Asymmetric -annulation/rearrangement

A hydrogen-bond-mediated asymmetric [4,l]-annulation/rearrangement cascade of stable sulfur ylides and nitroolefins has been developed in the presence of a C2-symmetric chiral urea catalyst This reaction provides a facile route to enantioenriched 4,5-substituted oxazolidinones with excellent stereocontrol (up to more than 95 5 dr and 97 3 er). The stereocontrolled modes and mechanism have been proposed to explain the origin of this stereochemistry. 

A hydrogen-bond-mediated asymmetric [4,l]-annulation/rearrangement cascade of sulfur ylides and nitroolefins provides a route to enantioenriched 4,5-substituted oxazolidi-nones (Scheme 40). ... 

Davies and co-workers [12, 35] have exploited one particular aspect of the asymmetric cyclopropanation of alkenes with vinyl diazoacetates, namely, application to substrates suitable for subsequent Cope rearrangement. Cyclopropanation of dienes with predominant cfs-1,2-divinyl diastereoselection makes possible subsequent facile [3,3]-sigmatropic rearrangement with entry to 1,4-cycloheptadienes or bicyclic dienes. Two such examples employing cyclopenta-diene and penta-l,3-diene as substrates and the rhodium(II) prolinate catalyst, Rh2(2S-TBSP)4 in Fig. 1, are shown in Eq. (6) and Eq. (7),respectively cfs-l,2-di-vinylcyclopropanes are presumed to be intermediates in these annulation reactions. In contrast, ethyl diazoacetate and styrene with the prolinate catalyst (Fig. 

Barluenga and co-workers extended the scope of [3+4] annulations of chromium vinylcarbenes by using 2-amino-1,3-butadienes as the diene component. In most cases the reaction with this diene is highly diastereoselective and favors the formation of cw-divinylcyclo-propanes that rapidly rearrange to the cycloheptadienes. Furthermore, the amino functionality can be a chiral auxiliary such that an asymmetric [3+4] annulation is achieved. Examples of this approach are shown in Scheme 17. Reaction of the chromium vinylcarbene complex... 

The intramolecular [3+4] annulation can be carried out with high asymmetric induction and this was used to synthesize the epi tre-mulenolide A skeleton, as illustrated in Scheme 39. ° Rh2(S-DOSP)4 catalyzed decomposition of 107, which contains a rranj-alkene nearest the tether results in the formation of the [3+4] annulation product 108 with full relative stereochemical control but in only 35% ee. A much more effective procedure is to use 109, which contains a trans- Vso nearest the tether, as the substrate. Rh2(5-DOSP)4 catalyzed decomposition of 109 results in the formation of the truns-divinylcyclopropane 110. On heating, 110 rearranges to the [3+4] annulation product 108, which is formed in 93% ee. The rearrangement presumably occurs through initial equilibration of 110 to the ds-divinylcyclopropane. However, as the equilibration occurs with epimerization of only two of the stereocenters in 110, the enantiose-lectivity of the cyclopropanation is maintained in the [3+4] annulation product 108. 

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