Opening Lewis acid promotion

Lewis acid promoted condensation of silyl ketene acetals (ester enolate equiv.) with aldehydes proceeds via "open" transition state to give anti aldols starting from either E- or Z- enolates. 

The conversion of anomerically linked enol ethers 29 into either the cis- or trans-substituted pyranyl ketones with high diastereoselectivity and yield involves a Lewis acid-promoted O —> C rearrangement (Scheme 19) <00JCS(P1)2385>. Under similar conditions, homoallylic ethers 30 ring open and the oxonium ions then recyclise to new pyran derivatives 31. Whilst the product is a mixture of alkene isomers, catalytic hydrogenation occurs with excellent diastereoselectivity (Scheme 20) <00JCS(P1)1829>. 

The four possible transition-state structures for the Lewis acid-promoted Diels-Alder reaction of cyclopentadiene with propynal are depicted in Fig. 7. In the chiral Lewis acid-promoted reaction, the enantiomeric excess of an adduct originates in the enantiofacial selectivity of cyclopentadiene, which has prochiral reactive centers. The enantioselective pathway presupposes three characteristics (i) the chiral Lewis acid must sterically shield one enantioface of the coordinated propynal because the open acetylenic jr-face in the chiral catalyst-dienophile complex approaches one face of... 

Equation 54 describes the preparation of dienes by a similar cyclopropane cleavage followed by proton transfer and elimination of sulfinate. The weak donor ability of a (trimethylsilyl)methyl group also assists Lewis acid-promoted ring cleavage according to equation 55, which allows synthesis of y, -unsaturated ketones Interestingly, the related ester opens smoothly only if fluoride reagents are employed (equation 56) ... 

Lewis acid promoted ring opening of oxiranes with cvgano-silanes or -stannanes is reported. TMS-CN cleaves the C—O bond in the presence of AlCb, Et2AlCl, Ti(OPr )4 or LnX3, and 3-hydroxy-... 

Ring-opening polymerization of cyclic ethers and lactones has been a major area of research in Lewis acid-promoted reactions. In particular, aluminum compounds have been investigated in depth not only because of their high oxophilicity and ability to initiate polymerization but also because of their commercial availability and low cost. 

Alkenylgermanes have been synthesized highly stereoselectively by addition of triethylgermyl lithium to cyclobutyl ketones and subsequent Lewis acid-promoted stereospecific ring opening of the resulting cyclobutylethanol derivatives (Scheme 11.43) [67]. 

Generation of alkoxycarbenium ions is often carried out by treatment of the corresponding acetals or their derivatives with a Lewis acid in the presence of a carbon nucleophile. This type of reaction serves as an efficient method for carbon-carbon bond formation under acidic conditions. Mechanistic studies of Lewis-acid promoted reactions of acetals using NMR spectroscopy showed the presence of Lewis acid-acetal complexes in the solution, but alkoxycarbenium ions were never detected. The absence of signals resulting from alkoxycarbenium ions in the NMR spectra, however, does not necessarily rule out their intermediacy in the reactions with nucleophiles. Therefore, it was imperative to accomplish the reactions of spectroscopically characterized, nonsta-bilized alkoxycarbenium ions with carbon nucleophiles. The cation-pool method made this possible and opened a new chapter in the chemistry of alkoxycarbenium ions. 

In 1992, Nugent [12] reported a significant advance in the Lewis acid-promoted opening of meso epoxides (Scheme 2). A novel chiral zirconium complex catalyses epoxide opening with hindered silyl azides iii excellent ee. The catalyst is derived from zirconium tert-h xi-oxide and the tetradentate C3-symmetric ligand 1, available by combination of (5)-pro-pene oxide with (S)-l-aminopropan-2-ol. Al-... 

A group of polycyclic polyether natural products are of special interest owing to their fascinating structure and biological activities. One of the proposed biosynthetic origins of these molecules features an epoxide-opening cascade pathway. Shi asymmetric epoxidation of un-activated alkenes has been frequently employed in the preparation of polyepoxide intermediates. McDonald and co-workers studied a series of tandem e 7o-selective and stereospecific oxacyclization of polyepoxides mediated by Lewis acid. Polyepoxides, such as 64, can be obtained from the epoxidation of triene 63 with ketone 2. Furthermore, a cascade cyclization, initiated by a Lewis acid-promoted epoxide opening of 64, furnished the desired polyether 65. 

---