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Monday, February 9, 2009
Douglass F. Taber
University of Delaware

C-O Ring Containing Natural Products: Paeonilactone B (Taylor), Deoxymonate B (de la Pradilla), Sanguiin H-5 (Spring), Solandelactone A (White), Spirastrellolide A (Paterson)

Richard J. K. Taylor of the University of York has developed (Angew. Chem. Int. Ed. 2008, 47, 1935. ) the diasteroselective intramolecular Michael cyclization of phosphonates such as 2. Quenching of the cyclized product with paraformaldehyde delivered (+)-Paeonilactone B (3).

Roberto Fernández de la Pradilla of the CSIC, Madrid established (Tetrahedron Lett. 2008, 49, 4167. ) the diastereoselective intramolecular hetero Michael addition of alcohols to enantiomerically-pure acyclic sulfoxides such as 4 to give the allylic sulfoxide 5. Mislow-Evans rearrangement converted 5 into 6, the enantiomerically-pure core of Ethyl Deoxymonate B (7).

The ellagitannins, represented by 10, are single atropisomers around the biphenyl linkage. David R. Spring of the University of Cambridge found (Org. Lett. 2008, 10, 2593. ) that the chiral constraint of the carbohydrate backbone of 9 directed the absolute sense of the oxidative coupling of the mixed cuprate derived from 9, leading to Sanguiin H-5 (10) with high diastereomeric control.

A key challenge in the synthesis of the solandelactones, exemplified by 14, is the stereocontrolled construction of the unsaturated eight-membered ring lactone. James D. White of Oregon State University found (J. Org. Chem. 2008, 73, 4139. ) an elegant solution to this problem, by exposure of the cyclic carbonate 11 to the Petasis reagent, to give 12. Subsequent Claisen rearrangement delivered the eight-membered ring lactone, at the same time installing the ring alkene of Solandelactone E (14).

AD-mix usually proceeds with only modest enantiocontrol with terminal alkenes. None the less, Ian Paterson, also of the University of Cambridge, observed (Angew. Chem. Int. Ed. 2008, 47, 3016, ; Angew. Chem. Int. Ed. 2008, 47, 3021, ) that bis-dihydroxylation of the diene 17 proceeded to give, after acid-mediated cyclization, the bis-spiro ketal core 18 of Spirastrellolide A Methyl Ester (19) with high diastereocontrol.

D. F. Taber, Org. Chem. Highlights 2009, February 9.