Synthesis of a novel N-hydroxypyrrolidine using enzyme catalysed asymmetric carbon-carbon bond synthesis

Article abstract of DOI:10.1016/S0040-4039(00)00645-6

N-Hydroxypyrrolidine 5 has been prepared in nine steps starting from 3- O-benzylglyceraldehyde 13. The synthetic route employs Escherichia coli transketolase mediated C-C bond synthesis to establish the absolute stereochemistry and a subsequent ring contraction of a 1,2-oxazine 17 to provide the N-hydroxypyrrolidine nucleus. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00645-6

Tetrahedron Letters 41 (2000) 4481±4485
Synthesis of a novel N-hydroxypyrrolidine using enzyme
catalysed asymmetric carbon±carbon bond synthesis
Andrew J. Humphrey, Simon F. Parsons, Mark E. B. Smith and Nicholas J. Turner*
Edinburgh Centre for Protein Technology, Department of Chemistry, The University of Edinburgh, King's Buildings,
West Mains Road, Edinburgh EH9 3JJ, UK
Received 23 February 2000; accepted 20 April 2000
Abstract
N-Hydroxypyrrolidine 5 has been prepared in nine steps starting from 3-O-benzylglyceraldehyde 13. The
synthetic route employs Escherichia coli transketolase mediated C±C bond synthesis to establish the absolute
stereochemistry and a subsequent ring contraction of a 1,2-oxazine 17 to provide the N-hydroxypyrrolidine
nucleus. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: imino sugars; transketolase; asymmetric carbon±carbon bond synthesis.
The class of compounds known as 1,1-dideoxyiminoalditols, in which the ring oxygen atom of
a sugar is replaced by NH, have proven to be potent inhibitors of a wide range of glycosidases.
1
For example, deoxynojirimycin 1 is a powerful inhibitor of a-glucosidases. The mechanistic basis
for the inhibition is believed to be due to a combination of the structural similarity between 1 and
d-glucosides 2, and also the ability to mimic the oxonium ion intermediate 3 by protonation on
the nitrogen atom.
Many types of imino sugars have been reported in the past few years, each with a di€erent
spectrum of activity against a range of glycosidases. Recently the synthesis of some O-alkylated
N-hydroxypiperidines 4 was described and the compounds shown to be active against glycosidases.2
*
Corresponding author.
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00645-6

4
482
Herein we describe the ®rst synthesis of an N-hydroxypyrrolidine 5 which has the same peripheral
stereochemistry found in d-glucose.
The synthesis of 5 arose out of an attempt to prepare the related imino sugar nectrisine 6. Our
initial planned route to 6 is shown in Scheme 1. Transketolase mediated condensation of (þ)-3,3-
diethoxy-2-hydroxypropanal 7 with hydroxypyruvate 8 a€orded the triol 9 in 56% yield (based
3
on the reactive enantiomer) which was silylated using TBSOTf and Et N (74%) followed by
3
treatment with hydroxylamine hydrochloride and KHCO in methanol to give the oxime 10 in
3
1
2% yield. Reduction of oxime 10 using Raney nickel proved capricious giving yields of the
8
diastereomeric mixture of amines 11 of up to 65%. The unreliability of the oxime reduction
inevitably presented a major obstacle to the successful synthesis of nectrisine, and severely limited
the availability of material for subsequent studies. The mixture of diastereomeric amines 11 was
readily cyclised (97%) by treatment with iodotrimethylsilane in anhydrous CH Cl to give a 3:2
2
2
mixture of cyclic imines from which the major diastereomer 12, bearing the stereochemistry found
in nectrisine 6, was isolated. Unfortunately, treatment of protected imine 12 under a range of
desilylation conditions (e.g. TBAF; AcOH/H O/THF; ¯uoride resin; HF/acetonitrile) failed to
2
yield a pure sample of nectrisine.
2+
.
3 2 3
N; (iii) NH OH HCl, KHCO ;
Scheme 1. Reagents: (i) transketolase, TPP, Mg , pH 7.0 (pH stat); (ii) TBSOTf, Et
iv) H /Raney Ni; (v) TMSI; (vi) SiO chromatography
(
2
2
In view of the problems with the oxime reduction step we turned our attention to an alternative
route that began with transketolase mediated coupling of (þ)-3-O-benzylglyceraldehyde 13 with
4
hydroxypyruvate yielding 5-O-benzyl-d-xylulose 14 in 80% yield on a 2±3 g scale (Scheme 2).
Triol 14 was converted to silylated oxime 15 by a sequence of silylation (TBSOTf, Et N, 83%
yield), oxime formation (hydroxylamine hydrochloride, KHCO , 71% yield, 2:1 mixture of
3
3
(E)-and (Z)-geometric isomers) and ®nally treatment with TBSOTf and Et N (95% yield).
3
Debenzylation of oxime ether 15 proved surprisingly problematic. The most e€ective catalyst
proved to be 10% palladium-charcoal which resulted in complete debenzylation of 15 within 24 h
under an atmosphere of H , though appreciable amounts of catalyst (40±50 weight%) were found to
2
be necessary to obtain a rapid, ecient conversion. The alcohol product was isolated by ®ltration

