Radical-chain redox rearrangement of cyclic benzylidene acetals to benzoate esters in the presence of thiols

Deoxy sugars. General methods for carbohydrate deoxygenation and glycosidation

“…This reaction was first described by Roberts’ group. 161–163 In the presence of a thiol, such as ( t -BuO) 3 SiSH (TBST) or (i-Pr) 3 SiSH (TIPS), and di- tert -butyl peroxide or 2,2-bis( tert -butylperoxy)butane as radical initiators, these acetals rearrange, resulting in the deoxygenation of one of the hydroxyls involved in the acetal and the formation of a benzoate group in the other one (Fig. 6A).…”

2 Synthetic methods : Part (iii) Protecting groups

“…75 An interesting method for the rearrangement of benzylidene acetals of 1,2-and 1,3diols to yield a Bz mono-ester via a radical redox process has been reported. 76 The procedure involves refluxing in dry octane with 2,2-di-tert-butylperoxybutane (DBPB, 5 mol%), tert-dodecanethiol (TDT, 5 mol%) and collidine (10 mol%) for 3 h. This radical rearrangement results in preferential homolysis of the C-O bond bearing the most substituted carbon with concomitant conversion of the remaining alcohol to a Bz ester. The role of the thiol is as a protic 'polarity-reversal catalyst' for H-atom transfer from the methine group of the acetal to the 3Њ or 2Њ alkyl radical (Scheme 8).…”

Deoxygenation of carbohydrates by thiol-catalysed radical-chain redox rearrangement of the derived benzylidene acetals

“…Our continuing interest in applications of the principle of polarity-reversal catalysis 1 (PRC) has led us to explore the use of thiols to promote the radical-chain deoxygenation of alcohols 2 and of diols. [3][4][5] It has proved possible to devise methodology for deoxygenation of the ROH function through the use of suitable derivatives of the general type ROCHXY that can be induced to undergo a thiol-catalysed radical-chain redox decomposition to give RH and XYC᎐ ᎐ O, without the need for any other stoichiometric reagent. 2 For mono-deoxygenation of 1,2-or 1,3-diol functionality, we have reported 3-5 that the corresponding redox rearrangement of the derived benzylidene acetals, 6,7 as exemplified by the conversion of 2-phenyl-4,4-dimethyl-1,3-dioxane 1 to isopentyl benzoate 2 (Scheme 1), 3 is effectively catalysed by thiols.…”

“…However, the thermodynamic driving forces behind such potentially competitive homolytic cleavage processes do not necessarily always follow the 'expected' order 3Њ-C-O > 2Њ-C-O > 1Њ-C-O and we have recently discussed the various factors that determine the regioselectivity of the β-scission stage. 5 The mono-deoxygenation of diol functionality is of particular relevance in the area of carbohydrate chemistry and, in preliminary communications, 3,4 we have described the application of our methodology to the redox rearrangement of some carbohydrate benzylidene acetals. For example, when the glucopyranoside 5 in octane-chlorobenzene (1 : 1) was heated at ca.…”