Synthesis of all diastereomeric methyl 2,5-anhydro-3-deoxy-hexonates: Precursors to C-2-deoxynucleosides and THF-templated γ- and δ-amino acids

Article abstract of DOI:10.1016/S0040-4039(03)01416-3

Efficient syntheses of all diastereomers of methyl 2,5-anhydro-3-deoxy-hexonate from mannono- or gulono-lactones provide precursors for C-nucleosides of 2-deoxyribose and for THF-templated γ- and δ-amino acids.

Full text of DOI:10.1016/S0040-4039(03)01416-3

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 5853–5857
Synthesis of all diastereomeric methyl
2,5-anhydro-3-deoxy-hexonates: precursors to
C-2-deoxynucleosides and THF-templated g- and d-amino acids
Mark P. Watterson,a Alison A. Edwards,a John A. Leach,a Martin D. Smith,a Osamu Ichiharab and
George W. J. Fleeta,*
aDyson Perrins Laboratory, South Parks Road, University of Oxford, Oxford OX1 3QY, UK
bEvotec OAI, 151 Milton Park, Abingdon, Oxfordshire OX14 4SD, UK
Received 18 September 2002; revised 29 May 2003; accepted 5 June 2003
Abstract—Efficient syntheses of all diastereomers of methyl 2,5-anhydro-3-deoxy-hexonate from mannono- or gulono-lactones
provide precursors for C-nucleosides of 2-deoxyribose and for THF-templated g- and d-amino acids.
© 2003 Elsevier Ltd. All rights reserved.
1. Introduction
libraries have been reported based on pyranose ana-
logues having the same three functionalities7 and with
five points of diversification.8 Additionally the THF-2-
carboxylates 2 are of interest as intermediates for the
synthesis of C-nucleosides of the stereoisomers of 2-
deoxy-ribose.9
The preceding paper1 described routes to tetra-
hydrofuran-templated g-amino acids from pentono-1,5-
lactones, as building blocks for carbopeptoids with
predisposition towards well-defined secondary struc-
tures (foldamers).2 This paper describes the synthesis of
analogues 3 bearing a C-5 hydroxymethyl group from
hexono-1,4-lactones (g-lactones) 1 via 3-deoxy-THF-2-
carboxylates 2 (Scheme 1). Sugar amino acids are an
excellent source of stereodiverse polyfunctional scaf-
folds for the design of peptidomimetics3,4 and combina-
torial libraries thereof.5,6 The THF-templated amino
acids 3 and their regioisomers 4 provide three points for
diversification with orthogonal chemical reactivity;
The THF-2-carboxylates 2 are accessible via the high
yielding rearrangement of 2-O-sulfonate derivatives of
3-deoxy-g-lactones under acidic conditions.10 For the
synthesis of THF g-amino acids 3, the g-lactone struc-
ture 1 precludes introduction of a C-4 nitrogen sub-
stituent prior to the formation of the THF ring. Within
this approach to THF g-amino acids 3, there is flexibil-
ity for C-3 deoxygenation either before (Strategy 1,
Scheme 1. Synthetic routes to THF-templated g- and d-amino acids from hexono-1,4-lactones.
* Corresponding author.
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)01416-3

