Stereodivergent synthesis of D,D- and L,L-glycero-β-alloa- heptopyranoses on a dioxanone scaffold

Article abstract of DOI:10.1055/S-0032-1290461

We report a stereodivergent synthesis of both enantiomers of glycero-allo-heptose from 2,2-dimethyl-1,3-dioxan-5-one, a readily available nonchiral scaffold, and from two different synthetic equivalents of glyoxal: dimethoxyacetaldehyde and 1,3-dithiane-2-carboxaldehyde. The short synthetic sequence involves first a proline-mediated, and then a lithium enolate mediated aldol reaction at the a- and α'-positions of the dioxanone ring, respectively, and demonstrates the complementary nature of organocatalysis and metal enolate based methods. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/S-0032-1290461

LETTER
▌2367
lSettteer reodivergent Synthesis of D,D- and L,L-glycero-β-allo-Heptopyranoses on a
Dioxanone Scaffold
Stereodivergent Synthesis of Heptoses
Nagarjuna Palyam, Izabella Niewczas, Marek Majewski*
Department of Chemistry, University of Saskatchewan, 110 Science Place, Saskatoon, SK, S7N 5C9, Canada
Fax +1(306)9664730; E-mail: marek.majewski@usask.ca
Received: 05.06.2012; Accepted after revision: 20.07.2012
synthesis of D,D- and L,L-glycero-β-allo-heptoses follow-
Abstract: We report a stereodivergent synthesis of both enantio-
ing the strategy summarized in Scheme 1, we have fo-
mers of glycero-allo-heptose from 2,2-dimethyl-1,3-dioxan-5-one,
cused our attention on compound 8 which is synthetically
equivalent to the meso-dialdehyde 7 and should be acces-
a readily available nonchiral scaffold, and from two different syn-
thetic equivalents of glyoxal: dimethoxyacetaldehyde and 1,3-dithi-
ane-2-carboxaldehyde. The short synthetic sequence involves first sible by reduction of the corresponding double aldol de-
a proline-mediated, and then a lithium enolate mediated aldol reac-
tion at the α- and α′-positions of the dioxanone ring, respectively,
rivative 9 (Scheme 2).
and demonstrates the complementary nature of organocatalysis and
metal enolate based methods.
O
O
H
O
H
Key words: aldol reaction, organocatalysis, carbohydrates, hetero-
cycles, stereoselective synthesis
+
+
OPG2
O
O
PG1O
R1 R2
1
3
2
The use of symmetry in carbohydrate chemistry fascinat-
ed chemists for a long time and, over the years, manifested
itself in elegant solutions to synthetic and analytical prob-
lems to mention only the classic intellectual tour de force
known as Fischer’s deduction of glucose stereochemis-
try.1 During studies aimed at developing new approaches
to the synthesis of higher carbohydrates we became at-
tracted to the possibility of constructing both enantiomers
of an odd-carbon higher sugar from one symmetrical pre-
cursor: 2,2-dialkyl-1,3-dioxan-5-one (1, dioxanone).2
Thus an aldoheptose (5 or 6) could, in principle, be con-
structed from three building blocks: a dioxanone 1 and
two different synthetic equivalents of glyoxal 2 and 3
(Scheme 1). Aldoheptoses are synthetic targets of interest
and considerable literature exists on their isolation, syn-
thesis, and biological significance.3
OH OH OH
PG1O
O
O
OPG2
R1 R2
4
OH OH OH
OH OH OH
OH OH OH
O
O
OH OH OH
5
6
Scheme 1 Aldoheptose–dioxanone retrosynthetic analysis
OBn OBn OTBS
OH OH OH
The sequence in which the two terminal hydroxyls in the
key intermediate 4 are deprotected dictates which isomer
of the carbohydrate will be produced. The control of ste-
reochemistry rests in the choice of the method for discrim-
ination between the two enantiotopic nucleophilic sites in
the ketone (C4 or C6), the relative stereochemistry of both
aldol reactions (syn or anti), and the disposition of both
newly attached side chains on the ring (cis or trans). A
combination of these elements such that 4 has CS symme-
try would lead to compounds 5 and 6 being enantiomers,
that is, to a stereodivergent synthesis.
