Subject stereoselective addition

The fourth chapter gives a comprehensive review about catalyzed hydroamina-tions of carbon carbon multiple bond systems from the beginning of this century to the state-of-the-art today. As was mentioned above, the direct - and whenever possible stereoselective - addition of amines to unsaturated hydrocarbons is one of the shortest routes to produce (chiral) amines. Provided that a catalyst of sufficient activity and stabihty can be found, this heterofunctionalization reaction could compete with classical substitution chemistry and is of high industrial interest. As the authors J. J. Bmnet and D. Neibecker show in their contribution, almost any transition metal salt has been subjected to this reaction and numerous reaction conditions were tested. However, although considerable progress has been made and enantios-electivites of 95% could be reached, all catalytic systems known to date suffer from low activity (TOP < 500 h ) or/and low stability. The most effective systems are represented by some iridium phosphine or cyclopentadienyl samarium complexes. 

This compilation embraces a wide variety of subjects, such as solid-phase and microwave stereoselective synthesis asymmetric phase-transfer asymmetric catalysis and application of chiral auxiliaries and microreactor technology stereoselective reduction and oxidation methods stereoselective additions cyclizations metatheses and different types of rearrangements asymmetric transition-metal-catalyzed, organocatalyzed, and biocatalytic reactions methods for the formation of carbon-heteroatom and heteroatom-heteroatom bonds like asymmetric hydroamina-tion and reductive amination, carboamination and alkylative cyclization, cycloadditions with carbon-heteroatom bond formation, and stereoselective halogenations and methods for the formation of carbon-sulfur and carbon-phosphorus bonds, asymmetric sulfoxidation, and so on. 

The general trend is that boron enolates parallel lithium enolates in their stereoselectivity but show enhanced stereoselectivity. There also are some advantages in terms of access to both stereoisomeric enol derivatives. Another important characteristic of boron enolates is that they are not subject to internal chelation. The tetracoordinate dialkylboron in the cyclic TS is not able to accept additional ligands, so there is no tendency to form a chelated TS when the aldehyde or enolate carries a donor substituent. Table 2.2 gives some typical data for boron enolates and shows the strong correspondence between enolate configuration and product stereochemistry. 

In the synthesis in Scheme 13.46, a stereoselective aldol addition was used to establish the configuration at C(2) and C(3) in Step A. The furan ring was then subjected to an electrophilic addition and solvolytic rearrangement in Step B. 

The Corey allylation system based on a chiral bis(sulfonamide) auxiliary was put to use with success in a number of synthetic efforts, including the total synthesis of the anticancer agent leucascandrolide (Scheme 13). Chiral reagent 152 is added to an achiral aldehyde, 3-(/ -methoxybenzyloxy)propanal, affording intermediate 153 in high stereoselectivity. The latter is transformed into a pyranyl aldehyde, which is subjected to a second allylation (this time, a doubly diastereoselective addition) en route to the completion of leucascandrolide. 

When the cyclization substrate (A Scheme 3) contains stereogenic centers, and the formation of the C—Z bond generates a new stereogenic center, two diastereomers of the cyclization product (H) can be formed (stereospecific anti addition assumed). The factors which lead to high stereoselectivity in this process are of considerable importance and have been the subject of numerous studies in recent years. This reaction mechanism shows that all pathways leading to cyclic products are potentially reversible thus, the ratios of products in these reactions may be the result of thermodynamic rather than kinetic control. Unfortunately, many studies have not determined which type of control was operating under the reaction conditions used. 

The addition of caibenoids derived from alkyl diazoacetates to alkenes has been extensively studied. As two thorough reviews on the subject,1 2 dealing with a detailed comparison of the various catalysts, have recently appeared, only a general summary concerning regioselectivity, competing reactions, dia-stereoselectivity and enantioselectivity will be presented here. 

We focused on 3-cyanochromone derivatives, for the dienophilic reactivity of these compounds has gone unnoticed.18 In addition, these 3-cyanochromones could serve as excellent model systems for exploring the potential of our strategy and investigating the scope [i.e. regio- and stereoselectivity] of y-pyrones in [4 + 2] cycloadditions. As a result, this cycloaddition also became a subject of research by other groups.19... 

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