Stereoselective Synthesis of Diverse Lactones through a Cascade Reaction of Rhodium Carbenoids with Ketoacids

Article abstract of DOI:10.1021/acs.orglett.8b03327

A convergent cascade approach for the stereoselective synthesis of diverse lactones is described. The Rh₂(TFA)₄-catalyzed cascade reaction proceeds via a carboxylic acid O-H insertion/aldol cyclization with high chemo-, regio-, and d

Full text of DOI:10.1021/acs.orglett.8b03327

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Stereoselective Synthesis of Diverse Lactones through a Cascade
Reaction of Rhodium Carbenoids with Ketoacids
Nicholas P. Massaro, Joseph C. Stevens, Aayushi Chatterji, and Indrajeet Sharma*
Department of Chemistry and Biochemistry, and Institute of Natural Products Applications and Research Technologies, University
of Oklahoma, 101 Stephenson Parkway, Norman, Oklahoma 73071, United States
*
S Supporting Information
ABSTRACT: A convergent cascade approach for the stereoselective
synthesis of diverse lactones is described. The Rh (TFA) -catalyzed
2
4
cascade reaction proceeds via a carboxylic acid O−H insertion/aldol
cyclization with high chemo-, regio-, and diastereoselectivity. The cascade
reaction provides quick access to highly functionalized γ-butyro- and δ-
valerolactones from readily accessible ketoacid and diazo synthons. To
demonstrate the utility of this approach, a thermally induced oxy-Cope
ring-expansion strategy has been incorporated in the cascade sequence to
access medium-sized lactones, which can undergo a serendipitous
rearrangement to form spiro-lactones through an intramolecular aldol/
trans-lactonization sequence. The reaction has proven to be general, with
a range of ketoacids and diazo carbonyls to provide functionalized
lactones of varying ring sizes.
4
panning the realm of complexity that exists within natural
and synthetic organic frameworks, the lactone motif is
precursor. The ring cyclization reactions work fine for small-
sized rings (5−7-membered) but often pose challenges for the
synthesis of medium-sized rings (8−12-membered) due to their
S
unquestionably prominent, useful, and attractive to the scientific
1
5
community. With ring sizes ranging from 4 to 60, the lactone
high entropic and enthalpic barriers. Ring-expansion reactions
that are insensitive to substrate conformational effects provide
an alternative to the conventional cyclization approach.
2
a
2b,c
ring is present in food additives, perfumes,
pharmaceuti-
2
d
2e−g
6
cals, and bioactive natural products (Figure 1).
In
However, the synthesis of appropriate precursors for ring-
expansion reactions often requires multiple steps and limits the
7
synthetic utility.
To overcome this synthetic challenge, we envision a carbene
cascade approach that utilizes readily accessible ketoacids and
diazo carbonyls as starting materials. Our hypothesis is inspired
from our recently developed carbene cascade reaction, which
proceeds through a carbene O−H insertion/aldol cyclization
sequence to provide a substituted tetrahydrofuran intermediate
that undergoes a thermally induced oxy-Cope rearrangement to
8
yield functionalized oxacycles. There have also been reports of
alcohol O−H insertion/aldol cyclization in the literature for the
9
synthesis of tetrahydrofurans (Scheme 1, eq 1); however, there
is no example in the literature of a ketoacid insertion/aldol
cyclization for the synthesis of the corresponding γ-butyr-
olactones, which are common structural motifs found in about
10
Figure 1. Lactone-based natural products and building blocks.
10% of all natural products. The incompatibility of carboxylic
acids in carbene cascade reactions may be attributed to the
competing insertion reaction via a proton transfer resulting from
addition, these compounds are also valuable synthons, as 3-
hydroxybutyrolactone has been utilized as an enantiopure
precursor for both Pfizer’s Lipitor and AstraZeneca’s Crestor.
Because of the importance of the lactone motif, their synthesis
remains an area of current interest to the chemical community.
