First one-pot synthesis of mikanecic acid derivatives from allylic phosphonates, via a Tandem-sequence Horner-Wadworth-Emmons and Diels-Alder reactions

Article abstract of DOI:10.1055/s-1999-3390

This paper describes a new, simple, general and efficient one-pot synthesis of mikanecic acid derivatives from allylic phosphonates, ethyl chloroformate and aqueous formaldehyde. The overall process involves a cascade sequence linking together metallation

Full text of DOI:10.1055/s-1999-3390

282
PAPER
First One-Pot Synthesis of Mikanecic Acid Derivatives from Allylic
Phosphonates, via a Tandem-Sequence HornerÐWadworthÐEmmons
and DielsÐAlder Reactions
Hashim Al-Badri, No‘l Collignon*
Laboratoire dÕHŽtŽrochimie Organique de lÕIRCOF, UniversitŽ de Rouen-INSAR, Place E. Blondel, BP 08, F-76131 Mont-Saint-Aignan
Cedex, France
Fax +(33)0235522959; E-mail: No‘l.Collignon@insa-rouen.fr
Received 18 June 1998
at Ð70¡C, then with ethyl chloroformate at the same tem-
Abstract: This paper describes a new, simple, general and efficient
perature and finally with the commercial aqueous solution
one-pot synthesis of mikanecic acid derivatives from allylic phos-
of formaldehyde near 0ûC, led directly to the correspond-
phonates, ethyl chloroformate and aqueous formaldehyde. The
ing mikanecic acid derivatives 1 in high yield (Scheme 2,
way a and Tables 1 and 2). This procedure exemplifies the
first synthesis of 1 via the tandem-sequence HornerÐWad-
worthÐEmmons (HWE) and DielsÐAlder reactions, real-
overall process involves a cascade sequence linking together metal-
lationÐalkoxycarbonylation, HornerÐWadworthÐEmmons and
DielsÐAlder reactions.
Key words: allylphosphonates, α-ethoxycarbonylallylphospho-
nates, mikanecic acid derivatives, alkenation, cycloaddition, one- ized in situ from the readily available phosphonates 3.20
The overall reaction was monitored by 31P NMR spectros-
pot synthesis
copy: in the first step, phosphonate 3 was deprotonated
with one equivalent of LDA, as previously described;20
4-Vinylcyclohex-1-ene-1,4-dicarboxylic acid or Òmi-
kanecic acidÓ (I, R1 = R2 = R = H) is a terpenoid dicarbox-
ylic acid isolable from natural alkaloids such as
Mikanoidine1 or Sarracine.2 It has been the object of sev-
eral synthesis attempts during the past three decades. All
the methods already proposed for the synthesis of this nat-
ural product and its derivatives involved the in situ gener-
ation of an isoprenic precursor II [Z = CO2R, CN,
C(O)R], which spontaneously dimerizes by a selective
head-to-head DielsÐAlder reaction,3 leading to I, after
possible functional group interconversion (FGI), (Scheme
1). The precursory 1,3-dienes II have been generated ei-
ther by thermolysis of suitable functional strained cycles
as cyclopropanes,3,4 cyclobutenes5 and sulfolenes,6Ð8 or
more generally, by an elimination procedure applied to an
a,b-unsaturated ester (or ketone) bearing a convenient
leaving group (halogeno,3,9Ð11 acetate,2 mesylate12Ð14 or
carbamate15), in allylic or homoallylic16 position. Finally,
various functional substituted allenes have been also em-
ployed as sources of dienes II.17Ð19 Most of the above-
mentioned routes to mikanecic acid derivatives suffer ei-
ther from difficulty of access to starting materials, or from
drastic reaction conditions, low yields and often also,
from lack of structural generality.
