Terpenic Acids by Cyclopentane Annulation of Exocyclic Dienes. Synthesis of Triquinane Portion of Retigeranic Acid

Article abstract of DOI:10.1021/jo00347a031

Tricyclic ester 5 has been prepared in 13 steps from (+)-pulegone.The key step in the synthesis of 5 involved the condensation of keto ester 15 with ethyl bromocrotonate under Reformatsky conditions to yield lactone 16 as one diastereomer.Either lithium diisopropylamine or 1,8-diazabicyclo<5.4.0>undec-7-ene was used to effect the elimination to the dienic acid 17, obtained as a mixture of diastereomers in an 80:20 ratio.Conditions were developed for the cyclopropanation of 18 in good isolated yields.The pyrolysis of 19 gave tricyclic ketone 20 in 50 percent yield.The ¹³C NMR data are provided for all relevant intermediates.The deoxygenation of this ketone provided ester 5.The structural and stereochemical assignments are based on data amassed previously on appropriate model compounds.Potential use of 5 in a convergent synthesis of retigeranic acid (3) is indicated.The concept of generality of this method in the context of total synthesis of other naturally occuring cyclopentene carboxylates is introduced.