Building higher carbohydrates via dioxanone aldol chemistry: The α,α′-bisaldol approach

Article abstract of DOI:10.1002/ejoc.200800844

A synthetic approach to higher carbohydrates via the sequence of two aldol reactions that proceed at the α and α′ positions of 2,2-dimethyl-1,3-dioxan-5-one (dioxanone) is described. As reported before, the first aldol reaction works well under organocata

Full text of DOI:10.1002/ejoc.200800844

SHORT COMMUNICATION
DOI: 10.1002/ejoc.200800844
Building Higher Carbohydrates via Dioxanone Aldol Chemistry:
The α,αЈ-Bisaldol Approach
Izabella Niewczas[a] and Marek Majewski*[a]
Keywords: Aldol reactions / Organocatalysis / Dioxanone / Carbohydrates
A synthetic approach to higher carbohydrates via the se-
quence of two aldol reactions that proceed at the α and αЈ
positions of 2,2-dimethyl-1,3-dioxan-5-one (dioxanone) is de-
scribed. As reported before, the first aldol reaction works
well under organocatalytic conditions (proline catalysis), this
is followed by protection of the hydroxy, deprotonation of the
having a straight-chain carbon skeleton with an oxygen-
based functional group at each carbon. Most of the groups
are protected. The utility of this strategy is illustrated by short
syntheses of 6-C-phenyl-
D-glycero-D-allo-hexopyranose and
D
-erythro- -allooctopyranose
D
resulting compound using excess of LDA, and the second al- (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
dol addition. This sequence of reactions gives compounds
Germany, 2009)
compounds.[3] A number of research groups have published
in this area, especially since it was discovered that diox-
anones are good substrates for organocatalytic aldol reac-
tions,[4] and the relevant chemistry has been reviewed from
two different points of view.[5] The work of Enders’ group
involving SAMP/RAMP derivatives of dioxanones is espe-
cially noteworthy.[5a,6] However, the sequential aldol-aldol
approach to the synthesis of carbohydrate derivatives from
dioxanones has not been fully realized as yet, despite the
fact that some preliminary observations pertaining to such
reactions in the dioxanone system were published a few
years ago.[3c,7] Below, we describe our recent studies on de-
velopment of the dioxanone bisaldol methodology via a se-
quence of two aldol reactions – the first catalyzed by proline
and the second proceeding via the corresponding lithium
enolate and we highlight examples of stereocontrolled syn-
thesis of higher sugars and their derivatives.
Introduction
Carbohydrates having a carbon backbone that is longer
than six carbon atoms (“higher monosaccharides” or
“higher-carbon sugars”) have attracted much attention re-
cently due to their interesting biological properties. Exam-
ples include higher aldoses that occur as chiral fragments
in antibiotics, e.g., hikosamine (C11) and lincosamine (C8),
and ketose-derived sialic acids such as KDO (C8) and KDN
(C9). Even a brief review of relevant chemistry is beyond the
scope of this communication (which focuses on dioxanone-
based synthesis); interested readers are directed to recent
reviews on the subject of synthesis of higher-carbon
sugars;[1a–1d] it should also be noted that two thematic is-
sues of Chemical Reviews are devoted to carbohydrate
chemistry and glycobiology.[1e,1f] Briefly, most of the ap-
proaches to higher monosaccharides involve simple aldoses
as the starting materials and either homologation by adding
one-, two- or three-carbon fragments, or coupling of carbo-
hydrate-derived building blocks as synthetic strategies, but
some “totally synthetic” routes were described as well.[2]
Due to their biological importance sialic acids have been
studied extensively from different vantage points, including
biological studies, modeling, design of non-natural analogs
and stereoselective synthesis.[1b,1g–1k]
Results and Discussion
Our synthetic approach is shown in Scheme 1. The sym-
metrical dioxanone starting material (CS or C2v) was used
as the substrate in the aldol reaction with a suitably chosen
aldehyde. This first aldol reaction could give up to sixteen
stereoisomers (relative stereochemistry being syn or anti
and cis or trans with respect to each stereogenic element in
the structure, if R1 = R2 the number of possible stereoiso-
mers decreases to eight) but it had been established before
that the selectivity of the reaction (both enantio- and dia-
stereoselectivity) could be efficiently controlled to give
either enantiomer of the anti mono-aldol by means of the
enantioselecitve deprotonation methodology,[3] by the chiral
auxiliary method,[6,7] or by organocatalysis with proline.[4]
The challenge was in developing conditions for the second
In the context of carbohydrate synthesis we are interested
in exploring the use of 2,2-dialkyl-1,3-dioxan-5-ones as at-
tractive scaffolds for ketoses and other poly-oxygenated
[a] Department of Chemistry, University of Saskatchewan,
110 Science Pl., Saskatoon, SK, S7N 5C9, Canada
Fax: +1-306-9664730
E-mail: marek.majewski@usask.ca
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
Eur. J. Org. Chem. 2009, 33–37
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
33

I. Niewczas, M. Majewski
SHORT COMMUNICATION
aldol reaction that should proceed in a reasonably high Table 1. Aldol addition reaction of dioxanone derivative 6a to iso-
butyraldehyde (Scheme 2).
