30 research outputs found
A Concise Total Synthesis of (--)-Maoecrystal Z
The first total synthesis of (--)-maoecrystal Z
is described. The key steps of the synthesis include a
diastereoselective Ti^(III)-mediated reductive epoxide coupling reaction and a diastereoselective Sm^(II)-mediated reductive cascade cyclization reaction. These transformations enabled the preparation of (--)-maoecrystal Z in only 12 steps from (--)-γ-cyclogeraniol
A Unified Strategy to ent-Kauranoid Natural Products: Total Syntheses of (−)-Trichorabdal A and (−)-Longikaurin E
The first total syntheses of (−)-trichorabdal A and (−)-longikaurin E are reported. A unified synthetic strategy is employed that relies on a Pd-mediated oxidative cyclization of a silyl ketene acetal to generate an all-carbon quaternary center and build the bicyclo[3.2.1]octane framework. These studies, taken together with our previous synthesis of (−)-maoecrystal Z, demonstrate that three architecturally distinct ent-kauranoids can be prepared from a common spirolactone intermediate
A unified strategy for the synthesis of (−)-maoecrystal Z, (−)-trichorabdal A, and (−)-longikaurin E
Herein we describe in full our investigations that led to the completion of the first total syntheses of (−)-maoecrystal Z, (−)-trichorabdal A, and (−)-longikaurin E. The unified strategy employs a Ti^(III)-mediated reductive epoxide coupling to rapidly prepare a key spirolactone. Highly diastereoselective Sm^(II)-mediated reductive cyclizations and a Pd^(II)-mediated oxidative cyclization enable the construction of three architecturally distinct ent-kauranoid frameworks from this common intermediate
Vacuum Induced Coherences in Radiatively Coupled Multilevel Systems
We show that radiative coupling between two multilevel atoms having
near-degenerate states can produce new interference effects in spontaneous
emission. We explicitly demonstrate this possibility by considering two
identical V systems each having a pair of transition dipole matrix elements
which are orthogonal to each other. We discuss in detail the origin of the new
interference terms and their consequences. Such terms lead to the evolution of
certain coherences and excitations which would not occur otherwise. The special
choice of the orientation of the transition dipole matrix elements enables us
to illustrate the significance of vacuum induced coherence in multi-atom
multilevel systems. These coherences can be significant in energy transfer
studies.Comment: 13 pages including 8 figures in Revtex; submitted to PR
Theory of Pseudomodes in Quantum Optical Processes
This paper deals with non-Markovian behaviour in atomic systems coupled to a
structured reservoir of quantum EM field modes, with particular relevance to
atoms interacting with the field in high Q cavities or photonic band gap
materials. In cases such as the former, we show that the pseudo mode theory for
single quantum reservoir excitations can be obtained by applying the Fano
diagonalisation method to a system in which the atomic transitions are coupled
to a discrete set of (cavity) quasimodes, which in turn are coupled to a
continuum set of (external) quasimodes with slowly varying coupling constants
and continuum mode density. Each pseudomode can be identified with a discrete
quasimode, which gives structure to the actual reservoir of true modes via the
expressions for the equivalent atom-true mode coupling constants. The quasimode
theory enables cases of multiple excitation of the reservoir to now be treated
via Markovian master equations for the atom-discrete quasimode system.
Applications of the theory to one, two and many discrete quasimodes are made.
For a simple photonic band gap model, where the reservoir structure is
associated with the true mode density rather than the coupling constants, the
single quantum excitation case appears to be equivalent to a case with two
discrete quasimodes
Planetary science and exploration in the deep subsurface: results from the MINAR Program, Boulby Mine, UK
The subsurface exploration of other planetary bodies can be used to unravel their geological history and assess their habitability. On Mars in particular, present-day habitable conditions may be restricted to the subsurface. Using a deep subsurface mine, we carried out a program of extraterrestrial analog research – MINe Analog Research (MINAR). MINAR aims to carry out the scientific study of the deep subsurface and test instrumentation designed for planetary surface exploration by investigating deep subsurface geology, whilst establishing the potential this technology has to be transferred into the mining industry. An integrated multi-instrument suite was used to investigate samples of representative evaporite minerals from a subsurface Permian evaporite sequence, in particular to assess mineral and elemental variations which provide small-scale regions of enhanced habitability. The instruments used were the Panoramic Camera emulator, Close-Up Imager, Raman spectrometer, Small Planetary Linear Impulse Tool, Ultrasonic drill and handheld X-ray diffraction (XRD). We present science results from the analog research and show that these instruments can be used to investigate in situ the geological context and mineralogical variations of a deep subsurface environment, and thus habitability, from millimetre to metre scales. We also show that these instruments are complementary. For example, the identification of primary evaporite minerals such as NaCl and KCl, which are difficult to detect by portable Raman spectrometers, can be accomplished with XRD. By contrast, Raman is highly effective at locating and detecting mineral inclusions in primary evaporite minerals. MINAR demonstrates the effective use of a deep subsurface environment for planetary instrument development, understanding the habitability of extreme deep subsurface environments on Earth and other planetary bodies, and advancing the use of space technology in economic mining
Single molecules put a ring on it
A variant of a classical reaction has been used to generate short-lived chemical species called arynes, allowing the one-step synthesis of structurally complex benzene derivatives from simple precursors
A Unified Strategy to <i>ent</i>-Kauranoid Natural Products: Total Syntheses of (−)-Trichorabdal A and (−)-Longikaurin E
The
first total syntheses of (−)-trichorabdal A and (−)-longikaurin
E are reported. A unified synthetic strategy is employed that relies
on a Pd-mediated oxidative cyclization of a silyl ketene acetal to
generate an all-carbon quaternary center and build the bicyclo[3.2.1]octane
framework. These studies, taken together with our previous synthesis
of (−)-maoecrystal Z, demonstrate that three architecturally
distinct <i>ent</i>-kauranoids can be prepared from a common
spirolactone intermediate