Flow synthesis of substituted γ-lactones by consecutive photocatalytic/reductive reactions

Full text of DOI:10.1002/adsc.201300859

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DOI: 10.1002/adsc.201300859
Flow Synthesis of Substituted g-Lactones by Consecutive
Photocatalytic/Reductive Reactions
Maurizio Fagnoni,a Filippo Bonassi,a Alessandro Palmieri,b,* Stefano Protti,a,*
Davide Ravelli,a and Roberto Ballinib
a
PhotoGreenLab, Department of Chemistry, University of Pavia, Viale Taramelli 12, 27100 Pavia, Italy
Fax : (+39)-03-8298-7323; phone: (+39)-03-8298-7316; e-mail: prottistefano@gmail.com
Green Chemistry Group, School of Science and Technology, Chemistry Division, University of Camerino, Via S. Agostino
b
1, 62032 Camerino (MC), Italy
E-mail: alessandro.palmieri@unicam.it
Received: September 23, 2013; Revised: December 17, 2013; Published online: March 3, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300859.
The extensive occurrence of the g-lactone ring in bio- ed opening-lactonization of g-epoxy esters,[11] the
logically active compounds or in their precursors,[1] in- asymmetric Michael addition of a,a-disubstituted al-
cluding derivatives with antifungal,[2] antibiotic,[3] anti- dehydes to maleimides,[12] the [Cu
ACTHNUTRGNE(UNG acac)2]-catalyzed
tumor,[4] anti-inflammatory[5] and cytotoxic[6] activity, 1,5-electrocyclic ring closure of conjugated esters with
makes its preparation an appealing target in organic dimethyl diazomalonate[13] and the reduction of g-
chemistry.
acylsuccinates.[14]
The synthetic routes reported so far made use of
Photochemical procedures have been employed
different approaches, such as three-component reac- only in a few cases. Representative examples are the
À
À
tions starting from aldehydes or ketones (Scheme 1, tandem C C and C O bond formations via addition
paths a, b),[7,8] the gold-catalyzed tandem cycloisome- of a photogenerated aryl cation to w-alkenoic acids
rization/oxidation of homopropargylic alcohols (path (path e)[15] and the (substituted) benzophenone or
c)[9] and the AgOTs-catalyzed silylene transfer to a- acetone (R’2CO) photomediated addition of an alco-
keto esters followed by an iodolactonization reaction hol[16,17c] onto unsaturated acids in the synthesis of
(path d).[10] Other methods include the SmI2-promot- (functionalized) terebic acids (Scheme 1, path f).[16a,c]
Scheme 1. Representative approaches for the synthesis of the g-lactone ring.
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Scheme 2. Retrosynthetic analysis of the g-lactone core V
proposed in this paper.
The application of flow synthesis (both thermal and
photochemical) for the preparation of these lactones,
however, has been only sparsely reported.[17]
Flow reactors have emerged in the last two decades
as a promising technology for the production of fine
chemicals,[18] especially in multistep procedures.[19]
Moreover, these devices are particularly attractive
when applied to photochemical reactions,[20] since the
thin diameter of the tube allows a deeper penetration
of light and thus a more uniform irradiation of the re-
action mixture.
We thus explored a two-step preparation of the g-
lactone ring (V in Scheme 2) under flow conditions
having a photocatalytic reaction as the first step.