5854
M. P. Watterson et al. / Tetrahedron Letters 44 (2003) 5853–5857
Scheme 1) or after (Strategy 2, Scheme 1) formation of
the THF ring. Selective transformation of either the
C-4 or C-6 functionality of the 3-deoxy-C-glycosyl
derivative 2 provides access to THF-templated g- and
d-amino acids 3 and 4, respectively. The 3-deoxy THF
d-amino acids 4 provide further opportunities for evalu-
ating the influence of ring stereochemistry11 on the
conformational properties of carbopeptoids derived
2. Strategy 1: Deoxygenation after THF formation.
-ribo g-Azido ester 17 and -xylo d-azido ester 19
D
D
In the synthesis of the
D
-xylo THF-2-carboxylate 13
deoxygenation at the C-3 position of the carbohydrate
skeleton was accomplished after formation of the THF
ring (Scheme 2). Regioselective O-sulfonation of the
kinetic monoacetonide 5 derived from
D-gulono-1,4-
from them. Routes to 2,5-anhydro-3-deoxy-
D
-xylo- and
lactone13 with triflic anhydride at −40°C gave the stable
triflate 6 {mp 103–104°C; [h]2D4 −15.7 (c 1.0)}14 in 72%
yield. Treatment with an acidic methanol solution con-
D
-ribo-hexonic acids and an O-benzylated-D-arabino
analogue9b have been reported previously from both
carbohydrate12 and non-carbohydrate9a starting materi-
als. This paper describes the synthesis of derivatives of
verted the lactone 6 to the
D
-ido THF carboxylate 8 in
87% yield {oil; [h]2D4 −29.3 (c 1.3 in MeOH)} pre-
sumably via an open chain hydroxy triflate 7. The
4,6-diol unit of 8 was subsequently protected as the
isopropylidene ketal 9 in 86% yield {mp 93–95°C (ethyl
acetate); [h]2D5 −29.5 (c 1.0 in MeOH)}. Activation of
the C-3 hydroxyl group of 9 with triflic anhydride at
methyl 2,5-anhydro-3-deoxy-hexonates 2 of
D-xylo, D-
ribo, -lyxo and -arabino configuration and their con-
L
L
version to all diastereomeric 4-azido and 6-azido
derivatives, precursors to THF g- and d-amino acids 3
and 4.
Scheme 2. Reagents and conditions: (i) Tf2O, pyridine, DCM; (ii) HCl, MeOH; (iii) Me2CO, Me2C(OMe)2, CSA; (iv) NaN3, DMF;
t
(v) H2, Pd black, MeOH; (vi) 80% v/v aq. AcOH; (vii) BuMe2SiCl, imidazole, DMF; (viii) DEAD, PPh3, PhCO2H, THF; (ix)
HCl, MeOH then NaOMe, MeOH; (x) MsCl, DMAP, pyridine.

M. P. Watterson et al. / Tetrahedron Letters 44 (2003) 5853–5857
5855
3. Strategy 2: Deoxygenation before THF formation
−30°C gave the unstable triflate 10 which was smoothly
converted to the crude elimination product 11 on addi-
tion of sodium azide in DMF. No b-azido ester prod-
ucts arising from substitution of the triflate ester
functionality were detected, in sharp contrast to an
example given in the preceding paper.1 Alkene 11
underwent stereoselective hydrogenation in the presence
of a palladium catalyst to afford the fully protected
3.1.
29
D
-xylo g-Azido ester 27 and
D
-ribo d-azido ester
The
D
-ribo diol 21 was subsequently synthesised via a
shorter route in which C-3 deoxygenation preceded
formation of the THF ring (Scheme 3). The
D-arabino
mesylate 23 {mp 142–144°C (MeOH), lit.17 142–
143.5°C (MeOH); [h]2D3 −25.6 (c 0.9), lit.17 [h]2D3 −23 (c
0.86)} was prepared from the kinetic monoacetonide 22
D
-xylo ester 12 {98% from 9; oil; [h]2D0 −20.4 (c 1.0 in
acetone)}. The 2,5-cis-stereochemistry of 12 was confi-
rmed from NOE difference data.