Scheme 1
Pursuing our work on the reactivity of phosphonoallylic
20Ð22
carbanions,
we found that treating allylic phospho-
Scheme 2
nates 3 with an excess of lithium diisopropylamide (LDA)
Synthesis 1999, No. 2, 282–285 ISSN 0039-7881 © Thieme Stuttgart á New York

PAPER
First One-Pot Synthesis of Mikanecic Acid Derivatives from Allylic Phosphonates
283
then, the addition of ethyl chloroformate to the lithiated very good yield (Tables 3 and 4).23 Interestingly, when the
phosphonate, in the presence of the second equivalent of quenching agent was the aqueous solution (37 wt %) of
LDA, led quantitatively to the intermediate carbanion 4 formaldehyde, the formation of diethyl phosphate [31P
[31P NMR, d (THF) ~ 35], whose formation was proved NMR, d (THF) ~ Ð1.7], as by-product of the HWE reac-
by quenching the resulting mixture with aqueous HCl, tion, was complete after about 15 min at 0 to 5¡C.24 Sub-
giving the corresponding allylic phosphonate a-ester 5 in sequent usual work up furnished the expected compound
1, likely resulting from spontaneous DielsÐAlder dimer-
ization of the transient 2-carboalkoxybuta-1,3-diene 2, the
main product of the HWE reaction. We also showed that
Table 1 Synthesis and Physical Data of Compounds 1
Product
Yield (%)a
Bp °C/0.05 mm Hgb
(Mp °C)
isolated phosphonates 5 could be readily transformed into
mikanecic derivatives 1 when treated by aqueous formal-
25
Way a
Way b
dehyde in the presence of K2CO3 (Scheme 2, way b),
however the overall yield of the sequence 3 → 1 in way b
was always lower than that obtained by the one-pot pro-
cess (Table 1).
1a
1b
1c
1d
78
81
86
84
57
62
74
71
87c
106
(119–120)d
115e
As previously reported,13 the DielsÐAlder dimerization of
the expected intermediate diene 2 was highly regio- and
stereoselective. Thus, in each example studied in the
a
Yield of isolated product, calculated from starting allylic phospho-
nates 3. Purification by column chromatography over silica gel
(eluent: petroleum ether/Et2O). Satisfactory microanalysis (1c) or
HRMS: (1a, 1b, 1d) were obtained.
Determined by bulb-to-bulb distillation.
Lit. (ref. 11): 55°C/0.001 mm Hg.
1
present work, careful examination of H as well as 13C
NMR spectra (Table 2) showed a sole regioisomer having
the typical Òpara-diesterÓ structure 1, resulting from the
selective head-to-head DielsÐAlder process. Moreover,
for product 1b a mixture of two isomers was obtained with
a diastereomeric ratio higher than 95: 5, while for 1c only
b
c
d
e
Lit. (ref. 13): 111–113°C.
Lit. (ref. 5): 121–123/0.1 mm Hg.
Table 2 1H and 13C NMRa and MS Data of Compounds 1
Pro-
duct
1H NMR (CDCl3/TMS)
d, J (Hz)
13C {1H} NMR (CDCl3/TMS)
d, J (Hz)
MS (70 eV)
m/z (%)
1a
1.15–1.4 (m, 6 H, CH3CH2O), 1.7–2.85 14.0, 14.15 (CH3CH2O), 21.6 (C-6), 29.4 (C-5), 252 (M+, 3), 207 (9), 206 (69), 179
(m, 6 H, H2C-3, H2C-5, H2C-6), 4.15 (q, J 32.05 (C-3), 47.1 (C-4), 60.25 and 60.9 (50), 151 (45), 133 (100), 105 (65),
= 7.1, 4 H, CH3CH2O), 5.15 (dd, J = 10.8, (CH3CH2O), 115.0 (HC=CH2), 130.4 (C-1), 79 (36), 53 (17).
1.6, 1 H, HC=CHE), 5.55 (dd, J = 17.5, 136.7 (HC=CH2), 139.5 (C-2), 166.8 (O=C–C-
1.6, 1 H, HC=CHZ), 5.9 (dd, J = 17.5, 1), 174.2 (O=C–C-4).
