Regio- and stereoselectivities in Diels–Alder cyclodimerizations of orthoquinonoid cyclohexa-2,4-dienones
摘要:
The [4+2] cyclodimerization of cyclohexa-2,4-dienone derivatives of the orthoquinone monoketal and orthoquinol types has been the topic of numerous investigations over the last 50 years in the aim of rationalizing the extraordinary level of regio-, site-, and stereoselectivities observed in these processes. In particular, the double diastereo-pi-facial differentiation expressed in cyclodimerizations of chiral orthoquinols (i.e., 6-alkyl-6-hydroxycyclohexa-2,4-dienones) is an important aspect of these transformations, for they relate to the construction of several natural products. The experimental and theoretical results that are described in this article offer a comprehensive understanding of the factors controlling these site- specific regio- and diastereoselectivities. Our interpretation of these results relies on a combination of Wood-ward-Hoffmann and Salem-Houk secondary orbital interactions and Cieplak-type hyperconjugative effects in bispericyclic C-2-symmetric transition states. (c) 2007 Elsevier Ltd. All rights reserved.
作者:Laurent Pouységu、Tahiri Sylla、Tony Garnier、Luis B. Rojas、Jaime Charris、Denis Deffieux、Stéphane Quideau
DOI:10.1016/j.tet.2010.05.078
日期:2010.7
Both lambda(3)- and lambda(5)-iodanes have proven to be useful reagents in the oxygenative dearomatization of phenols, and exploitations of their chemistry in the conception of both substrate- and reagent-controlled asymmetric variants of such a transformation of great value for natural product synthesis have shown evident signs of success. Moreover, the use of stabilized IBX (i.e., SIBX) in our methodology for O-demethylation of 2-methoxyphenols, which relies on the same key oxygenating dearomatization event, is reported here to be much more efficacious than that of 1BX itself in the chemoselective one-step conversion of homovanillyl alcohol into hydroxytyrosol, and bergenin into norbergenin. (C) 2010 Elsevier Ltd. All rights reserved.