A novel case of diastereoselection in 5-exo radical cyclization promoted by hydrogen bonding

Article abstract of DOI:10.1002/1099-0690(200011)2000:22<3727::AID-EJOC3727>3.0.CO;2-C

Moderate stereocontrol in 5-exo radical cyclizations of N,N-disubstituted (-)-8-amino menthol derivatives, promoted by tributyltin hydride and AIBN, was achieved. The stereoselection was explained in terms of hydrogen bond formation in the 1,3-amino alcohol derivative. The presence of one additional stereocenter at the allylic chain enhanced the stereoselection giving a single cyclization stereoisomer. The cyclization products were easily converted into enantiopure 3-alkyl- or 2,3-dialkyl-substituted pyrrolidines.

Full text of DOI:10.1002/1099-0690(200011)2000:22<3727::AID-EJOC3727>3.0.CO;2-C

FULL PAPER
A Novel Case of Diastereoselection in 5-exo Radical Cyclization Promoted by
Hydrogen Bonding
[a]
[a]
[a]
[a]
´
´
Rafael Pedrosa,* Celia Andres,* Juan P. Duque-Soladana, and Pilar Mendiguchıa
´
Dedicated to Professor Jose Barluenga on the occasion of his 60th birthday
Keywords: Radical reactions / Cyclizations / Asymmetric synthesis / Nitrogen heterocycles / Hydrogen bonds
Moderate stereocontrol in 5-exo radical cyclizations of N,N-
disubstituted (−)-8-amino menthol derivatives, promoted by
tributyltin hydride and AIBN, was achieved. The stereoselec-
tion was explained in terms of hydrogen bond formation in
the 1,3-amino alcohol derivative. The presence of one addi-
tional stereocenter at the allylic chain enhanced the stereose-
lection giving a single cyclization stereoisomer. The cycliza-
tion products were easily converted into enantiopure 3-alkyl-
or 2,3-dialkyl-substituted pyrrolidines.
Introduction
Results and Discussion
The design of new chiral auxiliaries for enantiocontrolled
radical cyclization, leading to nitrogen-containing molec-
ules, is a current topic of increasing interest. In recent years,
chiral auxiliaries have been incorporated at the nitrogen
atom of some α-haloamides, which can then undergo dias-
tereoselective radical cyclizations. Homolysis of the
carbonϪhalogen bond promoted by tributyltin hydride gen-
erates α-carbamoyl radicals, which add intramolecularly to
a suitably placed carbon-carbon multiple bond. The forma-
tion of optically active β-lactams by 4-exo ring closure[1]
and chiral pyrrolidinones[2] through 5-exo or even 5-endo
cyclizations has been described following this protocol. The
incorporation of the chiral appendage on the nitrogen atom
in aminobromodienes has also been used in the stereocon-
The starting compound was prepared by reductive ring
opening of perhydro-1,3-benzoxazine (1), obtained from the
condensation of 8-amino-N-(phenylselenoethyl) menthol
with 3,3-dimethylacroleine, with DIBALH at 0 °C.
Treatment of 8-amino-N-phenylselenoethyl-N-prenyl
menthol (2) with tributyltin hydride and AIBN in refluxing
benzene (slow addn. 8 h) provided a mixture of two 5-exo
diastereomers 3 and 4 in a 38:62 ratio (1H NMR) and 95%
combined yield. The same yield and selectivity was ob-
tained using tris(trimethylsilyl)silane[8] instead of tributyltin
hydride under the same experimental conditions.
