Synthesis and conformational analysis of seco C-nucleosides and their diseco double-headed analogues of the 1,2,4-triazole, 1,2,4-triazolo[3,4-b]1,3,4-thiadiazole1Parts of this work were presented at 19th International Carbohydrate Symposium, San Diego, CA, USA, August 1998 and XIII International Ro