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
Variability of Reflectance Measurements with Sensor Altitude and Canopy Type
Data were acquired on canopies of mature corn planted in 76 cm rows, mature soybeans planted in 96 cm rows with 71 percent soil cover, and mature soybeans planed in 76 cm rows with 100 percent soil cover. A LANDSAT band radiometer with a 15 degree field of view was used at ten altitudes ranging from 0.2 m to 10 m above the canopy. At each altitude, measurements were taken at 15 cm intervals also a 2.0 m transect perpendicular to the crop row direction. Reflectance data were plotted as a function of altitude and horizontal position to verify that the variance of measurements at low altitudes was attributable to row effects which disappear at higher altitudes where the sensor integrate across several rows. The coefficient of variation of reflectance decreased exponentially as the sensor was elevated. Systematic sampling (at odd multiples of 0.5 times the row spacing interval) required fewer measurements than simple random sampling over row crop canopies.