Chemoenzymatic synthesis of the enantiomers of desoxymuscarine

Article abstract of DOI:10.1016/S0957-4166(98)00010-X

Two different chemoenzymatic approaches allowed the preparation of the enantiomers of desoxymuscarine 5, a muscarinic receptor agonist. Transesterification of racemic 5-hexen-2-ol 7 with vinyl butyrate under the catalysis of Candida antarctica B lipase wa

Full text of DOI:10.1016/S0957-4166(98)00010-X

Pergamon
Tetrahedron: Asymmetry 9 (1998) 657–665
Chemoenzymatic synthesis of the enantiomers of
desoxymuscarine
Paola Conti,a Clelia Dallanoce,a Marco De Amici,a,∗ Carlo De Micheli,a Giacomo Carrea b
and Francesca Zambianchi b
aIstituto di Chimica Farmaceutica e Tossicologica, Università di Milano, via le Abruzzi 42, 20131 Milano, Italy
bIstituto di Chimica degli Ormoni, C.N.R., via Mario Bianco 9, 20131 Milano, Italy
Received 16 December 1997; accepted 9 January 1998
Abstract
Two different chemoenzymatic approaches allowed the preparation of the enantiomers of desoxymuscarine 5, a
muscarinic receptor agonist. Transesterification of racemic 5-hexen-2-ol 7 with vinyl butyrate under the catalysis
of Candida antarctica B lipase was the key step for the preparation of (−)-5 (2R,5R). On the other hand, lipase PS-
catalyzed hydrolysis of iodo butyrate (ꢀ)-14 was utilized to obtain (+)-5 (2S,5S). Both enantiomers were prepared
with enantiomeric excesses higher than 98%. © 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction
Whenever target chiral compounds with high enantiomeric purity are needed, enzyme-catalyzed
processes may represent an alternative (or complementary) approach to other strategies, i.e. asymmetric
synthesis and conventional resolution methods. Among the different classes of enzymes, hydrolytic
enzymes are by far the most versatile biocatalysts to afford kinetic resolutions of racemates and
asymmetrizations of meso compounds, thus providing enantiomerically pure synthetic intermediates
amenable to further functionalization.1–4
Stereochemistry is an essential feature of the molecular structure of biologically active chiral com-
pounds, since opposite absolute configurations at pharmacophoric groups are usually responsible for dif-
ferences in the biological response, mainly in terms of potency, toxicity and receptor subtype selectivity.5
Therefore, the stereoisomeric composition of drug substances is currently receiving considerable atten-
tion owing to its pharmacological as well as industrial and regulatory implications.6,7
As a part of our investigation of the relantionship between structure and activity/selectivity of
cholinergic ligands, we synthesized and tested a series of analogs of natural muscarine (+)-1, i.e.
∗
Corresponding author. E-mail:carlo.demicheli@unimi.it
0957-4166/98/$19.00 © 1998 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(98)00010-X

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P. Conti et al. / Tetrahedron: Asymmetry 9 (1998) 657–665
Fig. 1.
the four stereoisomers of muscarone,8 methylenemuscarone,9 and difluoromuscarine.10 The absolute
configurations of the most potent stereoisomers of muscarone (−)-2, methylenemuscarone (−)-3, and
difluoromuscarine (+)-4 are depicted in Fig. 1.
With the aim of studying further the nature of the interaction involving the binding of the hydroxy
group of muscarine to the complementary receptor subsite (‘muscarinic subsite’),8–11 we planned the
synthesis of the two enantiomers of desoxymuscarine 512 (Fig. 1), a relatively potent muscarinic agonist
with a tenfold selectivity for M3 (ileal) versus M2 (cardiac) muscarinic receptor subtype.13
In previous papers we reported the synthesis of the enantiomers of compounds provided with a variety
of biological activities, by taking advantage of enantioselective enzyme-catalyzed reactions.14–18 As a
part of an ongoing program, we describe the application of the chemoenzymatic strategy to the synthesis
of both enantiomers of 5. Furthermore, our approach allowed the assignment of the absolute configuration
of (+)-5 and (−)-5.
2. Results and discussion
We tackled the synthesis of the two enantiomers of desoxymuscarine by checking the feasibility of the
two chemoenzymatic sequences reported in Schemes 1 and 2.
