Base-induced decarboxylation of polyunsaturated α-cyano acids derived from malonic acid: Synthesis of sesquiterpene nitriles and aldehydes with β-, φ-, and ψ-end groups

Article abstract of DOI:10.1002/hlca.201200162

Catalytic base-induced decarboxylation of polyunsaturated α-cyano-β-methyl acids derived from malonic acid led to the corresponding nitriles 3 (Schemes 2 and 3), 6 (Scheme 5), and 9 (Scheme 6). This decarboxylation occurred with previous deconjugation of

Full text of DOI:10.1002/hlca.201200162

Helvetica Chimica Acta – Vol. 96 (2013)
259
Base-Induced Decarboxylation of Polyunsaturated a-Cyano Acids Derived
from Malonic Acid: Synthesis of Sesquiterpene Nitriles and Aldehydes with
b-, f-, and y-End Groups
by Laurent Dufosse´*a), Dominique Cartierb), Benoist Vallab), Mireille Fouillauda), Roger Labiab), and
Alain Vallab)
a
´
) Laboratoire de Chimie des Substances Naturelles et des Sciences des Aliments, Universite de La
´
´
Reunion, ESIROI-IDAI, 2 rue Joseph Wetzell, F-97490 Sainte-Clotilde, La Reunion
(laurent.dufosse@univ-reunion.fr)
b
´
) Chimie et Biologie des Substances Naturelles, CNRS FRE 2125, 6 rue de lꢀUniversite,
F-29000 Quimper
Catalytic base-induced decarboxylation of polyunsaturated a-cyano-b-methyl acids derived from
malonic acid led to the corresponding nitriles 3 (Schemes 2 and 3), 6 (Scheme 5), and 9 (Scheme 6). This
decarboxylation occurred with previous deconjugation of the a,b-alkene moiety of the a-cyano-b-methyl
acid, leading to an a-cyano-b-methylene propanoic acid which was easily decarboxylated (see Scheme 2).
b-Methylene intermediates, in some cases, could be isolated; mechanistic pathways are proposed. The
nitriles 3, 6, and 9 were reduced to the sesquiterpene aldehydes 4 (b-end group), 7 (f-end group), and 10
(y-end group), respectively.
Introduction. – Decarboxylations are important in biological systems and occur
frequently in primary and secondary metabolisms: glycolysis, gluconeogenesis,
glycogenolysis, and glycogenesis, degradation and synthesis of fatty acids, Krebs citric
acid cycle (including pyruvate to acetyl-CoA, oxalosuccinate to a-ketoglutarate, a-
ketoglutarate to succinyl-CoA). Other decarboxylations are important, such as
formation of significant amines from amino acids, 5-HTP to serotonin, l-DOPA to
dopamine.
We have worked on the base/acid-induced decarboxylation of polyunsaturated
malonic acid derivatives for many years. Thus, in a strong acidic medium and depending
on the substitution pattern of the side chain, we have found that g-lactones or d-
lactones could be obtained [1] (Scheme 1,a).
In another study related to new syntheses of retinoids and carotenoids [2], we have
demonstrated that stereospecific/stereoselective monodecarboxylations were possible,
depending on the base used [3][4] (Scheme 1,b). In a recent work, we have revealed
that bis-decarboxylation of some malonic acid derivatives could occur easily, and that
this reaction is accompanied by formation of (E)- and (Z)-configured mono-acids [5]
(Scheme 1,c). According to Corey and Fraenkel [6], these decarboxylations of malonic
acid derivatives require a previous isomerization to an intermediary b-unsaturated d-
lactone (Scheme 1,d). In the series of corresponding cyanoacetic acid derivatives with
b-, f-, and y-end groups, different mechanistic pathways were proposed, which may
explain the process of decarboxylation.
ꢁ 2013 Verlag Helvetica Chimica Acta AG, Zꢂrich

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Scheme 1
Sesquiterpenes are usually found as natural products [7] and could also be of
interest in chemical synthesis. C15-Sesquiterpene aldehydes are commonly used for the
syntheses of diterpenes (especially related to retinoic acids, retinals, and retinols), and
numerous references show them to be exceptional intermediates [8]. This paper reports
on new syntheses of some b-, f-, and y-sesquiterpene aldehydes as useful synthons for
retinoid syntheses.
