S- 1-Oxido-2-pyridinyl -1,1,3,3-tetramethylthiouronium Hexafluorophosphate HOTT

GesJChem. Ben 1986, 779,729. (c) Schonauer, K. J. Walter, P. Noe, washed with CHCI3, and then filtered to give a white solid of 

Hammerschmidt, E Vollenkle, H. Justus Liebigs Ann. ChemAJehigs Ann. Chem. 1989, 577. 

Greene, T. W. Wuts, P. G. M. Protective Groups in Organic Synthesis 2nd ed. Wiley New York, 1991 pp 37-38. 

Barton Esterification Reductive Decarboxylation. O-Acyl thiohydroxamates or Barton esters are useful precursors of carbon-centered radicals via thermolysis or photolysis. Several different methods are available for converting carboxylic acids into Barton esters (eq 1). These reactions generally proceed via the attack of a 2-mercaptopyridine-N-oxide salt on an activated carboxylic acid that has either been preformed (acid chloride, mixed anhydride) or generated in situ (with 1,3-dicyclohexylcarbodiimide or tri-n-butylphosphine + 2,2 -dithiodipyridine-l,r-dioxide). However, HOTT has the distinct advantages of (1) being easy to prepare and handle without the need for any special precautions, (2) facilitates efficient Barton esterification of carboxylic acids, and (3) simplifies subsequent work-up and purifications by avoiding the need to remove by-products like 1,3-dicyclohexylurea. 

The HOTT reagent has been shown to significantly improve the yields of reductive decarboxylations of 2,3 4,6-di-0-isopropylidene-2-keto-L-gulonic acid (eq 2) and peracetylated N-acetylneuraminic acid (eq 3). In both cases, the yield of reduced product nearly doubled when HOTT was used to esterify these hindered carboxylic acids. 

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S- 1 -Oxido-2-pyridinyl)-1,1,3,3-tetramethylthiouronium Hexafluorophosphate (HOTT), 463 CioH,602... 

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