In the first approach, depicted in Scheme 1, we submitted racemic 5-hexen-2-ol 7 to a transester-
ification reaction with vinyl butyrate in petroleum ether under the catalysis of Candida antarctica B
lipase. The substrate was in turn easily obtained by reducing an ether solution of commercially available
5-hexen-2-one (6) with lithium aluminum hydride. The enzymatic reaction was quite enantioselective
(enantiomeric ratio,1 E ≥300) with an enantiopreference for the R isomer, typical of Candida antarctica
B lipase.19 This result relies on the assignment of the absolute configuration to the enantiomers of 7.20,21
The degree of conversion and the enantiomeric excess (e.e.) of the produced butyrate were evaluated
by chiral GLC analysis (see Experimental section). For preparative purposes, we stopped the reaction at
42% conversion and obtained R-(−)-8, after purification by column chromatography. The e.e. value of R-
(−)-8 was higher than 99%. Butyrate R-(−)-8 was then hydrolyzed, in the presence of the same enzyme,
to enantiomerically pure R-(−)-7. Treatment of R-(−)-7 with sodium hydride and 2,6-dichlorobenzyl
(DCB) bromide afforded the corresponding ether R-(−)-9, which was submitted to a iodocyclization
reaction following the procedure described by Bartlett.22 The electronic and steric properties of the
substituent at oxygen control the stereochemical outcome of the cyclization,22,23 to yield (2R,5R)-cis-
2-iodomethyl-5-methyltetrahydrofuran (+)-10 in a highly diastereoselective process. On the other hand,
the corresponding benzyl ether gave rise to (+)-10 contaminated by a substantial amount (35%) of the
trans-(2S,5R)-isomer.
The synthetic sequence was then carried out to completion by reacting iodo derivative (+)-10 with
dimethylamine at 100°C followed by treatment with iodomethane, to give (2R,5R)-desoxymuscarine (−)-

P. Conti et al. / Tetrahedron: Asymmetry 9 (1998) 657–665
659
Scheme 1.
5. The absolute configuration at the two stereocenters was the result of both the R enantiopreference of the
initial enzymatic transesterification and the stereoselective iodoetherification reaction. The relationship
between the sign of specific rotation of (−)-5 and absolute (2R,5R)-configurations is further confirmed
by comparison to muscarine stereoisomers24 and related analogs.10
It is worth noting that the reactions listed in Scheme 1 may be conveniently utilized also to synthesize
the dextrorotatory enantiomer of desoxymuscarine. By extending transesterification slightly beyond 50%
conversion, S-(+)-7 was obtained with an e.e. value higher than 99%. Hence, the sequence illustrated
above for the preparation of (−)-5 was applied to accomplish the synthesis of enantiomerically pure
(+)-5.
With the aim of reaching our synthetic goal in a more expeditious way, in a parallel study we
investigated an alternative chemoenzymatic approach to the enantiomers of 5, by selecting prochiral
diester 12 as a substrate (Scheme 2). Derivative 12 already has the desired cis stereochemistry. Based
on published results,25 we tried at first pig liver esterase (PLE) induced asymmetrization of meso diester
12, in turn prepared from cis-2,5-bis(hydroxymethyl)tetrahydrofuran 11.26 In our hands this attempt was
unfruitful, owing to the poor selectivity of the PLE-catalyzed hydrolysis of 12. Moreover, chiral GLC
analysis did not allow separation of the enantiomers of the produced monoester, thus preventing any
direct evaluation of the enantiomeric excess of the product. The e.e. value of such a compound was
determined after transformation into iodo butyrate 14 (Scheme 2). The use of other biocatalysts, such as
Candida rugosa lipase, was similarly unsuccessful.
Conversely, lipase from Pseudomonas cepacia (lipase PS) catalyzed with acceptable selectivity (E=14)
the hydrolysis of butyrate (ꢀ)-14. The degree of conversion as well as the e.e. of the residual substrate
and the produced alcohol were evaluated by chiral GLC (see Experimental section). By extending the
hydrolysis at 65% conversion, iodo butyrate (+)-14 (2S,5R) was isolated with an e.e. value higher than
98%. Lithium aluminum hydride reduction of both the iodomethyl group and the ester function gave

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P. Conti et al. / Tetrahedron: Asymmetry 9 (1998) 657–665
Scheme 2.
the volatile (2S,5S)-2-hydroxymethyl-5-methyltetrahydrofuran, which was readily converted into the
corresponding mesylate (+)-16. The preparation of desoxymuscarine (+)-5 was then accomplished by
standard reactions. Absolute values of the specific rotation of the two desoxymuscarine enantiomers,
obtained through the two different routes, were superimposable.
