The photooxidation of toluene and p-xylene with molecular oxygen and visible light has been investigated in several cation-exchanged zeolites. In general, the yield of the photooxidation products, for fixed irradiation time and intensity, was found to correlate with the electric field intensity at the cation sites within the zeolites. On the basis of measurements of CO vibrational frequencies, electric fields of approximately 3-7 V nm -1 are indicated for the cation-exchanged zeolites X, Y, ZSM-5, and Beta used in these studies. These large electric fields are thought to promote photooxidation by stabilizing an intermolecular charge transfer state (R + ‚O 2 -) that is formed upon excitation with visible light. The measured electric field was found to correlate with the product yield and was highest in divalent cation-exchanged zeolites with high Si/Al ratios, such as BaZSM-5 and BaBeta. For zeolites containing the same cation, the selectivity of toluene to form benzaldehyde and p-xylene to form p-tolualdehyde was found to be higher in zeolites X and Y (g87%) compared to ZSM-5 and Beta (<35%). In each case, the presence of residual Brønsted acid sites in these zeolite materials was found to correlate with the loss of selectivity. The electric field strength, the presence of Brønsted acid sites, and other physicochemical properties that affect yield and selectivity in these photooxidation reactions are explored.
Tetracycline (1) 1 has been one of the major antibiotics for almost half a century. The difficult problems implied by its sensitive functionality and demanding stereochemistry have, predictably, attracted the attention of chemists for several decades. 2 Muxfeldt's construction of terramycin, 2a 5a-hydroxytetracycline, wassand issan impressive achievement in total synthesis. It, nevertheless, did not control the introduction of the center at C4a and would not, in any case, be easily extended to tetracycline itself. A remarkable reconstruction of tetracycline has been achieved recently 3 from anhydrotetracycline. If the 12a-hydroxylation 4 of synthetically available 2c 12a-deoxyanhydrotetracycline could be verified, this would constitute a formal, if indirect, total synthesis of tetracycline.We describe here a stereospecific total synthesis of (()-12adeoxytetracycline (2) which solves the longstanding problem of establishing the proper relative stereochemistry of the C5a and C4a centers.
Spirastrellolide A is an antimitotic polyketide that shows potent and selective inhibition of protein phosphatase 2A (IC50 = 1 nM) and causes premature entry of cells into mitosis (Figure 1). 1 This 38 membered macrocycle contains both 6,6-and 5,6,6spiroketal units, a cis-2,6-disubstituted tetrahydropyran, and a multitude of additional functionality. The acyclic side chain, appended at C38, contains a skipped diene and terminates in an -hydroxy carboxylic acid. The complex architecture and interesting biological profile of this and other spirostrellolides 2 have captured the attention of the organic chemistry community. Total syntheses of spirastrellolide A and F methyl esters have appeared, 3 as have a number of reports describing the preparation of spirastrellolide fragments. 4
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