Current Organic Chemistry - Volume 7, Issue 3, 2003

Fluorescence spectra of molecular guests at several temperatures are useful technique to study several types of polymer properties. In particular, it has been often employed to study polymer relaxation processes either in static or in dynamic conditions. In this work some applications of the steady-state fluorescence spectroscopy of different guests in polyethylene and in particular reports the photophysical behavior of 1,3-di(1-pyrenyl)propane (1Py(3)1Py) sorbed in low density polyethylene are shown. This molecular probe differs of pyrene by its ability to form intramolecular excimer species and we discuss the temperature dependence of this formation. In this study we recorded simultaneously the fluorescence spectra and the differential scanning calorimetry (dsc) traces. The relative dependence of fluorescence intensities on temperature of the higher energy vibronic band at 367 nm, of the isolated choromophore emission and of the excimer emission were discussed and associated with the polyethylene polymer relaxation processes. The influence of the macroscopic stretching upon the polymer relaxation processes and the fluorescence emission was also investigated and compared with previous results.

Further to a previous contribution to this journal (Kinghorn et al., Curr. Org. Chem. 1998, 2, 597), in the present review we describe a continuation of efforts to obtain novel potential cancer chemopreventive agents from mainly edible plants using selected in vitro bioassays to monitor chromatographic fractionation. Altogether, 52 further active compounds have been isolated from eight plant acquisitions, of which 19 are new natural products. Members of a total of eleven plant secondary metabolite classes have been isolated and characterized, namely, benzofuranoids, ceramides, flavonoids, lignans, a phenol, a porphyrin, sesquiterpenoids, stilbenoids, a stilbenolignan, triterpenoids, and withanolides. Several of these in vitro- active compounds have been found to be active in a follow-up mouse mammary organ culture model, and a number have been selected for more advanced biological testing. Examples include the withanolide, ixocarpalactone A, from the aerial parts of Physalis philadelphica, which has shown promising activity in a quinone reductase induction assay, and four flavonoid constituents from the whole plants of Broussonetia papyrifera, with potent aromatase inhibitory activity.

Over the course of the last several decades, a number of strategies have been designed for synthesizing spiroketals stereoselectively. This review looks at selected highlights of these strategies as they relate to recent synthetic efforts directed at natural products. While particular emphasis will be given to some newer strategies and innovations, examples of many of the strategies described herein have been documented in previous reviews. In these cases, recent applications of these established methods will be described. Sections will be categorized according to strategy, and not natural product or spiroketal ring size. Reference to the same natural product may therefore be found in different sections. Finally, a section will be included which reviews examples of recentlyreported reactions of spiroketals.

The chemical manifestations of orbital interactions through four σ-bonds in the base-induced and -directed reactions of monosulfonated trans-perhydronaphthalene- and norbornane-1,4-diols are discussed. Through-bond interactions (TBI) and σ-participation are the most important stereoelectronic effects which principally determine their reactivity. Dependent on the geometry of the relaying C-C bonds, these compounds react by three chief pathways: rearrangement, elimination, and homofragmentation. Substituents at carbon atoms adjacent to the carbon atom bearing the sulfonate ester group increase the contribution of σ-participation, which is expressed in a higher reactivity of these compounds. In cases where the ideal W arrangement is not distorted by alkyl substituents, the co-operation of TBI and σ-participation results in a fast homofragmentation. Through-space interactions (TSI), if occurring next to TBI, have only a modest influence on the reactivity of these compounds. Secondary monosulfonate esters of rigid 1,4-diols do show these typical reactions after induced ionization but the ionization of primary sulfonate esters can not be induced in a similar way. The results obtained from the reactions on monomesylated cholestane-3,7-diols, as model systems for cyclic 1,5-diol monosulfonate esters, show that chemical consequences of TBI and σ-participation still can be observed in these systems, but play only a minor role in their reaction patterns. Applications of reaction strategies in which orbital interactions through four σ-bonds play an essential role are demonstrated in the total syntheses of cis-fused guaiane sesquiterpenes, alloaromadendrenes, α- santalanes and marasmanes.

The aromatic metalation - cyclization sequence is a valuable protocol for the regiospecific construction of carbocyclic and heterocyclic systems. Aromatic lithiation can be carried out by lithium - hydrogen or lithium - halogen exchange. The latter procedure, though mechanistically controversial, is the method of choice to introduce specifically a lithium atom into a given, non-activated position of the aromatic ring. Once generated, the aryllithiums may react with external or internal electrophiles and thus give rise to cyclization reactions, which are known as Parham cyclization. In addition to the carbon - carbon bond formation, the aromatic lithiation allows the introduction of further functionality to the molecule. Therefore, strategies based aromatic lithiation can play a crucial role in natural products synthesis.In this review we will survey some recent advances on the application of the aromatic lithiation - cyclization sequence for the synthesis of different types of carbocycles and heterocycles. We have limited it to the metalation reactions, where the aryl and hetero-aryllithium intermediates are generated by the metal - halogen exchange reaction. Major emphasis will be placed on stereoselectivity of the metalation - cyclization sequence, since it has received only sporadic attention.