Recently a novel method for the construction of the furo[2,3&]quinoxaline ring system was achieved 1,2 by the dehydrative cyclisation of analogues of 2 during the acylation reactions under which condition 2 itself undergoes dehydration to give 8 or 9 depending upon the reagent used'. On the other hand, the structure 4 was reported -3 l4 for the product from the reaction of 1 with o-phenylenediamine and phenylhydrazine. The structure was based on the formation of a di-0-acetyl derivative4. However, the structure 2 was given for that product based on the results of periodate oxidation studies9,14, and its acetyl derivative was revised' to be a hemihydrate of its tri-0-acetyl derivative whose structure was confirmed by X-ray crystallography'.
In the present work, it seemed feasible that partial protection of the hydroxyl groups of 2 may afford derivatives such as 6, whose acylation would form the furo[2,3-blquinoxaline ring system. Consequently, the isopropylidenation of 2 and its D-erythro analogue 10 has been studied whereby a novel approach for constructing the furo[2,3-blquinoxaline was achieved.
The isopropylidenation of 3-[l-(phenylhydrazono)-t_-rhreo-2,3,4-trihydroxybutyllquinoxalin-2-one (2) had been expected to form either the corresponding o-threo-1,3-dioxolane, the cY-terminal-1,3-dioxolane, or the p-terminal-1,3-dioxane ring system. However, isopropylidenation of 2 with acetone and a catalytic amount of sulfuric acid did not afford any of the three anticipated products, but instead gave 2,2'-anhydro-[3-(l-(phenylhydrazono)-L-threo-3,4-O-isoprapylidene-2,3,4trihydroxybutyljquinoxaline (5). This result indicated that a novel dehydrative cyclisation had taken place in addition to the terminal isopropylidenation. The structure of 5 was confirmed by a combination of chemical and physical methods. Compound 5 resisted acetylation with acetic anhydride in pyridine, indicating the absence of hydroxyl groups. Aqueous acetic acid caused hydrolysis of the isopropylidene ring to give 2-2'-anhydro-[3-(l-(phenylhydrazono)-L-threo-2,3,4-trihy-