A gold-catalyzed alkyne-diol cycloisomerization for the synthesis of oxygenated 5,5-spiroketals

Article abstract of DOI:10.3762/bjoc.7.66

A highly efficient synthesis of oxygenated 5,5-spiroketals was performed towards the synthesis of the cephalosporolides. Gold(I) chloride in methanol induced the cycloisomerization of a protected alkyne triol with concomitant deprotection to give a strate

Full text of DOI:10.3762/bjoc.7.66

A gold-catalyzed alkyne-diol cycloisomerization for
the synthesis of oxygenated 5,5-spiroketals
Sami F. Tlais and Gregory B. Dudley*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2011, 7, 570–577.
Department of Chemistry and Biochemistry, Florida State University,
Tallahassee, FL 32306-4390 USA, Fax: (850) 644-8281
doi:10.3762/bjoc.7.66
Received: 15 December 2010
Accepted: 14 February 2011
Published: 04 May 2011
Email:
Gregory B. Dudley* - gdudley@chem.fsu.edu
* Corresponding author
This article is part of the Thematic Series "Gold catalysis for organic
synthesis".
Keywords:
alkynes; cyclocondensation; cycloisomerization; gold-catalyzed;
5,5-spiroketals
Guest Editor: F. D. Toste
© 2011 Tlais and Dudley; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
A highly efficient synthesis of oxygenated 5,5-spiroketals was performed towards the synthesis of the cephalosporolides. Gold(I)
chloride in methanol induced the cycloisomerization of a protected alkyne triol with concomitant deprotection to give a strategi-
cally hydroxylated 5,5-spiroketal, despite the potential for regiochemical complications and elimination to furan. Other late tran-
sition metal Lewis acids were less effective. The use of methanol as solvent helped suppress the formation of the undesired furan
by-product. This study provides yet another example of the advantages of gold catalysis in the activation of alkyne π-systems.
Introduction
Spiroketals, exemplified by structure shown in Figure 1, are
prominent structural features of many biomedically relevant
natural and non-natural target structures [1-4]. As such, the syn-
thesis of spiroketals has received considerable attention, with
most progress having been made on systems that include at least
one six-membered ring [5]. 5,5-Spiroketals (m, n = 0, Figure 1),
particularly oxygenated 5,5-spiroketals such as are found in the
Figure 1: Common spiroketal motifs.
cephalosporolides (Figure 2), are the focus of this study.
A variety of synthetic methods are available for the synthesis of oxidative spirocyclization of tetrahydrofuryl propanols [17-20],
5,5-spiroketals, including cyclocondensation of ketone diols and others. Cyclocondensation of ketone diols is perhaps the
[6,7], the cycloisomerization of alkyne diols (Scheme 1) [8-16], most straighforward and the most used method, but the alter-
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Beilstein J. Org. Chem. 2011, 7, 570–577.
Figure 2: Spiroketal-containing cephalosporolide natural products.
Scheme 1: Cyclocondensation vs. cycloisomerization for the synthesis of spiroketals.
native procedures offer specific advantages. For example, the For this thematic issue on gold catalysis in organic synthesis,
cycloisomerization of alkyne diols is more exothermic we detail here the challenges and considerations involved in the
(Scheme 1) [21] and atom economical [22], and non-polar cycloisomerization of alkynes to oxygenated spiroketals and
alkyne π-bonds are more compatible than ketones (kinetically outline our screening of various late transition metal catalysts
stable) towards a number of common reaction conditions. and conditions that ultimately resulted in the acquisition of our
Conversely, the use of alkynes in the synthesis of spiroketals target structures [28]. Gold(I) chloride emerged as the best
introduces regiochemistry concerns as to which of the two choice for the desired transformation.
alkyne carbons becomes the spiroketal carbon, and the kinetic
stability of alkynes must be overcome when alkyne reactivity is The key precedents for the desired cycloisomerization are
desired.
