doi.org/10.26434/chemrxiv.7853528.v1

Programmable Synthesis of 2-DeoxyglycosidesSeth Herzon, Kevin M. Hoang, Nicholas Lees

Submitted date: 15/03/2019 • Posted date: 18/03/2019Licence: CC BY-NC-ND 4.0Citation information: Herzon, Seth; Hoang, Kevin M.; Lees, Nicholas (2019): Programmable Synthesis of2-Deoxyglycosides. ChemRxiv. Preprint.

Control of glycoside bond stereochemistry is the central challenge in the synthesis of oligosaccharides.2-Deoxyglycosides, which lack a C2 substituent to guide stereoselectivity, are among the most difficultclasses of glycoside bond constructions. Here we present a method to synthesize 2-deoxysaccharides withspecified glycoside bond stereochemistry using a nucleophilic carbohydrate residue and the syntheticequivalent of an alcohol electrophile. Because the configuration of the nucleophile can be precisely controlled,both α- and β-glycosides can be synthesized from the same starting material in nearly all cases examined.Stereoselectivities in these reactions are often greater than 50:1 and yields typically exceed 70%. Thisstrategy is amenable to the stereocontrolled syntheses of trisaccharide diastereomers, and a tetrasaccharide.Our method offers a fundamentally new approach to O-glycoside synthesis to enable downstreambiochemical and natural product applications.

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Programmable synthesis of 2-deoxyglycosides.

Kevin M. Hoang,1 Nicholas R. Lees,1 and Seth B. Herzon*1,2

1Department of Chemistry, Yale University, New Haven, Connecticut 06520, United States.

2Department of Pharmacology, Yale School of Medicine, New Haven, Connecticut 06520,

United States.

*Correspondence to: seth.herzon@yale.edu.

Abstract: Control of glycoside bond stereochemistry is the central challenge in the synthesis of

oligosaccharides. 2-Deoxyglycosides, which lack a C2 substituent to guide stereoselectivity, are

among the most difficult classes of glycoside bond constructions. Here we present a method to

synthesize 2-deoxysaccharides with specified glycoside bond stereochemistry using a

nucleophilic carbohydrate residue and the synthetic equivalent of an alcohol electrophile.

Because the configuration of the nucleophile can be precisely controlled, both α- and β-

glycosides can be synthesized from the same starting material in nearly all cases examined.

Stereoselectivities in these reactions are often greater than 50:1 and yields typically exceed 70%.

This strategy is amenable to the stereocontrolled syntheses of trisaccharide diastereomers, and a

tetrasaccharide. Our method offers a fundamentally new approach to O-glycoside synthesis to

enable downstream biochemical and natural product applications.

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Text:

O-Glycosides are found in the extracellular matrix of all living organisms and mediate a wide

range of biological processes,1 and many small molecule natural products contain glycoside

residues that are essential for biological activity.2 In contrast to amide and phosphate bonds, O-

glycoside bonds can exist as two stable non-interconverting isomers, termed α and β, that differ

at the C1 (anomeric) position. Because these diastereoisomers have different physical and

biochemical properties, glycoside bond stereochemistry has been called the “central topic” of

synthetic carbohydrate chemistry.3

Most synthetic glycosylations involve the addition of a nucleophilic alcohol (acceptor) to a

carbohydrate electrophile (donor, Fig. 1A) and lie along a continuum bound by SN1 and SN2

pathways.4 The anomeric stereoselectivity of the reaction then derives from the facial selectivity

in this addition. Many effective strategies to control selectivity based on the nature of the C2

substituent have now been developed.5 2-Deoxyglycosides, such as olivose and digitoxose, are

an important constituent of biological polysaccharides and lack a C2 oxygen atom.6 In the

absence of this substituent, control of anomeric stereochemistry is considerably more

challenging. In particular, synthetic methods to directly access the thermodynamically- and

kinetically-

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Fig. 1. A. The most common approach to the synthesis of O-glycosides involves the addition of a

nucleophilic alcohol to an electrophilic carbohydrate donor. When X H, the C2 substituent may

be used strategically to control diastereoselectivity in the reaction. B. Anomeric anions can be

generated as α- or β-diastereomers by reductive lithiation of thiophenyl glycosides. We envisioned

that reaction with an electrophilic alcohol equivalent would provide access to either α- or β-

glycoside products.

disfavored β-glycoside are rare, but this linkage is very common in nature [for selected

examples, see refs. 7; for a review, see: 8]. Perhaps more significantly, methods that provide

controlled access to either diastereomer from a single precursor do not exist, to our knowledge,

although nature flexibly combines both linkages into single polymers.