This modest stereodifferentiation is somewhat striking be-
cause it has been reported[9] that there is no extraanular di-
astereoselective 1,4-induction when 1-phenylethylamine is
used as an auxiliary. Instead, the stereoselectivity in the cyc-
lization of our compounds could be attributed to the pres-
ence of the hydroxyl group. It has been demonstrated that
the formation of intramolecular H-bonds can direct the ste-
reochemical outcome of the reaction[10] and it plays an im-
portant role in the asymmetric induction of a wide range of
radical reactions.[11] The same effect was observed when the
reactions were carried out in the presence of Lewis acids.[12]
Unfortunately, changing the reaction solvent from ben-
zene to ethyl acetate or 2-propanol did not change the ratio
of diastereomeric 3 and 4, although the chemical yields de-
creased to 90% in both cases. The absence of the hydrogen
atom bonded to the oxygen, and consequently the disap-
pearance of the possible H-bond, modified the stereodiffer-
entiation. To confirm this fact, compound 2 was converted
into the O-TBDMS derivative 5 by reaction with TBDMS-
Cl. Compound 5 was then reacted with Bu3SnH to give 6
and 7. After desilylation with tetrabutylammonium fluor-
ide, an equimolar mixture of pyrrolidinyl menthols 3 and 4
was isolated, showing a total loss of stereoselectivity
(Scheme 1).
[3]
trolled synthesis of pyrrolidines.
One of the favorite auxiliaries involved in this type of
stereoselective transformation is (S)-1-phenethylamine, al-
though the observed stereoinduction in the radical cycliza-
tions is low, especially in the case of 5-exo ring closures.[4]
However, a family of more bulky auxiliaries has been
claimed to improve the stereoselectivity of such cycliza-
tions.[5]
Recently we have reported the diastereoselective intramo-
lecular radical cyclizations of bromoaryl-[6] and phenylse-
lenyl-substituted chiral perhydro-1,3-benzoxazines[7] de-
rived from (Ϫ)-8-amino menthol, and now our interest in
the preparation of enantiopure pyrrolidines has prompted
us to study the stereoselectivity of the cyclization in the
N,N-disubstituted 8-amino menthol 2.
[a]
´
´
Departamento de Quımica Organica, Facultad de Ciencias,
Universidad de Valladolid,
Dr. Mergelina s/n, 47011 Valladolid, Spain
Fax: (internat.) ϩ34-983/423-013
E-mail: pedrosa@qo.uva.es
Eur. J. Org. Chem. 2000, 3727Ϫ3730
WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000
1434Ϫ193X/00/1122Ϫ3727 $ 17.50ϩ.50/0
3727

´
R. Pedrosa, C. Andres, J. P. Duque-Soladana, P. Mendiguchia
FULL PAPER
Assuming that the formation of the H-bond leads to two
different conformations A and B (Scheme 2) in both the
ground and transition states for compound 2, the stereo-
chemistry of the major diastereomer can be interpreted, fol-
lowing the Beckwith rules, from the most stable[13] con-
formation A through a chair-like transition state. The minor
component 3 will be formed from the less stable structure
B, also in a chair-like transition state.
It has been well established that the stereoinduction of
cyclization of substituted 5-hexenyl radicals is dictated by
both the relative position of the substituent and the stereo-
chemistry of the stereocenter. In this way, 4-alkyl-5-hexenyl
radicals provide a high level of 4,5-trans stereoselection[14]
as a consequence of relative asymmetric induction.
In our case, and with the objective of preparing enantiop-
ure 2,3-dialkyl-substituted pyrrolidines, the cyclization of
the 8-amino menthol derivative 8 with a stereocenter of
known configuration at the allylic position was examined.
The starting compound was prepared, as a single stereoiso-
mer, by reaction of 1 with methylmagnesium iodide.[15]
When radical cyclization of 8 was promoted under stand-
ard conditions (Bu3SnH, AIBN, slow addition, 6 h) a single
diastereomeric pyrrolidinylmenthol 9 was obtained in excel-
lent yield. (Scheme 3).
Scheme 1. Preparation and radical cyclizations of compounds 2
and 5
This fact demonstrates the role that intramolecular H-
bonding may play in controlling conformational mobility
favoring a pseudo six-membered ring, and interestingly, the
stereochemistry of the major isomer 4 is the same as that
obtained in the cyclization of 2-vinyl-substituted perhydro-
N-(phenylselenyl)-ethyl-1,3-benzoxazines.[7b]
Scheme 3. Synthesis and radical cyclization of compound 8
The complete stereoinduction can be interpreted as a
consequence of the allylic 1,3-strain. This interaction favors
the chair-like transition state generated from radical B
(Scheme 2), where the methyl group is equatorial, and the
interaction with the double bond is therefore minimized,
and is consequently lower in energy than the transition state
generated from A.