S
OMe
S
O
O
OMe
O
O
O
O
8
7
O
OBn
O
OTBS
S
OMe
O
O
S
O
O
OMe
9
1a
Scheme 2 Key intermediates – latent symmetry and retrosynthesis
The utility of dioxanones as versatile synthetic building
blocks has been demonstrated before.2,4,5 Recently, we
have reported on two consecutive aldol reactions involv-
ing these compounds.4e In the current project, aimed at a
Organocatalytic aldol reactions involving dioxanones and
dimethoxyacetaldehyde, that is synthetically equivalent to
a protected glyoxal (Scheme 1), have been reported be-
fore.6 In our hands this reaction, catalyzed by (S)-proline,
proceeded in good yield and high selectivity affording the
anti-aldol 11 as the major product (Scheme 3). Compound
11 is a derivative of D-ribose, and in order to confirm the
SYNLETT 2012, 23, 2367–2370
Advanced online publication: 24.08.2012
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0032-1290461; Art ID: ST-2012-S0479-L
© Georg Thieme Verlag Stuttgart · New York

2368
N. Palyam et al.
LETTER
structure we have subjected this compound to reduction allo-heptopyranose (ent-18) in 20% overall yield in 13
followed by hydrolysis. As expected, based on the litera- steps starting from dioxanone 1. The opposite signs of op-
ture precedents, the reduction gave selectively the syn- tical rotation values of 18 and ent-18 were observed, and
diol 13.6 Hydrolysis of the diol yielded selectively D-ri- the specific rotation value was in good agreement with
bopyranose (14) or the corresponding methyl-D-ribofu- that reported for D-glycero-D-allo-heptose.3g The identical
ranoside (15) in high yield depending on the reaction 1H and 13C NMR spectroscopic data for both compounds
conditions (Scheme 3).
confirmed the stereodivergent synthesis of the L,L- and
D,D-isomers.
O
OH
O
OH
1a or 1b
(S)-proline
LiCl, DMSO
O
OMe
O
OH
OTBS
OMe
OMe
12 (syn)
MeO
+
S
OMe
H
O
O
OMe
4 d, 5 °C
67–73%
O
O
MeO
S
O
O
OMe
R1 R2
R1 R2
11 (anti)
10
9
a: R1, R2 = Me; 11a/12a = 92:8; 98:2 er (anti)
b: R1 = t-Bu, R2 = Me; 11b/12b = 96:4; 98:2 er (anti)
1. NaBH(OAc)3
2. BnBr, NaH
76%
OBn OBn OTBS
O
OH OH
HO
HO
S
OMe
OH
OMe
OH
AH, H2O
NaBH(OAc)3
OH
S
O
O
OMe
14
11
O
O
OMe
1. AcOH aq
NBS, AgNO3
MeCN/H2O
+
2. NaBH4
3. 2,2-DMP, H+
R1 R2
85%
8
OMe
76%
O
13a: 73%; 98:2 dr
13b: 81%; 98:2 dr
O
OBn
OBn
OH
OH
OBn OBn OTBS
O
S
15
H
OCH3
OCH3
AH = CF3COOH, 90 °C, 14/15 = 92:8
AcOH, 0–40 °C, 14/15 = 6:94
O
O
S
O
O
O
19
16
Scheme 3 Organocatalytic aldol reaction of dioxanones
NBS, AgNO3 99%
1. NaBH4
94%
Compound 8, corresponding to the key intermediate 4 in
Scheme 1, was synthesized from 11 via a sequence in-
volving protection and LDA-mediated aldol addition to
give the corresponding monoprotected bisaldol 9 fol-
lowed by a reduction of the ketone carbonyl and ben-
zylation using the methods described before.4e The
unmasking of one of the two aldehyde groups embedded
in compound 8 played the central role in carrying out the
stereodivergent synthesis of both enantiomers of glycero-
allo-heptose (Scheme 4). Dithiane hydrolysis of com-
pound 8 using Corey’s protocol (NBS/AgNO3 in aq
MeCN)7 gave the corresponding aldehyde 16. This com-
pound was reduced to the corresponding alcohol and then
both acetal groups as well as the tert-butyldimethylsilyl
(TBS) group were removed by hydrolysis to give com-
pound 17. Acetylation followed by debenzylation and yet
another acetylation afforded the hexaacetate of D-glycero-
β-D-allo-heptopyranose (18) in 22% overall yield in elev-
en steps starting from dioxanone 1.
