Stereoselective construction

H )-Euranones are useful building blocks in the synthesis of a variety of organic compounds. In addition, they often serve as valuable synthetic intermediates in the stereoselective construction of substituted y-butyrolactones via conjugated addition to the Q ,/3-unsaturated carbonyl moiety or catalytic hydrogenation of the double bond (88JOC1560). 

For the stereoselective construction of a structural unit of type 1 from aldehydes or ketones, the synthon 2 is required. It is delivered by the /-hetero-substituted allylmetal (E)- or (Z)-3 with allylic inversion. 

Stereoselective construction of tetrasubstituted exocyclic alkenes from the [4 + 2] cycloaddition of vinylallenes [155]... 

Tokoroyama T. Synthesis of Clerodane Diterpenoids and Related Compounds -Stereoselective Construction of the DecaUn Skeleton With Multiple Contiguous Stereogenic Centers Synthesis 2000 611-633... 

Mehta, G. Reddy, D.S. (1998) Reserpine Synthesis A Protocol for the Stereoselective Construction of the Densely Functionalized Ring-E. Journal of the Chemical Society Perkin Transactions I, 2125-2126. 

Another challenge associated with the synthesis of strychnine, as well as other Strychnos alkaloids, is the stereoselective construction of the F-hydroxyethylidene side chain (Scheme 4.5). Strategies that rely on a late-stage olefination reaction of a C20 ketone often suffered from low diasterocontrol for the newly formed alkene, with the best exception provided by Kuehne (15 > 16) [10a, b, d, f]. Overman,... 

Intramolecular C-H insertion reactions of metal carbenoids have been widely used for the stereoselective construction of substituted lactams, lactones, cyclopentanones, benzofurans, and benzopyrans. Several excellent reviews have been published covering the general aspects of intramolecular C-H insertion by metal carbenoids.46,47 62 71 99-104 The following section highlights the major advances made since 1994, especially in asymmetric intramolecular C-H insertion. 

The stereospecific preparation of aldehyde ester 462, already comprising five of the six stereo centers of the molecule and, furthermore, the stereoselective construction of the epialloyohimbane skeleton with Ec2 conformation are regarded the remarkable features of Woodward s approach. 

Scheme 6.41. Stereoselective construction of a quaternary stereocenter by allylic substitution of mesylate 195 with a boron trifluoride-modified cyano-Gilman cuprate reagent.

The validity of the oxauracil strategy for a stereoselective construction of the azabicyclic core has been proven for the syntheses of biologically important polyguanidinium alkaloids, namely, batzelladine A and D [58]. Reaction (7.48) shows that treatment of 41 under the standard free-radical conditions furnished the desired azabicycle in a 77 % yield, in a high diastereoisomeric ratio. Here it can be seen the effect of H donation from the more accessible face of the molecule. 

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. 

From a synthetic point of view the stereoselective construction of the tri-cyclodecane skeleton and the installation of the methyl group at C-5 and of the 2-propenyl group at C-7 pose significant challenges which were addressed differently in the five syntheses of kelsoene known to date. 

Somfai s group has demonstrated that the aza-[2,3]-Wittig rearrangement of the viny-laziridines 134 to 135 is useful for the stereoselective construction of the indolizidine alkaloids 136 . ... 

The stereoselective construction of nitrogen heterocycles remains a topic of intense synthetic interest [39]. Evans and Robinson described the combination of the stereospecific aUylic amination with ring-closing metathesis as a strategy for the constmction of mono- and disubstituted azacycles, which they demonstrated with the stereospecific construction of cis- and tra s-2,5-disubstituted pyrrolines [40]. Furthermore, this approach provided an ideal system for the determination of whether the enantiospecific rhodium-catalyzed aUyhc amination with an enantiomerically enriched nucleophile experiences a matched and a mismatched reaction manifold. 

Scheme 10.10 Stereoselective construction of dihydrobenzo[fa]indolines using allylic amination with anilines.

Scheme 10.11 Stereoselective construction of benzo[b]furans using allylic etherification with phenols.

In contrast, Hirama and co-workers stereoselectively constructed BCD ring moiety 8, utilizing intramolecular hydrogen bonding between the primary and... 

Stereoselective construction of (Z)-fluoroalkene depsipeptide (60) through a Cu(l)-mediated allylic substitution reaction... 

M. Okada, Y. Nakamura, A. Saito, A. Sato, H. Horikawa, T. Taguchi, Stereoselective construction of functionalized (Z)-fluoroalkenes directed to depsipeptide isosteres. Tetrahedron Lett. 43 (2002) 5845-5847. 

Deprotonation at an activated 4-position has been employed extensively in asymmetric synthesis, which is the key step in the Seebach protocol for the preparation of a-alkyl amino acids.The existing alkyl group at the 5-position acts as a directing group for the alkylation and is oriented trans to the new alkyl group (Scheme 8.118). This reaction provides an efficient methodology for normally difficult stereoselective construction of a quaternary chiral center. 

Yet, despite a broad spectrum of classical and more modern procedures for the chemical connection of aromatic moieties2-4, the development of efficient aryl coupling methods that allow the directed, namely regio- and especially stereoselective, construction of even sterically highly hindered biaryls, has only recently begun these stereoselective reactions will be dealt with in this section. 

Reddy, P. G., Baskaran, S. (2004) Epoxide-initiated cationic cyclization of azides a novel method for the stereoselective construction of 5-hydroxymethyl azabicyclic compounds and application in the stereo- and enan-tioselective total synthesis of (+)- and ( —) -indolizidine 167B and 209D. J Org Chem 69, 3093-3101. 

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