Unexpected Formation of Oxetanes during the Synthesis of Dodeco-6,7-diuloses

Abstract

During the synthesis of symmetrical dodeco-6,7-diuloses that are potential candidates for inhibition of glycosidases, an unanticipated epoxide-oxetane rearrangement was observed. A bicyclic sugar consisting of a glycal moiety and an anomeric esterified furanose was oxidized under epoxidation conditions (mCPBA/KF). The isolation of the pure epoxide was not possible since a rapid reversible conversion accompanied by the migration of the ester group took place and resulted in the formation of an unusual oxetane-bridged disaccharide scaffold. X-ray diffractometric structure elucidation and the suggested mechanism of the rearrangement are provided.

1. Introduction

Oxetane rings attached directly to a carbohydrate unit rarely occur in literature, due to steric reasons [1]. The most common representatives are 3,5-anhydrofuranose derivatives, which are formed by nucleophilic [2,3] or additive [4] intramolecular ring closure reactions. In addition to the furanoid 3,5-linked oxetanes, there also exists a limited number of 1,2-fused or anomeric spiro-oxetanes having a pyranoid constitution. These highly strained scaffolds are prepared photochemically [5,6,7] or via template-directing C-glycosylations [8,9]. In synthesis, sugar oxetanes are used as versatile precursors since the rigid conformation and steric repulsion of the bridge favor high stereoselectivities [10]. Furthermore, the detection of antimicrobial properties has brought these structures closer to the focus of current research [4]. In this communication, we introduce a new method for the generation of anomerically bridged oxetanes whose formation was initiated by an unexpected rearrangement of an epoxide assisted by an adjacent ester group.

2. Results and Discussion

Starting from the known compounds 1 [11] and 2 [12] a base-induced coupling of two anomeric centers analogous to Shiozaki’s protocol [13] was performed (Scheme 1). Contrary to Shiozaki’s procedure, which involved the use of toxic stannylated glycals, we could employ 1-phenylsulfinyl glycals since it is known that they are easily lithiated in a similar fashion with one equivalent of phenyllithium [14]. Sulfoxide 1 was treated with phenyllithium at –78 °C followed by quenching with manno-lactone 2 after 5 minutes. The observance of this reaction time is mandatory since a longer reaction time results in the irreversible formation of the protonated glycal species in increasing amounts leading to by-products. The coupled disaccharide 3 was isolated as an anomeric mixture (α/β ratio 1:5) in 80% yield. Next, the anomeric hydroxyl group was masked in order to avoid its coordinating effect on the envisaged stereoselective epoxidation. For this purpose, we decided to choose the ester protecting groups acetyl and benzoyl. Thus, the fully protected disaccharides 4a and 4b were obtained after the acylation step in 96% and 68% yield, respectively. In both cases solely the β-anomers were obtained which was proven by H,H-NOESY NMR spectroscopy (see Supplementary Materials).

Furthermore, the configuration could be confirmed by growing crystals of 5a suitable for X-ray diffraction by recrystallization of the latter from n-hexane and ethyl acetate. Compound 5a was obtained through desilylation of 4a with tetra-n-butylammonium fluoride (Scheme 1). The moderate yield of 71% can be explained by detectable partial cleavage of the acetyl group in alkaline milieu during desilylation. The benzoyl protecting group was significantly more stable under the same conditions resulting in a yield of 89% for the conversion of 4b to 5b. In our efforts to synthesize symmetrical dodeco-6,7-diuloses both sugar moieties need to be manno-configured. Therefore, the next step was to stereoselectively convert the glycal entity into a mannose derivative. Conventional methods for the implementation of stereoselective dihydroxylation on similar allyl alcohol systems use substrate-directing reagents such as molybdenum catalysts [15], vanadyl acetylacetonate (VO(acac)₂) [16,17] or meta-chloroperbenzoic acid (mCPBA) [17,18] for epoxidation. Subsequent epoxide opening leads to the desired diol derivatives. When these methodologies were applied to disaccharides 5a and 5b the best results were achieved using the Camps reagent (mCPBA/KF) [18,19]. The addition of potassium fluoride reduces the solubility of mCPBA and mCBA and thus, inhibiting nucleophilic epoxide opening by the acids. However, we could not isolate the pure epoxides 6a and 6b for a rapid rearrangement of the latter to oxetane species 7a and 7b occurred. Upon early quenching of the epoxidation reaction, only 8:1 mixtures of epoxide and oxetane could be isolated in yields of 94% for 6a and 90% for 6b, respectively. In order to enable the complete conversion of the epoxide to the oxetane derivatives the reaction times were extended and 7a and 7b could be obtained neatly in 70% and 63% yields, respectively. Crystals of compound 7a suitable for X-ray crystallography could be obtained by overlaying a saturated solution of 7a in methylene chloride with n-heptane and slowly evaporating the methylene chloride. Thus, the oxetane-bridged disaccharide structure of 7a with two anomeric α-configurations could be unambiguously verified (Scheme 1). For the formation of oxetanes 7 we propose a mechanism deduced from the X-ray data in which ester group migration from position 7 to 6 occurs simultaneously with O-heterocyclic ring extension of the oxirane ring to the less strained oxetane ring (Figure 1). The rearrangement process can also be visualized by ¹H-NMR spectroscopy since the H-5 signal of oxetane 7a (4.96 ppm, 4.6 Hz) is significantly shifted to lower field compared to the characteristic doublet of H-5 in epoxide 6a (3.66 ppm, 2.4 Hz) (Figure 1). To some extent, this rearrangement resembles the Ferrier rearrangement of acetylated glycals. Alternatively, a stepwise mechanism via the intermediate formation of carbenium ions can also be envisaged.