Results and Discussion. – b-End Group. Knoevenagel condensation of b-ionone
with cyanoacetic acid led to the a-cyano-b-methyl acid 1 [9], which, after decarbox-
ylation in pyridine furnished the b,g-unsaturated nitrile 2 (Scheme 2). In a previous
report, Smit [10] considered this step as an abnormal decarboxylation. Nitrile 2 was
isomerized to the a,b-unsaturated isomers 3 ((2E,4E)/(2Z,4E) 4 :1) by heating in

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261
KOH/MeOH. In this case, we think that the decarboxylation occurred in a regular
manner, due to the presence of the stronger electron-withdrawing effect of the nitrile
group, which could induce the suggested process, i.e., abstraction of a H-atom from
MeꢀC(3) by the base prior to loss of CO2 (Scheme 2).
Scheme 2
We now developed an improved process which allowed the formation of nitrile 3 in
a ꢃone-potꢀ procedure, with a good regioselectivity. Thus, condensation of b-ionone with
cyanoacetic acid in piperidine/benzene under reflux (DeanꢀStark; 6.5 h) led to 3 nearly
quantitatively ((2E,4E)/(2Z,4E) 95 :5 to 98 :2; Scheme 3). Piperidine (8 equiv.) and
benzene (as solvent) seemed to be indispensable because other bases and solvents led
to a mixture of nitriles with poor regioselectivity. Under these experimental conditions,
the initially formed cyanoacetic acids were concomitantly decarboxylated in the
reaction mixture. The (2E,4E)-nitrile 3 was easily purified by column chromatography
(SiO2, CH2Cl2). Thus, the crude C15-nitrile 3 was reduced by diisobutylaluminium
hydride (DIBAL-H) in toluene at 08 (30 min) to yield the sesquiterpenene aldehyde 4
((2E,4E)/(2Z,4E) 95 :5 to 98 :2). The crude aldehyde was purified by column
chromatography (SiO2, CH2Cl2) to provide (2E,4E)-4 as a yellow oil in 82% yield
from 3 [10].
Scheme 3
f-End Group. We have also recently reported that oxidation of b-ionone and a-
ionone having the respective b- and e-end group of carotenoids led to the correspond-
ing C13 aromatic ketone 5. By an improved procedure, under particularly mild
conditions, ketone 5 could be obtained from retro-ionone, an isomer of b- and a-
ionones (Scheme 4). Hence retro-ionone was heated at 608 for 1 h with 3 equiv. of 4,5-
dichloro-3,6-dioxocyclohexa-1,4-diene-1,2-dicarbonitrile (DDQ) in 1,2-dichloroethane
to produce the aromatic ketone 5 in 50% yield [11]. In this rapid process, extraction and
purification were easier. This reaction could be integrated into a new biomimetic
aromatization of a b-end group to a f-end group, with regiospecific migration of one of
the Me groups [2].

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Scheme 4
As described above (Scheme 3), a Knoevenagel condensation of aromatic ionone 5
with cyanoacetic acid in piperidine/benzene under reflux (DeanꢀStark) led to the
sesquiterpene nitrile 6 ((2E,4E)/(2Z,4E) 95 :5) (Scheme 5). The (E,E)-nitrile was
easily purified by recrystallization from pentane/Et2O 70 :30 and was further reduced
by DIBAL-H in toluene at 08 (30 min) to the corresponding aldehyde 7 (85%).
Scheme 5
y-End Group. Condensation of citral with methyl cyano(isopropylidene)acetate,
under Stobbeꢀs conditions (tBuOK, MeOH, 08, then 24 h r.t.) led to cyano acid 8 ((E)/
(Z)-isomer ca. 4 :1), which was further decarboxylated in pyridine/piperidine under
reflux (2 – 3 h) to provide nitrile 9 as a 7:3 mixture of (E)- and (Z)-isomers
(Scheme 6). After reduction with DIBAL-H at 08 (30 min), aldehyde 10 was obtained
as a 7:3 mixture of (E)- and (Z)-isomers which were separated by chromatography.
