CARBOCYCLIC CIS-[1.1.1]-TRIS-SIGMA-HOMOBENZENES - SYNTHESES BY TRIPLEEPOXIDE -] CYCLOPROPANE CONVERSIONS, STRUCTURAL DATA, [SIGMA-2S-2S+SIGMA-2S] CYCLOREVERSIONS(SIGMA)

By triple epoxide --> cyclopropane transformation starting from cis-be nzene trioxide the 3 alpha,6 alpha,9 alpha-tris(TIPS-ethynyl) (1b), -t ris(alkoxycarbonyl) (Id, e), -tricyano (1f), and -tris(ammoniomethyl) (1j) derivatives of the still elusive parent cis-[1.1.1]-tris-sigma-ho mobenzene (1a) have been synthesized, Out of a number of carbon nucleo philes tested, a weakly basic mixed cyanocuprate of Li-TIPS-propyne pr oved the reagent of choice: Eliminations were largely avoided, the -2, 4,6-tris(TIPS-propargyl)cyclohexane-1,3,5-triol (13a), obtained in a s tereoselective one-pot threefold epoxide opening (up to 69%), allowed the pertinent functional group manipulations, The selectivity of the b ase-induced triple cyclopropane ring closure in the flexible cyclohexa ne precursors strongly depends on the activating groups and is promote d by fixation of the ''all-axial'' conformation as an adamantoid ortho phosphate, The Delta G(not equal) values determined for the generally neat 3 sigma --> 3 pi isomerizations [28.5 (le), 29.0 (1f), 22.6 (1j) kcal mol(-1)] are in line with concerted reaction courses ([sigma 2s+s igma 2s+sigma 2s]). For the trinitrile 1f an X-ray structure analysis confirmed a nearly planar cyclohexane ring and a relatively large cycl ohexane/cyclopropane interplanary angle (113 degrees).