Glycals in Organic Synthesis: A Systematic Strategy for the Preparation of Uncommon Piperidine 1,2‐Dideoxy‐ L ‐azasugars and 2‐Deoxy‐1,5‐anhydro‐ L ‐hexitols

A systematic synthetic strategy has been developed for producing uncommon piperidine 1,2‐dideoxy‐ L ‐azasugars. This method involves the formation of open intermediates such as 2 , 7 , and 10 easily by ring‐opening of D ‐glycals with aqueous mercury(II) acetate/sodium borohydride. A concise sequence of regioselective amination and cyclization reactions then allowed us to prepare the cyclic compounds 5a and 5b , L enantiomers of naturally occurring fagomine congeners such as 3‐ epi ‐fagomine ( II ) and 3,4‐di‐ epi ‐fagomine ( III ), from D ‐glucal and D ‐galactal, respectively. The unnatural 3,4‐di‐ epi ‐6‐deoxyfagomine 9 was obtained from L ‐rhamnal by the same reaction sequence. This straightforward chemistry has been shown to be useful for preparing glycosyl derivatives of 1,2‐dideoxy‐ L ‐azasugars starting from glycosyl glycals such as D ‐lactal, D ‐cellobial, D ‐maltal, and D ‐melibial, thus avoiding the usually lengthy glycosylation procedures. The flexibility of our protocol has been demonstrated by the easy conversion of the above‐described open intermediates into uncommon 2‐deoxy‐1,5‐anhydro‐ L ‐hexitols, never previously described. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)