Synthesis of the 10-oxabicyclo[5.2.1]decane framework present in bioactive natural products

Article abstract of DOI:10.1039/c8ob00194d

The present work deals with the synthesis of the 10-oxabicyclo[5.2.1]decane framework present in bioactive natural products like physalins, with potential as antitumor agents. This synthetic methodology involves several key reactions: (a) synthesis of polyfunctionalized cycloheptenones by [4 + 3] cycloaddition reactions of furan precursors with oxyallyl cations; (b) Nicholas reaction with propargyl cations stabilized as dicobalt hexacarbonyl complexes; (c) demetallation and hydration of the resulting acetylenes; (d) stereoconvergent aldol cyclization to generate a key oxatricyclic intermediate and (e) a β-fragmentation process that affords, under hypoiodite photolysis, the desired product with moderate to good yield. The final compounds are the result of a radicalary β-fragmentation at the level of C2-C6 with respect to the tertiary hydroxyl group on C6, with an unexpected contraction from a ten- to a nine-membered ring system, via a radical addition to the carbonyl group on C4. The synthetic methodology has been scaled up to multigram level with good overall yield. Further biological, biochemical and biophysical studies are being carried out in our laboratory on these 1,7-epoxycyclononane derivatives to determine the potential of this kind of oxabicyclic compound as future hits and/or leads for the development of new anticancer drugs. The preliminary evaluation of the anticancer activity of the representative synthesized compounds, against the leukaemia cancer cell lines K-562 and SR, shows a promising activity with a GI₅₀ = 0.01 μM and a LC₅₀ = 7.4 μM for a conveniently functionalized 10-oxabicyclo[5.2.1]decane.

Full text of DOI:10.1039/c8ob00194d

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DOI: 10.1039/C8OB00194D
Synthesis of the 10ꢀoxabicyclo[5.2.1]decane framework,
present in bioactive natural products
Ángel M. Montaña [a][b]*, Stefano Ponzano[c], MariaꢀFilomena Sanasi[d] and
Gabriele KociokꢀKöhn [e]
[a]
Industrial and Applied Organic Chemistry Research Unity, Department of Inorganic
and Organic Chemistry, Universidad de Barcelona, Martí Franquès 1ꢀ11, 08028ꢀ
Barcelona, Spain.
[b] Consolidated Research Group of Proteins Engineering and Proteomics, Universidad
Autónoma de Barcelona, Bellaterra, 08193 Barcelona, Spain.
[c] European Medicines Agency, 30 Churchill Place, Canary Wharf, London E14 5EU,
United Kingdom
[d]Agenzia Italiana del Farmaco, Via del Tritone, 181 ꢀ 00187 Roma, Italy
[e] Department of Chemistry, University of Bath, Bath, BA2 7AY, UK.
(*) To whom correspondence should be addressed. ORCID: 0000ꢀ0001ꢀ8777ꢀ6888
Eꢀmail: angel.montana@ub.edu
Phone: (+34)ꢀ934.021.681
ABSTRACT
The present work deals with the synthesis of the 10ꢀoxabicyclo[5.2.1]decane framework
present in bioactive natural products like physalins, with potential as antitumor agents.
This synthetic methodology involves several key reactions: a) Synthesis of
polyfunctionalized cycloheptenones by [4+3]cycloaddition reactions of furan precursors
with oxyallyl cations. b) Nicholas reaction with propargyl cations stabilized as dicobalt
hexacarbonyl complexes. c) Demetallation and hydration of the resulting acetylenes. d)
Stereoconvergent aldol cyclization to generate a key oxatricyclic intermediate and e) A
βꢀfragmentation process that affords, under the hypoiodite photolysis, the desired
product with moderate to good yield. The final compounds are the result of a radicalary
βꢀfragmentation at the level of C2ꢀC6 respect to the tertiary hydroxyl group on C6, with
an unexpected contraction from a tenꢀ to a nineꢀmembered ring system, via a radical
addition to the carbonyl group on C4. The synthetic methodology has been scaled up to
multigram level with good overall yield. Further biological, biochemical and
biophysical studies are being carried out in our laboratory on these 1,7ꢀ
epoxycyclononane derivatives, to determine the potential of this kind of oxabicyclic
1

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compounds as future hits and/or leads for the development of new anticancer drugs. The
preliminary evaluation of the anticancer activity of representative synthesized
compounds, against the leukaemia cancer cell lines Kꢀ562 and SR, shows a promising
activity with GI50 = 0.01 µM and a LC50 = 7.4 µM for a conveniently functionalized 10ꢀ
oxabicyclo[5.2.1]decane.
Keywords: Synthesis of 10ꢀoxabicyclo[5.2.1]decane, physalins, anticancer agents,
[4+3]cycloaddition, Nicholas reaction, aldol cyclization, hypoiodite reaction, betaꢀ
fragmentation.
1. Introduction
The 10ꢀoxabicyclo[5.2.1]decane skeleton is present in a number of interesting natural
products with important bioactivity.1 Thus, for example, physalins2 and secoꢀphysalins3
(Fig.1) are a family of natural secoꢀsteroids,4,5 with a wide range of biological activities,
containing this oxabicyclic framework (Fig. 1). More than 60 physalins and
neophysalins have been identified and characterized as natural products since physalin
A was first isolated from Physalis alkekengi in 1969.6 The family of physalins have
remarkable biological properties, including cytotoxic,7 antiꢀinflammatory,8
antinociceptive,9 and immunomodulatory10 activities. These important biological
activities have generated a special interest among the scientific community, which has
stimulated the synthesis and biological evaluation of analogues of this type of
compounds. Thus, the 10ꢀoxabicyclo[5.2.1]decane framework is involved in a wide
variety of synthetic works related to natural products chemistry.11
O
Me
O
H
OH
H
O
O
HO
R
Me
O
O
O
Me
Physalin N: R, R' = CH2=
Physalin O: R= OH; R'= Me
R'
Figure 1. Examples of physalins with biological activity.
In this context, a different number of protocols can be found in the literature in
order to reach the oxabicyclic substructure (i.e. 10ꢀoxabicyclo[5.2.1]decane subunit).
Some selected important methodologies developed for this purpose are represented in
Fig. 2. Many of these methodologies can be used only for a certain type of substrates or
2

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they are not compatible with some functional groups. For these reasons, it is necessary
to consider all possible synthetic solutions to face the synthetic and structural challenges
represented by the aforementioned target molecules. Among these synthetic
methodologies are worth to mention the following:
(a) 1,4ꢀElimination reactions, mediated by a base in the presence of a good leaving
group12 (this is a kind of Grob fragmentation13 that affords good results if the
disposition of the hydrogen atom to be abstracted and the leaving group are
adequate).
