Enantioselective Iridium-Catalyzed Allylation of Acetylenic Ketones via 2-Propanol-Mediated Reductive Coupling of Allyl Acetate: C14-C23 of Pladienolide D

Article abstract of DOI:10.1002/anie.201908939

Highly enantioselective catalytic reductive coupling of allyl acetate with acetylenic ketones occurs in a chemoselective manner in the presence of aliphatic or aromatic ketones. This method was used to construct C14-C23 of pladienolide D in half the steps previously required.

Full text of DOI:10.1002/anie.201908939

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Accepted Article
Title: Enantioselective Iridium-Catalyzed Allylation of Acetylenic
Ketones via 2-Propanol-Mediated Reductive Coupling of Allyl
Acetate: C14-C23 of Pladienolide D
Authors: Michael Joseph Krische, Gilmar Brito, Woo-Ok Jung, and
Minjin Yoo
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http://dx.doi.org/10.1002/ange.201908939

10.1002/anie.201908939
Angewandte Chemie International Edition
Enantioselective Iridium-Catalyzed Allylation of Acetylenic Ketones via 2-Propanol-
Mediated Reductive Coupling of Allyl Acetate: C14-C23 of Pladienolide D
Gilmar A. Brito, Woo-Ok Jung, Minjin Yoo and Michael J. Krische*
University of Texas at Austin, Department of Chemistry
105 E 24th St. (A5300), Austin, TX 78712-1167 (USA)
<Graphical Abstract>
Alkyne-Directed Chemo- and Enantioselectivity. Highly enantioselective catalytic reductive
coupling of allyl acetate with acetylenic ketones occurs in a chemoselective manner in the
presence of aliphatic or aromatic ketones. This method was used to construct C14-C23 of
pladienolide D in half the steps previously required.
Keywords: Enantioselective, Iridium, Ketone Allylation, π-allyl, Transfer Hydrogenation.
This article is protected by copyright. All rights reserved.