Scheme 6
Taking into account the results of the above series (Scheme 6), two routes, a) and
b), can be competing in the acyclic series as shown in Scheme 7, and two nitriles may be
predicted as intermediates (unfortunately not detected under our experimental
conditions in the decarboxylation process).

Helvetica Chimica Acta – Vol. 96 (2013)
263
Scheme 7
Conclusion. – This work and previous ones reported elsewhere showed that
decarboxylation of malonic acid derivatives or analogs can be dependent on many
factors, such as base effect, steric hindrance, electron-withdrawing effect of the malonic
acid derived moiety, and others.
Experimental Part
General. Starting materials and solvents were obtained from Aldrich (Germany) and were used
without further purification. All reactions were carried out under Ar. M.p.: Leitz-350-microscope heating
stage; not corrected. IR Spectra: Bruker-IF-55 spectrometer; ˜n in cmꢀ1 1H- and 13C-NMR Spectra:
.
Bruker-Avance-DPX-400 spectrometer; at 400 (1H) and 100 MHz (13C); in CDCl3; d in ppm rel. to Me4Si
as internal standard, J in Hz.
(2E,4E)- and (2Z,4E)-2-Cyano-3-methyl-5-(2,6,6-trimethylcyclohex-1-en-1-yl)penta-2,4-dienoic
Acid (1). Its synthesis is reported in [9][10][12]: (2E,4E)-1 was obtained as orange crystals after
recrystallization from benzene, m.p. 1748. The mother liquor (benzene) was diluted with petroleum ether
to furnish (2Z,4E)-1 as orange crystals, m.p. 1248.
(4E)-3-Methylene-5-(2,6,6-trimethylcyclohex-1-en-1-yl)pent-4-enenitrile (2). The crude a-cyano
acids 1 were refluxed in pyridine for 2 h, the pyridine was distilled off under reduced pressure, and the
oily mixture was acidified with 1n HCl and extracted with Et2O. The extract was purified by filtration
through SiO2, the filtrate evaporated, and the residue distilled in vacuo to give 2. Colorless oil (90%). IR
(film): 2212. 1H-NMR1): 6.11 (s, HꢀC(7), HꢀC(8)); 5.37, 5.33 (2s, CH2ꢀC(9)); 3.33 (s, CH2(10)); 2.02 (m,
CH2(4)); 1.70 (s, MeꢀC(5)); 1.63 (m, CH2(3)); 1.47 (m, CH2(2)); 1.02 (s, 2 MeꢀC(1)). 13C-NMR : 132.7;
129.3; 117.9; 39.8; 33.3; 29.2; 22.0; 21.6; 19.6.
(2E,4E)- and (2Z,4E)-3-Methyl-5-(2,6,6-trimethylcyclohex-1-en-1-yl)penta-2,4-dienenitrile (3). a)
Nitrile 2 (5 g) was isomerized in 2m NaOH (100 ml) at r.t. overnight: (2E,4E)/(2Z,4E)-3 4 :1. This
mixture was separated by CC (SiO2, CH2Cl2).
(2E,4E)-3: IR (film): 2210, 1610. 1H-NMR1): 6.50 (d, J ¼ 16, HꢀC(7)); 6.08 (d, J ¼ 16, HꢀC(8)); 5.10
(s, HꢀC(10)); 2.20 (s, MeꢀC(5)); 2.00 (m, CH2(4)); 1.70 (s, MeꢀC(9)); 1.60 (m, CH2(3)); 1.40 (m,
CH2(2)); 0.92 (s, 2 MeꢀC(1)). 13C-NMR (CDCl3): 156.9; 136.5; 135.3; 132.5; 132.2; 128.1; 96.2; 39.3; 34.0;
33.0; 26.7; 21.5; 16.3.
