flections may have suffered a phase change so that the information in the appropriate frequency range is transferred with reversed contrast. The second stage in image analysis involves image reconstruction, where the electron micrograph now serves as object. Filtering of Fourier frequencies in reciprocal space (secondary diffraction pattern) is performed generally with the aid of a computer. The various stages of frequency filtering are very complex and must be treated with careful consideration of the physics involved. Otherwise, artifacts can easily be introduced. The safest way to guard against this is to compare the original image (which must be of very high quality) with the computer-treated image, and to ensure that defects have not been introduced which were not in the original. Details regarding these procedures will be published elsewhere [16].
The convergent synthesis of macrolide soraphen A 1 α is described starting from glucose (western part) and mannose (eastern part). Mannose was converted into a 2-deoxyribohexopyranoside that could be methylated and reduced stereoselectively. Chain elongation at CÐ6 was carried out by stereoselective addition of a magnesium acetylide. The two fragments (western and eastern) were assembled by a Julia olefination followed by macrolactonization. The introduction of the methyl group at CÐ2 of norsoraphen occurred stereoselectively for thermodynamic reasons.
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