"Front Matter". In: Organosilanes in Radical Chemistry - Index of
"Front Matter". In: Organosilanes in Radical Chemistry - Index of "Front Matter". In: Organosilanes in Radical Chemistry - Index of
62 Reducing Agents (TMS) 3Si + R X R' S H2 stepwise S H2 synchronous R R' X Si(TMS) 3 (TMS) 3 SiXR' + Scheme 4.2 Possible mechanistic paths for the displacement of a chalcogen group (R 0 X, where X ¼ S or Se) by (TMS) 3Si: radicals 4.3.3 DEOXYGENATION OF ALCOHOLS (BARTON–MCCOMBIE REACTION) A relevant reductive process, which has found wide application in organic synthesis, is the deoxygenation of alcohols introduced in 1975 by Barton and McCombie [58]. Reaction (4.28) shows that the thiocarbonyl derivatives, easily obtained from the corresponding alcohol, can be reduced in the presence of Bu3SnH under free radical conditions. The reactivity of xanthates and thiocarbonyl imidazolides [58] was successfully extended to O-arylthiocarbonates [59] and O-thioxocarbamates [60]. Several reviews have appeared on this subject, thus providing an exhaustive view of this methodology and its application in natural product synthesis [61–64]. ROH ROC(S)X Bu 3SnH X = SR, OAr, NHPh, N N R RH (4.28) (TMS) 3SiH replaced successfully tin hydrides and often proved to be a superior reagent [65]. In the initial work, it has been shown that (TMS) 3SiH and its silylated by-products are not toxic [66] and that the reaction is efficient also at room temperature [67]. The reduction of thiono esters of cholesterol to cholestene (Reaction 4.29) has been used for evaluation of by-products in biological assays [66]. The effect of substituents on the phenyl ring of thiono ester has also been evaluated, see for example Reaction (4.30) [67]. An improved procedure for the homolytic deoxygenation of aminoacid derivatives has also been reported (Reaction 4.31). It consists of the treatment with (TMS) 3SiH (without purification), followed by an acid treatment by TFA and an aqueous extraction, which was sufficient to separate the product from the silyl and sulfur-containing by-products [68]. The dideoxygenation of 1,6-anhydro-dglucose with (TMS) 3SiH has been described to afford the desired product in a
Tris(trimethylsilyl)silane 63 86% yield (Reaction 4.32), whereas other radical-based reducing systems give much poorer yields [69]. PhOC(S)O (TMS) 3 SiH AIBN, 80 �C 94% O O O O O (TMS) 3SiH O O O O O Et3B, O2 , r.t. O O S C6H4-p-F 94% O P(OMe) 2 (TMS) 3SiH CH-OC(S)OPh CH AIBN, 80 �C 2 CH-C(O)OBu-t NHBoc O OH PhOC(S)O OC(S)OPh O P(OMe) 2 TFA CH2 CH2 CH-C(O)OBu-t NHBoc (TMS) 3SiH AIBN, 80 �C O P(OMe) 2 CH2 CH2 CH-C(O)OBu-t NH + 3 CF3CO2− 72% (2 steps) O OH 86% (4.29) (4.30) (4.31) (4.32) Radical deoxygenation via thioimidazolyloxy derivatives is also found to be efficient process. Two examples are reported in Reactions (4.33) and (4.34) [70,71]. In particular, dideoxygenation was useful to prove the structure of the stemodane ring system, achieved by other routes. BnO N O N O S O O (TMS) 3SiH AIBN, 80 �C BnO O 92% O O (4.33)
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62 Reduc<strong>in</strong>g Agents<br />
(TMS) 3Si + R X R'<br />
S H2<br />
stepwise<br />
S H2<br />
synchronous<br />
R R'<br />
X<br />
Si(TMS) 3<br />
(TMS) 3 SiXR' +<br />
Scheme 4.2 Possible mechanistic paths for the displacement <strong>of</strong> a chalcogen group (R 0 X,<br />
where X ¼ S or Se) by (TMS) 3Si: radicals<br />
4.3.3 DEOXYGENATION OF ALCOHOLS<br />
(BARTON–MCCOMBIE REACTION)<br />
A relevant reductive process, which has found wide application <strong>in</strong> organic<br />
synthesis, is the deoxygenation <strong>of</strong> alcohols <strong>in</strong>troduced <strong>in</strong> 1975 by Barton and<br />
McCombie [58]. Reaction (4.28) shows that the thiocarbonyl derivatives, easily<br />
obta<strong>in</strong>ed from the correspond<strong>in</strong>g alcohol, can be reduced <strong>in</strong> the presence <strong>of</strong><br />
Bu3SnH under free radical conditions. The reactivity <strong>of</strong> xanthates and thiocarbonyl<br />
imidazolides [58] was successfully extended to O-arylthiocarbonates [59]<br />
and O-thioxocarbamates [60]. Several reviews have appeared on this subject,<br />
thus provid<strong>in</strong>g an exhaustive view <strong>of</strong> this methodology and its application <strong>in</strong><br />
natural product synthesis [61–64].<br />
ROH ROC(S)X<br />
Bu 3SnH<br />
X = SR, OAr, NHPh, N<br />
N<br />
R<br />
RH (4.28)<br />
(TMS) 3SiH replaced successfully t<strong>in</strong> hydrides and <strong>of</strong>ten proved to be a<br />
superior reagent [65]. <strong>In</strong> the <strong>in</strong>itial work, it has been shown that (TMS) 3SiH<br />
and its silylated by-products are not toxic [66] and that the reaction is efficient<br />
also at room temperature [67]. The reduction <strong>of</strong> thiono esters <strong>of</strong> cholesterol to<br />
cholestene (Reaction 4.29) has been used for evaluation <strong>of</strong> by-products <strong>in</strong><br />
biological assays [66]. The effect <strong>of</strong> substituents on the phenyl r<strong>in</strong>g <strong>of</strong> thiono<br />
ester has also been evaluated, see for example Reaction (4.30) [67]. An improved<br />
procedure for the homolytic deoxygenation <strong>of</strong> am<strong>in</strong>oacid derivatives has also<br />
been reported (Reaction 4.31). It consists <strong>of</strong> the treatment with (TMS) 3SiH<br />
(without purification), followed by an acid treatment by TFA and an aqueous<br />
extraction, which was sufficient to separate the product from the silyl and<br />
sulfur-conta<strong>in</strong><strong>in</strong>g by-products [68]. The dideoxygenation <strong>of</strong> 1,6-anhydro-dglucose<br />
with (TMS) 3SiH has been described to afford the desired product <strong>in</strong> a
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