(2Z,4E)-3: IR (film): 2210, 1610. 1H-NMR1): 6.60 (d, J ¼ 16, HꢀC(8)); 6.50 (d, J ¼ 16, HꢀC(7)); 5.00
(s, HꢀC(10)); 2.00 (s, MeꢀC(5) and m, CH2(4)); 1.70 (s, MeꢀC(9)); 1.60 (m, CH2(3)); 1.50 (m, CH2(2));
1.02 (s, 2 MeꢀC(1)).
1
)
Trivial atom numbering.

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b) b-Iodone (6 g) was condensed with cyanoacetic acid (1.1 equiv.) in piperidine (20 ml) and benzene
(20 ml) under reflux (Dean-Stark apparatus): (2E,4E)/(2Z,4E)-3 4 :1 (nearly quant.). This mixture was
separated by CC (SiO2, CH2Cl2).
(2E,4E)- and (2Z,4E)-3-Methyl-5-(2,6,6-trimethylcyclohex-1-en-1-yl)penta-2,4-dienal (4). At 08, 1,2m
DIBAL-H in toluene (23.5 ml, 28.2 mmol, 1.1 equiv.) was added slowly to 3 (5.06 g) in toluene (20 ml).
After 30 min, the reaction was quenched with 1m H2SO4. The salts were filtered off, and Et2O (40 ml) and
H2O (20 ml) were added. The org. layer was washed twice with brine, dried (MgSO4), and concentrated.
The crude (2E,4E)/(2Z,4E)-4 49 :1 was separated by CC (SiO2, CH2Cl2): (2E,4E)-4 (82%). Yellow oil.
IR: 1668. 1H-NMR1): 10.11 (d, J ¼ 8.16, HꢀC(11)); 6.73 (d, J ¼ 16.2, HꢀC(7)); 6.20 (d, J ¼ 16.2, HꢀC(8));
5.92 (d, J ¼ 8.16, HꢀC(10)); 2.3 (s, MeꢀC(9)); 2.03 (m, CH2(4)); 1.71 (s, MeꢀC(5)); 1.6 (m, CH2(3)); 1.47
(m, CH2(2)); 1.03 (s, 2 MeꢀC(1)). 13C-NMR: 135.7 (CH); 135.4 (CH); 128.7 (CH); 39.7 (CH2); 33.4
(CH2); 19.4 (CH2); 28.9 (Me); 21.6 (Me); 12.9 (Me).
(3E)-4-(2,3,6-Trimethylphenyl)but-3-en-2-one (5). retro-a-Ionone (9.62 g; 50 mmol) and 2,3-di-
chloro-5,6-dicyano-1,4-benzoquinone (DDQ; 34 g, 3 equiv.) in 1,2-dichloroethane (100 ml) were heated
for 1 h at 658. The mixture was filtered, the filtrate concentrated, and the crude product purified by CC
(SiO2, pentane/1,2-dichloroethane 1:1): 5 (50%). Yellow oil. IR (film): 1680. 1H-NMR1): 7.72 (d, J ¼ 16,
HꢀC(7)); 7.05, 6.95 (2d, J ¼ 7, HꢀC(3), HꢀC(4)); 6.25 (d, J ¼ 16, HꢀC(8)); 2.40, 235, 2.30, 2.25 (4s, 4 Me).
13C-NMR (CDCl3): 188.0 (C¼O); 134.6 (C); 134.5 (C); 133.5 (C); 143.1 (CH); 133.4 (CH); 129.8 (CH);
127.5 (CH); 27.4 (Me); 20.8 (Me); 16.9 (Me).
(4E)-3-Methyl-5-(2,3,6-trimethylphenyl)penta-2,4-dienenitrile (6). In a DeanꢀStark apparatus, a soln.