The absolute stereochemistry of compounds 3 and 4 was
established by comparison of their spectroscopic properties
with those previously described.[7a] Compound 9 was ident-
ical to that obtained from the cyclized product 10[7b] with
methylmagnesium iodide in diethyl ether.
Finally, compounds 3 and 9 were transformed into en-
antiopure (3S)-3-isopropyl pyrrolidine and (2S,3S)-2-
methyl-3-isopropyl pyrrolidine, which were isolated as the
tosylates 11 and 12 (Scheme 4). PCC oxidation of 3 and
9, followed by treatment of the corresponding pyrrolidinyl
Scheme 2. Proposed stereochemical outcome for cyclizations of 2
and 8
3728
Eur. J. Org. Chem. 2000, 3727Ϫ3730

Diastereoselection in Radical Cyclization Promoted by Hydrogen Bonding
FULL PAPER
sphere of argon. Then, the crude mixture was poured into saturated
ammonium chloride and extracted with diethyl ether. The organic
extracts were dried over anhydrous MgSO4, and the solvent was
removed under vacuum yielding 5 (150 mg, 0.28 mmol, 77%) as a
colorless oil. Ϫ [α]2D5 ϭ Ϫ20.7 (c ϭ 1.0, CH2Cl2). Ϫ 1H NMR
(CDCl3): δ ϭ 0.07 (s, 3 H), 0.09 (s, 3 H), 0.74Ϫ1.04 (m, 3 H), 0.89
(s, 12 H), 0.91 (d, J ϭ 6.6 Hz, 3 H), 0.97 (s, 3 H), 1.16 (s, 3 H),
1.27 (m, 1 H), 1.36Ϫ1.45 (m, 2 H), 1.56 (s, 3 H), 1.56Ϫ1.69 (m, 1
H), 1.68 (s, 3 H), 1.87 (m, 1 H), 2.08 (m, 1 H), 2.58Ϫ2.67 (m, 1
H), 2.80Ϫ2.96 (m, 2 H), 3.03 (m, 2 H), 3.54 (dt, J ϭ 3.9 Hz, J ϭ
10.2 Hz, 1 H), 5.18 (t, J ϭ 6.5 Hz, 1 H), 7.22 (m, 3 H), 7.50 (m, 2
H). Ϫ 13C NMR (CDCl3): δ ϭ Ϫ2.9, Ϫ2.3, 18.8, 19.1, 22.3, 23.2,
24.8, 26.8, 26.9, 27.1 (3C), 30.4, 33.1, 35.9, 47.7, 48.0, 51.2, 53.0,
60.4, 75.2, 126.7, 127.3, 129.9 (2 C), 131.4, 132.4, 133.2 (2 C). Ϫ
IR (neat): ν˜ ϭ 3100, 740, 690 cmϪ1. Ϫ C29H51NOSeSi (536.77):
calcd. C 64.89, H 9.58, N 2.61; found C 65.04, H 9.76, N 2.38.
menthones with a solution of KOH in THF/MeOH, led to
(ϩ)-pulegone and the free pyrrolidine derivatives, which
were isolated as the tosylates following treatment with TsCl
in pyridine. Further NOESY experiments on 12 demon-
strated the trans relationship of the substituent at the pyrro-
lidine nucleus, confirming the S configuration at C-2 and
C-3.
