Approaches to terpenoid natural products using reactive intermediates

Two attempted syntheses of a functionalized ring system common to kaurenoid natural products is presented. One of the attempts employed a bridgehead enone as a dienophile in a Diels-Alder reaction. One example in this series resulted in the synthesis of the tetracyclic ring system, but the angular C-8 (kaurene numbering) methyl group could not be added. Another example in this series resulted in the synthesis of a tricyclic intermediate containing the carbons to form the fourth ring, but this could not be cyclized. The other approach employed a Michael addition followed by an oxy-ene reaction for the synthesis of the tetracyclic framework, but a suitable precursor for the oxy-ene reaction was not realized. Also described is an attempted cyclization of a bicyclic intermediate to a tricyclic intermediate that contained functionality to prepare the bicyclo (3.2.1) octene ring system using a variety of reagents, but with no success;A synthesis of an advanced intermediate toward the synthesis of trixikingolide is also described. This synthesis employed bridgehead carbocation chemistry in the construction of a key intermediate. A tricyclic intermediate is prepared by a regioselective intramolecular aldol condensation. This intermediate contains the necessary functionality to synthesize the pentacyclic ring system common to this class of natural products

Molecular Chemistry to the Fore: New Insights into the Fascinating World of Photoactive Colloidal Semiconductor Nanocrystals

Colloidal semiconductor nanocrystals possess unique properties that are unmatched by other chromophores such as organic dyes or transition-metal complexes. These versatile building blocks have generated much scientific interest and found applications in bioimaging, tracking, lighting, lasing, photovoltaics, photocatalysis, thermoelectrics, and spintronics. Despite these advances, important challenges remain, notably how to produce semiconductor nanostructures with predetermined architecture, how to produce metastable semiconductor nanostructures that are hard to isolate by conventional syntheses, and how to control the degree of surface loading or valence per nanocrystal. Molecular chemists are very familiar with these issues and can use their expertise to help solve these challenges. In this Perspective, we present our group\u27s recent work on bottom-up molecular control of nanoscale composition and morphology, low-temperature photochemical routes to semiconductor heterostructures and metastable phases, solar-to-chemical energy conversion with semiconductor-based photocatalysts, and controlled surface modification of colloidal semiconductors that bypasses ligand exchange

Approaches to terpenoid natural products using reactive intermediates

Two attempted syntheses of a functionalized ring system common to kaurenoid natural products is presented. One of the attempts employed a bridgehead enone as a dienophile in a Diels-Alder reaction. One example in this series resulted in the synthesis of the tetracyclic ring system, but the angular C-8 (kaurene numbering) methyl group could not be added. Another example in this series resulted in the synthesis of a tricyclic intermediate containing the carbons to form the fourth ring, but this could not be cyclized. The other approach employed a Michael addition followed by an oxy-ene reaction for the synthesis of the tetracyclic framework, but a suitable precursor for the oxy-ene reaction was not realized. Also described is an attempted cyclization of a bicyclic intermediate to a tricyclic intermediate that contained functionality to prepare the bicyclo (3.2.1) octene ring system using a variety of reagents, but with no success;A synthesis of an advanced intermediate toward the synthesis of trixikingolide is also described. This synthesis employed bridgehead carbocation chemistry in the construction of a key intermediate. A tricyclic intermediate is prepared by a regioselective intramolecular aldol condensation. This intermediate contains the necessary functionality to synthesize the pentacyclic ring system common to this class of natural products.</p

