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Although direct, asymmetric, halonium-induced cyclizations have proven difficult to achieve in the absence of enzymes, this report provides a two-step alternative based on reacting polyenes with chiral mercury(II) complexes to afford a number of polycyclic organomercurials that can be subsequently converted, with retention, into their corresponding chlorine, bromine, and iodine derivatives in good yield and enantioselectivity. A five-step asymmetric total synthesis of the natural product 4-isocymobarbatol is also described.

The biosynthetic pathways of sesquiterpene lactones are described in conjunction with recent developments in the total syntheses of various biologically active guaianolides. Different strategies towards the 5,7,5-membered ring system are highlighted. Oxidative diol cleavages, aldol reactions, and intramolecular cyclopropanations were used as key reactions to construct the racemic guaianolide core system. Radical cyclizations, as well as ring closing metathesis, were successfully applied in asymmetric approaches. Biomimetic reactions were also applied as versatile tools for the construction of these highly complex natural products.

A systematic study of a ring-closing metathesis towards a tetrasubstituted double bond as part of a seven-membered ring in a 5.7.5-tricyclic guaianolide system is described. By combining two techniques, namely sparging an inert gas through the solution together with dielectric heating via microwave irradiation a high-yielding ring-closing metathesis reaction in this particularly challenging case was achieved. The results obtained compare favorably with conventional heating conditions or with microwave irradiation in a closed system. The key aspects seem to be that rapid microwave irradiation diminishes catalyst decay by allowing the required high reaction temperature to be reached quickly and homogeneously and thereby providing enough energy for a successful metathesis reaction, while inert gas sparging is purging off evolving ethylene to shift the equilibrium to the product.

Although direct, asymmetric, halonium-induced cyclizations have proven difficult to achieve in the absence of enzymes, this report provides a two-step alternative based on reacting polyenes with chiral mercury(II) complexes to afford a number of polycyclic organomercurials that can be subsequently converted, with retention, into their corresponding chlorine, bromine, and iodine derivatives in good yield and enantioselectivity. A five-step asymmetric total synthesis of the natural product 4-isocymobarbatol is also described.

The biosynthetic pathways of sesquiterpene lactones are described in conjunction with recent developments in the total syntheses of various biologically active guaianolides. Different strategies towards the 5,7,5-membered ring system are highlighted. Oxidative diol cleavages, aldol reactions, and intramolecular cyclopropanations were used as key reactions to construct the racemic guaianolide core system. Radical cyclizations, as well as ring closing metathesis, were successfully applied in asymmetric approaches. Biomimetic reactions were also applied as versatile tools for the construction of these highly complex natural products.

2,2-Dimethyl-propanedioyl dichloride (l)-Valinol Keywords: bis(4-isopropyloxazoline); ligands; bis(oxazolines); amino alcohols; acid chlorides; condensation; cyclization; tert-leucine; waste disposal

A new method for the synthesis of 2,3-disubstituted dihydrobenzofurans is reported making use of radical formation via translocation by 1,6-hydrogen atom transfer and subsequent 5-exotrig cyclization.

A strategy towards the stereoselective synthesis of bi-and tricyclic sesquiterpene lactones is reported. As key step radical cyclizations of appropriately functionalized trans-4,5-disubstituted γ-butyrolactones, which arereadily available from methyl 2-furoate, were carried out to give rise to 5,6-, 5,7- and 5,7,5-ring systems in diastereo- and enantiomerically pure form.