Synthesis of Aminomenthol (8):
A
solution of 1[7b] (2.0 g,
4.75 mmol) in dry diethyl ether was slowly added to a 1  solution
of MeMgI (20 mL) in dry diethyl ether, and the mixture was stirred
for 1 h at room temperature. After this time, the reaction mixture
was quenched with a saturated ammonium chloride solution and
then extracted with diethyl ether. The organic layer was dried over
anhydrous MgSO4, and the solvent was evaporated affording 1.87 g
(5.3 mmol, 91%) of pure amino menthol 8 as a pale yellow oil. Ϫ
[α]2D5 ϭ Ϫ7.2 (c ϭ 1.0, CH2Cl2). Ϫ 1H NMR (CDCl3): δ ϭ
0.68Ϫ0.95 (m, 3 H), 0.88 (s, 3 H), 0.90 (d, J ϭ 6.6 Hz, 3 H),
0.97Ϫ1.15 (m, 1 H), 1.00 (d, J ϭ 7.0 Hz, 3 H), 1.13 (s, 3 H),
1.32Ϫ1.50 (m, 2 H), 1.52Ϫ1.67 (m, 1 H), 1.58 (s, 3 H), 1.65 (s, 3
H), 1.93 (m, 1 H), 2.91 (m, 3 H), 3.20Ϫ3.40 (m, 1 H), 3.53 (dt, J ϭ
4.0 Hz, J ϭ 10.5 Hz, 1 H), 3.95 (quintet, J ϭ 7.2 Hz, 1 H), 5.3 (m,
1 H), 7.27 (m, 3 H), 7.62 (m, 2 H), 7.97 (br.s., 1 H). Ϫ 13C NMR
(CDCl3): δ ϭ 17.7, 20.7, 22.1, 22.7, 23.2, 25.7, 25.9, 30.7, 30.9,
35.0, 44.8, 44.9, 48.6, 50.5, 62.5, 73.0, 125.1, 127.3, 129.0 (2 C),
129.2, 130.2, 133.8. Ϫ IR (neat): ν˜ ϭ 3200, 750, 700 cmϪ1. Ϫ
C24H39NOSe (436.54): calcd. C 66.03, H 9.00, N 3.21; found C
65.89, H 9.16, N 3.38.
Scheme 4. Elimination of the chiral appendage
Experimental Section
1
General: H NMR and 13C NMR were recorded at 300 MHz and
75 MHz, respectively, with CDCl3 as the solvent and chemical
shifts are given in ppm relative to TMS as the internal standard.
Optical rotations were measured on a digital polarimeter in a 1-dm.
cell, and concentrations are given in g/100 mL. Chromatographic
separations were carried out by flash chromatography using Merck
silica gel (240Ϫ400 mesh), and TLC analysis was performed on
Merck 0.25 mm silica gel plates (60F-254) using mixtures of hexane
and EtOAc or CH2Cl2 as eluents. Melting points were determined
in open capillary tubes and are uncorrected. All the reactions were
carried out in oven-dried glassware under an atmosphere of argon.
Solvents were dried and distilled prior to use.
Radical Cyclization of Aminomenthols ؊ General Method: To a so-
lution of the corresponding 8-amino menthol derivatives 2, 5 or 8
(0.12 mmol) in refluxing benzene (6 mL) was slowly added (by syr-
inge pump) a solution of tributyltin hydride (45 µL, 0.18 mmol)
and AIBN (2 mg) in benzene (6 mL) over 5Ϫ7 h. The heating was
continued until the reaction was finished (TLC). The mixture was
then diluted with diethyl ether and extracted with 2  hydrochloric
acid. The aqueous layer was neutralized with NaOH solution, and
extracted with chloroform. The organic layer was dried over anhyd-
rous MgSO4, and the solvent evaporated under vacuum. The res-
idue was purified by flash chromatography (silica gel, EtOAc/hex-
ane: 1:20) to give the known pyrrolidinylmenthols 3[7b] and 4 in
95% combined yield. The same treatment of 5 yielded 6 and 7,
which, without separation, were then transformed into an equimo-
lar mixture of 3 and 4 by desilylation with tetrabutylammonium
fluoride. The cyclization reaction of 8 under identical conditions
led to 9 in 98% yield.