yield and diastereoselectivity to give the corresponding bi-
Entry Conditions
Products
Yield (%)[a]
saldol.
1
2
Proline,[b] DMSO, 5 °C
–
–
24
TiCl4,[c]
8 only
(iPr)2EtN
Chx2BCl,[d]
Et3N, –78 °C
MgI2,[e]
3
4
5
7aca/7ata/(7sca + 7sta)
13:86:1
8 only
96
15
74
(iPr)2EtN
LDA[f]
7aca/7ata/(7sca + 7sta)
65:35:0
[a] Combined yield of all isolated products. [b] For details on or-
ganocatalytic conditions cf. ref.[4e] [c] Conditions: cf. ref.[9d] [d] Con-
ditions: cf. ref.[3c] [e] Method description ref.[11] [f] For detailed con-
ditions see the following section.
before,[3,7] predominantly the anti-trans-anti isomer of the
double aldol product 7ata with good selectivity and in good
yield (note that only two of the four possible aldols were
observed within NMR detection limit). However, boron
Scheme 1. Synthetic strategy towards higher aldoses.
The bisaldol moiety (i.e., the β,βЈ-dihydroxycarbonyl enolates did not work well with aldehyde building blocks
fragment) occurs in natural products, such as epothilones that contained sulfur moieties and this limitation rendered
and myriaporones,[8] and a number of synthetic approaches the boron enolate method poorly suited to our synthetic
to compounds containing such juxtaposition of functional objectives (vide infra).
groups were reported.[8,9] We first investigated different con-
We then turned our attention to lithium enolates, the
ditions in the aldol addition reaction of trisisopropylsilyl “tried and true” intermediates in aldol chemistry.[12] Initial
(TIPS) protected pure anti isomer of the mono-aldol 6a attempts did not look promising, but after some experimen-
(Scheme 2, Table 1).
tation we have established that the reaction works well if an
excess of the base and an excess of the aldehyde are used
(Scheme 3, Table 2). The first equivalent of the base pre-
sumably complexes to Lewis basic sites in the substrate and
does not participate in deprotonation, upon addition of the
aldehyde the first molar equivalent of the aldehyde is con-
sumed in the addition reaction with LDA, which is a well
known process.[13]
Scheme 3. The second aldol reaction via lithium enolate.
Table 2. Aldol reaction of lithium enolate of 6a with benzaldehyde
(Scheme 3).
Entry
Equiv.
LDA
Equiv. PhCHO
Yield (%)
9aca/9ata
Scheme 2. The second aldol reaction.
1
2
3
4
5
6
1.1
2.2
3.3
1.1
2.2
3.3
1
1
1
3
3
3
9
14
10
2.7:1
1.9:1
1.8:1
–
8:1
7.3:1
A number of attempts to run the reaction under organo-
catalytic conditions failed (Table 1, Entry 1). This was, per-
haps, not surprising, because if the second aldol addition
was facile one would expect double aldol products to mani-
fest themselves in “normal” proline catalytic cycles and this
has been rarely observed.[10] Reactions involving titanium
–
Ͼ 90
Ͼ 90
Having elaborated the conditions for the second aldol
enolate or magnesium enolate of compound 6a afforded reaction via lithium enolates we then investigated a number
only small amounts of dehydrated double aldol 8. Experi- of sequential processes where the first aldol reaction was
ments involving boron enolates (Entry 3) gave, as reported done under organocatalytic conditions (proline, DMSO,
34
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The α,αЈ-Bisaldol Approach
5 °C, 1–4 d), the product was purified and protected as the compounds.[14] The bisaldol strategy offers a quick access
corresponding TIPS ether (one pure isomer), and was then to these modified carbohydrates as shown in Scheme 4. The
subjected to enolization with LDA (2–3 equiv., THF, anti-cis-anti compound 16aca was obtained as described in
–78 °C), followed by addition of the second aldehyde. The the preceding section and isolated in 50% yield. Reduction
results are summarized in Table 3.
with sodium tris(acetoxy)borohydride[4b,15] gave the corre-
sponding diol in 83% yield and 30:1 diastereoisomeric ratio
(dr). Deprotection of the three hydroxy groups and the for-
myl group proceeded smoothly in “one pot” upon treat-
ment with HCl affording the carbohydrate 18, that was
characterized as the acetate derivative, 6-C-phenyl--gly-
cero--allohexopyranose pentaacetate 19 (25% yield).
