Hemisynthesis of 2,3,4-13C3-1,4-androstadien-3,17-dione: A key precursor for the synthesis of 13C3-androstanes and 13C3-estranes

Article abstract of DOI:10.1021/acs.joc.7b03216

In this contribution, we describe two simple and efficient routes for the preparation of keto-aldehyde 1, a key intermediate for the synthesis of ¹³C3-androstanes and ¹³C3-estranes. In the first route, the targeted aldehyde 1 was obtained in 40% overall yield from 1,4-androstadien-3,17-dione (3 mmol scale) via a two-step sequence involving a one-pot, abnormal ozonolysis/sulfur oxidation/retro-Michael/ozonolysis process. Alternatively, a second route from 4-androsten-3,17-dione, using a six-step sequence, was optimized to produce 40 mmol batches of the key intermediate 1 in 42% overall yield. At the final stage, the A-ring was reconstructed through a Wittig reaction with the 1-triphenylphosphoranylidene-¹³C3-2-propanone 2, followed by an intramolecular condensation assisted by thioacetic acid via a Michael addition/retro-Michael reaction sequence to provide 2,3,4-¹³C3-1,4-androstadien-3,17-dione.

Full text of DOI:10.1021/acs.joc.7b03216

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Article
Hemisynthesis of 2,3,4-13C3-1,4-androstadien-3,17-dione: A key
precursor for the synthesis of 13C3-androstanes and 13C3-estranes
Clément Berthonneau, Pierrick Nun, Matthieu Rivière, Mickael Pauvert, Fabrice Dénès, and Jacques Lebreton
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b03216 • Publication Date (Web): 01 Mar 2018
Downloaded from http://pubs.acs.org on March 2, 2018
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Hemisynthesis of 2,3,4-13C3-1,4-androstadien-3,17-dione: A key precursor for the
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synthesis of 13C3-androstanes and 13C3-estranes.
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Clément Berthonneau‡,§, Pierrick Nun§, Matthieu Rivière§, Mickael Pauvert‡, Fabrice Dénès§ and Jacques
Lebreton‡,§*
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§Université de Nantes, Chimie et Interdisciplinarité: Synthèse, Analyse, Modélisation (CEISAM), UMR
CNRS 6230, 2, rue de la Houssinière – BP 92208 – 44322 NANTES Cedex 3 (France)
‡Atlanchim Pharma, 3, Rue Aronnax, 44800 SAINT HERBLAIN (France)
jacques.lebreton@univꢀnantes.fr
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jacques.lebreton@atlanchimpharma.com
Abstract
In this contribution, we describe two simple and efficient routes for the preparation of ketoꢀaldehyde 1, a
key intermediate for the synthesis of 13C3ꢀandrostanes and 13C3ꢀestranes. In the first route, the targeted
aldehyde 1 was obtained in 40% overall yield from 1,4ꢀandrostadienꢀ3,17ꢀdione (3 mmol scale) via a twoꢀ
step sequence involving a oneꢀpot, abnormal ozonolysis/sulfur oxidation/retroꢀMichael/ozonolysis process.
Alternatively, a second route from 4ꢀandrostenꢀ3,17ꢀdione, using a sixꢀstep sequence, was optimized to
produce 40 mmol batches of the key intermediate 1 in 42% overall yield. At the final stage, the Aꢀring was
reconstructed through a Wittig reaction with the 1ꢀtriphenylphosphoranylideneꢀ13C3ꢀ2ꢀpropanone 2,
followed by an intramolecular condensation assisted by thioacetic acid via a Michael addition/retroꢀMichael
reaction sequence to provide 2,3,4ꢀ13C3ꢀ1,4ꢀandrostadienꢀ3,17ꢀdione.
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Key words: Steroids, 13Cꢀlabeled compounds, hemisynthesis, oxidation, ozonolysis, Wittig reaction,
Palladiumꢀcatalyzed decarbonylative dehydration.
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Introduction
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Due to the wide spectrum of biological activitites of both natural and synthetic steroids,1 the detection and
quantification of steroidal residues in various biological materials are crucial for clinical and
pharmacological studies. For instance, it is extremely useful for absorption, distribution, metabolism, and
excretion (ADME) studies leading to the development of novel steroidal drugs2 and for doping prevention to
fight against the use of anabolic steroids3 in sports competitions. In addition, the quantification of steroid
traces is an issue for environmental protection. For example, it has been demonstrated that estrogen
contained in birth control pills, and thus eliminated by the body via the urinary system and discharged into
wastewaters, is an endocrine disruptor of aquatic fauna.4
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The analytical technique of stable isotopic dilution5 (SID) by Gas ChromatographyꢀMass Spectrometry (GCꢀ
MS) or Liquid ChromatographyꢀMass Spectrometry (LCꢀMS) is widely employed today to conduct accurate
and reproducible steroid quantification in clinical and biochemical studies. This firstꢀchoice technique
requires the use of an internal standard, an isotopologue labeled with stable isotopes (usually 13C or
deuterium) with a high isotopic purity and presenting a difference of at least three mass units with the parent
unlabeled compound. This second criterion nullifies the effect of the natural abundance of heavy isotopes
contained in the labeled standard. Deuteriumꢀlabeled standards can be readily accessed by postꢀsynthetic
hydrogenꢀdeuterium exchange reactions catalyzed by transition metals (mainly palladium and platinum),
using deuterium oxide (D2O) as the cheapest deuterium source. Compared to conventional synthetic
approaches, postꢀsynthetic hydrogenꢀdeuterium exchange is an easy and economical methodology to prepare
multiꢀdeuterated compounds. Since the pioneering work of Sajiki,6 many deuterated compounds have been
prepared by hydrogenꢀdeuterium exchange catalyzed by transition metals, with D2O as a deuterium source,
under a hydrogen atmosphere.7 To avoid the handling of gaseous hydrogen, Derdau and Atzrodt8
significantly improved this methodology by using deuteride donors, such as NaBD4, for preꢀactivation of the
catalyst. Nevertheless, the disadvantages of hydrogenꢀdeuterium exchange include nonꢀspecific and
incomplete deuteration affording deuterated standards with an inevitably uneven isotope distribution.
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Moreover, it has been frequently pointed out that the use of deuterated standards suffers from deuteriumꢀ
hydrogen back exchange at different steps of the analysis. For these reasons, 13Cꢀlabeled standards are
usually preferred to deuteriumꢀlabeled standards, due to the negligible isotopic effect during 13Cꢀlabeling
and the robustness of the standards during metabolic processes and/or preꢀanalytical chemical treatments of
the samples, and despite the cost of the 13Cꢀlabeled starting materials and/or reagents used for their
syntheses.
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To address the key issue of the synthesis of 13Cꢀlabeled steroids at reasonable cost and, ideally, with high
versatility, we describe here an efficient and short hemisynthesis of 2,3,4ꢀ13C3ꢀ1,4ꢀandrostadienꢀ3,17ꢀdione
(2,3,4-13C3-ADD) starting from commercially available unlabeled ADD and using the triply labeled 13C
[1,2,3ꢀ13C3]ꢀ1ꢀ(triphenylphosphoranylidene)ꢀ2ꢀpropanone 2 as the threeꢀcarbon building block (see Scheme
1). The latter was efficiently prepared in three steps, using wellꢀestablished procedures, from two relatively
inexpensive reagents: [13C2]ꢀacetyl chloride and 13Cꢀiodomethane. From a strategic point of view, it was
crucial to use this precious 13C3ꢀlabeled reagent at the final stage of our synthesis to incorporate the three
13C atoms into the framework of ADD.
Scheme 1: Retrosynthesis for 13C3-ADD.
It should be pointed out that the choice of ADD as target was dictated by the fact that the resulting 13C3-
ADD could be converted into other important members of this steroid family, such as 13C3ꢀandrogens and
13C3ꢀestrogens, using a limited number of wellꢀestablished transformations (see Scheme 2).9
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Scheme 2: Preparation of various [2,3,4ꢀ13C3]ꢀenriched steroids from [2,3,4ꢀ13C3]ꢀADD.10
Two synthetic routes to [3,4ꢀ13C2]ꢀenriched11 and [2,3,4ꢀ13C3]ꢀenriched12 steroids have been previously
reported. In the first one, the enone of the Aꢀring of ADD was opened either by ozonolysis11a followed by
treatment with hydrogen peroxide or by oxidative cleavage with KMnO4/KIO4.11b The treatment of the
resulting keto acid with Ac2O or AcCl in the presence of catalytic amounts of HClO4 led to the enol lactone
intermediate, which was converted into the corresponding lithium enolate, then subjected to acylation with
[1,2ꢀ13C2]ꢀacetyl chloride. Acidic treatment led to hydrolysis of the lactone and the resulting βꢀketo acid
underwent spontaneous decarboxylation, affording the dione. Subsequent Robinson annulation gave the
[3,4ꢀ13C2]ꢀenriched steroid. This efficient and straightforward fourꢀstep sequence allowed the incorporation
of only two 13C atoms into the steroid framework. In the second route, Vierhapper12 described the synthesis
of [2,3,4ꢀ13C3]ꢀenriched testosterone from unlabeled testosterone via a key 10ꢀformylꢀ5ꢀoxoꢀdesꢀA
intermediate similar to compound 1 depicted in Scheme 1. The first step of the sequence involved a tertꢀ
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butoxide/18ꢀCrownꢀ6 mediated autoxidation of the kinetic enolate of testosterone under a stream of
molecular oxygen over a long period of time (up to 90 hours). The resulting lactone intermediate was then
reduced in a second step into the corresponding 2ꢀoxasteroid. The targeted 10ꢀformylꢀ5ꢀoxoꢀdesꢀA
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intermediate was obtained following silylation of the 17βꢀhydroxyl, and ozonolysis and oxidation of the
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primary alcohol into the corresponding aldehyde.
