Synthetic studies in the tetranorditerpene lactone series. I. The total synthesis of dihydro-8-epi-acrostalidic acid

The total synthesis of dihydro-8-epi-acrostalidic acid ($\text{8b}$) and other related tetranorditerpene lactones are described.     

Chiral symmetry breaking and polymorphism in 1,1'-binaphthyl melt crystallization

We have studied chiral symmetry breaking in the melt crystallization of 1,1'-binaphthyl. We confirm that chiral symmetry breaking can be induced by stirring the melt as it crystallizes. We find an additional process of vapor crystallization to occur alongside the melt crystallization. This complicates the analysis of the enantiomorphism by introducing a further phenomenon: that of polymorphism. Crystallographic studies by X-ray diffraction reveal two polymorphs of 1,1'-binaphthyl that are made up of two different conformers of each of the two enantiomeric forms of the molecule. Crystals from the melt are generally chiral tetragonal crystals (P42(1)2(1)) composed of (R)- or (S)-1,1'-binaphthyl in a transoid conformer, while those from the vapor are racemic monoclinic crystals (C2/c) made up of the cisoid conformer of both (R)- and (S)-1,1'-binaphthyl enantiomers. The main intermolecular interactions in all these crystals are weak aromatic CH/pi hydrogen bonds, which are responsible for the enantiomeric discrimination in the molecular recognition during crystallization. A tendency for whisker crystal formation is notable in 1,1'-binaphthyl. In stirred crystallization, fluid and mechanical forces can break off these whiskers, which provide secondary nuclei for further crystallization. This autocatalytic mechanism induces chiral symmetry breaking during the crystallization.     

A new titanosilicate umbite membrane for the separation of H2

A new microporous titanosilicate umbite membrane has been synthesized on commercial porous TiO2 tubular supports. This membrane is able to separate H2/N2 mixtures (with selectivities as high as 48) even in the presence of water.     

Optimization based on the minimum maximal k-partial-matching problem of finite states machines with input multiplexing

Design Automation for Embedded Systems - Finite State Machines with Input Multiplexing (FSMIMs) were proposed in previous work as a technique for efficient mapping Finite State Machines (FSMs) into...     

Cyclopropanation with Fischer acyloxycarbene complexes: preparation of cyclopropane and cycloheptane-fused gamma-lactones

[reaction: see text] A sequential acylation-intramolecular cyclopropanation reaction takes place upon treatment of a series of tetraalkylammonium acylchromates with beta,gamma-unsaturated acyl chlorides at -10 degrees C. The reaction leads to 2-oxabicyclo[3.1.0]hexan-3-ones with exo selectivity in good yields. The diastereoselectivity of the reaction allows the preparation of cis-divinyl cyclopropanes, which evolve via Cope sigmatropic reaction toward cycloheptadiene derivatives. Furthermore, the aromatic Cope rearrangement of a series of cis-aryl vinyl cyclopropanes prepared by means of this methodology has been studied.     

Duty cycle and modulation efficiency of two-channel hadamard transform time-of-flight mass spectrometry

Hadamard transform time-of-flight mass spectrometry (HT-TOFMS) is based on the pseudorandom gating of ion packets into a time-of-flight mass-to-charge analyzer. In its typical implementation, the technique is able to monitor continuous ion sources with a 50% duty cycle, independent of all other figures of merit. Recently, we have demonstrated that the duty cycle can be extended to 100% using patterned, two-channel detection. Two-channel HT-TOFMS involves the simultaneous optimization of paired one-channel experiments and imposes more stringent conditions to achieve high-quality spectra. An ion modulation device, known as Bradbury-Nielson Gate (BNG), is central to HT-TOFMS. It is an ideal deflection plate, capable of transmitting or deflecting an ion beam according to a known binary sequence without changing the times-of-flight of the ions. Analytical equations are derived that accurately describe the ion modulation process of the BNG as confirmed by good agreement with SimIon simulations and ion beam imaging experiments. From these expressions, the duty cycle and ion modulation efficiency were calculated for various BNG parameters, ion beam characteristics, and detector dimensions, which permit the optimum conditions to be chosen for the two-channel experiment. We conclude that the outer detector should be three times the maximum deflection angle to detect all deflected ions (100% duty cycle) and that the difference between the modulated ion counts in the sequence elements 0 and 1 should be maximized to achieve high modulation efficiency. This condition is best achieved by tight focusing of the ion beam in the center of the inner detector. When both channels are optimized, the two-channel advantage can be exploited to achieve a further improvement over a single-channel experiment.     

