Stereoselective coupling

The ene reaction of an alkyne and an alkene produces a 1,4-diene. An important application, the regio- and stereoselective coupling of 17-(Z)-ethylidene steroids and alkynes to give cholane-type 16,22-dienes, is described in section 4.5.2. 

The elaboration of the polyunsaturated side chain of asteltoxin requires a stereoselective coupling of aldehyde 2 with a suitable synthetic equivalent for the anion of 4-formyl-1,3-butadiene (see intermediate 3 in Scheme 4). Acid-induced skeletal reorganization of the aldehyde addition product, followed by an intermolecular... 

The preparation of rigid alicyclic molecules bearing effector groups from alkene blocks using s-tetrazines and 1,3,4-triazines as stereoselective coupling agents [67]... 

The influence of iron(III) salts on coupling reactions of alkynes and aldehydes (Scheme 10, routes B and C) was also explored. In these routes, a new stereoselective coupling of alkynes and aldehydes was unmasked, which led to ( ,Z)-1,5-dihalo-1,4-dienes (route B, Scheme 10) and/or ( )-a,p-unsamrated ketones (route C, Scheme 10) [27]. 

Based on a similar literature method,116 stereoselective coupling reactions between various 3-acetoxy-4-alkyl-/3-lactams and in situ-generated titanium enolates of cyclohexanone117 or propiophenone derivatives were developed, yielding the corresponding a,/3-disubstituted /3-lactams. 

DIrIgent protein Free radical capture and stereoselective coupling... 

Example 3 different activators derived from 4,5-dicyanoimidazole were prepared and used by Just and Lu in the stereoselective coupling reactions of phosphoroamidites with nucleosides [19]. An easily removable chiral auxiliary derived from d-xylose was synthesized and used for stereoselective construction of dithymidine phosphorothioate. The part of this study is illustrated below. 

The synthesis of PS chiral analogues of biophosphates via P compounds required as therapeutic continues to attract attention. It is apparent that future perspectives in this fascinating area will focus on finding easy access to chiral P compounds other than the separation of diastereoisomers and improvements in the stereoselective coupling procedures. So far research efforts in synthesis of new drugs with a phosphorus backbone has paid insufficient attention to cost and difficulties of large scale production [110]. This chapter illustrated the trends with a series of selected examples. 

A series of bridged dinuclear [(bpy)2Ru(p-L)Ru(bpy)2] " and [(bpy)2Ru(/i-L)Os(bpy)2] complexes have been prepared in which the bridge coordination domains are phen ligands and the spacer is constructed by the stereoselective coupling of units through norbornene and epoxide functionalities. " " ... 

Stereodefined spiroketals are a common structural motif in physiologically-active natural products. Richard P. Hsung of the University of Minnesota recently reported (Organic Lett. 2005, 7, 2273) that Tf,NH is a particulary effective Brpnsted acid mediator for the stereoselective coupling of vinyl lactols such as 11 with homoallylic acids such as 12. The axial ethers so produced undergo smooth ringclosing metathesis to the spiroketals. 

E. J. Enholm and S. Jiang, Highly stereoselective coupling of carbohydrate lactones with terpene ketones promoted by SmI2, Tetrahedron Lett. 55 6069 (1993). 

The reaction of 1-iodonaphthalene with chiral-assisted imide enolate ion 3 provides an interesting example of the stereoselective coupling of an aromatic radical with a nucleophile. In this reaction, the diastereomeric isomers of the substitution compound are formed (43-64%), while the seledivity observed is highly dependent on the metal counterion used and its chelation properties (Scheme 10.11). All of the ions studied (Li, Na, K, Cs, Ti(IV)) are seledive however, the highest stereoseledivity... 

Stereoselective McMurry coupling. Clive and co-workers1 report that a modified McMurry reagent prepared in DME with C8K and TiCl3 ( 2 1) can effect highly stereoselective coupling of keto aldehydes. Thus the key step in a synthesis... 

The (E)-bromides in 1,1-dibromo-l-alkenes can be stereoselectively coupled with aryl- or al-kenylboronic acids to give the corresponding (Z)-l-aryl(or alkenyl)-l-bromo-l-alkenes. Tris(2-... 

Catalysts include copper pyrazolylborates (TpJCutQHt), Ru11 porphyrins, and Rh11 carboxylates.98 Metal carbene intermediates are likely such species are also present in the ruthenium-catalyzed stereoselective coupling of a-diazocarbonyl compounds 99... 

The precatalyst 17 was reported to promote a coupling reaction of two molecules of phenylacetylene or its derivatives 65 with carboxylic acids, leading to (l ,3i )-l,4-diaryl-l-acyloxybuta-1,3-dienes 66 in various yields (Scheme 4.24) [57]. Amino acids, as well as diacids, can also be employed as carboxylic acid components. A mechanism involving the addition of a carboxylic acid to the ruthenacydopentatriene intermediate 67 was proposed for this stereoselective coupling. 

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