In summary, in this paper the results of two independent chemoenzymatic syntheses of the enantiomers
of desoxymuscarine, a muscarinic receptor agonist, are reported and discussed. The key step of both
the described sequences is an efficient enzyme-catalyzed kinetic resolution. The first strategy was more
convenient, on account of the high enantioselectivity of its biocatalyzed transformation coupled to a
remarkable diastereoselective iodocyclization process. Furthermore, this approach allowed the assign-
ment of the absolute configuration to the enantiomers of (−)- and (+)-5. Based on the kind of reactions
performed and the values of the polarimetric data on final compounds, we believe that each enantiomer
has completely retained the enantiomeric purity induced by the enzyme-catalyzed transformations. The
results of the pharmacological investigation of the two enantiomers of desoxymuscarine will be reported
in due course.

P. Conti et al. / Tetrahedron: Asymmetry 9 (1998) 657–665
661
3. Experimental section
Immobilized Candida antarctica B lipase (Novozym 435) was purchased from Novo and lipase from
1
Pseudomonas cepacia (lipase PS) from Amano. Organic solvents were reagent grade. H NMR spectra
were recorded at 200 MHz in CDCl3 (unless otherwise specified) solutions; chemical shifts (δ) are
expressed in ppm and coupling constants (J) in hertz. Chiral GLC analyses were conducted on a gas
chromatograph equipped with a CP-Cyclodextrin-2,3,6-M-19 column (50 m, 0.25 mm). H2 was used
as the carrier gas at a flow rate of 1.2 mL/min; experimental conditions are reported in the appropriate
paragraph. Boiling points of liquid compounds were determined either by conventional or Kugelrohr
distillations. Rotary power determinations were carried out with a Perkin–Elmer 241 polarimeter coupled
with a Haake N3-B thermostat. TLC analyses were performed on commercial silica gel 60 F254 aluminum
sheets; spots were further evidenced by spraying with a dilute alkaline potassium permanganate solution.
Microanalyses (C, H, N) of new compounds agreed with the theoretical value ꢀ0.4%.
3.1. Synthesis of (RS)-5-hexen-2-ol (ꢀ)-7
To a suspension of lithium aluminum hydride (4.0 g) in anhydrous ether (300 mL) was added dropwise
an ethereal solution (50 mL) of 5-hexen-2-one (14.0 g, 0.143 mol) at 0°C under stirring. The reaction
mixture was stirred at room temperature for an additional 3 h, then water (25 mL) was cautiously
added at 0°C and inorganic salts were separated by suction filtration. The remaining solution, dried over
anhydrous sodium sulphate, was concentrated at atmospheric pressure, then distilled under vacuum to
yield 12.73 g (89%) of the desired secondary alcohol. (RS)-5-Hexen-2-ol (ꢀ)-7: bp 83–84°C/125 mmHg
(lit.20 138–140°C); 1H NMR 1.19 (d, 3, CH3; J=6.6); 1.47–1.58 (m, 3, H-3 and OH); 2.15 (m, 2, H-4);
3.82 (m, 1, H-2); 5.01 (m, 2, H-6); 5.83 (m, 1, H-5).
3.2. Enzymatic transesterification of (ꢀ)-7
A solution of 10.69 g (0.107 mol) of (ꢀ)-7 in petroleum ether (300 mL) was treated with vinyl butyrate
(15 mL, 0.118 mol) in the presence of Candida antarctica B lipase (70 mg). The reaction was stopped
after about 3 h (42% conversion), the enzyme was filtered off and the solution was concentrated at
atmospheric pressure. The residue was submitted to silica gel column chromatography (eluant: petroleum
ether:diethyl ether=3:1) to afford 6.97 g (0.041 mol) of R-(−)-827 and 5.41 g (0.054 mol) of S-(+)-7.
