation[14] of the isolable inter-
mediate 13, resulted in the extremely rapid (20 min; the
reaction normally requires > 15 h) and highly diastereose-
lective[15] formation of the boronic ester 14 (Scheme 2).
Several pieces of evidence suggest that the unusually fast rate
and the high degree of diastereoselectivity observed in the
Miyaura reaction of 13 may be ascribed to a directing effect of
The total synthesis of (ꢀ)-cylindricine (Scheme 3) started
with the desilylation of 14 followed by treatment of the
Scheme 3. a) TBAF, THF, 95% for 20, 96% for 3; b) NaBH(OAc)3, AcOH
(cat.), CH2Cl2, ꢀ788C, 73%; c) tBuPh2SiCl, imidazole, DMF, 95%; d) H2O2,
NaOH, THF, 08C 97%; e) Dess-Martin periodinane, CH2Cl2, room tempera-
ture, 94%.
Scheme 2. a) tBuLi, THF, ꢀ788C, (ꢁ)-1-octene oxide, BF3OEt2;
b) Dess–Martin periodinane, CH2Cl2, room temperature, 88% over two
steps for 12; 94% for 16; c) 1. DBU, DMF, then 2. bis(pinacolyl)dibor-
onate, CuCl, KOAc, room temperature, 86%; d) 1. NaBH3CN, AcOH,
MeOH, 08C, then 2. H2O2, NaOH, 80%; e) TBAF, THF, 96%.
DBU=1,8-diazabicyclo[5.4.0]undec-7-ene, TBAF=tetra-n-butylammo-
nium fluoride
resultant product 20 with NaBH(OAc)3 and a catalytic
quantity of AcOH. This induced a highly stereoselective[15]
Evans-type directed reduction[20] of a presumed iminium ion
intermediate 19. Such a directed reduction of an iminium ion
appears to be undocumented. The emerging product 21 was
readily transformed into 3 as shown in Scheme 3.[21]
Angew. Chem. Int. Ed. 2004, 43, 4336 –4338
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Communications
[12] a) M. J. Eis, J. E. Wrobel, B. Ganem, J. Am. Chem. Soc. 1984,
106, 3693; b) B. Achmatowicz, S. Marczak, J. Wicha, J. Chem.
Soc. Chem. Commun. 1987, 1226.
[13] D. B. Dess, J. C. Martin, J. Org. Chem. 1983, 48, 4155 – 4156.
[14] K. Takahashi, T. Ishiyama, N. Miyaura, J. Organomet. Chem.
2001, 625, 47.
The applicability of the newtechnology to other synthetic
problems is currently under study and will constitute the
subject of future reports.
Received: March 31, 2004 [Z460178]
[15] Only one stereoisomer of the product was apparent within the
1
limits of H- (300 MHz) and 13C NMR (75 MHz) spectrometry.
Keywords: antitumor agents · cyclization · spiro compounds ·
sulfonamides · total synthesis
.
[16] For amine-directed organocuprate additions, see: a) D. K.
Hutchinson, S. A. Hardinger, P. L. Fuchs, Tetrahedron Lett.
1986, 27, 1425; b) for an excellent bibliography on general
directed organocuprate addition reactions,see: B. Breit, P.
Demel, Tetrahedron 2000, 56, 2833.
[1] S. Canesi, P. Belmont, D. Bouchu, L. Rousset, M. A. Ciufolini,
Tetrahedron Lett. 2002, 43, 5193.
[2] a) A. J. Blackman, C. Li, D. C. R. Hockless, B. W. Skelton, A. H.
White, Tetrahedron 1993, 49, 8645; b) C. Li, A. J. Blackman,
Aust. J. Chem. 1994, 47, 1355; c) C. Li, A. J. Blackman, Aust. J.
Chem. 1995, 48, 955.
[17] For related processes, see reference [3g].
[18] For a discussion, see: R. V. Stevens, Acc. Chem. Res. 1984, 17,
289.
[19] The 1H NMR spectra of (ꢀ)-4 synthesized herein ([a]2D5 = ꢀ398;
c = 0.5 in CH2Cl2), could be superimposed on those of (ꢁ )-2-
epicylindricine C published by Weinreb and co-workers[3e] (see
Supporting Information).
[3] Total synthesis of (ꢀ)-cylindricine C: a) G. A. Molander, M.
Roenn, J. Org. Chem. 1999, 64, 5183; total synthesis of (+)-
cylindricines C, D, and E: b) B. M. Trost, M. T. Rudd, Org. Lett.
