Synthesis and reactivity of endocyclic α-cyanoenamines in the phenyloxazolopiperidine series

Article abstract of DOI:10.1081/SCC-100106032

Using the capto-dative character of piperidine building block 1, the diastereomeric α-cyanoenamines 2a and 2b accompanied by dimer 5 were prepared through a SET pathway in a one-step procedure. Adjustment of the reaction conditions afforded either 2a or 2

Full text of DOI:10.1081/SCC-100106032

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SYNTHESIS AND REACTIVITY OF ENDOCYCLIC α-
CYANOENAMINES IN THE PHENYLOXAZOLOPIPERIDINE
SERIES
Marie-Christine Lallemand a , David François a , Ingrid Laimer a , Nicole Kunesch b &
Henri-Philippe Husson c
a UMR 8638 associée au CNRS et à 1'Université René-Descartes , Faculté des Sciences
Pharmaceutiques et Biologiques, 4, Avenue de 1'Observatoire, Paris, Cedex 06, 75270,
France
b UMR 8638 associée au CNRS et à 1'Université René-Descartes , Faculté des Sciences
Pharmaceutiques et Biologiques, 4, Avenue de 1'Observatoire, Paris, Cedex 06, 75270,
France
c UMR 8638 associée au CNRS et à 1'Université René-Descartes , Faculté des Sciences
Pharmaceutiques et Biologiques, 4, Avenue de 1'Observatoire, Paris, Cedex 06, 75270,
France
Published online: 20 Aug 2006.
To cite this article: Marie-Christine Lallemand , David François , Ingrid Laimer , Nicole Kunesch & Henri-Philippe Husson
(2001) SYNTHESIS AND REACTIVITY OF ENDOCYCLIC α-CYANOENAMINES IN THE PHENYLOXAZOLOPIPERIDINE SERIES,
Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 31:21,
3231-3239, DOI: 10.1081/SCC-100106032
To link to this article: http://dx.doi.org/10.1081/SCC-100106032
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SYNTHETIC COMMUNICATIONS, 31(21), 3231–3239 (2001)
SYNTHESIS AND
REACTIVITY OF ENDOCYCLIC
a-CYANOENAMINES IN THE
PHENYLOXAZOLOPIPERIDINE
SERIES
Marie-Christine Lallemand,David Franc ois,
Ingrid Laimer,Nicole Kunesch, *
and Henri-Philippe Husson*
UMR 8638 associee au CNRS et a 1’Universite
Rene-Descartes, Faculte des Sciences Pharmaceutiques
et Biologiques, 4, Avenue de 1’Observatoire,
75270 Paris Cedex 06, France
ABSTRACT
Using the capto-dative character of piperidine building block
1, the diastereomeric a-cyanoenamines 2a and 2b accompa-
nied by dimer 5 were prepared through a SET pathway in a
one-step procedure. Adjustment of the reaction conditions
afforded either 2a or 2b as the major constituent. g-Alky-
lation of 2b with benzyl bromide proceeded smoothly.
*Corresponding authors. E-mail: kunesch@pharmacie.univ-paris5.fr
3231
Copyright & 2001 by Marcel Dekker, Inc.
www.dekker.com

