De Meijere

The majority of preparative methods which have been used for obtaining cyclopropane derivatives involve carbene addition to an olefmic bond, if acetylenes are used in the reaction, cyclopropenes are obtained. Heteroatom-substituted or vinyl cydopropanes come from alkenyl bromides or enol acetates (A. de Meijere, 1979 E. J. Corey, 1975 B E. Wenkert, 1970 A). The carbenes needed for cyclopropane syntheses can be obtained in situ by a-elimination of hydrogen halides with strong bases (R. Kdstcr, 1971 E.J. Corey, 1975 B), by copper catalyzed decomposition of diazo compounds (E. Wenkert, 1970 A S.D. Burke, 1979 N.J. Turro, 1966), or by reductive elimination of iodine from gem-diiodides (J. Nishimura, 1969 D. Wen-disch, 1971 J.M. Denis, 1972 H.E. Simmons, 1973 C. Girard, 1974),... 

The growing importance of cyclopropane derivatives (A. de Meijere, 1979), as synthetic intermediates originates in the unique, olefin-like properties of this carbocycle. Cyclopropane derivatives with one or two activating groups are easily opened (see. p. 69f.). Some of these reactions are highly regio- and stereoselective (E. Wenkert, 1970 A, B E. J. Corey, 1956 A, B, 1975 see p. 70). Many appropriately substituted cyclopropane derivatives yield 1,4-difunctional compounds under mild nucleophilic or reductive reaction conditions. Such compounds are especially useful in syntheses of cyclopentenone derivatives and of heterocycles (see also sections 1.13.3 and 4.6.4). 

For a review of various descriptions of the bonding in cyclopropane, see A. de Meijer, Angew. Chem. Int. Ed. Engl. 18 809 (1979) K. B. Wiberg, in The Chemistry of the Cyclopropyl Group, Z. Rappoport, ed., John Wiley Sons, New York, Chapter 1, 1987 B. Rozsondai, in The Chemistry of the Cyclopropyl Group, Vol. 2, Z. Rappoport (ed.), John Wiley Sons, New York, Chapter 3, 1995. 

T. Hirao, in Small Ring Compounds in Organie Synthesis. V (A. de Meijere, ed.) [Topies in Current Chemistry, Vol. 178], p. 99. Springer-Verlag, Berlin, 1996. 

This reaction also represents an example of the intramolecular Heck reaction, a variant that has gained some importance in recent years. Another instructive example of the potential of this reaction for the construction of ring systems has been reported by de Meijere and coworkers, taking advantage of a sequence of four consecutive intramolecular Heck reactions. The bromodiene-yne 18 reacts in a sequence of domino reactions within 3 d at 80 °C under Heck conditions to give the tetracyclic product 19 in 74% yield ... 

To illustrate the utility of the Heck reaction in organic synthesis, we will address a few of the many spectacular achievements reported thus far. For a much more thorough treatment of this important subject, we direct the reader s attention to a recent review by de Meijere and Meyer.12a... 

Scheme 2. de Meijere s fourfold Heck coupling-electrocyclization process. 

This reaction mode of alkynylcarbene complexes of type 23 undoubtedly provides the most convenient access to /J-amino-substituted a,/J-unsaturated Fischer carbene complexes 27 (X=NH2, NHR2, NR2). Fischer et al. reported the very first such addition of an amine to an alkynylcarbene complex of type 23 and observed a temperature-dependent competition between 1,4- and 1,2-addition [12]. In a later systematic study, de Meijere et al. found that in addition to the 1,4-addition products 30,1,2-addition-elimination (formal substitution)... 

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