It must also be mentioned that the rearrangement appeared to be reversible for in attempts to recrystallize 7a from boiling n-hexane the precipitation of an amorphous substance which turned out to be epoxide 6a was observed. Studies are ongoing to evaluate both the effect of the temperature and of the solvent on the equilibrium ratio between compounds 6a and 7a. After removal of the ester protecting groups of 7a and 7b with ammonia in methanol, oxetane 8 was obtained from both compounds in 99% and 80% yield respectively (Scheme 1). Oxetanes 8 did not recede to the corresponding epoxides according to TLC monitoring. Consequently, it was proven that 7b is also present as an oxetane-fused disaccharide.

3. Materials and Methods

All reactions were performed under an atmosphere of nitrogen using solvents dried by standard procedures. Reaction progress was monitored by TLC on Polygram SIL G/UV₂₅₄ silica gel plates from Macherey and Nagel, Germany. Detection of spots was affected by carbonizing with sulfuric acid (5% in EtOH), staining by spraying the plates with an alkaline aqueous solution of potassium permanganate or by inspection of the TLC plates under UV light (254 nm). Preparative flash chromatography was performed on silica gel (0.032–0.063 mm) from Macherey-Nagel, using plastic cartridges from Götec. The flowrate was regulated by a Sykam S1122 solvent delivery system. Nuclear magnetic resonance (NMR) spectra were recorded with a Bruker Avance 400 spectrometer and calibrated for the solvent signal (¹H: CDCl₃: δ = 7.26 ppm; acetone-d₆: δ = 2.05 ppm; DMSO-d₆: δ = 2.50 ppm; ¹³C: CDCl₃: δ =77.16 ppm; acetone-d₆: δ = 29.92 ppm; DMSO-d₆: δ = 39.52 ppm). All NMR-assignments were proven by 2D-experiments to be correct. ESI-TOF-HRM spectrometry was performed on a Bruker MAXIS 4G spectrometer. Elemental analyses were obtained from a HEKA tech Euro EA 3000 apparatus. Optical rotations were determined with a Perkin-Elmer Polarimeter 341 in a 10 cm cuvette at 20 °C with a wavelength of 589 nm (Na-lamp). Melting points were measured with a Büchi Melting Point M-560 apparatus.

2,6-Anhydro-5-deoxy-4-O-(tert-butyldimethylsilyl)-1,3:8,9:11,12-tri-O-isopropylidene-d-arabino-l-gulo-dodeco-7-ulo-6-enitol (3)

To a suspension of 1 (2.62 g, 6.18 mmol) and grounded molecular sieve (3 Å, 30 mg) in THF (40 mL) was added phenyllithium (1.9 M in Bu₂O, 3.57 mL, 6.79 mmol) at −78 °C over a period of 15 min. After 5 min a further solution containing 2 (1.75 g, 6.79 mmol) in THF (20 mL) was added dropwise over a period of 30 min. Subsequently, the reaction mixture was stirred for 2 h at the same temperature before the cooling bath was removed. The reaction was quenched by addition of sat. aq. NH₄Cl-solution (60 mL) at ambient temperature and the molecular sieve was filtered off. The organic layer was separated, the aqueous layer was extracted with CH₂Cl₂ (3 × 20 mL) and the combined organic layers were dried with Na₂SO₄. Removing of the solvent followed by column chromatography (methylene chloride/ethyl acetate 5:1) furnished 3 (2.77 mg, 4.97 mmol, 80%, anomeric mixture α:β, 1:5) as a colorless crystalline solid. R_f = 0.22–0.46 (methylene chloride/ethyl acetate 5:1).