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LALLEMAND ET AL.
The development of general synthetic methods involving chiral build-
ing units have attracted increasing attention in the recent years. In con-
nection with our methodology directed towards the asymmetric syntheses
of piperidine alkaloids, we have been engaged in the preparation of chiral
non-racemic building blocks related to 1. As a result various molecules have
been elaborated through the introduction of several substituents at the
a,a0,b,b0 positions relative to the tertiary nitrogen atom of 1,1 which may
clearly evidence the chemical potential of this compound.
To explore a new aspect of the versatile building block 1, we were
interested in the substitution at the g-position relative to the nitrogen
atom. This novel approach may involve a derivatization into an a-cyanoe-
namine 2a or 2b (Figure 1). Indeed, compounds 2a and 2b can be considered
as precursors of homoenolates, a series of synthetic intermediates that have
attracted much attention in virtue of their useful activities.2 In this context,
endocyclic enamines have recently been the subject of synthetic approaches.3
The chiral non-racemic endocyclic enamines 2a (7,8-dihydro-3-phenyl-
3R-[3a,8ab]-5H-oxazolo[3,2a]pyridine-5-carbonitrile) and 2b (7,8-dihydro-
3-phenyl-3R-[3a,8aa]-5H-oxazolo[3,2a]pyridine-5-carbonitrile) have already
been synthesized4 in our hands with modest yields through cyanation of 1
followed by thermic HCN elimination. The high enamine reactivity of both
compounds 2a and 2b was revealed by the [2 þ 2] cycloaddition reaction
with diethyl acetylendicarboxylate.4
In this paper we wish to describe a new access to a-cyanoenamines 2a
and 2b as well as their reactivity in g-alkylation reactions.
We reasoned that a-cyanoenamine 2a or 2b could be obtained in a
one-pot oxidation procedure through the lithiated anion 3. Since O2 oxida-
tion5 of 3 afforded lactam 4 in 46% yield, we thought to perform the
preparation of 2a by submitting a 2-substituted selenium derivative of 1
to an oxidative elimination. In fact, treatment of 3 with phenylselenium
bromide in oxygen-free conditions yielded directly a-cyanoenamine 2a and
a new compound (5), in addition to the recovered starting material 1
(Scheme 1).
Figure 1.

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3233
Scheme 1.
The dimeric structure of 5 was established by mass- and NMR-spec-
troscopies. Indeed, the mass spectrum obtained by chemical ionization
revealed the molecular peak at m/z 455 (M þ 1)+ accompanied by a
major fragment at m/z 401 (M þ 1À54)þ, due to the elimination of two
molecules of HCN. The 13C-NMR spectrum which differs mostly from
the spectrum of 1 by the presence of a quaternary carbon at 64.1 ppm and
by the deshielded resonance of the CN group (119.5 ppm versus 116.0 ppm),
confirmed the symmetrical feature of 5 and the newly created carbon-carbon
bond at C-5.
The oxidative dimerization reaction leading to the obtention of 5
was reminiscent of the well known dimerization of carbanions through a
radical process.6 The radical formation involves a single electron transfer
(SET)7 and is particularly well documented for the treatment of carbanions
with polyhaloalkanes or other donors of positive halogen.8 Very recently,
Gawley9 examined the reactions of unstabilized a-aminoorganolithiums
derived from pyrrolidines and piperidines with electrophiles and reported
a competition between electrophilic substitution reactions and single elec-
tron transfer reactions. It was suggested that SET is operative when the
lithiated anion is easily oxidized by the electrophile, depending on the
difference in redox potentials. This led us to take into account for the first
time in our study the capto-dative character of 1. The capto-dative concept
was formulated by Viehe10 as the result of electron-donating (dative) and
electron-withdrawing (capto) substituents giving easily access to stabilized
radicals.11 The cyano/dialkylamino pair appears to be the most effective
combination.12 It is now well established that capto-dative anions are
destabilized by electron-donating substituents such as amino groups and
consequently oxidize rapidly into the more stable capto-dative radical.10
The relative stability of capto-dative radicals was shown to favor radical
reaction at low energy activation such as dimerization.13 Thus, anion 3
would generate a capto-dative radical 6 leading to the dihydrodimer 5
while 2a could result of a disproportionation of the radical (Scheme 2).
Many such dimerization and disproportionation reactions of a-amino nitrile

