The biological and pharmacological activity as well as the therapeutic potential of l-ascorbic acid and its derivatives have been studied extensively. [1] However, despite the fact that l-ascorbic acid possesses several interesting functionalities, the organic chemistry and synthetic potential of this molecule and its derivatives have never been explored to any significant extent. The complex chemical properties [1] of lascorbic acid may have hindered developments in this regard. Alkylation of l-ascorbic acid has been extensively studied, and conditions for both O-and C-alkylations have been worked out. [2] With these alkylation reactions, natural products such as as delesserine, dilaspirolactone aglycon, leucodrin, leudrin, methyl rhodomelol, reflexin, and rhodomelol have been synthesized. [3] Apart from alkylation, however, virtually no other reactions of l-ascorbic acid have been documented, except for reports on the Claisen rearrangement of its O-allyl derivatives. [1] The conjugated electron-rich enediol unit in l-ascorbic acid is a rather unique and interesting functionality. Such an olefin should be amenable to various noteworthy transformations and might display intriguing reactivity. Enediol or 2-aminoenol moieties in the form of 1,3-dioxol-2-one, [4] 2,3-dihydrooxazol-2-one, [5] and 1,3-dioxole [6] have already yielded corresponding oxetanes through Paterno Á ± Büchi reactions. Enantiomerically pure oxetanes have been synthesized from chiral substrates [7] and with the aid of chiral auxiliaries. [8] Bach and co-workers investigated the stereochemical aspects of oxetane formation, including facial diastereoselection. [9] l-Ascorbic acid constitutes a convenient reactive chiral olefin that should also furnish chiral oxetanes through Paterno Á ± Büchi reactions. Further transformations of the resulting oxetanes by way of ring-opening reactions would afford novel C-2-branched 3-ketosugars and C-3-branched 2-ketosugars. We describe here the preliminary results of our investigations in this area.
Irradiation of a solution of 1 and benzaldehyde in benzene with UV light through a pyrex filter produced compounds 2 a and 2 b in 60 and 25 % yield, respectively (Scheme 1). Spectroscopic data and elemental analyses indicate that these Scheme 1. Paterno Á ± Büchi reactions of 1 and 3. a) hn, C 6 H 6 , N 2 , 30 ± 60 h. Bn benzyl.