A concise total synthesis of saliniketal B

Article abstract of DOI:10.1021/ja9061757

(Chemical Equation Presented) We report a concise, enantioselective, and highly efficient synthesis of the marine actinomycete-derived natural product saliniketal B. Our approach was motivated with an eye toward future structure-function studies of this inhibitor of phorbol ester-mediated ornithine decarboxylase induction via an unknown mechanism. Our strategy highlights the utility of Pt (II)-mediated cycloisomerization of alkynediols developed in our laboratory to construct the dioxabicyclo[3.2.1]octane ring system, a highly selective aldol fragment coupling whose stereochemical outcome is influenced by a γ-stereogenic methyl group, and an interesting one-pot desilylation/dihydropyranone fragmentation/amidation sequence. As such, saliniketal B was obtained in 11 steps and 23% overall yield from commercially available starting material via a convergent coupling of two equally complex fragments assembled in seven and eight steps (39 and 45%), respectively.

Full text of DOI:10.1021/ja9061757

Published on Web 08/18/2009
A Concise Total Synthesis of Saliniketal B
Jun Liu and Jef K. De Brabander*
Department of Biochemistry and Harold C. Simmons ComprehensiVe Cancer Center, The UniVersity of Texas
Southwestern Medical Center at Dallas, 5323 Harry Hines BouleVard, Dallas, Texas 75390-9038
Received July 23, 2009; E-mail: jef.debrabander@utsouthwestern.edu
Scheme 2. Synthesis of Fragments 3 and 4a
The groundbreaking work of Fenical and co-workers1 demon-
strated that obligate marine actinomycetes are a rich source of novel
bioactive natural products. In 2007, they reported the isolation of
the polyketides saliniketal A (1) and B (2) from the marine
actinomycete Salinispora arenicola,2 the structure of which was
confirmed by a total synthesis of Paterson and co-workers.3 Besides
unusual structural features, including a dioxabicyclo[3.2.1]octane
ring system, an E,Z-dienamide unit reminiscent of the ansa chain
of rifamycin, and nine stereocenters (eight of which are contiguous),
saliniketals are of biological interest because of their ability to
inhibit ornithine decarboxylase (ODC) induction. As the first
enzyme in the polyamine biosynthesis pathway and the direct
transcriptional target of the oncogene MYC, ODC has been shown
to be a potential target for chemotherapeutic or chemopreventive
intervention.4 Unlike R-DFMO, saliniketals do not inhibit ODC
enzyme activity but instead attenuate tumor-promoter-mediated
induction of ODC.2 Herein, we report a concise and flexible
synthesis of saliniketal B (2) that features a strategy aimed at
enabling future structure-function and mode-of-action studies.
Scheme 1. Structure of Saliniketals and Synthetic Strategy
a Reagents and conditions: (a) (COCl)2, DMSO, Et3N, CH2Cl2, -78 °C,
2 h, 92%; (b) Sn(OTf)2 (1.05 equiv), Et3N (1.05 equiv), CH2Cl2, -20 °C,
then -78 °C, 7 (2 equiv), 82%; (c) Na(AcO)3BH, HOAc, 0 °C to rt, 79%;
(d) TBAF, THF, 3 min, 94%; (e) [PtCl2(CH2CH2)]2 (5 mol %), THF, 5
min, quantitative; (f) MeONHMe ·HCl (3 equiv), AlMe3 (3 equiv), THF, 0
°C; (g) EtMgBr (3 equiv), THF, 0 °C to rt, 2 h, 87% (two steps); (h)
4-MeOBnOC(NH)CCl3, PPTS, CH2Cl2, rt, 18 h, 87%; (i) DIBAL-H,
CH2Cl2, -78 °C, 2 h, 93%; (j) (+)-MeOB(Ipc)2, allylMgBr, 0 °C, add 12,
-98 °C, then NaOH, 30% H2O2, Et2O, reflux, 90%; (k) paraformaldehyde
(10 equiv), DABCO (0.5 equiv), dioxane/H2O (1:1), 72 h; (l) TIPSCl, imid.,
DMAP, CH2Cl2, 0 °C to rt, 1 h, 79% (two steps); (m) LiOH, THF/H2O
(1:1), rt, 36 h, 92%; (n) DCC, DMAP, CH2Cl2, 0 °C to rt, 12 h, 82%; (o)
Grubbs-II (10 mol %), CH2Cl2, reflux, 14 h, 67% plus 15% recovered
starting material; (p) DDQ, CH2Cl2/H2O (20/1), rt, 1 h, 91%; (q)
Dess-Martin periodinane, NaHCO3, CH2Cl2, rt, 30 min, quantitative.