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LALLEMAND ET AL.
Scheme 2.
radicals have already been reported.14 A recent study of the kinetics of
thermolysis concerning capto-dative trimethylsilyl cyanohydrin radicals
demonstrated a simultaneous disproportionation and dimerization.15
Although the dimerization occurs mostly 10–50 times faster than dispropor-
tionation, this latter process is favored by steric effects and by the presence
of vicinal hydrogens leading to b-elimination.16 In our case, the three
compounds of the reaction, 1, 2a, and 5 can be considered as the result of
a simultaneous disproportionation and dimerization mechanism of radical 6
(Scheme 2). Since the reversible dimerization does not proceed with stereo-
specificity,17 the relative stereochemistry of the carbon centers in dimer 5 is
not ascertained.
In the light of the mechanism we repeated the reaction with iodine, a
classical reagent for the oxidation of capto-dative anions.6,10 In the presence
of an equivalent of iodine compound 3 afforded in 5 min a mixture of 1
(25%), 2a (25%), and 5 (25%). Other agents (tosyl chloride, ICN, NBS,
NIS, benzoylperoxide) led to the same yield of dimer 5 and to the additional
formation of a-cyanoenamine 2b. In contrast, the yield of 2a was highly
dependent of the reaction conditions and oxidation agents. For instance,
when the reaction was conducted in the presence of air, the major isolated
product was the previously described lactam 4. The best conditions for the
obtention of 2a were a molar solution and a short reaction time, while a
longer reaction time afforded, in addition, the stereomeric a-cyanoenamine
2b as a minor compound.

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ENDOCYCLIC a-CYANOENAMINES
3235
The difficulties of the purification resulting from the simultaneous
formation of 1, 2a, 2b, and 5 were solved by treating the reaction mixture
with AgBF4. We reasoned that the silver salt would remove the more labile
cyano group borne by compounds 1 and 5 to yield water soluble salts, while
a-cyanoenamines 2a and 2b would not react in this process. Treatment of
the reaction mixture with AgBF4 followed by extraction with water/CH2Cl2
provided exclusively the thermodynamically more stable 2b (H-3/H-8a
trans) (20%).
As previously mentioned, 2a and 2b were ideal substrates for g-alkyla-
tion. Following Meyers’s methodology,3a,3d,18 the metalation of 2b was
carried out with LiTMPin THF/HMPA at À70ꢀC. Addition of benzyl
bromide followed by hydrolysis afforded the b-alkylated lactam 7 in 50%
yield as a single diastereomer (Scheme 3).
Scheme 3.
The equatorial position of the benzyl chain C-7 was deduced from
the H NMR spectrum. Indeed, while the signal of H-7 showed a complex
1
multiplet, a carefull NMR study including 2D experiments revealed the
axial position of H-7 through the large coupling constants exhibited by
the signals of the axial protons H-6 at 2.09 ppm (dd, J gem ¼ 17 Hz,
JH-6/H-7 ¼ 12 Hz) and H-8 at 1.31 ppm (ddd, Jgem ¼ 12.5 Hz, JH-8/H-7
¼
10 Hz, JH-8/H-8a ¼ 9.5 Hz). In addition, the large coupling constant between
H-8 axial and H-8a was in favor of the axial position of H-8a. The pseudo-
chair conformation as depicted in Scheme 3 was deduced from nOe-differ-
ence experiments and was in good agreement with the stereoselectivity of the
alkylation as previously reported.3a The same procedure applied to a-cya-
noenamine 2a failed to give an alkylated derivative and yielded the unsub-
stituted lactam 4 after hydrolysis.
Further studies are in progress in order to reach the isobenzomorphan
skeleton19 by the same alkylation procedure using a methoxy acti-
vated benzyl group in a Mannich type reaction with the C-8a iminium
functionality.

