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

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.


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.

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 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 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 1 H-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.
Molbank 2020, 2020, x FOR PEER REVIEW 3 of 9 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 1 H-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.

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 ( 1 H: CDCl3: δ = 7.26 ppm; acetone-d6: δ = 2.05 ppm; DMSO-d6: δ = 2.50 ppm; 13 C: CDCl3: δ =77.16 ppm; acetone-d6: δ = 29.92 ppm; DMSO-d6: δ = 39.52 ppm). All NMRassignments 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. -1,3:8,9:11,12- 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.

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/UV 254 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  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 Bu 2 O, 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. NH 4 Cl-solution (60 mL) at ambient temperature and the molecular sieve was filtered off. The organic layer was separated, the aqueous layer was extracted with CH 2 Cl 2 (3 × 20 mL) and the combined organic layers were dried with Na 2 SO 4 . Removing of the solvent followed by column chromatography 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. NH 4 Cl-solution (1 × 10 mL) and brine (1 × 10 mL). The solution was dried with Na 2 SO 4 , 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.

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 CH 2 Cl 2 (20 mL) and washed with brine (3 × 10 mL). The aqueous phase was extracted with CH 2 Cl 2 (2 × 20 mL) and the combined organic phases were dried with Na 2 SO 4 . The solvent was evaporated and the residue was purified by column chromatography (petroleum ether/ethyl acetate 2:1 containing 0.5% Et 3 N).