Our synthetic strategy was based on a convergent aldol coupling
of fragments 3 and 4 following an anti-selective reduction of
ꢀ-hydroxyketone 2 (Scheme 1). We envisioned a late-stage installation
of the E,Z-dienamide via an interesting but rarely utilized fragmentation
of a dihydropyranone.5 The 2,8-dioxabicylo[3.2.1]octane moiety was
to be assembled via cycloisomerization of alkynediol 5 by exploiting
methodology developed in our laboratory.6
An efficient synthesis of coupling partners 3 and 4 is depicted
in Scheme 2. The reagent-controlled aldol reaction of the stannyl
enolate derived from known oxazolidinone 87 with aldehyde 7,
obtained from oxidation of commercially available alkynol 6 (92%
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C O M M U N I C A T I O N S
yield), provided aldol product 9 in 82% yield and >16:1 dr. Anti-
selective reduction with Na(OAc)3BH (>20:1 dr)7a followed by
desilylation (74% yield, two steps) set the stage for a cycloisomer-
ization of alkynediol 5. Use of 5 mol % Zeise’s dimer6 afforded
2,8-dioxabicyclo[3.2.1]octane 10 in quantitative yield, and this was
processed to ketone 3 via Weinreb amide formation and Grignard
reaction with EtMgBr (87%, two steps).8
Dihydropyranone 4 was synthesized from ester 11 ($). According
to a sequence by Nicolaou and co-workers, p-methoxybenzyl ether
formation (87%) was followed by semireduction to aldehyde 12
(93%) and allylation with Brown’s reagent (90%).9 The resulting
syn-homoallylic alcohol 139,10 was esterified with acid 14, a
material prepared from methyl acrylate via Baylis-Hillman reac-
tion,11 silylation, and saponification (73%, three steps). Dihydro-
pyranone formation to give 15 was accomplished in 67% yield via
ring-closing metathesis with Grubbs’ second-generation catalyst
under high dilution conditions (15% starting material was recov-
ered).12 Final oxidative deprotection (DDQ, 91%) and oxidation
with Dess-Martin periodinane13 (quantitative) delivered aldehyde
4 in seven steps and 36-39% overall yield.
In summary, we have achieved a short, highly efficient synthesis
of saliniketal B (2) in 11 steps (longest linear) and 23% overall
yield. Our approach features the utility of our Pt(II)-catalyzed
cycloisomerization methodology for the construction of the
dioxabicyclo[3.2.1]octane core, a stereoselective aldol coupling
whose selectivity was positively influenced by the ketone γ-ste-
reocenter, and an unusual one-pot desilylation/dihydropyranone
fragmentation/amidation sequence.
Acknowledgment. Financial support was provided by the NIH
(CA90349) and the Robert A. Welch Foundation. We thank
Christopher F. Bender for helpful suggestions.
Scheme 3. Synthesis of Saliniketal Ba
Supporting Information Available: Experimental procedures and
characterization data for new compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
References
(1) (a) Mincer, T. J.; Jensen, P. R.; Kauffman, C. A.; Fenical, W. Appl. EnViron.
Microbiol. 2002, 68, 5005. (b) Marris, E. Nature 2006, 443, 904.
(2) Williams, P. G.; Asolkar, R. N.; Kondratyuk, T.; Pezzuto, J. M.; Jensen,
P. R.; Fenical, W. J. Nat. Prod. 2007, 70, 83.
(3) Paterson, I.; Razzak, M.; Anderson, E. A. Org. Lett. 2008, 10, 3295.
(4) Reviews: (a) Gerner, E. W.; Meyskens, F. L., Jr. Nat. ReV. Cancer 2004,
4, 781. (b) Basuroy, U. K.; Gerner, E. W. J. Biochem. 2006, 139, 27.
(5) Examples: (a) Corey, E. J.; Schmidt, G. Tetrahedron Lett. 1979, 20, 2317.
(b) Masamune, S.; Imperiali, B.; Garvey, D. S. J. Am. Chem. Soc. 1982,
104, 5528. (c) Roush, W. R.; Spada, A. P. Tetrahedron Lett. 1982, 23,
3773. (d) Nakata, T.; Hata, N.; Oishi, T. Heterocycles 1990, 30, 333.
(6) Liu, B.; De Brabander, J. K. Org. Lett. 2006, 8, 4907.