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LALLEMAND ET AL.
In summary we have described a very easy access to endocyclic chiral
non-racemic a-cyanoenamines. Starting from building block 1 obtained in
85% yield by reacting glutaraldehyde and (R)-(-)-phenylglycinol in the pres-
ence of KCN,20 the one-pot procedure consisting in metalation and iodine
oxidation led to a-cyanoenamine 2a, and dimer 5 in addition to the starting
material. Further treatment with AgBF4 afforded exclusively a-cyanoena-
mine 2b. The modest yield of the reaction is counterbalanced by the simpli-
city and the versatility of the procedure.
EXPERIMENTAL
7,8-Dihydro-3-phenyl-3R-[3a,8ab]-5H-oxazolo[3,2a]-
pyridine-5-carbonitrile (2a)
To a solution of diisopropylamine (1.47 mL, 9.6 mmol) in 5 mL of dry
tetrahydrofuran at 0ꢀC was added n-butyllithium (3.9 mL, 9.6 mmol, 2.5 M
in hexanes) under an argon atmosphere. The mixture was stirred for 20 min
at 0ꢀC and then cooled to À78ꢀC. Compound 1 (1 g, 4.38 mmol) in 15 mL of
dry tetrahydrofuran was added dropwise and the reaction mixture was stir-
red for 20 min. A solution of iodine in dry THF was transferred via canula
into the mixture that was stirred for 5 min at À78ꢀC. The reaction was
quenched with an aqueous buffered phosphate solution (pH 6.2) and the
aqueous solution extracted with methylene chloride. The combined organic
extracts were dried over Na2SO4 and concentrated under reduced pressure
to give a crude oil, which was purified by flash chromatography (8.5/1.5
hexane/acetone) to afford the diastereomers 2a and 2b, and the dimer 5 in
25% yield respectively.
7,8-Dihydro-3-phenyl-3R-[3a,8ab]-5H-oxazolo[3,2a]-
pyridine-5-carbonitrile (2a)
½aꢁD -297 (c 1 in CHCl3); m.p. 100–104ꢀC; IR (n cmÀ1): 2250, 1725,
20
1
1675, 1620; H NMR (300 MHz; CDCl3): d 1.72 (1H, m, H-8), 2.31 (3H,
m, H-8 and H-7), 4.09 (1H, dd, J ¼ 1.5, 8.5 Hz, H-2), 4.29 (1H, dd, J ¼ 6,
8.5 Hz, H-2), 4.72 (1H, dd, J ¼ 1.5, 6.0 Hz, H-3), 4.81 (1H, dd, J ¼ 4.0,
10.0 Hz, H-8a), 5.32 (1H, br s, H-6), 7.3–7.5 (5H, m, aromatics);
13C NMR (75 MHz; CDCl3): d 21.2 (C-8), 25.9 (C-7), 61.6 (C-3), 73.6
(C-2), 87.4 (C-8a), 114.0 (C-6), 115.1 (CN), 115.7 (C-5), 127.6, 127.9,
128.6 (CH aromatics), 141.0 (C aromatic); m/z (CI) 227 (M þ 1)þ; Anal.

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3237
Calcd for C14H14N2O: C, 74.31; H, 6.24; N, 12.38; Found: C, 74.21; H, 6.39;
N, 12.21.
7,8-Dihydro-3-phenyl-3R-[3a,8aa]-5H-oxazolo[3,2a]-
pyridine-5-carbonitrile (2b)
20
½aꢁD -245 (c 1 in CHCl3); IR (n cmÀ1): 2224, 1720, 1670, 1610; 1H NMR
(300 MHz; CDCl3): d 1.7–1.8 (1H, m, H-8), 2.0–2.4 (3H, m, H-8 and H-7),
3.80 (1H, dd, J ¼ 7.0, 8.0 Hz, H-2), 4.34 (1H, dd, J ¼ 7.0, 8.0 Hz, H-2), 4.65
(1H, t, J ¼ 7.0 Hz, H-3), 4.95 (1H, dd, J ¼ 3.0, 7.0 Hz, H-8a), 5.67 (1H, t,
J ¼ 4.5 Hz, H-6), 7.3–7.5 (5H, m, aromatics); 13C NMR (75 MHz; CDCl3): d
19.9 (C-8), 24.6 (C-7), 64.4 (C-3), 72.2 (C-2), 87.8 (C-8a), 115.5 (CN), 118.1
(C-6), 119.5 (C-5), 126.7, 127.9, 128.6 (CH aromatics), 139.5 (C aromatic);
m/z (CI) 227 (M þ 1)þ; HRMS (CI, CH4) m/z (M þ 1)þ; Calcd for
C14H14N2O: 227.1184 Found: 227.1177.
Dimer 5
½aꢁD -77 (c 1 in CHCl3); m.p. 185–188ꢀC (MeOH/aceton); IR (n cmÀ1):
20
1
2177; H NMR (300 MHz; CDCl3): d 1.6–2.3 (6H, m, H-6, H-7 and H-8),
3.94 (1H, d, J ¼ 9.0 Hz, H-2), 4.13 (1H, dd, J ¼ 7.0, 9.0 Hz, H-2), 4.81 (1H, d,
J ¼ 7.0 Hz, H-3), 4.98 (1H, t, J ¼ 7.0 Hz, H-8a), 7.3–7.5 (5H, m,
aromatics); 13C NMR (75 MHz; CDCl3): d 17.5 (C-7), 29.3 (C-8), 30.8
(C-6), 58.7 (C-3), 64.1 (C-5), 75.1 (C-2), 86.8 (C-8a), 119.5 (CN), 126.9,
127.6, 128.6, (CH aromatics), 141.0 (C aromatic); m/z (CI) 455 (M þ 1)þ;
401 (M þ 1-54)þ; HRMS (CI, CH4) m/z (M þ1)þ; Calcd for C28H30N4O2:
455.2447 Found: 455.2456.
Alkylation of Cyanoenamine 2b
To
a stirred solution of 2,2,6,6-tetramethylpiperidine (105 mL,
0.622 mmol) in 1.5 mL of anhydrous THF at À78ꢀC were added successively
n-BuLi (0.25 mL, 0.622 mmol, 2.5 M solution in hexanes) and HMPA
(105 mL, 0.622 mmol) under an argon atmosphere. The reaction mixture
was then allowed to warm to 0ꢀC, stirred for 15 min and then cooled to
À78ꢀC. Cyanoenamine 2b (86 mg, 0.389 mmol) in 0.5 mL of anhydrous THF
was added dropwise. The mixture was stirred at À78ꢀC for 20 min and
benzylbromide (93 mL, 0.78 mmol) was added dropwise over 2 min. The
mixture was stirred for 1 h at À78ꢀC. The reaction was then quenched

