Efficient methodology for the preparation of the artificial levoglucosenone-type chiral building blocks having a bicyclo [3. 2. 1] octane framework has been developed along with two new routes to both enantiomers of levoglucosenone. Because of their inherent stereochemical and chemical nature with a sterically biased structure containing an enone, a masked formyl and a 1, 2-glycol, and a hydroxymethyl functionalities, these chiral building blocks allow the diastereo- and enantio-controlled construction of a wide variety of natural products.
Organotellurium compounds reversibly generate carbon-centered radicals upon photolysis and thermolysis, and the radicals thus generated can be used for carbon-carbon bond formation with a variety of radical acceptors. For example, a variety of organotellurium compounds react with quinones to give the corresponding adducts, including polyprenyl qnuinonoids. The reaction of telluroglycosides and alkynes also gives the corresponding C-glycosides bearing vinyl telluride moiety. Isonitriles are also good acceptors and gives group-transfer products. Because the coupling reaction proceeds in a group-transfer manner, the carbon-tellurium bond in the products can be used for further synthetic transformation. Thus, the radical-mediated coupling followed by an adequate transformation provides a new versatile method in organic synthesis.
Dioxiranes are remarkably versatile oxidizing agents that show great potential for asymmetric synthesis, particularly asymmetric epoxidation of olefins. This article outlines some recent progress in this area from the author's laboratories.
This article reviews synthetically efficient, elegant, and ingenious examples on the biomimetic syntheses of many skeletal types of alkaloids, covering the literature mainly published since 1997. The contents are arranged in the order of the biogenetic origin of the nitrogen in each alkaloid, i.e., the alkaloid derived from aliphatic amino acids and from aromatic amino acids, and then the pseudoalkaloids.
The importance of a reaction for construction of the cis, trans diene moiety of the general structures 1 and 2 is described with the molecules shown in Figure 1. In the first section, several reactions useful for construction of these dienes are presented. Such reactions are : (1) Wittig reaction; (2) Sonogashira reactions followed by cis hydrogenation; (3) - (6) cross-coupling reactions of various types. Stereoselectivity of these reactions and availability of the reaction partners required for the reactions are briefly discussed. In the remaining sections, synthesis of leukotriene B4 (LTB4), 10, 11-dihydro-LTB4, didemnilactone A, korormicin, and fostriecin is presented with the observed efficiency in construction of the conjugated olefin moieties.
We have systematically synthesized 2α-functionalized 1α, 25-dihydroxyvitamin D3 [1α, 25 (OH) 2D3] 1 based on a convergent method using Pd-catalyzed alkylative cyclization with the A-ring precursor enynes and the CD-ring bromoolefin 5 in the following three categories : 2α-alkyl, 2α-hydroxyalkyl, and 2α-hydroxyalkoxyl derivatives, in order to study the structure-activity relationships of the natural hormone 1α, 25 (OH) 2D3, particularly on the A-ring. First, the 2-methyl analogues 2 were designed and all eight possible diastereomers on the A-ring stereochemistry at C 1, C 2, and C 3 were synthesized from the A-ring precursor enynes 22a-h, which were prepared from methyl (S) -and (R) -3-hydroxy-2-methylpropionate. Biological activities including affinities to vitamin D receptor (VDR) and vitamin D binding protein (DBP), elevation of serum Ca level, induction of HL-60 cell differentiation, and apoptosis are discussed. Next, on the basis of biological activities of the 2α-methyl derivative, the other types of 2α-functional groups were introduced stereoselectively into the 1α, 25 (OH) 2D3 skeleton using the A-ring precursor enynes derived from D-xylose or D-glucose. Five of these synthetic 2α-modified analogues showed higher VDR binding affinity than that of the natural hormone. Docking studies of the synthetic ligands to VDR based on Moras' X-ray results and remarkably high biological activities are described.
In the modern highly selective organic synthesis, organometallic reagents are indispensable. Although the plausible mechanism and structure of these reagents are also demonstrated based on the single crystal X-ray analysis, only a few efforts has been tried to analyse their structure in the solution. In this sense, there is no great difference between the reaction vessel of the modern chemists and the cauldron of the ancient alchemists. We introduce the direct method to analyse the organometallic reagents in solution by X-ray analysis.