High-productivity synthesis

The intramolecular Sakurai reaction allows for the synthesis of functionalized bicyclic systems. By proper choice of the reaction conditions, especially of the Lewis acid or fluoride reagent used, high stereoselectivity can be achieved, which is an important aspect for its applicability in natural products synthesis. 

High-pressure synthesis Here austenitic steels are applied to autoclaves and pressure-vessel linings, pipelines, valves and other equipment dealing with a variety of products, often at quite high temperatures. 

In a catalytic asymmetric reaction, a small amount of an enantio-merically pure catalyst, either an enzyme or a synthetic, soluble transition metal complex, is used to produce large quantities of an optically active compound from a precursor that may be chiral or achiral. In recent years, synthetic chemists have developed numerous catalytic asymmetric reaction processes that transform prochiral substrates into chiral products with impressive margins of enantio-selectivity, feats that were once the exclusive domain of enzymes.56 These developments have had an enormous impact on academic and industrial organic synthesis. In the pharmaceutical industry, where there is a great emphasis on the production of enantiomeri-cally pure compounds, effective catalytic asymmetric reactions are particularly valuable because one molecule of an enantiomerically pure catalyst can, in principle, direct the stereoselective formation of millions of chiral product molecules. Such reactions are thus highly productive and economical, and, when applicable, they make the wasteful practice of racemate resolution obsolete. 

This highly convergent synthesis amply demonstrates the utility of Evans s asymmetric aldol and alkylation methodology for the synthesis of polypropionate-derived natural products. By virtue of the molecular complexity and pronounced lability of cytovaricin, this synthesis ranks among the most outstanding synthetic achievements in the macrolide field. 

Alkylations of 4-cyano-l,3-dioxanes (cyanohydrin acetonides) represent a highly practical approach to syn-l,3-diol synthesis. Herein we present a comprehensive summary of cyanohydrin acetonide chemistry, with particular emphasis on practical aspects of couplings, as well as their utility in natural product synthesis. Both 4-acetoxy-l,3-dioxanes and 4-lithio-1,3-dioxanes have emerged as interesting anri-l,3-diol synthons. The preparation and utility of these two synthons are described. 

This section reviews recent developments in the chemistry of 4-acetoxy-1,3-diox-anes. Highly selective Lewis acid catalyzed nucleophilic additions are described, including their potential utility in natural product synthesis. 

Carbenoid N-H insertion of amines with diazoacetates provides a useful means for the synthesis of ot-amino esters. Fe(III) porphyrins [64] and Fe(III/IV) corroles [65] are efficient catalysts for N-H carbenoid insertion of various aromatic and aliphatic amines using EDA as a carbene source (Scheme 16). The insertion reactions occur at room temperature and can be completed in short reaction times and with high product yields. It is performed in a one-pot fashion without the need for slow... 

Chakraborty has described the highly diastereoselective. Barrero and his group developed an approach to functionalized six-membered rings with exocyclic olefins from a-oxygenated derivatives of geraniol. The diastereo-selectivity observed is reasonable and thus the method holds promise for natural product synthesis [105]. 

The final example demonstrates that microwave irradiation allows a perfect fine-tuning of reaction conditions to obtain different products from the same starting materials. In the procedure developed by Garcfa-Tellado and coworkers [41], two domino processes were coupled. The first process consists of a high-yielding synthesis of enol-protected propargylic alcohols 10-111 starting from alkyne 10-109 and aldehyde 10-110 (Scheme 10.28). In the second process, transformation into... 

Occurence of olefins which are, formally speaking carbene dimers, as well as of similar products (R2C=N—N=CR2, R2CH—CHR2) represents an usually unwanted side-reaction which the chemist endeavors to suppress as far as possible. Nevertheless, conditions for high-yield synthesis of carbene dimers from several diazo compounds have been reported in the past13,141. Some novel examples, published since the last review14) was written, are listed in Table 22. 

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