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Results and discussion
In the light of these previous works, it was clear to us that the 10ꢀformylꢀ5ꢀoxoꢀdesꢀA intermediate was the
best common intermediate to prepare a large variety of [2,3,4ꢀ13C3]ꢀenriched steroids. Following
Vierhapper’s work,12 we became interested in both shortening the synthetic route and finding an alternative
to the tricky tertꢀbutoxide/18ꢀCrownꢀ6 mediated autoxidation used in the first step of the sequence to
provide a safe and reliable route to large quantities of 10ꢀformylꢀ5ꢀoxoꢀdesꢀA intermediate 1.
To this end, we initially focused our attention on the double oxidative cleavage of the two carbonꢀcarbon
double bonds in the Aꢀring of ADD. This strategy would rapidly deliver the desired ketoꢀaldehyde 1
(Scheme 3). Unfortunately, the Aꢀring of ADD was found to be completely inert under standard ozonolysis
conditions and under oxidative cleavage conditions with potassium permanganate. Breaking the electronic
conjugation of the Aꢀring by a regioselective reduction of carbonyl at Cꢀ3, with concomitant reduction of the
carbonyl at Cꢀ17, has been considered.10c The resulting cyclohexadienol 3 was found to be rather unstable
and led to either aromatization of the Aꢀring by a benzeneꢀdienol rearrangement13 or a complex mixture of
degradation products under oxidative cleavage conditions.
Scheme 3: Initial attempts at the double oxidative cleavage of the Aꢀring of ADD and cyclohexadienol 3.
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Turning to this critical double simultaneous oxidative cleavage of the Aꢀring carbonꢀcarbon double bonds of
ADD, we envisaged that the cleavage could be achieved in a sequential manner using a oneꢀpot procedure
depicted in Scheme 4. A regioselective thioꢀ or selenoꢀMichael addition onto the more reactive carbonꢀ
carbon double bond of ADD was considered as a temporary protection. This would allow the oxidative
cleavage of the remaining 4,5ꢀcarbonꢀcarbon double bond by ozone to be achieved. Under these oxidative
conditions, we expected to form in situ the corresponding sulfoxide (or selenoxide), which could regenerate
spontaneously the masked carbonꢀcarbon double bond via a βꢀelimination reaction. The resulting α,βꢀ
unsaturated acid 5 was expected to undergo oxidative cleavage under the atmosphere of ozone and thus
provide the desired 10ꢀformylꢀ5ꢀoxoꢀdesꢀA intermediate 1 following this oneꢀpot multiꢀstep process.
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Scheme 4: Plausible synthetic route to ketoꢀaldehyde 1 involving a oneꢀpot multiꢀstep sequence from 4a–d.
Four heteroꢀMichael adducts 4a–d of ADD were prepared to evaluate our strategy. Following known
procedures,14 thioacetate 4a and the phenyl sulfide 4b adducts were obtained in high yields from ADD (see
Table 1). In the case of compound 4b, slightly modified reaction conditions were employed to avoid the use
of thiophenol as solvent and metallic sodium. Under the same conditions, the ethanethiol adduct 4c was
obtained with a moderate yield of 43%. To our delight, the selenoꢀMichael addition promoted by a catalytic
amount of γꢀcyclodextrin gave the selenoꢀadduct 4d in 92% yield under smooth reaction conditions. It is
worth noting that the use of βꢀcyclodextrin did not lead to a complete conversion of ADD.15 In all cases,
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only the diastereoisomer resulting from the nucleophilic attack on the less hindered αꢀside was obtained.
This preference for the attack on the αꢀside was confirmed by an Xꢀray diffraction crystallography analysis
of the thiophenol adduct 4b (see Supporting Information).
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Table 1: Preparation of adducts 4a-d from ADD.
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Entry
Conditions
Yield a
4a: 89 %
4b: 83 %
4c: 43 %
AcSH (10.3 equiv), THF/MeOH/H2O (10:10:1), rt, 12 h
PhSH (10 equiv), PhSNa (5 equiv), THF, rt, 48 h
EtSH (4 equiv), NaH (1 equiv), THF, rt, 96 h
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PhSeH (1 equiv), γꢀCD (10 mol%), H2O/Acetone (15:2), rt, 6 h 4d: 92 %
a Isolated yield.
With these four heteroꢀMichael adducts 4a-d in hand, we investigated the abnormal ozonolysis16,17 of the
remaining enone function (see Table 2). We started our investigations with thiophenol adduct 4b as the
model substrate. When a short contact (3 min) with ozone was applied in dichloromethane at –78 °C,
followed by purging the system with Argon to remove the excess of ozone, then warming to room
temperature, only a mixture of the starting material 4b and ADD was obtained, with no traces of the desired
ketoꢀaldehyde 1 or alternatively, ketoꢀacid intermediate 5, being detectable in the crude reaction mixture
(Table 2, Entry 1). This result clearly indicated that the first transformation occurring in this process is the
oxidation of the sulfide into the corresponding sulfoxide (or sulfone), which undergoes a retroꢀMichael
reaction to afford ADD. At this stage it remained unclear whether the βꢀelimination occurred at low
temperature or, as expected, during the warm up to room temperature. Ketoꢀacid 5 was observed as the
minor product in the reaction mixture (28%) when ozone bubbling was applied for a longer period of time
(30 min) at –78 ºC, with the major component still being ADD (Table 2, Entry 2). As ADD was found to be
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unreactive under ozonolysis conditions, the formation of α,βꢀunsaturated acid 5 (in its Eꢀconfiguration) in
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this second experiment tends to indicate that the βꢀelimination of the sulfoxide moiety occurred upon
warming to room temperature, after ozone has been removed from the system, and not at low temperature.
The use of methanol and acetic acid as coꢀsolvents did not improve the conversion (Table 2, Entries 3 and
4). Interestingly, the use of ethyl acetate as a solvent for the ozonolysis considerably improved the
conversion (Table 2, Entries 5–7) and, after one hour of reaction at –78 ºC then warming to room
temperature, only the expected ketoꢀacid 5 was observed in the crude mixture (Table 2, Entry 7).
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Table 2: Study of the reaction conditions for the ozonolysis of 4b.
Entrya
Conditions
O3, DCM, 3 minc
Treatment
Me2S (10 equiv)
Me2S (10 equiv)
NaOH aq.
Products (%)b
ADD (55): 4b (45)
ADD (72): 5 (28)
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O3, DCM, 30 min
O3, DCM, MeOH, 45 min
ADD (80): 5 (traces)d
ADD (60): 5 (traces)d
O3, DCM, MeOH, AcOH, 1 h 30 NaOH aq.
min
O3, AcOEt, 45 min
O3, AcOEt, 45 min
O3, AcOEt, 1 h
Me2S (10 equiv)
ADD (18): 5 (82) c
ADD (15): 5 (85)d
ADD (–): 5 (>95)
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NaOH aq.
Me2S (10 equiv)
a All reactions were performed on a 3 mmol scale, first at –78 °C under an atmosphere of O3 for the period
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of time indicated, then at room temperature under an atmosphere of Argon; Ratios were determined by 1H
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NMR of the crude mixture; The deep blue coloration, characteristic of a saturated ozone medium, was
reached after 3 min of ozone bubbling; d Several unidentified degradation products were formed.
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Having secured the abnormal ozonolysis/sulfur oxidation/retroꢀMichael sequence to form the α,βꢀ
unsaturated acid 5 from 4b, we next evaluated the oneꢀpot process in which the first ozonolysis intermediate
5 would be directly involved in a second ozonolysis reaction to provide the key ketoꢀaldehyde 1. The
doubleꢀozonolysis sequence was carried out on the four heteroꢀMichael adducts 4a–d. For the first part of
this oneꢀpot, multiꢀstep protocol, we used the optimized conditions described in Table 2 to reach
intermediate 5. The reaction mixture was then cooled down back to –78 °C, treated with ozone for one hour
and finally reacted with an excess of dimethyl sulfide. As presented in Table 3, the phenylsulfide 4b gave
the best yield (43%) (Table 3, Entry 2), while other thioꢀ or selenoꢀMichael adducts 4a and 4c–d were found
to be less efficient (Table 3, Entries 1, 3 and 4). To explain the formation of the ketoꢀaldehyde 1, we
proposed the mechanism depicted in the supporting information.18
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Table 3: Oneꢀpot process for the synthesis of 1 from 4a-d.
Entry a
Substrate
R
Yield (%) b
AcS
PhS
EtS
0
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4b
4c
4d
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PhSe
a All reactions were performed on 3 mmol of adduct 4; b Isolated yield.
To the best of our knowledge, this approach represents the shortest access reported so far to the ketoꢀ
aldehyde 1, a key precursor of 13C3ꢀlabeled steroids, from a commercially available steroid. Overall, this
oneꢀpot, multiꢀstep sequence proceeded in an acceptable 36% yield on a 3 mmol scale from ADD, via the
thiophenyl sulfide 4b intermediate. This sequence provided ca. 500 mg of pure ketoꢀaldehyde 1, which in
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the field of the synthesis of labeled compounds is a reasonable scale. Unfortunately, on larger scales the
overall yield dropped to 20–25%.
