Diazoalkanes react with electrophilic cyclobutenes by a 1,3-dipolar cycloaddition to Δ−1 pyrazolines. After photolysis and acidic solvolysis polyfunctionalized diquinanes are obtained.
Synthesis of polyquinanes via bicyclo [2.1.0] pentane intermediates. An easy access to optically pure diquinane alcohols of known absolute configuration.
The resolution of the cyclobutenic ester 1 using chiral lactols is described. After a stereospecific reaction sequence (cyclopropanation followed by an acidic solvolysis), the enantiomerically pure diquinanes 10 are obtained.
The cyclobutenic ester 1, readily available by thermal [2+2] cycloaddition of the silyl enol ether derived from cyclopentanone with ethyl propynoate, is easily transformed into the diquinanic alcohol 3 via the bicycle [2.1.0] pentane intermediate 2. After protection as the thexyldimethylsilyl ether, an allylic oxidation stereospecifically introduces a hydroxyl group at position 8. Following formation of the benzyl ether, the diquinane 9 was then transformed in four steps into the triquinane 20 using a silyl-assisted Nazarov type cyclisation. From this intermediate, (+/-)-silphinene has previously been synthesized. (C) 1997, Elsevier Science Ltd.