Calculation of the magnetic field penetration depth for high-Tc cuprate superconductors based on the Interlayer Pair Tunneling model /

In this work, the magnetic field penetration depth for high-Tc cuprate superconductors is calculated using a recent Interlayer Pair Tunneling (ILPT) model proposed by Chakravarty, Sudb0, Anderson, and Strong [1] to explain high temperature superconductivity. This model involves a "hopping" of Cooper pairs between layers of the unit cell which acts to amplify the pairing mechanism within the planes themselves. Recent work has shown that this model can account reasonably well for the isotope effect and the dependence of Tc on nonmagnetic in-plane impurities [2] , as well as the Knight shift curves [3] and the presence of a magnetic peak in the neutron scattering intensity [4]. In the latter case, Yin et al. emphasize that the pair tunneling must be the dominant pairing mechanism in the high-Tc cuprates in order to capture the features found in experiments. The goal of this work is to determine whether or not the ILPT model can account for the experimental observations of the magnetic field penetration depth in YBa2Cu307_a7. Calculations are performed in the weak and strong coupling limits, and the efi"ects of both small and large strengths of interlayer pair tunneling are investigated. Furthermore, as a follow up to the penetration depth calculations, both the neutron scattering intensity and the Knight shift are calculated within the ILPT formalism. The aim is to determine if the ILPT model can yield results consistent with experiments performed for these properties. The results for all three thermodynamic properties considered are not consistent with the notion that the interlayer pair tunneling must be the dominate pairing mechanism in these high-Tc cuprate superconductors. Instead, it is found that reasonable agreement with experiments is obtained for small strengths of pair tunneling, and that large pair tunneling yields results which do not resemble those of the experiments.

Towards the enantioselective synthesis of lycoricidine alkaloids /

Two efficient, regio- and stereo controlled synthetic approaches to the synthesis of racemic analogs of pancratistatin have been accomplished and they serve as the model systems for the total synthesis of optically active 7-deoxy-pancratistatin. In the Diels-Alder approach, an efficient [4+2] cycloaddition of 3,4-methylenedioxyco- nitrostyrene with Danishefsky's diene to selectively form an exo-nitro adduct has been developed as the key step in the construction of the C-ring of the target molecule. In the Michael addition approach, the key step was a conjugate addition of an organic zinc-cuprate to the 3,4-methylenedioxy-(B-nitrostyrene, followed by a diastereocontroUed closure to form the cyclohexane C-ring of the target molecule via an intramolecular nitro-aldol cyclization on a neutral alumina surface. A chair-like transition state for such a cyclization has been established and such a chelation controlled transition state can be useful in the prediction of diastereoselectivity in other related 6-exo-trig nitroaldol reactions. Cyclization of the above products fi^om both approaches by using a Bischler-Napieralski type reaction afforded two lycoricidine derivatives 38 and 50 in good yields. The initial results from the above modeling studies as well as the analysis of the synthetic strategy were directed to a chiral pool approach to the total synthesis of optically active 7-deoxy-pancratistatin. Selective monsilylation and iodination of Ltartaric acid provided a chiral precursor for the proposed key Michael transformation. The outlook for the total synthesis of 7-deoxy-pancratistatin by this approach is very promising.A concise synthesis of novel designed, optically pure, Cz-symmetrical disulfonylamide chiral ligands starting from L-tartaric acid has also been achieved. This sequence employs the metallation of indole followed by Sfj2 replacement of a dimesylate as the key step. The activity for this Cz-symmetric chiral disulfonamide ligand in the catalytic enantioselective reaction has been confirmed by nucleophilic addition to benzaldehyde in the disulfonamide-Ti (0-i-Pr)4-diethylzinc system with a 48% yield and a 33% e.e. value. Such a ligand tethered with a suitable metal complex should be also applicable towards the total synthesis of 7-deoxy-pancratistatin.

Synthetic studies on prostaglandins: "synthesis of a new thia-PGEI analog"

A PGE1 analog, namely (±)-trans-2-(6'-carbomethoxyhexyl)-3- (E-3"-thia-1 "-octene)-4-hydroxycyclopentanone 71, has been prepared for the first time. Towards the synthesis of this compound, several synthetic approaches aimed at the preparation of the required acetylenic and E-halovinylic sulfides as building blocks were investigated. Among all the methods examined, it appeared evident that the best route to ethynyl n.pentyl sulfide 81 is via a double dehydrohalogenation of the corresponding 1,2-dibromoethyl sulfide with sodium amide in liquid ammonia. In addition, the isomerically pure E-2-iodoethenyl n.pentyl sulfide 85 is conveniently prepared in high yield and stereoselectivity by hydrozirconation-iodination of the terminal ethynyl sulfide 81. The classical hydroalumination and hydroboration reactions for the preparation of vinyl halides from alkynes gave only small yields when applied as methods towards the synthesis of 85 . The building block 2-(6'-carbomethoxyhexyl)-4-hydroxy-2- cyclopentenone (±)-1 carrying the upper side-chain of prostaglandin E 1 was prepared by a step-wise synthesis involving transformations of compounds possessing the required carbocyclic framework (see scheme 27). The synthesis proved to be convenient and gave a good overall yield of (±)-1 which was protected as the TH P-derivative 37 or the siloxy derivative 38. With the required building blocks 81 and 37 in hand, the target 1S-thia-PGE1 analog (±)-71 was prepared via the in situ higher cuprate formation-conjugate addition reaction. This method proved to be convenient and stereospecific. The standard cuprate method, involving an organocuprate reagent generated from an isolated vinyl iodide, did not work well in our case and gave a complicated mixture of products. The target compound will be submitted for assessment of bio log ical activity.