3β*(major anomer): ¹H-NMR (400 MHz, CDCl₃): δ(ppm) = 5.04 (d, J_5,4 = 2.2 Hz, 1H, H-5), 4.82 (dd, J_9,8 = 5.7 Hz, J_9,10 = 3.5 Hz, 1H, H-9), 4.53 (d, J_8,9 = 5.9 Hz, 1H, H-8), 4.38–4.44 (m, 1H, H-11), 4.33–4.35 (m, 1H, H-4), 4.15 (dd, J_10,11 = 7.2 Hz, J_10,9 = 3.5 Hz, 1H, H-10), 4.07–4.11 (m, 1H, H-12a), 4.03 (dd, J = 8.7 Hz, J = 4.9 Hz, 1H, H-12b), 3.86–3.95 (m, 2H, H-1a, H-1b), 3.71–3.76 (m, 2H, H-2, H-3), 2.77 (s, 1H, OH), 1.50, 1.44, 1.42, 1.40, 1.38, 1.31 (6s, 18H, C(CH₃)₂), 0.87 (s, 9H, SiC(CH₃)₃), 0.07, 0.06 (2s, 6H,SiCH₃). ¹³C-NMR (101 MHz, CDCl₃): δ(ppm) = 150.4 (C-6), 113.3, 109.1 (C(CH₃)₂), 104.3 (C-5), 103.4 (C-7), 99.4 (C(CH₃)₂), 86.7 (C-8), 80.0 (C-9), 79.4 (C-10), 73.2 (C-11), 72.9 (C-3), 70.2 (C-2), 68.2 (C-4), 66.7 (C-12), 61.7 (C-1), 28.9, 26.8, 25.8 (C(CH₃)₂), 25.7 (SiC(CH₃)₃), 25.4, 25.0, 19.1 (C(CH₃)₂), 18.1 (SiC(CH₃)₃), –4.4, –4.8 (SiCH₃).

3α*(minor anomer): ¹H-NMR (400 MHz, CDCl₃): δ(ppm) = 5.07 (d, J_5,4 = 2.1 Hz, 1H, H-5), 4.79 (dd, J_9,8 = 6.0 Hz, J_9,10 = 3.7 Hz, 1H, H-9), 4.62 (d, J_8,9 = 6.0 Hz, 1H, H-8), 4.34–4.35 (m, 2H, H-4, H-11), 4.10–4.11 (m, 1H, H-12a), 4.01–4.02 (m, 1H, H-12b), 3.85–3.88 (m, 2H, H-1a, H-1b, 3.76–3.80 (m, 3H, H-2, H-3, H-10), 2.77 (s, 1H, OH), 1.57, 1.49, 1.42, 1.40, 1.36 (5s, 18H, C(CH₃)₂), 0.88 (s, 9H, SiC(CH₃)₃), 0.08, 0.07 (2s, 6H, SiCH₃). ¹³C-NMR (101 MHz, CDCl₃): δ(ppm) = 150.8 (C-6), 113.6, 109.3 (C(CH₃)₂), 103.1 (C-5), 100.6 (C-7), 99.6 (C(CH₃)₂), 80.7 (C-8), 80.0 (C-9), 78.6 (C-10), 73.2 (C-11), 72.8 (C-3), 70.3 (C-2), 67.8 (C-4), 67.2 (C-12), 61.7 (C-1), 28.9, 26.9, 25.9 (C(CH₃)₂), 25.8 (SiC(CH₃)₃), 25.2, 24.6, 19.0 (C(CH₃)₂), 18.2 (SiC(CH₃)₃), –4.4, –4.8 (SiCH₃). HRESIMS m/z 581.27570 (calcd for C₂₇H₄₆O₁₀SiNa, 581.27524); anal. C 58.37, H 8.21, calcd for C₂₇H₄₆O₁₀Si, C 58.04, H 8.39. *Anomers 3β and 3α could be interchanged

2,6-Anhydro-5-deoxy-7-O-acetyl-4-O-(tert-butyldimethylsilyl)-1,3:8,9:11,12-tri-O-isopropylidene-β-d-manno-d-lyxo-dodeco-6-enitol (4a)

A solution of 3 (300 mg, 0.537 mmol) and 4-dimethylaminopyridine (13 mg, 0.107 mmol) in triethylamine (7 mL) was treated with acetic anhydride (508 μL, 5.37 mmol) at 0 °C. After stirring for 12 h at ambient temperature until TLC indicated complete transformation of the starting material, the reaction mixture was diluted with ethyl acetate (20 mL). The organic phase was washed with 1 N hydrochloric acid (3 × 10 mL), sat. aq. NH₄Cl-solution (1 × 10 mL) and brine (1 × 10 mL). The solution was dried with Na₂SO₄, the solvent was removed and the crude product was purified by column chromatography (petroleum ether/ethyl acetate 5:1) to afford 4a (309 mg, 0.515 mmol, 96%) as a colorless crystalline solid. R_f = 0.82 (methylene chloride/ethyl acetate 5:1). ${\left[\mathsf{\alpha }\right]}_{\mathrm{D}}^{20}$ = +64.3 (c = 1.0, CHCl₃). M.p. 52 °C (n-hexane). ¹H-NMR (400 MHz, CDCl₃): δ (ppm) = 4.99 (d, J_5,4 = 2.2 Hz, 1H, H-5), 4.86 (dd, J_9,8 = 5.6 Hz, J_9,10 = 3.4 Hz, 1H, H-9), 4.69 (d, J_8,9 = 5.6 Hz, 1H, H-8), 4.42 (ddd, J = 8.3 Hz, J = 5.9 Hz, J = 3.9 Hz, 1H, H-11), 4.35 (dd, J_4,3 = 6.7 Hz, J_4,5 = 2.1 Hz, 1H, H-4), 4.09–4.15 (m, 1H, H-12a), 3.99–4.07 (m, 2H, H-10, H-12b), 3.80–3.89 (m, 2H, H-1aH-1b), 3.61–3.73 (m, 2H, H-2, H-3), 2.01 (s, 3H, COCH₃), 1.48, 1.47, 1.45, 1.37, 1.37, 1.33 (6s, 18H, C(CH₃)₂), 0.83 (s, 9H, SiC(CH₃)₃), 0.03, 0.03 (2s, 6H, SiCH₃).¹³C-NMR (101 MHz, CDCl₃): δ(ppm) = 168.3 (CO), 146.9 (C-6), 113.9, 109.4 (C(CH₃)₂), 107.4 (C-7), 104.8 (C-5), 99.3 (C(CH₃)₂), 86.4 (C-8), 81.2 (C-10), 79.6 (C-9), 73.0 (C-3), 72.8 (C-11), 70.2 (C-2), 68.2 (C-4), 67.1 (C-12), 61.7 (C-1), 28.9, 27.0, 25.8 (C(CH₃)₂), 25.6 (SiC(CH₃)₃), 25.1, 25.1 (C(CH₃)₂), 21.5 (COCH₃), 19.1 (C(CH₃)₂), 18.0 (SiC(CH₃)₃), –4.4, –4.9 (SiCH₃). HRESIMS m/z 623.28604 (calcd for C₂₉H₄₈O₁₁SiNa, 623.28581); anal. C 58.10, H 8.21, calcd for C₂₉H₄₈O₁₁Si, C 57.98, H 8.05.