8-Amino-N-Phenylselenoethyl-N-prenylmenthol (2): A cooled solu-
tion (0 °C) of perhydrobenzoxazine 1[7b] (1.0 g, 2.4 mmol) in tolu-
ene (80 mL), was treated with a 1  solution of DIBALH (7.2 mL,
3 equiv.) in toluene under an atmosphere of argon. After 15 mi-
nutes at this temperature, the mixture was quenched by the addition
of saturated ammonium chloride and then extracted with ethyl
acetate. The organic layer was dried over anhydrous Na2SO4, and
the solvent was evaporated yielding the amino menthol derivative
2 (704 mg, 1.7 mmol, 70%) which was purified by recrystallization.
Ϫ Colorless solid, m.p. 64Ϫ65 °C (from pentane). Ϫ [α]2D5 ϭ Ϫ23.7
1
(c ϭ 1.0, CH2Cl2). Ϫ H NMR (CDCl3): δ ϭ 0.83Ϫ1.00 (m, 3 H),
0.90 (d, J ϭ 6.5 Hz, 3 H), 0.91 (s, 3 H), 1.13 (s, 3 H), 1.38Ϫ1.70
(m, 4 H), 1.54 (s, 3 H), 1.65 (s, 3 H), 1.94 (m, 1 H), 2.31Ϫ2.75
(broad, 1 H), 2.75Ϫ3.50 (m, 5 H), 3.58 (dt, J ϭ 4.2 Hz, J ϭ
10.5 Hz, 1 H), 5.20 (t, J ϭ 6.2 Hz, 1 H), 7.23 (m, 3 H), 7.49 (m, 2
H), 7.95 (br. s., 1 H). Ϫ 13C NMR (CDCl3): δ ϭ 17.8, 20.3, 21.1,
22.1, 25.8 (2 C), 30.9, 35.0, 44.6, 46.7, 47.7, 61.6, 72.4, 126.8, 129.0,
129.4, 132.7. Ϫ IR (nujol): ν˜ ϭ 3200, 730, 690 cmϪ1. Ϫ
C23H37NOSe (422.51): calcd. C 65.38, H 8.83, N 3.32; found C
65.54, H 8.48, N 3.41.
8-[(2ЈS,3ЈS)-3Ј-Isopropyl-2Ј-methylpyrrolidinyl] Menthol (9): Color-
1
less oil. Ϫ [α]2D5 ϭ Ϫ17.8 (c ϭ 1.0, CH2Cl2). Ϫ H NMR (CDCl3)
δ ϭ 0.80Ϫ1.01 (m, 3 H), 0.87 (d, J ϭ 6.6 Hz, 3 H), 0.91 (d, J ϭ
8.0 Hz, 3 H), 0.92 (s, 3 H), 0.95 (d, J ϭ 6.0 Hz, 3 H), 1.18 (d, J ϭ
6.0 Hz, 3 H), 1.20 (s, 3 H), 1.30Ϫ1.40 (m, 1 H), 1.40Ϫ1.60 (m, 5
H), 1.62Ϫ1.79 (m, 2 H), 1.86Ϫ1.97 (m, 1 H), 2.77Ϫ2.85 (m, 1 H),
8-Amino-O-TBDMS-N-Phenylselenoethyl-N-prenylmenthol (5): A
mixture of 2 (150 mg, 0.36 mmol), imidazole (68 mg, 1.0 mmol) 2.86Ϫ3.01 (m, 2 H), 3.60 (dt, J ϭ 4.0 Hz, J ϭ 10.5 Hz, 1 H), 9.05
and tert-butyldimethylsilyl chloride (78 mg, 0.50 mmol) in DMF
(s, 1 H). Ϫ 13C NMR (CDCl3) δ ϭ 19.3, 19.7, 21.6, 21.8, 22.0, 25.6
(1 mL) was stirred for 12 h at room temperature under an atmo-
(2 C), 27.1, 29.1, 30.9, 35.1, 44.3, 46.2, 49.1, 54.6, 56.0, 60.4, 72.2.