Most of the methods for the synthesis of lactones rely on
intramolecular cyclization reactions of an appropriate linear
low pK values of protonated carboxylic acids in the zwitterionic
a
11
3
intermediate (Scheme 1, eq 2). In this work, we have
demonstrated that proton transfer could be delayed to facilitate
Received: October 17, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03327
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 1. Recent Applications of O−H Insertion/Aldol
Cyclization
examined different dirhodium carboxylates known to decom-
13
pose diazoacetates (Table 1, entries 2−5). Among them,
Rh (TFA) bearing electron-deficient trifluoroacetate ligands
2
4
was found to be the most efficient and provided the
corresponding aldol product 4a in good yield as a single
diastereomer (entry 4). We also achieved some success with
Rh (HFB) (entry 5). With hope of achieving the cascade
2
4
sequence with earth-abundant transition-metal catalysts known
to effectively decompose diazo compounds, we also screened
copper and iron salts but did not achieve any success (entries 6−
1
4
8
). Finally, the cascade was attempted under metal-free
1
5
conditions in refluxing CH Cl , which led to diazo
2
2
decomposition to exclusively provide insertion product 3a
(
entry 9).
With the optimized conditions in hand, we then investigated
the scope of the carbene cascade sequence using Rh (TFA) as a
2
4
catalyst (Figure 2). Interestingly, the cascade proceeded in
excellent yield with the cinnamoyl derivative (4b, Figure 2)
presumably due to the lower steric interference of the styryl
16
substituent compared to the methyl group. Both the electron-
withdrawing and -donating groups were tolerated on the
aromatic ring of diazoacetates (4c,d, Figure 2). Notably, the
cascade reaction also accommodated diazoketones, which are
prone to undergo Wolff rearrangement (4e, Figure 2). The
cascade reaction also proceeded in good yield with isatin diazo
the aldol cyclization by simply changing the electronics of the
diazo-derived rhodium carbenoid (Scheme 1, eq 2).
12
17
As β-ketoacids are prone to decarboxylation, we decided to
initiate our optimization reaction with γ-ketoacids. For the initial
optimization, commercially available 2-acylbenzoic acid 1a and
diazo 2a were selected as model substrates and exposed to our
previously developed conditions for ketoalcohol O−H
to provide the corresponding spirocyclic oxindole (4f, Figure
18
2). The relative stereochemistry of spirooxindole 4f was
determined using single-crystal X-ray crystallography. As
expected, the resulting hydroxyl group and carbonyl moiety of
the diazo were found to be in a cis configuration as reported
previously in the literature, including our own work on the
8
insertion/aldol cyclization (Table 1, entry 1). As anticipated,
Rh (OAc) in refluxing CH Cl exclusively provided insertion
2
4
2
2
product 3a. In order to obtain the cascade reaction, we then
8
,9
carbene−heteroatom insertion/aldol cyclizations. We also
attempted a reaction with acceptor and acceptor/acceptor diazo
compounds ethyl diazoacetate and benzyl diazoacetoacetate,
respectively, but we only observed the insertion product without
any aldol cyclization product.
Table 1. Efficiency of Metal Salts in Carbene-Initiated
Ketoacid O−H Insertion/Aldol Cyclization
With an underlying goal of accessing medium-sized rings, we
19
then turned our attention to vinyldiazoacetates, which will
result in aldol products having the divinyl functionality necessary
8
,20
for the thermally induced oxy-Cope ring expansion
to
provide access to 10-membered benzannulated lactones. To our
delight, the cascade proceeded in good yield with high
diastereoselectivity to provide the corresponding δ-lactones
bearing two vinyl substituents. A variety of aryl substituents on
ketoacids were also tolerated, bearing electron-withdrawing and
electron-donating groups (4g−l, Figure 2). The compounds
b
c
entry
catalyst
3a/4a
4a yield (%)
1
2
3
4
5
6
7
8
9
Rh (OAc)
100:0
100:0
100:0
0
0
0
60
46
0
0
0
0
2
4
Rh (esp)
2
2
4
g−l were then subjected to the thermally induced oxy−Cope
Rh (TPA)
2
4
ring expansion reaction in refluxing toluene (boiling point 110
Rh (TFA)
Rh (HFB)
37:63
50:50
100:0
100:0
100:0
100:0
2
4
4
°
C) as previously reported for the synthesis of medium-sized
2
8
,20
(CuOTf) benzene
oxacycyles.
To our surprise, the reaction was found to be
2
Cu(acac)2
Fe(TPP)Cl
sluggish in refluxing toluene.