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Given the difficulties encountered in carrying out the direct double oxidative cleavage of the cyclic dienone
function of ADD (or its dienol derivative), we examined an alternative route to produce ketoꢀaldehyde 1
from commercial 4ꢀandrostenꢀ3,17ꢀdione (AD), without the need for any ozonolysis steps. We focused our
attention on the use of the diketoꢀacid 6, which is easily accessible in a single step by an oxidative cleavage
of the 4ꢀenꢀ3ꢀone function of AD19 (Scheme 5). In their previous works, Vierhapper and coꢀworkers
attempted to convert carboxylic intermediates such as compound 6 into the corresponding vinyl derivatives
via an oxidative decarboxylation reaction using either lead tetraacetate or diacetoxyꢀiodobenzene (DAIB).12b
Despite their efforts to optimize these oxidative decarboxylation reactions, the best yields with lead
tetraacetate and DAIB remained modest (42 and 34%, respectively) and the targeted terminal olefin was
obtained only in small amounts.
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Inspired by these seminal works, we evaluated the palladiumꢀcatalyzed decarbonylative dehydration of the
carboxylic acid 7 as a key step to furnish the key vinyl 8. The latter could then lead to ketoꢀaldehyde 1 after
a second oxidative cleavage. Although this reaction has been known for a long time and used for the
production of olefins from biomass feedstocks,20 surprisingly we found very few examples of metalꢀ
catalyzed decarbonylative dehydration applied in a multiꢀstep synthesis.21 First, the palladiumꢀcatalyzed
decarbonylative dehydration conditions published by Scott22 on aliphatic carboxylic acids were evaluated on
our substrate 7. It should be pointed out that both ketone functions at Cꢀ3 and Cꢀ17 on compound 6 were
protected to avoid the formation of the cylic enolꢀlactone intermediate11a during the activation of the
carboxylic acid with Ac2O. Starting material 7 remained essentially unchanged even after prolonged reaction
times (up to three days) and only low yields of alkene 8 were obtained (Table 4, Entries 1–4). The
quaternary center in acid 7 could be responsible for the lack of reactivity compared to the results reported in
the literature with linear aliphatic carboxylic acids. Gratifyingly, after some experimentation, we noted that
higher reaction temperatures (up to 245 °C) provided a good conversion. In particular, we found that the
combination of catalytic amounts of palladium dichloride and triphenylphosphine at 250 °C previously
described by Miller23 was effective in furnishing the expected olefin 8 in good yields (Table 4, Entries 5–7).
Interestingly, decarbonylation of the acyl palladium intermediate was initiated when the reaction
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temperature reached 245 °C, as noted by the appearance of an intense red coloration concomitantly with the
liberation of CO from the reaction flask.
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Scheme 5: Synthesis of 1 from AD. Reaction conditions: a) KMnO4 (6 mol%), NaIO4 (5.7 equiv), Na2CO3
(1.2 equiv), iꢀPrOH/H2O (1:1), 70 °C, 4 h; b) Ethylene glycol (3 equiv), (EtO)3CH (3 equiv), PTSA.H2O (10
mol%), DCM, reflux, 16 h; c) Ac2O (1.1 equiv), PdCl2 (3 mol%), PPh3 (0.75 equiv), DMPU, 250 °C, 75 min
; d) AcOH/H2O (1:1), reflux, 1 h; e) OsO4 (5 mol%), NMO (4 equiv), 2,6ꢀlutidine (2 equiv), Acetone/H2O
(2:1), rt, 60 h; f) SiO2ꢀsupported NaIO4 (1.7 equiv), DCM, rt, 4 h.
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Table 4
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Yield
(%)b
16
Entrya
Additives
Ligand
Solvent
T °C, Time
Ac2O (1 equiv), TEA (1 equiv)
Ac2O (1 equiv), TEA (1 equiv)
Ac2O (1 equiv), TEA (1 equiv)
Ac2O (1 equiv), TEA (1 equiv)
Ac2O (1 equiv)
DPEꢀPhos
DPEꢀPhos
DPEꢀPhos
DPEꢀPhos
PPh3
DMPU
DMPU
DMPU
CH3CN
110 °C, 14 h
115 °C, 18 h
120 °C, 3 d
82 °C, 18 h
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5
6
7
19
20
18
DMPU 250 °C, 30 min
DMPU 250 °C, 75 min
DMPU 250 °C, 75 min
50
Ac2O (1 equiv)
PPh3
62
Ac2O (1.1 equiv)
PPh3
73
a All reactions were performed on 1 mmol of 7 with 3 mol% of PdCl2; b Isolated yield.
This palladiumꢀcatalyzed decarbonylative dehydration reaction was routinely carried out on a 35 mmol scale
to provide the desired vinyl intermediate 8. The latter was then directly treated in refluxing aqueous acetic
acid to afford olefin 9 in 68% yield over the two steps, after purification by flash chromatography on silica
gel. Oxidative cleavage of 9 was successfully accomplished in two steps by Upjohn dihydroxylation,
followed by treatment using silica gelꢀsupported sodium metaperiodate,24 to give the key ketoꢀaldehyde 1 in
75% overall yield. It should be pointed out that dihydroxylation of the vinyl compound 9 afforded the
expected diastereomeric diols, which were present in solution mostly in their corresponding hemiketal forms
10.
Having secured two efficient routes to the key ketoꢀaldehyde 1 following either a twoꢀstep sequence from
ADD on a small scale (ca. 3 mmol) or a robust, ozoneꢀfree, sixꢀstep sequence from AD on a larger scale (ca.
40 mmol), delivering 1 in 36% and 42% overall yields, respectively, the last phase of our investigation was
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The Journal of Organic Chemistry
the reconstruction of the Aꢀring to provide the targeted 13C3ꢀADD. To this end, the preparation of 1ꢀ
triphenylphosphoranylideneꢀ13C3ꢀ2ꢀpropanone 2, necessary for the incorporation of three 13C atoms via a
Wittig olefination, was accomplished in two steps,25 starting from inexpensive 13Cꢀiodomethane and 13C2ꢀ
acetyl chloride. In order to address the issue of the cost of the synthesis, an improved sequence involving a
13C2ꢀacetyl imidazole intermediate was used (see Supporting Information for details).
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The Wittig olefination of ketoꢀaldehyde 1 with 1ꢀtriphenylphosphoranylideneꢀ13C3ꢀ2ꢀpropanone 2 was
carried out using reported reaction conditions, furnishing exclusively the corresponding Eꢀenone 11 in 75 %
yield (Scheme 6). Extensive experimentation to increase the yield of this transformation under microwave
activation,26 or by using additives such as benzoic acid 27 or silica gel,28 or by replacing the organic solvent
by water,29 proved unsuccessful. On the contrary, a negative effect was observed in most cases, as these
conditions tended to favor the simple deformylation reaction of ketoꢀaldehyde 1.30
Scheme 6: Synthesis of 13C3ꢀADD from ketoꢀaldehyde 1. Reaction conditions: a) 2 (1 equiv), pꢀxylene, 150
°C, 7 days; b) AcSH (1.2 equiv), PTSA.H2O (0.3 equiv), toluene, 75 °C, 18 h, then DBU (2 equiv), rt, 30
min.
The last challenge of this synthesis was to develop a ringꢀclosing ketolization of the (E)ꢀenone 11 under
reaction conditions enabling the isomerization of the (E)ꢀcarbonꢀcarbon double bond into the (Z)ꢀisomer in
order to furnish the targeted 13C3-ADD in a single step. To the best of our knowledge, only one report
describes the ketolization of an (E)ꢀenone leading to the formation of a cyclic dienone under a refluxing
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methanolic solution of potassium hydroxide.31 Not surprisingly, these harsh conditions applied to (E)ꢀenone
11 led to complete degradation. The geometry issue was addressed by treating (E)ꢀenone 11 with a slight
excess of thioacetic acid under acidic conditions. Under these conditions, the thioꢀMichael adduct underwent
cyclization and the resulting intermediate was then treated with an excess of DBU to promote the retro thioꢀ
Michael reaction. Following this oneꢀpot, threeꢀstep process, the targeted 13C3ꢀADD was obtained in good
yield (75 %).
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Conclusion
With current analytical techniques, appropriately 13Cꢀlabeled steroids with a high isotopic purity and at least
three 13Cꢀlabels are crucial as internal standards for the analysis and quantification of steroids present in
trace amounts in various matrices or as probes for clinical or biological investigations. Although there are
several syntheses of 13Cꢀlabeled steroids, their preparation requires a significant number of steps and
supplies the quantities needed for routine use with difficulty. In this context, we have optimized a sequence
to prepare 13Cꢀlabeled steroids based upon ketoꢀaldehyde 1, which is routinely obtained by degradation of
the Aꢀring of commercially available ADD or AD. Thus, we have developed two different routes; on a 3
mmol scale, a twoꢀstep sequence afforded this key compound 1 in 36% overall yield from ADD, whereas a
more convenient largeꢀscale synthesis (ca. 40 mmol) provided compound 1 following a sixꢀstep sequence in
42% overall yield from AD. Both strategies represent a significant improvement in existing methods in
terms of efficiency, scale and cost. As an illustration of our investigations in this field, 13C3ꢀADD was
prepared from ketoꢀaldehyde 1 by reconstruction of the Aꢀring via a Wittig reaction with the 1ꢀ
triphenylphosphoranylideneꢀ13C3ꢀ2ꢀpropanone 2 followed by the intramolecular aldolizationꢀcrotonization
condensation assisted by thioacetic acid.
