Search

Herein we report on the self-assembly, in a low polarity solvent, of a pseudorotaxane species comprising binaphthyl-26-crown-8 (BN26C8) as the macrocyclic host and anthracenyl-benzylammonium as the threadlike positively charged guest (ABH+). Absorption and luminescence data reveal a very efficient energy transfer process occurring from the binaphthyl to the anthracene singlet excited states. The self-assembly is highly dependent on the nature of the counteranion confirming the crucial role played by it in the competition between the self-assembly process and the formation of ion pairs (ABH+X-). This behavior can be readily evidenced in dilute solutions from the analysis of the luminescence properties of the system. The complexation of chloride, sulfate and hexafluorophosphate salts of ABH+ by racemic BN26C8 as well as the complexation of ABH+ salts of the chiral anion tris[tetrachlorobenzenediolato]phosphate(V) (TRISPHAT-) by (+)BN26C8 are described. The efficiency in complexation follows the trend PF6- > TRISPHAT- > Cl-, SO42-. The use of the chiral anion TRISPHAT allowed us to investigate the possibility of inducing stereoselective control on the formation of the interpenetrating assembly. Preliminary 1H NMR spectroscopic evidence supports the fact that the chiral anion is paired to the supramolecular complex and that its configuration influences the recognition process.

The A*[s]U(*) dinucleosides 1 and 2 form thermoreversible gels in organic solvents. The basis of the gelation is the formation of linear aggregates by base pairing following desolvation of the nucleobases. This is evidenced by the absence of gel formation by the C(6)-deaminated analogue 3 of 1, the correlation of gelation with the anti-conformation, as preferred for 1, and the temperature-, concentration-, and time-dependent CD spectra. The gels were also characterized by the minimum gelation concentration, the gel–sol transition (melting) temperature, and rheological properties.

Reactions between allenylzinc carbenoid 1 and various imines were examined; trans-substituted alkynylaziridines were produced with excellent diastereoselectivity. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Wheat (Triticum aestivum L.) varieties with the dwarfing Rht (reduced height) alleles have reduced sensitivity to endogenous or exogenous gibberellin (GA). The Rht isolines are also less sensitive to the effects of chlormequat and other compounds which cause dwarfing by depressing GA synthesis. Abscisic add (ABA) inhibits cereal seed germination and is in dynamic tension with GA, which enhances germination. Tetcyclacis is a cytochrome P450 mono-oxygenase inhibitor of GA synthesis. It also inhibits ABA hydroxylase, thus extending ABA activity. In preliminary experiments, wheat varieties differing in Rht complement responded differently to ABA and/or tetcyclacis. We therefore investigated the effect of seed applications of ABA (100 μM), with or without tetcyclacis (25 μM), on germination and initial growth of Rht wheat isolines Rht-B1b, Rht-B1a (dwarf); Rht-B1b,Rht-D1b (dwarf); Rht-B1b + Rht-D1b (double-dwarf); and Rht-B1a, Rht-D1a (tall) to determine a possible genetic basis for the observed varietal differences. Treating seeds with ABA, tetcyclacis, or both, delayed emergence by 1, 1.8, or 3.7 d, respectively, compared with controls. ABA had little or no effect on final emergence, shoot height, or fresh weight, but led to a slight reduction in dry weight of Rht-D1b and Rht-B1b+D1b. Tetcyclacis reduced growth rate (mm/ d), shoot height, fresh weight, and percentage dry weight in proportion to Rht dosage, as is typical of GA synthesis inhibitors. Synergistic effects of ABA and tetcyclacis were not detected for any parameter measured, except for rate of emergence. ABA did not exert a profound effect on seedling germination or initial growth of the Rht wheat isolines examined in this study.

The metallation of 3-trimethylsilyl-1-chloroprop-2-yne and the subsequent reaction with N-benzyl imines affords stereospecifically trans propargylic aziridines with good to excellent yields.

A series of 3-halo-2( 1H)-pyridinones 4 have been synthesized by treatment of δ-dienaminoesters 1 with N-halosuccinimide in basic medium. The chemoselectivity of the reaction is discussed.

We have studied the reactivity of β-enaminoesters 1 (R 2 =OMe) or β-enaminoketone 1 (R 2 =Me), derived from ( S )-phenylglycinol, with activated acrylate derivatives. Substrates 1 (R 1 =Me) afforded by aza-annulation oxazololactams 2 and 3,4-dihydro-2-pyridones 3 . In the same conditions β-enaminoesters 1 (R 1 =H) furnish N -acylated oxazolidines 4 .

Diastereoselective aza-annulation of seven-membered β-enaminolactones 2 gives access to bicyclic heterocyles 5. Fragmentation of molecule 5a with lithium methoxide affords cis or trans bicyclic lactams 8 with excellent stereoselectivities.

Chiral non-racemic bicyclic lactams 2b,c , derived from (R)- and (S)-phenylglycinol, were used in the enantioselective synthesis of (−)-lupinine and 5-epitashiromine, respectively. The efficiency of the synthesis relied on the high diastereoselectivities of formation and reduction of compounds 2b,c .

A series of polysubstituted pyrroles 3 have been synthesized efficiently in two or three steps starting from primary amines 1. The key step of this process is the bromocyclization of δ-dienamino esters 2. The chemoselectivity of this reaction is discussed. AM I calculations confirmed the observed reactivity.

Seedling establishment of dryland crops in semiarid and arid zones is limited by precipitation. Spring wheat (Triticum aestivum L. emend. Thell.) generally is sown in dry soil in the dryland regions of Israel before the rainy season starts. We compared the effects of no-tillage, conventional tillage, and plant growth regulators on wheat seedling growth to identify the optimal crop management system for seedling establishment in dryland farming. Experiments were conducted during 1998 and 1999 at the Gilat Experimental Station located in southern Israel (annual precipitation of 222 and 72 mm for 1998 and 1999, respectively; soil type is sandy loam loess-Torrifluvents). Neither inhibitors of gibberellin synthesis (chlorocoline chloride, padobutrazol, and trinexapac-ethyl) nor monopotassium phosphate (KH 2 PO 4 ) enhanced seedling survival under drought stress when sprayed on seedlings at the two-leaf stage. No-tillage led to increases in water content in the upper (0-30 cm) soil layer and in seedling water content and seedling biomass. No-tillage management also maintained seedling viability compared with the control, with seedlings surviving as long as 35 d without precipitation. No-tillage management allows successful seedling establishment and growth after a dry period that follows germination.