shown in Scheme 3, although many methods are available [30-
34] and no consensus option has emerged. Utimoto studied the
As an off-shoot of our program devoted to the synthesis of palladium-catalyzed cycloisomerization [8] and reported that a
functionalized alkynes by fragmentation reactions [23-27], we range of spiroketals are available in excellent yield (e.g.,
became interested in the application of alkyne-diol cycloiso- Scheme 3, Reaction 1). However, regiochemistry is sometimes
merization to the synthesis of the cephalosporolides and other difficult to control, and De Brabander later found variability in
oxygenated spiroketals. Our retrosynthetic analysis of the reaction selectivity using the Utimoto conditions to prepare 6,6-
reported structure of cephalosporolide H (1) is outlined in spiroketals. Therefore, he suggested the preferred use of Ziese’s
Scheme 2. We recently demonstrated the use of inter-cycle dimer, a platinum catalyst (Scheme 3, Reaction 2), for such
chelation effects to control the spiroketal stereochemistry cyclizations [9]. In an unrelated study that also bears on the
[28,29]. However, formation of the requisite oxygenated current work, Aponick and co-workers described a gold-
spiroketals (by cycloisomerization) posed significant chal- catalyzed cyclocondensation of alkyne diols to give substituted
lenges that required a focused study.
furans (Scheme 3, Reaction 3) [35].
Scheme 2: Retrosynthetic analysis of cephalosporolide H.
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Beilstein J. Org. Chem. 2011, 7, 570–577.
Scheme 3: Key precedents for the desired cycloisomerization.
Our objective, laid out in Scheme 4, was to initiate cycloiso- Results and Discussion
merization with a 5-endo-dig cyclization of the homopropargyl Initial studies on the cycloisomerization took advantage of
alcohol 6, followed by 5-exo-trig cyclization onto the resulting chiral propargyl alcohol 12, which is readily available from
dihydrofuran, whilst avoiding dehydration to furan 10. The use pantolactone (11, Scheme 5) [37]. An alkyne zipper reaction,
of alkyne-diol cycloisomerization instead of ketone-diol cyclo- protection, and coupling with propylene oxide gave homo-
condensation is important for the potential success of this ap- propargyl 13.
proach, since β-alkoxy ketone 9 (Scheme 4, inset) would be
more prone to undesired elimination than homopropargyl ether Cycloisomerization of 13 to the spiroketal (14) was investi-
6. We addressed regiochemistry by blocking one of the alco- gated under a variety of conditions, some of which are featured
hols as an acetal (the alcohol that otherwise could undergo in Table 1. Utimoto’s general conditions as reported (Table 1,
either 5-exo or 6-endo cyclization [9,13,36]), thus favoring the entry 1) resulted in decomposition of the substrate, but at room
initial 5-endo cyclization of the other. In this way we aimed to temperature the spiroketal was obtained in modest yield
ensure that the desired regioisomer could form, with the expec- (Table 1, entry 2). Reactions involving Ziese’s dimer were
tation of acetal hydrolysis during the course of the reaction. disappointing (Table 1, entry 3), but gold(I) chloride in meth-
Indeed, attempts to induce spiroketalization after removal of the ylene chloride (cf. Scheme 3, Reaction 3) gave more encour-
acetal resulted in complex product mixtures (not shown).
aging results. Other gold catalysts and solvents were screened,
Scheme 4: Proposed cycloisomerization with acetal hydrolysis.
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Beilstein J. Org. Chem. 2011, 7, 570–577.
Scheme 5: Synthesis of model cyclization substrate 13.
with the best results being achieved with a higher catalyst help suppress formation of the (undesired for our purposes)
loading of gold(I) chloride in methanol (Table 1, entry 10). The furan product.
need for higher catalyst loading is tentatively ascribed to some
form of instability of the gold catalyst in methanol, as pre- For the synthesis of cephalosporolide H, we prepared homo-
mixing the gold(I) chloride with methanol and aging this mix- propargyl alcohol 6 by a two-step inversion of 12, followed by
ture prior to adding the substrate results in a less efficient reac- an alkyne zipper reaction and coupling with nonene oxide
tion. This is not the first time that we have observed the impor- (Scheme 6) [28]. Treatment of 6 with 40 mol % gold(I) chlo-
tance of the order of addition in a gold-catalyzed reaction in a ride in methanol resulted in cycloisomerization with simulta-
protic solvent [38], but nonetheless we were satisfied with these neous hydrolysis of the PMP acetal. Meanwhile, cleavage of the
results for our current study. Furan 16, which presumably arises silyl ether also occurred under the reaction conditions, and
by analogy to Aponick’s cyclocondensation, was observed in spiroketal diol 17 was isolated in 80% yield as a roughly 1:1
varying amounts in many cases and was the major product in mixture of spiroketal epimers. This mixture of epimers led to a
Table 1, entry 11: The use of methanol as a solvent seems to single diastereomer upon chelation with zinc chloride. TEMPO
Table 1: Spiroketalization using late transition metal salt complexes.