Table 3. Bisaldol reactions of dioxanone 1 (R1 = R2 = Me) with
aldehydes 10–15.
Entry R3CHO
R4CHO
Products
aca/ata/(sca + sta)
Yield (%)[a]
1
2
3
4
5
6
7
8
9
10
10
10
10
11
13
13
13
13
13
12
10
11
14
14
10
14
12
11
15
12
63:37:0
80:11:9
86:13:1
63:34:3
64:27:9
91:9:0
78:22:0
64:34:2
70:30:0
98:2:0
56 (68)
99
68 (74)
53
54
86 (97)
45 (79)
75
39 (40)
83 (99)
[a] Combined yield. Values in brackets refer to yields calculated on
the basis of the recovered starting material (BORSM).
Scheme 4. Synthesis of 6-C-phenyl--glycero--allohexopyranose.
D-erythro-D-Allooctopyranose: Compound 20aca was iso-
During this study we paid special attention to aldehydes lated in 74% yield and was reduced to the corresponding
that are useful building blocks for carbohydrate synthesis: diol 21 (57% yield). The aldooctose was then deprotected
monoprotected glyoxal derivatives 13 and 14 and protected and characterized as the acetate derivative 23 (23% yield)
glyceraldehyde 12. While some systems clearly worked bet- Scheme 5.
ter than others, the following points should be noted: (i) in
the best systems (Entries 2, 6 and 7) the aldol addition reac-
tions proceeded in high yield, reasonable selectivity (the
major products anti-cis-anti were obtained as pure com-
pounds after chromatography). While the level of diastereo-
selectivity needs to be improved, the simplicity of this ap-
proach compensates for less than ideal distribution of iso-
meric products. The reaction with formaldehyde is note-
worthy (Entry 9). In our studies with dioxanones we had
never been able to accomplish a reaction with formalde-
hyde, despite trying a number of different conditions includ-
ing organocatalysis. However, this reaction does proceed as
the second aldol addition, albeit in low yield. It should be
noted that a number of isolated aldol anti-cis-anti products
comprise the complete carbon skeleton of carbohydrates
and have the necessary oxygen functional groups in the
right positions. Thus, easy to envisage functional group ma-
Scheme 5. Synthesis of a -erythro--allopyranose derivative.
nipulations of these compounds (reduction-deprotection se-
quences) offer access to hexoses or their C-6 derivatives
(Entries 3–5, 8–9), heptoses (Entry 6), octoses (Entry 7) and
nonoses (Entry 10). We highlight selected conversions of
these bisaldol products to the corresponding carbohydrates
Conclusions
below.
In summary the two sequential aldol reactions on the
Synthesis of 6-C-Phenyl-D-glycero-
D-allo-hexopyranose: dioxanone scaffold, the first catalyzed by proline and the
Derivatives of simple aldohexoses having an alkyl or aryl second involving the lithium enolate of the protected first
group connected to C-6 are important biologically active aldol product, proceeded stereoselectively and formed a
Eur. J. Org. Chem. 2009, 33–37
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35

I. Niewczas, M. Majewski
SHORT COMMUNICATION
cornerstone of the synthetic strategy towards diastero- and
enantioselective synthesis of higher carbohydrates. In this
system proline organocatalysis and the lithium enolate
methodology are complementing each other. We are work-
ing on expanding this methodology to allow access to more
carbohydrate stereoisomers at higher levels of selectivity.
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Experimental Section
Second Aldol Reaction. Typical Procedure
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4-(1Ј-tert-Butyldimethylsilyloxy-2Ј,2Ј-dimethoxy)ethyl-6-[hydroxy-
(phenyl)]methyl-2,2-dimethyl-1,3-dioxan-5-one
(16aca):
nBuLi
(0.37 mL, 0.88 mmol, 2.38  solution in hexanes) was added drop-
wise to a stirred solution of diisopropylamine (0.18 mL, 0.96 mmol,
2.4 equiv.) in THF (10 mL) at 0 °C under nitrogen. After 30 min
the mixture was cooled to –78 °C and a solution of the TBS-pro-
tected aldol substrate (139 mg, 0.40 mmol, 1.00 equiv.) in THF was
added slowly. The mixture was stirred for 40 min at –78 °C.
