Of Wittig rearrangement

Generally, ester enolates of structure (202 R = M, R = Oalkyl) rearrange via a 3,3-shift, whereas the corresponding amide enolates (202 R = M, R = N(alkyl)2) and acid dianions (202 R = M, R = OM) prefer the 2,3-pathway (equation 20). Both pathways have been observed with ketone enolates (202 R = M, R = alkyl). With substrate (179), Koreeda and Luengo observed only traces of Wittig rearrangement product (205), except for the lithium enolate, where (205) accounted for up to 20% of the reaction mixture (equation 21). ° Thomas and Dubini, however, reported predominant formation of 2,3 Wittig products (207) and (209) under base treatment of ketones (206) and (208) (equation 22). ... 

Scheme 3 Transition states rationalizing cis and syn selectivity of Wittig rearrangements...

Scheme 11 Transition states rationalizing the diastereoselectivity of Wittig rearrangements with EWG = acyl...

Axial ViS. equatorial selectivities of Wittig rearrangements were studied with the f-butylcyclohexane derivatives (89 equation 22) (Table 8). The lithio dianion of acid (89a) prefers equatorial attack. The Bu 02C (89b) and MesSiCsC substituted ethers (89c) undergo nonstereoselective rearrangements. The Still-Wittig reaction of (89d) is the only one among all known 2,3-sigmatropic shifts on this skeleton that prefers axial attack (67 33). 

Garst, J. F., Smith, C. D. Mechanisms of Wittig rearrangements and ketyl-alkyl iodide reactions. J. Am. Chem. Soc. 1973, 95, 6870-6871. 

Wittig rearrangements. Solvent effects dominate the mode of Wittig rearrangement of benzyl propargyl ethers. 

Antithetic conversion of a TGT by molecular rearrangement into a symmetrical precursor with the possibility for disconnection into two identical molecules. This case can be illustrated by the application of the Wittig rearrangement transform which converts 139 to 140 or the pinacol rearrangement transform which changes spiro ketone 141 into diol 142. 

The aza-Wittig rearrangement in synthesis and transformations of N-hetero-cycles 97S497. 

Wittig rearrangement with participation of 0-heterocycles 97LA1275. 

The rearrangement of an ether 1 when treated with a strong base, e.g. an organo-lithium compound RLi, to give an alcohol 3 via the intermediate a-metallated ether 2, is called the Wittig rearrangement. The product obtained is a secondary or tertiary alcohol. R R can be alkyl, aryl and vinyl. Especially suitable substrates are ethers where the intermediate carbanion can be stabilized by one of the substituents R R e.g. benzyl or allyl ethers. 

Recently this [2,3]-Wittig rearrangement has received much attention and has been developed into a useful method for the stereoselective synthesis of homoal-lylic alcohols. 

Isomerization of vinylaziridines is widely used in organic synthesis. Six types of isomerization of vinylaziridines are shown in Scheme 2.40. Outlined in this section are i) azepine formation by aza-[3,3]-Claisen rearrangement of 1,2-divinyl- or 2,3-divinylaziridines 153 (Section 2.4.1), ii) pyrroline formation from 155 (Section 2.4.2), Hi) aza-[2,3]-Wittig rearrangement of anionic species 157 (Section 2.4.3),... 

Scheme 2.52 Aza-[2,3]-Wittig rearrangements of vinylaziridines 213 and alkaloid syntheses.

The aza-[2,3]-Wittig rearrangement of a vinylaziridine-derived quaternary azir-idinium ylide (i.e., [2,3]-Stevens rearrangement) has recently been reported (Scheme 2.53) [86], The aziridinium ylide 219, generated by the intramolecular reaction of a copper carbenoid tethered to a vinylaziridine, underwent a [2,3]-Ste-vens rearrangement to furnish the bicydic amine 220 with the indolizidine skeleton. 

In contrast to the intermediate hydroxystannanes, O-protected stannanes 7 are stable compounds which can be distilled or chromatographed and stored under nitrogen for months. Treatment of 7 with butyllithium in tetrahydrofuran at — 78,JC results in rapid tin/lithium exchange (< 1 min). No products resulting from Wittig rearrangement or formation of an ate complex 8 could be detected9. 

Of these reactions, the [2,3] Wittig rearrangement in particular has often been used as a means of transferring chirality. The product of this reaction has potential chiral centers at C-3 and C-4 (if R ), and if the starting... 

The aza-[2,3]-Wittig rearrangement [47] and the related aza-[3,3]-Claisen rearrangement [48] of vinylaziridines are elegant examples of expansion of the aziridine ring in a stereocontrolled fashion (Scheme 38). 

The stereochemistry of the Wittig rearrangement can be predicted in terms of a cyclic five-membered TS in which the a-substituent prefers an equatorial orientation.293... 

Sigmatropic rearrangements of anions of (V-allyl amines have also been observed and are known as aza-Wittig rearrangements.291 The reaction requires anion stabilizing substituents and is favored by (V-benzyl and by silyl or sulfenyl substituents... 

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