Thispaper is dedicated to Prof. Raymond U . Lemieux on the occasion of his 60th birthday VANGA S. RAo and ARTHUR S. PERLIN. Can. J. Chem. 59,333 (1981).The reaction between 2,3,4,6-tetra-0-benzyl-1,5-di-O-mesyl-~-glucitol(2) and potassium superoxide resulted in the loss of H-4 and the 5-mesyloxy (as well as 1-mesyloxy) substituent, and an almost quantitative conversion into en01 ether 4, i.e., 1,3,4,5-tetra-O-benzyl-3-dehydro-2-deoxy-~-threo-hex-2-enitol. When the reaction was performed with 8, in which the 1-0-mesyl group (of 2) is replaced by 0-(methoxy)trityl, the outcome was wholly different: an olefin was formed through the removal of a primary (H-6) rather than a secondary (H-4) proton, and nucleophilic displacement also took place. Results similar to those for 8 were obtained with the 2-epimer of 2, i.e., 2,3,4,6-tetra-O-benzyl-1,5-di-O-mesyl-~-rnannitol (12). It is suggested that selective displacement of the 1-0-mesyl group of 2 by superoxide generates a 1-peroxy anion (6) that abstracts H-4 intramolecularly, promoting concomitant loss of the 5-mesyloxy group. The transition state for proton abstraction within anion 6 appears to be more stable than that of the corresponding anion in the manno series, accounting for the different reaction routes for 2 and 12. Elimination occurred also in the reduction of 2,3,4,6-tetra-0-benzyl-D-glucopyranose with sodium borohydride in 2-propanol, en route to 2, affording 2,4,6-tri-0-benzyl-2-dehydro-3-deoxy-~-threo-hex-2-enitol (21). The 'H nmr spectra of alditol derivatives 2, 8, and 12 show that these molecules depart substantially from extended zigzag conformations, in contrast to configurationally related peracetylated acyclic derivatives. 1,3,4,5 dehydro-3 deoxy-2 L-thrio-hexbne-2 itol. Si I'on effectue la meme reaction sur le compose 8qui correspond au remplacement du groupe 0-mesyl-1 (de 2) par un groupe 0-(methoxy)trityl, la reaction est tres differente. I1 se forme une olefine par I'intermediaire d'une elimination de I'hydrogeneprirnaire (H-6) plutBt que parcelle du proton (H-4) et il se produit ainsi un deplacement nucleophile.
IAvec I'Cpimkre du compose 2, soit le tetra-0-benzyl-2,3,4,6 di-0-m6syl-1,5 D-mannitol (l2), on obtient des resultats semblables a I ceux que donne le compose 8. On suggere que le d6placement selectif du groupe 0-mesyl-1 du compos6 2 par le superoxyde, engendre un anion peroxy (6) qui enleve intramoleculairement H-4 en favorisant la perte concomitante du groupe mesyloxy en position 5. L'etat de transition lors de l'elimination du proton dans I'anion (6) semble itre plus stable que celui derivant de I'anion correspondant des series manno; ceci explique les reactions differentes de 2 et 12. Lors de la reduction du tetra-0-benzyl-2,3,4,6 D-gl~copyI'ano~e par le borohydrure de sodium dans le propanol-2, il se produit egalement une elimination qui conduit au tri-0-benzyl-2,4,6dChydro-2deoxy-3 D-thrio-hexene-2 it01 (21) plutBt qu'au compose 2 attendu. Les spectres rmn du 'H des derives alditol2,8, et 12 indiquent que ces molCcules s'eloignen...