Therefore, we decided to increase the reaction temperature
and switched the solvent to chlorobenzene (boiling point 131
°
C). As expected, the oxy-Cope ring expansion proceeded
a
All optimization reactions were performed by adding a 0.24 M
solution of 2a (24.0 mg, 0.12 mmol, 2.0 equiv) to a 0.12 M solution of
a (15.0 mg, 0.06 mmol, 1.0 equiv) with catalysts via a syringe pump
cleanly in good yield to their corresponding benzannulated
decanolides (5a−e, Figure 3). To our surprise, when the aldol
product 4l was subjected to the same reaction conditions, we
obtained a spirophthalolactone bearing a highly functionalized
cyclopentane ring (6a, Figure 3). The structure of 6a was
determined on the basis of the nuclear Overhauser effect (NOE)
correlations and was further confirmed by single-crystal X-ray
1
for 1.5 h. After the addition of diazo compound, all reactions were
b
refluxed for an additional 30 min. The percent ratio of 3a and 4a was
1
c
determined by crude H NMR. Isolated yields of 4a obtained after
column chromatography. TPA = triphenylacetate; TFA = trifluor-
oacetate; HFB = heptafluorobutyrate; acac = acetylacetonate; TPP =
21
5
,10,15,20-tetraphenyl-21H,23H-porphine.
crystallography.
B
DOI: 10.1021/acs.orglett.8b03327
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Figure 3. Scope of thermal oxy-Cope ring expansion strategy; all
reactions were performed by heating a 0.01 M solution of 4 in
chlorobenzene for 12−18 h at 140 °C in a sealed tube. (a) The reaction
was heated at 200 °C for 24 h.
Figure 2. Scope of Rh (TFA) -catalyzed carbene carboxylic acid O−H
2
4
insertion/aldol cascade sequence; all reactions were performed by
adding a 0.24 M solution of 2 (2.0 equiv) to a 0.12 M solution of 1 (1
equiv) over 1.5 h via a syring pump. After the addition of a diazo
compound, all reactions were refluxed for an additional 30 min. (a) The
reaction was also performed on a 1 mmol scale with an isolated yield of
83%.
As the functionalized cyclopentanes are present in numerous
22
bioactive natural products, we decided to test the substrate
scope of this serendipitous cascade. We synthesized aldol
products 4m−o with high diastereoselectivity using a methyl-
substituted vinyldiazoacetate. As expected, the corresponding δ-
lactones 4m−o underwent the serendipitous cascade in good
yield as single diastereomers (6b−d, Figure 4), but at a higher
reaction temperature with prolonged reaction time.
We rationalized that the additional methyl substituent on the
vinyldiazoacetate might be the cause of this unexpected
serendipitous cascade reaction. The product 6a is presumably
formed through the ring-rearrangement reaction of the
corresponding decanolide, which will have a significant Prelog
strain due to the additional methyl group causing the
decanolide to undergo an intramolecular aldol cyclization
followed by the translactonization to provide the spirophthalo-
lactone 6 (Scheme 2). Both the methyl and aryl substituents
Figure 4. Scope of cascade sequence to access spirocyclic fused
phthalolactones; all reactions were performed by adding a 0.24 M
solution of 2 (4.0 equiv) to a 0.12 M solution of 1 (1 equiv) over 1.5 h
via a syring pump. After the addition of a diazo compound, all reactions
were refluxed for an additional 30 min. (a) The reaction was performed
by heating a 0.01 M solution of 4 in chlorobenzene for 1−3 d at 200 °C
until full conversion of starting material.
2
3
C
DOI: 10.1021/acs.orglett.8b03327
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 2. Plausible Mechanism of Spirophthalolactone-
Fused Cyclopentanes
Scheme 4. One-Pot Synthesis of Decanolides
a
The reaction was performed by adding a 0.24 M solution of 2 (2.0
equiv) in chlorobenzene into a 0.12 M solution of 1 (1 equiv) at 60
°
C over 1.5 h via a syring pump. After the addition of a diazo
compound, the reaction was refluxed for an additional 24 h.
To further demonstrate the utility of this cascade, we also
attempted the reaction with β-ketoacids, which are prone to
decarboxylation. To our delight, the cascade accommodates β-
were found to be anti to each other in spirophthalolactones,
12
suggesting that the oxy-Cope rearrangement may proceed via a
24
ketoacids with a wide range of diazo compounds to provide
chair-type transition state.