Experimental Section
General information:
Waterꢀsensible reactions were performed under an argon atmosphere in flameꢀdried glassware. All solvents
used were reagent grade. Solvents for waterꢀsensible reactions were purchased in anhydrous grade and were
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The Journal of Organic Chemistry
used without further purification. 13Cꢀiodomethane (isotopic purity, 99 atom % 13C) and 13C2ꢀacetyl chloride
(isotopic purity, 99 atom % 13C) were purchased from common suppliers. TLC were performed on 60 F 254
silicaꢀcovered aluminum sheets. Eluted TLC were revealed using UV radiation (λ= 254 nm), vanillin,
phosphomolybdic acid, or potassium permanganate solutions. Column chromatography were performed on
silica gel 40ꢀ63 ꢁm. Ozone gas for ozonolysis reactions was generated by an air cooled ozone generator.
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NMR spectra were recorded at 400 MHz for H, 100 MHz for 13C at 303 K, on samples dissolved in
appropriate deuterated solvent. Used references were deuterated solvent signal for H and 13C. Chemical
1
displacement values (δ) are expressed in parts per million (ppm), and coupling constants (J) in Hertz (Hz).
Lowꢀresolution mass spectra were performed on a UPLC/MS in positive electrospray ionization (ESI+).
Some mass spectra were performed on a quadripolar in electron impact (EI) (70 eV). Highꢀresolution mass
spectrometry (HRMS) analyses were performed at the « plateforme de spectrométrie de masse de l’IRSꢀUN
à Nantes » on a LCꢀQꢀTof spectrometer in positive electrospray ionization (ESI+) or at LABERCA
(« laboratoire d'étude des résidus et contaminants dans les aliments ») on a LTQꢀOrbitrap spectrometer in
electrospray ionization. Melting points were measured on both a Kofler bench and a Tottoli system and are
uncorrected. Optical rotations were determined with a polarimeter equipped with a Na lamp (589 nm) at a
concentration c given in g/100mL in CHCl3.
S-((1S,8S,9S,10R,13S,14S)-10,13-dimethyl-3,17-dioxo-2,3,6,7,8,9,10,11,12,13,14,15,16,17-
tetradecahydro-1H-cyclopenta[a]phenanthren-1-yl) ethanethioate (4a)32
To a solution of ADD (1.56 g, 5.5 mmol, 1.0 equiv) in 42 mL of a THF/MeOH/H2O mixture (10:10:1), was
added thioacetic acid (4.0 mL, 56.5 mmol, 10.3 equiv) and the reaction mixture was stirred under argon at
room temperature. After all starting material was consumed (ca. 12 h, as indicated by TLC analysis), the
reaction mixture was diluted with dichloromethane (100 mL) and washed three times with a saturated
aqueous solution of NaHCO3. Resulting organic phase was dried over anhydrous Na2SO4 and concentrated
under vacuum. The resulting yellow solid was triturated with 5 mL of Et2O for 3 h at room temperature and
the solid was filtered and rinsed with a mixture of cyclohexane/Et2O (3:1). The resulting white solid was
1
dried under vacuum to furnish thioacetate 4a (1.77 g, 89 %). H NMR (400 MHz; CDCl3) δH 5.73 (bs, 1H),
4.04 (t, 1H, J = 3.3 Hz), 2.95 (dd, 1H, J = 17.2, 3.7 Hz), 2.55 (ddd, 1H, J = 17.2, 2.0, 0.8 Hz), 2.44 (dd, 1H,
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J = 19.1, 8.8 Hz), 2.41–2.36 (m, 2H), 2.29 (s, 3H,), 2.13–1.89 (m, 3H), 1.79 (dt, 1H, J = 12.9, 3.3 Hz), 1.69
(ddd, 1H, J = 22.0, 10.9, 3.3 Hz), 1.60–1.50 (m, 2H), 1.42ꢀ1.17 (m, 4H), 1.38 (s, 3H), 1.13–1.02 (m, 1H),
0.88 (s, 3H). 13C NMR (100 MHz; CDCl3) δC 219.9 (C), 195.9 (C), 194.4 (C), 167.6 (C), 124.8 (CH), 50.9
(CH), 49.4 (CH), 48.8 (CH), 47.5 (C), 41.8 (CH2), 41.6 (C), 35.8 (CH2), 35.3 (CH), 32.4 (CH2), 31.2 (CH3),
31.1 (CH2), 29.8 (CH2), 21.8 (CH2), 19.6 (CH2), 19.5 (CH3), 13.7 (CH3). HRMS (ESI+): calcd. for
C21H28O3SNa [(M+Na+]: 383.1657; Found: 383.1659. Mp 196ꢀ198 °C. [α]D20 = 177.3 (c 0.23, CHCl3).
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(1S,8S,9S,10R,13S,14S)-10,13-dimethyl-1-(phenylthio)-7,8,9,10,11,12,13,14,15,16-decahydro-1H-
cyclopenta[a]phenanthrene-3,17(2H,6H)-dione (4b)14
To a solution of ADD (8.53 g, 30 mmol, 1.0 equiv) in anhydrous THF (150 mL) under argon was added
thiophenol (30.6 mL, 300 mmol, 10.0 equiv) and sodium thiophenolate (19.81 g, 150 mmol, 5.0 equiv). The
resulting mixture was stirred for 48 h at room temperature, diluted with 250 mL of dichloromethane and 100
mL of water were added. The organic layer was washed five times with 150 mL of a halfꢀsaturated aqueous
solution of Na2CO3, and then washed with water and brine before dried over anhydrous Na2SO4. After
filtration and evaporation, the pale yellow solid residue was triturated in a minimum of Et2O for 5 h at room
temperature. Filtration and high vacuum drying afforded the desired sulfide 4b (9.80 g) in 83 % yield as a
white crystalline solid. 1H NMR (400 MHz; CDCl3) δH 7.41–7.38 (m, 2H), 7.33–7.25 (m, 3H), 5.82 (bd, 1H,
J = 0.9 Hz), 3.54 (t, 1H, J = 3.25 Hz), 2.77 (dd, 1H, J = 16.9, 0.9 Hz), 2.59 (ddd, 1H, J = 16.9, 3.0, 0.9 Hz),
2.49 (ddd, 1H, J = 19.1, 9.1, 1.1 Hz), 2.46–2.42 (m, 2H), 2.12 (dt, 1H, J = 19.1 Hz, 9.1 Hz), 2.05–1.84 (m,
5H), 1.76 (ddd, 1H, J = 11.6, 11.6, 10.5 Hz), 1.64–1.33 (m, 4H), 1.38 (s, 3H), 1.29–1.13 (m, 1H), 0.93 (s,
3H). 13C NMR (100 MHz; CDCl3) δC 220.2 (C), 196.1 (C), 165.7 (C), 134.0 (C), 133.7 (2CH), 129.4 (2CH),
128.1 (CH), 124.8 (CH), 54.5 (CH), 50.9 (CH), 48.0 (CH), 47.6 (C), 42.8 (C), 39.8 (CH2), 35.9 (CH2), 35.4
(CH), 32.5 (CH2), 31.2 (CH2), 29.8 (CH2), 21.9 (CH2), 20.0 (CH3), 19.7 (CH2), 13.9 (CH3). HRMS (ESI+):
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calcd. for C25H31O2S [(M+H)+]: 395.2045; Found: 395.2029. Mp 188.1ꢀ189.3 °C. [α]D = 97.7 (c 0.15,
CHCl3).
(1S,8S,9S,10R,13S,14S)-1-(ethylthio)-10,13-dimethyl-7,8,9,10,11,12,13,14,15,16-decahydro-1H-
cyclopenta[a]phenanthrene-3,17(2H,6H)-dione (4c)
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To a suspension of sodium hydride, 60 % dispersion in mineral oil, (200 mg, 5.0 mmol, 1.0 equiv) in
anhydrous THF (10 mL) under argon was carefully added ethanethiol (0.6 mL, 20.0 mmol, 4.0 equiv) at 0
°C. A solution of ADD (1.42 g, 5.0 mmol, 1.0 equiv) in anhydrous THF (10 mL) was then added dropwise.
The reaction mixture was stirred for 4 days at room temperature, and quenched with a saturated aqueous
solution of NH4Cl. The aqueous phase was extracted twice with ethyl acetate (50 mL) and the combined
organic layers were washed sequentially with water and brine and then dried over anhydrous Na2SO4. After
evaporation of volatiles under vacuum, NMR analysis of crude product showed the presence of a 1:1
mixture of starting material and expected product 4c. The crude mixture was subjected to column
chromatography on silica gel (cyclohexane:EtOAc, 7:3 to 6:4) to afford the sulfide compound 4c as a white
powder (745 mg, 43 %). 1H NMR (400 MHz; CDCl3) δH 5.76 (bd, 1H, J = 0.9 Hz), 3.17 (t, 3H, J = 3.3 Hz),
2.90 (dd, 1H, J = 16.8, 3.3 Hz), 2.71 (ddd, 1H, J = 16.8, 3.3, 0.9 Hz), 2.59–2.43 (m, 3H,), 2.41–2.37 (m,
2H), 2.10 (dt, 1H, J = 19.1, 9.0 Hz), 2.03–1.95 (m, 1H), 1.93ꢀ1.82 (m, 3H), 1.75–1.66 (m, 2H), 1.57 (tt, 1H,
J = 12.4, 9.0 Hz), 1.48–1.31 (m, 3H), 1.36 (s, 3H), 1.22 (t, 3H, J = 7.4 Hz), 1.20–1.07 (m, 1H), 0.91 (s, 3H).
