Ethynylation of 1,2:5,6-di- O-isopropylidene-α- D-ribo-hexofuranos-3-ulose ( 1) gave the 3- C-ethynyl allo derivative 2, together with an adduct ( 3) resulting from interaction of two molecules of 1 with one of acetylene. Lithium aluminum hydride reduced the acetylenes 2 and 3 to the corresponding alkenes 4 and 8; on sequential ozonolysis-borohydride reduction, these both gave 3- C-(hydroxymethyl)-1,2:5,6-di- O-isopropylidene-α- D-allofuranose ( 6), further characterized as its 3,3 1-cyclic carbonate 9. Ozonolysis of the acetylene 2 gave the 3 1,5-lactone ( 5) of the 3- C-carboxy analog, thus establishing the stereochemistry of 2, which was independently established by n.m.r. spectroscopy employing a lanthanide shift-reagent. Treatment of 2 with mercuric acetate in ethyl acetate, followed by hydrogen sulfide, gave a mixture of the 3- C-acetyl-3- O-acetyl derivative 10 and a product ( 11) derived from internal cyclization of 5,6-deacetonated, O-deacetylated 10. Reduction of 10 with lithium aluminum hydride gave a separable mixture of diastereoisomeric 3- C-(l-hydroxy-ethyl) derivatives ( 12a, 12b) that were individually converted into their corresponding 3,3 1-cyclic carbonates 13a and 13b, products that contain the branch functionality of the unusual, branched-chain sugar aldgarose.