Intramolecularly competitive Ireland-Claisen rearrangements: scope and potential applications to natural product synthesis.
A variety of bis-allylic esters were prepared by vinylmetal addition to cycloalkenones followed by esterification either in situ or in a separate operation. For chiral cyclohexenones, the vinyl additions generally occurred with >10:1 diastereoselectivity. Although in some cases the bis-allylic esters proved to be sensitive to silica gel or other adsorbents, all of the esters examined could be isolated in acceptable purity. The Ireland-Claisen rearrangement of the bis-allylic esters occurred with complete regioselectivity via the exocyclic alkene. The alkene stereochemistry and the stereochemistry at C-2 and C-3 of the pentenoic acid products were consistent with a chairlike transition state in the rearrangement. Substituents at the carbons adjacent to the allylic carbinol carbon (i.e., C-2 or C-6 in cyclohexenone-derived substrates) directed the stereochemical course of the rearrangement. The rearrangements generally proceeded so as to place the larger of the C-2 or C-6 substituents in the pseudoequatorial position with respect to the chairlike transition state. For a bis-allylic ester bearing both a C-2-CH(3) and a C-6-OMEM substituent, the rearrangement product resulted from the nominally smaller OMEM substituent occupying a pseudoequatorial position with respect to the chairlike transition state.
Hong Sp
Sang-phyo
SP
Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, Arkansas 72701, USA.
Lindsay
Harriet A
HA
Yaramasu
Tripura
T
Zhang
Xiaowei
X
McIntosh
Matthias C
MC
D001685:Biological Factors / Q000138:chemical synthesis*; D002384:Catalysis; D002625:Chemistry, Organic / Q000379:methods; D018360:Crystallography, X-Ray; D004952:Esters / Q000138:chemical synthesis* / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D013237:Stereoisomerism
D001685:Biological Factors; D004952:Esters