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The presence of an isopropylidene ketal protection of an internal diol in 3,4-O-isopropylidene-D-arabino-1-C-phenyl hexanone locks it in a conformation that prevents its cyclization to a pyranose ring. 相似文献
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Umpolung catalysis is studied by a sequence of model reactions (CPCM in THF, B3LYP/6-31G*) with different aldehydes and catalysts. We involved addition of the catalyst to the aldehyde and 1,2-H-migration to form a carbanionic d1-species, which is the crucial intermediate according to the Lapworth- and Breslow-mechanisms. Cyanide, N-methylthiazol-2-ylidene, and a glycol-based phosphite perform as umpolung catalysts, formaldehyde, acetaldehyde, benzaldehyde, and acrolein are substrates in this study. In these aldehyde substrates, alkyl-substitution disfavors but π-conjugation favors formation of the carbanionic d1-intermediate. The nucleophilic carbene, N-methylthiazol-2-ylidene, is the strongest umpolung catalyst, while the phosphite is about as active as cyanide.
Figure Transitions structure for the umpolung of formaldehyde with a glycol phosphinite catalyst
Dedicated to Professor Dr. Paul von Ragué Schleyer on the occasion of his 75th birthday 相似文献
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