13C NMR (100 MHz; CDCl3) δC 220.3 (C), 196.4 (C), 166.2 (C), 124.5 (CH), 50.9 (CH), 49.5 (CH), 47.8
(CH), 47.6 (C), 42.7 (C), 40.0 (CH2), 35.9 (CH2), 35.3 (CH), 32.6 (CH2), 31.2 (CH2), 29.7 (CH2), 25.5
(CH2), 21.9 (CH2), 20.0 (CH3), 19.6 (CH2), 14.6 (CH3), 13.8 (CH3). HRMS (ESI+): calcd. for C21H30O2SNa
[(M+Na)+]: 369.1864; Found: 369.1861. Mp 174.5ꢀ176.1 °C. [α]D20 = 212.2 (c 0.115, CHCl3).
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(1S,8S,9S,10R,13S,14S)-10,13-dimethyl-1-(phenylselanyl)-7,8,9,10,11,12,13,14,15,16-decahydro-1H-
cyclopenta[a]phenanthrene-3,17(2H,6H)-dione (4d)
γꢀCyclodextrine (130 mg, 0.1 mmol, 10 mol%) was dissolved in water (15 mL) at room temperature. A
solution of benzeneselenol (157 mg, 1.0 mmol, 1.0 equiv) in acetone (1 mL) and a solution of ADD (284
mg, 1.0 mmol, 1.0 equiv) in acetone (1 mL) were added dropwise. The reaction mixture was stirred for 6 h
at room temperature. After this time, TLC analysis indicated the complete conversion of starting material.
Dichloromethane (30 mL) was then added and the layers were partitioned into a separation funnel. Aqueous
layer was reꢀextracted with 30 mL of dichloromethane and the combined organic layers were washed with
brine. After drying over anhydrous Na2SO4 and evaporation under vacuum, the crude product was purified
by column chromatography on silica gel (cyclohexane:EtOAc, 7:3) to yield the expected phenyl selenide
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1
compound 4d as a white powder (406 mg, 92%). H NMR (400 MHz; CDCl3) δH 7.55–7.52 (m, 2H), 7.33ꢀ
7.25 (m, 3H), 5.82 (bd, 1H, J = 0.9 Hz), 3.61 (t, 1H, J = 3.3 Hz), 2.99 (dd, 1H, J = 17.2, 3.6 Hz), 2.67 (ddd,
1H, J = 17.2, 3.0, 0.9 Hz), 2.48 (dd, 1H, J = 19.2, 8.6 Hz), 2.44–2.40 (m, 2H), 2.12 (dt, 1H, J = 19.2, 8.6
Hz), 2.04ꢀ1.92 (m, 2H), 1.89–1.83 (m, 2H), 1.79–1.70 (m, 2H), 1.58 (tt, 1H, J = 12.2, 12.2 Hz), 1.54–1.31
(m, 3H), 1.36 (s, 3H), 1.25ꢀ1.08 (m, 1H), 0.92 (s, 3H). 13C NMR (100 MHz; CDCl3) δC 220.1 (C), 196.3 (C),
166.2 (C), 136.0 (2CH), 129.4 (2CH), 128.8 (C), 128.4 (CH), 124.8 (CH), 51.6 (CH), 50.8 (CH), 50.3 (CH),
47.5 (C), 43.1 (C), 40.8 (CH2), 35.9 (CH2), 35.5 (CH), 32.5 (CH2), 31.1 (CH2), 29.8 (CH2), 21.9 (CH2), 19.8
(CH2), 19.2 (CH3), 13.9 (CH3). HRMS (ESI+): calcd for C25H31O276Se [(M+H)+]: 439.1516; Found:
439.1507. Mp 192.1ꢀ193.8 °C. [α]D20 = 128.4 (c 0.116, CHCl3).
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(E)-3-((3aS,5aS,6R,9aS,9bS)-3a,6-dimethyl-3,7-dioxododecahydro-1H-cyclopenta[a]naphthalen-6-
yl)acrylic acid (5)
Phenyl sulfide 4b (1.2 g, 3.04 mmol, 1.0 equiv) was dissolved in 100 mL of EtOAc under argon atmosphere.
This solution was cooled to –78 °C and treated with an ozone bubbling for 1 h at –78 °C. The reaction
mixture was then purged with an argon bubbling for 30 min and dimethyl sulfide (2.23 mL, 30.4 mmol, 10.0
equiv) was added and the mixture was allowed to reach room temperature. Stirring was maintained 14 h at
room temperature and a 1 N solution of Na2CO3 (100 mL) was added. The biphasic solution was stirred
vigorously for 15 minutes at room temperature and the two phases were separated. The aqueous layer was
washed with ethyl acetate (100 mL) and then acidified to pH= 2 with a 37 % aqueous solution of
hydrochloric acid. After extraction with dichloromethane (2 × 150 mL), the resulting organic phases were
combined, washed with water (150 mL), brine (250 mL) and then dried over anhydrous Na2SO4. The solvent
was removed by evaporation, and the crude expected acid 5 was obtained as white powder. The crude
mixture was subjected to column chromatography on silica gel (AcOEt/MeOH, 10:0 to 8:2) to afford the
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acid 5 as a white powder (0.95g, 95%, E:Z = 92:8). H NMR (400 MHz; CDCl3) δH 7.05 (d, 1H, J = 16.1
Hz), 5.82 (d, 1H, J = 16.1 Hz), 2.66 (td, 1H, J = 14.4, 6.3 Hz), 2.50 (dd, 1H, J = 19.2, 8.6 Hz), 2.42 (ddd,
1H, J = 14.5, 4.4, 2.4 Hz), 2.19–1.86 (m, 4H), 1.83ꢀ1.79 (m, 1H), 1.68–1.23 (m, 7H), 1.35 (s, 3H), 0.93 (s,
3H), 0.90ꢀ0.77 (m, 1H). 13C NMR (100 MHz; CDCl3) δC 219.7 (C), 211.8 (C), 170.6 (C), 154.1 (CH), 121.3
(CH), 54.7, 50.91, 50.83, 47.9, 37.7, 35.8, 34.4, 31.0, 30.4, 21.99, 21.93, 15.5 (CH3), 13.9 (CH3). HRMS
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The Journal of Organic Chemistry
(ESI+): calcd for C18H24O4Na [(M+Na)+]: 327.1572; Found: 327.1570. Mp 201 °C. [α]D = 130.3 (c 0.056,
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CHCl3).
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(3aS,5aS,6S,9aS,9bS)-3a,6-dimethyl-3,7-dioxododecahydro-1H-cyclopenta[a]naphthalene-6-
carbaldehyde (1) 12a
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Procedure 1:
Phenyl sulfide 4b (1.2 g, 3.04 mmol, 1.0 equiv) was dissolved in 100 mL of EtOAc under argon atmosphere.
This solution was cooled to –78 °C and treated with an ozone bubbling for 1 h at –78 °C. The reaction
mixture was then purged with an argon bubbling for 30 min and allowed to reach room temperature. The
resulting yellow solution was stirred for 1 h at 25 °C. The mixture was then cooled to –78 °C and treated
with ozone bubbling for 1 h. After a purge by argon bubbling, dimethyl sulfide (2.23 mL, 30.4 mmol, 10.0
equiv) was added and the mixture was allowed to reach room temperature. Stirring was maintained 14 h at
room temperature and water (100 mL) was added. The organic phase was washed with brine and dried over
anhydrous sodium sulfate and concentrated under vacuum. The yellow oily residue was subjected to column
chromatography on silica gel (cyclohexane:EtOAc, gradient 7:3 to 6:4) to afford the expected diketoꢀ
aldehyde 1 as a white crystalline solid in 43 % yield.
Procedure 2:
Preparation of silica gel supported sodium periodate: Sodium periodate (25.7 g, 120 mmol) was dissolved
in 50 mL of hot water (80 °C). To this hot solution was added silica gel (230ꢀ400 mesh, 100 g) with
vigorous stirring and shaking. The resultant powder and freeꢀflowing silica gel coated with NaIO4 was
placed in a mortar and was finely grounded with a pestle. The resulting fine powdered solid can be stored in
a closed bottle for months. Oxidative cleavage: Compound 10 (5.53 g, 18.78 mmol) was dissolved in
dichloromethane (240 mL) and silica gelꢀsupported sodium periodate (50 g) was added. The reaction
mixture was stirred vigorously at room temperature for 4 h. At this point, TLC analysis showed complete
conversion of starting material. Dichloromethane was removed under vacuum and the resulting crude
product adsorbed on silica gel was subjected to a short column chromatography on silica gel
(cyclohexane:EtOAc, 65:35) to afford diketoꢀaldehyde 1 as a white crystalline solid (4.53 g, 92 %). 1H NMR
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(400 MHz; CDCl3) δH 9.53 (s, 1H), 2.58 (dt, 1H, J = 14.4, 6.5 Hz), 2.51 (dd, 1H, J = 19.0, 8.8 Hz), 2.42
(ddd, 1H, J = 14.4, 4.5, 2.4 Hz), 2.21–2.00 (m, 3H), 1.96–1.80 (m, 3H), 1.64 (tt, 1H, J = 12.3, 8.9 Hz), 1.55–
1.38 (m, 3H), 1.35–1.25 (m, 1H), 1.30 (s, 3H), 1.24–1.16 (m, 1H), 0.93 (s, 3H). 13C NMR (100 MHz;
CDCl3) δC 219.3 (C), 212.2 (C), 201.1 (C), 62.5 (C), 50.7 (CH), 47.9 (C), 46.1 (CH), 38.1 (CH2), 35.6
(CH2), 34.0 (CH), 30.9 (CH2), 30.0 (CH2), 21.9 (CH2), 21.8 (CH2), 13.9 (CH3), 12.6 (CH3). HRMS (ESI+):
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calcd for C16H22O3Na [(M+Na)+]: 285.1467; Found: 285.1465. Mp 199.5ꢀ201.0 °C. [α]D = 75.6 (c 0.348,
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CHCl3).
