A versatile approach to novel homo-C-nucleosides based on aldehydes and acetylenic ketones derived from ribo-and 2-deoxy-ribofuranose C-glycosides

A series of ribofuranosyl-and 2-deoxyribofuranosyl homo-and spacered-C-nucleosides have been synthesized by reaction of fully protected 3-(1-deoxy- - D -ribofuranosyl-1-yl)propanal ( 1 ), 3-(1,2-dideoxy- - D -ribofuranos-1-yl)propanal ( 14 ), 1-(1-desoxy- - D -ribofuranos-1-yl)pent-4-yn-3-on ( 19 ), 1-(1-desoxy- - D -ribofuranos-1-yl)-5-phenyl-pent-4-yn-3-on ( 20 ), 1-(1,2-didesoxy- - D -ribofuranos-1-yl)pent-4-yn-3-on ( 29 ), and 1-(1,2-didesoxy- - D -ribofuranos-1-yl)-5-phenyl- pent-4-yn-3-on ( 30 ) with different nucleophiles. The preparation of 1 and 14 proceeds by Knoevenagel reaction with malononitrile, cyanoacetamide and 2-cyano-N -(4-methoxy-phenyl)acetamide and subsequent cyclization with sulphur to thiophenes 5 , 7 , 8 , 16 and then by cyclization with triethyl orthoformate to give thienopyrimidine 6 and thienopyrimidinone 9 , 10 , and 17 . Treatment of acetylenic ketones 19 , 20 , 29 , and 30 with acetamidinium chloride, benzamidinium chloride, and S -methylisothiouronium sulphate provided the corresponding pyrimidines 21 – 26 , 31 , 32 . Finally, the use of 4 H -1,2,4-triazol-3-amine and 2-aminobenz-imidazole as 1,3-N , N’ -dinucleophiles afforded the triazolopyrimidines 35 , 39 and the pyrimidobenzimidazoles 36 , 37 , and 40 , respectively. Deprotection of a selected number of C-nucleosides was achieved by one or two steps procedure without serious problems. That makes these C-nucleosides promising candidates for the synthesis of monomers suitable for solid phase nucleic acid oligomerization.


Introduction
The interest in nucleoside analogues is unbroken and their design and synthesis have been done with quite different intentions e.g.[8][9][10] Pursuing a program directed at the synthesis of homo-C-nucleosides, we have described previously an efficient route for the preparation of -allyl C-glycosides of D-ribofuranose and 2deoxy-D-ribofuranose. 11Recently we have reported the synthesis of consecutive compounds e.g.alcohols, amines, aldehydes, and acetylenic ketones. 12In this contribution we present our results on transformation of the furnished aldehydes (1,14) and acetylenic ketones (19, 20, 29, 30) as versatile intermediates into a selected number of different heterocycles to make the synthetic potential of these precursors visible.

Synthesis of thienopyrimidine homo-C-nucleosides
The synthesis started from the aldehyde 1 which was readily obtainable from -allyl C-glycoside of D-ribose 11 via hydroboration/ oxidation and selective oxidation of the corresponding alcohol. 12reatment of 1 with malononitrile, cyanoacetamide and 2-cyano-N-(4-methoxyphenyl)acetamide provided the corresponding Knoevenagel products 2-4 in 60%, 51%, and 77%, respectively (Scheme 1).The reaction was carried out by using an excess of the CH-acidic compounds and basic aluminium oxide in boiling toluene. 13The reaction time varied between 2 and 16 h as monitored by TLC.All analytical data were in accordance with the proposed structures.Additionally, the values for the coupling constants JH2-CN and JH2-C=O (13-14 Hz and 5-6 Hz, respectively) determined from coupled 13 C NMR spectra confirmed the E-configuration of structures 3 and 4. When the compounds 2-4 were treated with elemental sulphur and triethylamine in N,N-dimethylformamide (DMF) 13,14 for 2 h at r.t. the light yellow aminothiophenes 5, 7, and 8 were obtained in about 80% yield.The 1 H NMR spectra showed signals at δ 2.81-3.02and the 13 C NMR spectra provided signals at δ 33.7-33.9characteristic of the methylene unit of homo-C-nucleosides.For the synthesis of 4-aminothienopyrimidine 6 compound 5 was reacted with triethyl orthoformate under reflux for 2 h.Without purification the resulting formimidate was treated with a saturated ethanolic ammonia solution to afford the desired compound 6 in 74% overall yield. 15Cyclization to the thienopyrimidinones 9 and 10 were achieved when a mixture of compounds 7 or 8 and triethyl orthoformate were heated at reflux in DMF for 7-10 h.In spite of the drastic reaction conditions the desired derivatives 9 and 10 were obtained in about 70% yield.In the 1 H NMR spectra singlets were observed at δ 8.05 and 8.02 characteristic of H-2 of 9 and 10, respectively.Finally, treatment of 9 and 6 with 90% trifluoroacetic acid in CH2Cl2 removed both the silyl and isopropylidene protecting groups, providing the unprotected derivatives 11 and 13 in 90 % yield.In contrast, deprotection of thiophene 5 required a two step procedure.Firstly, the tertbutyldiphenylsilyl group (TBDPS) was cleaved off by treatment of 5 with a solution of tetrabutylammonium fluoride (TBAF) in 1,4-dioxane.After 5 h, the isopropylidene group was then removed with 90% trifluoroacetic acid in CH2Cl2 to give 12 in 87% overall yield.In a previous paper, 11 we described an efficient route to transfer -allyl C-glycoside of Dribose into the corresponding 2-deoxy ribofuranose.Employment of exactly the same conditions of hydroboration-oxidation and consecutive selective oxidation of the corresponding alcohol furnished aldehyde 14. 12 The versatility of 14 was demonstrated by the preparation of thieonopyrimidinone 18 by the same sequence of reaction steps as described for compound 13 (Scheme 2).Even the reaction conditions were identical only slightly differences of reaction time and yield occur.Thus, Knoevenagel product 15 was obtained in 52% yield.Transformation of 15 into thiophene 16 (73%) was followed by ring closure reaction to provide 17 in 69% yield.Deprotection was simply achieved by treatment of 17 with TBAF in a solution of 1,4-dioxane.After 2 h at r.t.derivative 18 was obtained in 78% yield.
As described previously, 12 aldehydes 1 and 14 can be converted to acetylenic ketones 19, 20 and 29, 30 by reaction with ethynylmagnesium bromide or phenylethynyllithium, respectively, and followed by oxidation of the diastereomeric alcohols.Analytical sample of 20 was obtained by crystallization from ethyl acetate-n-hexane.Its constitution was confirmed by X-ray diffraction studies (Figure 1).