In this context, V was thought to be obtained from
Figure 1. Flow photocatalytic reactor used for the prepara-
tion of g-keto esters 3. Further notice that during irradia-
tion, both photochemical flow reactor and feeding tubes
were covered with an aluminum foil to avoid cross irradia-
tion.
g-keto ester III by reduction of the ketone moiety fol- a flow-through back-pressure unit was mounted at the
lowed by acid-promoted cyclization of the resulting g- outlet of the photochemical reactor. The overall pres-
hydroxy ester IV. In turn, the preparation of III in- sure in the system has been set to ca. 2 atm during the
volved the conjugate radical addition of a photogener- experiment.
ated acyl radical (from aldehyde I) to a a,b-unsaturat-
The first part of the study was aimed to find out
ed ester II. The synthesis of g-keto esters III has al- the best conditions for the photochemical step, taking
ready been reported under batch conditions and is as a model the reaction of heptanal (1a) with diethyl
based on the tetrabutylammonium decatungstate maleate (2a) to give the corresponding acylsuccinate
(TBADT, (nBu4N)
of aldehydes (either aliphatic or aromatic) to elec-
tron-poor olefins in a multilamp apparatus,[21] as well reactor equipped with ten 15 W phosphor-coated
as under natural sunlight irradiation.[22]
lamps; emission centered at 310 nm) of 1a (0.1M) in
[W10O32]) photocatalyzed addition 3aa (Table 1).
Thus, irradiation under batch conditions (multilamp
In order to move from batch to flow conditions, we acetonitrile with an equimolar amount of 2a in the
took inspiration from the photo-reactor optimized by presence of 2 mol% of TBADT afforded 3aa in a dis-
Booker-Milburn and co-workers,[23] consisting of coils crete yield (44% yield, entry 1). 24 h of irradiation
of UV-transparent tubing (made of a fluoropolymer) were required in order to obtain a complete conver-
wound around a traditional water-cooled immersion sion of the reactants. Irradiation of the solution
well apparatus. Thus, the designed photochemical re- placed in a test tube carried out by using the same
actor (R1; volume=12 mL) was assembled by using medium pressure Hg vapor lamp as in Figure 1 al-
a polytetrafluoroethylene tubing (Algoflon PTFE; lowed us to increase the yield of 3aa to 66% and re-
outer diameter: 1.6 mm; inner diameter: 1.3 mm) and quired a shorter irradiation time (6 h, entry 2). When
a medium pressure (125 W) Hg vapor lamp as shown shifting to the flow reactor with a 0.1 mLminÀ1 flow
in Figure 1.
rate, 3aa was formed in 39% yield when using
The solution to be irradiated was charged into a re- a quartz cooling vessel and 67% when the cooling
servoir (see the Supporting Information for details) vessel was made of pyrex (entry 3).
and circulated by means of a Hewlett–Packard HPLC
By using the latter experimental set-up, the yield
pump. In order to assure a constant flow rate and was not affected upon doubling the concentration of
avoid the formation of bubbles during the reaction, the reactants (while maintaining the same TBADT
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Flow Synthesis of Substituted g-Lactones
Table 1. Optimization of the photocatalyzed synthesis of acyl succinate 3aa.
[a]
Reaction performed in a quartz test tube by means of a multilamp reactor equipped with 10 phos-
phor-coated lamps (lIRR =310 nm), 24 h irradiation. TABT=(n-Bu)4ACHTUNTRGNEUNG[W10O32].
Reaction performed in a quartz test tube by means of a medium pressure Hg vapor lamp equipped
with a merry-go-round apparatus, 6 h irradiation.
Reaction performed by using a quartz cooling vessel.
Reaction performed by using a pyrex cooling vessel.
