A convenient synthesis of new 6-substituted purinylcarbanucleosides on cyclopenta [ b ] thiophene

The first members of a new family of heterocarbobicyclic nucleoside analogues have been synthesized from the cis/trans mixture of (4-amino-5,6-dihydro4H-cyclopenta[b]thiophen-6-yl)methanols cis/trans-7. The separation of cis and trans intermediates during preparation of the 6-chloropurine derivatives allowed separate preparation of the purine heterocarbanucleosides cis-10 and trans-11, from which cis(12-14) and trans-(16-18) were obtained by replacement of the 6-chloro substituent with amino, hydroxy and cyclopropylamino groups. Additionally, the 6-phenyl-purinyl analogues cis-15 and trans-19 were prepared from cis-10 and trans-11 using SuzukiMiyaura methodology.


Introduction
The development of new antiviral drugs is a dynamic process driven by the identification of new molecular targets and the emergence of problems associated with drugs in current clinical use (resistance, toxicity, etc.). [1]In recent decades, many research programs have sought non-toxic antiviral drugs that act by selective inhibition of kinases or polymerases. [2]Among the most extensively and intensively studied compounds are the nucleoside analogues, which become active when acted upon by kinases after entry into a target cell.A number of these prodrugs have been found to have antiviral activity and/or antitumoral activity, and some are in clinical use. [3]igure 1 The many structural modifications that have been made to natural nucleosides in the search for desired biological properties include the introduction of a double bond between positions 2' and 3' of the sugar ring.The seminal work of Balzarini et al. [4] showed that when the parent compound is a pyrimidine nucleoside the resulting cytosine and thymidine analogues (respectively d4C (1) and d4T (2); see Figure 1) are active against human immunodeficiency virus (HIV), and d4T is still in clinical use under the commercial name Stavudine ® . [5]The antiviral activity of these compounds is thought to be related to the conformational restriction imposed on the pseudosugar moiety by its double bond, [6] which also increases the lipophilicity of the molecule and thereby facilitate its access to the central nervous system, a major reservoir of the HIV virus. [7] carbanucleosides (CNs), [8] the D-ribose moiety of the natural nucleosides is replaced by an aliphatic or aromatic carbocycle.CNs include the well-known anti-HIV agents carbovir (3) [9] and abacavir (4, marketed as Ziagen ® ); [10] the purine derivative BMS-200475 (5), [11] which is active against hepatitis B virus (HBV); and the pyrimidine derivative carba-BVDU (6), which is active in vitro against herpes simplex virus 1 (HSV-1) (Figure 2). [12]Once in a cell, they are active for longer than analogues with the endocyclic oxygen of natural nucleosides because they resist the phosphorylases and hydrolases that cleave the glycoside bonds of the latter.Furthermore, replacement of the endocyclic furanose oxygen by a methylene group makes CNs less toxic than their parent compounds.In previous papers our research group reported the synthesis of abacavir analogues in which a purine or pyrimidine base was linked to an indane system. [13]Some of these compounds showed considerable cytostatic activity against human T lymphoblastic leukaemia cell lines (Molt4/C8 and CEM/0) and murine leukaemia cells (L1210/0), [14] and many of the active purine derivatives featured an oxo or amino group at position 6 that would allow hydrogen bonding between the nucleobase and the polymerases and other enzymes involved in nucleic acid metabolism. [15] the search for ways of increasing the lipophilicity and polar interactions of the pseudosugar while maintaining the rigidity of this moiety, our group has now begun to explore a new class of analogues in which the aromatic ring of the indane system has been replaced by a heterocyclic aromatic ring system. [16]The results have been encouraging: for example, preliminary biological assays of purinylmethyl derivatives of 2-benzylcyclopenta[c]pyrazoles have found them to possess high activity against varicella zoster virus and cytomegalovirus at subcytotoxic concentrations, [17] and derivatives of 1-methylcyclopenta[c]pyrazoles have cytostatic activity. [18]We are currently exploring the case in which the pseudosugar is cyclopenta[b]thiopene; in the work reported here, we examined the dependence of the biological activity of some members of this family on the stereochemistry of the pseudosugar-nucleobase linkage.
13c,14a] In this work we chose the former approach not only because it allows divergent synthesis of multiple CNs from a single pseudosugar intermediate, but also because it would hopefully allow easy separation of stereoisomers with cis and trans pseudosugar-nucleobase linkages (whereas in previous work [20] direct separation of the mixture of amino pseudosugar isomers cis/trans-7 had been found to afford only moderate combined yields).Our hopes in this respect were fulfilled, flash column chromatography of the heterocarbanucleoside precursors cis-8 and trans-9 (Scheme 1) was accomplished more eficiently (96%).These CNs have been synthesized in its racemic form, so the resolution of the enantiomers was planned for a later work, once the biological activities of the racemic mixtures were known.

Results and discussion
The aminoalcohol mixture cis/trans-7 was synthesized as before, [20] in spectroscopically determined 1:1 isomer ratio and 55% combined yield, by reduction of the corresponding hydroxyiminoester in refluxing THF with AlH 3 that had been freshly prepared in quantitative yield from LiAlH 4 and H 2 SO 4 . [21]Reaction of cis/trans-7 for 45 hours with 5-amino-4,6-dichloropyrimidine and Et 3 N in refluxing n BuOH afforded a mixture of cis-8 and trans-9 in 41% and 36% yield, respectively (the combined yield of 77% was less with shorter reaction times), and flash column chromatography of this mixture using 40:1 CHCl 3 /MeOH as eluent efficiently separated cis-8 from trans-9 (in addition, a small amount of starting material was recovered and traces of 5-amino-4chloro-6-hydroxypyrimidine were detected).Treatment of cis-8 and trans-9 with triethyl orthoformiate under the classical conditions for synthesis of the 5-membered ring of purine [22] afforded the corresponding 6-chloropurines, cis-10 and trans-11, in 80% and 87% yield, respectively (Scheme 2).

General
Melting points are uncorrected and were determined in a Reichert Kofler Thermopan or in capillary tubes in a Büchi 510 apparatus.Infrared spectra were recorded in a Perkin-Elmer 1640 FTIR spectrophotometer. 1 H NMR spectra (300 MHz) and 13 C NMR spectra (75 MHz) were recorded in a Bruker AMX 300 spectrometer using TMS as internal reference (chemical shifts in δ values, J in Hz).EI and FAB mass

Figure 3 .
Figure 3. MERCURY projection (with 40% probability ellipsoids) of the molecular structure of trans-18 (the atomic numbering is arbitrary).