A synthetic approach to chiral carbocyclic nucleosides of varied ring-sizes using carbon framework of D-glucose

Synthesis of enantiomerically pure carbocyclic nucleoside analogues 10-16 with five-, six-and seven-membered rings has been achieved starting from D-glucose derived tetracyclic isoxazolidinocarbocycle precursors 1-3 . Cyclization of 6-chloro pyrimidine derivatives 7-9 to purine derivatives was found to be accomplished by nucleophilic displacement of 6-chloro substituent (by dimethylamino and/or methoxy groups). Apparently, hydrogen bonding between N-3 of the purine ring and a hydoxy substituent at C-2 / plays a crucial role in this transformation.


Introduction
After the discovery of the two natural carbocyclic nucleosides (-)-aristeromycin 1 and (-)neplanocin A 2 possessing antineoplastic activity, considerable attention was paid towards the synthesis of carbocyclic nucleosides with cyclopentane ring [3][4] over the last decade.But little effort has been paid towards the synthesis of such molecules with different ring sizes 5 , particularly with six-and seven-membered rings.Besides, the realization that the biological activity normally resides in a particular enantiomer has given an impetus to the task of generating enantiomerically pure carbocyclic nucleosides.For an easy access to this class of chiral nucleosides, the synthesis of appropriate aminocarbocycles/hydroxycarbocycles in optically pure form is desirable.Though cyclopentadiene is often used for asymmetric synthesis of chiral amino-or hydroxy-cyclopentane derivatives 4 , the other procedures to prepare these carbocycles involve (i) ring closing metathesis reaction between two olefins 6 , (ii) manipulative degradation of norbornadiene skeleton and construction of 5-membered ring 7 , (iii) construction of cyclopentane ring system through Michael-Aldol cyclization of appropriate intermediates 8 and (iv) preparation of requisite substrates from commercially available carbohydrates 9 .It is conceivable that carbohydrates could be ideal precursors for preparing chiral carbocyclic nucleosides.An additional advantage of this approach in using carbohydrates lies in the fact that the optically pure aminocarbocycles are occasionally utilized as potential glycosidase inhibitors and as antibiotics as well.
We had previously shown that chiral carbocycles of different ring sizes 10 fused to furanose ring could be constructed through the application of intramolecular nitrone cycloaddition reaction on glucose-derived nitrones.As a part of our programme in search of analogues of newer chiral carbocyclic nucleosides, we wish to report herein the useful conversion of these furanocarbocycles towards the synthesis of such nucleosides with five-, six-and sevenmembered carbocyclic rings.

