Barbier-Grignard type addition

The intramolecular variant of the chromium(U) ion mediated Barbier-Grignard-type addition reaction was first described by Still and Mobilio in an elegant synthesis of ( )-Asperdiol (122 R = H). Qm>-mium(II)-mediated cyclization of (121 R = Bn) provided a 4 1 mixture of (122) and (123) in 64% yield (equation 50). The relative topicity of the process is ik. This result is in accord with a synclinal chair transition structure (124) in which both hydrocarbon chains diverge from the reacting centers pseudo-... 

Mediated by Other Metals. In addition to the metals discussed above, other metals have been found to mediate the Barbier-Grignard-type conversions in water, but investigations on these metals are relatively limited. 

The Reformatsky reactions are run following two basic procedures (i) a two-step Grignard-type protocol which first involves the formation of an organometallic zinc eno-late derivative followed by addition of the electrophile, and (ii) a Barbier-type protocol where the bromoester and the electrophile are simultaneously exposed to the action of zinc metal. 

A Lewis acid push-pull mechanism proposed for the magnesium bromide promoted Barbier-type intramolecular cyclization of Grignard reagents derived from halo-substituted acetals, ketals, and orthoesters as depicted in (Scheme 9). Stereoselective addition of chiral a-aminoorganometallics to aldehydes has been reviewed. Chiral 1,4-dihydropyridines have been synthesized with good regio- and... 

---