Propargylic acetates reactions with carbonyl compounds

The reactions of allenyl and propargyl derivatives of boron, silicon or tin with carbonyl compounds or acetals take place with double-bond migration (Se reactions). Therefore, allenyl derivatives will lead to homopropargyl alcohols and propargyl analogs to allenyl alcohols. 

Propargylic lithium alanates or lithium borates react with allylic halides or with carbonyl compounds in a regioselective manner to furnish 1,1 -disubstituted allenes (Scheme The reaction between alkyl halides and aluminum metal is the basis of the oldest method for the synthesis of organoaluminum compounds. For example, propargylic bromides react with aluminum in ether giving organoaluminum compounds that on treatment with acetals yield solely a-allenic ethers (equation 1)." However, the reaction of simple alkyl halides with aluminum metal requires a long reaction time. 

An additional advantage of the intramolecular protocol stems from the opportunity to prepare easily the required polyfunctional precursors via cobalt carbonyl stabilized propargyl cations. The approach based on the tandem utilization of Co-mediated alkylation and Pauson-Khand annulation was developed in Schreiber s studies to elaborate short pathways for the synthesis of polycyclic compounds. An example of the efficiency of this protocol is the two-step transformation of the acyclic precursor 409 into the tricyclic derivative 410. The cobalt-complexed acetal 409 was first transformed into the cyclooctyne derivative 411 via intramolecular reaction of the in situ generated propargyl cation 409a with the allylsilane moiety. Cyclooctyne 411 underwent smooth cycloaddition in the presence of carbon monoxide to give the target compound 410 with excellent stereoselectivity. 

Acetals prepared from chiral diols and carbonyl compounds serve as a chiral synthetic equivalent of aldehydes or ketones. 1,3-Dioxanes synthesized from chiral 2,4-pentanediols are especially useful, and high asymmetric inductions are observed in the Lewis acid promoted reactions of a variety of organometallic compounds. After the removal of the chiral auxiliary by the oxidation and -elimination procedures, optically active alcohols are obtained. Optically active propargylic alcohols and cyanohydrins are synthesized from organosilane compounds, TMS-C CR or TMS-CN in the presence of TiCU (Scheme 24). 1 6-138 Reactive wganometals such as alkyl-lithiums, -magnesiums or -coppers also react with chiral... 

Formation of a,p-unsaturated carbonyl compounds from propargylic alcohols was described in 2007 by Chung et al. [119] and from propargylic acetates by Nolan and co-workers (Scheme 8) with [(NHC)Au ] complexes [120]. The presence of water was required for the formation of the desired products. Steric hindrance of the ligand appeared to be crucial for the selectivity of the reaction. The reaction was not... 

---