Retro rhodium-catalyzed reaction

The retro-Diels-Alder reaction of dimer 220 led to monomer 221, which underwent a [4 + 2]-cycloaddition with the cyclopentadienone formed in situ by dehydrogenation of 222. The crude product was subsequently treated with BBtj to afford (+)-chamaecypanone C (223) in 53% yield over two steps. Starting material 220 was readily accessible in three steps and 99% ee from 2-hydroxy-5-isopropylbenzaldehyde. In advanced studies, a rhodium-catalyzed dehydrogenation has been applied to the synthesis of chamaecypanone C analogs ]91]. 

Rhodium catalyzes retro-allylation of homoallylic alcohols to generate nucleophilic allylic rhodium species. A combination of [RhCl(cod)]2, PMcj, and cesium carbonate catalyzes crotyl transfer from homoallylic alcohol to benzaldehyde in dioxane at 100 "C to afford the corresponding homoallylic alcohol (Scheme 5.37) [26]. Like allylic nickel, allylic rhodium experiences rapid and E-Z interconversions under the reaction conditions, which prevents stereoselectivity of the crotylation. Interestingly, a similar crotylation is followed by isomerization into saturated ketone with the aid of a bulkier catalyst [RhCl(cod)]2/PtBu3 at a higher temperature. 

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