Grignard reagents, with acetals mechanism

The high diastereoselectivity which is found in the nucleophilic addition of Grignard reagents to chiral 2-0x0 acetals can be explained by a chelation-controlled mechanism. Thus, coordination of the magnesium metal with the carbonyl oxygen and the acetal moiety leads to a rigid structure 3A in the transition state with preferred attack of the nucleophile occurring from the S/-side. 

Ally) ethers are selectively cleaved with titanium(lV) isopropoxide and commercially available Grignard reagents like /i-butyl- or cyclohexylmagnesium chloride [Scheme 4.229].432 Neither benzylidene acetals nor more highly substituted allylic ethers suffer under the reaction conditions. A mechanism for the reaction implicates formation of the titanacyclopropane intermediate 229.1 as the first step. Ligand exchange with an unsubstituted allyl ether affords intermediate 229.2. -Elimination to the allyltitanium(lV) alkoxide 2293 followed by hydrolysis returns the deprotected alcohol. The reaction closely resembles an earlier method based on zirconium.433... 

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... 

The second is really carbocation chemistry- When Nuc — H is R OH, or R SH, the product in the reaction shown above may have its OH group replaced by a second Nuc. This reaction occurs by the SnI mechanism and leads to relatively stable acetal or thioacetal products. As the text section shows, these latter two kinds of compounds are resistant to attack by many kinds of basic or nucleophilic reagents, such as RLi, Grignards, and hydrides. Conversion of an aldehyde or ketone C=0 to an acetal or thioacetal is a handy way of protecting it when you need to react some other functional group in a molecule with a strong ba.se or nucleophile. 

---