Synthesis of Bradyrhizose from D-Glucose

Abstract

We describe the synthesis of the unusual bicyclic sugar bradyrhizose in 14 steps and 6% overall yield from D-glucose. The synthesis involves the elaboration of a trans-fused carbocyclic ring onto the pre-existing glucopyranose framework followed by adjustment of the oxidation levels. Key steps include radical extension of the glucopyranose side chain, ring closing metathesis, allylic oxidation, Luche reduction, hydroxy-directed epoxidation and acid-catalyzed epoxide opening at the more substituted position.

Graphical Abstract

Bradyrhizose, a rare bicyclic sugar, is an important component of the lipopolysaccharide O-antigen side chain of Bradyrhizobium species (BTAil and ORS278 strains). As a molecular signal, it is thought to be involved in facilitating the fixation of nitrogen in tropical leguminous Aeschynomenes species especially Aeschynomene indica and sensitiva through a symbiotic process.^1–5 Isolation and characterization of this sugar revealed that the monomeric unit is made up of an inositol-type backbone which is trans-fused to a pyranose ring resulting in a bicyclic compound, whose α-(1→7)-linked oligomer adopts a compact two-fold right-handed helicoidal structure.⁶

The unusual structural of bradyrhizose coupled with the need to investigate its biological properties have stimulated interest in its chemical synthesis. A first synthesis was achieved by Yu and coworkers in 9% overall yield and 26 steps from D-glucal; the key steps being Ferrier rearrangement of a ramified glucopyranoside derivative and subsequent adjustment of the oxidation levels (Scheme 1).⁷ A second synthesis was reported by Lowary and coworkers, who employed myo-inositol as starting material, to which a glucopyranose ring was appended with an overall yield of 6% in 25 steps (Scheme 2).⁸ The construction of bicyclic mimetics of bradyrhizose and of related compounds has also been reported.⁹

Scheme 1.

Key steps of the Yu synthesis from D-glucal

Scheme 2.

Key steps of the Lowary synthesis from myo-inositol

Our laboratory has been interested in the synthesis of structurally related oxabicyclic motifs in connection with studies on the influence of pyranose side chain conformation on anomeric reactivity,¹⁰ and as analogues of the structurally unique aminoglycoside antibiotic apramycin with its broad spectrum of activity toward multidrug, carbapenem and aminoglycoside resistant enterobacteriaceae and ESKAPE pathogens.^{11, 12, 13} Building on aspects of the chemistry developed in these projects, and inspired by earlier syntheses of apramycin,^{14, 15} and of inositol variants from cis-arene glycols by the Carless,¹⁶ Ley,¹⁷ Hudlicky,¹⁸ and Motherwell laboratories,¹⁹ we envisioned a synthesis of bradyrhizose from glucose comprised of i) chain elongation of at the glucopyranose 6-position; ii) annelation of an unsaturated six-membered carbocyclic ring spanning the glucopyranose 4- and 6-positions; and iii) adjustment of the oxidation level in the newly appended ring to afford the target compound. We report here on the reduction of this concept to practice, resulting in a 14 step synthesis of bradyrhizose from D-glucose.

Following the literature protocol, D-glucose 1 was subjected to Fischer glycosylation²⁰ in benzyl alcohol at reflux giving 2. Subsequent selective installation of a 4,6-O-benzylidene acetal under standard conditions gave 3α in 85% yield and the corresponding β-anomer in 6% yield.²¹ In principle a mixture of the two anomeris of 3 could be advanced through the synthesis, but in order to simplify the NMR spectra only the major anomer was taken forward. Thus, the α-anomer of 3 was subjected to benzylation with benzyl bromide and sodium hydride at 0 °C affording 4.²² Acid-catalyzed removal of the benzylidene acetal,²³ was followed by selective Appel²⁴ iodination of the primary hydroxyl group resulting in compound 6 in excellent yield (Scheme 3).

Scheme 3.

Derivatization of D-glucose

With 6 in hand, we explored conditions for the side chain elongation with various methallylation protocols. Attempts at cross coupling reactions using methallyl Grignard reagents in the presence of metal catalysts such as (Pd(PPh₃))_4, CuI,²⁵ or Li₂CuCl₄²⁶ resulted only in the recovery of starting materials. Consequently, we turned our attention to the use of radical allylation reactions,²⁷ and in particular the use of methallylsulfones (Table 1). The use of triethylborane²⁸ as initiator in the presence of oxygen at room temperature and below was inefficient and resulted in the formation of significant quantities of the simple reduction product 8. Attention was therefore focused on thermal conditions, with initiation by lauroyl peroxide, and azoisiobutyronitrile. α,α,α-Trifluorotoluene was selected as an environmentally friendly radical-compatible solvent for these reactions.²⁹ The use of ethyl methallyl sulfone under conditions prescribed by Zard and coworkers³⁰ gave only a low yield of the desired product. The optimum yield of 68% (Table 1, entry 6), however, was obtained under photocatalytic conditions with the blue light irradiation of 6 and 2-pyridyl methallyl sulfone in acetonitrile with fac-Ir(ppy)₃ as the catalyst.³¹

Table 1.

