Anomeric Anhydro Sugars
3.7
749
⊡ Scheme 21
⊡ Scheme 22
and 1,6-anhydro-β-D-glucofuranose (7) ( > Scheme 4). Dehydration of methyl α-D-galactopyranoside with CSA yields an equimolar mixture of the furanose and pyranose anhydro
derivatives [21]. In an alternative approach, synthesis of 1,6-anhydro-α-D-galactofuranose
(19) through stannic chloride catalyzed ring-closure of the fully protected galactofuranose
(20) was achieved in two steps in excellent yield ( > Scheme 22) [64]. It has been observed
that the trans-oriented 2,3-diols of these bicyclic compounds possess a remarkable resistance
to both periodate and lead tetraacetate oxidation, a consequence of the rigidity imposed upon
these molecules by the 1,6-anhydro bridge.
4.4 Levoglucosenone
Levoglucosenone is an enone-containing 1,6-anhydro sugar, the considerable differences of
which merit a separate section to 1,6-anhydro sugars, although only a brief overview of the
extensive chemistry pertaining to this compound is possible here [65]. The presence of the
acetal and enone moieties of levoglucosenone provides ready access to a wide range of functionalized sugars from a single chemical feedstock. Halpern and co-workers properly characterized levoglucosenone in 1973, correcting several structures proposed by other workers [66]. Levoglucosenone may be prepared by the Brønsted-acid catalyzed pyrolysis of cellulosic materials and is easily isolated in multi-gram quantities from the pyrolyzate of waste
paper [67]. The 1,6-anhydro bridge sterically hinders the endo-face of levoglucosenone and,
as a consequence, nucleophilic addition is directed to the non-hindered exo-face. For example, reduction of the carbonyl group with sodium borohydride or lithium aluminum hydride
leads to the expected allylic alcohol in good yield [68,69]. Carbon nucleophiles derived from
Grignard or Gilman reagents add to the β-carbon yielding the corresponding Michael adducts.
In this way, stereospecific addition of methylmagnesium iodide or lithium dimethylcuprate
3.7
749
⊡ Scheme 21
⊡ Scheme 22
and 1,6-anhydro-β-D-glucofuranose (7) ( > Scheme 4). Dehydration of methyl α-D-galactopyranoside with CSA yields an equimolar mixture of the furanose and pyranose anhydro
derivatives [21]. In an alternative approach, synthesis of 1,6-anhydro-α-D-galactofuranose
(19) through stannic chloride catalyzed ring-closure of the fully protected galactofuranose
(20) was achieved in two steps in excellent yield ( > Scheme 22) [64]. It has been observed
that the trans-oriented 2,3-diols of these bicyclic compounds possess a remarkable resistance
to both periodate and lead tetraacetate oxidation, a consequence of the rigidity imposed upon
these molecules by the 1,6-anhydro bridge.
4.4 Levoglucosenone
Levoglucosenone is an enone-containing 1,6-anhydro sugar, the considerable differences of
which merit a separate section to 1,6-anhydro sugars, although only a brief overview of the
extensive chemistry pertaining to this compound is possible here [65]. The presence of the
acetal and enone moieties of levoglucosenone provides ready access to a wide range of functionalized sugars from a single chemical feedstock. Halpern and co-workers properly characterized levoglucosenone in 1973, correcting several structures proposed by other workers [66]. Levoglucosenone may be prepared by the Brønsted-acid catalyzed pyrolysis of cellulosic materials and is easily isolated in multi-gram quantities from the pyrolyzate of waste
paper [67]. The 1,6-anhydro bridge sterically hinders the endo-face of levoglucosenone and,
as a consequence, nucleophilic addition is directed to the non-hindered exo-face. For example, reduction of the carbonyl group with sodium borohydride or lithium aluminum hydride
leads to the expected allylic alcohol in good yield [68,69]. Carbon nucleophiles derived from
Grignard or Gilman reagents add to the β-carbon yielding the corresponding Michael adducts.
In this way, stereospecific addition of methylmagnesium iodide or lithium dimethylcuprate