Stereochemical Course of Wittig Rearrangements of Dihydropyran Allyl Propargyl Ethers

Abstract

2,3]-Wittig rearrangements of sugar-derived dihydropyran allyl propargyl ethers located at the 2-or 4-position have been studied as useful means for extending the carbon chains of the 4-or 2-position with chirality transfer. The stereochemical course of these reactions depends on the following factors: (1) deprotonation of pro-R or pro-S-H, (2) equilibration of the lithiated stereogenic carbanion, (3) conformational inversion during the rearrangement, and (4) concerted [2,3]-or [1,2]-Wittig rearrangement. In some cases, a stepwise mechanism that involves the allyl-C−O bond cleavage is shared as the first step by both the [2,3]-and [1,2]-Wittig rearrangements. The stereochemical courses of the rearrangements are compared among the lithiated reactants to determine the reaction pathways. These mechanisms in the polyoxygenated dihydropyran ring system were further supported by DFT calculations.

[1,2]- vs [2,3]-Wittig Rearrangement in Carbohydrate Derived Alkenyl Systems

“…Recently, the application of WR on pyranosederived allyl propargyl ethers was reported. 13 We have been exploring the synthesis of various bioactive/ important carbon-branched sugars 14−17 involving Claisen rearrangement. 18 In connection to that, we were prompted to investigate the WR on a carbohydrate scaffold.…”

“…In this connection, the [1,2]- − or [2,3]- − WR of carbohydrate acetals has been studied to convert the O -glycosides into C -glycosides. Recently, the application of WR on pyranose-derived allyl propargyl ethers was reported …”

Asymmetric [2,3]-Wittig Rearrangement: Synthesis of Homoallylic, Allenylic, and Enynyl α-Benzyl Alcohols

“…Nevertheless, methods based on the use of (i) strong Brønsted bases, such as n -BuLi or t -BuLi, in the presence of a stoichiometric ligand, and (ii) stoichiometric amounts of a chiral boron reagent have been reported . Moreover, stereoselective [2,3]-rearrangements with substrates bearing either existing stereocenters , or chiral auxiliaries , (Scheme a) to control the configuration of the newly formed σ-bond have been studied. Although rare, more recently, different groups − have described interesting organocatalyzed asymmetric [2,3]-rearrangement methodologies of allyloxy derivatives for the asymmetric synthesis of allylic alcohols.…”

Synthesis of a Tiacumicin B Protected Aglycone

“…By forming homoaggregated organolithium species, LiCl has similar properties, and actually its addition leads to spectacular yield and stereoselectivity improvements (85%, 93/7: syn / anti , entry 7) even reproducible on a 3 g scale. It has been reported that LiBr suppressed concurrent [1,2]-Wittig rearrangement, but the effect of Li + on syn / anti selectivity has not been reported before. The use of MeLi·LiBr (entry 10) still led to a good yield, but with a lower selectivity.…”