2,6-Anhydro-5-deoxy-7-O-benzoyl-4-O-(tert-butyldimethylsilyl)-1,3:8,9:11,12-tri-O-isopropylidene-β-d-manno-d-lyxo-dodeco-6-enitol (4b)

A solution of 3 (930 mg, 1.67 mmol) and DMAP (41 mg, 0.333 mmol) in pyridine (15 mL) was treated with benzoyl chloride (1.92 mL, 16.7 mmol) at 0 °C. After stirring for 48 h at ambient temperature TLC indicated complete transformation of the starting material and the reaction mixture was diluted with ethyl acetate (60 mL). The organic layer was washed with 1 N hydrochloric acid (3 × 30 mL), sat. aq. NaHCO₃-solution (3 × 20 mL) and brine (1 × 20 mL). The solution was dried with Na₂SO₄, the solvent was removed and the crude product was purified by column chromatography (toluene/ethyl acetate, gradient: 100:1 – 20:1) to afford 4b (749 mg, 1.13 mmol, 68%) as a colorless crystalline solid. R_f = 0.66 (toluene/ethyl acetate 4:1). ${\left[\mathsf{\alpha }\right]}_{\mathrm{D}}^{20}$ = +57.1(c = 1.0, CHCl₃). M.p. 58 °C. ¹H-NMR (400 MHz, CDCl₃): δ(ppm) = 7.90–8.02 (m, 2H, Ph), 7.52–7.62 (m, 1H, Ph), 7.39–7.48 (m, 2H, Ph), 5.11 (d, J_5,4 = 2.3 Hz, 1H, H-5), 4.90–5.01 (m, 2H, H-8, H-9), 4.47 (ddd, J = 8.2 Hz, J = 6.0 Hz, J_11,12b = 4.0 Hz, 1H, H-11), 4.41 (dd, J_4,3 = 6.7 Hz, J_4,5 = 2.2 Hz, 1H, H-4), 4.08–4.17 (m, 2H, H-10, H-12a), 4.05 (dd, J_12b,12a = 9.1 Hz, J_12b,11 = 4.0 Hz, 1H, H-12b), 3.66–3.84 (m, 4H, H-1a, H-1b, H-2, H-3), 1.52, 1.48, 1.42, 1.38, 1.37 (5s, 18H, C(CH₃)₂), 0.86 (s, 9H, SiC(CH₃)₃), 0.06, 0.06 (2s, 6H, SiCH₃).¹³C-NMR (101 MHz, CDCl₃): δ(ppm) = 164.0 (CO), 147.3 (C-6), 133.4, 130.2, 129.9, 128.5 (Ph), 114.2, 109.6 (C(CH₃)₂), 108.2 (C-7), 105.1 (C-5), 99.5 (C(CH₃)₂), 86.7 (C-8), 81.8 (C-10), 80.0 (C-9), 73.3 (C-3), 73.0 (C-11), 70.4 (C-2), 68.5 (C-4), 67.3 (C-12), 61.8 (C-1), 29.1, 27.1, 26.0 (C(CH₃)₂), 25.8 (SiC(CH₃)₃), 25.4, 25.3, 19.3 (C(CH₃)₂), 18.2 (SiC(CH₃)₃), –4.2, –4.7 (SiCH₃). HRESIMS m/z 685.30154 (calcd for C₃₄H₅₀O₁₁SiNa, 685.30146); anal. C 61.26, H 8.02, calcd for C₃₄H₅₀O₁₁Si, C 61.61, H 7.60.

