An Asymmetric Synthetic Approach to the A-ring of the Taxol Family of Anti-Cancer Compounds*
A synthetic route developed for the preparation of the A-ring of Taxol family of molecules is reported. By means of an intramolecular Diels-Alder reaction an asymmetric approach to this ring has been accomplished. Also, initial studies to prepare the A ring using an intramolecular Diels-Alder reaction have been successful.
1,7-SIGMATROPIC HYDROGEN SHIFTS OF A-NORVITAMIN D ANALOGUES : RING SIZE AND SUBSTITUENT EFFECTS ON THE PREVITAMIN D-VITAMIN D EQUILIBRIUM
The [1,7]-sigmatropic hydrogen shift of the 11α-OH-A-norprevitamin D analogue and its 11β-epimer to the vitamin forms, respectively, was determined to be ∼20 and ∼10 times faster than that of previtamin D 3 to vitamin D 3 . In addition, the location or absence of the hydroxyl group relative to the triene moiety of A-norprevitamins was determined to have diverse effects on the position of the previtamin-vitamin equilibrium. It appears that the smaller A-ring size favors the previtamin structure and the larger A-ring favors the vitamin form, but small structural changes can readily influence the equilibrium. While the observed equilibrium changes can be rationalized on the basis of nonbonded substituent steric effects superimposed upon ring size (strain) effects, the origin of the observed rate enhancements of the [1,7]-shift in the A-nor series compared to the six-membered A-ring, natural series is less clear
The unexpected intramolecular C-arylation of 2-0-benzylated cyclic sugar derivatives: a useful 1,2-cis-C-glycosylation reaction
Upon treatment with tin(IV) chloride, perbenzylated glycofuranosyl acetates such as 3 or 6 undergo an unusual, intramolecular α-C-arylation resulting from the internal Friedel-Crafts alkylation of the 2-O-benzyl substituent.