10.1002/anie.201908939
Angewandte Chemie International Edition
Enantioselective Iridium-Catalyzed Allylation of Acetylenic Ketones via 2-Propanol-
Mediated Reductive Coupling of Allyl Acetate: C14-C23 of Pladienolide D**
Gilmar A. Brito, Woo-Ok Jung, Minjin Yoo and Michael J. Krische*
[*]
Dr. Gilmar A. Brito, Ms. Woo-Ok Jung, Ms. Minjin Yoo and Prof. M. J. Krische
University of Texas at Austin, Department of Chemistry
105 E 24th St. (A5300), Austin, TX 78712-1167 (USA)
Email: mkrische@mail.utexas.edu
[**]
The Welch Foundation (F-0038) and the NIH (RO1-GM069445) are acknowledged for partial support of this research.
Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) is acknowledged for postdoctoral fellowship
support (G.B. 2017/00734-5).
The catalytic enantioselective addition of C-nucleophiles to ketones represents a
persistent challenge in chemical synthesis.[1] Differentiation of the acyl substituents by the chiral
catalyst is often incomplete, leading to low C=O π-facial enantioselectivities. Additionally, for
stabilized C-nucleophiles, ketone addition is frequently reversible, which erodes kinetic
stereoselectivity. Consequently, work in this area is relegated to the use of non-stabilized C-
nucleophiles in the form of stoichiometric organometallic reagents or related metalloids, which
are often prepared via successive transmetallation.[2,3] In the specific context of enantioselective
ketone allylation, reagents based on B, Si, and Sn have been described.[4] Metal-catalyzed
carbonyl reductive couplings of π-unsaturated feedstocks potentially provides an alternative to
the use of premetalated C-nucleophiles.[5] However, the requisite reductants utilized in these
processes are typically metallic (Zn, Mn), pyrophoric (ZnEt2, BEt3), expensive or mass-intensive
(R3SiH).[6] To address these limitations, we have
Figure 1. Asymmetric allylation of acetylenic
ketones.
developed metal-catalyzed carbonyl reductive
couplings that utilize elemental hydrogen, 2-
propanol or formate as terminal reductant, as
well as related hydrogen auto-transfer processes
wherein alcohols serve as hydrogen donor and
carbonyl
proelectrophile.[7]
While
this
technology has enabled diverse catalytic
enantioselective aldehyde allylations[7f] and
propargylations,[7g]
asymmetric
transfer
hydrogenative allylations of ketones are
restricted to vicinal dicarbonyl compounds.[8]
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10.1002/anie.201908939
Angewandte Chemie International Edition
It was posited that acetylenic ketones might participate in efficient alcohol-mediated
carbonyl allylation due to electrophilic activation associated with the negative inductive effect of
the alkyne.[9] Although isolated examples have been reported,[10] only one systematic study on
the asymmetric allylation of acetylenic ketones appears in the literature, which exploits an
allylzinc reagent bound by a stoichiometric quantities of a chiral modifier.[11] Given this lack of
prior art and the utility of the resulting 1,5-enynes vis-á-vis diverse carbophilic metal-catalyzed
transformations,[12] the 2-propanol-mediated transfer hydrogenative allylation of acetylenic
ketones was explored. Here, we report that such processes not only occur with high levels of
enantioselectivity, but may be conducted in a chemoselective manner in the presence of aliphatic
and aromatic ketones (Figure 1).
Guided by our prior work on the transfer hydrogenative allylation of acetylenic
i
aldehydes,[9] methyl ketone 1a (100 mol%, [Si] = Pr3Si) was exposed to allyl acetate 2a (200
mol%) and 2-propanol (200 mol%) in the presence of Cs2CO3 (50 mol%) and the commercially
available π-allyliridium-C,O-benzoate modified by (S)-SEGPHOS, (S)-Ir-I (5 mol%), in
anhydrous THF (Table 1, entry 1). The desired product of ketone allylation 3a was not observed.
However, upon introduction of water (100 mol%), which may facilitate alkoxide exchange at the
metal center and solubilize the carbonate base, the tertiary homoallylic alcohol 3a was obtained
in 44% yield in highly enantiomerically enriched form (Table 1, entry 2). The mass balance in
this experiment consisted primarily of unreacted 1a and the corresponding product of silyl
cleavage. As acetylenic C-H moieties are not tolerated by the iridium catalyst, an effort was
made to accelerate carbonyl addition with respect silyl cleavage through the introduction of a
more inductive silyl groups. While use of the triphenylsilyl group, as in 1a ([Si] = Ph3Si), only
exacerbated silyl cleavage to prevent formation of 3a (Table 1, entry 3), the corresponding tert-
t
butyl-diphenylsilyl compound, 1a ([Si] = BuPh2Si), better balanced inductive activation and
stability toward cleavage, enabling formation of 3a in 48% yield (Table 1, entry 4). While
adjusting the loading of water did not increase efficiency (Table 1, entries 5 and 6), decreased
loadings of 2-propanol (120 mol%) and use of a slightly weaker base, K2CO3 (50 mol%) each
led to modest improvements (Table 1, entries 7 and 8). Finally, introduction of 3-NO2-4-CN-
BzOH (10 mol%), which presumably stabilizes the catalyst and attenuates silyl cleavage by
buffering the medium, led to a much cleaner reaction, delivering 3a in 66% yield and 96%
enantiomeric excess (Table 1, entry 9).
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10.1002/anie.201908939
Angewandte Chemie International Edition
Table 1. Selected optimization experiments in the enantioselective π-allyliridium-C,O-benzoate
catalyzed allylation of acetylenic ketone 1a.a
aYields are of material isolated by silica gel chromatography. Enantioselectivities were determined by chiral stationary phase
HPLC analysis. See Supporting Information for further experimental details. b3-NO2-4-CN-BzOH (10 mol%).
Table 2. Enantioselective π-allyliridium-C,O-benzoate catalyzed allylation of acetylenic ketones