of
procedure of Chittenden.17 Treatment of 23 with an
acidic methanol solution afforded the -ribo THF car-
D
-mannono-1,4-lactone18 via a modification of the
Acidic hydrolysis of the isopropylidene moiety of 12
D
generated the -xylo diol 13 {quantitative; mp 57–58°C
D
boxylate 21 in 78% yield. A similar approach to the
synthesis of the carboxylic acid corresponding to 21 has
been described already by Pedersen in which 23 is
(ethyl acetate); [h]2D3 −19.8 (c 1.0 in H2O)} and derivati-
sation with tert-butyldimethylsilyl chloride and imida-
zole at 0°C occurred selectively to give a single
monosilyated product 14 in 92% yield {mp 55–56°C;
[h]2D6 +29.9 (c 1.1)}. Conversion of 14 to the mesylate 15
{99% yield; mp 64–65°C; [h]2D6 +67.4 (c 1.1)}, followed
by treatment of 15 with sodium azide in DMF at 100°C
generated instead from
Although the efficiency of the route from
tone does not match Pedersen’s methodology, the ready
availability of -mannono-1,4-lactone makes the
present route equally applicable to the synthesis of the
enantiomeric -ribo carboxylate.
D
-glucono-1,5-lactone.12b
D
-mannolac-
L
L
generated the O-protected
89% yield {oil; [h]2D6 +15.8 (c 1.1)}. Acidic methanolysis
of 16 gave the corresponding unprotected -ribo target
17 in high yield (94%).15 The -xylo d-azido ester 1916
D
-ribo g-azido ester 16 in
Introduction of nitrogen at the C-4 position of the
-ribo diol 21 followed a similar protocol to that
described above for the -xylo diol 13. In this instance,
D
D
D
D
was prepared from the -xylo diol 13 in moderate yield
D
(44% from 13; oil; [h]2D3 −40.2 (c 1.4)} via azide displace-
ment of the 6-O-methanesulfonyl intermediate 18 (92%
from 13; oil; [h]2D3 −29.0 (c 0.5)}.
use of the more bulky tert-butyldiphenylsilyl ether pro-
tecting group avoided problems of deprotection during
the azide displacement step. Accordingly 21 was con-
verted to the monosilylated product 24 in 84% yield
{mp 62–64°C; [h]2D4 +24.9 (c 1.1)}. Esterification of 24
with methanesulfonyl chloride afforded the mesylate 25
{100%; oil; [h]2D4 +31.6 (c, 1.0)} which was converted to
The silyl ether 14 was also used to prepare the
D-ribo
THF-2-carboxylate 21, epimeric at C-4 to 13 (Scheme
2). Inversion of configuration at the C-4 position of 14
was performed in highest yield under Mitsunobu condi-
tions to give the inversion ester 20 {92%; oil; [h]2D6 +13.7
(c 0.8)}. O-Deprotection of 20 by alternate treatment
the
D
-xylo g-azido ester 26 by the addition of sodium
azide {70%; oil; [h]2D4 −18.2 (c, 1.2)}. Treatment of 26
with TBAF in THF to remove the 6-O-TBDPS group
led to partial hydrolysis of the ester functionality. By
stirring the mixture of crude cleavage products with a
methanolic solution of HCl, the required unprotected
g-azido ester product 2719 was isolated in good yield
with acidic and basic methanol provided the
D-ribo
configured diol 21 {81% from 20; oil; [h]2D4 +27.7 (c 0.8
in MeOH)}.
Scheme 3. Reagents and conditions: (i) HCl, MeOH; (ii) TBDPSCl, imidazole, DMF; (iii) MsCl, DMAP, pyridine; (iv) NaN3,
DMF; (v) TsCl, pyridine.

5856
M. P. Watterson et al. / Tetrahedron Letters 44 (2003) 5853–5857
Scheme 4. Reagents and conditions: (i) HCl, MeOH; (ii) TBDPSCl, imidazole, DMF; (iii) MsCl, DMAP, pyridine; (iv) NaN3,
DMF; (v) TsCl, pyridine; (vi) Tf2O, pyridine, DCM; (vii) CsO2CCF3, butanone.
after chromatography (77%). The
D
-ribo d-azido ester
tion of configuration. Esterification of the C-4 hydroxyl
group of 33 with triflic anhydride and displacement
with caesium trifluoroacetate23 gave the inverted alco-
hol 36 in 92% yield {oil; [h]2D3 −27.7 (c 1.0)}. Treatment
of the corresponding 4-O-mesylate derivative with
sodium azide generated the g-azido ester 37 with the
2920 was generated in 72% yield over two steps from the
diol 21 via regioselective esterification with p-toluene-
sulfonyl chloride and subsequent treatment of the
monosulfonate ester product 28 {mp 73–75°C; [h]D24
+38.2 (c 1.0)} with sodium azide.