(b) Retroaldol reactions.14
(c) Cyclobutane thermolysis by a [2+2] cycloreversion reaction.15
(d) Transannular ring expansion induced by electrophiles.16
(e) Transannular cyclization in acidic media.17
(f) Ring expansion mediated by hydrogen peroxide.18
(g) Intramolecular reductive coupling mediated by SmI2.19
(h) Anionic oxyꢀCope rearrangement20 (this reaction is less reversible than the
conventional Cope rearrangement because the initially formed enol tautomerizes
to afford the corresponding ketone).
(i) The hypoiodite reaction.21
Figure 2. Significant methodologies for the synthesis of oxabicyclic structures.
3

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Considering physalins as a reference and following a fragmentꢀbased drug
design approach we envisioned the synthesis of conveniently functionalized molecules
having the 10ꢀoxabicyclo[5.2.1]decane framework as potential hits of a new family of
anticancer agents. The rationale of choosing the previous core molecules as a model for
the design of molecules with anticancer activity was because the SAR studies2,7 carried
out on the aforementioned natural secoꢀsteroids showed that the the 10ꢀ
oxabicyclo[5.2.1]decane subunit was fundamental for the anticancer activity of these
natural products. On the other hand, the presence of a hydroxymethyl or alkoxymethyl
group on C2 is also important. In addition, a hydroxycarbonylic or carboxylic function
on C6 of the oxabicyclic system has also a decisive influence on the cytotoxic activity
of this kind of molecules (see green colored subunit in Fig. 1). Thus, continuing with
our efforts to develop new organic22 and/or metalꢀbased23 drugs with anticancer activity,
we have developed a versatile synthetic methodology to prepare a chemical library of
derivatives having this oxabicyclic framework, in order to carry out, in the future,
structureꢀactivity relationship studies based on in vitro cytotoxicity tests on cancer cells.
What were the reasons to develop the hereꢀinꢀdescribed synthetic methodology and its
potential advantages compared to those methods briefly described in Fig. 2. The first
reason was the deep knowledge of the knowꢀhow by our research group of the reactions
involved in the synthetic pathway. The second reason, and most important, was the
potential and advantages and of our method: starts from cheap and commercially
available substrates. The intermediates are very versatile and may be transformed by
standard reactions in a wide variety of derivatives (molecular diversity), which is very
important in rational design of drugs and SAR studies. The synthetic steps may be
easily optimized and scaledꢀup to multigram level with high yield and may be
developed in an enantioselective way, as discussed below.
The synthetic methodology is illustrated in Fig. 3 and involves the preparation of
a sevenꢀmembered synthon via [4+3] cycloaddition reaction of a furan derivative and an
oxyallyl cation generated in situ from an α, α’ꢀdihaloketone and a reducing metal24, 25
,
methodology that has been widely in our research group.26 The resulting
cycloheptenone 5 was converted to a silylenol ether 6, to facilitate its further αꢀ
alkylation. In a parallel way, the dibenzylacetal of the propargyl aldehyde 10 was
prepared from 2ꢀpropenal 7 and its corresponding dicobalthexacarbonyl complex 11
was formed by reaction with Co2(CO)8. The treatment of this complex with a Lewis
acid generates, in situ,
a
propargyl cation, conveniently stabilized as a
dicobalthexacarbonyl complex. In a convergent way, the building blocks 6 and the
propargyl cation generated from 11 were reacted under the Nicholas’ reaction
conditions,27 affording intermediate 12, which was demetallated to get the free terminal
acetylene 13, which was further hydrated to generate methylketone 14. Aldol
cyclization with NaOH in ethanol, under controlled conditions generated the key
oxatricyclic intermediate 15, as a single diastereoisomer, due to a process of
stereoconvergence during the aldol cyclization, which is discussed below.
4

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Figure 3. Synthetic strategy to obtain new 10ꢀoxabicyclo[5.2.1]decane derivatives.
Finally, the oxatricyclic intermediate 15 was converted in the final products by a
βꢀfragmentation process mediated by the hypoiodite reaction under thermal and/or
photochemical conditions.
In order to simplify and facilitate the development of this synthetic methodology
up to the final oxabicyclic products of interest, we have worked with the simple starting
materials: 3ꢀpentanone 1 (as precursor of the oxyallyl cation), furan 3 (as dienophile)
and acrylic aldehyde 7 as precursor of dibenzyl acetal of propargyl aldehyde 10. An
important issue is that this synthetic path has been scaled up. We have worked at 100ꢀ
150 g scale to prepare key intermediates 4 and 10, which may be stored in the
refrigerator without alteration for very long periods of time. In the case of 4 for years
and in the case of acetylene 10 for many months. This last compound becomes even
more stable after complexation with dicobaltꢀoctacarbonyl. For the remaining steps of
the synthetic sequence, the scale of work has ranged from a few milligrams up to 20 g,
depending on the strategy of the experiment. Thus, if the experiment was a trial with a
5

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new reagent or to evaluate new conditions, in these cases we were conservative, but if it
was an operation to prepare a considerable amount of a key intermediate, as in the case
of 15, the scale of work was from 1 to 20 g. We have performed all steps many times
and at different scales and under different conditions, but in the experimental section we
only quote those scales and conditions that afforded the better yields.
In addition, it is worth to mention that the complete synthetic process may be
carried out on an enantioselective fashion, because the early step of [4+3]ꢀcycloaddition
reaction may be performed in an enantioselective way by using an adequate chiral
auxiliary on the oxyallyl cation or on the furan ring.28 Also, chiral organocatalysts may
be used instead of chiral auxiliaries.29 On the other hand, the symmetrical ketone 5 may
be desymmetrized in the process of formation of the silyl enol ether 6 by using an
enantiopure chiral base,30 approach that has been carried out previously with exit in our
lab.31 This enantioselective version adds value to the here presented methodology, in the
case that in the future a chiral recognition of the oxabicyclic active principle at the
active site of the target molecule should condition the cytotoxicity. That will make
necessary the synthesis of the adequate eutomer.32
2. Results and discussion
2.1. The [4+3] cycloaddition reaction between furan and 2,4-dibromo-3-pentanone
This reaction has been frequently used in our laboratory since several years ago
to obtain a variety of products in good yields.22, 25, 26 The methodology adopted was
originally established by Hoffmann.33 Haloketone 2 was obtained in 80% yield by
bromination of cheap and commercially available 3ꢀpentanone 1 under acid catalysis.34
The reaction was optimized and scaled up to 100 g scale. Once the 2,4ꢀdibromoꢀ3ꢀ
pentanone has been freshly distilled, it was reacted with furan in the presence of a
reducing metal, in our case copper powder, and sodium iodide. Mechanistically, the
reaction involves two very fast SN2 displacements of bromide by iodide ions, as seen by
the precipitation of sodium bromide. A subsequent slower nucleophilic attack of excess
iodide ion on the positively polarized iodine of the diiodoketone is visualized to yield an
allylic iodide, which forms a 2ꢀoxyallyl cation in a facile E1 reaction (Fig. 4).
