Approaches to the synthesis of cytochalasans. Part 5. Studies on the selectivity of the Diels-Alder reaction leading to the tetrahydroiso-indolinone sub-unit

The stereo- and regiochemical course in the [2+4]cycloaddition of the chiral alkylidene malonic ester 1 to selected derivatives of (2E, 4E)-4-methyl-2, 4-hexadien-l-ol ( 2 ) and (2E, 4E)-4-methyl-2, 4-hexadien-l-al ( 12 ) has been investigated. The results are discussed on the basis of semiquantitative PMO theory.     

Isolation of 2'-dehydroverrucarin A as metabolite of Myrothecium roridum Tode ex Fr.,species type by Fries. Verrucarin and roridin, 13

From cultures of Myrothecium roridum TODE ex FRIES 2′-dehydroverrucarin ( 1 ) was isolated. 1 is a genuin metabolite of the micro-organism.     

Absolute calibration of Hartmann-Shack wavefront sensor by spherical wavefronts

A method for absolute calibration of Hartmann-Shack wavefront sensor (HSWFS), in which the wavefront differences of several spherical wavefronts are used to determine parameters of HSWFS, is proposed in this paper. The calibration method is introduced and the experiment results and error analysis are presented. Across a pupil with diameter of 2.6 mm, a lenslet array of 20 × 20 sub-apertures with square configuration, and focal length 4 mm, is used to sample the incident wave. The results indicate the uncertainty of the Hartmann-Shack wavefront sensor calibrated by the proposed method, is improved to less than λ/60 PV value and λ/500 RMS value (λ = 635 nm) with modal reconstruction method. Furthermore, the factors affected the results are analyzed. The error analysis suggested that the influences of the factors on the accuracy of reconstruction can be controlled to an accept level.     

Biosynthesis of the cytochalasans. Part III. C-NMR. of cytochalasin B (phomin) and cytochalasin D. Incorporation of [1-13C]- and [2-13C]-sodium acetate

Sequential single frequency decoupling and partially relaxed Fourier transform (PRFT) are used to assign the natural abundance ¹³C-NMR. spectra of cytochalasin B (phomin) ( 1 ) and cytochalasin D ( 2 ). Cultures of Phoma spec. S 298 were fed [2-¹³C]-sodium acetate, and the distribution of this precursor in cytochalasin B (phomin) ( 1 ) was determined by ¹³C-NMR. spectroscopy. Likewise, the labelling patterns in cytochalasin D (zygosporin A) ( 2 ) from Zygos-posium masonii could be identified after incorporation of [2-¹³C]-sodium acetate and [l-¹³C]-sodium acetate. The results confirm previous proposals for the biogenesis of the cytochalasans from phenylalanine, methionine, and a C₁₈, or C₁₆, polyketide part.     

Preparation of cyclophanes by room-temperature ring-closing alkyne metathesis with imidazolin-2-iminato tungsten alkylidyne complexes

Room-temperature ring-closing alkyne metathesis of 1,2-, 1,3-, and 1,4-bis(3-pentynyloxymethyl)benzenes has been investigated in the presence of catalytic amounts of an imidazolin-2-iminato tungsten alkylidyne complex. The m- and p-diynes selectively form the respective [10]metacyclophane or [10.10]paracyclophane, respectively, whereas a mixture of monomeric and dimeric cycloalkynes is obtained in the case of the o-diyne. DFT calculations reveal that the different selectivities can be attributed to the relative thermodynamic stability of the emerging cyclophanes.     

Cu(I)-promoted one-pot click-SNAr reaction of nitrobenzaldehydes

A series of 1,4-disubstituted 1,2,3-triazoles containing formyl was synthesized from a variety of readily available nitrobenzaldehydes and alkynes via a convenient one-pot, click-S_NAr reaction with moderate to excellent yields. The reactions were easily carried out in hexamethyl phosphoramide in the absence of a base at 60 °C.     

Crystal orientation-ordered ZnS nanobelt quasi-arrays and their enhanced field-emission

Crystal orientation-ordered ZnS nanobelt quasi-arrays were fabricated using a non-catalytic and template-free thermal evaporation process; field-emission measurements show that these novel arrays are decent field emitters possessing a current density more than 20 times higher than that of randomly-oriented ZnS nanobelt ensembles at a macroscopic field of 5.5 V microm(-1).     

Si nanowire semisphere-like ensembles as field emitters

Silicon nanowires assembled in micro-sized semispheres were synthesized through simple thermal evaporation without using any templates and metal particle catalysts; electron microscopy revealed that the nanowires within semisphere ensembles are well-aligned and evenly distributed; a typical nanowire array density was of approximately 4 x 10(9) cm(-2); field-emitting characteristics of the arrays were analyzed.