Stereoselective oxidative ring-contraction

The first asymmetric total synthesis of the macrocyclic lactone metabolite (+)-pyrenolide D was accomplished in the laboratory of D.Y. Gin. The natural product has a densely functionalized polycyclic structure and its absolute configuration had to be established. The key step of the synthesis was a stereoselective oxidative ring-contraction of a 6-deoxy-D-gulal, which was prepared from anomeric allylic sulfoxide via the Mislow-Evans rearrangement. 

Anionic rearrangement of sulfonium ylides and anions of sulfides are well exemplified. It has been reported that base-catalyzed ring contraction of 3-alkyl- and 2,3-dialkyIthietane 1-oxides, prepared by peroxyacid oxidation of the corresponding substituted thietanes, gave substituted cyclopropanes with stereoselectivity controlled by the configuration of the sulfur... 

Under similar conditions, the four diastereomers of 3-hexyl-2-methylthietane 1-oxides underwent ring contraction stereoselectively to give 2 and in two cases, (la,2, 3a)-2 and (la,2a,3a)-2, regioselectively. 

Carvone-derived 2,3-epoxy alcohol derivatives (317) have been found to rearrange with stereoselective formation of ring-contracted ketones (318) in a process where the stereochemical result seems to be independent of epoxide configuration. On the basis of this study, the authors concluded that the rearrangement of tetrasubstituted cyclic epoxy alcohol derivatives remains a transformation for which both regiose-lectivity and stereochemical outcome are difficult to predict. New solid acid catalysts based on silica-supported zinc triflate have been prepared for use in the rearrangement of a-pinene oxide to campholenic aldehyde. ... 

Danishefsky and coworkers has approached Terreulactone A, with their efforts outtined in Scheme 12.58 [107]. The enol intermediate 242 was readily obtained from the Wieland-Miescher ketone 241 by chemo- and stereoselective reduction, protection of die resultant alcohol, dimethy-lation, followed by a two-step oxidation sequence. Then the enol intermediate 242 was subjected to a stereoselective ring contraction condition to afford 243. Lactonization of 243 followed by appropriate functional group management provided the A, B, C rings of terreulactone A 245, which they hoped to convert to 240 and expected to couple 240 with a suitable E,F progenitor 239 by the [3+3] cycloaddition strategy. Their completion of the total synthesis of terreulactone A 238 is under way. 

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