2003, 5, 4599; total synthesis of (ꢁ )-cylindricines A, D, and E:
c) B. B. Snider, T. Liu, J. Org. Chem. 1997, 62, 5630; total
synthesis of (ꢁ )-cylindricines A and B: d) J. F. Liu, C. H.
Heathcock, J. Org. Chem. 1999, 64, 8263; synthesis of (ꢁ )-4-
epicylindricine C: e) K. M. Werner, J. M. de los Santos, S. M.
Weinreb, M. Shang, J. Org. Chem. 1999, 64, 4865; synthetic
studies: f) W. Oppolzer, C. G. Bochet, Tetrahedron: Asymmetry
2000, 11, 4761; g) W. H. Pearson, N. S. Barta, J. W. Kamp,
Tetrahedron Lett. 1997, 38, 3369.
[20] D. A. Evans, J. S. Clark, R. Metternich, V. J. Novack, G. S.
Sheppard, J. Am. Chem. Soc. 1990, 112, 866.
[21] The 1H NMR spectra of (ꢀ)-3 synthesized herein could be
superimposed on those of (ꢀ)-cylindricine C reported by
Molander and Roenn[3a] (see Supporting Information), and the
specific rotations were also essentially identical: [a]2D5 = ꢀ668
(c = 0.5 in CH2Cl2) for 3 versus [a]2D5 = ꢀ648 (c = 0.2 in CH2Cl2)
for reference [3a].
[4] For synthetic studies on the structurally related alkaloids,
fasicularin and lepadiformine, see: a) H. Abe, S. Aoyagi, C.
Kibayashi, J. Am. Chem. Soc. 2000, 122, 4583; b) T. J. Greshock,
R. L. Funk, Org. Lett. 2001, 3, 3511; c) J.-H. Maeng, R. L. Funk,
Org. Lett. 2002, 4, 331; d) W. H. Pearson, Pure Appl. Chem. 2002,
74, 1339; e) H. Abe, S. Aoyagi, C. Kibayashi, Angew. Chem.
2002, 114, 3143, Angew. Chem. Int. Ed. 2002, 41, 3017; f) S. M.
Weinreb, Acc. Chem. Res. 2003, 36, 59; g) R. Hunter, P. Richards,
Synlett 2003, 271; h) C. Kibayashi, S. Aoyagi, H. Abe, Bull.
Chem. Soc. Jpn. 2003, 76, 2059; i) M. D. Fenster, G. R. Dake,
Org. Lett. 2003, 5, 4313.
[5] Unpublished results from these laboratories; details will be
provided in a forthcoming full paper.
[6] The absolute configuration of cylindricine C is unknown.[2] We
chose to prepare (ꢀ)-4 from d-homotyrosine for comparison
purposes because at the beginning of our investigations the only
enantioselective synthesis of (ꢀ)-4 reported in the literature was
that by Molander and Roenn.[3a]
[7] a) SOCl2/MeOH; b) MsCl/TEA (excess), CH2Cl2, 08C, (forma-
tion of the N,O-dimesyl derivative), 91% over two steps;
c) NaBH4/EtOH/THF, 94%; d) NaOH/dioxane, 808C, 90%.
Ms = methanesulfonyl, TEA = triethylamine.
[8] Under identical conditions, cyclization of the corresponding
methyl ether proceeded with d.r. = 3.5:1 and that of the TBDMS
ether with d.r. = 4.5:1.
[9] The structure of this material was confirmed by an X-ray
crystallographic study of its desilylated analogue. The selectivity
of the 1,4-addition of PhSH to 8 appears to be due to a Felkin–
Anh-type stereoelectronic effect created by the strongly electro-
negative sulfonamide nitrogen atom. For examples, see: a) E. L.
Eliel, S. H. Wilen, Stereochemistry of Organic Compounds,
Wiley, NewYork, 1994, p. 875; b) S. Bennabi, K. Narkunan, L.
Rousset, D. Bouchu, M. A. Ciufolini, Tetrahedron Lett. 2000, 41,
8873, and references therein.
[10] R. Mozingo, D. E. Wolf, S. A. Harris, K. Folkers, J. Am. Chem.
Soc. 1943, 65, 1013.
[11] Weaker bases were ineffective for deprotonation of 10, in accord
with: L. A. Paquette, S. C. Ra, J. D. Schloss, S. M. Leit, J. C.
Gallucci, J. Org. Chem. 2001, 66, 3564.
4338
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Angew. Chem. Int. Ed. 2004, 43, 4336 –4338