Insertion of unsaturated organic electrophiles into molybdenumbond-alkoxide and rheniumbond-alkoxide bonds of neutral,stable carbonyl complexes

Alkoxo complexes [Re(OR)(CO)(3)(N-N)] (R=Me, Et, tBu; N-N=2,2'-bipyridine (bipy), 4,4'-dimethyl-2,2'bipyridine (bipy'), 1,10-phenanthroline (phen)) and [M(OMe)(eta(3)-allyl)(CO)(2)(phen)] (M=Mo, W) have been synthesized in good yields and using mild conditions by the reaction of sodium alkoxides with [Re(OTf)(CO)(3)(N-N)] and [MCl(eta(3)-allyl)(CO)(2)(phen)] precursors. These have been characterized by IR and NMR spectroscopy as well as by X-ray diffraction for [W(OMe)(eta(3)-allyl)(CO)(2)(phen)] (10). The reactions of the molybdenum and rhenium alkoxo complexes with isocyanates, R'NCO, yield [L(n)M[N(R')C(O)OR]] complexes; the carbamate ligand, which results from an R'NCO insertion into the Mbond;OR bond, is monodentate through the nitrogen atom. The solid-state structures of Mo and Re examples have been determined by X-ray diffraction. The geometry around the carbamate nitrogen of these compounds is planar, and the distances indicate delocalization of the nitrogen lone pair involving mainly the carbonyl groups. Experiments carried out with the Re complexes showed that aryl isocyanates are more reactive than their alkyl counterparts, and that bulky R' groups led to slow rates of insertion. Insertion reactions were also observed with isothiocyanates, although here it is the Sbond;C bond that inserts into the Mbond;OR bond, and the resulting ligand is bound to the metal by sulfur. Competition experiments with the Re compounds indicate that isocyanates are more reactive than isothiocyanates towards the Rebond;OR bonds. Tetracyanoethylene inserts into the Rebond;OMe bond of [Re(OMe)(CO)(3)(bipy')], forming a complex with a 2-methoxytetracyanoethyl ligand; the structure of which was determined by X-ray diffraction. The formation of the xanthato complex [Re(SC(S)OtBu)(CO)(3)(bipy)] (20) by reaction of [Re(OTf)(CO)(3)(bipy)] with CS(2) and NaOtBu, but not by the reaction of CS(2) and [Re(OtBu)(CO)(3)(bipy)] (5 a), suggests that the insertion reactions do not take place by ionization of the alkoxo complexes to give the free alkoxide ion.     

Stereoselective synthesis of pyrrolidinyl glycines from nitrones: complementarity of nucleophilic addition and 1,3-dipolar cycloaddition

Epimeric pyrrolidinyl glycines, a sort of conformationally constrained α,β-diaminoacids, were stereoselectively prepared using complementary approaches based on nitrone chemistry. Nucleophilic additions to pyrrolidinyl nitrones and 1,3-dipolar cycloadditions of l-serine derived nitrones to form the corresponding hydroxylamines and isoxazolidines, respectively, provided key intermediates for the synthesis of the target compounds. Whereas the nucleophilic addition route afforded the syn adduct, the 1,3-dipolar cycloaddition approach furnished the precursor for the preparation of the corresponding anti compound.     

Communication: polymer-assisted solution-phase synthesis of 4,5-dihydro-1,4-benzoxazepin-3(2H)-ones

The polymer assisted solution phase (PASP) synthesis of 4,5-dihydro-1,4-benzoxazepin-3(2H)-ones is described. Using salicylic aldehydes, alpha-bromo acetic acid esters, and primary amines as broadly variable building blocks, the target molecules were obtained in a straightforward manner. The use of polymer bound reagents and scavengers greatly simplified workup, and avoided the use of protecting groups. A small library was prepared, showing the feasibility of the synthetic concept for the generation of larger sets of screening compounds.     

Mechanism of anionic dearomatizing reactions of diphenylphosphinamide derivatives: a theoretical and experimental study

The mechanism of the anionic dearomatisation of phosphinamide derivatives has been investigated both theoretically and experimentally. The potential-energy surface of model reactions was studied at the Becke3LYP/6-31+G* level of theory, and according to this study, a pre-reactive complex is formed between the alkyllithium and the phosphinamide. This complex evolves preferentially through NC(alpha)-metalation of the phosphinamide. The intramolecular nucleophilic addition of the carbanion to the ortho position of the aromatic ring leads to the dearomatised products, in a reaction that has been shown to be under thermodynamic control. Coordinating co-solvents, such as hexamethyl phosphoramide (HMPA) or N,N'-dimethyl-N,N'-propylene urea (DMPU), appear to influence the reaction by favouring the formation of solvent-separated ion pairs. The cyclisation reaction of allylphosphinamide derivatives was also studied. It was found that both the alpha- and gamma-attack of the allyl anion can take place, however the formation of the seven-membered ring products derived from the gamma-attack are clearly favoured.