[b]
[c]
[d]
molar concentration, viz. 2ꢁ10À3 M corresponding to 3, we decided to adopt a sodium borohydride solution
1 mol%, entry 4). On the other hand, a higher flow in absolute ethanol to perform the desired transfor-
rate (0.2 mLminÀ1) was detrimental since the reaction mation. This solution was charged in a reservoir and
yield decreased somewhat (from 67 to 60%, entry 5). circulated in a PTFE tubing (with the same specifica-
A lower amount of the photocatalyst (0.5 mol%, tions as above) by means of a second HPLC pump
entry 6) was insufficient to assure a satisfying acyla- (see again the Supporting Information for details)
tion yield (35% of 3aa formed). The space-time yield and the same flow rate (0.1 mLminÀ1) adopted in the
(STY) was then calculated in each case. As it is ap- photochemical step. In detail, after removing the
parent from Table 1, flow reactions showed consis- back-pressure unit, a three-way valve (T1) was placed
tently higher STY values than the batch conditions at the outlet of the photochemical reactor (R1) and
despite a similar acylation yield (compare entries 2 connected with the reservoir containing the NaBH4
and
4,
STY=11.00 mmolLÀ1 hÀ1
and solution by means of another HPLC pump. A second
67.00 mmolLÀ1 hÀ1, respectively). We then chose the reactor (R2; volume=10 mL) was then mounted at
conditions described in Table 1, entry 4, since the flow the outlet of T1 and a back-pressure unit for the
rate of 0.1 mLminÀ1 was found to be the best suited whole system was inserted. Using this apparatus, we
for carrying out the ensuing reduction step.
treated the photolyzed solution containing crude 3aa
Having optimized the photochemical transforma- with a solution of NaBH4 (0.2–0.5M) in absolute
tion (Scheme 3, path a’), we switched our attention to EtOH. A maximum yield of lactone 4aa (65% overall
the optimization of the synthesis of 4 (path b’) under yield on the basis of the amount of 1a used, see also
flow conditions, starting from the photolyzed solution. Table 2) was obtained when using a 0.4M solution of
Thus, since the second step required the reduction of NaBH4 (2 equiv.) with 50 min of residence time
Scheme 3. Multistep synthesis of g-lactones 4.
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Table 2. Synthesis of g-lactones 4.
of this approach with respect to the usual multilamp
batch reactor[21a,b] is apparent from Table 1, showing
that g-keto ester 3aa was formed in a higher yield and
in a significantly shorter time (2 h vs. 24 h) by using
the apparatus described above. Moreover, the use of
the flow reactor allows us to reduce the electric
energy demand due to the irradiation source as is ap-
parent by comparing the “specific productivity” (de-
fined as the number of mmol of product produced
with respect to the energy consumed by the light
source).[24] This parameter under flow conditions re-
sults to be around 6.4ꢁ10À3 mmol WhÀ1 whereas that
of the batch reactors are consistently lower viz. 3.6ꢁ
10À3 mmol WhÀ1, (determined by considering an
“ideal” irradiation of 300 mL by using the multilamp
reactor, entry 1), and 5.3ꢁ10À3 mmol WhÀ1, (deter-
mined by considering an “ideal” irradiation of 60 mL
by using the merry-go-round apparatus, entry 2, see
the Supporting Information for further details).
The apparatus shown in Figure 1 is simple, versatile
and capable of being scaled, allowing us to obtain the
desired products on a gram scale (ca. 5.5 g dÀ1 of keto
ester 3aa can be obtained). In addition, this set-up
takes an additional advantage from the use of PTFE
tubing in place of the usual (more expensive) FEP
(fluorinated ethylene propylene) tubing.[20c]
[a]
Isolated yield by silica gel chromatography; R1=12 mL
(residence time: 120 min), R2=10 mL (residence time:
50 min).
cis-Isomer has been observed as the most abundant.
R2=14 mL (residence time: 70 min).
The present synthesis of g-lactones compares favor-
ably with other ketone photomediated reactions such
as the photoaddition of alcohols onto unsaturated
acids followed by cyclization of the hydroxyl es-
[b]
[c]
(Scheme 3). The use of a higher NaBH4 concentration
ter,[16a,c,17c] since the latter approaches make use of
ACHTUNGTRENNUNG
was detrimental due to extensive bubbling in the reac- a large excess of low-boiling alcohols, such as 2-prop-
tor. Interestingly, when the same reduction was car- anol [mainly as (co)solvents].[16,17c] Moreover, the use
ried out under batch conditions on the crude photoly- of benzophenone as photomediator leads to the for-
zate (see the Supporting Information), 4aa was ob- mation of several by-products (e.g., benzopinacol)
tained in a lower yield (49% overall).
