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81.
Ivar Elvers 《Nordic Journal of Botany》1984,4(1):61-64
Observations of living flower–heads of Tussilago farfara with ultraviolet light have confirmed the previously known distinction between the UV absorbing disc–flowers and the UV reflecting ray–flowers. A further differentiation of the yellow colours in the capitulum is that the anther walls fluoresce when exposed to UV light. 相似文献
82.
The reaction of methyl 2,3:4,6-di-O-benzylidene-α-D-mannopyranoside (5) with N-bromosuccinimide gave mainly three, isomeric dibromo dibenzoates, identified as the 3,6-dibromo-altro (1), 3,6-dibromo-manno (2), and 4,6-dibromo-ido (3) derivatives by subsequent chemical transformation and by extended n.m.r.-spectral studies. The reaction of methyl 2,3:4,6-O-benzylidene-α-D-allopyranoside (22) with N-bromosuccinimide gave two isomeric dibromo dibenzoates, the 2,6-dibromo-altro (23) and 3,6-dibromo-gluco (24) products, and their structures were similarly assigned. A similar reaction-sequence with methyl 2,3:4,6-di-O-benzylidene-α-D-glucopyranoside (32), however, yielded two isomeric monobenzoates 33 and 34, which could be identified straightforwardly. The results are consistent with the intermediate formation of benzoxonium ions that undergo favored axial attack. Thus the observed products and their ratios in reactions of 5 and of 22 with N-bromo-succinimide are explicable. Compound 32, however, does not appear to react by way of an intermediate, five-membered 2,3-trans benzoxonium ion. 相似文献