3-((3aS,5aS,6R,9aR,9bS)-3a,6-dimethyl-3,7-dioxododecahydro-1H-cyclopenta[a]naphthalen-6-
yl)propanoic acid (6)33
In a 2L roundꢀbottom flask was placed a solution of AD (11.45 g, 40.0 mmol, 1.0 equiv) in iꢀPrOH (460
mL). A solution of sodium carbonate (5.09 g, 48.0 mmol, 1.2 equiv) in water (40 mL) was added and the
mixture was heated at 70 °C. In a separate flask, potassium permanganate (379 mg, 2.4 mmol, 6 mol%) and
sodium periodate (48.77 g, 228.0 mmol, 5.7 eq.) were dissolved in hot water (360 mL, 70 °C) and this
purple solution was carefully and slowly added to the reaction flask. The resulting mixture was vigorously
stirred at 70 °C until TLC analysis revealed complete consumption of starting material (ca. 4 h). The
reaction mixture was cooled to room temperature and filtered through a pad of Celite®. The pad was rinsed
with DCM (100 mL) and then with a 0.5 N aqueous solution of NaOH (200mL). Dichloromethane and iꢀ
PrOH were removed under reduced pressure and the resulting basic aqueous solution was washed with
dichloromethane (2 × 200 mL) and then acidified to pH= 2 with a 37 % aqueous solution of hydrochloric
acid. After extraction with dichloromethane (2 × 250 mL), the resulting organic phases were combined,
washed with water (150 mL), brine (250 mL) and then dried over anhydrous Na2SO4. The solvent was
removed by evaporation, and the expected acid 6 (12.24 g, white foam) was used in the next step without
further purification. An analytical sample was purified by column chromatography on silica gel
(cyclohexane:EtOAc, 3:7 to 1:9). 1H NMR (400 MHz; CDCl3) δH 10.90 (bs, 1H), 2.53 (td, 1H, J = 14.4 Hz,
6.3 Hz), 2.42 (dd, 1H, J = 19.2, 8.7 Hz), 2.32–2.24 (m, 2H), 2.21ꢀ2.13 (m, 1H), 2.10–2.00 (m, 3H), 1.97ꢀ
1.77 (m, 3H), 1.60–1.44 (m, 4H), 1.31–1.18 (m, 4H), 1.08 (s, 3H, H19), 0.87 (s, 3H, H18). 13C NMR (100
MHz; CDCl3) δC 220.2 (C), 214.0 (C), 179.3 (C), 50.7 (CH), 50.4 (C), 48.0 (CH), 47.6 (C), 37.7 (CH2), 35.7
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The Journal of Organic Chemistry
(CH2), 34.4 (CH), 30.9 (CH2), 29.9 (CH2), 29.2 (CH2), 29.1 (CH2), 21.8 (CH2), 20.7 (CH2), 20.3 (CH3), 13.7
(CH3). HRMS (ESI+): calcd for C18H26O4Na [(M+Na)+]: 329.1729; Found: 329.1741. Mp 107.8ꢀ109.7 °C.
[α]D20 = 105.1 (c 0.216, CHCl3).
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3-((3a'S,5a'S,6'R,9a'R,9b'S)-3a',6'-dimethyldecahydro-5'H-dispiro[[1,3]dioxolane-2,3'-
cyclopenta[a]naphthalene-7',2''-[1,3]dioxolan]-6'-yl)propanoic acid (7)
The crude diketoꢀacid 6 (12.24 g, 39.95 mmol, 1.0 equiv) was dissolved in dichloromethane (100 mL) under
argon. Ethylene glycol (6.7 mL, 120.0 mmol, 3.0 equiv), triethyl orthoformate (20.0 mL, 120.0 mmol, 3.0
equiv) and pꢀtoluenesulfonic acid monohydrate (761 mg, 4.0 mmol, 0.1 equiv) were added and the reaction
mixture was heated at reflux for 16 h and then cooled to room temperature. Water (100 mL) was added and
the organic layer was separated. Aqueous layer was extracted twice with DCM (2 × 100 mL) and the
combined organic layers were washed with brine (100 mL) and dried over Na2SO4. After filtration and
solvent evaporation under vacuum, the residue was dissolved in methanol (100 mL). A 2 N aqueous solution
of NaOH (50 mL) was added and the mixture was heated at reflux with vigorous stirring for 1.5 h and then
cooled room temperature. Water (250 mL) was added and methanol was evaporated under vacuum. The
aqueous layer was washed twice with DCM (2 × 200 mL) and acidified to pH= 2–3 with a 37 % aqueous
solution of hydrochloric acid. After two extractions with DCM (2 × 200 mL) the obtained organic phases
were combined, washed with water (200 mL), brine (250 mL) and dried over anhydrous Na2SO4. The
solvent was evaporated under vacuum, and the expected product 7 (13.7 g, 87 %, white foam) was used in
the next step without further purification. An analytical sample was purified by column chromatography on
1
silica gel (cyclohexane:EtOAc, 6:4). H NMR (400 MHz; C6D6) δH 3.65–3.30 (m, 8H), 2.9–2.7 (m, 1H),
2.6–2.4 (m, 1H), 2.3–2.0 (m, 2H), 1.95–1.05 (m, 15H), 0.93 (s, 3H), 0.72 (s, 3H). 13C NMR (100 MHz;
C6D6) δC 181.8 (C3), 119.5 (C), 113.9 (C), 65.3 (CH2), 64.7 (CH2), 64.2 (CH2), 64.0 (CH2), 50.2 (CH), 46.4
(C), 46.1 (CH), 43.2 (C), 35.4 (CH), 34.7 (CH2), 31.1 (CH2), 31.0 (CH2), 30.4 (CH2), 30.0 (CH2), 27.7
(CH2), 23.1 (CH2), 21.6 (CH2), 18.5 (CH3), 14.8 (CH3). HRMS (ESI+): calcd for C22H34O6Na [(M+Na)+]:
417.2253; Found: 417.2246. Mp 92.5 °C. [α]D20 = 12.9 (c 0.14, CHCl3).
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(3a'S,5a'S,6'R,9a'S,9b'S)-3a',6'-dimethyl-6'-vinyldecahydro-5'H-dispiro[[1,3]dioxolane-2,3'-
cyclopenta[a]naphthalene-7',2''-[1,3]dioxolane] (8)
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9
Diketal 7 (13.7 g, 34.73 mmol, 1.0 equiv) was dissolved in DMPU (70 mL) under argon and acetic
anhydride (3.61 mL, 38.20 mmol, 1.1 equiv) was added. The resulting solution was stirred at room
temperature for 10 min and triphenylphosphine (6.83 g, 26.04 mmol, 0.75 equiv) was added followed by
palladium dichloride (185 mg, 1.04 mmol, 3 mol%). The reaction mixture was degassed (vacuum/argon,
three times) and heated to 250 °C. The rate of decarbonylation is followed by liberation of CO (reaction
begins about 245 °C with apparition of an intense red coloration). Heating was maintained 75 min at 250 °C
after beginning of bubbling and cooled to room temperature. The resulting dark mixture was filtered through
a pad of Celite® and the pad was rinsed with Et2O (300 mL). Water (250 mL) was added to the filtrate and
layers were separated. Aqueous layer was reꢀextracted by Et2O (300 mL) and combined organic layers were
washed with a 2 N aqueous solution of NaOH (250 mL), water (200 mL), brine (200 mL) and dried over
Na2SO4. Solvent was evaporated to afford 15.7 g of crude product 8, which was used in the next step
without further purification. An analytical sample was purified by column chromatography on silica gel
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(cyclohexane:EtOAc, 9:1). H NMR (400 MHz; CDCl3) δH 5.83 (dd, J = 17.7, 11.0 Hz, 1H), 5.09 (dd, J =
11.0, 1.7 Hz, 1H), 5.03 (dd, J = 17.7, 1.7 Hz, 1H), 3.95–3.79 (m, 8H), 1.98 (ddd, J = 14.2, 11.4, 2.8 Hz, 1H),
1.83–1.17 (m, 14H), 1.10 (s, 3H), 0.83 (d, J = 0.5 Hz, 3H). 13C NMR (100 MHz; CDCl3) δC 144.0 (CH),
119.4 (C), 114.6 (CH2), 113.0 (C), 65.4 (CH2), 65.31 (CH2), 65.27 (CH2), 64.7 (CH2), 50.0 (CH), 48.0 (C),
47.7 (CH), 46.4 (C), 34.8 (CH), 34.4 (CH2), 31.4 (CH2), 30.7 (CH2), 27.7 (CH2), 22.7 (CH2), 22.0 (CH2),
14.6 (CH3), 13.5 (CH3). HRMS (ESI+): calcd for C21H32O4Na [(M+Na)+]: 371.2198; Found: 371.2186. Mp
153.3ꢀ156.7 °C. [α]D20 = 25.4 (c 0.063, CHCl3).