making purification of the end product trouble-
ACHTUNGTRENNUNG
To demonstrate the scope of our multistep protocol,
some.[16e] In some cases, the incorporation of the pho-
we investigated different combinations of aldehydes tocatalyst itself in the final product has also been ob-
1 and esters 2 (Table 2). Ethyl paraconate 4ba was served.[16b] The photocatalyst employed here, TBADT,
formed in a comparable yield (58%) when using is superior to those previously employed in related
hexanal 1b. The reaction conditions are tolerant to syntheses of lactones since it is more robust, it can be
the presence of a cyano group tethered to the unsatu- used in a lower mol% amount and it is easily re-
rated ester, as in the acylation of isopropylidenecya- moved in the purification step.
noacetate 2b with 1b to give the corresponding 3-
The synthesis of lactones 4 can be actually consid-
cyano-g-lactone 4bb in 51% yield (Vol. R2=14 mL in ered as one of the rare cases reported so far where
this case). The yields of lactones were slightly in- two consecutive reactions (one of which is photo-
creased when shifting to 3-phenylpropanal 1c, which chemical) were carried out under flow conditions.[25]
gave 4ca and 4cc in the reaction with diethyl maleate The acylation reaction was easily applied to substitut-
and methyl crotonate (Vol. R2=14 mL in the latter ed a,b-unsaturated esters and when using maleate 2a,
case) in 63% and 68% yields, respectively. The pres- the process gave access to paraconic acid derivatives,
ent approach was particularly advantageous in the important compounds having antitumor and antibiotic
case of aromatic aldehyde 1d, since the reaction time activities.[26a]
(2 h) for the synthesis of 3da was markedly shortened
when compared to the same reaction carried out used for the preparation of enantiopure phaseolinic
under batch conditions (30 h).[22] acid.[26b] The application of the apparatus presented
In particular, the cis-isomer of compound 4ba was
This is the first application of a TBADT photocata- here to other multistep consecutive reactions is cur-
lyzed reaction under flow conditions. The advantage rently under investigation in our laboratories, as well
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Flow Synthesis of Substituted g-Lactones
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Typical Procedure for the Synthesis of 4
Aldehyde 1 (2 mmol, 0.2M) and ester 2 (2 mmol, 0.2M) in
the presence of a catalytic amount of TBADT (0.02 mmol,
2ꢁ10À3 M) were dissolved in 10 mL of acetonitrile. Sodium
borohydride (4 mmol, 0.4M) was dissolved in 10 mL of ab-
solute ethanol. The two solutions were charged in two reser-
voirs and pumped by means of two HPLC pumps (model:
Hewlett–Packard HPLC pump series 1050; flow rate:
0.1 mLminÀ1) through the apparatus described in the text
(see also the Supporting Information, Figure S1). The pres-
sure was maintained at the constant value of ca. 2 atm by
means of a flow-through back-pressure unit mounted at the
output of the apparatus. The final solution was quenched in
a 0.1M HCl solution/ethyl acetate biphasic system. The
aqueous phase was extracted with ethyl acetate (3ꢁ15 mL)
and the combined organic portions were dried over MgSO4.
After removal of the solvent under reduced pressure, the
residue was purified by column chromatography (cyclohexa-
ne:ethyl acetate as the eluants).
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See the Supporting Information for experimental details,
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1
characterization of products and copies of H and 13C NMR
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Acknowledgements
The authors thank the Universities of Pavia and Camerino
and MIUR – Italy (FIRB National Project “Metodologie di
nuova generazione nella formazione di legami carbonio-car-
bonio e carbonio-eteroatomo in condizioni eco-sostenibili”)
for financial support. We are grateful to Prof. Angelo Albini,
Prof. Mariella Mella (University of Pavia), and to Dr. Elisa-
betta Sartirana (Solvay Special T Polymers, Italy) for fruitful
discussions.
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2014, 356, 753 – 758