Stereoselectivity Stereospecific synthesis

The trans/cis ratio of the product must, therefore, be determined at an earlier reaction stage and most probably by the ratio of species 27a and 27b. Steric or electronic factors affecting this ratio will influence the trans/cis ratio of the resulting 1,4-hexadiene. The phosphine and the cocatalyst effect on the stereoselectivity can thus be interpreted in terms of their influence on the mode of butadiene coordination. Some earlier work on the stereospecific synthesis of polybutadiene by Ni catalyst can be adopted to explain the effect observed here, because the intermediates that control the stereospecificity of the polymerization should be essen-... 

Keywoids Chiral synthon. Chiral building blocks. Stereospecific synthesis. Stereoselective degradation. Microbial transformation. 

The first publications on the successful stereospecific synthesis of uleine (7a) and epiuleine (31) appeared in 1971 (71JA2492). Using 31 as an example, the key step lies in the formation of transoid piperidone 37, whose conformational stability allows the subsequent transformations to be carried out stereoselectively (Scheme 6). 

Overman, L.E., Petty, C.B., Doedens, R.J. Synthetic application of N-(acylamino)-1,3-dienes. Control of endo stereoselectivity by the acyl substituent, stereospecific synthesis of the analgesic tilidine, J. Org. Chem. 1979, 44, 4183-4185. 

Hydroboration of terminal acetylenes with boron halides permits the stereospecific synthesis of ( )-l-chloro(or bromo)alk-l-enes919. BBr3 adds to terminal acetylenes in a regio- and stereoselective manner to yield /Lhalovinylboranes, which are very useful syn-thons92a 923. B-bromo- and B-iodo-9 borabicyclo[3.3.1]nonane react similarly924"926. 

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