6

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Figure 4. In situ oxyallyl cation formation mechanism from dibromoketone 2 in the [4+3]
cycloaddition reaction.
The oxyallyl cation generated in situ reacts with furan and generates two
possible isomers of the cisꢀcycloadduct: the diequatorial 4a and diaxial 4b in a ratio of
92.5 : 7.5, respectively (regarding the relative position of the methyl groups) (Fig. 5) in
a 98% yield (and 100% conversion) at a 10 g scale of work. No presence of the trans
isomer has been observed in the crude mixture. Both stereoisomers were easily
separated by conventional flash column chromatography on silica gel or by
crystallization in hexane at –20 °C and both were physically and spectroscopically
characterized.
Figure 5. [4+3] cycloaddition reaction scheme.
Under Hoffmann’s reaction conditions, the cycloaddition follows a concerted
mechanism, yielding preferentially the thermodynamically more stable adduct with the
allyl cation in a Wꢀconfiguration and compact transition state via an endo approach.22ꢀ27
This explains the formation of only the cisꢀcycloadduct in the form of the diequatorial
stereoisomer as the major product (Fig. 6).
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O
O
O
O
O M
Endo
approach
O
diequatorial
COMPACT
transition state
M
4a
O
O
+
O
O
O
O
M
Exo
approach
O
O
diaxial
4b
EXTENDED
trasition state
Figure 6. The two different approaches of furan diene and the oxyallyl cation.
The stereochemistry of the isolated diastereoisomers 4a and 4b was stablished
1
by a correlation of Hꢀ and 13C NMR data with several stereoꢀelectronic differential
effects at the minimum energy conformation of both isomers.35
2.2. Hydrogenation of the C6-C7 double bond in 4a: synthesis of 5a
The next step was to hydrogenate the double bond present between the carbons
Cꢀ6 and Cꢀ7. Since in the previous reaction the major product resulted to be the
diequatorial stereoisomer in a 92:8 ratio, it was decided to continue the synthetic
pathway with only the major pure isomer 4a. This way of working facilitated the
evaluation of the results, since the reaction crude was much simpler than in the case of
working with a mixture of stereoisomers as a substrate.
Figure 7. Cycloadduct hydrogenation.
This reaction was carried out by using hydrogen and Pd/C (10 %) as a catalyst in
ethanol and the yield resulted to be almost quantitative (see Fig. 7).36 Since the
conversion was complete and no byproduct was detected by TLC and 1H NMR, the loss
of a 2% of product during recovery (even using sonication) was assumed to be due to its
adsorption on the surface of Pd and/or charcoal particles from the catalyst.
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2.3. Synthesis of the silyl enol ether 6a.
Since in ketone 5a there is no difference between the two hydrogens situated in
α respect to the carbonyl group, either the thermodynamic or the kinetic approaches
could be used to prepare the desired silyl enol ether. The thermodynamic approach is
nevertheless disadvantageous since it requires high temperatures and long reaction time,
while the kinetic method37 is much faster and it results in complete conversion of the
starting material, requiring low reaction temperatures. This last method consists in three
steps: a) Preparation of LDA by reacting diisopropylamine with BuLi at –78 °C. (b)
Once the base is formed, the cycloadduct was added and the proton in α to the carbonyl
group is abstracted generating the corresponding enolate. c) Trimethyl silyl chloride
(TMSCl) was subsequently added to form the desired product. The reaction was
performed under strict anhydrous conditions and Ar atmosphere. The conversion was
complete and the resulting yield 99% (Fig. 8).
Figure 8. Synthesis of the silyl enol ether 6.
2.4. Synthesis of dibenzyl acetal of propynal
The synthesis of the dicobalt complex starts from acrolein,7, which was reacted
first with bromine, in order to dibrominate the double bond, and subsequently with
trimethyl orthoformate to protect the aldehyde group as a dimethyl acetal 8.38 The next
step consisted in a double bromine elimination of 8, by using tetrabutylammonium
hydroxide, which was generated in situ from tetrabutylammonium hydrogen sulfate and
sodium hydroxide, affording dimethyl acetal of propynal 9 (Fig. 9).
Figure 9. 3,3ꢀDimethoxyꢀpropyne synthesis.
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Intermediate 10 was then synthesized by transacetallization of 9 with benzyl alcohol to
generate the corresponding dibenzyl acetal (Fig. 10), in the presence pꢀtoluenesulfonic
acid, as a catalyst and in cyclohexane as a solvent, which allows the azeotropic
separation of water and ethanol to shift the equilibrium towards the desired product.39
OH
O
O
CH3
pꢀTsOH
C6H12
+
2
2 EtOH
+
O
CH3
O
Y= 99,8 %
C= 100 %
9
10
Figure 10. Transacetallization equilibrium to obtain 10 from 9.
The next step consisted in the protection of the triple bond to generate the
hexacarbonyl µꢀ{η−[1,1ꢀdibenzyloxyꢀ2ꢀpropyne]}dicobalt(CoꢀCo) 11. The reaction was
carried out by using dicobalt octacarbonyl Co2(CO)8, in hexane for two hours, obtaining
the product in high yield (93.4 %)(Fig. 11).
O
O
H
O
O
Co2(CO)8
Hexane,2h
OC Co
OC
Co
OC
CO
Y= 93,4 %
C= 100 %
CO
CO
10
11
Figure 11. Synthesis of the dicobalt complex 11.
2.5 Coupling the dicobalt complex (11) with the silyl enol ether (6a). The Nicholas
reaction
The Nicholas propargylation of 6a with propargyl cation complex 11, generated
in situ from cobalt complex 11 and F3B·OEt2, under strict anhydrous conditions,40
afforded in 47ꢀ67% yield (depending on the scale of work) a 3:1 diastereoisomeric
mixture of 12a/12b, epimers at Cꢀ1'. In this reaction, two stereocenters were generated,
thus, four diastereoisomers could be theoretically formed. However, due to the
bulkiness of the organocobalt cluster, the attack of propargylium cation is only possible
by the exo face of the πꢀsystem of silyl enol ether 6, forming only two exo
10

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diastereoisomers due to the attack through the two prochiral faces of the propargyl
cation (see Fig. 12). The preferred attack by the proS face of propargyl cation, leading to
the major product 12a, is due to the lower steric demand exerted by the Co2(CO)6
cluster and the benzyl group of cation 11’ and the oxygen bridge and methyl group of
Cꢀ9 of silyl enol ether 6a.
