Structure of Flavan-3,4-diols

During the same period there were numerous reports of the isolation of leuco-cyanidins (5,7,3, 4 -tetrahydroxyflavan-3,4-diols) as natural products from a variety of plants. Unfortunately, none of these compounds was fully characterized (i.e. no comparison of heterocyclic proton coupling constants with data from methylated synthetic racemates (11) and none of the reported melting points or optical rotations coincided with each other or, more importantly, with properties of synthetic compounds of determined structure) (301, 304, 377). These discrep- 

Despite the extreme sensitivity of the leucocyanidins. Porter et al. (305) have recently crystallized 2/ ,35,4i -leucocyanidin as the dihydrate from aqueous solu- 

4 -tri-0-methylgleditsin OH OH OCH3 H OCH3 OCH3 H R S S Neorautanenia am-boensis 282 

4 -penta-0-methyl- OCH3 OCH3 OCH3 H OCH3 OCH3 H R s s N. amboensis 282 

Reduction of (+)-dihydroquercetin with enzymes from cell suspension give the 2/ ,3S,4S-(2,3-/m 5-3,4-c/5)-flavan-3,4-diol (339, 342). H-NMR coupling constants for the tetra-O-methyl ether and the tetra-O-methyl ether diacetate derivatives as well as molecular mechanical studies indicate that these compounds exist exclusively in half-chair -conformations. The 2,3-cis flavan-3,4-diol methyl ethers also occur in solution predominantly in half-chair -conformations (304). 

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