Radical homologation of 6 to give 7

entry	sulfone (R)	initiator	solvent	temp (°C)	% yield 7	% yield 8
1	2-pyridyl	Et3B	PhCF3	0	40	51
2	2-pyridyl	lauroyl peroxide	PhCF3	80	54	36
3	2-pyridyl	AIBN	PhCF3	80	0	0
4	3-pyridyl	lauroyl peroxide	PhCF3	80	50	38
5	ethyl	AIBN	heptane/PhCl	80	10	6
6	2-pyridyl	fac-Ir(ppy)3 (5 mol%), Bu3N	acetonitrile	rt	68a	13

^a)Conducted under argon with irradiation by 12V blue LED strips at room temperature.

Subsequently, oxidation of 7 under Parikh-Doering conditions³² gave ketone 9 in good yield, which on treatment with with vinyl magnesium bromide afforded alcohol 10 in 97% yield as a single diastereoisomer consistent with earlier observations in related systems.³³ The requisite bicyclic scaffold was obtained by ring closing methathesis with the Grubbs 2^nd generation catalyst in dichloromethane at 45 °C, affording 11 in 85% yield (Scheme 4), and consistent with the literature precedent for the effective formation of trisubstituted cyclohexenes from methallyl type precursors.³⁴

Scheme 4.

Construction of the bicyclic scaffold

Allylic oxidation of 11 with SeO₂ in 1,4 dioxane at 80 °C gave enone 12 in 70% yield, with regioselectivity in agreement with Guillemonat’s rules and subsequent work.³⁵ Regio- and stereoselective reduction of the enone derivative 12 was achieved under Luche’s conditions,³⁶ resulting in the isolation of the allylic alcohol 13 in 86% yield as a single diastereomer. Treatment of 13 with m-CPBA at room temeprature in dichloromethane gave epoxide 14 in 83% yield as a single diastereoisomer, as a result of steering by hydrogen bonding by either of the two allylic alcohols (Scheme 5).³⁷ The relative configuration of 14 was confirmed by the observation of strong NOE correlations of H9 with each of the C8-methyl group, H3, H5 and H7.

Scheme 5.

Adjustment of oxidation levels and completion of the synthesis.

The regioselective opening of 14 was explored under a variety of acidic conditions (Table 2). In dry solvents the enone 12 was returned as the only product, as the result of a 1,2-hydride shift followed by β-elimination. Reactions did not proceed under simple aqueous acidic conditions without excessive heating for reasons of insolubility. The use of aqueous 1,4-dixoane as solvent proved more successful when the mixture was adjusted to contain the maximum amount of water consistent with a homogeneous solution. The desired regio- and stereoselective ring opening of the epoxide was best achieved by employing sulfuric acid as catalyst in a 1:3 dioxane:water mixture at 65 °C (Table 2, entry 4) when the tetraol 15 was obtained in 50% yield alongside 45% of the enone 12, which could be recycled. The relative configuration of 15 rests on the spatial proximity of H9 to H3, H5 and H7, and of the axial C8 methyl group to H6_axial, as established by the presence of strong NOE correlations.

Table 2.

Optimization of the epoxide ring opening

entry	reagent	temp (°C)	solvent	% yield 16	% yield 13
1	wet triflic acid	0–45	dichloromethane	0	0
2	conc H2SO4	65	1,4-dioxane:water (1:1)	20	68
3	conc H2SO4	65	1,4-dioxane:water (1:2)	37	35
4	conc H2SO4	65	1,4-dioxane:water (1:3)	50	45
5	conc H2SO4	65	1,4-dioxane:water (1:4)	13	0
6	conc H2SO4	65	1,4-dioxane:water (1:6)	10	0
7	Sc(SO3CF3)3	rt-45	THF	0	0

While the regiochemical outcome of the epoxide opening under acid-catalyzed conditions deviates from the Fürst-Plattner rule,³⁸ it was anticipated on the basis of the well-known anti-attack of nucleophiles at the more substituted position of trisubstituted epoxides under acidic conditions (Scheme 6).³⁹

Scheme 6.

Regio and stereoselective epoxide ring opening

The synthesis of bradyrhizose was completed by hydrogenolysis of the benzyl ethers over 10% Pd/C in methanol in essentially quantitative yield (Scheme 5). The spectral data of the synthetic material were consistent with the literature data and the existence of bradyrhizose as a mutarotating mixture of the two pyranose forms and two furanose forms, rich in the former.⁷

In conclusion, we have succeeded in developing an effective synthesis of bradyrhizose from D-glucose in 14 steps and 6% overall yield. The approach employed differs conceptually from the existing syntheses in that it is based on the elaboration of the carbocyclic ring onto an existing pyranose framework rather the fusion of a pyranose ring onto a preformed inositol skeleton; it is also some ten steps shorter than either of the two existing syntheses. Relatively minor variations on the sequence of reactions from the key bicyclic intermediate 12 are expected to afford a variety of stereoisomers of bradyrhizose itself.