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LALLEMAND ET AL.
with saturated KHCO3 (5 mL) and extracted with ethyl ether (2 ꢂ 15 mL).
The combined organic layers were concentrated under reduced pressure,
treated with 5 mL of THF and 5 ml of a 1N aqueous solution of HCl
(5 mL) and stirred for 15 h at room temperature. After extraction with
ethyl ether and evaporation to dryness the crude residue was purified by
flash chromatography (6 : 1 hexanes : ethyl acetate) and provided 66 mg
(55%) of lactam 7 and 8 mg (10%) of lactam 4.
Lactam 7
20
½aꢁD -24 (c 1 in CHCl3); IR (n cmÀ1): 1644; 1H NMR (300 MHz;
CDCl3): d 1.31 (1H, ddd, J ¼ 12.5, 10, 9.5 Hz, H-8ax), 2.09 (1H, dd,
J ¼ 17, 12 Hz, H-6ax), 2.15–2.30 (1H, m, H-7), 2.35 (1H, ddd, J ¼ 12.5,
4.5, 2 Hz, H-8eq), 2.55 (1H, dd, J ¼ 17, 4.5 Hz, H-6eq), 2.69 (1H, dd,
J ¼ 13.5, 6.5 Hz, Ph-CH2), 2.72 (1H, dd, J ¼ 13.5, 6 Hz Ph-CH2), 4.49 (1H,
dd, J ¼ 9, 8 Hz, H-2), 4.98 (1H, dd, J ¼ 9.5, 4.5 Hz, H-8a), 5.24 (1H, dd,
J ¼ 8 Hz, H-3, 1H), 7.1–7.4 (5H, m, aromatics); 13C NMR (75 MHz;
CDCl3): d 31.7 (C-7); 34.1 (C-8); 38.0 (C-6); 41.8 (Ph-CH2); 57.8 (C-8a);
72.6 (C-2); 88.2 (C-3); 125.9; 126.4; 127.5; 128.4; 128.7; 129.0 (CH aro-
matics); 138.5; 139.3 (C aromatics); 168.3 (C¼O); m/z (CI) 308 (M þ 1)þ;
HRMS (CI, CH4) m/z (M þ 1)þ; Calcd for C20H21NO2: 308.1650 Found:
308.1655.
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Received in the UK November 8, 2000

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