This Nicholas reaction was evaluated under different reaction conditions
observing that the ratio between both epimers may vary with reaction conditions
(temperature and reaction time) but both diastereoisomers are useful for our synthetic
purposes as explained below. At this stage, the reaction was scaled up and the synthesis
of the silyl enol ether was carried out at high scale to get 10.1 g of product 6. The
obtained silyl enol ether was divided in three fractions to perform three trials of the
Nicholas alkylation reaction under the previously optimized conditions. The results are
shown in Table 1.
Figure 12. Synthesis of the two possible exo diastereoisomers 12a and 12b obtained by the Nicholas
reaction.
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Table 1. Results of the Nicholas reaction giving the two possible isomers 12a and 12b.
Trial*
Silyl enol
ether
(g)
Dicobalt
complex
(g)
BF3.OEt2 Reagents molar
ratio
Products
ratio
12a: 12b (%)
Conv. Yield**
(mL)
SEE : dicobalt
complex :
(%)
(%)
BF3.OEt2
1.93
4.07
4.12
5.50
11.00
10.97
3.23
6.78
6.87
1 : 1.2 : 3
1 : 1.2 : 3
1 : 1.2 : 3
78:22
76:24
79:21
100
100
100
47
64
67
1
2
3
(*): Reaction conditions: CH2Cl2 as a solvent, 0 ºC, 2 h. (**) Isolated yield.
The yield slightly increased with the amount of starting material (see Trials 2
and 3 in Table 1) but, in general, remained lower than in case of performing the reaction
with, the lower steric demanding, dimethoxy or dimethoxy acetals analogue of 11.41
The stereochemistry of major product 12a and its epimer 12b, was stablished by
1
13
correlation of Hꢀ and C NMR data42 and it was confirmed for the isomer 12a by Xꢀ
ray diffraction analysis. Thus, 12a was crystallized in a mixture of hexane/diethyl ether
to get suitable crystals for Xꢀray diffraction analysis. The complete relative
stereochemistry of 12a was determined as illustrated in the thermal ellipsoid
representation of Fig.13. The Crystal data and structure refinement for 12a are quoted in
Table 2.
Figure 13. Thermal ellipsoid representation of 12a structure, determined by Xꢀray diffraction analysis on
a single crystal.
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Table 2. Crystal data and structure refinement for 12a.
Empirical formula
C25H22Co2O9
Formula weight
Temperature
584.29 g·mol‒1
150(2) K
Wavelength
0.71073 Å
triclinic
Crystal system
Space group
P 1
Unit cell dimensions
a = 8.57600(10) Å α = 81.0490(10) deg.
b = 9.05900(10) Å β = 89.2630(10) deg.
c = 16.3000(3) Å
γ = 78.6610(10) deg
1226.30(3) Å3
2
Volume
Z
Calculated density
Absorption coefficient
F(000)
1.582 Mg·m‒3
1.405 mm‒1
596
Crystal size
0.50 x 0.30 x 0.13 mm
4.13 to 30.10 deg.
Theta range for data collection
Limiting indices
‒12 ≤ h ≤ 12, ‒12 ≤ k ≤ 12, ‒22 ≤ l ≤ 23
31059 / 7141 [R(int) = 0.0461]
30.10 99.0 %
Reflections collected / unique
Completeness to θ
Max. and min. transmission
Refinement method
Data / restraints / parameters
Goodnessꢀofꢀfit on F2
Final R indices [I > 2σ(I)]
R indices (all data)
Largest diff. peak and hole
0.8385 and 0.5402
Fullꢀmatrix leastꢀsquares on F2
7141 / 0 / 325
1.012
R1 = 0.0305, wR2 = 0.0712
R1 = 0.0448, wR2 = 0.0763
0.381 and ‒0.494 e· Å‒3
2.6. Demetallation of the dicobalt complexes 12a and 12b
Demetallation reaction of the dicobalt complexes 12a and 12b was carried out
by using cerium ammonium nitrate (CAN) as oxidizing agent, which was added
portionꢀwise (one equivalent every 45 minutes) in order to check the evolution of the
reaction and to avoid the possible formation of secondary products resulting from overꢀ
oxidation (Fig. 14).
Figure 14. Demetallation reaction of the dicobalt complexes 12a and 12b.
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The reaction was performed in a first trial at the scale of 50 mg with the two
different isomers, independently, and then the scale up was carried out at the level of
grams. The reaction was controlled by TLC and the disappearance of the starting
material was noticed after the addition of the fourth equivalent of CAN. The crude was
then percolated directly through a column having a short pad of alumina (Al2O3) to
remove the excess of cerium ammonium nitrate and most of other cerium species, which
were retained in the column, to avoid overꢀoxidation of the final products of interest.
Other improvement of the workꢀup was to add triethylamine, prior to do the aqueous
workꢀup, in order to avoid the influence of a low pH on the stability of acetylenic
products. Having removed the largest part of excess of CAN, the extraction of the crude
did not present emulsions, and centrifugation of the solution could be avoided. The
analysis of the crude, revealed the complete demetallation of the complex and the
formation of triethylamine oxide which could be easily separated by column
chromatography. A trial was also done, without adding the triethylamine to avoid the
formation of its oxide, but in this case, a new product, the 4ꢀhydroxyꢀ4ꢀmethylꢀ2ꢀ
pentanone, was formed due to the autoꢀcondensation of acetone in an acidic medium.
This product was also easily separated by column chromatography. Even though the
reaction gave excellent results (see exp. 4, Table 3) without the addition of NEt3, it was
preferred to perform the last scaleꢀup reaction (exp. 5, Table 3) in the presence of NEt3
to avoid secondary reactions of acetylenes in an acidic medium, as well as a possible
enolization of the cycloheptanone. The results from the demetallation reactions are
shown in Table 3.
Table 3. Results of the demetallation reactions of complexes 12a and 12b.
Experiment 12a 12b NEt3 Conversion
Yield
(%)
(g)
(g) (eq)
(%)
13a 13b
90.6
0.05
2
100
100
100
100
100
1
2
3
4
5
0.05
2.72
2
2
0
2
100
99.8
4.01
5.56
85.1
95.6
2.7. Hydration of the acetylenic products 13a and 13b
The two obtained acetylene isomers were submitted to the hydration reaction,
using mercury (II) pꢀtoluenesulfonamidate as reagent to transform the terminal triple
bond into the corresponding methyl ketone.43 The two isomers gave excellent results,
showing a complete conversion of the starting material and affording a yield going from
82 % for the isomer 14b to 100 % for the isomer 14a (see Fig. 15 and Table 4).