10.8, 1 H, HC=CH2), 6.85–6.95 (m, 1 H,
HC-2).b
1bc
0.95 (d, J = 6.8, 3 H, H3CC-3), 1.2–1.4 14.05, 14.2 [14.0, 14.1] (CH3CH2O), 16.2 [15.9] 280 (M+, 21), 265 (1), 234 (96),
(m, 6 H, CH3CH2O), 1.5–2.4 (m, 4 H, (H3CC-3), 18.2 [16.83] (HC=CHCH3), 22.05 207 (78), 188 (36), 161 (100), 140
H2C-5, H2C-6), 1.65 (d, J = 5.9, 3 H, [21.55] (C-6), 24.8 [23.95] (C-5), 35.5 [36.8] (41), 133 (73), 91 (51), 67 (42), 39
H3CC=C), 2.5–3.4 (m, 1 H, HC-3), 4.0– (C-3), 50.0 [49.8] (C-4), 60.3, 60.7 [60.2] (19).d
4.25 (m, 4 H, CH3CH2O), 5.35 (d, J = (CH3CH2O), 126.2 (HC=CHCH3), 128.2 [128.8]
15.9, 1 H, HC=CHCH3) 5.55 (dm, J = (C-1), 131.5 [132.0] (HC=CHCH3), 143.4
15.9, 1 H, HC=CHCH3), 6.8–6.95 (m, 1 [141.7] (C-2), 167.2 [167.18] (O=CC-1), 174.9
H, HC-2).
[174.8] (O=CC-4).
1ce
1.2–1.4 (m, 6 H, CH3CH2O), 1.8–1.75 14.0, 14.1 (CH3CH2O), 22.05 (C-6), 24.75 (C-5), 404 (M+, 2 ), 373 (2), 315 (4), 232
(m, 4 H, H2C-5, H2C-6), 4.1–4.3, (m, 4 H, 47.6 (C-3), 51.05 (C-4), 60.3, 61.1 (CH3CH2O), (5), 188 (4), 128 (7), 104 (100), 77
CH3CH2O), 4.4 (d, J = 4.9, 1 H, HC-3), 126.2, 127.1, 127.3, 127.8, 128.3, 129.35, 130.5, (7), 51 (4).
5.75 (d, J = 16.4, 1 H, HC=CHPh), 6.2 (d, 131.6 (Co-, m-, p-arom and HC=CPh), 130.0 (C-1),
J = 16.4, 1 H, HC=CHPh), 7.0–7.3 (m, 11 136.7, 138.1 (Ci-arom), 139.1 (C-2), 166.8 (O=CC-
H, Harom and HC-2).
1), 173.9 (O=CC-4).
1d
1.0, 1.22 (2s, 6 H, (H3C)2C-3), 1.3–1.4 14.2, 14.25 (CH3CH2O), 18.5, 21.7 ((H3C)2C-3), 308 (M+, 27), 263 (11), 235 (9),
(m, 6 H, CH3CH2O), 1.45, 1.7 (2s, 6 H, 21.7 (C-6), 24.3, 26.8 ((H3C)2C=C), 27.2 (C-5), 189 (16), 154 (100), 125 (49), 108
(H3C)2C=C), 1.9–2.4 (m, 4 H, H2C-5, 38.8 (C-3), 51.3 (C-4), 60.4 (CH3CH2O), 124.15 (32), 81 (32), 55 (18), 41 (27).
H2C-6), 4.1–4.3 (m, 4 H, CH3CH2O), (HC=C(CH3)2),
5.15 (s, 1 H, HC=C(CH3)2), 6.6 (s, 1 H, (HC=C(CH3)2), 147.2 (C-2), 167.3 (O=CC-1),
HC-2). 175.5 (O=CC-4).
128.0
(C-1),
134.0
a
For numbering of the carbon atoms of cyclohexene ring, see Scheme 2.
60 MHz 1H NMR spectrum of 1a was given in ref. 3 and 9.
b
c
As a mixture of two diastereomers, in a ratio higher than 95:5 (determined by GC measurements). Main data refer to the major diastereomer;
values in square brackets, in the 13C{1H} NMR spectrum, refer to the minor one.
Not any significant difference was observed between the two diastereomers.
As a sole diastereomer.
d
e
Synthesis 1999, No. 2, 282–285 ISSN 0039-7881 © Thieme Stuttgart · New York

284
H. Al-Badri, N. Collignon
PAPER
ppm relative to TMS for 1H and 13C nuclei and to H3PO4 for 31P nu-
cleus; coupling constants (J) are given in Hz; coupling multiplici-
ties are reported using conventional abbreviations. Starting allylic
phosphonates 3 were prepared according to ref. 20.