(3aS,5aS,6R,9aS,9bS)-3a,6-dimethyl-6-vinyloctahydro-1H-cyclopenta[a]naphthalene-3,7(2H,3aH)-
dione (9)
Crude vinylic compound 8 (15.7 g) obtained in the previous step was placed in an AcOH:H2O mixture (1:1,
350 mL) under argon atmosphere. The reaction mixture was heated at reflux temperature for 1 h and then
cooled to room temperature. At this point TLC showed complete conversion of starting material. The half
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The Journal of Organic Chemistry
initial volume of solvent was removed under vacuum and a saturated aqueous solution of sodium carbonate
(800 mL) was slowly added via a dropping funnel at 0 °C. The resulting aqueous solution was extracted
three times with Et2O (3 × 500 mL) and the combined organic layers were washed with water (300 mL) and
brine (300 mL) before drying over Na2SO4. Solvent was removed under vacuum and the yellow solid
residue was subjected to a short column chromatography on silica gel (cyclohexane:EtOAc, 75:25) to afford
expected diketoꢀolefin 9 as a white crystalline solid (6.11 g). (Overall yield from AD: 59 %). 1H NMR (400
MHz; CDCl3) δH 5.78 (dd, 1H, J = 17.6, 10.9 Hz), 5.25 (dd, 1H, J = 10.9, 1.0 Hz), 5.03 (dd, 1H, J = 17.6,
1.0 Hz), 2.62 (td, 1H, J = 14.3, 6.3 Hz), 2.48 (ddd, 1H, J = 19.1, 9.0, 0.8 Hz), 2.38 (ddd, 1H, J = 14.3, 4.5,
2.5 Hz), 2.15–2.06 (m, 2H), 2.02–1.95 (m, 1H), 1.91 (bdq, 1H, J = 10.7, 3.6 Hz), 1.79 (ddd, 1H, J = 13.0,
3.8, 2.8 Hz), 1.61 (tt, 1H, J = 12.2, 9.0 Hz), 1.55–1.20 (m, 5H), 1.26 (s, 3H), 0.92 (d, 3H, J = 0.6Hz). 13C
NMR (100 MHz; CDCl3) δC 219.9 (C), 213.6 (C), 141.4 (CH), 114.9 (CH2), 54.6 (C), 51.1 (CH), 51.0 (CH),
48.0 (C), 37.8 (CH2), 35.8 (CH2), 34.5 (CH), 31.1 (CH2), 30.4 (CH2), 21.9 (CH2), 21.7 (CH2), 15.2 (CH3),
13.9 (CH3). HRMS (ESI+): calcd. for C17H24O2Na [(M+Na)+]: 283.1674; Found: 283.1665. Mp 142.3ꢀ143.9
°C. [α]D20 = 88.2 (c 0.11, CHCl3).
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(3aS,3bS,5aS,8aS,8bS)-3,10a-dihydroxy-3a,5a-dimethyldodecahydro-2H-cyclopenta[5,6]naphtho[2,1-
b]furan-6(7H)-one (10)
In a 250 mL roundꢀbottom flask, compound 9 (6.37 g, 24.46 mmol, 1.0 equiv) was dissolved in acetone (100
mL) and water (50 mL) was added followed by 2,6ꢀlutidine (5.7 mL, 48.93 mmol, 2.0 equiv), Nꢀ
methylmorpholineꢀNꢀoxide (11.46 g, 97.86 mmol, 4.0 equiv) and a 4% aqueous solution of osmium
tetroxide (7.8 mL, 1.22 mmol, 5 mol%). The flask was hermetically closed under an argon atmosphere and
the solution was stirred for 60 h at room temperature. At this point, TLC analysis showed complete
conversion of starting material. The reaction mixture was poured into a saturated aqueous solution of sodium
thiosulfate (100 mL) and the resulting mixture was stirred vigorously for 30 min. Acetone was removed
under vacuum and the resulting aqueous phase was extracted four times with dichloromethane (4 × 200 mL).
The combined organic layers were dried over Na2SO4 and solvent was removed under reduced pressure and
the residue was subjected to column chromatography on silica gel (cyclohexane:EtOAc, 2:8) to afford 5.54 g
(76 % yield) of a mixture of 2 diastereoisomers of hemiketal 10 and 2 diastereoisomers of open form of the
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Page 24 of 31
expected dihydroxylated compound as viscous oil. The following spectral data refers to the major isomer
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hemiketal form (ca. 80 % in CDCl3). H NMR (400 MHz; CDCl3) δH 4.24 (dd, 1H, J = 10.4 5.4 Hz), 4.03
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9
(bdd, 1H, J = 9.0, J = 5.4 Hz), 3.92 (dd, 1H, J = 10.4, 1.0 Hz), 3.19 (s, 1H), 3.06 (d, 1H, J = 9.0 Hz), 2.45
(bdd, 1H, J = 19.6, 8.6 Hz), 2.07 (dt, 1H, J = 19.6, 9.0 Hz), 1.98–1.91 (m, 2H), 1.83 (bdt, 1H, J = 13.0, 3.4
Hz), 1.69–1.41 (m, 6H), 1.27–1.18 (m, 2H), 1.18–1.12 (m, 1H), 1.12 (s, 3H), 0.89 (s, 3H), 0.75 (bdt, 1H, J =
11.1, 4.6 Hz). 13C NMR (100 MHz; CDCl3) δC 220.6 (C), 107.2 (C), 77.3 (CH), 74.3 (CH2), 51.1 (CH), 50.9
(C), 47.9 (C), 46.0 (CH), 35.9 (CH2), 34.7 (CH), 31.69 (CH2), 31.66 (CH2), 26.6 (CH2), 22.3 (CH2), 21.9
(CH2), 13.9 (CH3), 10.1 (CH3). HRMS (ESI+): calcd. for C17H26O4Na [(M+Na)+]: 317.1729; Found:
317.1718.
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[13C2]-acetylimidazole (12)34
To a solution of 1Hꢀimidazole (2.79 g, 40.99 mmol, 2.0 equiv) in a THF:Et2O mixture (25 mL, 1:1) at 0 °C,
was slowly added [13C2]ꢀacetyl chloride (1.65 g, 20.49 mmol, 1.0 equiv) and the resulting suspension was
vigorously stirred for 2 h. The salts were removed by filtration of the mixture on Büchner, washed twice
with 30 mL of diethyl ether and the solution was evaporated under vacuum. The white solid was dried over
P2O5 under vacuum to yield 91 % (2.09 g) of the expected labeled compound 12, which was used in the next
step without further purification. 1H NMR (400 MHz; CDCl3) δH 8.13 (bs, 1H), 7.46 (bs, 1H), 7.10 (bs, 1H),
2.60 (dd, 3H, JCꢀH = 130.1 Hz, JCꢀH = 6.7 Hz). 13C NMR (100 MHz; CDCl3) δC 166.5 (d, JCꢀC = 53.3 Hz, 13C),
136.5 (CH), 131.3 (CH), 116.3 (CH), 23.0 (d, JCꢀC = 53.3 Hz, 13CH3).
[13C]-methyltriphenylphosphonium iodide (13)35
Triphenylphosphine (13,9 g, 52.8 mmol, 1.5 equiv) was solubilized in anhydrous toluene (50 mL) under
argon and the solution was cooled to 0 °C. 13CꢀIodomethane (5.0 g, 35.0 mmol, 1.0 equiv) was quickly
added and the flask was hermetically closed. The solution was stirred for 12 h at room temperature and the
resulting white suspension was filtered on Büchner. The white solid was rinsed three times with 50 mL of
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anhydrous toluene and then dried under vacuum over P2O5 to afford Cꢀlabeled phosphonium iodide 13 in
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quantitative yield (14.1 g). H NMR (400 MHz; DMSO d6) δH 7.93ꢀ7.86 (m, 3H), 7.80ꢀ7.74 (m, 12H), 3.17
(dd, 3H, JCꢀH = 134.9 Hz, JPꢀH = 14.6 Hz). 31P NMR (162 MHz, DMSO d6): δP 22.65 (d, JCꢀP = 55.5 Hz). 13C
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The Journal of Organic Chemistry
NMR (100 MHz; DMSO d6) δC 134.8 (d, JCꢀP = 2.9 Hz, 3CH), 133.2 (d, JCꢀP = 10.8 Hz, 3CH), 130.1 (d, JCꢀP
= 12.7 Hz, 3CH), 119.9 (d, JCꢀP = 88.1 Hz, 3C), 7.31 (d, JCꢀP = 55.5 Hz, 13CH3).