Synthesis of pyrimidine-spacered C-nucleosides
7][18][19][20] Herein we report efficient short synthesis of substituted pyrimidines and triazolopyrimidines.Following the strategy of Addlington et al, 17 who reported the reaction of acetylenic ketones with amidinium salts using ethyl acetate/water as solvent and sodium carbonate as base, 19 and 20 were treated with acetamidinium chloride, benzamidinium chloride, and S-methylisothiouronium sulphate to give the corresponding pyrimidines 21-26 separated from the tetrahydrofuran ring by an ethylene group (Scheme 3).All reactions proceeded in good (60%) to excellent yield (quantitative).As expected, all analytical data were in agreement with the proposed structures.We examined the stepwise and complete deprotection of the obtained pyrimidines using the example of compound 25.Treatment of 25 with aq HCl in EtOH resulted in simultaneous removal of both silyl and isopropylidene protecting groups to give 28 in 74% yield.On the other hand, in the presence of TBAF in 1,4-dioxane only the TBDPS group was removed and the partial protected derivative 27 was obtained in 85% yield.Again, the 2-deoxy acetylenic ketones 29 and 30 were allowed to react with selected amidinium salts e.g.S-methylisothiouronium sulphate and acetamidinium chloride to provide the pyrimidines 31 and 32, respectively (Scheme 4).Deprotection of 31 with TBAF in 1,4-dioxane afforded 33 in 78% yield.

Synthesis of triazolo-and pyrimidinobenzimidazole-spacered C-nucleosides
Nucleophilic attack of 4H-1,2,4-triazol-3-amine on the triple bond of ynone 19 in boiling EtOH resulted in compound 34 in 89% yield (Scheme 5). 21The 1 H and 13 C NMR spectra of 34 were fully consistent with the assigned structure.As expected, no signals of acetylenic carbon atoms were observed in the 13 C-NMR spectra, but a resonance was visible at δ 151.7 for C-3'' of the triazole ring.The E-configuration of the addition product was evident from the large coupling constant 3 J4,5 13.3 Hz in the 1 H NMR spectra.In order to prepare a fused heterocycle enone 34 was treated with sodium ethanolate at r.t. for 1 h to afford triazolopyrimidine 35 in 62% yield.Analogous to that addition reaction 2-aminobenzimidazole was used as 1,3-N,N'-dinucleophile and allowed to react with ynone 19 and 20.Suprisingly, the TLC of the reaction solution showed in each case the formation of a mixture two products after reflux (EtOH) for 2 h.The stepwise formation of compound 35 strongly suggested that here a mixture of an addition products (in parenthesis Scheme 5) and the fused heterocycles 36 and 37 were observed.Indeed, treatment of the unseparated reaction mixture with sodium ethanolate at r.t. for 1 h caused disappearance of the side-products and provided 36 and 37 in 76% and 80% yield, respectively.The protocol of the formation of triazolopyrimidine and pyrimidobenzimidazole by reaction of 4H-1,2,4-triazole-3-amine and 2-aminobenzimidazole, respectively, was now transferred to the ynone 29 (Scheme 6).Fortunately, the course of all reactions was comparable to ynone 19.Consequently, triazolopyrimidine 39 and pyrimidobenzimidazole 40 were obtained in 69% and 63%, respectively.The regioselectivity of the ring closure reactions was evident from NOESY and HMBC experiments.In summary, we have shown that fully protected ribofuranosylpropanal (1), 2-deoxyribofuranosylpropanal ( 14) and the corresponding alkynyl ketones 19, 20, 29, and 30 are suitable intermediates for the preparation of homo-and spacered C-nucleosides.The tetrahydrofuran ring and the protecting group pattern is stable enough even under drastic reaction conditions to allow the synthesis of a broad variety of heterocyclic systems.In one of our next papers, we will describe the conversion of some of our C-nucleosides into building blocks suitable for solid phase synthesis of nucleic acid oligomers.

Deprotection of compounds (6) and (9)
90% Trifluoroacetic acid (25 mL) was added to a solution of compound 6 (576 mg, 1.0 mmol) or compound 9 (576 mg, 1.0 mmol) in CH2Cl2 (10 mL).After stirring at r.t.(monitored by TLC), the reaction mixture was concentrated.Traces of acid were removed by evaporation with repeated addition of toluene.The residue was then subjected to flash chromatography.