Scheme 1
In the cyclization reaction the presumed chloropurine derivatives generated in situ underwent facile transformation to the respective dimethylaminopurine nucleosides 10, 12 and 15 conceivably through nucleophilic displacement of the chloro group by dimethylamine derived from DMF.The formation of these products may be due to H-bonding between N-3 of purine ring and a hydroxy substituent at C-2 / facilitating nucleophilic attack at C-6 by nucleophiles.The minor products 11 and 13 were found to be methoxypurine analogues, which were formed during purification of nucleosides by reversed phase HPLC using H 2 O-MeOH solvent system.The chloronucleoside 14 was, however, unchanged during chromatography and in this case no methoxypurine nucleoside was isolated.
To find out the possible role of the hydroxyl group adjacent to the amino group in influencing the ease of substitution of chloro group by NMe 2 and methoxy groups, we wanted to protect the 2 / -hydroxyl groups in 4 and 5.While 4 was easily diallylated to 17, attempted diallylation of the hydroxyl groups in 5 furnished only the monoallylated derivative 18 presumably due to steric hindrance offered by the bridged ring.The compounds 17 and 18 on transfer hydrogenolysis with cyclohexene-Pd/C treatment furnished crude di-n-propyl aminocyclopentane carbocycles, which on cyclization by the usual method furnished only the chloronucleoside analogues 20 and 21 (Scheme 2).
Removal of the acetonide functionality from each of the compounds 1-3 by acid treatment was evident from the disappearance of two distinctive methyl signals at δ ~ 1.30 and 1.50.The generation of two hydroxyl groups in 4-6 was confirmed after their conversion into diacetates.In each case two acetoxy methyl peaks at ~ δ 2.1-2.2 were observed in their 1 H NMR spectra.The trans-disposition of the 2,3-hydroxyl groups in 5 and 6 was indicated by the recovery of the starting diols from attempted periodate oxidation.However, the same periodate oxidation reaction on 4 resulted in the formation of a dialdehyde characterized by the peak at 1730 cm -1 in the IR spectrum of the crude product indicating the cleavage of the cyclopentane ring.During the reaction sequences, the isoxazolidine ring was not disturbed as was evident from the presence of aromatic and benzylic protons in the 1 H NMR spectra of the compounds.However, the absence of aromatic as well as benzylic proton signals in the 1 H NMR spectra of the crude products obtained by hydrogenolysis of 4-6 suggested the cleavage of the isoxazolidine rings accompanied by debenzylation.
Formation of the products 7-9 from the coupling reactions of the aminocarbocycles with 5amino-4, 6-dichloropyrimidine was evident from the appearance of a one-proton singlet at δ ~7.8 in the 1 H NMR spectrum which is characteristic for the aromatic H-2.On cyclization, the two aromatic proton singlets appeared at δ ~8.1-8.5 in all the purine nucleosides.The characteristic feature in the 1 H NMR spectrum (in DMSO-d 6 ) of the compounds 10, 12 and 15 was the presence of a very broad signal at  δ ~3.45 assigned to NMe 2 .At 60 0 C the broad peak changed to a sharp singlet.Additionally, the 13 C signal for NMe 2 of these compounds (in DMSO-d 6 ) could not be detected at normal temperature but appeared as a sharp singlet at δ ~ 38.8 at an elevated temperature (60 0 C).The 1 H NMR spectrum of 11 and 13 exhibited a sharp singlet at δ  4.10, characteristic for the OCH 3 , in addition to the aromatic proton signals at δ 8.39 and 8.51 (for 11) and 8.36 and 8.50 (for 13).The presence of 13 C peaks at δ ~53.8 (OMe), 143.3, 151.0 (aromatic CH) in addition to other appropriate signals are also in conformity with the assigned structures.The mass spectra (FABMS) of 14 and 16 exhibited molecular ion peaks at m/z 299 and 301 (for 14) and at 280 (for 16).
The 1 H NMR signals of the two methylene groups of the compound 18 appeared at δ 1.74, 2.06 and 2.36; N-CH (isoxazolidine ring juncture) as well as CH-O allyl signals were found at δ 3.61, CH-O (isoxazolidine ring juncture) signal at δ 4.58 and CH-OH signal at δ 4.65.In confirmation, the proton signal at δ 4.65 shifted downfield to δ 4.85 after acetylation.Finally, upon irradiation of the signal at δ 4.85 of the acetylated product 19, the doublet of a double doublet (ddd) at δ 2.02 (J =15, 7, 2.5 Hz) for one of the methylene protons changed to a broad doublet (J =16 Hz).Similarly, upon irradiation of the peak at δ 4.58, the same methylene signal appeared as dd (J =7, 15 Hz), confirming the substitution pattern.

Scheme 2
The 1 H NMR spectrum of 20 did not show any signal for NMe 2 group and the FABMS showed the molecular ion peak at m/z 369 (MH + for Cl 35 ) and 371 (MH + for Cl 37 ).Similar results were obtained with the nucleoside 21 (protonated molecular ion peaks at m/z 327 and 329).Thus, the participation of 2 / -OH group in generating dimethylaminopurine and methoxypurine nucleoside analogues through nucleophilic displacement of chloro group is obvious.
In conclusion, the present work mainly deals with an efficient synthetic route to enantiomerically pure carbocyclic nucleoside analogues with five-, six-and seven-membered rings starting from D-glucose derived precursors.The neighbouring 2 / -OH group is involved in forming hydrogen bond with N-3 of purine nucleus and helps to displace C-6 chloro group by nucleophiles like dimethyl amino and methoxy groups.

Experimental Section
General Procedures.Melting points were taken in open capillaries and are uncorrected.IR spectra were measured on a JASCO 700 spectrophotometer. 1 H and 13 C NMR spectra were measured either on a JEOL FX-100 or on a Bruker AM 300 L spectrometer using TMS as internal standard.Mass spectra were obtained using a JEOL AX-500 spectrometer operating at 70 eV.Optical rotations were measured in a JASCO DIP 360 polarimeter.HPLC was performed on µ Bondapak TM C 18 column (7.8x300 mm).Flash chromatography was carried out on LiChroprep R RP-18 (Merck).
To this ketone dissolved in MeOH (40 mL) at 10 o C was added NaBH 4 (2x125 mg) portionwise and the mixture was stirred for 5 h.The solvent was evaporated, brine (20 mL) was added to the residue, and the crude product was extracted with CHCl 3 (2x50 mL).The combined CHCl 3 extract was washed with H 2 O (2x25 mL), dried with anhydrous Na 2 SO 4 and evaporated to give a residue, which was purified by column chromatography, eluting with CHCl 3 -MeOH (98:2) mixture to afford 4 (257 mg, 71%): mp 127-128 o C; 1  The compound 5 (in 64% yield) was prepared following the method as used in the preparation of 4 using the same protocol.