4
483
1
of the reaction mixture through Celite and found to be >90% pure by H and C NMR analysis
1
13
1
(
H NMR suggesting an 1:1 mixture of double bond isomers), and was isolated in near-quantitative
yield as a clear or yellow oil which slightly solidi®ed after prolonged evacuation on a high-vacuum
line. Oxidation of the alcohol was accomplished either under Swern conditions (40±60%) or by
using NaOCl and TEMPO (cat.) in a two-phase reaction mixture (66%) to give aldehyde 16.
2
+
.
Scheme 2. Reagents: (i) transketolase, TPP, Mg , pH 7.0 (pH stat); (ii) TBSOTf, Et
iv) TBSOTf, Et N (v) H /Pd; (vi) NaOCl, TEMPO or Swern; (vii) (EtO) CH, pTsOH; (viii) NaCNBH
HF
3
N; (iii) NH
2
OH HCl, KHCO
; (ix) H /Pd (x)
3
(
3
2
3
3
2
The aldehyde 16 was then treated with triethyl orthoformate and p-toluenesulfonic acid (cat.)
in EtOH with the intention of preparing the corresponding diethyl acetal although the reaction
resulted in the unexpected formation of the oxazine 17. Assignment of the 1,2-oxazine structure
1
17 was made on the basis of the following evidence. H NMR integrals were consistent with the
presence of three TBS groups and a single ethoxy group per molecule; a molecular ion of m/z 534
+
[
MH ] consistent with the molecular formula C H NO Si was observed in the CI mass spec-
25 55 5 3
^1
trum; furthermore a CˆN stretch at ꢀ
1585 cm (comparable to that reported for the CˆN
max
5
stretches of a range of unsaturated 1,2-oxazines ) was observed, implying that addition or other
reaction across the oxime double bond had not taken place.6
1
7
Reduction of 1,2-oxazines using hydrogenation over Raney nickel had been shown to yield
highly-substituted ®ve-membered heterocycles (proline analogues), useful in the synthesis of
ACE inhibitors. Treatment of 1,2-oxazine 17 under analogous conditions yielded the trisilylated
derivatives 19 as a mixture of diastereomers (2:1) in 59% overall yield. Of potentially greater
interest, however, were reductive processes which did not cleave the N±O bond, and consequently
left the oxazine ring structure intact. The reduction of the CˆN bond of 6-silyloxy-and 6-alkoxy-
1
,2-oxazines with sodium cyanoborohydride in acetic acid has been reported to proceed without
8
cleavage of the N±O bond. Reduction of 1,2-oxazine 17 with sodium cyanoborohydride in acetic
acid yielded the N-hydroxypyrrolidine 18 as a single diastereoisomer and crystalline solid (37%).
The X-ray structure of 18 (R-factor of 18%) showed unambiguously that a ring contraction had
occurred and that the product contained a ®ve-membered ring. The stereochemistry at the new
chiral centre in the product was clearly shown to be (R), with all substituents adopting a pseudo-
equatorial arrangement about the ®ve-membered ring (Fig. 1).

4
484
Figure 1. X-Ray structure of protected N-hydroxypyrrolidine 18
Ring contractions of 1,2-oxazines upon treatment with NaBH CN have not previously been
3
9
reported. However similar processes have been observed under acidic conditions yielding
nitrones or pyridine N-oxides, depending on the nature of the substituent at C-6 of the oxazine,
and in the reduction of 6-silyloxy-1,2-oxazines under aprotic conditions using DIBAL-H to yield
N-hydroxypyrrolidines.10
Finally desilylation of 18 was achieved by treatment with aqueous HF in 1:1 acetonitrile/THF
which yielded the fully desilylated product 5 in quantitative yield. Evaluation of the activity of 5
as a glycosidase inhibitor is currently in progress.
11
Acknowledgements
We are grateful to the BBSRC for a studentship (A.J.H.) and postdoctoral fellowship
M.E.B.S.).
(
References
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3
ꢀ
dissolved in 1:1 ethyl acetate/toluene (160 cm ) and cooled to 0 C with stirring. Benzyl trimethylammonium
3
chloride (523 mg, 2.81 mmol), NaBr (335 mg, 3.25 mmol) and saturated aqueous NaHCO (35 cm ) were added.
3
3
The resulting two-phase mixture was treated dropwise with a solution made up of 1.1 M NaOCl solution (15 cm ),
3
3
3
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3
3
aq. NaHCO
3
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4
), concentrated in vacuo, and puri®ed by chromatography
(300 MHz;
D O) 1.26 (6H, t, J 6.0, CH ), 3.57 (1H, dd, J 4.0 and 5.5, CH(OD)), 3.59±3.73, 3.74±3.90 (4H, m, CH CH ), 4.64
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H
2
3
3
2
(
1H, d, J 5.5, CH(OEt)
2
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2
); ꢁ
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(
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3 2 2 2

4
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7
8
9
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1
1
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1. Experimental data for N-hydroxypyrrolidine 5: ꢁ (200 MHz; D
), 3.75 (1H, dd, J 5.5, 11.0, 3-H), 4.05 (1H, m, 4-H); ꢁ
H
2
O) 2.69 (1H, dt, J 6.5, 11.0, 2-H), 3.08 (2H, d, J
(150.8 MHz; D O)
9.6, 63.0, 74.7, 75.1, 77.4; m/z (CI) 150 [MH ], 98, 82; (Found: m/z (CI) [MH ] 150.0766. C H NO requires m/z
7
.5, 5-H), 3.66 (2H, d, J 6.5, CH
2
O
H
C
2
+
+
5
5
11
4
+
(
CI) [MH ] 150.0766).