Radical Rearrangements of 2‐O‐(Diphenoxyphosphoryl)glycosyl Bromides

Abstract

A variety of 2-deoxy-1-0-diphenoxylphosphoryl-hexopyranoses were generated in situ by a radical 2+ 1 migration of the phosphate group. The structures of the reactive rearrangement products were fully elucidated by NMR spectroscopy. Rate constants for this new rearrangement were determined for a variety of substrates.Introduction. -The radical 2+ 1 migration of acyloxy groups in glycosyl radicals is a well known reaction [I]. Recently, we and Crich and Yao published studies [2] on the hitherto unknown radical rearrangement of diphenoxyphosphoryl groups. As radical precursors, 3,4,6-tri-O -acetyl-2-0 -(diphenoxyphosphoryl)-a -D-glucopyranosyl bromide (1 ; -+ 2, see Scheme I) and 3,5-di-0 -benzoyl-2-0 -(diphenoxyphosphoryl)-D-ribofuranosy1 bromide were examined. The full extent of this new rearrangement has now been demonstrated by studies on other pyranose radical precursors.

Chemistry of β-(Acyloxy)alkyl and β-(Phosphatoxy)alkyl Radicals and Related Species: Radical and Radical Ionic Migrations and Fragmentations of Carbon−Oxygen Bonds

“…97 Further examples include galactose, 6-deoxyglucose, and mannose derivatives 187, 189, and 191, respectively as well as the ribofuranose 194 (Table 5, entries [11][12][13][14]. 97,99 In each case, yields were high and the migration follows the now familiar pattern of suprafacial migration along the carbohydrate framework. The driving force for these carbohydrate based migrations is again thought to be the formation of the anomeric C-O bond.…”

“…103 This rate constant is several orders of magnitude greater than those recorded for comparable acyloxy migrations in benzene (Table 2). In the same study, rate constants were determined for the migration of a series of para-substituted diphenylphosphatoxy groups ( 97,99 Again, the rate constants are orders of magnitude greater than those for directly comparable carbohydrate based acyloxy shifts (Table 2). However, there is not the distinct spread observed in the acyloxy series and the rate constants for rearrangement of the 2-glucosyl and galactosyl diphenylphosphates 206 and 210, respectively, differ by less than a factor of 2 ( Table 6, entries 7 and 9).…”

An Unusual Route to Deoxysugars by Hydrogen Atom Transfer from Cyclohexane. Possible Manifestation of Polar Effects in a Radical Process

“…We have shown, over the past few years, that xanthates represent a synthetically useful source of a variety of radical species . As part of this work, we considered the possibility of obtaining 2-thiosugars by generating an anomeric radical 2 from the corresponding xanthate 1 , allowing it, by analogy with the work of Giese and his collaborators, , to migrate to the 2-position as in 3 , and finally letting the transfer of the xanthate group occur to give the isomeric xanthate 4 , as outlined in Scheme . Carbohydrates containing a xanthate group in the anomeric position are known and readily accessible .…”

Some intriguing mechanistic aspects of the radical chemistry of xanthates

“…A plausible mechanism is provided in Scheme , whereby the anomeric radical 38 undergoes a well‐known 1,2‐shift reported by Giese and colleagues in the carbohydrate field, and the resulting radical 39 abstracts a hydrogen atom from cyclohexane to furnish peracetylated 2‐deoxyglucose 40 and a cyclohexyl radical that propagates the chain. The unexpected feature is the ease and efficiency with which the hydrogen atom abstraction occurs.…”