Iminosugars,also referred to as azasugars,iminocyclitols,or polyhydroxylated alkaloids,have attracted considerable attention due to their intriguing biological properties ^[1]. These ‘‘nitrogen- in-the-ring’’ analogues of pyranoses and furanoses are potent inhibitors of glycosidases and other glycosyl processing enzymes, and have potential as pharmaceuticals ^{[1, 2]}. Polyhydroxylated pyrrolidines carrying an aromatic substituent on the iminosugar ring are a rare class of iminosugars found in nature (Fig. 1) ^[3]; these alkaloids exhibit antihypertensive pharmacological activity without influencing the central nervous system in animal tests. Manymethods have been developed to access these alkaloids ^[4a], and a number of derivatives have been synthesized ^[5]. In contrast, 2-aryl polyhydroxylated piperidines have not yet been isolated from nature; in view of this,this paper reports the synthesis and glycosidase inhibition profile of a number of aryl piperidine analogues.

Fig. 1

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Fig. 1. Radicamines and related natural compounds.

Nitrones have been shown as very versatile synthetic intermediates for the construction of structurally complex molecules ^{[5g, 6, 7]} because they are capable of undergoing a variety of synthetically useful transformations,such as 1,3-dipolar cycload- ditions ^[8],nucleophilic additions ^{[9, 10]},and pinacol-type coupling reactions ^[11].

In the context of our interest in the synthesis of bioactive azasugars ^{[4k, 12]},we have developed a practical method to synthesize a series of five- and six-membered polyhydroxylated cyclic nitrones on a large scale ^[13],which are versatile synthons in the construction of novel iminosugars ^[14]. As a part of our continuing interest in the synthesis of iminosugar analogues, herein we report the synthesis of 2-aryl polyhydroxylated piperidine iminosugars (compounds 6-8,and ent-(6-8),Fig. 2) froma pair of enantiomeric six-membered polyhydroxylated cyclic nitrones 12 and ent-12,and their glycosidase inhibitory activity.

Fig. 1

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Fig. 2. 2-Aryl polyhydroxylated piperidine iminosugars.

General procedure for the synthesis compounds 6-8 and ent- (6-8): A solution of the cyclic six-membered nitrone 12 or ent-12 (1.0 mmol) in THF (10 mL) was cooled to 0 ℃ and aryl Grignard reagents (5.0 mmol) in THF (5 mL) was added under an inert N₂ atmosphere. The mixture was kept at this temperature for 2 h,and then quenched by adding saturated aqueous NH₄Cl solution (15 mL). The aqueous phasewas extractedwith EtOAc (3 ×10 mL), the combined extractswere dried (MgSO₄) and concentrated under reduced pressure. The residue was purified by flash column chromatography (silica gel,petroleum ether/EtOAc = 15/1) to give the addition compound. Aqueous HCl (6 mol/L,1.0 mL) was added to the suspension of the addition product (0.23 mmol) and Pd/C (40 mg) in CH3OH (20 mL),then the mixture was subjected to hydrogenation under H2 atmosphere for 24 h. The catalyst was filtered and washed with methanol (5 mL). The filtrate was neutralized by adding NH₃·H₂O,then concentrated,the residue was purified by ion exchange resin to give the target product 6-8 or ent-(6-8).

7:[α] 20.74 (c 1.35,CH₃OH); mp: 233-234 ℃; IR (KBr, cm^-1 ): 3366 (br s),2918 (m),1637 (m),1496 (m),1455 (m),1145 (m),1091 (s),866 (m),761 (s),701 (m); ¹H NMR (300 MHz,D₂O): δ 7.16 (d,2H,J = 8.06 Hz,Ph-),6.77 (d,2H,J = 8.07 Hz,Ph-),3.97 (s, 1H,H-5),3.76 (t,1H,J = 9.67 Hz,H-3),3.53 (dd,1H,J₁ = 2.78 Hz, J₂ = 9.43 Hz,H-4),3.30 (d,1H, J = 9.99 Hz,H-2),2.93 (d,1H, J = 14.08 Hz,H-6e),2.76 (d,1H, J = 14.17 Hz,H-6a); ¹³C NMR (75 MHz,D₂O): d 155.8,130.4,129.4,115.8 (Ph-C),74.4 (C-4),71.7 (C-3),69.0 (C-5),64.0 (C-2),48.8 (C-6); HRMS (ESI-MS), m/z 226.10719 [M+H]⁺ (C₁₁H₁₆NO₄ calcd. 226.10738).

