6698-34-6Relevant academic research and scientific papers
Stereochemical dependence of the mechanism of deoxygenation, with lithium triethylborohydride, in 4,6-O-benzylidenehexopyranoside p-toluenesulfonates
Baer, Hans H.,Mekarska-Falicki, Miroslawa
, p. 3043 - 3052 (2007/10/02)
Lithium triethylborohydride was shown to react with methyl 4,6-O-benzylidene-α-D-hexopyranoside 2- and 3-tosylates, and 2,3-ditosylates, in the manno, allo, and altro configurational series both by O-S fission (O-desulfonylation) and by C-O fission (C-desulfonyloxylation), to produce carbinol and deoxy functions, respectively.The results were compared with those previously obtained with the corresponding gluco and galacto isomers, and the degree of facility of the cleavage reactions was seen to depend on the position of the sulfonic ester groups and the overall configuration of the molecules.The mechanism of reductive desulfonyloxylation also depended on configuration and was demonstrated to involve intermediary epoxide formation or displacement by internal hydride shift as the principal paths; competing elimination and direct nucleophilic displacement were found to occur in the allo series, whereas reduction accompanied by ring contraction has thus far been encountered only in the conformationally less constrained, cis-fused acetal system of the galacto series.Like the borohydride reagent, lithium aluminum hydride was found to react (though much more slowly) with the altro 2,3-ditosylate by the epoxide-mediated mechanism, although the latter hydride is known to desulfonyloxylate the α-D-gluco-isomer by a different, intramolecular reduction mechanism.
DESULFONYLOXYLATION OF SOME SECONDARY p-TOLUENESULFONATES OF GLYCOSIDES BY LITHIUM TRIETHYLBOROHYDRIDE; A HIGH-YIELDING ROUTE TO 2- AND 3-DEOXY SUGARS
Baer, Hans H.,Hanna, Hanna R.
, p. 19 - 42 (2007/10/02)
Lithium triethylborohydride (LTBH) reacts readily with p-toluenesulfonates of methyl 4,6-O-benzylidene-α-D-glucopyranoside (4) to give deoxyglycosides in >90percent yield.Thus, the 2,3-ditosylate (1) and the 3-monotosylate (2) thereof afford methyl 4,6-O-benzylidene-2-deoxy-α-D-ribo-hexopyranoside (7) in highly regio- and stereo-selective reactions that proceed via methyl 2,3-anhydro-4,6-O-benzylidene-α-D-allopyranoside (6), and the 2-monotosylate (8) of 4 gives the 3-deoxy-α-D-arabino isomer (12) of 7 via the corresponding 2,3-anhydro-α-D-mannopyranoside 11.In the series of the corresponding β anomers, the 3-monotosylate 14 and the 2-monotosylate 16 are similarly desulfonyloxylated, with equal ease, but furnish mixtures of regioisomeric deoxyglycosides, namely, the 3- and 2-deoxy-β-D-ribo derivatives 20 and 21, and the 2- and 3-deoxy-β-D-arabino derivatives 22 and 23, respectively.It could be shown that this difference is due to the failure of the intermediary, β-glycosidic epoxides 18 and 19 (the anomers of 6 and 11) to obey the Fuerst-Plattner rule in their reductive ring-opening with LTBH.The β-glycosidic 2,3-ditosylate 15 reacts less readily, and gives 20-23, with 20 preponderating.The 2-O-methyl-3-O-tosyl-β-D-glucopyranoside 24 is partly desulfonylated and partly desulfonyloxylated, whereas its 3-O-methyl-2-O-tosyl isomer 27 undergoes desulfonylation exclusively.The reductions of 1, 2, and 8 by LTBH are compared with those previously effected by lithium aluminum hydride, which are slower, involve considerable desulfonylation, and afford lower yields of deoxyglycosides, with the main products differing from those obtained by the action of LTBH.Mechanistic differences associated with the two reductants are discussed.