Entry
Conditions
Major Product
Yield
1
1% PdCl2, CH3CN, reflux, 1 h
1% PdCl2, CH3CN, rt, 1.5 h
—
14
—
14
14
—
—
—
15
15
16
—a
2
43%b
—a
3
1% [Cl2Pt(CH2=CH2)]2, Et2O, rt, then CSA
5% AuCl, CH2Cl2, rt, 6 h
4
36%
37%
—a
5
5% AuCl, PPTS, CH2Cl2, rt, 14 h
5% AuCl(PPh3)3, CH2Cl2, rt
5% AuCl(PPh3)3, AgSbF6, CH2Cl2, rt, 12 h
5% AuCl3, CH2Cl2, rt, 12 h
6
7
—a
8
—a
9
5% AuCl, MeOH, rt, 12 h
35%
68%
18%
10c
11
25% + 25% AuCl, MeOH, rt, 12 h
35% AuCl, MeCN, rt, 4 h
acomplex mixture of products was observed, bno increase in yield after a longer reaction time, ca second portion of AuCl (25 mol %) was added after
1 h to achieve full conversion.
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Beilstein J. Org. Chem. 2011, 7, 570–577.
Scheme 6: Synthesis of reported structure of cephalosporolide H.
oxidation gave lactone 1, which corresponds to the reported to hydrolyze the secondary silyl ether group in a separate event:
structure of cephalosporolide H. A more detailed discussion is The reaction time was intentionally extended to ensure
found in our earlier report [28].
complete desilylation.
Two mechanistic alternatives (Scheme 7) are proposed for the A second mechanistic alternative, path b, cannot be ruled out at
conversion of 6 → 17 in methanol. Path a, which corresponds this time. Path b involves gold-activation of the alkyne fol-
roughly to our original experimental designs, involves initial lowed by 5-exo-dig nucleophilic attack of the acetal oxygen.
gold-catalyzed 5-endo-dig cyclization to dihydrofuran 18. Once Methanolysis of the acetal and spirocyclization would quickly
the regiochemistry is established, any number of condensation follow. Although this pathway seems unlikely to compete effec-
pathways would lead to spiroketal 17. For example, protona- tively with path a, a control experiment suggests that path b is
tion of the enol ether could assist in the opening of the acetal, feasible under certain conditions (Scheme 8, Reaction 4): We
with simultaneous formation of spiroketal 19. Any carbenium subjected terminal alkyne 21 to gold(I) chloride in methylene
intermediates could be intercepted reversibly by methanol. The chloride and observed the formation of furan 23 in low yield,
acidity of the gold(I) chloride in methanol mixture is sufficient along with other products.
Scheme 7: Proposed mechanism.
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Beilstein J. Org. Chem. 2011, 7, 570–577.
Scheme 8: Control experiment for gold-activation of the alkyne.
Conclusion
Characterization data for 17a: 1H NMR (600 MHz, CDCl3) δ
Gold(I) chloride effectively catalyzed the cycloisomerization of 4.26 (m, 1H), 3.98 (ddd, J = 13.1, 9.4, 6.1 Hz, 1H), 3.59 (d, J =
homopropargyl alcohol 6 to spiroketal 17 in good yield, despite 3.1 Hz, 1H), 3.51 (d, J = 10.9 Hz, 1H), 3.46 (d, J = 10.9 Hz,
the potential for regiochemical complications and elimination to 1H), 2.15 (dd, J = 13.4, 4.3 Hz, 1H), 2.10–1.91 (m, 4H),
give furan by-products. Other late transition metal Lewis acids 1.75–1.66 (m, 2H), 1.55–1.47 (m, 1H), 1.40–1.20 (m, 13H),
were less effective. This study provides yet another example of 1.09 (s, 3H), 1.02 (s, 3H), 0.87 (t, J = 7.0 Hz, 4H); 13C NMR
the advantages of gold catalysis in the activation of alkyne (150 MHz, CDCl3) δ 114.1, 91.0, 81.3, 73.3, 71.1, 43.9, 38.0,
π-systems.
37.3, 36.1, 31.8, 30.6, 29.5, 29.2, 26.3, 23.4, 22.6, 20.9, 14.1;
IR (Neat): 3280, 2926, 2857, 1461, 1334, 1108; HRMS (ESI+):
calcd. for C18H34O4Na 337.2354, found: 337.2354.