(7) Oxazolidinone 8 was obtained in two steps from commercially available
(4S)-N-propionyl-4-Bn-2-oxazolidinone. See: (a) Evans, D. A.; Clark, J. S.;
Metternich, R.; Novack, V. J.; Sheppard, G. S. J. Am. Chem. Soc. 1990,
112, 866. (b) Evans, D. A.; Ng, H. P.; Clark, S.; Rieger, D. L. Tetrahedron
1992, 48, 2127.
(8) The stereochemistry was confirmed by NMR analysis of the acetonide
derived from 5 and comparison of the primary alcohol obtained from
reduction of oxazolidinone 10 to the same alcohol prepared independently
by Paterson et al.3 (see the Supporting Information).
a Reagents and conditions: (a) LiHMDS (1.2 equiv), -78 °C, 1 h, 4
(1.4 equiv), THF, 81%; (b) Me4N(AcO)3BH, MeCN/HOAc (1/1), -20 °C,
48 h, 89%; (c) TBAF (10 equiv), THF, 48 h; then NH3 (gas), HOBt
(2 equiv), EDC (2 equiv), rt, 72%; (d) (MeO)2CMe2, PPTS, acetone, rt,
87%.
(9) Nicolaou, K. C.; Patron, A. P.; Ajito, K.; Richter, P. K.; Khatuya, H.;
Bertinato, P.; Miller, R. A.; Tomaszewski, M. J. Chem.sEur. J. 1996, 2,
847.
The final aldol coupling between ethyl ketone 3 and aldehyde 4
yielded the anti-Felkin adduct 2 with high selectivity (>11:1 dr) in
81% yield (Scheme 3). The stereochemical outcome of this reaction
deserves some comment. The Z(O)-lithium enolates of syn-R-Me,ꢀ-
alkoxy-substituted ethyl ketones typically yield the 1,3-anti-1,4-
anti-aldol adducts,14 a situation that is mismatched with the inherent
anti-Felkin bias of aldehyde 4.10 We surmise that the observed high
selectivity for our reaction can be attributed to the presence of the
additional γ-Me stereocenter. As shown in eq 1, the Si-enolate face
is normally exposed via conformation A, minimizing A1,3-strain in
the transition state, whereas the additional γ-Me group disfavors
this conformation (A′, eq 2) as a result of unfavorable syn-pentane
interactions. This exposes the enolate Re-face via B′ for a matched
reaction with aldehyde 4.15 Next, reduction of ꢀ-hydroxy ketone 2
delivered anti-diol 16 (89%, >20:1 dr).16,17 Finally, fluoride-
mediated desilylation and concomitant fragmentation5d of dihy-
dropyranone 16 followed by in situ amidation of the liberated
carboxylic acid provided saliniketal B (2) with a yield of 72% for
this one-pot operation.18
(10) It should be noted that the stereochemical outcome is not important because
the resulting stereocenter is destroyed during the final dihydropyranone
fragmentation reaction (Scheme 3). However, homogeneous material
facilitates characterization, and targeting the syn stereoisomer ensures
maximal stereocontrol during the aldol fragment coupling (Scheme 3) in
this double-diastereodifferentiating process. See: (a) Roush, W. R. J. Org.
Chem. 1991, 56, 4151. (b) Evans, D. A.; Dart, M. J.; Duffy, J. L.; Yang,
M. G. J. Am. Chem. Soc. 1996, 118, 4322.
(11) Yu, C.; Liu, B.; Hu, L. J. Org. Chem. 2001, 66, 5413.
(12) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999, 1, 953.
(13) Dess, D. B.; Martin, J. C. J. Am. Chem. Soc. 1991, 113, 7277.
(14) (a) Gustin, D. J.; VanNieuwenhze, M. S.; Roush, W. R. Tetrahedron Lett.
1995, 36, 3447. (b) Evans, D. A.; Yang, M. G.; Dart, M. J.; Duffy, J. L.
Tetrahedron Lett. 1996, 37, 1957.
(15) We are currently testing this hypothesis by preparing the corresponding
γ-desmethyl and epi-γ-methyl congeners of ketone 3, which are expected
to give lower selectivity in the aldol reaction with aldehyde 4.
(16) Evans, D. A.; Chapman, K. T.; Carreira, E. M. J. Am. Chem. Soc. 1988,
110, 3560.
(17) The relative configuration was confirmed via acetonide 17. See: (a)
Rychnovsky, S. D.; Skalitzky, D. J. Tetrahedron Lett. 1990, 31, 945. (b)
Evans, D. A.; Rieger, D. L.; Gage, J. R. Tetrahedron Lett. 1990, 31, 7099.
(18) The spectroscopic data and optical rotation of synthetic saliniketal B (2)
are in full agreement with literature data reported for natural2 and synthetic3
2. See the Supporting Information for details.
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