General Procedure A for Desilylation of 4a and 4b

Tetra-n-butylammonium fluoride (1.0 M in THF, 1.5 eq.) was added to a solution of 4a or 4b (1.0 mmol) in THF (10 mL) at 0 °C. The reaction mixture was warmed to room temperature and stirring was continued until complete conversion of the starting material was detected by TLC. Subsequently, the solution was diluted with CH₂Cl₂ (20 mL) and washed with brine (3 × 10 mL). The aqueous phase was extracted with CH₂Cl₂ (2 × 20 mL) and the combined organic phases were dried with Na₂SO₄. The solvent was evaporated and the residue was purified by column chromatography (petroleum ether/ethyl acetate 2:1 containing 0.5% Et₃N).

2,6-Anhydro-5-deoxy-7-O-acetyl-1,3:8,9:11,12-tri-O-isopropylidene-β-d-manno-d-lyxo-dodeco-6-enitol (5a)

Following general procedure A 5a was obtained as a white amorphous solid and was prepared from 4a in 71% yield stirring the reaction mixture for 20 h. R_f = 0.28 (petroleum ether/ethyl acetate 1:1). ${\left[\mathsf{\alpha }\right]}_{\mathrm{D}}^{20}$= +98.0 (c = 1.0, CHCl₃). M.p. 179 °C (n-hexane/ethyl acetate). ¹H-NMR (400 MHz, CDCl₃): δ(ppm) = 5.14 (d, J_5,6 = 2.2 Hz, 1H, H-5), 4.88 (dd, J_9,8 = 5.7 Hz, J_9,10 = 3.5 Hz, 1H, H-9), 4.71 (d, J_8,9 = 5.7 Hz, 1H, H-8), 4.37–4.45 (m, 2H, H-4, H-11), 4.01–4.12 (m, 3H, H-10, H-12a, H-12b), 3.85–3.94 (m, 2H, H-1a, H-1b), 3.68–3.80 (m, 2H, H-2, H-3), 2.02 (s, 3H, COCH₃), 1.53, 1.47, 1.45, 1.42, 1.37, 1.33 (6s, 18H, C(CH₃)₂). ¹³C-NMR (101 MHz, CDCl₃): δ (ppm) = 168.2 (CO), 148.0 (C-6), 113.7, 109.4 (C(CH₃)₂), 107.1 (C-7), 102.9 (C-5), 99.7 (C(CH₃)₂), 86.3 (C-8), 81.5 (C-10), 79.6 (C-9), 73.2 (C-3), 72.8 (C-11), 69.9 (C-2), 67.8 (C-4), 67.0 (C-12), 61.5 (C-1), 28.9, 27.0, 25.8, 25.1, 24.9 (C(CH₃)₂), 21.6 (COCH₃), 19.2 (C(CH₃)₂). HRESIMS m/z 509.19928 (calcd for C₂₃H₃₄O₁₁Na, 509.19933); anal. C 56.57, H 7.17, calcd for C₂₃H₃₄O₁₁, C 56.78, H 7.04.

2,6-Anhydro-5-deoxy-7-O-benzoyl-1,3:8,9:11,12-tri-O-isopropylidene-β-d-manno-d-lyxo-dodeco-6-enitol (5b)

Following general procedure A 5b was obtained as a colorless amorphous solid and was prepared from 4b in 89% yield stirring the reaction mixture for 24 h. R_f = 0.37 (petroleum ether/ethyl acetate 1:1). ${\left[\mathsf{\alpha }\right]}_{\mathrm{D}}^{20}$ = +89.4 (c = 1.0, CHCl₃). ¹H-NMR (400 MHz, CDCl₃): δ(ppm) = 7.92–7.99 (m, 2H, Ph), 7.54–7.60 (m, 1H, Ph), 7.41–7.47 (m, 2H, Ph), 5.26 (d, J_5,4 = 2.2 Hz, 1H, H-5), 4.99 (dd, J_9,8 = 5.9 Hz, J_9,10 = 3.6 Hz, 1H, H-9), 4.95 (d, J_8,9 = 5.9 Hz, 1H, H-8), 4.41–4.50 (m, 2H, H-4, H-11), 4.09–4.14 (m, 2H, H-10, H-12a), 4.04 (dd, J = 9.1 Hz, J = 4.2 Hz, 1H, H-12b), 3.72–3.88 (m, 4H, H-1a, H-1b, H-2, H-3), 2.11 (br. s., 1H, OH), 1.53, 1.52, 1.42, 1.41, 1.38, 1.36 (6 s, 18H, C(CH₃)₂).¹³C-NMR (101 MHz, CDCl₃): δ(ppm) = 163.9 (CO), 148.4 (C-6), 133.5, 130.1, 129.9, 128.6 (Ph), 114.0, 109.6 (C(CH₃)₂), 107.9 (C-7), 103.2 (C-5), 99.9 (C(CH₃)₂), 86.6 (C-8), 82.0 (C-10), 79.9 (C-9), 73.4 (C-3), 73.0 (C-11), 70.2 (C-2), 68.1 (C-4), 67.2 (C-12), 61.6 (C-1), 29.1, 27.1, 26.0, 25.2, 25.1, 19.4 (C(CH₃)₂). HRESIMS m/z 571.21543 (calcd for C₂₈H₃₆O₁₁Na, 571.21498); anal. C 61.18, H 6.77, calcd for C₂₈H₃₆O₁₁, C 61.30, H 6.61.