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Figure 15. Hydration of acetylenes 13a and 13b.
Table 4. Results of the hydration reactions of isomers 13a and 13b.
Experiment 13a 13b
Hg(II) Conversion
Yield
(g)
(g)
amidate
(%)
(%)
(eq)
14a 14b
94.8
0.10
0.39
1.55
2.30
1.5
1.5
1.5
1.5
1.5
100
100
100
100
100
1
2
3
4
5
96.5
98.1
99.0
1.39
82
2.8. Aldol cyclization of methyl ketones 14a and 14b to obtain the oxatricyclic key
intermediate 15
The reaction was carried out with separated isomers 14a and 14b and also with
the mixture 14a/14b. A very important issue is that before doing the workꢀup, the crude
mixture should be neutralized, with an HCl (2M) solution, prior to concentration to
dryness. In this case, the analysis of the crude mixture revealed the presence of a small
amount of starting material and a unique cyclization product 15, the same in all three
cases (Fig. 16). Neutralization prior concentration of the very basic crude mixture is of
capital importance in order to avoid the Curtin rearrangement of the benzyl group.44
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Figure 16. Results of the independent aldol cyclization of 14a, 14b and 14a/b (1:1), with the
neutralization of the crude before the workꢀup.
The obtained crude was fractionated by flash column chromatography in order to
separate the products and to characterized them. The aldol cyclization product 15 was
isolated and characterized (1H NMR , 13C NMR, COSY, HSQC) and its structure was
confirmed by Xꢀray diffraction analysis.44a
A series of cyclization reactions were carried out with 14a, 14b and 1:1 mixtures
of 14a/b, increasing the scale of work in order to optimize the results, which are shown
in Table 5. As may be observed, in all cases the conversion reached, after 15 h of
reaction time, was around 70% and the yield of aldol 15 increased considerably with the
scale of work.
In order to optimize the aldol cyclization reaction, a kinetic study was carried
out starting from compound 14a. The evolution of the reaction was followed by gas
chromatography, and samples of the reaction mixture were taken and injected in the
GCꢀapparatus every hour, along the reaction time. As it can be seen from the graph of
Fig 17, the reaction shows clearly from the beginning, that compound 14a epimerizes to
generate compound 14b, which is indeed the species that undergo the aldol cyclization
to afford product 15. The driving force of the reaction reaches an end when the
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proportion of 14a becomes lower than that of 14b and the system reaches a final
equilibrium, with the relative proportion: 14a / 14b / 15 = 1: 2.5 : 6, after 6 h of reaction
time.
Table 5. Results obtained with the aldol cyclization of isomers 14a and 14b to obtain 15.
Substrate
Aldol 15
Yield***
(S x C)
(%)
14a
(g)
14b
(g)
14a/b
(g)
Conversion
(%)
Selectivity**
(%)
Exp*
0.13
0.34
0.93
1.50
70,2
68,0
67,6
64,9
70,3
70,5
64,9
66,2
67,6
61,4
68,7
33,6
41,0
55,9
90.0
60,3
67,0
74,8
88,8
96,6
52,2
91,7
23,6
1
2
3
23,8
37,8
58,6
35,4
40,2
48,6
37,8
65,3
32,1
63,0
4
1.12
1.44
1.50
4.50
4.60
5
6
7
8
9
0.72
3.22
10
11
(*) Reaction conditions: Absolute ethanol as a solvent, room temperature, 15 h reaction time.
The workꢀup was the same in all cases, with a previous quenching of excess of NaOH with
aqueous HCl 2M at 0 ºC. (**) This value corresponds to the yield of product 15 with respect to
the converted starting material, taking into account that unconverted starting material 14 is
completely recovered by column chromatography. (***) Isolated yield.
1
0.9
Proportion
in the
reaction
mixture
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
14b
14a
15
0
1
2
4
6
7
8
9
10 11
Reaction time (h)
Figure 17. Kinetic study of the aldol cyclization reaction starting from 14a. Proportion of components of
the reaction mixture along reaction time.
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It is worth noting, from the stereochemical point of view, that compound 15 is
necessarily formed from 14b (not from 14a). Thus, it is clear the existence of an
epimerization process of 14a into 14b. The relative proportion of 14a and 14b in the
equilibrium may be rationalized based on their formation enthalpies, at least at the level
of calculation accuracy got by standard MOPAC PM7 calculations (Fig. 18).45 Looking
at the pKa values of the three possible hydrogen types in alpha to the carbonyl groups,
three enolates may be theoretically formed.46 The acidity of methyl ketone is about 107
times higher than that of hydrogen geminal to benzyloxy group and much more
accessible, however, aldol 15’ is not formed, because neither has been detected by both
GC and high field 1H NMR on the reaction crude, nor isolated after a careful preparative
chromatography at high scale of work (gram scale). This results made us to think of a
thermodynamic control of the reaction, which is supported by the differences of
formation enthalpy of 14a versus 14b and that of 15 versus 15’. On the other hand,
compounds 15’’ to 15v (Fig. 18) have not been detected probably due to the lower
accessibility of hydrogen Hꢀ4 of cycloheptanone in 14a and /or 14b, the higher cycle
strain in enolates C and C’ and also the steric strain of the final products.
The stereoselectivity in the formation of the stereocenter at the Cꢀ6 carbon atom
of aldol 15 is due to the attack of enolate by the exo face of the carbonyl group of
cycloheptanone, because steric constrains. Apart from the thermodynamic reasons, aldol
15 is formed in opposition to 15’ because in enolate B the exo attack on ketone of Cꢀ3 is
not affected by the steric hindrance exerted by the benzyloxy group and, in addition, the
bridging oxygen and the oxygen of benzyloxy group are far apart from each other.
However, in the hypothetical formation of 15’, the approach of enolate of methylketone
to the carbonyl on Cꢀ3 of enolate D creates a strong steric interaction between the
benzyloxy group and the oxygen bridge and the intermediate is not favored. In addition,
both ethereal oxygen atoms are almost eclipsed, which generated a destabilizing
repulsive electronic interaction between both dipole moments (see Fig. 19).
The reaction profile diagram of the aldol cyclization is illustrated in Fig.20,
where it is possible to appreciate how the product 15, the only aldol isolated from the
reaction, is indeed the most stable product.
