Monoalkylation organic synthesis with

The potential utility of cyclopentadienylthallium(I) in organic synthesis, and its advantages over the much more commonly employed sodium salt, have been illustrated recently 22). Treatment of (I) (Scheme 2) with chloromethyl methyl ether in dry ether resulted in smooth monoalkylation... 

The synthesis of enantiomerically pure compounds is the challenging problem for organic chemists. The synthesis becomes obsolete if the intermediates produce racemic mixtures. The problem is particularly acute when the asymmetric centers do not reside in a rigid cyclic or polycyclic framework. To be able to carry out efficient syntheses of complex molecules, chemists have to control the sense of chirality at each chiral center as it is introduced in the course of synthesis. Monoalkyl- or dialkyl-boranes exhibit a remarkable chemo-, stereo-, and regioselectivity for the hydroboration of unsaturated compounds. This property, coupled with the capability for asymmetric creation of chiral centers with chiral hydroboration agents, makes the reaction most valuable for asymmetric organic synthesis. In some of the cases, however, this has been achieved by diborane itself as shown in the synthesis of monensin by Kishi et al. A stereospecific synthesis of its seven carbon. component has been accomplished by two hydroboration reactions (Eq. 129) 209. ... 

The development of the synthesis of peptides on solid supports has also had a strong impact on other areas of organic synthesis. From the very beginning, some research groups tried, with some success, to export the idea from peptide synthesis to, for instance, the monoalkylation and monoacylation of carboxylic acids,the synthesis of aldehydes and ketones, or to the use of resin-bound dienes or dienophiles to trap reactive intermediates.PI This pioneering work has been fundamental to the much more recent explosion of combinatorial chemistry on solid supports. Synthesis on solid supports has also found application in the synthesis of oligonucleotides, PI oligosaccharides,PI peptide-DNA hybrids,P°l and peptide nucleic acids (PNAs).P l... 

The role of palladium in organic synthesis continues to be explored and exploited. Enol stannanes are monoalkylated by allylic acetates in the presence of tetrakis(triphenylphosphine)palladium, Enol stannanes give higher selectivity for monoalkylation than enolate ions or silyl enol ethers. High regioselec-tivity is observed for alkylation at the less substituted end of the allyl moiety. Olefins, after complexation to palladium(ll), alkylate enolate anions. The organopalladium product may be converted into saturated ketones, or into enones by /3-elimination, or acylated with carbon monoxide (Scheme... 

Sulphinyl ylides, the carbanions obtained from sulphoxides by proton abstraction, continue to be of limited use in organic synthesis but of increasing interest in stereochemical studies. The thioacetal monosulphox-ide (95) could be dialkylated in the presence of n-butyl-lithium, then cleaved with HgCh to afford a ketone. The monoalkylation product could be intercepted readily, thus permitting the effective dialkylation of C=0 and a novel initiated synthesis of ketones. The stereochemistry of the methyla-tion of 4-t-butyl-l-thiacyclohexane oxide depends on the orientation of the sulphoxide oxygen. Using methyl iodide or trideuteriomethyl iodide in the... 

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