of 5 (6 g) and cyanoacetic acid (8.21 g, 3 equiv.) in benzene (30 ml) was cooled to 08, and piperidine
(25.43 ml, 8 equiv.) was slowly added. Then, the mixture was refluxed for 4 h. The benzene was
evaporated, the crude product extracted with Et2O, the org. layer washed with brine and H2O, dried
(MgSO4), and concentrated, the crude product extracted with CH2Cl2, and this extract quickly filtered
over a mixture of basic alumina and SiO2 40 :60: (2E,4E)/(2Z,4E)-6 95 :5 (96%; ratio determined by
1H-NMR). Yellow ochre crystals which were purified by recrystallization from Et2O/pentane 30 :70:
major isomer (2E,4E)-6. M.p. 628. IR (film): 2206. 1H-NMR1): 7.02 (d, J ¼ 16.3, HꢀC(7)); 6.98 (d, J ¼ 7.5,
HꢀC(3)); 6.96 (d, J ¼ 7.5, HꢀC(4)); 6.28 (d, J ¼ 16.3, HꢀC(8)); 5.25 (s, HꢀC(10)); 2.32 (s, MeꢀC(2)); 2.26
(s, MeꢀC(9)); 2.25 (s, MeꢀC(5)); 2.2 (s, MeꢀC(1)). 13C-NMR (CN): 118.0; 157.1 (C); 136.0 (C); 134.9
(C); 134.7 (C); 133.8 (C); 135.9 (CH); 134.5 (CH); 129.7 (CH); 127.8 (CH); 98.3 (CH); 21.2 (Me); 20.7
(Me); 17.4 (Me); 16.9 (Me).
(4E)-(2,3,6-Trimethylphenyl)penta-2,4-dienal (7). At 08, 1.2m DIBAL-H in toluene (24 ml, 1.2
equiv.) was slowly added under vigorous stirring to 6 (5 g) in toluene. The mixture was stirred for 2 h and
then hydrolyzed by 2m H2SO4. After filtration of the aluminium salts, Et2O (50 ml) and H2O (50 ml)
were added. The org. layer was washed with brine and H2O, dried (MgSO4), and concentrated and the
crude product purified by CC (SiO2, CH2Cl2): major isomer (2E,4E)-7 (85%). Yellow oil. IR (film):
1659. 1H-NMR1): 10.20 (d, J ¼ 8.1, HꢀC(11)); 7.18 (d, J ¼ 16.4, HꢀC(7)); 7.01 (2d, J ¼ 7.6, HꢀC(3),
HꢀC(4)); 6.36 (d, J ¼ 16.4, HꢀC(8)); 6.14 (d, J ¼ 8.1, HꢀC(10)); 2.44 (s, MeꢀC(9)); 2.26 (s, MeꢀC(1),
MeꢀC(2)); 2.23 (s, MeꢀC(5)). 13C-NMR: 191.3 (C¼O); 154.1 (C); 135.9 (C); 134.5 (C); 134.4 (C); 133.5
(C); 137.2 (CH); 135.2 (CH); 129.7 (CH); 129.2 (CH); 127.5 (CH); 20.8 (Me); 20.4 (Me); 13.0 (Me); 9.7
(Me).
(2E,4E,6E)- and (2Z,4E,6E)-2-Cyano-3,7,11-trimethydodeca-2,4,6,10-tetraenoic Acid (8) and
(2E,4E,6E)- and (2Z,4E,6E)-3,7,11-Trimethydodeca-2,4,6,10-tetraenenitrile (9). At 08, a mixture of citral
(1.52 g) and methyl cyano(isopropylidene)acetate (¼ methyl 2-cyano-3-methylbut-2-enoate; 3; 2.325 g,
1.5 equiv.) was added to tBuOK (1.12 g, 1 equiv.) in MeOH (25 ml). The soln. was allowed to stay at r.t.
for 24 h. The MeOH was evaporated and ice (100 g) was added. The mixture was extracted with Et2O, the
aq. layer acidified and extracted with Et2O, and this extract washed with H2O, dried (MgSO4), and
concentrated: crude 8 as a mixture of isomers.
A soln. of crude 8 (1 g) in piperidine (50 ml) and pyridine (50 ml) was refluxed until the flow of CO2
ceased (2 – 3 h). The bases were evaporated and the crude product was extracted with Et2O. The extract
was washed successively with a 1m HCl and H2O, dried (MgSO4) and concentrated: (2E,4E,6E)/
(2Z,4E,6E)-9 7:3. This mixture was separated by CC (SiO2, CH2Cl2).

