Alkoxy migration

OR, -SEt, and -NR2 migration from a tertiary phosphine to phosphenium P (from 32 to 31 in Scheme l).33 Another one is 1,3-alkyl migration from a stannyl ligand to phosphenium P (from 36 to 39 in Scheme 2).23,24 34,36 48 In this reaction, a phosphenium complex is converted into a stannylene complex. This reaction corresponds to migration of a double bond from M=P to M=Sn. The last one is 1,3-alkoxy migration from a phosphite... 

We propose that the reaction of acetals of 2-bromopropiophenone constitutes a test to distinguish between the different types of acid sites in a metal ion exchanged large pore zeolite. Thus, while Bronsted or hard Lewis sites produce hydrolysis of the acetal moiety, 1,2-phenyl shift leading to 2-phenylpropanoate is catalyzed by soft and borderline Lewis sites. Moreover, the ratio between phenyl or alkoxy migration products appears to be controlled by the softness-hardness of the centers. A new large pore zeolite with two framework compositions (ETAS and ETS) have also been studied and it has been found that their chemical behavior is Intermediate between Y and X faujasite. 

A-aryl ketenimines, substituted at the ort/io-position either with noncyclic acetalic functions, undergo thermal rearrangement to 3,4-dihydroquinolines involving 1,5 hydride or alkoxy migration followed by subsequent 6jt electrocyclic ring closure (Scheme 65)7 ... 

An unusual 1,4-migration of a trifluoromethyl group was observed when azomethine imines were synthesized from hexafluoroacetone azine and alkoxy-acetylenes The rearrangement, which occurs at temperatures as low as 0 "C, results in the formation of A-(perfluoro-ferf-butyl)pyrazoles [207] (equation 46)... 

These data demonstrate that a photorearrangement reaction of 1-alkoxy group actually occurs. We continue to speculate that any substituents at the 1 position of indoles may migrate to 3, 4, and/or 6 positions upon photoirradiation. 

A novel approach was developed very recently by Kita et al. [15]. DKR of allylic alcohols was performed by combining a lipase-catalyzed acylation with a racemization through the formation of allyl vanadate intermediates. Excellent yields and enantioselectivities were obtained. An example is shown in Figure 4.4. A limitation with this approach for the substrates shown in Figure 4.4 is that the allylic alcohol must be equally disubstituted in the allylic position (R = R ) since C—C single bond rotation is required in the tertiary alkoxy intermediate. Alternatively, R or R can be H if the two allylic alcohols formed by migration of the hydroxyl group are enantiomers (e.g. cyclic allylic acetates). 

The preference for the /3-silyl isomer product complements methods available for hydrostannation of alkynes, for which the a-stannyl regioisomer is formed preferentially.70 7011 70c In addition, the /3-silyl products serve as the platform for a tertiary alcohol synthesis (Scheme 15). Upon treatment of vinylsilanes such as B with tetrabutylam-monium fluoride (TBAF) in DMF at 0 °C, a 1,2 carbon-to-silicon migration occurs, affording the tertiary heterosilane E. Oxidation of the C-Si bond then provides the tertiary alcohol. Good 1,2-diastereocontrol has been demonstrated for y-alkoxy substrates, as in the example shown. The studies suggest that the oxidation of the sterically demanding silane intermediate is facilitated by the intramolecular formation of a silyl hemiketal or silyllactone for ketone or ester substrates, respectively.71... 

Syntheses of simple diols are described first, followed by a multistep synthesis of ribose. These syntheses illustrate the compatibility of alkoxy substituents on boronic esters with their reaction with (dihalomethyl)lithium. The limitations of this compatibility, as well as failure of a (chloro-allyl)boronic ester to undergo substitution by alkoxides, are also noted. Retention of configuration of the migrating group is a consistent and repeated feature of these syntheses. 

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