(R)-5-Hexen-2-ol butyrate (−)-8: bp 100–110°C/22 mmHg; Rf 0.85 (petroleum ether:diethyl
1
ether=3:1); [α]20 −9.11 (c 1.032, CHCl3); e.e.>99%; H NMR 0.94 (t, 3, CH2CH3; J=7.5); 1.20 (d,
D
3, CH3; J=6.6); 1.42–1.80 (m, 4, H-3 and CH2CH3); 2.07 (m, 2, H-4); 2.25 (t, 2, OCOCH2; J=7.2);
4.82–5.09 (m, 3, H-2 and H-6); 5.79 (m, 1, H-5). Anal. calcd for C10H18O2: C, 70.53; H, 10.66. Found:
C, 70.44; H, 10.87.
(S)-5-Hexen-2-ol (+)-7: Rf 0.21 (petroleum ether:diethyl ether=3:1); [α]25 +8.97 (c 1.026, Et2O);
D
e.e.=73% [after conversion into the corresponding S-(+)-8].
Capillary GLC analysis. Temperature program: 80°C (1 min.), then from 80°C to 95°C (0.5°C/min.).
Injection volume: 10 µl. Relative retention times (min): R-(−)- and S-(+)-7, 6.85; S-(+)-8, 20.87, R-(−)-8,
21.72.
The same enzymatic transesterification was carried out on 2.14 g (21.4 mmol) of (ꢀ)-7. The reaction
was stopped at 55% conversion and, after the above described work up, 0.79 g (37% yield) of S-(+)-7
were obtained.

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P. Conti et al. / Tetrahedron: Asymmetry 9 (1998) 657–665
(S)-5-Hexen-2-ol (+)-7: [α]25 +17.28 (c 1.054, Et2O) [lit.20 [α]25 +21.8 (Et2O)]; e.e.>99% [after
D
D
conversion into the corresponding S-(+)-8]. S-(+)-8: [α]20D +9.52 (c 1.040, CHCl3); e.e.>99%.
3.3. Synthesis of (R)-5-hexen-2-ol (−)-7
5.0 g (29.4 mmol) of (R)-(−)-8 was dissolved in 150 mL of 0.1 M potassium phosphate buffer (pH
8) in the presence of Candida antarctica B lipase powder (100 mg). The hydrolysis was complete in
about 4 h. The reaction mixture was extracted with ether (4×100 mL), the organic phase was dried over
anhydrous sodium sulphate and the solvent was eliminated at atmospheric pressure. The residue was
distilled at reduced pressure to give 2.23 g (76%) of (−)-7. (R)-5-Hexen-2-ol (−)-7: [α]25 −17.62 (c
D
1.016, Et2O) [lit.20 [α]25 −13.7 (Et2O)].
D
3.4. Synthesis of (R)-5-hexen-2-ol-2,6-dichlorobenzyl ether (−)-9
To a cooled (−10°C) stirred suspension of sodium hydride (1.3 g of a 50% dispersion in mineral
oil) in anhydrous THF (25 mL) was added 0.776 g (2.1 mmol) of tetra-n-butylammonium iodide under
nitrogen. A solution of 2,6-dichlorobenzyl bromide (5.03 g, 21 mmol) in THF (20 mL) was then added
dropwise. After 15 min, a solution of 2.10 g (21 mmol) of (S)-(−)-7 in THF (5 mL) was added dropwise
at −10°C. The reaction mixture was maintained under stirring at 0°C for about 5 h, then water (20 mL)
was cautiously added and the aqueous phase was extracted with ether (4×20 mL). After the usual work
up, the residue was column chromatographed (eluant: petroleum ether/5% diethyl ether) to yield 4.68
g (86%) of the desired compound. (R)-5-Hexen-2-ol-2,6-dichlorobenzyl ether (−)-9: bp 140–145°C/7
mmHg (see literature22 for racemate); Rf 0.52 (petroleum ether/5% diethyl ether); [α]20 −35.69 (c
D
1.048, CHCl3); 1H NMR 1.25 (d, 3, CH3; J=6.3); 1.50 (m, 1, H-3); 1.68 (m, 1, H-30); 2.16 (m, 2, H-4);
3.57 (m, 1, H-2); 4.70 and 4.80 (d, 2, CH2Ar; J=10.1); 4.95 (m, 2, H-6); 5.78 (m, 1, H-5); 7.09–7.35 (m,
3, arom.).
