Abstract
The work in this thesis is divided into two parts. The first part describes the stereoselective
synthesis of carbohydrate-based thiochromenes from pyranothiochroman precursors. The
substrate pyrano thiochromans were prepared setereoselectivley in good to excellent yields in
gram quantities starting from iodomethyl glycosyl acetates.
Treatment of the iodomethyl glycosyl acetate with arylthiolates at 0 oC produces aryl
thiomethyl glycosides via SN2 mechanism. The resulting aryl thiomethyl glycosides were
treated with BF3.Et2O in dry CH2Cl2 at 0 oC to afford the corresponding pyranothiochromans.
The iodomethyl glucosyl acetate affords α-1,2-aryl-C-glucoside pyranothiochromans while the
mannosyl analogue prepared by opening of a β-1,2-cyclopropanated sugar (mannose) with NIodosuccinimide
(NIS) in the presence of water as a nucleophile generated a β-1,2-aryl-Cmannoside
pyranothiochroman with opposite stereochemistry at positions C1 and C2 relative
to the former analogue.
These pyranothiochromans were treated with oxidizing agents to afford the corresponding
sulfoxides. The sulfoxide derivatives were then transformed into thiochromenes via Pummerer
rearrangement followed by hydrolysis. The reaction pathways followed by the glucosyl (α-1,2-
aryl-C-glucoside) and mannosyl (β-1,2-aryl-C-mannoside) sulfoxides under these conditions
are different and led to two different sets of thiochromene derivatives. The mechanisms and
plausible reasons that lead to these differences are discussed.
The second part of the thesis describes the synthesis of O-, S- and C-glycosides. There have
been several methods reported on the synthesis of these types of glycosides. 2,3- Unsaturated
glycosides have been prepared by Ferrier rearrangement using various promoters. The search
to find a better promoter has been an on-going endeavor. In this aspect this thesis evaluated the
use of NaHSO4 supported on silica gel as an alternative catalyst for the formation of 2,3-
unsaturated O- and S- glycosides. The catalyst was found to be as good as and in some cases
superior to the existing Ferrier rearrangement reaction promoters. The catalyst is a new entrant
to the already reported acids that catalyze the Ferrier rearrangement with the added advantage
of being easy to handle and environmental benigness.
On the other hand 2-C-branched-α-aryl-C-glycosides were synthesized by careful
desulfurization of pyranothiochroman precursors. In this aspect two desulfurizing agents were employed...
Ph.D. (Chemistry)