Carbocyliccis‐[1.1.1]‐Tris‐σ‐homobenzenes – Syntheses by Triple Epoxide → Cyclopropane Conversions, Structural Data, [σ2s+σ2s+σ2s] Cycloreversions

Abstract

By triple epoxide ---3 cyclopropane transformation starting from cis-benzene trioxide the 3~~,6a,Sa-tris(TIPS-ethynyl) (lb), -tris(alkoxycarbonyl) (Id, e), -tricyano (If), and -tris(ammoniomethyl) ( l j ) derivatives of the still elusive parent cis-[ 1.1.11-tris-o-homobenzene (la) have been synthesized. Out of a number of carbon nucleophiles tested, a weakly basic mixed cyanocuprate of Li-TIPS-propyne proved the reagent of choice: Eliminations were largely avoided, the scyllo[*l-2,4,6-tris(TIPS-propargyl)cyclohexane-1,3,5-triol (13a), obtained in a stereoselective one-pot threefold epoxide opening (up to 69%), allowed the pertinent functional group manipulations. The selectivity of the base-induced triple cyclopropane ring closure in the flexible cyclohexane precursors strongly depends on the activating groups and is promoted by fixation of the "all-axial" conformation as an adamantoid orthophosphate. The AG# values determined for the generally neat 30 + 371 isomerizations [28.5 (le), 29.0 (If), 22.6 (11) kcal mol-l] are in line with concerted reaction courses ([o2s+o2s+o2s]). For the trinitrile If an X-ray structure analysis confirmed a nearly planar cyclohexane ring and a relatively large cyclohexane/cyclopropane interplanary angle (1 13").

Evidence for Endocyclic Cleavage of Conformationally Restricted Glycopyranosides

“…The unknown C 3 v ‐symmetric all ‐ cis triscyclobutenocyclohexane 1 1 is of fundamental interest as a key structure on which to probe the mechanism of [2+2+2]cycloreversions of cyclopropa‐ and cyclobuta‐fused cyclohexanes (“tris‐σ‐homobenzenes”) 1a. 2 Thus, various examples of the cyclopropa‐fused derivatives undergo facile [σ 2 s +σ 2 s +σ 2 s ] retrocyclization through aromatic transition states, whereas all ‐ cis triscyclobutacyclohexane is deflected to a radical decomposition pathway,3 which is the result of unfavorable through‐bond interactions 2c,d.…”

The Thermal Retro[2+2+2]cycloaddition of Cyclohexane Activated by Triscyclobutenannelation: Concerted All‐Disrotatory versus Stepwise Conrotatory Pathways to Fused [12]Annulenes

“…The unknown C 3 v ‐symmetric all ‐ cis triscyclobutenocyclohexane 1 1 is of fundamental interest as a key structure on which to probe the mechanism of [2+2+2]cycloreversions of cyclopropa‐ and cyclobuta‐fused cyclohexanes (“tris‐σ‐homobenzenes”) 1a. 2 Thus, various examples of the cyclopropa‐fused derivatives undergo facile [σ 2 s +σ 2 s +σ 2 s ] retrocyclization through aromatic transition states, whereas all ‐ cis triscyclobutacyclohexane is deflected to a radical decomposition pathway,3 which is the result of unfavorable through‐bond interactions 2c,d.…”

Three-Membered-Ring-Based Molecular Architectures

“…Prinzbach et al demonstrated that electronegative substituents on the cyclopropane rings can efficiently stabilize cis -[1.1.1]-tris-σ-homobenzene derivatives, and they synthesized a series of such compounds starting from cis -trioxatris-σ-homobenzene 297 , which, in turn, was prepared from 3,6-dibromo-5,6-epoxycyclohexene ( 295 ) in 24% overall yield (Scheme ) 44 …”

“…The anomalously facile cleavage of all three cyclopropane rings in cis - 139 was rationalized in terms of through-bond interactions involving the breaking σ-bonds and the neighboring groups. 103d, In contrast to this, the cis -tris-σ-homobenzenes 302 -R, 304 , and 306 rearrange to the corresponding cyclononatriene derivatives only above 180 °C and cis -trioxatris-σ-homobenzene 297 even above 200 °C 224a. The importance of electronic influences of substituents is best illustrated by the fact that 3-fold reduction of tris(methoxycarbonyl)- cis -tris-σ-homobenzene 302 -Me to tris(hydroxymethyl)- cis -tris-σ-homobenzene, that is, transformation of the three electron-withdrawing into electron-donating substituents, is accompanied by immediate [σ 2 s + σ 2 s + σ 2 s ] cycloreversion, occurring at 0 °C, so that the 3,6,9-tris(hydroxymethyl)cyclonona-1,4,7-triene ( 312 ) was the only isolated product (85% yield) (Scheme ) .…”