2. AcOH (aq)
HO
O
OBn
OBn
O
H
BnO
O
O
O
O
20
BnO
OH
1. AcOH (aq)
2. NaOAc
Ac2O
77%
OAc
OH
OH
17
OAc
OBn
1. NaOAc, Ac2O
2. H2, Pd/C
3. NaOAc, Ac2O
O
69%
AcO
BnO
AcO
21
OAc
88%
AcO
AcO
1. H2, Pd/C
2. NaOAc
Ac2O
O
OAc
OAc
OAc
OAc
OAc
18
O
OAc
ent-18
[α]D25 +5.0 (c 1.0, C6H6)
AcO
AcO
[α]D25 –5.7 (c 1.1, C6H6)
OAc
A similar approach was applied in the synthesis of the
L-enantiomer (ent-18, Scheme 4). Intermediate 8 was sub-
jected to hydrolysis followed by NaBH4-mediated reduc-
tion of the aldehyde intermediate and protection of the
resulting tetraol moiety using 2,2-dimethoxypropane. The
subsequent hydrolysis of the dithiane moiety in com-
pound 19 gave the aldehyde 20 that was subjected to a
‘one-pot’ acid-catalyzed ketal deprotection–cyclization
followed by acetylation to afford compound 21 (α/β =
14:86, 77% overall yield). Debenzylation followed by fur-
ther acetylation gave the hexaacetate of L-glycero-β-L-
OH OH OH
OH OH OH
H
R
R
H
S
S
S
S
S
R
R
R
O
OH OH OH
OH OH OH
O
5
6
L-glycero-β-L-alloheptose
D-glycero-β-D-alloheptose
Scheme 4 Stereodivergent synthesis of glycero-allo-heptoses
In summary, both enantiomers of glycero-β-allo-heptopy-
ranose (18 and ent-18) were synthesized starting from the
readily available dioxanone 1. Further expansion of the
Synlett 2012, 23, 2367–2370
© Georg Thieme Verlag Stuttgart · New York

LETTER
Stereodivergent Synthesis of Heptoses
2369
give the crude diol. Diastereoselectivity of the reaction was mea-
sured by integrating peaks at δ = 3.45 and 3.56 ppm and was found
to be syn/anti = 2:98. The crude product was purified by flash col-
umn chromatography using silica gel (hexane–EtOAc = 8:2) to give
pure diol 9a as a colorless oil (1.11 g, 85%). [α]D25 +23 (c 1, C6H6)
and [α]D25 +7 (c 1.05, CHCl3). 1H NMR (500 MHz, CDCl3): δ = 4.93
[br s, 1 H (OH)], 4.25 (d, J = 3.8 Hz, 1 H), 4.24 (d, J = 2.6 Hz, 1 H),
4.20 (br s, 1 H), 4.06 (dd, J1 = 2.6 Hz, J2 = 7.8 Hz, 1 H), 3.90 (dd,
J1 = 7.8 Hz, J2 = 9.2 Hz, 1 H), 3.88 (dd, J1 = 1.0 Hz, J2 = 9.1 Hz, 1
H), 3.84 (dd, J1 = 1.0 Hz, J2 = 3.8 Hz, 1 H), 3.80 (dd, J1 = 9.1 Hz,
J2 = 9.2 Hz, 1 H), 3.56 (s, 3 H), 3.42 (s, 3 H), 3.12–3.02 (m, 2 H),
2.84–2.79 (m, 1 H), 2.72–2.67 (m, 1 H), 2.07–1.96 (m, 2 H), 1.45
(s, 3 H), 1.32 (s, 3 H), 0.88 (s, 9 H), 0.10 (s, 3 H) 0.07 (s, 3 H) ppm.