General Procedure B for Epoxidation of 5a and 5b and Further Rearrangement to the Oxetanes

To a solution of mCPBA (2.5 eq.) in CH₂Cl₂ (10 mL) was added potassium fluoride (5 eq.) and the resulting suspension was stirred for 30 min at ambient temperature. A solution containing 5a or 5b (0.5 mmol) in CH₂Cl₂ (10 mL) was added in one portion and stirring was continued for 2 h (epoxidation). To enforce the rearrangement to the oxetanes the reaction mixture was stirred for 20 h, the white precipitate was filtered off and the solution was stirred for further 20 h. The reaction was quenched by addition of sat. aq. NaHCO₃-solution (10 mL) and sat. aq. Na₂S₂O₃-solution (10 mL). Subsequently, the aqueous layer was extracted by CH₂Cl₂ (3 × 10 mL), the combined organic layers were dried with Na₂SO₄, the solvent was removed and the crude product was purified by column chromatography.

5,6-Anhydro-7-O-acetyl-1,3:8,9:11,12-tri-O-isopropylidene-β-d-manno-β-d-manno-dodeco-6,7-diulo-2,6-pyranose-7,10-furanose (6a)

Following general procedure B a mixture of 6a and 7a (8:1) was obtained after quenching the reaction after 2 h as a white amorphous solid and was prepared from 5a in 94% yield. A mixture of petroleum ether/ethyl acetate 3:1 containing 0.5% Et₃N was used as the eluent for column chromatography. R_f = 0.58 (petroleum ether/ethyl acetate 1:2). ¹H-NMR (400 MHz, CDCl₃): δ(ppm) = 4.96 (d, J_8,9 = 6.0 Hz, 1H, H-8), 4.91 (dd, J_9,8 = 6.0 Hz, J_9,10 = 3.9 Hz, 1H, H-9), 4.31–4.38 (m, 1H, H-11), 4.20 (dd, J_10,11 = 6.6 Hz, J_10,9 = 3.9 Hz, 1H, H-10), 3.85–4.06 (m, 5H, H-1a, H-3, H-4, H-12a, H-12b), 3.77 (dd, J = 10.7 Hz, J = 10.7 Hz, 1H, H-1b), 3.66 (d, J_5,4 = 2.4 Hz, 1H, H-5), 3.49 (ddd, J = 10.2 Hz, J = 10.2 Hz, J = 5.7 Hz, 1H, H-2), 2.38 (br. s., 1H, OH), 2.06 (s, 3H, COCH₃), 1.57, 1.48, 1.42, 1.39, 1.35, 1.33 (6s, 18H, C(CH₃)₂). ¹³C-NMR (101 MHz, CDCl₃): δ(ppm) = 169.2 (CO), 114.2, 109.2 (C(CH₃)₂), 107.9 (C-7), 99.7 (C(CH₃)₂), 86.2 (C-8), 85.0 (C-6), 82.9 (C-10), 79.9 (C-9), 73.2 (C-11), 73.0 (C-3), 70.6 (C-2), 69.9 (C-4), 66.4, 61.7 (C-1), 57.3 (C-5), 29.1, 27.0, 25.6, 25.3, 24.6 (C(CH₃)₂), 21.9 (COCH₃), 19.3 (C(CH₃)₂). HRESIMS m/z 525.19448 (calcd for C₂₃H₃₄O₁₂Na, 525.19425).

5,6-Anhydro-7-O-benzoyl-1,3:8,9:11,12-tri-O-isopropylidene-β-d-manno-β-d-manno-dodeco-6,7-diulo-2,6-pyranose-7,10-furanose (6b)

Following general procedure B, a mixture of 6b and 7b (8:1) was obtained after quenching the reaction after 2 h as a white amorphous solid and was prepared from 5b in 90% yield. A mixture of petroleum ether/ethyl acetate 3:1 containing 0.5% Et₃N was used as the eluent for column chromatography. R_f = 0.64 (petroleum ether/ethyl acetate 1:2). ¹H-NMR (400 MHz, CDCl₃): δ(ppm) = 7.92–7.99 (m, 2H, Ph), 7.59–7.63 (m, 1H, Ph), 7.45–7.50 (m, 2H, Ph), 5.18 (d, J_8,9 = 6.0 Hz, 1H, H-8), 5.03 (dd, J_9,8 = 6.1 Hz, J_9,10 = 4.0 Hz, 1H, H-9), 4.34–4.41 (m, 1H, H-11), 4.31 (dd, J_10,11 = 6.6 Hz, J_10,9 = 3.9 Hz, 1H, H-10), 4.13–4.18 (m, 1H, H-4), 4.03 (dd, J = 8.9 Hz, J = 6.2 Hz, 1H, H-12a), 3.91–3.99 (m, 2H, H-3, 12b), 3.73–3.83 (m, 3H, H-1a, H-1b, H-5), 3.54 (ddd, J = 10.0 Hz, J = 10.0 Hz, J = 6.0 Hz, 1H, H-2), 2.42 (br. s., 1H, OH), 1.62, 1.48, 1.39, 1.37, 1.34 (5s, 18H, C(CH₃)₂). ¹³C-NMR (101 MHz, CDCl₃): δ(ppm) = 164.7 (CO), 133.8, 129.9, 129.8, 128.8 (Ph), 114.3, 109.2 (C(CH₃)₂), 108.5 (C-7), 99.7 (C(CH₃)₂), 86.4 (C-8), 85.3 (C-6), 83.2 (C-10), 80.0 (C-9), 73.2 (C-11), 73.0 (C-3), 70.7 (C-2), 69.9 (C-4), 66.4 (C-12), 61.6 (C-1), 57.4 (C-5), 29.0, 26.9, 25.6, 25.3, 24.6, 19.3 (C(CH₃)₂). HRESIMS m/z 587.21029 (calcd for C₂₈H₃₆O₁₂Na, 587.20990).