We considered alternative mechanistic manifolds to circumvent the recurring challenges

associated with the direct synthesis of 2-deoxyglycosides. After evaluating several strategies, we

developed an approach to 2-deoxyglycosides based on the coupling of a nucleophilic

carbohydrate C1 anion with the synthetic equivalent of an alcohol electrophile (Fig. 1B). This

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approach was inspired by several key findings. In 1980, Cohen reported that axial 2-

lithiotetrahydropyrans are accessible by reductive lithiation of 2-thiophenyl-tetrahydropyrans9

and later demonstrated10 that the kinetically-favored axial anion could be thermally equilibrated

to the equatorial diastereomer in conformationally-restricted pyrans. In another important

advance, Rychnovsky showed that both axial and equatorial anions were accessible in high

diastereoselectivity from a range of flexible monocyclic tetrahydropyrans, including a glycoside

derivative.11 Anomeric anions undergo stereoretentive addition to a range of electrophiles,12 and

have found use in the preparation glycosyl sulfides.13 Thus, we recognized that these anomeric

anions might form the basis for a method to access both α- and β-2-deoxyglycosides from a

single starting material. Additionally, the configuration of the starting thiophenyl glycoside is

inconsequential because the reductive lithiation proceeds by stepwise electron transfer.10 This

anomeric anion approach constitutes a general strategy to build 2-deoxysaccharides of specified

glycoside stereochemistry.

To realize this strategy, the synthetic equivalent of an alcohol electrophile needed to be

identified. Reagents that transfer “R–O+” to carbanions are rare. Tetrahydropyranyl (THP)

monoperoxy acetals, pioneered by Dussault, have emerged as perhaps the most useful and

general reagents for formal alkoxenium ion transfer to organolithium and Grignard reagents

(typical yields of 50–70%).14 Using these as a starting point, we evaluated the reductive lithiation

of 3,4-dimethyl-1-thiophenyl-L-olivose (1, Fig. 2A) with lithium di-tert-butylbiphenylide

(LiDBB),15 followed by the addition of benzyl tetrahydropyranyl monoperoxy acetal (2). Under

these conditions, the product 3 was isolated in 6% yield as a single detectable α-diastereomer (1H

NMR analysis). Benzaldehyde and the protodethiolated sugar 4 were each formed in 34% yield,

presumably by Kornblum–DeLaMare elimination.16 To circumvent proton abstraction from the

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acidic benzylic position, we employed the 3-phenylpropanol monoperoxy acetal 5a (Fig. 2B).

Addition of 5a to the α-anion derived from reductive lithiation of 1 provided the α-glycoside 6 in

81% yield and >50:1 α:β selectivity. A 91% yield of 6 (>50:1 α:β) was obtained when the novel

2-methyl-tetrahydrofuranyl (MTHP)

Fig. 2. A–C. Optimization of the glycosylation of the α-anomeric anion derived from 1 using alkyl

THP and alkyl MTHP monoperoxy acetals. D. Optimized conditions for the generation of the β-

glycoside 8β (see Table S1 for optimization studies).

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Fig. 3. Preliminary scope of the inverse glycosylation reaction. A. Structures of thiophenyl

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glycosides used in this study. B. Structures of the MTHP monoperoxy acetals used in this study.

C. Products derived from coupling of 2,6-dideoxy donors and primary MTHP monoperoxy

acetals. D. Products derived from 2,6-dideoxydonors and secondary MTHP monoperoxy acetals.

E. Products derived from 2-deoxy donors and primary MTHP monoperoxy acetals. F. Products

derived from 2-deoxy donors and secondary MTHP monoperoxy acetals. G. Products derived

from 2-deoxy-6-hydroxy donors and primary MTHP monoperoxy acetals.

monoperoxy acetal 5b was used as the electrophile, presumably by suppressing minor amounts

of elimination by proton abstraction from the THP ring. This modification may also improve the

yields of product when more hindered coupling partners are employed (see below). To test the

feasibility of this reaction in the synthesis of a disaccharide, we prepared the carbohydrate-

derived MTHP monoperoxy acetal 7 (Fig. 2C). Glycosylation with the anomeric anion derived

from 1 generated the product 8α in 77% yield and >50:1 dr. Increasing the reaction time to 3 h

increased the yield of the product to 89% (>50:1 α:β).