one diastereomer was detected.26 In each case, NMR spec-
tra of the major diastereomer are in good agreement with
those of the corresponding dimethyl esters,13 in which the
ester group is exo and the alkenyl chain is endo with re-
spect to the roof-like cyclohexene moiety (Scheme 3; only
one enantiomer was represented), as that was already
proved by Hoffmann et al.13 In particular, the chemical
shifts of the carbons of the cyclohexene part (C-1 to C-6)
Diethyl 1-Ethoxycarbonylalk-2-enylphosphonates 5; General
Procedure
To a solution of LDA (22 mmol) in THF (15 mL) at Ð70¡C was
added a solution of phosphonate 3 (10 mmol) in THF (5 mL) over
20 min. Then, a solution of ethyl chloroformate (2.17 g, 20 mmol)
in THF (5 mL) was added dropwise, and the mixture was stirred for
2h at Ð70¡C and for 15 mn atÐ 20¡C. The resulting mixture was
then quenched at 0¡C by aqueous 4M HCl, until pH ~1. Et2O
(10 mL) was added and the aqueous layer was extracted with
CH2Cl2 (3 × 15 mL). The combined organic phases were dried
(MgSO4) and evaporated to give the crude product, which was pu-
rified by flash chromatography over silica gel (eluent: CH2Cl2) lea-
ding to the pure phosphonate 5 (Tables 3 and 4).
Table 3 Synthesis, Physical and 31P NMR Data of Compounds 5
Product
31P NMR (CDCl3)
Yield (%)a
Bp °C/mm Hgb
5a
5b
5c
5d
17.8
17.9
16.8
18.0
88
86
81
88
90/0.01
110/0.5
150/0.3
105/0.25
a
Yield of purified products (oils). Purification by flash chromatogra-
phy (silica gel; eluent: CH2Cl2). Satisfactory microanalyses ob-
tained: C ± 0.31, H ± 0.49.
b
Determined by bulb-to-bulb distillation.
Diethyl Mikanecic Acid Esters and Substituted Analogues 1;
General Procedure
Way a: To a solution of the in situ generated lithiated derivative of
phosphonate 5 (prepared as above, at Ð20¡C), was added, rapidly
and with vigorous stirring, an aqueous solution (37 wt %) of form-
aldehyde (15 mmol). A fast increase in temperature of the mixture
and formation of a white precipitate were observed. Stirring was
continued at 5¡C for about 15 min. Then, H2O (5 mL) and Et2O
(15 mL) were added at r.t. The organic layer was washed with H2O
(5 mL) and dried (MgSO4). The solvent was evaporated under re-
duced pressure (15 mm Hg) at 50¡C and the remaining volatiles were
removed by bulb-to-bulb distillation at 70¡C under reduced pressure
(2.5 mm Hg). The resulting crude product, which was analyzed by
GC for possible diastereoselectivity determinations, was then puri-
fied by column chromatography over silica gel (eluent: petroleum
ether / Et2O: 1/1), giving the pure product 1 (Tables 1 and 2).
Way b : To pure phosphonate 5 (10 mmol), was added at r.t. dried
K2CO3 (30 mmol) and then a solution (37% wt) of formaldehyde
(20 mmol) in H2O, under magnetic stirring. An exothermic reaction
developed over a period of about 10 min, at the end of which 31P
NMR spectrum showed a single peak at d ~1.5 ppm. The mixture
was then hydrolyzed with a saturated solution of NH4Cl (7 mL) and
extracted with Et2O (3 × 15 mL). Subsequent work-up and purifica-
tion were carried out as described above in way a.
Scheme 3
of the two diastereomers of 1b (major and minor) or of the
sole diastereomer of 1c, are in very good agreement with
those reported for the dimethyl esters and they can be
therefore securely used as criteria for stereochemical as-
signment.27
In conclusion, we describe here a new, efficient and high-
ly diastereoselective one-pot synthesis of mikanecic acid
derivatives from readily available allylic phosphonates
and standard materials, which illustrates again the advan-
tage of the in situ generation of HWE reagents. The exten-
sion of this methodology is currently being investigated in
our laboratory.
