J . Am. Chem. SOC. natural tylosin (vide infra). To complete the synthesis, 6 was selectively reduced (4.0 equiv of Dibal, CH2C12, -78 "C, reduction of dienone and y-lactone) and oxidized (1.3 equiv of DDQ, benzene, 25 "C) to produce 0-mycinosyltylonolide (1) in 76% overall yield. Synthetic 1 was identical in all respects with an authentic sample obtained by degradation of tylosin as described below.Mild acid hydrolysis of tylosin (7) (Scheme I) (dilute HC1, THF, 25 OC or AcOH-THF-H20, 25 "C) detaches only mycarose from the molecule producing 0-mycinosyl-0-mycaminosyltylonolide (8) in 90% yield.I0 Both the highly methoxylated mycinose and the basic, N-containing mycaminose are resistant to acid hydrolysis under normal conditions that will allow survival of the rest of the molecule. A Polonovski type degradation of 8 [(a) 1.1 equiv of m-CPBA, CH2C12, 0 OC, 0.5 h followed by 8.0 equiv of (CF3C0),0 and 8.0 equiv of pyridine, 0 "C, 0.5 h and then aqueous KHC03 workup and 1 .O equiv of anhydrous K2C03, MeOH, 0 "C, 1 h)], however, furnished 0-mycinosyltylonolide (1) in 76% yield after chromatographic purification. The same compound (1) was also produced by direct degradation of tylosin (7) under the above conditions (65% yield)." M n 0 2 (excess, CH2C12, 48 h) oxidation of 0-mycinosyltylonolide (1) led to compound 6 in high yield (87%).So that the potential of Omycinosyltylonolide (1) as a precursor to tylosin (7) could be demonstrated, a number of final chemical maneuvers were performed. Thus, reaction of (1) with ethylene glycol in the presence of camphorsulfonic acid as catalyst yielded the acetal 9 as a major product (65%),12 liberating the C-5 hydroxyl for selective glycosidation. Although molecular models as well as chemical evidence indicate that no protection of the C-3 hydroxyl group should be needed, protection of the mycinose-bound hydroxyl might be necessary for steric-reactivity reasons for such glycoside formation attempts. Selective blockade of this hydroxyl was, therefore, sought and achieved by treating 9 sequentially with (a) phenylboronic acid (1.2 equiv, benzene, azeotropic removal of water at reflux to form a cyclic boronate ester at C-3 and C-5), (b) tert-butyldimethylsilyl chloride (excess imidazole, DMF, 25 "C), and (c) acetone-H,O (3:1, 25 "C, 1 h) to remove the boronate furnishing compound 10 in 72% overall yield.This highly efficient and stereocontrolled total synthesis of Omycinosyltylonolide (1) clearly demonstrates the advantageous utilization of carbohydrates in the construction of the 16-membered ring macrolide antibiotic^.'^-'^ At present, no practical methodology exists for completing the remaining disaccharide, mycaminose-containing unit, due to the well-recognized problems of glycosidation of N-containing sugars.I6 New strategies directed toward this goal are currently under way in these laboratories.Four research groups have suggested that "polypyrrole"' formed by anodic polymerization of pyrrole, eq 1, can be used to protect '0 0.3 positive M [ E t q N ] E F q / C H~C~ potential

A study has been made of the electrochromic behaviour of polyaniline films of potential use in passive display devices, and a preliminary examination of the properties of substituted polyanilines has also been carried out. Films of 'emeraldine-type' conducting polyanilines have been grown electrochemically from aqueous solutions onto glass substrates coated with gold or indium tin oxide. The authors have studied the electrochemistry of their redox reactions in acidic media to examine the kinetics and reversibility. Cyclic voltammetry was used to determine the electrode potentials for growth and redox switching and to obtain information on the electrode kinetics. The effect of electrode potential on the optical absorption spectra was observed, and was found to be consistent with polaron and bipolaron formation. The electrochromic colour contrast and switching times were measured as a function of pH; the requirements were somewhat conflicting, but a satisfactory compromise could be obtained around pH=0. By switching the electrodes over a constant potential step but from a variety of starting potentials, the influence on switching times of the initial oxidation state, and hence of the film conductivity, was investigated. Under suitable circumstances electronic conductivity, ionic transport or interfacial charge-transfer can limit the switching speed, but response times (for 50% transmission change) as short as 15 ms have been obtained without full optimisation. Of the numerous substituted polyanilines tried, only simple alkyl and alkoxy derivatives formed good polymeric films, and only the 2-ethoxy and 2-methoxy polymers gave good electrochromic behaviour. Switching times for these two were very fast, being less than 2.5 ms for both oxidising and reducing potential changes.