We then proceeded to optimize formation of the β-product with respect to yield and

stereoselectivity (Fig. 2D and Table S1). The α-anion formed upon reductive lithiation of 1 at –

78 °C could be equilibrated to the β-isomer by warming to –20 °C. In accord with Rychnovsky’s

observations,11 we found that a tetrahydrofuran–pentane solvent mixture provided the fastest

equilibration rates while minimizing proton transfer from the solvent. Under optimized

conditions, the β-linked product 8β was obtained in 82% yield and >50:1 dr by reductive

lithiation of 1 (1.5 equiv), equilibration to the β-anion (–20 °C, 1 h), re-cooling to –78 °C, and

addition of the MTHP monoperoxy acetal 7.

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The preliminary scope of this glycosylation reaction is shown in Fig. 3. The 2,6-

dideoxyglycosides 8 and 16 were obtained in 79–90% yield using the primary MTHP

monoperoxy acetal 7 as the electrophile and the thiophenyl glycosides 1 or 9 as pronucleophiles

(Fig. 3C). The secondary MTHP monoperoxy acetals 14 and 15 were also competent and

provided the α- or β-linked 2,6- dideoxyglycosides 17–20 in 53–90% yield (Fig. 5D). As

expected, the less-hindered MTHP monoperoxy acetal 14 transformed more efficiently than 15.

The selectivities in the formation of 8 and 16–20 exceeded 19:1, and were >50:1 for most

products.

2-Deoxydonors such as 10, 11, and 12 also reacted smoothly. For example, both the α- and β-

products derived from reaction of the trimethoxy donor 10 with the primary MTHP monoperoxy

acetal 7 were prepared in high yield and with complete control of anomeric stereochemistry

(21α: 84%, >50:1; 21β: 75% <1:20). However, reactions of the 2-deoxydonor 10 with secondary

MTHP monoperoxy acetals were more challenging (Fig. 3F). While the α-linked products 24α

and 25α were obtained in 85% (>50:1 α:β ) and 37% (>50:1 α:β) yields, respectively, reactions

of the β-anions proceeded more slowly. The yields of the β-linked products 24β or 25β were

53% and 21%, respectively, although the stereoselectivities remained high (<1:50 α:β). The α-

linked glycosides 22α and 23α could be prepared in 80% (>30:1 α:β) and 74% (>30:1 α:β)

yields, respectively, but attempts to form the diastereomeric β-products were unsuccessful.

To gain insight into the relative reactivity of the 2-deoxy and 2,6-dideoxy donors we calculated

the energies of the C1-lithio derivatives in the gas phase (MP2/6-311+G(d,p); Fig. 4). For both

series’, the β-anion was found to be lower in energy than the α-anion, as expected, but the

conformations of the lowest energy β-anions were distinct. In the 2,6-dideoxyglycoside series,

the equatorial β-anion was more stable than the axial α-anion and the axial β-anion by 1.6 and

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Fig. 4. A. Relative energies and structures of the β axial, α axial, and β equatorial 2-

deoxyglycoside anions. B. Relative energies and structures of the β equatorial, β axial, and α

axial 2,6-dideoxyglycoside anions. All geometries and energies were optimized using the

MP2/6-311+G(d,p) level of theory in the gas phase. C. Single-flask synthesis of the

trisaccharides 27α,α, 27β,α, 25α,β, and the tetrasaccharide 28α,α,α.

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3.0 kcal/mol, respectively. By comparison, in the 2-deoxyglycoside series, the axial β-anion was

calculated to be 10.7 kcal/mol lower in energy than the equatorial conformer, and 7.2 kcal/mol

lower in energy than the axial α-anion (Fig. 4A). Thus, in the 2-deoxyseries the more stable β-

anion adopts an all-axial chair conformation due to a stabilizing interaction between the C6(O)

and C2(O) substituents with the axial lithium atom (interatomic distances = 1.91 and 1.97 Å,

respectively). The formation of this unexpected yet more stable conformation provides a

plausible explanation for the lower reactivity of these substrates.