Synthesis, characterization, and reactivity of unusually basic prophosphatranes

The prophosphatranes upon which this thesis focusses, P(NMeCH[subscript]2CH[subscript]2)[subscript]3N,P(NHCH[subscript]2CH[subscript]2)[subscript]3N, and P(NBnCH[subscript]2CH[subscript]2)[subscript]3N have been found to be at least 10[superscript]7times more basic than any known phosphine derivative. While the methyl analogue was reported from these laboratories prior to the present work, the other two prophosphatranes are new;The basicities of these species were compared to each other by spectroscopic means and by competitive deprotonation experiments, as discussed in Parts I and II. The unexpected basicity order, in which the hydrogen analogue was found to be more basic than the methyl analogue which was more basic than the benzyl analogue, could not be explained sufficiently by either steric or inductive arguments. However the observed trend may be explained by the extra stability found in electron delocalization;The unusually high basicities observed for the prophosphatranes has been postulated to be due to a combination of the formation of three chelate rings and the ability to form a three-center-four-electron molecular orbital system in the conjugate-acid phosphatranes. An attempt to separate these two effects is described in Part III. Thus in (Me[subscript]2N)[subscript]2P(NMeCH[subscript]2CH[subscript]2)NMe[subscript]2, there is the possibility of forming one chelate ring upon the formation of a 3-center-4-electron molecular orbital system with a Lewis acid such as a proton. If such a transannular interaction occurred in this compound, it did not offer enough stability in the conjugate acid to prevent the evolution of HNMe[subscript]2, giving the phosphenium cation, (Me[subscript]2N)[overline]P(NMeCH[subscript]2CH[subscript]2N[superscript]+Me[subscript]2), strongly indicating that chelate rings contribute substantially to the unusual stability of phosphatrane cations;Some metal chemistry of P(NMeCH[subscript]2CH[subscript]2)[subscript]3N is discussed in Part IV. This prophosphatrane (a) coordinates to Re(CO)[subscript]5Br, displacing a CO cis to the bromide; (b) causes disproportionation of ClHgMe to give HgMe[subscript]2 and Cl[subscript]2Hg (P(NMeCH[subscript]2CH[subscript]2)[subscript]3N) [subscript]2; (c) induces a redox reaction giving H[overline]P(NMeCH[subscript]2CH[subscript]2)[subscript]3N[superscript]+, Hg[superscript]0, and a peroxide; and (d) reduces mercury (II) to give the dioxaphosphetane dimer, (HMe[subscript]2NCH[subscript]2CH[subscript]2[overline]N(CH[subscript]2CH[subscript]2MeN)[subscript]2PO] [subscript]2(OTf)[subscript]4.;In an attempt to prepare 1,1,2,2-tetraethylcarboxylatocyclobutane, the heretofore unknown tetraester, tricyclo (4.2.1.1[superscript]2,5) -1,2,5,6-tetraethylcarboxylatodecane-9,10-dione, was synthesized. The ester was probably formed from the dimerization of 2,5-diethylestercyclopentanone;Synthetic and spectroscopic data are reported on all of the above compounds and crystallographic determinations for H[overline]P(NHCH[subscript]2CH[subscript]2)[subscript]3N[superscript]+Cl[superscript]-, (Me[subscript]2N)(:)[overline]PNMeCH[subscript]2CH[subscript]2NMe[subscript]2(BF[subscript]4), Cl[subscript]2Hg (P(NMeCH[subscript]2CH[subscript]2)[subscript]3N) [subscript]2, (HMe[subscript]2NCH[subscript]2CH[subscript]2[overline]N(CH[subscript]2CH[subscript]2MeN)[subscript]2PO] [subscript]2 (OTf)[subscript]4, cis-ReBr(CO)[subscript]4 (P(NMeCH[subscript]2CH[subscript]2)[subscript]3N), and tricyclo (4.2.1.1[superscript]2,5]-1,2,5,6-tetraethylcarboxylatodecane-9,10-dione are presented.</p

Organic synthesis using bridgehead carbocations and bridgehead enones

The development of new pathways by which organic molecules can be stereoselectively constructed remains an important direction for organic chemistry. The palladium-mediated cyclizations of Trost,' Stille? Hegedus,3 Larock," Negishi: and Overman6 and the radical cyclization reactions of Keck,' Stork: Hart: Curran,Io Barton," and Beckwith12 represent important new strategies. A comparatively unstudied strategy is the formation of bicyclic or tricyclic systems via reactive bridgehead intermediates.Reprinted (adapted) with permission from Chemical Reviews, 89(7); 1591-1598. Doi: 10.1021/cr00097a013. Copyright 1989 American Chemical Society.</p