Lewis acid promoted transformation

A new oxidative rearrangement has led to the synthesis of (+)-lycoramine (219), and a related Lewis-acid-promoted transformation has allowed the preparation of mesembrine (92) and crinane in racemic form (220). 

Lewis acids promote migration of fluorine m halofluoroalkenes to yield isomers, which can be transformed easily into perfluorinated alkynes [27. 22] (equation 6)... 

The synthesis of fenozan BO-7 4 involves two key steps, the first of which employs a 4 + 2 cycloaddition of singlet oxygen to the diene 83a122-20 123. This provides the endoper-oxide 83b that can be transformed into the target cis-fused 1,2,4-trioxane by treatment with the Lewis acid, TMSOTf, in the presence of a carbonyl compound. The reaction proceeds by Lewis acid promoted heterolysis of the C—O bond to give an intermediate peroxy allyl cation 83c that is captured by the carbonyl compound (in this case, cyclopen-tanone) to give the product (Scheme 30). A number of different carbonyls have been used in this reaction along with a number of different endoperoxide templates and detailed SAR have been developed (Scheme 30). 

The intramolecular Alder-ene reaction (enyne cydoisomerization reaction) with alkynes as the enophiles has found wide application compared with diene systems. The reason may be the ready chemo-differentiation between alkene and alkyne functionality and the more reactive alkyne moiety. Furthermore, the diene nature of the products will promote further applications such as Diels-Alder reactions in organic synthesis. Over the past two decades the transition metal-catalyzed Alder-ene cycloisomerization of l,n-enynes (typically n= 6, 7) has emerged as a very powerful method for constructing complicated carbo- or heterocydic frameworks. The transition metals for this transformation indude Pd, Pt, Co, Ru, Ni-Cr, and Rh. Lewis acid-promoted cydoisomerization of activated enynes has also been reported [11],... 

All syn-Z fi, 4- 3co-methylene tetrahydropyrans 367 can be readily assembled using a Lewis acid promoted Prins reaction between enol carbamates 368 and aldehydes (Equation 153). The presence of an o-methylene group and the labile carbamate in the product allows for simple transformation into tetrahydropyranones and tetrahydropyranols <2000TL7225, 2001TL8685, 2002JOC8744>. 

Two extreme pathways may be envisaged for this transformation. One involves a pericyclic reaction followed by desilylation and elimination of methoxide from the product silyl enol ether. The alternative proceeds via a Lewis acid promoted aldol reaction to give an intermediate which cyclizes to the product dihydropyrone (Figure Si3.6). The actual pathway in an individual case is affected to a considerable degree by the nature of the Lewis acid used and lies somewhere between these two extremes. 

Compounds as Lewis-Acidic Promoter for Organic Transformations 281... 

Specialized Topics - Recent Advances in Organoaluminum and Related Compounds as Lewis-Acidic Promoter for Organic Transformations... 

A more general approach to cyclopropanes from allylic ethers has recently been reported [28]. Two steps are involved in this one-pot transformation, i.e., hydrozirconation reaction followed by Lewis acid-promoted deoxygenative ring formation. The overall transformation is symbolized in Scheme 21, Eq. 1,... 

Aluminum chloride and its derivatives are the most familiar Lewis acids and are routinely employed in many Lewis acid-promoted synthetic transformations. The first polymer-supported metal Lewis acids to be studied were polymers attached by weak chemical or physical interactions to a Lewis acid. In the 1970s Neckers and coworkers reported the use of styrene-divinylbenzene copolymer-supported AlCl,- or BF3 as catalyst in condensations, esterifications, and acetalization of alcohols [11,12]. This type of polymer-supported AICI3 (1) is readily prepared by impregnation of a polystyrene resin with AICI3 in a suitable solvent. Subsequent removal of the solvent leaves a tightly bound complex of the resin and AICI3. The hydrophobic nature of polystyrene protects the moisture-sensitive Lewis acid from hydrolysis, and in this form the Lewis acid is considerably less sensitive to deactivation by hydrolysis. This polymer complex could be used as a mild Lewis acid catalyst for condensation of relatively acid-sensitive dicyclopropylcarbinol to an ether (Eq. 1) [13],... 

Yamamoto has investigated the intra-molecular condensation of y-(alkoxy)allylstaimanes with aldehydes as a general approach for the stereocontrolled synthesis of polycyclic ethers (Scheme 5.2.26). The strategy has been iteratively applied to the total synthesis of hemibrevetoxin B. i In this fashion, the Lewis acid-promoted reaction of 122 leads to formation of seven-membered ether 123, which is transformed into 124 for intra-molecular allylation to the fused-oxepane 125. 

Another point to be made in Lewis acid-promoted additions is that for most systems of the type shown in Eqs. (7)-(9), there will be a background reaction of addition that is independent of Lewis acid. In the special case in which a Lewis acid-promoted reaction occurs stereoselectively while the background reaction does not, it is essential that the rate and equilibrium constants for the system are such that the background reaction is minimized, since this reaction will degrade the overall stereocontrol of the transformation. 

Oxazolidinones used in conjunction with Lewis acids promote radical atom transfer sequences with inactivated alkenes, and these reactions proceed with good to excellent selectivity [32]. In the absence of Lewis acid, poor conversion was seen. Several common Lewis acids (e.g. MgBr2, Zn(OTf)2, and La(OTf)3), which are effective promoters of the allyl transfer reaction described in Eq. (22), failed to improve conversion significantly. Sc(OTf)3 performed moderately well with the 1° bromide while Yb(OTf)3 gave excellent yields of product. With terminal alkenes, the reactions were rapid and efficient complete conversion was seen within 15 min at room temperature. Internal alkenes were slower to react, but near quantitative yields were possible with cis alkenes in reactions promoted by Yb(OTf)3. 1,2-Di-chloroethane (DCE) appears to be a particularly good solvent for these transformations. 

Discovered in 1877, the Friedel-Crafts reaction is one of the more important and useful organic transformations. This class of reactions is generally considered to include all Lewis acid-promoted electrophilic alkylations and acylations of aromatic rings. Due to their synthetic utility, Friedel-Crafts reactions have been extensively studied and used across a broad and diverse area of chemical research. 

A large family of semipinacol rearrangements have been developed for the stereoselective transformation of non-racemic a-hydroxy epoxides. These reactions are usually Lewis acid-promoted, as in the case of the... 

A Lewis acid-promoted tandem rearrangement/reduction of non-racemic a-hydroxy epoxides yields 2-quartemary-l,3-diols. This reaction also successfully transforms a-hydroxy-A-tosyl aziridines into A-tosy 1-1,3-amino alcohols. 

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