required
L
-lyxo stereochemistry in 93% yield from 36
{oil; [h]2D6 +24.3 (c, 1.1)}. Acidic methanolysis of 37
gave the corresponding 6-hydroxy building block 38
(73%).24 Treatment of the 4-O-trifluoromethanesulfonyl
3.2.
L
-lyxo and
L
-arabino g-azido esters 38 and 35, and
L
-lyxo and
L
-arabino d-azido esters 40 and 41
derivative of the -lyxo d-azido ester 40 with caesium
L
Synthesis of
L
-arabino g-azido ester 35 and
L
-lyxo d-
trifluoroacetate gave the C-4 epimeric
ester 41 in 81% overall yield.25
L
-arabino d-azido
azido ester 40 followed an analogous protocol to that
given in Scheme 3, employing the
L
-xylo mesylate 31
{mp 115–116°C (MeOH), lit.17 114–115°C (MeOH);
[h]D22 −9.2 (c, 0.5), lit.17 [h]2D0 −9 (c, 1.6)}, available from
the monoacetonide 30 of
L-gulono-1,4-lactone by the
4. Summary
methodology of Chittenden17 (Scheme 4). Acidic
methanolysis of 31 formed the L-lyxo THF-2-carboxyl-
This paper has described routes to protected forms of
all diastereomeric methyl 4-amino and 6-amino-2,5-
anhydro-3-deoxy-hexonates starting from either gulono-
or mannono-1,4-lactones. Since both enantiomers of
these starting materials are readily available, this work
demonstrates the formal synthesis of all eight stereoiso-
mers of both the 4-amino- and 6-amino-THF-2-car-
boxylate scaffolds. These THF g- and d-azido esters
have been investigated as building blocks for THF-tem-
plated carbopeptoids up to the octamer level. Results of
conformational studies on these molecules will be
reported elsewhere.
ate 32 in 66% yield {oil; [h]2D3 +35.8 (c 1.1 in MeOH)}
and treatment with tert-butyldiphenylsilyl chloride and
imidazole gave the corresponding 6-O-silyl derivative
33 {90%; mp 49–50°C; [h]2D5 +11.9 (c 1.0)}. Mesylation
followed by azide displacement introduced the required
C-4 azido group with inversion of configuration,
affording the
from 33 {oil; [h]2D6 −20.3 (c 1.2)}. Acid-catalysed cleav-
age of the TBDPS protection gave the 6-hydroxy -ara-
bino compound 35 in 76% yield.21 The
-lyxo d-azido
ester 4022 was obtained from the
-lyxo diol 32 in 75%
L
-arabino g-azido ester 34 in 99% yield
L
L
L
overall yield via the 6-O-tosylate 39 {mp 85–87°C
(diethyl ether); [h]2D2 +20.4 (c 1.1)}.
Synthesis of the
d-azido ester 38 was achieved by manipulation of the
C-4 configuration (Scheme 4). Conversion of the -lyxo
silyl ether 33 to the -lyxo g-azido ester 38 demanded
introduction of a C-4 azido group with overall reten-
L
-lyxo g-azido ester 37 and L-arabino
Acknowledgements
L
L
Support from Evotec OAI, EPSRC and BBSRC is
gratefully acknowledged.

M. P. Watterson et al. / Tetrahedron Letters 44 (2003) 5853–5857
5857
References
(3H, s, COOCH3), 4.04 (1H, app-dt, J5,4 3.3, J 6.3, H-5),
4.35 (1H, br-app-t, J 3.7, H-4), 4.58 (1H, dd, J2,3 9.7, J2,3%
2.3, H-2). lC: 39.5 (CH2, C-3), 50.9 (CH2, C-6), 53.2 (CH3,
COOCH3), 72.3, 76.5, 83.8 (3×CH, C-2, C-4, C-5), 175.4
(C, C-1).