3.5. Synthesis of (2R,5R)-2-iodomethyl-5-methyltetrahydrofuran (+)-10
To a cooled (0°C) stirred solution of 4.50 g (17.37 mmol) of (−)-9 in 30 mL anhydrous acetonitrile,
a solution of iodine (6.0 g, 23.64 mmoL) in 80 mL dry acetonitrile was added dropwise under vigorous
stirring. The progress of the reaction was monitored by TLC. After completion of the reaction (about
5 h at 0°C), 50 mL of a saturated solution of Na2S2O3 was added at room temperature. Acetonitrile
was evaporated under vacuum and the aqueous layer was extracted with ether (3×30 mL). After the
usual work up, the residue was chromatographed on silica gel (eluant: petroleum ether:diethyl ether=9:1)
to yield 2.55 g (65%) of iododerivative (+)-10. (2R,5R)-2-Iodomethyl-5-methyltetrahydrofuran (+)-10:
bp 115–120°C/22 mmHg (see literature22,23 for racemate); Rf 0.55 (petroleum ether:diethyl ether=9:1);
[α]20D +11.24 (c 1.014, CHCl3); 1H NMR 1.26 (d, 3, CH3; J=6.2); 1.42–1.82 (m, 2, H-4); 1.88–2.17 (m,
2, H-3); 3.17 and 3.24 (dd, 2, CH2I; J=6.8, 9.8 and 4.8); 3.96 (m, 1, H-5); 4.07 (m, 1, H-2).
3.6. Synthesis of (2R,5R)-2-[(dimethylamino)methyl]-5-methyltetrahydrofuran methiodide (−)-5
A. A sealed metal container, filled with a solution of (+)-10 (2.40 g, 10.62 mmol) in methanol (10
mL) and a tenfold excess of anhydrous dimethylamine, was heated at 100°C for 5 h. The container was
cooled at 0°C, the solution was acified with 3 N HCl, and the volatiles were evaporated under vacuum.
The residual aqueous phase was treated with ether (4×10 mL), made alkaline by portionwise addition

P. Conti et al. / Tetrahedron: Asymmetry 9 (1998) 657–665
663
of solid K2CO3 and extracted with dichloromethane (4×10 mL). The pooled organic extracts were dried
over anhydrous sodium sulphate, the solvent was evaporated under vacuum at 10°C, and the residue
was Kugelrohr distilled to yield 0.94 g (62%) of tertiary base. (2R,5R)-2-[(Dimethylamino)methyl]-
5-methyltetrahydrofuran: bp 130–135°C/140 mmHg (lit.12 86–89°C/55 mmHg for racemate); Rf 0.46
1
(chloroform:methanol=4:1); [α]20 −18.96 (c 1.092, CHCl3); H NMR 1.23 (d, 3, CH3; J=6.0);
D
1.30–1.62 (m, 2, H-4); 1.83–2.08 (m, 2, H-3); 2.26 (s, 6, NMe2); 2.29 and 2.44 (dd, 2, CH2NMe2;
J=4.9, 12.5 and 7.5); 3.86–4.03 (m, 2, H-2 and H-5).
B. An ether solution of the tertiary amine was reacted with a fivefold excess of methyl iodide at room
temperature. The salt precipited quantitatively and was crystallized from 2-propanol:ether (1:1). (2R,5R)-
2-[(Dimethylamino)methyl]-5-methyltetrahydrofuran methiodide (−)-5: mp 141–142°C, dec., colorless
1
needles from 2-propanol–ether (lit.12 154–155°C for racemate); [α]20 −28.16 (c 1.110, MeOH); H
D
NMR (D2O) 1.24 (d, 3, CH3; J=6.2); 1.42–1.78 (m, 2, H-4); 1.93–2.27 (m, 2, H-3); 3.18 (s, 9, NMe3);
3.34–3.57 (m, 2, CH2NMe3); 4.16 (m, 1, H-5); 4.43 (m, 1, H-2). Anal. calcd for C9H20INO: C, 37.91;
H, 7.07; N, 4.91. Found: C, 38.15; H, 7.19; N, 4.75.
