How the polarity of carbohydrates can be used in chemistry - Carbohydrate Chemistry: Chemical and Biological Approaches - page 35

Scheme 4 Aqueous Claisen rearrangements of glycoorganic vinylallylethers.
improved facial selectivity was observed. In this latter example, out of the
four possible isomers, the major one was obtained with a 88% selectivity,
thanks to water-induced endo selectivity and to the sugar induced facial
selectivity. Other uses of the dienyl b-glucoside 2 later included an
aqueous 4 þ 6 cycloaddition 6 as well as aqueous hetero Diels Alder
reactions using diethylketomalonate as the heterodienophile, but rapid
transformation of the carbonyl group of the heterodienophile to its
hydrated form made this latter reaction less ecient.
The concept of glyco-organic compound was extended to Claisen re-
arrangements. This reaction also exhibits a negative activation volume,
and is therefore sensitive to high pressure conditions and solvent ef-
fects. 7,8 The vinyloxypropenyl glucosides 12 and 13 were prepared from
the unsaturated aldehyde obtained by anomeric alkylation with 3-tosyl-
oxyacrolein, followed by reduction to alcohol and enolether formation.
Both anomers were separated and each of them was submitted to Claisen
rearrangement, which afforded two diastereomers (resulting from the
attack on one face of the dienyl moiety or the other) which was
immediately reduced by sodium borohydride to the corresponding
homoallylalcohols. After separation of the diastereoisomers, enzymatic
hydrolysis led to enantiomerically pure (R)- or (S)-1, 3-diols 14 and 15
(Scheme 4). 9,10
2.2 Other types of carbohydrate-based dienes for aqueous
cycloadditions
Dihydroxyhexadiene 16 can be obtained by Wittig olefination of a pro-
tected glyceraldehyde and further deprotection. When reacted with
acrolein (Scheme 5, Table 3), a strong solvent effect using water vs.
toluene was observed (50 fold acceleration), a total endoselectivity, and
an increase in the facial selectivity due to a differentiation of both faces of
the diene with respect to hydrophobicity. This contrasted with the case of
the reaction of glucosyl diene 2 which did not show any facial selectivity
change with respect to solvent. For diene 16, the preferred attack by the