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19.
D
-xylo g-Azido ester 27: oil. [h]2D5 −92.0 (c, 1.7). lH: 2.38
(1H, ddd, J3,3% 13.5, J3,2 6.1, J3,4 4.8, H-3), 2.63 (1H, ddd,
J3%,3 13.5, J3%,2 8.8, J3%,4 6.8, H-3%), 2.86 (1H, app-t, J 6.2,
OH), 3.78 (3H, s, COOCH3), 3.82 (2H, app-t, J 5.4, H-6,
H-6%), 4.15 (1H, dd, J5,4 10.6, J 5.4, H-5), 4.22–4.27 (1H,
m, H-4), 4.56 (1H, dd, J2,3% 8.8, J2,3 6.1, H-2). lC: 35.7 (CH2,
C-3), 52.6 (CH3, COOCH3), 61.0 (CH, C-4), 61.3 (CH2,
C-6), 75.3 (CH, C-2), 82.0 (CH, C-5), 172.9 (C, C-1).
20.
21.
D
-ribo d-Azido ester 29: oil. [h]2D2 +75.9 (c, 1.0). lH:
2.24–2.28 (2H, m, H-3, H-3%), 3.40 (1H, dd, J6,6% 12.9, J6,5
5.1, H-6), 3.45 (1H, dd, J6%,6 12.9, J6%,5 5.5, H-6%), 3.76 (3H,
s, COOCH3), 4.04 (app-dt, 1H, J 3.1, J 5.3, H-5), 4.33 (1H,
m, H-4), 4.69 (1H, app-t, J 7.8, H-2). lC: 38.6 (CH2, C-3),
52.4 (CH3, COOCH3), 52.6 (CH2, C-6), 73.2 (CH, C-4),
76.5 (CH, C-2), 85.7 (CH, C-5), 172.5 (C, C-1).
L
-arabino g-Azido ester 35: oil. [h]2D5 −106.4 (c, 1.0). lH: 2.24
(1H, app-dt, J3,3% 13.4, J 5.2, H-3), 2.61 (1H, ddd, J3%,3 13.4,
J3%,2 8.4, J3%,4 7.6, H-3%), 3.66 (1H, d, J6,6% 12.2, H-6), 3.78
(3H, s, COOCH3), 3.84 (1H, dd, J6%,6 12.2, J6%,5 3.2, H-6%),
4.06 (1H, dt, J5,4 5.6, J5,6% 3.2, H-5), 4.12 (1H, app-dt, J4,3%
7.6, J 5.6, H-4), 4.64 (1H, dd, J2,3% 8.4, J2,3 5.2, H-2). lC:
35.9 (CH2, C-3), 52.4 (CH3, COOCH3), 60.4 (CH, C-4),
61.9 (CH2, C-6), 76.2 (CH, C-2), 84.3 (CH, C-5), 172.4 (C,
C-1).
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22.
L
-lyxo d-Azido ester 40: oil. [h]2D2 +47.9 (c, 1.3). lH: 2.27
(1H, ddd, J3,3% 13.6, J3,2 7.8, J3,4 2.0, H-3), 2.37 (1H, ddd,
J3%,3 13.6, J3%,2 7.8, J3%,4 2.0, H-3%), 3.57 (1H, dd, J6,6% 12.4,
J6,5 5.8, H-6), 3.62 (1H, dd, J6%,6 12.4, J6%,5 6.6, H-6%), 3.76
(3H, s, COOCH3), 4.21 (1H, ddd, J5,6% 6.8, J5,6 5.8, J5,4 4.0,
H-5), 4.52 (1H, m, H-4), 4.75 (1H, app-t, J 3.9, H-2). lC:
39.7 (CH2, C-3), 49.7 (CH2, C-6), 52.3 (CH3, COOCH3),
71.9 (CH, C-4), 75.6 (CH, C-2), 81.2 (CH, C-5), 173.2 (C,
C-1).