3.7. Synthesis of cis-2-(hydroxymethyl)-5-iodomethyltetrahydrofuran butyrate (ꢀ)-14
A. A 500 mL Erlenmeyer flask was charged with 1126 (8.0 g, 61 mmol), dichloromethane (250
mL), tetra-n-butylammonium hydrogen sulphate (2.55 g, 7.51 mmol) and 18 mL of a 15% aqueous
solution of NaOH. To the suspension a solution of tosyl chloride (12.70 g, 67 mmol) in dichloromethane
(75 mL) was added under vigorous stirring. The reaction mixture was stirred for 4 days at room
temperature, then the organic phase was separated and sequentially washed with dilute HCl (100 mL)
and a saturated solution of NaHCO3 (100 mL). After the usual work up, the desired monotosylate (7.63
g, 44% yield) was obtained as a light yellow viscous oil by silica gel column chromatography (eluant:
petroleum ether:ethyl acetate=1:4). cis-2-(Hydroxymethyl)-5-(tosyloxymethyl)tetrahydrofuran (ꢀ)-13:
1
Rf 0.41 (cyclohexane:ethyl acetate=1:4); H NMR 1.68–2.12 (m, 5, H-3, H-4 and OH); 2.44 (s, 3, CH3);
3.42 and 3.65 (dd, 2, CH2OH; J=5.7, 12.2 and 3.1); 3.93–4.21 (m, 4, H-2, H-5 and CH2OSO2); 7.35 and
7.80 (d, 4, arom.; J=8.2).
B. To a solution of (ꢀ)-13 (7.63 g, 26.67 mmol) and triethylamine (12.20 mL, 84 mmol) in anhydrous
dichloromethane (180 mL) was added dropwise a dichloromethane solution (40 mL) of butyryl chloride
(8.40 mL, 80 mmol) at 0°C. The disappearance of the starting material was monitored by TLC (eluant:
petroleum ether:ethyl acetate=1:4). The reaction mixture was acidified by addition of dilute HCl and the
organic phase was dried and concentrated. The residue was purified by silica gel column chromatography
(eluant: petroleum ether:ethyl acetate=7:3) to give 8.08 g (85%) of the expected diester as a light yellow
viscous oil. cis-2-(Butyryloxymethyl)-5-(tosyloxymethyl)tetrahydrofuran: Rf 0.37 (cyclohexane:ethyl
acetate=7:3); 1H NMR 0.94 (t, 3, CH2CH3; J=7.5); 1.66 (m, 2, CH2CH3); 1.68–2.08 (m, 4, H-3 and H-4);
2.29 (t, 2, OCOCH2; J=7.4); 2.45 (s, 3, CH3); 3.83–4.18 (m, 6, H-2, H-5, CH2OCO and CH2OSO2); 7.34
and 7.78 (d, 4, arom.; J=8.2).
C. A 250 mL Erlenmeyer flask, charged with cis-2-(butyryloxymethyl)-5-(tosyloxymethyl)tetrahydro-
furan (7.95 g, 22.33 mmoL), 180 mL ethyl methyl ketone and NaI (7.0 g, 46.70 mmoL), was heated
at reflux overnight under nitrogen. After cooling at room temperature and addition of 75 mL of a
saturated solution of Na2S2O3, the volatiles were evaporated under vacuum and the aqueous layer was
extracted with dichloromethane (3×50 mL). After the usual work up, the desired compound (5,50 g,
79% yield) was purified by silica gel column chromatograpy (eluant: petroleum ether/15% ethyl acetate).
cis-2-(Hydroxymethyl)-5-iodomethyltetrahydrofuran butyrate (ꢀ)-14: bp 155–160°C/2 mmHg; Rf 0.47
1
(cyclohexane:ethyl acetate=4:1); H NMR 0.94 (t, 3, CH2CH3; J=7.5); 1.66 (m, 2, CH2CH3); 1.68–1.83

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P. Conti et al. / Tetrahedron: Asymmetry 9 (1998) 657–665
(m, 2, H-3); 1.90–2.21 (m, 2, H-4); 2.32 (t, 2, OCOCH2; J=7.3); 3.17 and 3.26 (dd, 2, CH2I; J=6.9, 9.7
and 4.7); 3.95–4.08 (m, 2, H-2 and H-5); 4.10–4.28 (m, 2, CH2OCO). Anal. calcd for C10H17IO3: C,
38.48; H, 5.49. Found: C, 38.65; H, 5.12.
3.8. Enzymatic hydrolysis of (ꢀ)-14
A 500 mL Erlenmeyer flask was charged with (ꢀ)-14 (8.80 g, 28.20 mmol), lipase PS (200 mg), 0.1
M potassium phosphate buffer, pH 7 (200 mL) and acetone (30 mL). The mixture was stirred at room
temperature for about 6 h (65% conversion), then filtered and extracted with ethyl acetate (3×100 mL).