13C NMR (125 MHz, CDCl3): δ = 106.8, 98.7, 80.8, 76.2, 72.3, 70.5,
65.0, 58.6, 56.5, 47.5, 29.5, 29.1, 28.8, 26.2, 26.0, 19.5, 18.4, –4.2,
–4.8 ppm. HRMS (CI, Na): m/z calcd for (C21H42O7S2Si + Na)+ [M
+ Na]+: 521.2039; found: 521.2038. IR (KBr): 3457 (br), 2991,
2928, 2854, 1463, 1380, 1255, 1166, 1151, 1070, 1001, 836, 779,
731 cm–1.
scope of this methodology into synthesis of sialic acids is
under investigation.
Compound 11
2,2-Dimethoxyacetaldehyde (2a, 60% aq solution, 2.90 g, 16.9
mmol), (S)-proline (530 mg, 4.60 mmol), and LiCl (645 mg; 15.4
mmol) were added to a solution of 1a (2.00 g, 14.4 mmol) in dry
DMSO (5 mL). The resulting mixture was flushed with nitrogen,
stirred at r.t. for 15 min to dissolve all the reactants and refrigerated
at 5 °C until the reaction was complete as shown by TLC (ca. 72 h).
A sat. NH4Cl solution and EtOAc were added with vigorous stir-
ring, the mixture was extracted with EtOAc (3 × 50 mL), and the
combined organic layers were washed with brine. The organic
phase was dried (Na2SO4) and concentrated to afford the crude
product. The ratio of diastereomers was measured by 1H NMR spec-
troscopy on the crude product and was found to be anti/syn = 92:8.
Purification by flash column chromatography (hexane–EtOAc =
7: 3) provided the anti-aldol adduct 11 (2.4 g, 67%) as a pale yellow
oil. The enantiomeric ratio (er) was measured by 1H NMR spectro-
scopy in C6D6 with Eu(tfc)3 as a shift reagent. [α]D25 –134 (c 1.15,
CHCl3; 96% ee). 1H NMR (500 MHz, CDCl3): δ = 4.65 (d, J = 6.8
Hz, 1 H), 4.44 (dd, J1 = 1.3 Hz, J2 = 3.1 Hz, 1 H), 4.25 (dd, J1 = 1.5
Hz, J2 = 16.7 Hz, 1 H), 4.07 (dd, J1 = 3.1 Hz, J2 = 6.8 Hz, 1 H), 3.99
(d, J = 16.7 Hz, 1 H), 3.43 (s, 3 H), 3.38 (s, 3 H), 2.42 (br s, 1 H),
1.47 (s, 6 H). 13C NMR (125 MHz, CDCl3): δ = 206.5, 103.4, 100.5,
76.3, 71.2, 67.1, 55.4, 54.3, 25.0, 23.0. HRMS (CI, NH3): m/z calcd
for (C10H18O6 + NH4)+ [M + NH4]+’: 252.1447; found: 252.1451.
LRMS (CI, NH3): m/z (%) = 252 (20) [M + 18]+, 238 (22), 220 (56),
206(44), 188 (70), 152 (100), 148 (28). IR (KBr): 3377, 1747 cm–1.