26
functionalized γ-butyrolactones (Figure 5). As anticipated, the
To gain further insight into the reaction mechanism of the
ketoacid insertion/aldol cyclization, additional experiments
were carried out (Scheme 3). We attempted the same O−H
Scheme 3. Control Experiments
a
Reagents and conditions: reaction was performed by adding a 0.24
M solution of 2f (2.0 equiv) into a 0.12 M solution of 1h (1 equiv)
over 1.5 h via a syringe pump. After the addition of 2f, the reaction
was refluxed for an additional 30 min.
Figure 5. Scope of Rh (TFA) -catalyzed carbene carboxylic acid O−H
2
4
insertion/aldol cascade sequence for the synthesis of 3-hydroxy-γ-
lactones; all reactions were performed by adding a 0.24 M solution of 2
(
2.0 equiv) into a 0.12 M solution of 1 (1 equiv) over 1.5 h via a syringe
pump. After the addition of a diazo compound, all reactions were
insertion/aldol cascade with 3-benzoylpropionic acid, exclu-
sively yielding the insertion product 3b (Scheme 3, eq 1). This
result suggests that the aromatic ring constraint is necessary for a
refluxed for an additional 30 min.
6
-membered aldol cyclization. The aromatic ring brings the
cascade reaction proceeded in lower yield due to the instability
of the ketoacid starting materials under the reaction conditions.
The cascade reaction also proceeded in good yield with isatin
diazo to provide the corresponding spirocyclic oxindole (7e,
Figure 5). The relative stereochemistry of the spirocyclic
oxindole 7e was determined using single-crystal X-ray
electrophilic ketone moiety in closer proximity to rhoda-enolate
in the zwitterionic intermediate, lowering the entropic barrier of
ring cyclization associated with the corresponding linear
alternatives. Next, we performed two experiments with the
corresponding insertion product 3a (Scheme 3, eq 2). We
refluxed the insertion product in CH Cl with Rh (TFA) alone
2
5
27
crystallography. As expected, the resulting hydroxyl group
and the carbonyl moiety of diazo were found to be in a cis
configuration as reported previously in the literature, including
our own work on the carbene−heteroatom insertion/aldol
2
2
2
4
and with the addition of excess triethylamine. We did not
observe any conversion of insertion product 3a to aldol product
4
a, suggesting that a rhodium-bound zwitterionic intermediate
8,9,20
8
is necessary for the aldol cyclization.
cyclizations. With an underlying goal to access 9-membered
Finally, we attempted the synthesis of decanolides as a one-
pot cascade by performing the reaction in refluxing chlor-
obenzene. The synthesis of decanolide 5a was successful, but the
yield was diminished as compared to our two-step protocol
lactones, we also attempted a reaction of β-ketoacids with
vinyldiazoacetates; unfortunately, the reaction gave a complex
mixture without any trace of aldol product.
In conclusion, the reported rhodium carbenoid initiated
carboxylic O−H insertion/aldol cascade sequence is convergent
(Scheme 4).
D
DOI: 10.1021/acs.orglett.8b03327
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
in nature and uses readily accessible ketoacids and diazo-
carbonyls as starting materials to access highly functionalized
lactones of a variety of ring sizes. An important feature of this
transformation is its high chemo-, regio-, and stereoselectivity.
Applications of this cascade reaction to the corresponding
lactams are ongoing and will be reported in due course.
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important compounds (PDF)
Accession Codes
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981−10080.
CCDC 1844114, 1844116, and 1860137 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
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AUTHOR INFORMATION
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*
(14) (a) Zhu, Y.; Zhai, C.; Yang, L.; Hu, W. Chem. Commun. 2010, 46,
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Corresponding Author
E-mail: isharma@ou.edu.
(
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ORCID
770−773.
(
16) Mejuch, T.; Gilboa, N.; Gayon, E.; Wang, H.; Houk, K. N.;
Indrajeet Sharma: 0000-0002-0707-0621
Notes
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ACKNOWLEDGMENTS
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We thank Dr. Susan Nimmo, Dr. Steven Foster, and Dr. Douglas
R. Powell from the Research Support Services, University of
Oklahoma, for expert NMR, mass spectral, and X-ray crystallo-
graphic analyses, respectively. The work was supported by NSF
CHE-1753187, the Oklahoma Center for the Advancement of
Science and Technology (OCAST, HR16-095), and the
American Chemical Society Petroleum Research Fund (ACS-
PRF) Doctoral New Investigator grant (PRF No. 58487-DNI1).
We also thank Dr. Kiran Chinthapally for helpful discussions and
useful insight into the experimental design.
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