Radical chemistry Barton-McCombie deoxygenation

The Barton-McCombie deoxygenation reaction was invented for use in the manipulation of aminoglycoside antibiotics. It has become a popular method because of the mild conditions employed. Radical reactions have advantages over ionic reactions for carbohydrate chemistry. In this context, there is little neighboring group interference in cationic reactions and little elimination compared with normal nucleophilic displacement reactions. 

The other type of radical chemistry of importance in the carbohydrate field is one-electron reductions. A handful of these reactions (such as the metallic Zn reduction of acetobromoglucose to triacetylglucal) have been used in synthesis for decades, but, starting with the Barton-McCombie deoxygenation of sugars in the mid-1970s there has been an explosion of interest, as increasingly sophisticated cascades of elementary radical steps have been devised. Such reactions are driven by the homolysis of weak bonds such as Sn-H or N-O under conditions of photolysis or mild thermolysis. Nature uses a similar basic principle in Type II ribonucleotide reductases, where the weak bond in question is the cobalt-carbon a bond in the corrin cofactor. ... 

Addition of carbon radicals to carbon-carbon double bonds is an important reaction that can be carried out under hydrogen transfer conditions [27], Peroxides are usually used as radical precursors and an application of this chemistry is presented in Scheme 3 (Eq. 3.1). More recently, reduction of alkyl radical by C-H hydrogen donor has been examined in order to find an environmentally friendly alternative to tin hydride. Zard has reported a simple and cheap alternative to tin hydride for Barton-McCombie deoxygenation reactions [80]. Heating of xanthates derived from carbohydrates in 2-propanol in the presence of dilauroyl peroxide affords the de-oxygenated products in good yields (Scheme 17, Eq. 17.1). 2-Propanol functions as... 

Radical Chemistry. Treatment of secondary alcohols with 1 equiv of TCDI affords an imidazole-1-thiocarbonyl derivative (imidazolide), which can be reduced to a CH2 unit under Tri-n-butylstannane (TBTH) radical chain reaction conditions. - The deoxygenation of secondary alcohols by way of an imidazolide or other thiocarbonyl derivative is called the Barton-McCombie reaction (eq 4). Since imidazolide formation (TCDI, reflux, 65 °C) and the subsequent radical chemistry are done under neutral or near-neutral conditions, the overall reduction is tolerant of the presence of many sorts of functional groups. Furthermore, the low solvation requirements of radical species permits deoxygenation in sterically congested environments (eq 5). ... 

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