Eur. J. Org. Chem. 2000, 3727Ϫ3730
3729

´
R. Pedrosa, C. Andres, J. P. Duque-Soladana, P. Mendiguchia
FULL PAPER
Ϫ IR (film): ν˜ ϭ 3100, 1450, 1170 cmϪ1. Ϫ C18H35NO (281.48):
[2]
[3]
B. Cardillo, R. Galeazzi, G. Mobbili, M. Orena, Synlett 1995,
1159Ϫ1160.
calcd. C 76.81, H 12.53, N 4.98; found C, 76.99, H 12.74, N 5.03.
´
´
Y. Cancho, J. M. Martın, M. Martınez, A. Llebaria, J. M. Mor-
´
eto, A. Delgado, Tetrahedron 1998, 54, 1221Ϫ1232.
Elimination of the Chiral Appendage ؊ General Method: The pyrro-
lidinyl menthols 3 or 9 (1.3 mmol) were transformed into the cor-
responding pyrrolidinyl menthones by oxidation with PCC (1.1 g,
[4] [4a]
´
J. Aube, X. Peng, Y. Wang, F. Takusagawa, J. Am. Chem.
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5.3 mmol) and 4 A mol sieves in dichloromethane at room temper-
[5]
[6]
H. Ishibashi, Y. Fuke, T. Yamashita, M. Ikeda, Tetrahedron:
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solution, extracted with chloroform and the organic phase evapor-
ated to dryness. The residue was dissolved in a mixture containing
14% KOH (4 mL), THF (8 mL) and MeOH (4 mL) and stirred for
2 h. The mixture was extracted with diethyl ether to eliminate the
(ϩ)-pulegone and the corresponding pyrrolidine was isolated as the
hydrochloride. This hydrochloride was then transformed into the
N-tosylate by reaction with TsCl (510 mg, 2.65 mmol) and diisop-
ropylethylamine (1.1 mL, 6.36 mmol) in dichloromethane (20 °C,
48 h), which yielded pure 11[7b] (219 mg, 63%) or 12 (245 mg, 67%)
after flash chromatography (silica gel, CH2Cl2/hexane, 1:1).
Asymmetry 1996, 7, 2531Ϫ2538.
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C. Andres, J. P. Duque-Soladana, J. M. Iglesias, R. Pedrosa,
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(2S,3S)-3-Isopropyl-2-methyl-N-Tosylpirrolidine (12): Colorless oil.
Ϫ [α]2D5 ϭ ϩ72.2 (c ϭ 1.0, CH2Cl2). Ϫ 1H NMR (CDCl3): δ ϭ 0.64
(d, J ϭ 6.7 Hz, 3 H), 0.78 (d, J ϭ 6.7 Hz, 3 H), 1.00Ϫ1.20 (m, 2
H), 1.34 (d, J ϭ 6.3 Hz, 3 H), 1.40Ϫ1.50 (m, 1 H), 1.60Ϫ1.80 (m,
1 H), 2.40 (s, 3 H), 3.20Ϫ3.40 (m, 2 H), 3.40 (q, J ϭ 6.3 Hz, 1 H),
7.30 (d, J ϭ 8.0 Hz, 2 H), 7.70 (d, J ϭ 8.0 Hz, 2 H). Ϫ 13C NMR
(CDCl3): δ ϭ 19.4, 21.4, 21.6, 23.2, 27.4, 29.4, 47.8, 54.1, 59.3,
127.2 (2 C), 129.5 (2C), 135.4, 143.1. Ϫ IR (film): ν˜ ϭ 1600, 1340
cmϪ1. Ϫ C15H23NO2S (281.41): calcd. C 64.02, H 8.24, N 4.98;
found C 64.35, H 8.27, N 4.82.
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Acknowledgments
The financial support provided by the Spanish DGESIC (Project
PB98Ϫ0361) and Junta de Castilla y Leon (Project VA79Ϫ99) is
gratefully acknowledged. One of us (P.M.) thanks the Ministerio
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[13]
Calculations, performed using UHF/PM3 Hamiltonian and
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Received May 5, 2000
[O00229]
´
de Educacion y Cultura for a predoctoral fellowship.
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