Magnesium Monoperphthalate (MMPP): a Convenient Oxidant for the Direct Rubottom Oxidation of Malonates, β-Keto Esters, and Amides

Article abstract of DOI:10.1002/ejoc.202100098

A mild and convenient protocol for the α-hydroxylation of α-substituted malonates, β-ketoesters, and β-ketoamides is disclosed. Cheap and stable magnesium monoperphthalate (MMPP) was effective for a first direct α-hydroxylation protocol of the Rubottom oxidation, providing tartronic esters, cyclic α-hydroxy β-ketoesters and amides in good to high yields, when working at room temperature and in ethanol as the solvent. The protocol has been successfully scaled up to gram quantity.

Full text of DOI:10.1002/ejoc.202100098

Communications
doi.org/10.1002/ejoc.202100098
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Magnesium Monoperphthalate (MMPP): a Convenient
Oxidant for the Direct Rubottom Oxidation of Malonates,
β-Keto Esters, and Amides
Sara Meninno,*[a] Rosaria Villano,[b] and Alessandra Lattanzi*[a]
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Dedicated to Professor Franco Cozzi on the occasion of his 70th birthday.
A mild and convenient protocol for the α-hydroxylation of α-
substituted malonates, β-ketoesters, and β-ketoamides is dis-
closed. Cheap and stable magnesium monoperphthalate
(MMPP) was effective for a first direct α-hydroxylation protocol
of the Rubottom oxidation, providing tartronic esters, cyclic α-
hydroxy β-ketoesters and amides in good to high yields, when
working at room temperature and in ethanol as the solvent.
The protocol has been successfully scaled up to gram quantity.
Scheme 1. Examples of biologically active α-hydroxy-β-ketoesters and
amides.
The α-hydroxy carbonyl unit is a recurrent motif, found in
several bioactive compounds, pharmaceuticals and synthetically
been accomplished by a variety of oxidative systems, except
the most common laboratory oxidant such as MCPBA.[10] Indeed,
competitive oxidations can occur, affecting the selectivity and
applicability, thus the use of their silyl enol ethers as alternative
starting reagents is required (Scheme 2).[10b]
This transformation, well-known as the Rubottom
reaction,[17] although being a widespread and useful α-hydrox-
ylation reaction in organic synthesis,[18] requires prederivatiza-
tion of carbonyl compounds to silyl enol ethers and occasion-
ally TBAF deprotection of the firstly formed silyl ether, which
represent drawbacks from the step-economic point of view. In
this context, magnesium monoperphthalate (MMPP),[19]
although being cheaper, stable and practical peroxy acid than
popular MCPBA, was only briefly studied by Gannon and House
useful
intermediates,
exemplified
by
antibiotics
kjellmanianone,[1] hamigeran A[2] and antimicrobial agent pra-
manicin (Scheme 1).[3]
Moreover, α-hydroxy malonates, known as tartronic esters,
demonstrated to be useful building blocks for the synthesis of
drugs,[4] or bioderived intermediates for surfactant
preparation.[5] Unsurprisingly, direct α-hydroxylation of the
corresponding 1,3-dicarbonyl compounds was intensively inves-
tigated as the most straightforward process to prepare these
compounds.[6] Hence, numerous methodologies have been
developed over the last decades, mostly based on enolate
oxidation with different systems, including MoOPH,[7] Fe/H2O2,[8]
Ni/dimethyl dioxirane (DMDO),[9] MCPBA,[10] ROOH,[11] Oxone,[12]
Ce/O2,[13] IBX,[14] oxazirines.[15] The search for simple and
inexpensive methodologies, proceeding under convenient con-
ditions, using green solvents at room temperature and readily
available oxidants, has become an imperative to follow in
modern oxidation chemistry.[16]
The direct α-hydroxylation of α-substituted β-dicarbonyl
compounds, such as β-ketoesters/amides and malonates, has
[a] Dr. S. Meninno, Prof. Dr. A. Lattanzi
Dipartimento di Chimica e Biologia “A. Zambelli”
Università di Salerno
Via Giovanni Paolo II 132, 84084, Fisciano, Italy
E-mail: smeninno@unisa.it
lattanzi@unisa.it
https://docenti.unisa.it/003250/en/home
[b] Dr. R. Villano
Istituto di Chimica Biomolecolare - CNR,
Via Campi Flegrei 34, 80078 Pozzuoli, Italy
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejoc.202100098
Scheme 2. Routes for peroxy acid mediated α-hydroxylation of carbonyl
compounds.
Part of the “Franco Cozzi’s 70th Birthday” Special Collection.