5,7-Anhydro-6-O-acetyl-1,3:8,9:11,12-tri-O-isopropylidene-β-d-manno-β-d-manno-dodeco-6,7-diulo-2,6-pyranose-7,10-furanose (7a)

Following general procedure B 7a was obtained after quenching the reaction after 40 h as a white crystalline solid and was prepared from 5a in 70% yield. A mixture of petroleum ether/ethyl acetate 2:1 was used as the eluent for column chromatography. Crystals from 7a were grown by overlaying a saturated solution of 7a in methylene chloride with n-heptane and slowly evaporating the methylene chloride. R_f = 0.51 (petroleum ether/ethyl acetate 1:2). ${\left[\mathsf{\alpha }\right]}_{\mathrm{D}}^{20}$ = +18.5 (c = 1.0, CHCl₃). M.p. 151 °C (n-hexane/ethyl acetate). ¹H-NMR (400 MHz, CDCl₃): δ(ppm) = 5.04 (d, J_8,9 = 5.6 Hz, 1H, H-8), 4.96 (d, J_4,5 = 4.9 Hz, 1H, H-5), 4.70 (dd, J_9,8 = 5.6 Hz, J_9,10 = 4.0 Hz, 1H, H-9), 4.37–4.46 (m, 1H, H-11), 4.00–4.13 (m, 3H, H-3, H-12a, H12b), 3.86–3.97 (m, 3H, H-1a, H-4, H-10), 3.80 (dd, J = 10.5 Hz, 1H, H-1b), 3.65–3.74 (m, 1H, H-2), 2.14 (d, J_OH,₄ = 9.0 Hz, 1H, OH), 2.10 (s, 3H, COCH₃), 1.56, 1.55, 1.43, 1.43, 1.39, 1.36 (6s, 18H, C(CH₃)₂). ¹³C-NMR (101 MHz, CDCl₃): δ(ppm) = 169.4 (CO), 114.2 (C(CH₃)₂), 112.9 (C-7), 109.5, 99.9 (C(CH₃)₂), 98.3 (C-6), 80.9 (C-5), 80.0 (C-10), 79.1 (C-8), 78.6 (C-9), 73.5 (C-11), 71.6 (C-3), 69.1 (C-2), 68.9 (C-4), 67.1 (C-12), 61.8 (C-1), 29.1, 27.1, 25.9, 25.8, 25.4 (C(CH₃)₂), 21.2 (COCH₃), 19.2 (C(CH₃)₂). HRESIMS m/z 525.19318 (calcd for C₂₃H₃₄O₁₂Na, 525.19425); anal. C 55.08, H 7.11, calcd for C₂₃H₃₄O₁₂, C 54.97, H 6.82.

5,7-Anhydro-6-O-benzoyl-1,3:8,9:11,12-tri-O-isopropylidene-β-d-manno-β-d-manno-dodeco-6,7-diulo-2,6-pyranose-7,10-furanose (7b)

Following general procedure B 7b was obtained after quenching the reaction after 40 h as a white amorphous solid and was prepared from 5b in 63% yield. A mixture of petroleum ether/ethyl acetate 2:1 was used as the eluent for column chromatography. R_f = 0.60 (petroleum ether/ethyl acetate 1:2). ${\left[\mathsf{\alpha }\right]}_{\mathrm{D}}^{20}$= +15.7 (c = 1.0, CHCl₃). ¹H-NMR (400 MHz, acetone-d₆): δ(ppm) = 8.04–8.10 (m, 2H, Ph), 7.68–7.74 (m, 1H, Ph), 7.53–7.59 (m, 2H, Ph), 5.16 (d, J_8,9 = 5.6 Hz, 1H, H-8), 5.10 (d, J_5,4 = 4.5 Hz, 1H, H-5), 4.82 (dd, J_9,8 = 5.6 Hz, J_9,10 = 3.7 Hz, 1H, H-9), 4.36 (ddd, J = 6.5 Hz, J = 6.5 Hz, J = 5.1 Hz, 1H, H-11), 4.15–4.26 (m, 2H, H-3, OH), 3.99–4.06 (m, 3H, H-4, H-10, H-12a), 3.80–3.97 (m, 4H, H-1a, H-1b, H-2, H-12b), 1.52, 1.51, 1.37, 1.35, 1.34, 1.28 (6s, 18H, C(CH₃)₂). ¹³C-NMR (101 MHz, acetone-d₆): δ(ppm) = 165.7 (CO), 135.0, 130.8, 130.2, 129.7 (Ph), 114.2 (C(CH₃)₂), 113.5 (C-7), 109.5, 100.2 (C(CH₃)₂), 99.5 (C-6), 83.3 (C-5), 80.1 (C-10), 79.6 (C-9), 79.6 (C-8), 74.5 (C-11), 72.1 (C-3), 70.7 (C-2), 69.4 (C-4), 67.1 (C-12), 62.4 (C-1), 29.5, 27.2, 26.3, 26.2, 25.7, 19.3 (C(CH₃)₂). HRESIMS m/z 587.20977 (calcd for C₂₈H₃₆O₁₂Na, 587.20990); anal. C 59.64, H 6.86, calcd for C₂₈H₃₆O₁₂, C 59.57, H 6.43.