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Fig 18. Stereoconvergent equilibria involved in the aldol cyclization reaction of 14a and 14b to generate product 15 as a unique stereoisomer.
19

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Benzyl group not interfering
Region
sterically crowded
Oxygen atoms
in closer proximity
2
3
Oxygen atoms
far apart
2
3
1
1
Enolate B
Enolate D
Figure 19. Different approach in the aldol cyclization of enolates B and D in the minimum energy
conformation.
This phenomenon of stereoconvergence in the aldol cyclization is of capital
importance in our synthetic pathway because we can work with the mixture of
alkylation products resulting from the Nicholas reaction (see synthetic scheme in Fig. 3
and Fig. 12) and to go all the way to final compounds 16 and 17, because in the aldol
cyclization step we get a unique stereoisomer 15 instead of the eight theoretically
possible. This approach simplifies considerably the synthetic work.
Figure 20. Reaction profile diagram of the aldol cyclization step.
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2.9. Synthesis of the benzylic 1,7-epoxy-cyclononane derivatives 16 and 17
The new oxatricyclic substrate 15 was submitted to the hypoiodite reaction
conditions,21 using three equivalents of lead tetraacetate, one equivalent of iodine,
dissolved in anhydrous benzene and under irradiation with two 100 watt lamps (natural
or visible light), to synthesize the corresponding epoxycyclononane molecules 16a,b
and 17a,b. These molecules, having the 10ꢀoxabicyclo[5.1.1]decane framework, are
formed by the βꢀfragmentation of the C2─C6 bond and the subsequent rearrangement to
a nine membered ring structure.
The two pairs of isomeric products (epimers) 16a,b and 17a,b (Fig. 21) were
separated by flash column chromatography and conveniently purified and physically
and spectroscopically characterized.
Figure 21. Synthesis of new benzylic 1,7ꢀepoxycyclononanes 16a,b and 17a,b.
The relative stereochemistry of βꢀfragmentation products was stablished by a
1
13
careful H and C NMR data comparison, once all signal were unequivocally assigned
1
by homoꢀ and heterocorrelated 2D NMR experiments. By correlating the ꢁδ of both H
and 13C NMR spectra with stereoelectronic effects responsible for the α, β, γ and δ
interactions, it was possible to assign the relative configuration of molecules. A similar
protocol was used to assign the stereochemistry of an analogue molecule and the
protocol was validated by confirmation of the structure by Xꢀray analysis.41d
The stereoelectronic effects were analyzed for the minimum energy
conformation of epimers 16a,b and 17a,b, whose molecular population contribute the
most to the chemical shift data. The 1H and 13C NMR correlation studies carried out for
the first pair of iodinated stereoisomers is reported in Table 6 where the chemical shifts
(ppm) for every hydrogen of the two molecules are listed in the second and third
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columns. The fourth column represents the difference, in ppm, between the respective
chemical shifts of 17a and 17b.
As shown in the Table 6, the main differences affect the hydrogens which are
present on the side chain of the molecule (H1' and H1''') and the one present at the head
of the oxygenated bridge (H1''). Also the hydrogen atom, geminal to the iodine atom,
maintaining the configuration of the C4’’ center, results affected by the change of
configuration at C1’ due to the different influence of the anisotropy cones of methyl
ketone and benzyl groups in 17a versus 17b (at the minimum energy conformation,
resulting from the rotation around the C2''─C1' axis). The remaining hydrogens of the
molecules do not change very much their δ values, which indicates that the only carbon
that underwent a stereochemical change is the one relative to the C1' center in the side
chain, and that the rest of the molecule did not undergo any substantial variations.
A support to this assignment are the observations resulting from the 13C NMR
correlation study, where significant ꢀδ were appreciated for C1', C1’’, C1’’’, C4’, C2
and C3’’. All of them related to the side chains or to the atoms in close proximity to the
stereocenter C1’ that has opposite configuration in 17a versus 17b.
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Table 6. 1H and 13C NMR correlation of product 16a,b and 17a,b.
17a
17b
16a
16b
H / C
δ(ppm)
ꢁδ(17aꢀ17b)
δ(ppm)
ꢁδ(16aꢀ16b)
1H NMR
21a
1.00
1.12
21b
1.00
ꢁδ
0
20a
0.89
1.07
20b
0.94
1.07
ꢁδ
‒0.05
0
12''
11''
9''
1.12
0
1.12ꢀ1.18
1.72ꢀ1.80
1.88ꢀ1.98
2.05
1.21ꢀ1.33
1.56ꢀ1.64
1.86ꢀ1.96
2.02
1.24ꢀ1.30
1.62ꢀ1.74
1.82ꢀ1.95
1.96
1.20ꢀ1.33
1.73ꢀ1.76
1.86ꢀ1.96
2.11
8''
8'' and 9''
2
0.03
‒0.02
0.33
0.09
0.19
0.12
‒0.14
ꢀ
‒0.15
0.12
0.27
‒0.07
0.27
0.17
0.18
0.44
‒0.12
3.33
3.35
3.30
3.18
6''
4.33
4.00
4.33
4.06
1''
4.47
4.38
4.42
4.49
7''
4.83
4.64
4.85
4.58
1'''
1'''
4''
4.87
4.75
4.85
4.68
5.70
5.84
3.00
2.82
ꢀ
ꢀ
4.58
4.14
4''
6.73
6.88
6.81
6.92
1'
‒0.15
7.30ꢀ7.40
7.25ꢀ7.34
7.28ꢀ7.46
7.25ꢀ7.33
Ph
13C NMR
12.08
12.48
21.84
25.85
26.82
33.92
53.42
64.17
73.35
82.62
83.86
96.60
127.70
128.13
128.62
137.35
171.35
197.17
201.71
11.76
12.62
21.10
25.84
26.90
32,68
53.18
63.74
72.39
82.14
83.91
95.96
127.99
128.26
128.68
136.93
171.14
196.64
201.75
0.32
‒0.14
0.74
0.01
‒0.08
1.24
0.24
0.43
0.96
0.48
‒0.05
0.64
‒0.29
‒0.13
0
11.56
11.79
21.06
25.88
26.90
57.93
54.26
63.27
73.17
82.87
83.98
97.77
127.70
128.08
128.60
137.45
171.12
199.64
203.07
11.53
12.03
21.28
25.81
26.75
57.38
53.92
62.81
71.56
82.43
83.87
95.65
127.84
128.34
128.46
136.80
170.88
198.75
203.04
0.03
‒0.24
‒0.22
0.07
0.15
0.55
0.34
0.46
1.61
0.44
0.11
2.12
‒0.14
‒0.26
0.14
0.65
0.24
0.89
0.03
12''
11''
2
8''
9''
4''
6''
2''
1'''
1''
7''
1'
3''' and 7'''
5'''
4''' and 6'''
0.42
0.21
0.53
‒0.04
2'''
1
3''
5’’
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This correlation study was also carried out for the other couple of isomers:
16a/16b, showing chemical shifts differences even more pronounced, in some cases, for
the same affected hydrogens and carbon atoms (Table 6). It is worth to mention the
difference ꢁδ = 2.12 for carbon C1’, center with opposite configuration in the pair of
epimers 16a/b. The shielding and deshielding effects observed for the carbon atoms in
13C NMR of 17a/b and 16a/b are due to several interactions that act in a concomitant
way and for which it is very difficult to evaluate their individual contribution. Among
these effects, analyzed in 3D for the minimum energy conformations of all these
stereoisomers, are worth to mention: δ
ꢀgauche shielding effect47, 48a, deshielding effect
exerted by iodine atom, steric compression (responsible for the pullꢀpush electron
density shift)48a, 48c, 47, electric field interaction47, 48a, 48c, and deshielding effect produced
by the oxygen atoms of ethers (bridging oxygen and benzyloxy group), among other
main physical phenomena.35, 42, 48
The beforeꢀmentioned results could be interpreted based on the following
proposed mechanism. In this mechanism, the hypoiodite reaction21 is a key step in
which the tertiary alcohol of 15 reacts with an iodonium ion, generated in situ from
LTA and iodine, and it forms the corresponding hypoiodite A (see Fig. 22).