The diol 9a (0.6 g, 1.2 mmol) in dry THF (10 mL) was added to a
stirred suspension of NaH (80% w/w, 91 mg, 23.0 mmol) in dry
THF (5 mL) at 0 °C under inert atmosphere. The resulting mixture
was stirred at 0 °C for 15 min, benzyl bromide (0.32 mL, 2.6 mmol)
and catalytic amount of TBAI (20 mg) were then added. The result-
ing mixture was stirred at ambient temperature for 12 h. After TLC
showed no starting material, the reaction was quenched with an aq
sat. NaHCO3 solution and extracted with EtOAc (3 × 50 mL). The
combined organic layers were washed with brine, dried (Na2SO4),
and concentrated to yield the crude product which was purified by
flash chromatography using SiO2; hexane–EtOAc (5–10%) to give
25
1
pure 8 (0.73 g, 89%). [α]D +14 (c 1.0, CHCl3). H NMR (500
MHz, CDCl3): δ = 7.44–7.22 (m, 10 H), 4.87 (dd, J1 = 5.5 Hz,
J2 = 11.6 Hz, 2 H), 4.77 (t, J = 12.0 Hz, 2 H), 4.45 (d, J = 10.1 Hz,
1 H), 4.28 (d, J = 9.4 Hz, 1 H), 4.25 (d, J = 8.2 Hz, 1 H), 4.05 (t,
J = 9.9 Hz, 1 H), 4.00 (t, J = 9.7 Hz, 1 H), 3.69 (d, J = 8.2 Hz, 1 H),
3.62 (d, J = 9.7 Hz, 1 H), 3.28 (s, 3 H), 3.27 (s, 3 H), 2.71 (m, 1 H),
2.61 (m, 2 H), 2.36 (m, 1 H), 1.94 (m, 1 H), 1.76 (m, 1 H), 1.46 (s,
3 H), 1.40 (s, 3 H), 0.88 (s, 9 H), 0.10 (s, 3 H), 0.07 (s, 3 H). 13C
NMR (125 MHz, CDCl3): δ = 138.9, 138.3, 128.3, 128.2, 128.1,
127.5, 127.3, 127.0, 105.6, 99.3, 81.7, 75.5, 74.7, 74.0, 73.7, 72.5,
71.1, 55.9, 54.8, 49.9, 40.6, 30.0, 29.4, 26.0, 19.2, 18.5, –4.2, –4.6.
HRMS (CI, Na): m/z calcd for (C35H54O7S2Si + Na)+ [M + Na]+:
701.2978; found: 701.2989. IR (KBr): 3060, 2993, 2928, 2854,
1457, 1380, 1095, 1076, 1000, 836, 778, 697 cm–1.
Compound 8
A solution of n-BuLi (4.80 mL, 10.1 mmol, 2.1 M solution in hex-
anes, 3.3 equiv) was added dropwise to a stirred solution of diiso-
propylamine (1.57 mL, 11.1 mmol, 3.6 equiv) in THF (10 mL) at
0 °C under nitrogen. After 30 min, a solution of TBS-protected 11
(1.07 g, 3.07 mmol, 1.00 equiv) in THF (5 mL) was added slowly,
and the mixture was stirred for 2 h at –78 °C. Dithiane carboxalde-
hyde (1.82 g, 12.3 mmol, 4.0 equiv) in dry THF (5 mL) was added,
and the mixture was stirred at –78 °C for 30 min. The reaction was
quenched with concentrated phosphate buffer (pH 7.5; 15 mL) and
extracted with Et2O (3 × 100 mL). The combined organic layers
were rinsed with sat. solution of NaHCO3, NaCl, and dried
(Na2SO4). The solvents were removed under reduced pressure, and
the diastereoselectivity of the reaction was determined by 1H NMR
spectroscopy on the crude product by integration of the peaks at δ =
3.43 and 3.35 ppm and was found to be anti-anti-trans/anti-anti-cis
aldols = 9:91. The crude reaction mixture was fractionated by flash
column chromatography (5–10% EtOAc in hexane) to give com-
pound 9 as a colorless oil (1.33 g, 87%). [α]D24 –3 (c 0.33, CHCl3).
1H NMR (500 MHz, CDCl3): δ = 4.50 (d, J = 7.4 Hz, 1 H), 4.37–
4.33 (m, 2 H), 4.31–4.27 (m, 1 H), 4.11–4.04 (m, 2 H), 3.73 (br s, 1
H), 3.39 (s, 3 H), 3.35 (s, 3 H), 3.20–3.10 (m, 2 H), 2.75–2.67 (m, 1
H), 2.64–2.56 (m, 1 H), 2.04–1.95 (m, 2 H), 1.50 (s, 3 H), 1.47 (s, 3
H), 0.84 (s, 9 H), 0.08 (s, 3 H), 0.06 (s, 3 H) ppm. 13C NMR (125
MHz, CDCl3): δ = 208.8, 105.5, 98.9, 79.8, 77.9, 76.0, 74.4, 56.0,
55.9, 44.6, 28.9, 28.8, 28.4, 26.0, 25.9, 20.6, 18.3, –4.4, –4.5 ppm.