23. (a) Kruizinga, W. H.; Strijtveen, B.; Kellog, R. M. J. Org.
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14. All optical rotations were recorded in CHCl3 unless
otherwise stated. All 1H and 13C NMR spectra were
recorded in CDCl3 at 400 and 100.6 MHz, respectively,
unless otherwise stated; coupling constants (J) are quoted
in Hz. Satisfactory elemental analysis or HRMS data has
been obtained for all compounds.
24.
L
-lyxo g-Azido ester 38: oil. [h]2D5 +57.4 (c, 0.96). lH: 2.30
(1H, s, OH), 2.39 (1H, ddd, J3,3% 13.8, J3,2 7.6, J3,4 6.0, H-3),
2.49 (1H, ddd, J3%,3 13.8, J3%,2 7.6, J3%,4 2.8, H-3%), 3.76 (3H,
s, COOCH3), 3.80 (1H, dd, J6,6% 11.4, J6,5 4.8, H-6), 3.87
(1H, dd, J6%,6 11.4, J6%,5 5.4, H-6%), 4.22–4.29 (2H, m, H-4,
H-5), 4.68 (1H, app-t, J 7.6, H-2). lC: 36.2 (CH2, C-3), 52.3
(CH3, COOCH3), 61.4 (CH2, C-6), 62.0 (CH, C-4), 75.4
(CH, C-2), 82.2 (CH, C-5), 172.7 (C, C-1).
15.
D
-ribo g-Azido ester 17: oil. [h]2D6 +21.0 (c, 0.9). lH: 2.40
(1H, ddd, J3%,2 7.6, J3%,3 13.3, J3%,4 6.2, H-3%), 2.45 (1H,
app-dt, J3,3% 13.3, J 6.6, H-3), 3.62 (1H, dd, J6,5 2.6, J6,6%
12.5, H-6), 3.80 (3H, s, COOCH3), 3.93 (1H, dd, J6%,5 2.7,
J6%,6 12.5, H-6%), 4.04 (1H, app-quintet, J 2.5, H-5), 4.18
(1H, app-q, J 5.9, H-4), 4.66 (1H, dd, J2,3 6.5, J2,3% 7.6, H-2).
lC: (CDCl3, 50.3 MHz): 36.5 (CH2, C-3), 52.6 (CH3,
COOCH3), 60.6 (CH, C-4), 62.0 (CH2, C-6), 75.8, 85.4
(2×CH, C-2, C-5), 173.9 (C, C-1).
25.
L
-arabino d-Azido ester 41: oil. [h]2D3 −104.0 (c, 0.9). lH: 2.18
(1H, app-dt, J3,3% 14.0, J 2.8, H-3), 2.50 (1H, ddd, J3%,3 14.0,
J3%,2 8.8, J3%,4 6.2, H-3%), 2.98 (1H, m, OH), 3.31 (1H, dd,
J6,6% 13.0, J6,5 4.2, H-6), 3.48 (1H, dd, J6%,6 13.0, J6%,5 4.2,
H-6%), 3.78 (3H, s, COOCH3), 4.26–4.31 (2H, m, H-4, H-5),
4.69 (1H, dd, J2,3% 8.8, J2,3 2.8, H-2). lC: 38.7 (CH2, C-3),
52.5 (CH2, CH3, C-6, COOCH3), 73.7, 86.5 (2×CH, C-4,
C-5), 76.9 (CH, C-2), 174.4 (C, C-1).
16.
D
-xylo d-Azido ester 19: oil. [h]2D3 −40.2 (c 1.4). lH: 2.23 (1H,
ddd, J3%,2 2.3, J3%,3 14.1, J3%,4 1.1, H-3%), 2.49 (1H, ddd, J3,2
9.6, J3,3% 14.1, J3,4 4.8, H-3), 3.62 (2H, m, H-6, H-6%), 3.80