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in the direct oxidation of two α-substituted β-diketones.[20] The
was isolated in 88% and 87% yield, respectively. Pleasingly, in
both cases, only a small amount (�10%) of the transesterifica-
tion α-hydroxylated product, was observed. These results are
noteworthy, since they indicate that transesterification of differ-
ent malonate esters would not occur when using EtOH as the
solvent. In order to reduce the amount of MMPP to 1
equivalent, inorganic bases (1 equiv) were added, with the aim
to help enolization, working in anhydrous ethanol. Pleasingly,
good conversions were observed, after reasonable reaction
times, when using Na2CO3 and NaOH (entries 10 and 11). The
use of 96% ethanol with NaOH provided lower conversion to
product 2a (entry 12). Pleasingly, when using NaHCO3 in
anhydrous ethanol, compound 2a was recovered in 84% yield
(entry 13). Given the presence of two peroxyacid molecules in
the MMPP salt, 0.5 equivalent of MMPP was used in the
oxidation (entry 14). After a comparable reaction time, the
product was isolated in 66% yield, suggesting that 1 equivalent
of MMPP is required to achieve higher conversion to the
product. Finally, MCPBA was checked in the hydroxylation of 1a
either in the presence or absence of NaHCO3 (entries 15 and
16). The results confirmed MCPBA to be an unsuitable oxidant,
given the presence of unreacted 1a in the reaction mixture.
With the optimized conditions in hands, a variety of α-
substituted malonates was investigated under the conditions
reported in entry 13 of Table 1 (Table 2).
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oxidation proved to be unselective, observing the formation of
α-hydroxy-β-diketones, together with the carboxylic acids and
α-hydroxy ketones. Although these results were not encourag-
ing, as part of our interest in the α-hydroxylation of β-
dicarbonyl compounds[21] and considering the practical features
of MMPP as an oxidant, we undertook an investigation focused
on the hydroxylation of α-substituted malonates, β-ketoesters
and amides. We herein report the preliminary results, showing
the suitability of MMPP to serve as a useful and practical
oxidant in the direct hydroxylation reaction, working under
green conditions.
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First, the optimization of model 2-benzyl malonate methyl
ester 1a was studied (Table 1). The reaction was carried out
°
with 2 equivalents of the oxidant at 70 C in MeOH, being
MMPP more soluble in polar media (entry 1). Pleasingly, after a
short time, α-hydroxylated product 2a was isolated in 64%
yield. Reducing the amount of MMPP (1.2 equiv) drastically
retarded the conversion to the product to only 10% (entry 1 in
parenthesis). Hence, the reactions were successively performed
with 2 equivalents of MMPP. The oxidation could be carried out
at room temperature to give 2a in 58% yield after 16 hours
(entry 2). Under more concentrated conditions, the conversion
to 2a was significantly accelerated (entry 3), although further
concentration of the reaction mixture, was not useful (entry 4).
At this point, green polar solvents were checked in the
oxidation performed at C=0.5 M of reagent 1a (entries 5–7).
Unfortunately, the hydroxylation did not proceed in dimethyl
carbonate (DMC), ethyl acetate and in water using tetrabutyl
ammonium chloride (nBu)4NCl as phase transfer catalyst.
Different α-alkyl substituted tartronic esters 2a–d were
isolated in good yields.[22] α-Functionalized malonates were also
converted into the α-hydroxyl derivatives 2e–h in fairly good
yields. The α-allyl substituted diethyl malonate 1i was then
subjected to oxidation to check the chemoselectivity of MMPP.
The α-hydroxylated product 2i was recovered as prevalent
compound in 52% yield, whereas product of further oxidation,
Interestingly, when using 96% ethanol (entry 8) or anhy-
drous ethanol (entry 9) after a short reaction time, product 2a
Table 1. Optimization of the α-hydroxylation of malonate ester 1a.
Entry[a]
Solvent
Additive
Conditions
2a [%][b]
[c]
1
MeOH
MeOH
MeOH
MeOH
DMC
AcOEt
H2O
–
–
–
–
–
–
70 C/3 h(5 h)
64(10)[c]
58
68
59
nd
–
°
°
2
25 C/16 h
3[d]
25 C/5.5 h
°
4[e]
25 C/4 h
°
5[d]
25 C/24 h
°
6[d]
25 C/24 h
°
7[d]
TBAC 20 mol%
–
–
25 C/7 h
–
°
8[d]
EtOH 96%
EtOH
25 C/4 h
88
87
73
69
60
84
66
–
°
9[d]
25 C/4 h
°
10[d,f]
11[d,f]
12[d,f]
13[d,f]
14[d,g]
15[h]
16[h]
EtOH
EtOH
EtOH 96%
EtOH
EtOH
EtOH
EtOH
Na2CO3 (1 equiv)
NaOH (1 equiv)
NaOH (1 equiv)
NaHCO3 (1 equiv)
NaHCO3 (1 equiv)
NaHCO3 (1 equiv)
–
25 C/7 h
°
°
25 C/3.5 h
°
25 C/4 h
°
25 C/8.5 h
°
25 C/8.5 h
°
25 C/6 h
°
20 C/16 h
–
[a] Reaction conditions: 1a (89 mg, 0.4 mmol), MMPP (technical �80%, 495 mg, 2 equiv) in 4 mL of solvent (at C=0.1 M [1a]). [b] Isolated yield after
chromatography. [c] MMPP (1.2 equiv) was used. [d] Reaction performed at C=0.5 M [1a]. [e] Reaction performed at C=1 M [1a]. [f] MMPP (1.0 equiv) was
used. [g] MMPP (0.5 equiv) was used. [h] MCPBA (1.0 equiv) was used.
