A New Method for the Stereoselective Synthesis of Nucleosides by Means of Sodium Borohydride Mediated Reductive C‐C or C‐N Bond Cleavage Reaction

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Abstract
New dienophiles, dimethyl acetoxymethylenemalonate and its analogues, have been synthesized. These dienophiles reacted with cyclopentadiene to give Diels‐Alder adducts, which served as versatile synthetic precursors for carbocyclic C‐nucleosides. The method has the following three characteristics: (1) the C‐C bond in the adduct originated from the dienophile can be cleaved stereoselectively by the reductive retrograde aldol (RRA) reaction, (2) the CC bond in the adduct serves as a latent functionality of 2′‐ and 3′‐ positions of the nucleoside, and (3) the malonate unit can be transformed to a variety of heterocycles. The high pressure mediated Diels‐Alder reaction of furan with the same dienophiles also gave the cycloadducts, which served as synthons for C‐nucleosides. Use of chiral dienophiles, such as di‐1‐menthyl acetoxymethylenemalonate, for the above Diels‐Alder reactions has offered a new route to the enantioselective synthesis of C‐nucleosides and their carbocyclic analogues. Essentially the same methodology has also been applied to the synthesis of carbocyclic nucleosides from 2‐azabicyclo[2.2.1]hept‐5‐en‐3‐one, in which a reductive amido bond cleavage reaction similar to (1) is involved. Synthesis of carbocyclic oxetanocin and its analogues is also described.

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