Helvetica Chimica Acta – Vol. 96 (2013)
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1
(2E,4E,6E)-9: (85%): IR(film): 2213. H-NMR1): 1.60 (s, MeaꢀC(1)); 1.68 (s, MebꢀC(1)); 1.87 (s,
MeꢀC(5)); 2.17 (m, CH2(3), CH2(4); 2.21 (s, MeꢀC(9)); 5.17 (m, HꢀC(2)); 5.29 (s, HꢀC(10)); 5.93 (d,
J ¼ 11.0, HꢀC(6)); 6.19 (d, J ¼ 15.2, HꢀC(8)); 6.76 (dd, J ¼ 15.2, 11.0, HꢀC(7)). 13C-NMR : 133.0 (CH);
130.2 (CH); 127.6 (CH); 123.8 (CH); 96.4 (CH); 40.7 (CH2); 26.8 (CH2); 26.1 (Me); 18.1 (Me); 17.0
1
(Me). (2Z,4E,6E)-9: H-NMR1): 1.56 (s, MeaꢀC(1)); 1.64 (s, MebꢀC(1)); 1.86 (s, MeꢀC(5)); 2.10 (m,
CH2(3), CH2(4)); 2.18 (s, MeꢀC(9)); 5.11 (m, HꢀC(2), HꢀC(10)); 6.04 (d, HꢀC(6)); 6.70 (d, J ¼ 15.0,
HꢀC(8)); 6.76 (dd, J ¼ 15.0, 11.0, HꢀC(7)). 13C-NMR: 133.5 (CH); 129.9 (CH); 125.7 (CH); 124.8
(CH); 95.0 (CH); 33.3 (CH2); 27.2 (CH2); 24.8 (Me); 19.8 (Me); 18.1 (Me).
(2E,4E,6E)- and (2Z,4E,6E)-3,7-Trimethyldodeca-4,6-trienal (10). As described for 7, with 9
(0.645 g) in toluene and DIBAL-H (1.1 equiv.) in toluene. After 30 min, the reaction was quenched with
1m H2SO4. After filtration and usual workup, the org. layer was rapidly filtered over Al2O3: (2E,4E,6E)/
(2Z,4E,6E)-10 7:3 (65%). This mixture was separated by CC (SiO2, CH2Cl2).
(2E,4E,6E)-10: 1H-NMR1): 1.60 (s, MeaꢀC(1)); 1.70 (s, MebꢀC(1)); 1.89 (s, MeꢀC(5)); 2.05 (m,
CH2(3), CH2(4)); 2.36 (s, MeꢀC(9)); 5.11 (m, HꢀC(2)); 5.96 (d, J ¼ 8.1, HꢀC(10)); 6.24 (m, HꢀC(3),
HꢀC(4)); 10.09 (d, J ¼ 8.0, HꢀC(11)). 13C-NMR: 191.6 (C¼O); 133.1 (CH); 129.1 (CH); 126.3 (CH);
125.3 (CH); 123.8 (CH); 40.7 (CH2); 26.8 (CH2); 26.1 (Me); 17.7 (Me); 13.5 (Me).
(2Z,4E,6E)-10: 1H-NMR1): 1.61 (s, MeaꢀC(1)); 1.68 (s, MebꢀC(1)); 1.87 (s, MeꢀC(5)); 2.15 (m,
CH2(3), CH2(4)); 2.3 (s, MeꢀC(9)); 5.08 (m, HꢀC(2)); 5.85 (d, J ¼ 8.0, HꢀC(10)); 5.95 (d, J ¼ 15.0,
HꢀC(6)); 6.98 (m, J ¼ 15.0, HꢀC(8)); 7.37 (m, HꢀC(7)); 10.18 (d, J ¼ 7.5, HꢀC(11)). 13C-NMR: 190.4
(C¼O); 134.0 (CH); 133.3 (CH); 127.9 (CH); 125.4 (CH); 125.1 (CH); 33.4 (CH2); 27.2 (CH2); 24.9
(Me); 21.6 (Me); 18.1 (Me).
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Received June 9, 2012