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Glycosylidene Carbenes. Part 14. Glycosidation of partially protected galactopyranose‐, glucopyranose‐, and mannopyranose‐derived vicinal diols
Author(s) -
Muddasani Pulla Reddy,
Bozó Eva,
Bernet Bruno,
Vasella Andrea
Publication year - 1994
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19940770128
Subject(s) - chemistry , diol , vicinal , regioselectivity , stereochemistry , organic chemistry , catalysis
Abstract The relation between H‐bonding in diequatorial trans ‐1,2 and axial, equatorial cis ‐1,2‐diols and the regioselectivity of glycosidation by the diazirine 1 was examined. H‐Bonds were assigned on the basis of FT‐IR and 1 H‐NMR spectra ( Fig. 1 ). Glycosidation by 1 of the gluco ‐configurated diequatorial trans ‐2,3‐diols 4–7 yielded the mono‐glucosylated products 16/17/20/21 (69–89%); 1,2‐/1,3‐linked products (37–46:63–54), 24/25/28/29 (60–63%; 1,2‐/1,3‐linked products 46–51:54–49), 32–35 (69–94%; 1,2‐/1,3‐linked products 45–52:55–48), and 36/37/40/41 (59–63%; 1,2‐/1,3‐linked products 52–59:48–41), respectively ( Scheme 1, Table 3 ). The disaccharides derived from 4, 5 , and 7 were characterized as their acetates 18/19/22/23, 26/27/30/31 , and 38/39/42/43 , respectively. Glycosidation of the galacto ‐configurated diequatorial 2,3‐diols 8 and 9 and the manno ‐configurated diequatorial 3,4‐diol 10 by 1 ( Scheme 2, Table 3 ) also proceeded in fair yields to give the disaccharides 44–47 (69–80%;1,2‐/1,3‐linked products ca. 1:1), 48–51 (51–61%;1,2/‐1,3‐linked products 54–56:56–54), and 56/57/60/61 (71–80%; 1,3‐/1,4‐linked products 49–54:51–46), respectively. The 1,3‐linked disaccharides 56/57 derived from the diol 10 were characterized as the acetates 58/59 . The regio‐ and stereoselectivities of the glycosidation by 1 were much better for the α‐ D ‐ manno ‐configurated axial, equatorial cis ‐2,3‐diol 11 and the galacto ‐configurated axial, equatorial cis ‐3,4‐diol 13 (1,2‐/1,3‐linked disaccharides ca. 3:7 for 11 and 1,3‐/1,4‐linked disaccharides ca. 4:1 for 13 ; Scheme 3, Table 4 ). The regio‐ and stereoselectivity for the β‐ D ‐ manno ‐configurated cis ‐2,3‐diol 12 were, however, rather poor (1,2‐/1,3‐linked products 48:52). The 1,2‐linked disaccharides 66/67 derived from 12 were characterized as the acetates 70/71 . Koenigs‐Knorr ‐type glycosidation of the cis ‐diols 11–13 by 2 or 3 proceeded with a similar regio‐ and a higher stereoselectivity (α‐ D > β‐ D with the donor 2 and α‐ D < β‐ D with the donor 3 ) than with 1 , with the exception of 12 which did not react with 2 . The regioselectivity of the glycosidations by 1 agrees fully with the H‐bonding scheme of the diols and with the hypothesis that the intermediate carbene is preferentially protonated by the most weakly H‐bonded OH group. The regioselectivity of the glycosidation by 2 and by 3 is determined by a higher reactivity of the equatorial OH groups and by H‐bonding. Several H‐bonded and equilibrating isomers of a given diol may intervene in the glycosidation by 1 , or by 2 and 3 , resulting in the same regioselectivity. The low nucleophilicity of 12 and the low degree of regioselectivity in its reaction with 3 show that stereoelectronic effects may also profoundly influence the nucleophilicity of OH groups.