HRMS (CI): m/z calcd for C21H40O7S2Si [M + H]: 497.2065; found:
497.2078. LRMS (CI, NH3): m/z (%) = 497 (56) [M+ + 1], 465 (44),
407 (8), 366 (17), 334 (98), 317 (100), 276 (44), 185 (20), 159 (32),
119 (72), 75 (84). IR (KBr): 3487, 1709 cm–1.
Spectroscopic Data for Compound 18
β-Anomer
[α]D24 5.1 (c 1.0, C6H6). 1H NMR (500 MHz, CDCl3): δ = 5.93 (d,
J = 8.6 Hz, 1 H), 5.65 (t, J = 3.0 Hz, 1 H), 5.19 (ddd, J1 = 2.8 Hz,
J2 = 4.6 Hz, J3 = 7.2 Hz 1 H), 5.00 (dd, J1 = 2.9 Hz, J2 = 10.4 Hz, 1
H), 4.92 (dd, J1 = 3.0 Hz, J2 = 8.6 Hz, 1 H), 4.25 (dd, J1 = 4.4 Hz,
J2 = 11.8 Hz, 1 H), 4.17 (dd, J1 = 2.9 Hz, J2 = 10.4 Hz, 1 H), 4.11
(dd, J1 = 7.2 Hz, J2 = 11.8 Hz, 1 H), 2.14 (s, 3 H), 2.09 (s, 3 H), 2.08
(s, 3 H), 2.01 (s, 6 H), 1.98 (s, 3 H) ppm. 13C NMR (125 MHz,
CDCl3): δ = 170.6, 170.1, 169.96, 169.3, 169.3, 169.2, 90.3, 72.5,
69.9, 68.4, 68.1, 66.9, 61.8, 21.17, 21.14, 20.95, 20.93, 20.78,
20.75. HRMS (CI, NH3): m/z calcd for (C19H26O13 + NH4)+:
480.1717; found: 480.1715. LRMS (CI, NH3): m/z (%) = 480 (100)
[M + 18]+, 403 (52), 215 (2), 152 (8), 139 (9), 110 (8), 77 (7), 60
(41). IR (KBr): 3476 (w), 2965, 1749, 1433, 1370, 1216, 1048, 948,
914, 601 cm–1.
AcOH (3.2 mL) and NaBH(OAc)3 (1.1 g, 5.2 mmol, 2 equiv) were
added at –20 °C to the solution of β-hydroxyketone 9 (1.3 g, 2.6
mmol, 1.0 equiv) in dry CH2Cl2 (20 mL). The mixture was stirred at
–20 °C for 12 h and then kept at that temperature for 3 d (TLC con-
trolled). The reaction was quenched with a sat. NaHCO3 solution
and extracted with CH2Cl2 (3 × 100 mL).The combined organic lay-
ers were washed with brine, dried (Na2SO4), and concentrated to
Spectroscopic Data for Compound ent-19
β-Anomer
24
[α]D –5.7 (c 1.1, C6H6). HRMS (CI, NH3): m/z calcd for
(C19H26O13 + NH4)+: 480.1717; found: 480.1710. The NMR and IR
spectra were identical with these of compound 18.
© Georg Thieme Verlag Stuttgart · New York
Synlett 2012, 23, 2367–2370

2370
N. Palyam et al.
LETTER
therein. Heptoses synthesis:. (k) Jaipuri, F. A.; Collet, B. Y.
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Acknowledgment
The authors thank NSERC (Canada) and the University of Saskat-
chewan for financial support and the Saskatchewan Structural Sci-
ences Centre for help in analytical measurements.
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