Addition reactions continued synthesis

Examples of nitrogen-containing heterocycle syntheses based on condensation reactions continue to be forthcoming. Examples include a tandem oxidation-annulation of propargyl alcohols in a one-pot synthesis of pyridines (Equation 148) <2003SL1443>, trifluoromethyl-substituted pyridines (Scheme 94) <2003S1531>, and standard malononitrile additions to a,/3-unsaturated ketones <1995JCM392>. 

Variations and improvements of the Pictet-Spengler reaction continued to be a popular area of research for the synthesis of isoquinolines. Ruchirawat et al. reported a new version of the Pictet-Spengler reaction where V-acylcarbamatcs 122 were partially reduced with DIBAL-H followed by the sequential addition of BF3-OEt2 to provide 1,2-disubstituted tetrahydroisoquinoline derivatives 123 <07TL8182>. 

Both intermolecular and intramolecular additions of carbon radicals to alkenes and alkynes continue to be a widely investigated method for carbon-carbon bond formation and has been the subject of a number of review articles. In particular, the inter- and intra-molecular additions of vinyl, heteroatomic and metal-centred radicals to alkynes have been reported and also the factors which influence the addition reactions of carbon radicals to unsaturated carbon-carbon bonds. The stereochemical outcome of such additions continues to attract interest. The generation and use of alkoxy radicals in both asymmetric cyclizations and skeletal rearrangements has been reviewed and the use of fi ee radical reactions in the stereoselective synthesis of a-amino acid derivatives has appeared in two reports." The stereochemical features and synthetic potential of the [1,2]-Wittig rearrangement has also been reviewed. In addition, a review of some recent applications of free radical chain reactions in organic and polymer synthesis has appeared. The effect of solvent upon the reactions of neutral fi ee radicals has also recently been reviewed. ... 

The industrial scale reaction of synthesis gas to ammonia in pressure reactors takes place in a cyclic process in which the ammonia formed is removed from the reaction gas and the unreacted synthesis gas returned to the reactor. In addition to the ammonia formed, inert gases and the liberated reaction heat have to be continuously removed from the cyclic process. The excess heat of the product gas is used to heat the feed synthesis gas to the reaction temperature in a heat exchanger integrated into the reactor. Additional waste heat can be utilized for steam generation. The pressure loss in the synthesis gas due to its passage through the synthesis loop is compensated for and the fraction of synthesis gas converted replaced by fresh compressed synthesis gas ( fresh gas ). 

New auxiliaries and reaction methods are now available for the stereoselective synthesis of all members of the stereochemical family of propionate aldol additions. These also include improvements on previously reported methods that by insightful modification of the original reaction conditions have led to considerable expansion of the versatility of the process. In addition to novel auxiliary-based systems, there continue to be unexpected observations in diastereoselective aldol addition reactions involving chiral aldehyde/achiral enolate, achiral aldehyde/chir-al enolate, and chiral aldehyde/chiral enolate reaction partners. These stereochemical surpri.ses underscore the underlying complexity of the reaction process and how much we have yet to understand. 

Radical-mediated reactions continue to be popular for the formation of heterocycles, and several additional methods for tetrahydrofuran synthesis using radicals have been published. Reductive cyclization using tri-n-butyltin hydride has been the most commonly used method, and Pezechk et al. have extended this... 

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