PhCHO (110 mg, 0.11 mL, 1.00 mmol, 2.5 equiv.) was then added.
After 20 min the reaction was quenched with concentrated phos-
phate buffer (pH 7.5; 10 mL) and extracted three times with diethyl
ether. The combined organic layers were rinsed with saturated
NaCl, dried with MgSO4, concentrated, and fractionated by FCC
(3–7% ethyl acetate in hexane) to give 16ata (44.5 mg, 0.10 mmol,
25%) as a pale yellow liquid, and 16aca (89.7 mg, 0.30 mmol, 50%)
as a pale yellow liquid. Diastereoselectivity of the reaction was
measured on the crude product by integration of peaks in 1H
NMR: δ = 5.12 (dd, J = 4.6 Hz), 4.89 (d, J = 8.4 Hz) 4.78 ppm (d,
J = 8.4 Hz) and was found to be 2:64:34 syn/aca/ata.
[3]
[4]
16aca: [α]2D2 = +17 (c = 1.5, CHCl ). IR: ν = 3535, 1737 cm–1. H
1
˜
3
NMR (500 MHz, CDCl3): δ = 7.42–7.24 (m, 5 H), 4.89 (d, J =
8.4 Hz, 1 H), 4.59 (d, J = 7.5 Hz, 1 H), 4.44 (dd, J1 = 1.2, J2
=
1.8 Hz, 1 H), 4.15 (dd, J1 = 1.2, J2 = 8.4 Hz, 1 H), 4.12 (dd, J1 =
1.9, J2 = 7.5 Hz, 1 H), 4.06 (br. s, 1 H), 3.46 (s, 3 H), 3.45 (s, 3 H),
1.41 (s, 3 H), 1.38 (s, 3 H) 0.90 (s, 9 H), 0.15 (s, 3 H), 0.11 (s, 3 H)
ppm. 13C NMR (125 MHz, CDCl3): δ = 208.97, 139.8, 128.1, 128.0,
127.6, 105.7, 98.7, 79.8, 79.1, 74.7, 73.7, 56.2, 56.0, 28.8, 26.1, 26.0,
20.6, 18.4, –4.2, –4.4 ppm. HRMS m/z calcd. for C23H38O7Si
472.2731 [M + NH4], found 472.2729 (CI).
[5]
[6]
[7]
[8]
16ata: [α]2D9 = –64 (c = 1.85, CHCl ). IR: ν = 3535, 1737 cm–1. H
1
˜
3
NMR (500 MHz, CDCl3): δ = 7.41–7.24 (m, 5 H), 4.78 (dd, J1 =
1.7, J2 = 8.4 Hz, 1 H), 4.52 (d, J = 7.5 Hz, 1 H), 4.31 (dd, J1 = 1.2,
J2 = 1.7 Hz, 1 H), 4.15 (dd, J1 = 1.2, J2 = 8.4 Hz, 1 H), 4.09 (dd,
J1 = 1.7, J2 = 7.5 Hz, 1 H), 3.63 (d, J = 1.7 Hz, 1 H), 3.45 (s, 3 H),
3.37 (s, 3 H), 1.32 (s, 3 H), 1.17 (s, 3 H), 0.88 (s, 9 H), 0.1 (s, 3 H),
0.09 (s, 3 H) ppm. 13C NMR (125 MHz, CDCl3): δ = 209.3,139.9,
128.2, 128.0, 127.2, 105.4, 101.7, 77.9, 75.1, 73.8, 72.4, 56.5, 55.5,
26.2, 26.0, 24.0, 23.5, 18.3, –4.3, –4.7 ppm. HRMS m/z calcd. for
C23H38O7Si 472.2731 [M + NH4], found 472.2731 (CI).
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this article): Procedures and spectroscopic data for for bis-aldols
and carbohydrate derivatives.
Acknowledgments
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Council of Canada for funding and the Saskatchewan Structural
Sciences Center for help with analytical and crystallographic mea-
surements.
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The α,αЈ-Bisaldol Approach
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Received: September 2, 2008
Published Online: November 21, 2008
Eur. J. Org. Chem. 2009, 33–37
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