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Table 2. Scope of the α-hydroxylation of malonate esters 1 to tartronic
Table 3. Scope of the α-hydroxylation of β-keto esters and amides 4.[a,b]
1
2
esters 2.[a,b]
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[a] Reaction conditions: 1 (0.4 mmol), MMPP (technical �80%, 247 mg,
1 equiv), NaHCO3 (34 mg, 1 equiv) in anhydrous EtOH (0.8 mL) at C=0.5 M
[1]. [b] Isolated yield after chromatography.
[a] Reaction conditions: 4 (0.4 mmol), MMPP (technical �80%, 247 mg,
1 equiv), NaHCO3 (34 mg, 1 equiv) in anhydrous EtOH (0.8 mL) at C=0.5 M
[4]. [b] Isolated yield after chromatography. [c] MMPP (0.5 equiv) was used.
the epoxy alcohol 3i, was isolated in 22% yield. Notably, the
oxidation proved to be chemoselective when carried out using
Finally, to study the applicability of this oxidative system,
model reaction on compound 1a was scaled up to 6 mmol
(Scheme 3). Pleasingly, after slightly prolonged reaction time,
compound 2a was isolated in excellent yield, demonstrating
the efficiency of MMPP mediated hydroxylation on a larger
scale. The result is of interest, considering that in contrast to
MCPBA, MMPP is a non-shock-sensitive and non-deflagrating
peroxy acid, with improved water solubility, making easier the
work-up procedure.
Ester analogues of compound 2a, obtained by α-hydrox-
ylation reaction of the corresponding malonates, have been
used by a Merck group as key-intermediates for the synthesis of
nonsteroidal mineralocorticoid receptor antagonists.[26]
The oxidation is thought to proceed via the generally
accepted one-pot Rubottom type-oxidation (Scheme 4).[6a,10b]
Enol tautomer of the starting 1,3-dicarbonyl compound, whose
presence is fostered by the basic additive, is epoxidized by
MMPP. The hydroxyl epoxide intermediate then rearranges to
the final α-hydroxy derivative.
°
1.5 equivalents of MMPP at 50 C. Indeed, compound 2i was
isolated in 65% yield[22] and traces of epoxy alcohol 3i were
detected. This result might be ascribed to increased enolization
of the reagent and then faster epoxidation of more nucleophilic
enol moiety (see Scheme 4). Interestingly, MMPP displayed
reversed chemoselectivity in the oxidation of compound 1i
with respect to DMDO.[23,9b] Indeed, DMDO did not provide
compound 2i, but the oxidation of terminal carbon-carbon
double bond occurred selectively to give the epoxide. Then,
further oxidation of epoxide with DMDO yielded product 3i.
Notably, MMPP appears to be the sole oxidant useful to obtain
the tartronic ester 2i in chemoselective manner.[24]
The study was then extended to α-substituted β-ketoesters
and amides. Under the optimized conditions, some cyclic β-
keto esters underwent α-hydroxylation to give the products
5a–c in good to high yields (Table 3). In the case of sensitive
cyclopentanone based β-keto ester 4d, the reaction was carried
out using 0.5 equivalent of MMPP. The expected product 5d
was isolated in 78%, observing only traces of further oxidative
cleavage side-products 6a/6b.[25]
Acyclic α-substituted cyclohexyl 3-oxo-3-phenylpropanoate
underwent the oxidation to compound 5e in 50% yield.
Unfortunately, α-methyl ethyl acetoacetate 4f did not
provide the expected compound, but further oxidation to low
molecular side-products was observed. Secondary aryl and alkyl
substituted β-ketoamides were converted into the hydroxylated
products 5g–i in fairly good yields.
Scheme 3. Scale-up of the model reaction on compound 1a.
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Scheme 4. Mechanism of hydroxylation mediated by MMPP.
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In conclusion, we disclosed a simple and useful method-
ology for the direct α-hydroxylation of malonates, β-ketoesters
and amides by MMPP, which proceeds at room temperature in
EtOH as green solvent. To the best of our knowledge this is a
first example of Rubottom hydroxylation, which do not require
prederivatization of the β-dicarbonyl compounds to silyl enol
ethers. Commercially available oxidant MMPP furnished the
products in good to high yields, achieving comparable results
also at gram-scale. This work adds to the list of oxidative
reactions enabled by using MMPP[19b,27] as a practical and mild
peroxy acid with a distinctive and advantageous behavior with
respect to MCPBA.[28] Further investigations on the reactivity of
MMPP, in direct hydroxylation reactions, are underway in our
laboratory.
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Acknowledgements
MUR, University of Salerno and ICB-CNR are acknowledged for
financial support. S.M. thanks MUR and European Union for AIM –
International attraction and mobility call for researchers funded
by PON RI 2014–2020. R.V. thanks ICB-CNR for technical support.
Conflict of Interest
The authors declare no conflict of interest.
Keywords: Green solvents · Hydroxylation · β-Ketoesters ·
MMPP · Tartronic esters
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Manuscript received: January 27, 2021
Revised manuscript received: February 19, 2021
Accepted manuscript online: February 24, 2021
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