1a-1n via 2-propanol-mediated reductive coupling with allyl acetate 2a.a
aYields are of material isolated by silica gel chromatography. Enantioselectivities were determined by chiral stationary phase
HPLC analysis. See Supporting Information for further experimental details.
With these optimal conditions in hand, the enantioselective allylation of diverse
acetylenic ketones was explored (Table 2). While simple alkyl-substituted acetylenic ketones 1a
and 1b provide moderate yields of the corresponding tertiary homoallylic alcohols 3a and 3b,
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10.1002/anie.201908939
Angewandte Chemie International Edition
acetylenic ketones 1c-1g, which incorporate substituents that exert a negative σ-inductive effect,
deliver the tertiary homoallylic alcohols 3c-3g, respectively, excellent yields. For α-alkoxy
ketones 1c and 1d, which exert a relatively strong negative σ-inductive effect, the less inductive
i
t
and more stable Pr3Si moiety can be used instead of the BuPh2Si moiety. Remarkably, ketone
1h, which incorporate acetylenic and vinylic acyl substituents, is an effective partner of carbonyl
allylation, forming the tertiary propargylic-allylic-homoallylic alcohol 3h in good yield.
Tolerance of N-heterocycles and acetals are illustrated by the formation of 3i and 3j,
respectively. Perhaps most significant, however, are the reactions of ketones 1k-1n. The acetylic
ketone moieties of 1k-1n undergo completely chemoselective allylation in the presence of
aliphatic or aromatic ketone moieties to form 3k-3n as single constitutional isomers. For all
adducts 3a-3n, uniformly high levels of enantioselectivity are observed. In lower yielding
transformations, unreacted ketone along with trace quantities of tert-butyldiphenylsilanol (<5 %)
were observed. The absolute stereochemistry of tertiary alcohols 3a-3n was assigned in analogy
to that determined for adducts 3g, which was determined by single crystal X-ray diffraction
analysis. Aryl substituted acetylenic ketones did not participate in allylation. Additionally,
formation of compounds 3a-3n from the propargyl alcohols corresponding to ketones 1k-1n via
hydrogen auto-transfer was inefficient, as the negative inductive effect of the alkyne impedes
dehydrogenation. Finally, acetylenic ketones are competent partners in related enantioselective
transfer hydrogenative allylations. For example, exposure of acetylenic ketone 1g to methallyl
chloride 2b and 2-propanol (300 mol%) in the presence of (S)-Ir-I (5 mol%) provides the
product of methallylation 4g in 91% yield and 92% ee (eq. 1).[13] Additionally, the reductive
coupling of acetylenic ketone 1g with isoprenyl tert-butoxy carbonate 2c mediated by 2-propanol
(300 mol%) and catalyzed by the corresponding (S)-DM-SEGPHOS-modified catalyst (S)-Ir-II
(5 mol%) provides the product of isoprenylation 5g in 64% yield and 89% ee (eq. 1).[14]
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10.1002/anie.201908939
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Scheme 1. Enantioselective synthesis of C14-C23 side chain of pladienolide D.
The utility of the present ketone allylation method to natural product total synthesis is
illustrated by the rapid conversion of adduct ent-3a to the C14-C23 side chain of pladienolide D
(Scheme 1). The pladienolides were isolated in 2004 from the culture broth of Mer-11107, an
engineered strain of Streptomyces platensis,[15] and were found to inhibit the proliferation of
multiple drug resistant human cancer cells with low nanomolar IC50 values.[16] These compounds
operate through a novel mechanism of action, involving binding to the splicing factor 3b (SF3b)
subunit of the spliceosome.[17] Clinical evaluation of pladienolide analogue E7107 was
undertaken but discontinued due to vision loss.[18] More recently, however, clinical trials were
initiated with another pladienolide analogue, H3B-8800, which was granted orphan drug status
by the FDA in August 2017 for treatment of myelogenous leukemia and chronic
myelomonocytic leukemia.[19] Cross-metathesis of ent-3a with compound 6 (conveniently
prepared via butadiene-mediated crotylation of propanal),[20] followed by Shi epoxidation[21] of
the resulting olefin, delivers acetylenic epoxide 7 with high levels of stereocontrol. To
corroborate the stereochemical assignment of acetylenic epoxide 7, it was subjected to silyl
deprotection and Lindlar reduction to furnish the previously reported tertiary allylic alcohol 8,[21]
which embodies C14-C23 of pladienolide D. Whereas the prior synthesis of tertiary allylic
alcohol 8 required 16 steps (LLS), the present synthesis of 8 is completed in only 8 steps (LLS).
In summary, we report the first catalytic enantioselective allylations of acetylenic
ketones. Specifically, using the commercially available π-allyliridium-C,O-benzoate modified by
(S)-SEGPHOS, (S)-Ir-I, as catalyst, allyl acetate 2a engages acetylenic ketones 1a-1n in
enantioselective 2-propanol-mediated reductive coupling to form tertiary propargyl alcohols 3a-
3n. Additionally, owing the highly chemoselective nature of this process, we demonstrate that
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10.1002/anie.201908939
Angewandte Chemie International Edition
acetylenic ketones undergo allylation in the presence of aliphatic or aromatic ketone functional
groups to form single constitutional isomers. Using this protocol, a concise enantioselective
synthesis of C14-C23 side chain of pladienolide D is achieved. More broadly, these studies
contribute to a growing body of alcohol-mediated carbonyl additions that collectively signify a
departure from the use of premetalated reagents C-C bond formation.[5d]
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