Subsequently, under irradiation with two 100 watt lamps (natural or visible light) and/or
heating, the molecule undergoes a homolytic breakdown of the O─I bond, forming an
alkoxy radical and releasing an iodine atom. This alkoxy radical undergoes a
βꢀ
fragmentation49 by a homolytic scission of the C2─C6 bond and a new carbon free
radical on position Cꢀ7 of intermediate C is formed.
To explain the ring contraction undergone by the intermediate C, it could be
envisioned an intramolecular attack by the free radical at Cꢀ7 on the ketone carbonyl at
Cꢀ5.50 The intermediate cyclopropyloxy free radical D then undergoes a
βꢀscission to
afford the cyclononane ring. The new secondary radical E could subsequently react with
an acetoxyl free radical, generated from the photodecomposition of LTA, to give the
nonꢀiodinated diketones (16a and 16b). The stereoselectivity in the formation of 20a as
a major isomer may be interpreted as the preferential attack of the acetoxyl radical by
the pro
R
face of the secondary free radical E (see Fig. 23).
The iodoꢀderivatives (17a and 17b) probably form via a ketoꢀenol
tautomerization and subsequent attack of iodine to the double bond of the corresponding
enol F. The configuration at the carbon bearing the iodine atom (C4) in 17a,b has been
confirmed by Xꢀray diffraction analysis of crystals of different analogues,41 thus the
stereoselectivity in the insertion of iodine atom could be interpreted as due to an endo
attack of iodine to the enolate F, because the exo face is hindered by the side chain
(acetoxy and benzyloxy groups) (see Fig. 23), taking into account the large size of
iodine atom.
In the aforeꢀmentioned mechanism proposal, the alkoxy group could play several
roles. In one hand it may facilitate the radical addition to the ketone on Cꢀ4, by
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activating the carbonyl group, and on the other hand it can assist in the ring opening of
the intermediate cyclopropyloxy free radical, by stabilizing the resulting free radical in
intermediate E (Fig. 23).50,51,52 In addition, a ThorpeꢀIngold effect53 could facilitate the
cyclization process to afford the cyclopropyloxy radical. On the other hand, because the
cycloaddition of free radicals to a carbonyl group are much slower than the ring opening
of the resultant cyclic alkoxy radical,54 thus, the driving force, to facilitate the approach
of the carbonyl carbon and the carbon wearing the free radical, could be the presence of
steric demanding substituents, such as alkoxy groups, at the alpha position of carbonyl
group, as takes place in our compounds.
Figure 22. Proposal of a formation mechanism of 16a,b and 17a,b from substrate 15.
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Intermediate free radical E
Intermediate enol F
Figure 23. Left: Explanation of stereoselectivity in the formation of 16a and 16b by attack of acetoxyl
radical to intermediate E. Right: Illustration of the preferential attack of iodine to enol F by the endo face
to afford endo 17a,b compounds. The images represent the minimum energy conformations of
intermediates E and F.
4.ꢀ Anticancer in vitro testing of compounds 15, 16a and 17a
The anticancer activity of compounds 15, 16a and 17a has been evaluated in vitro by
using the protocol included as supplementary material (Document S2) against the
leukaemia cancer cell lines Kꢀ562 and SR.
The preliminary evaluation of the anticancer activity of compounds 15, 16a and
17b, (see Table 12) shows that the oxatricyclic compound 15 results nonꢀactive against
the leukaemia cancer cell lines Kꢀ562 and SR, with a LC50 higher than 100 µM.
However 16a and, in particular, 17a show a promising activity. Thus 17a has a GI50
=
0.01 µM and a LC50 = 7.4 µM against SR cell line. These results encourage us to
continue this research project in order to find better hits and to carry out, in the future,
cytotoxicity evaluations on other cancer cell lines. This is also an interesting result
because cell replication stops (growth inhibition) at a much lower concentration than
cell death takes place (LC50). From this result, we may speculate that the therapeutic
index could be favorable for 17a.
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Table 12. Anticancer in vitro testing of 15, 16a and 17a against the leukaemia cancer cell lines Kꢀ562
and SR.
Leukaemia
cancer cell line
Compound
GI50 (µM)a
TGI (µM)b
LC50 (µM)c
Kꢀ562
SR
4.79
0.01
18.20
1.95
64.56
7.41
17a
16a
15
Kꢀ562
SR
31.62
21.38
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
Kꢀ562
SR
(a) GI50: cell Growth Inhibition 50; (b) TGI: Total cell Growth Inhibition; (c) LC50:
Lethal Concentration 50. For details, see document S2 in the supplementary data.