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[1,2,3-13C3]-1-(triphenylphosphoranylidene)-2-propanone (2)36,12a
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9
To a suspension of [13C]ꢀmethyltriphenylphosphonium iodide 13 (2.03 g, 5.0 mmol, 1.0 equiv) in anhydrous
THF (50 mL) under argon, was added dropwise a 1 N solution of LiHMDS in THF (5.0 mL, 5.0 mmol, 1.0
equiv) at room temperature. The resulting yellow solution was stirred for 30 min at room temperature and
then cooled to ꢀ78 °C. 1 N Solution of LiHMDS in THF (5.0 mL, 5.0 mmol, 1.0 equiv) was quickly added
followed by a slow addition of a solution of [13C2]ꢀacetylimidazole 12 (561 mg, 5.0 mmol, 1.0 equiv) in
anhydrous THF (10 mL). The reaction mixture was stirred for 4 h at 0 °C and then poured to a 2 N aqueous
solution of hydrochloric acid. The aqueous layer was washed with diethyl ether (2 × 100 mL) and then
poured to a 10 N aqueous solution of NaOH (200 mL). The aqueous layer was extracted twice with 200 mL
of a DCM:Et2O mixture (1:2) and the combined organic layers were dried over anhydrous Na2SO4. Solvents
were removed under reduced pressure to afford 1.4 g (90 %, purity = 93 % (UPLCꢀMS data)) of the
expected stabilized ylide 2 as pale yellow solid. This compound was used in the next step without further
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purification. H NMR (400 MHz, DMSO d6): δH 7.68ꢀ7.62 (m, 5H), 7.57ꢀ7.52 (m, 3H), 7.47ꢀ7.43 (m, 5H),
3.70 (bd, 1H, JCꢀH = 165.7 Hz), 2.10 (ddt, 3H, JCꢀH = 125.8 Hz, J = 5.1, 2.1 Hz). 31P NMR (162 MHz,
DMSO d6): δP (ppm) 14.3 (d, JCꢀP = 107.7 Hz). 13C NMR (100 MHz, CDCl3): δC 191.0 (ddd, JCꢀC = 64.5 Hz,
J
CꢀC = 41.5 Hz, JCꢀP = 2.1 Hz, 13C), 133.2 (d, JCꢀP = 10.1 Hz, 3CH), 132.1 (d, JCꢀP = 2.9 Hz, 3CH), 128.9 (d,
JCꢀP = 12.2 Hz, 3CH), 127.2 (d, JCꢀP = 90.6 Hz, 3C), 52.0 (ddd, JCꢀP = 107.6 Hz, JCꢀC = 64.4 Hz, JCꢀC = 19.2
Hz, 13CH), 28.6 (ddd, JCꢀC = 41.5 Hz, JCꢀC = 19.3 Hz, JCꢀP = 15.8 Hz, 13CH3).
[2,3,4-13C3]-(3aS,5aS,6R,9aS,9bS)-3a,6-dimethyl-6-((E)-3-oxobut-1-en-1-yl)octahydro-1H-
cyclopenta[a]naphthalene-3,7(2H,3aH)-dione (11)12a
To a solution of diketoꢀaldehyde 1 (1.22 g, 4.67 mmol, 1.0 equiv) in degassed pꢀxylene (70 mL) was added
[1,2,3ꢀ13C3]ꢀ1ꢀ(triphenylphosphoranylidene)ꢀ2ꢀpropanone 2 (1.50 g, 4.67 mmol, 1.0 equiv). The resulting
suspension was heated to reflux for 7 days. After this time, the mixture was allowed to reach room
temperature and 50 mL of Et2O were added. The solution was filtered through a pad of Celite® to remove
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precipitated solids and then concentrated under vacuum. The darkꢀbrown residue was subjected to column
chromatography on silica gel (cyclohexane:EtOAc, gradient 7:3 to 6:4) to afford the corresponding (E)ꢀ
enone 11 as a pale yellow oil (1.06 g, 75 %). 1H NMR (400 MHz; CDCl3) δH 6.75 (ddd, 1H, J = 16.5 Hz, JCꢀ
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= 6.7 Hz, JCꢀH = 1.4 Hz), 5.94 (ddt, 1H, J = 16.5 Hz, JCꢀH = 157,6 Hz, JCꢀH = 2.0 Hz), 2.62 (dt, 1H, J =
H
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14.4, 6.3 Hz), 2.43 (dd, 1H, J = 19.0, 9.0 Hz), 2.34 (bd, 1H, J = 14.4 Hz), 2.20 (ddd, 3H, JCꢀH = 127.4 Hz,
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J
CꢀH = 5.9 Hz, JCꢀH = 1.1 Hz), 2.13ꢀ1.84 (m, 4H), 1.73 (bd, 1H, J = 13.2 Hz), 1.57 (ddd, 1H, J = 21.6 Hz, J =
11.9 Hz, J = 9.3 Hz), 1.48–1.11 (m, 6H), 1.27 (s, 3H), 0.87 (s, 3H). 13C NMR (100 MHz, CDCl3) δC 219.4
(C), 211.9 (d, JCꢀC = 1.6 Hz, 13C), 198.0 (dd, JCꢀC = 52.8 Hz, JCꢀC = 42.4 Hz, 13C), 150.3 (dd, JCꢀC = 70.1 Hz,
J
CꢀC = 1.3 Hz, 13CH), 131.0 (dd, JCꢀC = 52.8 Hz, JCꢀC = 14.8 Hz, 13CH), 54.4 (d, JCꢀC = 5.9 Hz, C), 50.8 (d, JCꢀ
= 1.4 Hz, CH), 50.7 (CH), 47.7 (C), 37.5 (CH2), 35.6 (CH2), 34.2 (CH), 30.8 (CH), 30.2 (CH2), 27.1 (dd,
C
JCꢀC = 42.4 Hz, JCꢀC = 14.8 Hz, 13CH3), 21.8 (CH2) 21.7 (CH2), 15.4 (d, JCꢀC = 3.4 Hz, CH3), 13.8 (CH3).
HRMS (ESI+): calcd. for C1613C3H26O3Na [(M+Na)+]: 328.1880; Found: 328.1884. [α]D = 171 (c 1.1,
20
CHCl3).
[2,3,4-13C3]-(8R,9S,10R,13S,14S)-10,13-dimethyl-7,8,9,10,11,12,13,14,15,16-decahydro-3H-
cyclopenta[a]phenanthrene-3,17(6H)-dione (13C3-ADD)37
To a solution of enone 11 (60 mg, 0.20 mmol, 1.0 equiv) in anhydrous toluene (2 mL) was added thioacetic
acid (17 ꢁL, 0.24 mmol, 1.2 equiv) and PTSA.H2O (11 mg, 0.06 mmol, 0.3 equiv). The reaction mixture
was stirred at 75 °C for 18 h. The solution was cooled to room temperature and DBU (60 ꢁL, 0.39 mmol, 2.0
equiv) was added. The resulting solution was stirred for 30 min at room temperature and a saturated aqueous
solution of NaHCO3 (10 mL) was added. The aqueous layer was extracted with dichloromethane (2 × 10
mL) and the combined organic layers were washed with a saturated aqueous solution of NH4Cl (15 mL),
water (10 mL), brine (10 mL) and then dried over anhydrous Na2SO4. Solvents were removed under reduced
pressure and the residue was subjected to column chromatography on silica gel (cyclohexane:EtOAc,
gradient 7:3 to 6:4) to afford 13C3-ADD as a white crystalline solid (42 mg, 75 %). H NMR (400 MHz;
1
CDCl3) δH 7.02 (t, 1H, J = 10.1 Hz), 6.20 (bdd, 1H, J = 9.6 Hz, JCꢀH = 164.8 Hz), 6.05 (bd, 1H, JCꢀH = 161.5
Hz), 2.55–2.38 (m, 3H), 2.14–2.04 (m, 2H), 1.96 (dddd, 1H, J = 12.4, 8.8, 5.8, 0.7 Hz), 1.87–1.53 (m, 6H),
1.32–1.19 (m, 2H), 1.25 (s, 3H), 1.19–1.07 (m, 2H), 0.94 (s, 3H). 13C NMR (100 MHz, CDCl3) δC 219.8 (C),
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The Journal of Organic Chemistry
186.3 (dd, J = 54.0, 52.3 Hz, 13C), 168.2 (C), 155.2 (CH), 127.8 (m, 13CH), 124.2 (m, 13CH), 52.4 (CH),
50.5 (CH), 47.7 (C), 43.5 (C), 35.7 (CH2), 35.2 (CH), 32.6 (CH2), 32.4 (CH2), 31.3 (CH2), 22.2 (CH2), 22.0
(CH2), 18.8 (CH3), 13.9 (CH3). HRMS of nonꢀlabeled ADD (ESI+): calcd for C19H24O2Na [(M+Na)+]:
307.1674, Found: 307.1665. HRMS of 13C3-ADD (ESI+): calcd for C1613C3H24O2Na [(M+Na)+]: 310.1775,
Found: 310.1764. Mp 144.5ꢀ145.0 °C. [α]D20 = 113.3 (c 1.02, CHCl3).
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Acknowledgment
This work has been funded by Atlanchim Pharma through a CIFRE grant. The authors are deeply indebted
to Atlanchim Pharma, the Ministère de l’Enseignement Supérieur et de la Recherche, the Centre National de
la Recherche Scientifique (CNRS) and the Université de Nantes for their support. Dr. Laurence Arzel and
Julie Hémez are warmly acknowledged for highꢀresolution mass spectrometry and NMR spectroscopy
assistance, as well as Aurélien Planchat for Xꢀray structure determination of compound 4b. The authors also
thank all colleagues from Atlanchim Pharma for their valuable suggestions during the course of this
research.
Supporting Information
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Synthesis and experimental part of compounds 12, 13 and 2, H, C and P NMR spectra of 4a-d, 5, 1, 6-
11, 13C3-11, 1,4ꢀandrostadienꢀ3,17ꢀdione and 2,3,4ꢀ13C3ꢀ1,4ꢀandrostadienꢀ3,17ꢀdione, 12, 13 and 2, as well
as a proposed reaction pathway for the formation of ketoꢀaldehyde 1 from 4b (Scheme 1) and an ORTEP
drawing of thiophenol adduct 4b (Figure 1.) are presented in supporting information. This material is
available free of charge via the Internet at http://pubs.acs.org.
Author Information
Corresponding Author
*Eꢀmail for J. L.: jacques.lebreton@univꢀnantes.fr or jacques.lebreton@atlanchimpharma.com
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References and notes
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(a) Gentles, M. J.; Moss, J. B.; Herzog, H. L.; Hershberg, E. B. J. Am. Chem. Soc. 1958, 80, 3702–3705.
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The term abnormal ozonolysis refers to a transformation with ozone in which not only the CꢀC double
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18
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