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Scheme 5 Facial discrimination in cycloadditions of glyceraldehyde-derived diene 16.
Table 3 Influence of solvent on the selectivity of the cycloaddition of diene 16 with
acrolein.
Diastereofacial selectivity
endo Si/endo Re
Solvent
Conditions
Isolated yield (%)
Toluene
4 days at 60 1C
94
55/45
Neat
2 days at 60 1C
73
55/45
Water
24 h at 60 1C
82
65/35
Water
7 days at 0 1C
92
66/34
Scheme 6 Aqueous hetero-Diels-Alder approach to ulosonic acid analogs.
dienophile occurred on the more hydrophobic face, opposite to where
hydroxyl groups are concentrated.
11
The glyceraldehyde-derived diene 16, as well as pentadienol 17,ora
longer analog 18 obtained from a tetrose were used in hetero-Diels-Alder
reactions with sodium glyoxylate 19 in a strategy towards ulosonic acid
analogs. Glyoxylic acid is readily available as a solid or in solution in
water, whereas anhydrous glyoxylic esters are less available. This allowed
the preparation of a series of dihydropyrans (20-22) which were elabor-
ated to ulosonic acid analogues in the KDH 23, KDO 24 and KDN 25
series (Scheme 6).
12-14
The strategy was also applied to other aqueous
solutions of carbonyl compounds.
15,16
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