5. Conclusions and future work
Taking physalins, natural products with important anticancer properties, as a model and,
considering the fragmentꢀbased drug design approach, we envisioned the synthesis of
conveniently functionalized molecules having the 10ꢀoxabicyclo[5.2.1]decane
framework as potential hits of a new family of anticancer agents. A versatile synthetic
methodology has been developed involving several key reactions: a) Synthesis of
polyfunctionalized cycloheptenones by [4+3]ꢀcycloaddition reactions of furan
precursors with oxyallyl cations. b) Nicholas reaction with propargyl cations stabilized
as dicobalt hexacarbonyl complexes. c) Demetallation and hydration of the resulting
acetylenes. d) Stereoconvergent aldol cyclization to generate a key oxatricyclic
intermediate and e) A βꢀfragmentation process that affords, under the hypoiodite
photolysis, the desired bioactive compounds. All reactions have been optimized to get,
in general high yields, and all compounds have been properly purified and physically
and spectroscopically characterized. The stereochemistry has been assigned in most
cases by correlation of 1Hꢀ and 13C NMR data with related compounds and confirmed in
the key intermediates by Xꢀray diffraction analysis on single crystals. Moreover, the
whole process has been scaled up to multigrain scale, even at the level of hundreds of
grams in the initial steps. The yields resulted increased when raising the scale of work.
In also worth to mention that the final products may be prepared in an enantioselective
way, by inducing enantioselectivity in an early step in the synthetic pathway. This
aspect is not discussed in this paper but has been developed in our lab in the past. By
this versatile synthetic methodology, we pave the way to obtain a chemical library with
structural diversity in order to carry out SAR studies to optimize the antitumor activity
of these compounds. The preliminary evaluation of the anticancer activity of
compounds 15, 16a and 17b, shows that the oxatricyclic compound 15 results nonꢀ
active against the leukaemia cancer cell lines Kꢀ562 and SR, with a LC50 higher than
100 µM. However 16a and, in particular, 17a show a promising activity. Thus 17a has a
GI50 = 0.01 µM and a LC50 = 7.4 µM against cell line SR.
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6. Experimental section
6.1. General procedures
NMR spectra were recorded on a Varian Inova 200 or 300 MHz, on a Varian Mercury
400 MHz and/or in a Bruker DMX 500 MHz apparatus. Chemical shifts (δ) are
expressed in ppm versus tetramethylsilane as an internal standard. IR spectra were
recorded on a NICOLET 6700 FTꢀIR by film, KBr pellet or ATR (Attenuated Total
Reflectance) methods. Mass spectrometry was performed on a HewlettꢀPackard 5890
apparatus, generally under a CI (Chemical Ionization) method by using NH3 or CH4 or
by direct insertion under Electron Impact a 70 eV and 150 ºC. The elemental analyses
were obtained in a FISONS Elemental Analyzer, Model Naꢀ1500. The samples were
previously pyrolized al 1000 ºC, under oxygen atmosphere, and the content of carbon,
hydrogen and nitrogen were determined by evaluation of the combustion gases by gas
chromatography using a FID detector. Solvents were dried, according to standard
procedures, and distilled prior to use. All other major chemicals were obtained from
commercial sources and used without further purification. Gas chromatography was
performed by using a Shimatzu AOCꢀ20i apparatus with a capillary column (HPꢀ5
Crossꢀlinked 5% PheꢀMeꢀsiloxane, 0.25 ꢂm film thickness, 30 m length and 0.32 mm
diameter). Used carrier gas brands and pressures were: He = 5.5 bar (Linde, Helium
5.0), Air = 3 bar (Linde, synthetic air), H2 = 3 bar (Linde, Hydrogen 5.0). The
experimental conditions are specified in each case. Depending on the composition to be
analyzed, different working conditions were used, which are reported in the Table 13.
Table 13. GC operational conditions.
Initial
Initial
Gradient
rate
(°C/min)
Initial time
(minutes)
Final time
(minutes)
Type
Temperature Temperature
(°C)
30
(°C)
250
250
290
250
290
2
1
1
1
1
5
5
20
20
20
20
20
A
B
C
D
E
150
100
50
10
5
50
10
6.2. Molecular computer calculations
Geometry and energy calculations were preoptimized by molecular mechanics MM2
followed by semiempirical quantum mechanical PM7 algorithm,55 implemented in the
MOPACꢀ2016 software.56 This software was also used to calculate the formation
enthalpy. Density functional theory (DFT) based methods at the B3LYP functional
level57,58 were used for subsequent full refinements, within the Gaussianꢀ03W (Revision
E.01, version 6.1) software package.59 For carbon, hydrogen and oxygen atoms, the 6–
311++G(d, p) basis set was used. All calculations were performed on the isolated
molecules (gas phase), as consideration of solvation by the molecules of the solvent by
28

Page 29 of 48
Organic & Biomolecular Chemistry
View Article Online
DOI: 10.1039/C8OB00194D
a polarizable continuum model (PCM)60 produced a loss in computational performance
(increase of CPU calculation time and change of convergence behavior), but did not
result in significant changes of the calculation results.
6.3. X-ray experimental data acquisition for compound 12a
A single crystal (0.50 x 0.30 x 0.13 mm) of 12a was mounted on Nonius–Kappa CCD
diffractometer, equipped with an Oxford Cryosystem, using graphiteꢀmonochromated
MoꢀKα radiation (λ = 0.71073 Å). Intensity data were collected at 150 K and the data
were processed by using the Nonius software61 A symmetryꢀrelated (multiscan)
absorption correction was applied. 31059 reflections were measured in the range 4.13 ≤
θ
≤ 30.10 deg. 7141 reflections were assumed as observed applying the condition I >
2
σ
(I). The structures were solved by direct methods (SIRꢀ97)62 and refined by fullꢀ
matrix leastꢀsquares techniques against
F
2 (SHELXLꢀ2013)63 using the program
platform SHELXle.64 Nonꢀhydrogen atoms were anisotropically refined. Heteroatom
and hydrogen atoms were located in the difference Fourier map and were isotropically
refined all others were placed onto calculated positions. The final R(on F) factors were:
0.0305, wR(on | | ) = 0.0712 and goodness of fit = 1.012 for all observed reflections.
F 2
The number of refined parameters was 325 and the maximum and minimum peaks in
final difference synthesis was 0.381 and ‒0.494 e.Å‒3. The crystal data and a summary
of the intensity data collection for 12 are summarized in Table 2. Data for crystal and
structure refinements, atomic coordinates, bond lengths, bond angles, anisotropic
displacement parameters, and hydrogen coordinates may be obtained from the CIF file
deposited with The Cambridge Crystallographic Data Centre with the number CCDCꢀ
1562215. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via the Internet at www.ccdc.cam.ac.uk/data_request/cif.
The molecular illustration such as the thermal ellipsoid plot (Fig. 13) was made by
using MERCURY 3.9 software.65
6.4. Synthetic procedures
The protocols for the synthesis of compounds 4 to 9 are included in the supplementary
information as Sections 6.4.1 to 6.4.5 (Document S1).
6.4.6. Synthesis of 3,3ꢀdibenzyloxypropꢀ1ꢀyne, (10)
29