Esters phenoxythiocarbonyl

Deoxygenation of RiCHOH1 (11, 550). Radical deoxygenation of secondary alcohols via thiocarbonyl esters was introduced by Barton and McCombie. More recently, phenoxythiocarbonyl esters have been recommended for the radical deoxygenation (10, 306-307). Particularly rapid and quantitative reduction can be obtained using 2,4,6-trichlorophenoxy- or pentafluorophenoxythiocarbonyl esters. 

In another similar reaction, activation of the phenoxythiocarbonyl ester on C-2 of the thioman-noside 158 by iodonium ions leads to the anomeric thioethyl group migration to the C-2 position, yielding the l,2-fraw5 -glucoside 159 (O Scheme 71) [114]. 

Similarly, thioglycosides with a phenoxythiocarbonyl ester group at C-2 (109) undergo 1,2-migration of the anomeric phenyl- or ethylthio group when they are activated (110) by NIS/cat. TfOH. 

The C14-C20 segment [108] Stereocontrolled formation of the C15 and chiral centers was the major problem encountered in the synthesis of this segment. A route starting from benzoate 165 [111] was selected after several attempts (Scheme 21). Standard chemical operations led to allylic alcohol 166. Stereocontrolled approach from the re-re face of the double bond in 166 was insured by a diborane-oxidative treatment of the bulky bis-boronate ester 167. 1,2-Diol protection, phenoxythiocarbonylation-hydride reduction [112], and deprotection led to diol 169, representing the C14-C20 segment. 

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