B Unsaturated systems

there is a dichotomy in behaviour of fluorine because effects 1 and 2 lead to electron withdrawal whereas 3 leads to return of electron density from fluorine. 

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We have already seen evidence from the retrohydrocyanation reaction Ip. 222) that the ejection of a leaving group a to a carbonyl which yields an o,B-unsaturated system should also be stereoelectronically controlled, i.e. the C — Y bond of the leaving group should be parallel to the n system of the enolate ion (145 146). 

The stereochemical implication for the ejection of a leaving group in the B-position of a carbonyl group which yields an a,B-unsaturated system has been treated elsewhere (see p. 233). It may be pointed out here that double bond formation through the opening of a cyclopropane ring should also take place following the same stereoelectronic principle. One example of such a reaction is the transformation of B,r-cyclopropyl-s-hydroxyketone 299 which is smoothly converted into the dienone 300 (85) under acid conditions. 

The chemical shift of the 19-methyl is subject to the influence of 5,6- or 6,7-epoxides as well as the presence of the 4B-hydroxyl and the presence of B-unsaturated system or saturated -l-)ceto system in ring A. In the case of 6a, 7o-epoxides the 19-methyl generally comes around B1.20 while the same comes around 6 1.30 in 56,66-epoxides because of the deshielding influences of the 66-epoxy oxygen on the 19-methyl in the latter. In agreement with this the 19-methyl moved downfield to 6 1.32 in... 

In an analogous fashion to the hydroboration reaction, a variety of boron-containing substrates react with iminoboranes. Addition of X2B—Cl, X2B—N3, X2B—SR, X2B—NR2, and X2B—R to the unsaturated B—N system is called chloro-, azido-, thio-, amino-, and alkyloboration, respectively. The azidoboration and chloroboration of two iminoboranes are shown ia equations 23 and 24 (72). 

B. Radical Addition of Sulfonic Acid Derivatives to Unsaturated Systems. 189... 

B. Cathodic Activation of Unsaturated Systems by Sulphonyl Groups. . 1019... 

SUBSTITUENT EFFECTS IN NONAROMATIC UNSATURATED SYSTEMS 181 B. Composition of the Electrical Effect... 

P. B. de la Mare and R. Bolton, Electrophilic Additions to Unsaturated Systems, 2nd ed., Elsevier, New York, 1982. 

De la Mare, P.B.D. Bolton, R. Electrophilic Additions to Unsaturated Systems. Elsevier Scient. Publ. Co. Amsterdam, Holland, 1982 p 247. 

DE LA MARE, p. B. D. and BOLTON, J. ElcctrophiUc Addition to Unsaturated Systems (Elsevier, 2nd Edition, 1982). 

Wadsworth-Emmons olefination with a variety of aldehydes to afford (E)-a,B-unsaturated and diene acyl tetramic and tetronic acids in good to excellent yields upon treatment with potassium tert-butoxide (2 equiv) in tetrahydrofuran. For readily enolizable substrates use of the N-protected systems is generally required. The following compounds have been prepared, in the indicated yields, in this manner ... 

When a new carbon-carbon bond is produced by nucleophilic addition to conjugated systems, the process is called Michael addition. The generalised process involves an a, b-unsaturated compound and a compound containing an active hydrogen attached to a carbon atom (e.g., malonic ester, acetoacetic ester, nitrocompounds, aldehydes, ketones etc.) These are condensed in the presence of a base. The overall reaction and its mechanism can be represented as follows ... 

Gouverneur and coworkers showed that a,b-unsaturated esters can be cy-clized in good to excellent yields (Eq. 31) [156]. Optimization of the reaction conditions revealed that four equivalents of BQ were necessary to achieve good yields (73%). Aerobic conditions, without a copper cocatalyst, proved to be superior, resulting in 89% yield over extended reaction times. No cycli-zation product was observed with the Pd/pyridine catalyst system described above. 

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