Experimental
1H NMR and 13C NMR spectra were recorded in CDCl3 as the
deuterated solvent. The chemical shifts (δ) are reported in parts
per million (ppm) relative to the residual CHCl3 peak (7.26 ppm
for 1H NMR and 77.0 ppm for 13C NMR) with TMS as internal
standard. The coupling constants (J) are reported in hertz (Hz).
IR spectra were recorded on a FT-IR spectrometer (100). Mass
spectra were recorded either by electron ionization (EI) or fast-
atom bombardment (FAB). Yields refer to isolated material
judged to be ≥95% pure by 1H NMR spectroscopy following
silica gel chromatography. All solvents, solutions and liquid
reagents were added via syringe. Methanol (MeOH), methylene
chloride (CH2Cl2) and acetonitrile (CH3CN) were used without
any purification. All reactions were carried out under an inert
nitrogen atmosphere unless otherwise stated. Purifications were
performed by flash chromatography on silica gel F-254
(230–499 mesh particle size).
Characterization data for 17b (obtained as a mixture with 17a):
1H NMR (400 MHz, CDCl3) δ 4.39–4.33 (m, 1H), 4.07–3.97
(m, 1H), 3.67 (d, J = 10.7 Hz, 1H), 3.65 (d, J = 2.9 Hz, 1H),
2.44 (dd, J = 14.3, 5.5 Hz, 1H), 2.21–1.98 (m, 5H), 1.54–1.46
(m, 1H), 1.36–1.23 (m, 12H), 1.07 (s, 3H), 1.05 (s, 3H), 0.89 (t,
J = 6.8 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 113.2, 87.9,
78.3, 72.6, 69.7, 69.0, 46.4, 37.5, 36.9, 35.6, 31.8, 30.2, 29.7,
29.3, 25.8, 24.2, 22.7, 21.8, 14.1. IR (Neat): 3280, 2926, 2857,
1461, 1334, 1108; HRMS (ESI+): calcd. for C18H34O4Na
337.2354, found: 337.2354.
Typical procedure for gold-catalyzed spiroketalization: AuCl (8
mg, 0.036 mmol) was added to a solution of 6 (50 mg, 0.091
mmol) in MeOH (5 mL) at room temperature to give a black
mixture. After 4 h, the reaction mixture was filtered, mixed with
100 mg of silica gel, and concentrated under reduced pressure.
The silica gel admixed with the crude reaction mixture was
transferred to a silica gel column and eluted with 15% EtOAc in
hexane to afford pure product 17 (23 mg, 80%).
Characterization data for 14: 1H NMR (400 MHz, CDCl3) δ
4.49 (m, 1H) [Major], 4.18 (m, 1H) [ Minor], 3.76 (d, J = 6.9
Hz, 1H) [Minor], 3.60 (d, J = 6.5 Hz, 1H) [Major], 3.49 (m,
1H), 3.39–3.30 (m, 2H), 3.31–2.20 (m, 1H), 2.15–1.86 (m, 4H),
1.68 (m, 1H), 1.43 (m, 1H), 1.29 (d, J = 6.1 Hz, 3H) [Major],
1.21 (d, J = 6.2 Hz, 3H) [Minor], 0.87 (s, 9H) [Minor], 0.86 (s,
9H) [Major], 0.071 (d, J = 1.8 Hz, 6H) [Minor], 0.07(d, J = 7.4
Hz, 6H) [Major]; 13C NMR (101 MHz, CDCl3) δ 113.28,
112.79, 92.23, 89.87, 76.85, 74.43, 72.49, 71.61, 71.49, 71.43,
45.43, 45.15, 37.54, 37.18, 36.75, 36.70, 32.34, 31.77, 25.73,
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Beilstein J. Org. Chem. 2011, 7, 570–577.
25.70, 22.68, 22.39, 21.20, 21.04, 20.46, 19.73, 17.78, 17.70, Gedris (FSU) for assistance with NMR analysis and the FSU
−3.95, −4.01, −4.86, −4.95. HRMS (CI+): calcd. for High Performance Computing group for assistance with the
C18H37O4Si 345.2455, found: 345.2455.
calculations. Helpful discussions with Dr. Abdulkader Baroudi
(FSU) regarding computational chemistry and with Prof. Aaron
Aponick (UF) regarding gold-catalyzed cyclocondensations are
gratefully acknowledged.
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