Biology Reference
In-Depth Information
fruit and leaves of
Lagerstroemia speciosa
have blood sugar lowering
properties and are used to treat diabetes mellitus.
Apart from the regiochemical control issues fully addressed in the
previously described total syntheses, a special aspect of the synthesis of
lagerstannin C (
47
) was the construction of the
D
-gluconic acid unit by
oxidative opening of the
D
-glucopyranose ring (Fig. 5.9). We thus
planned to oxidise substrate
25
(see also Fig. 5.6) into the δ-lactone
42
,
followed by opening of the sugar ring under weakly basic conditions
(Kida
et al.
, 1990) in the aim of preparing the
D
-gluconic acid derivative
43
; strongly basic conditions might cleave the remaining ester groups in
the molecule. Dimethylsulfoxide (DMSO) is an oxidant that has become
very popular in carbohydrate chemistry, since its development in 1965
(Pfitzner and Moffatt, 1965a/b). In the presence of electrophilic
activating agents such as dicyclohexylcarbodiimide (DCC, Albright and
Goldman, 1965a/b, Pfitzner and Moffatt, 1965a/b), acetic anhydride
(Ac
2
O, Albright and Goldman, 1965a/b, 1967, Sowa and Thomas, 1966,
Kuzuhara and Fletcher, 1967), phosphorus pentoxide (Onodera
et al.
,
1965) or oxalyl chloride (Mancuso and Swern, 1981), and a proton
donor, primary and secondary hydroxyl functions are oxidized into a
carbonyl group (Lehman, 1996). Other classical oxidants for primary
and secondary alcohols are Cr(VI)-based reagents [CrO
3
.2pyridine,
pyridinium chlorochromate (PCC)] (Piancatelli and d'Auria, 1982,
Hollenberg
et al.
, 1978 Bissember and Wightman, 1980). However,
oxidation reactions of isolated secondary hydroxyl functions in
carbohydrates are often relatively slow, with low conversion rates and
modest yields (Czernecki
et al.
, 1985). Pyridinium dichromate (PDC)
then became the reagent of choice (Herscovici and Antonakis, 1980,
Herscovici
et al.
, 1982; Corey and Schmidt, 1979). Alcohols are
converted nearly quantitatively to the corresponding carbonyl functions
in an extremely fast and efficient reaction with PDC in the presence of
freshly activated molecular sieve powder (3Å) and catalytic quantities of
absolute acetic acid in dry dichloromethane. The addition of freshly
activated molecular sieve powder (3Å) leads to the immediate adsorption
of the chromate ester formed during the oxidation. It is thought that the
participating bonds of the intermediate product are oriented in such a
way by the adsorption that the C-H bond breaking at the alcohol carbon
