Stereoconvergent process

This reaction also gave the opportunity to open an access to various polycyclic derivatives, as illustrated by the transformation of the mixture of diaster eomers 133 into tricyclic derivative 134 as a single diastereomer, thanks to a completely stereoconvergent process. An interesting entry into carene 136 and carone derivatives 137 has also been disclosed by Fiirstner [88]. 1,5-Enynes have proven to be versatile precursors for the preparation of perfumery agents such as sabina ketone (Scheme 45). 

Although the subsequent discussion describes the stereoselection at the steady state through the example of radical reactions, the analysis and principles are general for any reaction profile that fits into the scheme of complex stereoselective reactions. In the process proposed and analyzed by Curran et al., the activation of compounds of type 1 is done, for example, by radical formation. The group selectivity in this first step has again no effect on the stereomeric nature of the product. To obtain a stereoconvergent process it is crucial, however, that the reaction is operating at the steady state. This means that the concentrations of the radial intermediates (compounds in brackets in Scheme 2) is low and stationary, while their absolute concentrations are determined by the different rates of reaction. 

During continuous research efforts, Fu designed and synthesized new tertiary phosphines of type 14 that made it feasible to develop catalytic diastereo-and enantioselective [3-t2] cycloadditions of racemic, ysubstituted allenoates and allenamides with functionalized electron-deficient olefins, thus providing stereoconvergent processes to access structurally and stereochemically diverse cyclopentenes with heteroatom (N, P, O, and S)-substituted quaternary stereocenters (Scheme 7.26) [45]. 

In two recent publications Curran et al. described the theoretical as well as the mathematical background of stereoconvergent reactions [2]. They give further evidence for their analysis by providing some examples from the field of radical chemistry to demonstrate this strategy called complex stereoselection. The process of stereoconver-... 

There are two distinct selectivities configurational selectivity relates to syn vs. anti addition regio (or directio) selectivity is concerned with 1-Nu, 2-E+ vs. 1-E+, 2-Nu addition. The resulting orientations will be labelled specific (0 or 100%) when one product or process is exclusive otherwise they may range from highly selective to non-selective (>0 or < 100%) - A reaction will be termed stereoconvergent if the same composition of product isomers is obtained on two or more reaction paths. 

Asymmetric transformation The conversion of a mixture (usually 1 1) of stereoisomers into a single stereoisomer or a mixture in which one isomer predominates. An asymmetric transformation of the first kind involves such a conversion without separation of the stereoisomers. An asymmetric transformation of the second kind also involves separation, such as an equilibration accompanied by selective crystallization of one stereoisomer [76]. The terms first- and second-order asymmetric transformations to describe these processes are inappropriate. See also stereoconvergent. 

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