Our synthesis started from the enantiomeric six-membered cyclic nitrones 12 and ent-12 (Scheme 1),prepared fromarabinose according to the reported procedures ^[13]. The addition of aryl Grignard reagents to the nitrone 12 gave compounds 13-15 with high 2,3-trans-selectivity (Table 1),although not as high as with the corresponding five-membered cyclic nitrones ^[4k],which gave exclusively trans-products. The results are summarized in Table 1. The configurations of the productswere determined either by ¹H-NMR data (13a: J_2-3(trans) = 9.69 Hz for example) or by COSY and NOESY spectra (see the Supporting information) (compound 8 for example: the NOE interactions between H-2/H-6a,H-2/H-5, and H-2/H-4,indicated that H-2,H-6a,H-5,and H-4 were on the same side of the ring).

Scheme 1

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Scheme 1. Reagents and conditions: (a) Grignard reagents,THF,ice-bath,2 h,68%-82%; (b) H2,Pd-C,6 mol/L HCl,CH3OH,89%-97%.

Table 1

Table 1 The addition of nitrones with aryl Grignard reagents.

Table 1 The addition of nitrones with aryl Grignard reagents.

Catalytic hydrogenation of the analogues 13a,14a and 15a catalyzed by 10% Pd-C afforded the deprotected 2-aryl piperidine iminosugars 6,7,8 (Table 2).

Table 2

Table 2 Catalytic hydrogenation of the Grignard addition products.

Table 2 Catalytic hydrogenation of the Grignard addition products.

The three enantiomeric piperidines were obtained similarly; ent-12 was treated with the above three aryl Grignard reagents to give the protected addition products ent-(13a-15a),which were converted,as described above,to the targeted compounds ent-(6- 8) respectively (Scheme 1,Tables 1 and 2).

The piperidine analogues of radicamine A and Bwere assayed as potential glycosidase inhibitors against 10 different enzymes. All the compounds tested exhibited weaker inhibition than radicamine A and B ^[3c] of α-glucosidase in spite of their structural similarities. 8 showed weak glycosidase inhibition of β-galactosidase (IC₅₀ 1080 μmol/L) isolated from rat intestinal lactases. The three pairs of compounds had no inhibition of β-mannosidase, while ent-(6-8) had better inhibition of a-L-fucosidase from bovine kidney than compounds 6-8.However,compounds6-8 were more potent than their enantiomers in inhibiting a-galactosidase from coffee beans and β-galactosidase from rat intestinal lactase. To study the effect of the size of the heterocyclic ring on glycosidase inhibitory activity,we will synthesize and survey other related compounds in future work.

In summary,we have synthesized three enantiomeric pairs of 2-aryl polyhydroxylated piperidines by the addition of aryl Grignard reagents to six-membered cyclic nitrones. Bioassay results indicated that these compounds exhibited no or only very weak inhibition of any of the glycosidases tested and are a much weaker class of inhibitors than the corresponding pyrrolidines.

Financial support from National Basic Research Program of China (Nos. 2012CB822101 and 2011CB808603),and the Natural Science Foundation of China (Nos. 21272240 and 21102149) is gratefully acknowledged.

Supplementary data associated with this article can be found,in the online version,at http://dx.doi.org/10.1016/j.cclet.2013.06.027.