Elsevier

Tetrahedron Letters

Volume 46, Issue 40, 3 October 2005, Pages 6897-6899
Tetrahedron Letters

Favorskii rearrangement of 23-oxo-3-epi-smilagenin acetate induced by iodosobenzene

Dedicated to the memory of our dearest colleague and friend Professor Amparo Tortajada López of the University of Valencia, Spain
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Abstract

Treatment of (25R)-3α-acetoxy-5β-spirostan-23-one with diacetoxyiodobenzene in KOH/MeOH led to F-ring contraction via Favorskii rearrangement.

Graphical abstract

Treatment of (25R)-23-oxosapogenin with diacetoxyiodobenzene in KOH/MeOH led to F-ring contraction via Favorskii rearrangement.

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Acknowledgments

We thank Dirección General de Asuntos del Personal Académico (DGAPA-UNAM) for support via project IN200105 and Professor Benjamin Ruiz Loyola (UNAM) for providing laboratory facilities. We are also indebted to Rosa I. del Villar Morales and Georgina Duarte Lisci (USAI-UNAM) for registering all NMR and Mass spectra.

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    When α-haloketones are treated with a nucleophilic base, carboxylic acids, esters and amides are usually obtained as reaction products via the C-C skeletal rearrangement. Besides base, by the employment of hypervalent iodine reagents,2 acid,3 enzyme,4 photochemistry5 or electrochemistry,6 some novel Favorskii rearrangements have been also developed. To date, some reviews by Butkus7 and Yus8 discussed synthetic applications of this rearrangement.

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    Provided that 2a can be produced by both, the CPM and the SBM, the 9/1 ratio in which 2a predominates over 2b, suggests that in this particular case both mechanisms occur simultaneously, although SBM predominates. On the other hand, the absence of α′-protons in compounds 3 and 5 (Eqs. 2 and 3), makes the exclusive participation of SBM mandatory and justifies the observed regioselectivity [16,17]. Examination of the literature indicates that when iodine III compounds are employed, FR of cyclic ketones is only a deviation of the more generally observed α-hydroxylation-ketalization course (Scheme 3, Eq. 4) [18].

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