Barton’s reagent

New innovations incorporated in the sequences shown in Eqs. (25) and (26) are an improved method for Barton ester formation by the use of a thiouronium derivative of Barton s reagent 42 [41] and new auxiliary groups 43 bearing a tertiary C-6 substituent, which can be prepared in either enantiomeric form. These new auxiliaries give product as a mixture of diastereomers in a ratio of 7 1 to 10 1. The fact that both enantiomers of 43 are available permits one to dial in the configuration of each stereogenic center produced in a complex product. 

Alternate procedures were also developed using Barton s reagent. In the first of these, coupling of the xanthate 3.1.4.3 obtained from 10-deacetylbaccatin III with protected side chain, followed by free radical deoxygenation and deprotection, gave 10-deacetoxy taxol (3.1.4.4) or 10-dehydroxydocetaxel (3.1.4.5), depending on the side-chain used (83). 

Compound 136 (Scheme 35) is classically used as a reagent for Barton s reductive decarboxylation, in the presence of triethylamine and/-BuSH <1995JOC6237, 1996JOC6189, 2003T6797>. 

S. Hanessian Recent studies in synthetic carbohydrate chemistry Sir Derek Barton New and selective reactions and reagents... 

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