5,7-Anhydro-1,3:8,9:11,12-tri-O-isopropylidene-β-d-manno-β-d-manno-dodeco-6,7-diulo-2,6-pyranose-7,10-furanose (8)

From 7a: 7a (50 mg, 0.10 mmol) was dissolved in methanol (2 mL) and ammonia (7 N in methanol, 280 μL, 1.99 mmol) was added at ambient temperature. TLC indicated complete transformation of the starting material after 5 h and the volatile components were removed. Neat 8 (46 mg, 0.099 mmol, 99%) remained as a white crystalline solid.

From 7b: 7b (20 mg, 0.0354 mmol) was dissolved in methanol (1 mL) and ammonia (7 N in methanol, 101 μL, 0.708 mmol) was added at ambient temperature. TLC indicated complete transformation of the starting material after 5 h and the volatile components were removed. Subsequently, the crude product was purified by column chromatography (petroleum ether/ethyl acetate 1:2) and 8 (13 mg, 0.282 mmol, 80%) was isolated as a white crystalline solid.

R_f = 0.32 (petroleum ether/ethyl acetate 1:3). ${\left[\mathsf{\alpha }\right]}_{\mathrm{D}}^{20}$= +17.5 (c = 1.0, CHCl₃). M.p. 122 °C (n-hexane). ¹H-NMR (600 MHz, DMSO-d₆): δ(ppm) = 7.09 (s, 1H, OH-6), 5.16 (d, J_OH,4 = 5.7 Hz, 1H, OH-4), 4.75 (d, J_8,9 = 5.9 Hz, 1H, H-8), 4.57–4.63 (m, 2H, H-5, H-9), 4.22 (ddd, J_11,10 = 7.3 Hz, J_11,12a = 6.4 Hz, J_11,12b = 5.1, 1H, H-11), 4.01 (dd, J_12a,12b = 8.4 Hz, J_12a,11 = 6.4 Hz, 1H, H-12a), 3.92 (dd, J_12b,12a = 8.4 Hz, J_12b,11 = 5.1 Hz, 1H, H-12b), 3.78–3.85 (m, 2H, H-1a, H-3), 3.76 (dd, J_10,11 = 7.3 Hz, J_10,9 = 3.7 Hz, 1H, H-10), 3.68 (dd, J_1b,1a = 10.5 Hz, J_1b,2 = 10.5 Hz, 1H, H-1b), 3.52 (ddd, J = 9.7 Hz, J_4,OH = 5.1 Hz, J = 4.4 Hz, 1H, H-4), 3.40 (ddd, J_2,1b = 9.9 Hz, J = 9.9 Hz, J = 5.8 Hz, 1H, H-2), 1.45, 1.39, 1.32, 1.31, 1.28, 1.26 (6s, 18H, C(CH₃)₂).¹³C-NMR (101 MHz, DMSO-d₆): δ(ppm) = 112.5 (C-7), 112.3, 108.2, 98.8 (C(CH₃)₂), 97.4 (C-6), 83.9 (C-5), 78.2 (C-9), 77.0 (C-8), 76.9 (C-10), 72.7 (C-11), 71.1 (C-3), 67.8 (C-4), 66.8 (C-2), 66.1 (C-12), 61.1 (C-1), 28.9,26.6, 25.8, 25.5, 25.2, 19.0 (C(CH₃)₂). HRESIMS m/z 483.18377 (calcd for C₂₁H₃₂O₁₁Na, 483.18368); anal. C 54.58, H 7.41, calcd for C₂₁H₃₂O₁₁, C 54.78, H 7.00.

4. Conclusions

In summary, we have demonstrated that the stability of 2,5-anhydro sugars depends on their chemical environment. In the case of epoxide 6a and 6b, where an ester group is adjacent to the epoxide group, reversible rearrangements to highly annulated oxetane-bridged disaccharides 7a and 7b occur. In further studies, these compounds serve as precursors for the synthesis of symmetrical dodeco-6,7-diuloses.