Since a large and growing number of natural products have been shown to contain fused cyclic acetals, such as furo [2,3-b]furans, as part structures [1] and since many of them have a variety of activities (e.g., antitumor, antimicrobial, and insect antifeedant), interest has rapidly grown and resulted in several strategies for the construction [2] of these systems. Herein, we describe a protocol, utilising an intramolecular radical cyclisation [3] of a-halogeno acetals [4] of chiral templates derived from the furanoid glycal 1, for the synthesis of fused bicyclic acetals 8-11.
The required furanoid glycal, 1,4-anhydro-2-deoxy-5,6-O-isopropylidene-o-arabinohex-l-enitol (1) [5] was prepared from D-mannose. Reaction of 1 with tert-butyldimethylsilyl chloride in the presence of imidazole in N,N-dimethylformamide afforded 2 in 83% yield. In the 1H NMR (200 MHz) spectrum of 2, H-1 and H-2 resonated at 6 6.55 (d, J1,2 2.5 Hz) and 5.1 (t), respectively. Treatment of 2 with N-bromosuccinimide [6] in the presence of 10 equivalents of the propargylic and allylic alcohols 2-propyn-l-ol, 2-methyl-3-butyn-2-ol, 2-propen-l-ol, and 2-methyl-2-propen-l-ol resulted in the stereo- , allyl 2-bromo-3-O-tert-butyldimethylsilyl-2-deoxy-5,6-O-isopropylidene-fl-D-glucofuranoside (5), and 2-methyl-2-propenyl 2-bromo-3-O-tert-butyldimethylsilyl-2-deoxy-5,6-O-isopropylidene-/3-~glucofuranoside (6), respectively, in good ~ IICT Communication No. 3321.