The pooled organic phases were dried over anhydrous sodium sulphate and the solvent was evaporated
at reduced pressure.The residue was submitted to silica gel column chromatography (eluant: petroleum
ether:ethyl acetate=4:1) to afford 2.68 g (8.59 mmol) of (+)-14 and 3.60 (14.88 mmol) of (−)-15.
(2S,5R)-(+)-14: [α]20D +32.61 (c 1.156, CHCl3); e.e.>98%.
(2R,5S)-2-(Hydroxymethyl)-5-iodomethyltetrahydrofuran (−)-15: (see literature28 for racemate) Rf
1
0.33 (cyclohexane:ethyl acetate 7:3); [α]20 −8.94 (c 1.044, CHCl3); e.e.=53%; H NMR 1.68–1.88
D
(m, 2, H-3); 1.90–2.18 (m, 3, H-4 and OH); 3.30 (d, 2, CH2I); 3.53 and 3.73 (dd, 2, CH2OH; J=5.1, 11.6
and 3.0); 3.90 (m, 1, H-5); 4.08 (m, 1, H-2).
Capillary GLC analysis. Temperature program: 150°C (80 min.). Injection volume: 10µl. Relative
retention times (min): (−)-15, 20.57; (+)-15, 20.90; (−)-14, 63.04; (−)-14, 63.65.
3.9. Synthesis of (2S,5S)-2-[(dimethylamino)methyl]-5-methyltetrahydrofuran methiodide (+)-5
A. To a suspension of lithium aluminum hydride (1.80 g) in anhydrous ether (170 mL) was added
dropwise an ethereal solution (20 mL) of (+)-14 (2.16 g, 6.92 mmol) at 0°C under stirring. The reaction
mixture was refluxed for an additional 2 h, then water (10 mL) was cautiously added at 0°C and
inorganic salts were separated by suction filtration. To the remaining solution, dried over anhydrous
sodium sulphate, was added triethylamine (4.82 mL, 34.6 mmol). A solution of methanesulfonyl
chloride (2.42 mL, 31.14 mmol) in dry ether (10 mL) was then added dropwise at 0°C, and the
reaction was stirred for about 20 h at room temperature. After addition of 3 N HCl (50 mL), the
organic layer was separated and the aqueous phase was extracted with dichloromethane (3×30 mL).
After the usual work up, the residue was submitted to silica gel column chromatography (eluant:
petroleum ether:ethyl acetate=3:2) to afford 0.835 g (62% overall yield) of the expected mesylate.
(2S,5S)-2-(Hydroxymethyl)-5-methyltetrahydrofuran mesylate (+)-16: bp 125–130°C/4 mmHg; Rf 0.37
1
(cyclohexane:ethyl acetate=3:2); [α]20 +21.90 (c 1.0, CHCl3); H NMR 1.25 (d, 3, CH3; J=5.9);
D
1.38–1.59 (m, 1, H-3); 1.65–1.84 (m, 1, H-30); 1.90–2.12 (m, 2, H-4); 3.07 (s, 3, OSO2CH3); 3.96–4.32
(m, 4, H-2, H-5 and CH2OSO2). Anal. calcd for C7H14O4S: C, 43.28; H, 7.26. Found: C, 43.54; H, 7.11.
B. A sealed metal container, filled with a solution of (+)-16 (0.80 g, 4.12 mmol) in methanol (5 mL)
and a tenfold excess anhydrous dimethylamine, was heated at 100°C for 5 h. Following the experimental
protocol described above for the preparation of the levorotatory enantiomer, 0.41 g (70% yield) of
dextorotatory tertiary base were isolated. (2S,5S)-2-[(Dimethylamino)methyl]-5-methyltetrahydrofuran:
[α]20D +18.52 (c 1.188, CHCl3).
C. Quaternary ammonium salt (2S,5S)-(+)-5 was prepared with the procedure described for (−)-5.
(+)-5: mp 141–142°C, dec. [α]20 +28.06 (c 1.110, MeOH). Anal. calcd for C9H20INO: C, 37.91; H,
D
7.07; N, 4.91. Found: C, 38.09; H, 7.27; N, 4.88.

P. Conti et al. / Tetrahedron: Asymmetry 9 (1998) 657–665
665
Acknowledgements
This research was financially supported by the National Council of Research (C.N.R., Rome).
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