Stereochemistry oxidative coupling

The regioselectivity and syn stereochemistry of hydroboration-oxidation coupled with a knowledge of the chemical properties of alkenes and boranes contribute to our under standing of the reaction mechanism... 

Bis-oxazoline ligands can also be produced by oxidative coupling of the copper derivative of diastereoisomerically pure 306 (Scheme 145) . Further lithiations of the product 317, which was produced as single diastereoisomer, occur (as in Scheme 143) at the second site adjacent to the oxazoline, giving, for example, 318, despite the (presumably) less favourable stereochemistry of the lithiation step. Bisoxazolines 318 direct the asymmetric copper-catalysed cyclopropanation of styrene using diazoacetate. 

Racemic yatein 43 was obtained by Michael addition of the anion of piperonaldehyde dithiomethyl acetal to 5/7-furan-2-one (butenolide), followed by trapping of the resulting enolate with 3,4,5-trimethoxybenzyl bromide (see section 3.2.2). This process gave 43 with the desired trans stereochemistry at the butyrolactone. Oxidative coupling of the two... 

A group of alkaloids with a unique skeleton known as the Erythrina alkaloids have been isolated from the seeds of Erythrina plants examples include erythraline and erysodienone obtained from the seeds of E. indica [1,2]. The relative and absolute stereochemistry of these alkaloids was determined by X-ray crystallography and chemical degradation studies [3].The chemical synthesis of the Erythrina alkaloid nucleus was achieved biomi-metically through phenol oxidative coupling [4]. 

A series of arylations of olefins by C-H bond cleavage without direction by an ortho functional group has also been reported, and these reactions can be divided into two sets. In one case, the C-H bond of an arene adds across an olefin to form an alkylarene product. This reaction has been called hydroarylation. In a second case, oxidative coupling of an arene with an olefin has been reported. This reaction forms an aryl-substituted olefin as product, and has been called an oxidative arylation of olefins. The first reaction forms the same t)q)es of products that are formed from Friedel-Crafts reactions, but with selectivity controlled by the irietal catalyst. For example, the metal-catalyzed process can form products enriched in the isomer resulting from anti-Markovnikov addition, or it could form the products from Markovnikov addition with control of absolute stereochemistry. Examples of hydroarylation and oxidative arylation of olefins are shown in Equations 18.63 - and 18.64. ... 

Two teracacidol-catechin dimers linked (4)ff->8) and (4)ff- 6) have been synthesized, but these compounds have not been reported as natural products (27, 28). A melacacidol-(4a- 6)-melacacidol dimer isolated from the heartwood of Prosopis glandulosa (mesquite) occurs in low concentration together with the oxidatively coupled flavan-3-ols described previously (174). Foo (103) has recently isolated a melacacinidin-(4a->6)-isomelacacidin dimer from Acacia melanoxylon, the first natural product that has the 2i ,3i ,4i -(2,3-d5-3,4-c/5)-stereochemistry. However, only its 2R,3R,4S (2y3-cis-3,4-t ans) diastereoisomer was obtained by synthesis. 

Secondary bromides and tosylates react with inversion of stereochemistry, as in the classical SN2 substitution reaction.24 Alkyl iodides, however, lead to racemized product. Aryl and alkenyl halides are reactive, even though the direct displacement mechanism is not feasible. For these halides, the overall mechanism probably consists of two steps an oxidative addition to the metal, after which the oxidation state of the copper is +3, followed by combination of two of the groups from the copper. This process, which is very common for transition metal intermediates, is called reductive elimination. The [R 2Cu] species is linear and the oxidative addition takes place perpendicular to this moiety, generating a T-shaped structure. The reductive elimination occurs between adjacent R and R groups, accounting for the absence of R — R coupling product. 

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