To investigate if conformationally-restricting the system might inhibit formation of the stable,

all-axial β-anion, we prepared the 2,3-butanedione acetal derivatives 13a and 13b (see Fig. 3A).

However, both substrates failed to produce the desired β-linked disaccharides. These results

point to a deeper effect invoked by the C6 oxygen atom. Further investigation led us to the C6

free hydroxy derivative 13c, whereupon we reasoned that prior removal of the acidic C6

hydroxyl proton would allow the glycosylation to proceed. To our delight, we were able to

restore β-reactivity and synthesize the glycosides 26α and 26β in 77% and 68% yields,

respectively, with >50:1 α:β and <1:8 α:β stereoselectivities. Furthermore, the presence of free

hydroxyl groups (and especially a primary C6 hydroxyl group) in synthetic glycosylations is

rare.17 Thus, this approach provides a handle for installation of a third sugar and suggests that

application of this chemistry to 2-hydroxy or 2-acetamido thioglycosides may also be possible.

We recognized that the addition of carbohydrate-based MTHP monoperoxy acetals containing an

anomeric thiophenyl residue (see 14 and 15, Fig. 3B) might allow us to extend our method to

sequential glycosylations in one flask. The synthesis of oligosaccharides in one flask from

carbohydrate monomers is a longstanding goal in synthetic carbohydrate chemistry, and

significant advances toward this end have been recorded.18 However, in most instances

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stereocontrol derives from the mode of activation or carbohydrate protecting group strategy. Our

approach was inspired by Aggarwal’s landmark studies demonstrating the power and control of

iterative lithiation–addition reactions,19 and we envisioned the generation of oligosaccharides

with control over the newly-formed glycoside linkages would be possible.

Toward this end, we synthesized the products shown in Fig. 4C. Reductive lithiation of 1 and

addition of the MTHP monoperoxy acetal 14, followed by sequential reductive lithiation of the

resulting disaccharide and addition of the MTHP monoperoxy acetal 7, resulted in the formation

of the α,α-linked trisaccharide 27α,α in 54% yield as a single detectable diastereomer by 1H

NMR (>20:1:1:1 dr, >73% per iteration). As a benchmark, Kahne’s pioneering sulfoxide-based

one-step synthesis of the related α,α-linked cyclamycin trisaccharide proceeds in 25% yield.18b

Following our equilibration procedure for the first glycosylation, the diastereomer 27β,α was

also accessible (30%, >20:1:1:1 dr). Alternatively, reductive lithiation of 1 and addition of the

MTHP monoperoxy acetal 14, followed by a second reductive lithiation and equilibration to the

β-anion, provided 27α,β in 13% yield (>20:1:1:1 dr). The lower yield obtained when the second

glycosylation involves equilibration to the β diastereomer was not entirely unexpected; in earlier

optimization studies, we had observed that the MTHP leaving group can act as an electrophile

and proton source. Warming to −20 °C in the presence of this leaving group from the first

iteration results in competitive protonation of the anomeric anion (we recovered 43% of

protodethiolated disaccharide). Alternative electrophiles that circumvent this issue are currently

under development. Finally, by executing three sequential reductive lithiations using the MTHP

monoperoxy acetal 7 twice as a building block we obtained the tetrasaccharide 28α,α,α in 25%

yield (>20:1:1:1:1:1:1:1 dr).

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In conclusion, we have developed a novel O-glycosylation protocol that provides efficient

stereocontrolled access to both α and β anomers of 2-deoxy and 2,6-dideoxyglycoside products

with high yield and exceptionally high diastereoselectivities from simple carbohydrate donors

that are free of directing groups and glycosyl promotors. Additionally, computational studies

have proven useful in elucidating the conformations and understanding the relative reactivity of

anomeric anions, and we used these to guide the glycosylation of carbohydrates bearing a C6

hydroxyl group. We envision this methodology will be amenable to the iterative construction of

polysaccharides with control over anomeric stereochemistry.

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Acknowledgments: Financial support from Yale University and the National Science

Foundation (Graduate Research Fellowship to K.M.H.) is gratefully acknowledged. We thank

Dr. Fabian Menges for HRMS analysis.

Author contributions: K.M.H., N.R.L., and S.B.H. designed the study. K.M.H. and N.R.L.

conducted the experiments. K.M.H., N.R.L., and S.B.H. wrote the manuscript.

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