ligand-based radical anion, arid the solubility of the polymerized metal complex units.(4) The polymerization efficiency, approaches but never exceeds unity. This argues for hydrodimerization, and against a chain-growth mechanism, as the dominant pathway for monomer coupling.(5) Charge-transport rates, film morphology, and dry-film conductivity measurements show the ((py)2C2)-based films to be similar to other metallopolymer films prepared by EP but not to poly (acetylene).

which qualitatively accounted for the rapidity of the S1 -Mg+.-H2-. step and the much slower (5 X lo9 s-l) decay of this E T product to X and the ground state (see Scheme I). An implicit assumption of this model was that the rate of decay of the S1 state to So was similar to that of other l(a,r*) decays in related porphyrins (i.e., k [ ,< lo9 s-l).This implies an E T quantum yield >99 % . Our measured value of 0.9 f 0.1 is in good agreement with this expectation.Electrochemical measurements have previously shown that -90% of the S1 state's free energy was stored in the Mg+-Hp photoproduct.' Thus, three important criteria for successfully replicating the primary E T step of PSI1 (and other photosystems as well) have now been satisfied for ET from S1 in the Mg-H2 model: (1) a >lo" s-l rate,(2) high energy-storage efficiency, and (3) near unity quantum yield. Recent work has also shown that by changing the solvent from CH2C12 to NJV-dimethylformamide one can extend the lifetime of the Mg+-Hp product from 200 ps to 1.3 1x3.~ This makes it likely that Mg+-Hp will also be able to reduce an associated secondary electron acceptor with a high quantum yield.Eledrochemical deposition of Pt onto the surface of n -M a or n-WS2 electrodes results in significant improvement in the efficiency of the photoelectrochemical oxidation of C1or Br-in aqueous or nonaqueous media. The optimum amount of Pt is in the 10-8-10-7 mol/cm2 range where electron microscopy and Auger and X-ray photoelectron spectroscopy show that the Pt incompletely covers the MS2 surfaces. Phenomenologically, the deposited F't behaves as a catalyat for the two-electron oxidation of X-and does not affect the interfacial energetics of the n-MS2/liquid electrolyte. Improvement in efficiency for X-oxidation results from enhanced photovoltages and fill factors. Improvement in the oxidation of C1-in 15 M LiCl is most notable; for one n-MoS2 electrode the platinization improved the energy conversion efficiency from 0.7% to 9.8% for 632.8 nm (36 mW/cm2) and for one n-WSz electrode the efficiency was improved from 7.4% to 13.4% for 632.8 nm (15 mW/cm2). The improved efficiency can be maintained at -10 mA/cm2 for periods of -1 h, but the Pt catalyst is slowly oxidatively dissolved and the output eventually declines to that associated with naked n-MS2 photoanodes. ~ ~~~~ (1) (a) Tributsch, H.; Bennett, J. C. Schneemeyer, L. S.; Lewerenz, H. J. Appl. Phys. Lett. 1981, 38, 949. (d) Lewerenz, H. J.; Ferris, S. D.; Doherty, C. J.; Leamy, H. J. Wrighton, M. S.; Stacy, A.; Sienko, M. J. App. Phys. Lett. 1980,36, 701. (c) Kubiak, C. P.; Schneemeyer, L. F.; Wrighton, M. S. J. Am. Chem. SOC. 1980,102, 6898. (d) Calabrese, G. S.; Wrighton, M. S. Ibid. 1981,103,6273. (5) (a) Kline, G.; Kam, K.; Canfield, D.; Parkinson, B. A. Sol. Energy Mater. 1981,3,301. (b) Canfield, D.; Parkinson, B. A. J. Am. Chem. SOC. 1981,103,1279. (c) Furtak, T. E.; Canfield, D.; Parkinson, B. A. J. Appl. Phys. 1980,51,8018. (d) Parkinson, B. A.; Furtak, T. E.; Canfield, D.; Kam, K.; Kline, G. Discuss. Farad...

A partially stabilized photoeletrochemical cell using hydrogenated amorphous silicon photoanodes coated with thin films of polypyrrole Appl. Phys. Lett. 40, 281 (1982); 10.1063/1.93039Stabilization of ntype semiconductors to photoanodic dissolution: II-VI and III-V compound semiconductors and recent results for ntype silicon