Total synthesis of hydroxymethyl isonucleosides as potential antiviral agents

Hydroxymethyl isodideoxynucleosides, designed as potential antiviral agents, have been synthesized through development of a multi-step procedure starting from furan and cyanovinyl acetate. Key steps in the synthesis include a Diels-Alder reaction, oxidative cleavage of alkene, a Mitsunobu reaction, stereospecific introduction of amino group and nucleobase construction


Introduction
Isomeric dideoxynucleosides as antiviral agents have been the subject of intense investigation in our laboratory 4 and have also been studied by others. 5Work in our laboratory led to the discovery of 4 (S)-(adenin-9-yl)-2(S)-hydroxymethyltetrahydrofuran [(S,S)-isoddA] (1, Figure 1) which exhibits potent anti-HIV activity against HIV-1, HIV-2, and HIV-resistant strains. 6Its triphosphate is among the strongest known inhibitors of HIV reverse transcriptase.In the design of new structures that may have anti-HIV activity, we decided to explore analogues of this lead compound by introducing functionalized substituents into the sugar moiety.Introduction of an additional hydroxymethyl group was of interest, not only because of the observation of anti-HIV activity associated with isomeric nucleosides in which the adenine ring and the -CH 2 OH have cis-1,3-and 1,2-relationships (2, Figure 1), 6,7 but also because of the known antiviral activity of natural oxetanocin and its ring expanded analog which bear an additional -CH 2 OH group. 8The molecules targeted for synthesis (3-5) are shown in Figure 1.

Results and Discussion
For the preparation of the proposed hydroxymethyl isodideoxynucleosides, the initial step of the procedure involved a Diels-Alder reaction (Scheme 1).Thus, furan 6 was treated with 1cyanovinylacetate (7) and catalyst, zinc diiodide, and stirred at room temperature for 4 days to give an adduct 8 in high yield (70%). 9,10 Adduct 8 was converted to ketone 9 by sequential treatment with 1N potassium hydroxide and formaldehyde. 10Ketone 9 was reduced stereoselectively to alcohol 10 by sodium borohydride in methanol in 65% yield from 8. 11 However, direct reduction of 8 with sodium borohydride gave even higher yields of 10 (89%) and with the same stereoselectivity.Protection of compound 10 with tert-butyldimethylsilyl chloride gave intermediate 11 (83%).
An attempt to convert olefin 11 to a dialdehyde 13 in one pot, using osmium tetroxidesodium periodate system, 1 did not give a clean reaction.Therefore, compound 11 was converted to the dialdehyde 13 in two discrete steps.First, olefin 11 was dihydroxylated to the triol derivative 12 using catalytic amounts of osmium tetroxide and stoichiometric amounts of Nmethyl morpholine N-oxide (88% yield). 1 Purified 12 was treated with sodium periodate in tetrahydrofuran-water to give the dialdehyde 13, which was immediately reduced to the triol derivative 14 in 76% yield by sodium borohydride.Because the initial Diels-Alder reaction produced a racemic adduct 8, the sequence of reactions from 8 would produce the racemic triol derivative 14.

Scheme 1
Intermediate 14 was protected to its ditrityl derivative 15 with trityl chloride (82%) which was desilylated using tetrabutylammonium fluoride (TBAF) in 85% yield (Scheme 2).An attempt to invert the chirality at the C-3 position of 17 using benzoic acid-PPh 3 -DEAD was not successful probably due to the bulky trityl groups.However, under similar conditions using PPh 3 and I 2 the iodide 20 with inverted configuration at the C-3 position was produced in 87% yield. 1 Coupling the iodide 20 with sodium azide in dimethyl sulfoxide (DMSO) gave the desired azide 22 but in moderate yields (58%).Elimination to produce an olefin byproduct 20 1 in 16% yield was also observed.The catalytic reduction of the azide 22 was compromised by partial removal of the trityl protection to give 56% yield of an amine 27 (Scheme 3).Thus, to avoid the problems derived from the trityl protection, the diol 14 was protected with MOMCl to give intermediate 16 which was desilylated to alcohol 18 in 79% yield from 14 (Scheme 2).Treatment of alcohol 18 under Mitsunobu conditions and in the presence of benzoic acid afforded benzoate 23, which was debenzoylated with ammonia in methanol to give the inverted alcohol 24.As previously mentioned, this conversion with inversion of stereochemistry could not be achieved with the trityl counterpart 17.Alcohol 24 was mesylated to 25 (98%) which was then treated with sodium azide in DMF to give azide 26 in 82% yield without the eliminative by-product.However, development of the pathway utilizing the MOM protecting group was done after the total synthesis was complete with the trityl protecting group in order to have a more efficient pathway for the synthesis of the key azido intermediate.

Scheme 3
Ammonolysis with methanolic ammonia gave the desired adenosine analogue 3 in 53% yield from 28.For the preparation of the uridine analogue, intermediate 27 was treated with 3methoxyacryloyl isocyanate, 17 freshly prepared from 3-methoxyacryloyl chloride and silver cyanate to give an acryloylurea 30, which was then cyclized to the uracil analogue 31 on treatment with ammonium hydroxide at 100 °C.Deprotection with trifluoroacetic acid gave the target uridine 4 (34% yield from 27).The cytidine analogue 5 was obtained from the uridine 31 by sequential treatment with 2,4,6-triisopropylbenzenesulfonyl chloride and ammonium hydroxide 18 followed by deprotection with dilute HCl in 46% yield.
In summary, target compounds, 3, 4, and 5, were synthesized as racemates from the key intermediate, the tetrahydrofuran triol 14, which was obtained efficiently using a Diels-Alder pathway from basic starting compounds, furan (6) and 1-cyanovinylacetate (7).Antiviral studies of the target compounds are currently in progress.

Experimental Section
General Procedures.Melting points reported are uncorrected and were determined on an Electrothermal Engineering Ltd. melting point apparatus.Nuclear magnetic resonance spectra were recorded on Bruker Model AC300 and WM 360 systems.Ultraviolet spectra were recorded on a Varian Cary Model 3 spectrophotometer.Flash chromatography used 230-400 mesh silica gel.HPLC analyses were carried out on a Beckman-Coulter instrument with C-18 reversedphase columns.