Stereochemical Process for Carbon-Carbon Bond Formation on Dihydropyran Ring System - Ferrier Rearrangement and Wittig Rearrangement

This thesis consists of two topics: I. Cobalt-Assisted Ferrier-Type Rearrangement to Construct Chiral Alkynyl Cyclohexanones Functionalized 1-C-alkynyl sugars are prepared through so called C-glycosidation from a D-glucose derivative, 2-acetoxy-glucal triacetate with TMS-acetylene under Lewis acid condition. Dicobalt complexes of these C-alkynyl products, sugar acetylenes possess a special chemical character to stabilize the propargylic carbenium cations (known as Nicholas cation) at the anomeric position. This cation could promote the Ferrier-type rearrangement through the above propargyl vinyl cyclic cationic intermediates to afford alkynyl cyclohexanones after decomplexation. High stereoselectivity resulted from the suitably oriented steric interaction between the C-2 O-protecting group and the alkynyl substituents due to gauche effect. Such an effect prompted us to focus on stereochemical course of this rearrangement particularly found in the dihydropyran ring system. II. Stereochemical Course of Wittig Rearrangement of Dihydropyran Allyl Propargyl Ethers [2,3]-Wittig rearrangements of sugar-derived dihydropyran allyl propargyl ethers located at the 2- or 4-position have been studied as a means for extending the carbon chains to the 4- or 2-position with chirality transfer. The stereochemical courses of these reactions depend on the following factors: (1) deprotonation of propargylic pro-R or pro-S-H, (2) equilibration of the lithiated stereogenic carbanion, (3) conformational inversion at the transition state, (4) [2,3]- or [1,2]-Wittig-rearrangement, and (5) workup. The equilibration step-(2) may be skipped depending on the orientational change for the step-(3) leading to a concerted [2,3]-Wittig process. In some cases, a new mechanism occurs leading to allyl-C-O bond-cleavage before either the [2,3]- or [1,2]-Wittig rearrangement. The stereochemical courses of the rearrangements are compared among the lithiated reactants for determining the reaction pathways. This unique mechanism in the poly-oxygenated dihydropyran ring system was further supported by DFT calculations. Indeed, some examples are reported of the [2,3]-process going through a non-concerted process in 2-steps due to strong chelation effects.