All reactions were carried out using standard techniques. Solvents
were purified by conventional methods prior to use. Reagents were
purchased from common commercial suppliers. TLC was per-
formed on Merck 60 F-254 silica gel plates and column chromatog-
raphy over silica gel (230Ð400 mesh). Elemental microanalyses
were carried out on a Carlo Erba EA 1110 analyser. Mass spectra
under electronic impact at 70 eV (m/z and relative abundance in
%, are given) were obtained with a GC/MS Hewlett Packard 5970
mass selective detector. HRMS measurements under chemical
ionisation at 200 eV were performed on a Jeol AX 500 spectrom-
eter. NMR spectra were recorded on a Bruker AC 200 spectrome-
ter operating at 200 MHz for proton, 50.3 MHz for carbon and
81.01 MHz for phosphorus; chemical shifts (d) are expressed in
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PAPER
First One-Pot Synthesis of Mikanecic Acid Derivatives from Allylic Phosphonates
285
Table 4 1H and 13C NMRa and MS Data of Compounds 5
Prod-
duct
1H NMR (CDCl3/TMS)
d, J (Hz)
13C{1H} NMR (CDCl3/TMS)
d, J (Hz)
MS (70 eV)
m/z (%)
5a
5b
5c
1.3 (m, 9 H, CH3CH2O), 3.7 (dd, J = 18.0, 14.8 (s, CH3CH2OC), 15.1 (d, J = 6.2, CH3CH2OP), 250 (M+, 11), 235 (94),
7.9, H, HC-1), 4.0–4.3 (m, H, 51.2 (d, J = 130.5, C-1), 61.5 (s, CH3CH2OC), 62.8, 205 (96), 204 (45), 177
CH3CH2O), 5.2–5.4 (m, 2 H, H2C-3), 5.9– 63.0 (2d, J = 7, 6.6, CH3CH2OP), 120.0 (d, J = 12.5, (73), 149 (100), 121 (83),
1
6
6.1 (m, 1 H, HC-2).
C-3), 128.0 (d, J = 11.3, C-2), 168.0 (d, J = 6.1, 81 (42), 68 (44), 39 (33).
C=O).
1.2–1.4 (m, 9 H, CH3CH2O), 1.75 (dd, J = 15.9–16.5 (m, CH3CH2OC, CH3CH2OP), 18.0 (d, J = 264 (M+, 4), 244 (3), 216
5.1, 4.8, 3 H, H3C–C-3), 3.7 (dd, J = 23.8, 2.5, H3C–C-3), 50.0 (d, J = 132.1, C-1), 63.0–64.0 (18), 191 (100), 177 (3),
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CH3CH2O), 5.6–5.8 (m, 2 H, HC-2, HC-3).
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C=O). (4).
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24.0, 9.5, 1 H, HC-1), 4.1–4.3 (m, 6 H, 50.1 (d, J = 130.5, C-1), 61.0 (s, CH3CH2OC), 62.8, 253 (19), 197 (27), 144
CH3CH2O), 6.35 (ddd, J = 15.9, 9.5, 6.2, 1 63.2 (2 d, J = 7.1, 6.8, CH3CH2OP), 118.2 (d, J = 9.0, (96), 115 (100), 81 (14),
H, HC-2), 6.6 (dd, J = 15.9, 4.9, 1 H, HC-3), C-2), 126.0, 127.7, 128.2 (3s, Co-, m-, p-arom), 134.2 (d, 65 (7), 40 (4).
7.2–7.5 (m, 5 H, Harom).
J = 13.0, C-3), 136.0 (s, Ci-arom), 167.0 (d, J = 5.8,
C=O).
5d
1.2–1.4 (m, 9 H, CH3CH2O), 1.7, 1.8 (2dm, 14.1 (s, CH3CH2OC), 16.2 (d, J = 6.1, CH3CH2OP), 278 (M+, 28), 232 (16),
J = 4.2, 3.7, 6 H, (H3C)2C-3), 3.95 (dd, J 18.2 and 25.9 (2d, J = 2.5, 3.1, (H3C)2C-3), 46.1 (d, J 205 (100), 177 (19), 149
= 23.9, 10.2, 1 H, HC-1), 4.15–4.3 (m, 6 H, = 132.7, C-1), 61.5 (s, CH3CH2OC), 62.8, 63.2 (2 d, (43), 138 (22), 111 (28),
CH3CH2O), 5.3–5.45 (m, 1 H, HC-2).
J = 7.2, 6.8, CH3CH2OP), 113.2 (d, J = 9.2, C-2), 67 (32), 39 (9).
138.0 (d, J = 13.1, C-3), 168.2 (d, J = 5.2, C=O).
a For numbering of the allylic carbon atoms, see Scheme 2.
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Synthesis 1999, No. 2, 282–285 ISSN 0039-7881 © Thieme Stuttgart · New York