The long‐sought dianion of the still unknown acepentalene 2 is formed surprisingly easily upon reaction of triquinacene 1 with n‐butylithium and potassium tert‐amyl alkoxide in hexane at room temperature. 22⊖ reacts with chlorotrimethylsilane to give a mixture of 1,4‐ and 1,10‐bis(trimethylsilyl)dihydroacepentalenes.

A versatile approach towards the highly strained acepentalene 3 via the readily accessible dipotassium acepentalenediide (10 a) and the highly strained tetraenes 7 is reported. An unexpected [42] cycloaddition dimer 14 is formed upon protonation of the dipotassium acepentalenediide (10 a) in 93 % yield, and the monomeric 4,7-dihydroacepentalene (7 a), as the reactive intermediate, can be trapped with anthracene to form the corresponding Diels ± Alder adduct 16 in 15 % yield. In contrast, the highly strained, but sterically protected monomeric bridgehead ± bridgehead alkenes 7 c, d can be isolated upon reaction of 10 a or 10 b with bulky electrophiles, such as Me 3 -SiCl and Me 3 SnCl, respectively. The Xray crystal structure analysis of 7 d exhibits a highly pyramidalized central double bond. The bisstannane 7 d is an ideal precursor for acepentalene (3), which would be formed by removal of the two trimethylstannyl substituents. It can also be transmetallated to give the pure crystalline dilithium acepentalenediide (10 b) in 78 % yield. According to its 1 H, 7Li and 13 C NMR spectra, the bowl-shaped 12 p-dianion in 10 b is an aromatic species, and it undergoes a rapid bowl-to-bowl inversion at room temperature.

Dihydroacepentalendiid 22e, formal ein ethenoiiberbriicktes Dihydropentalendiid''], sollte wie dieses['**] nach semiempirischen Rechnungen ein geschlossenschaliges System seini31. Allerdings diirfte seiner Bildung die hohe Spannungsenergie in dem cyclisch und gekreuzt konjugierten Geriist rnit drei angular anellierten Flinfringenf4I entgegenstehen. Bisher war es offenbar nicht gelungen, 22e zu erzeugen, obwohl sich die Theoretiker schon lange fur Acepentalen (Cyclopenta[cd]pentalen) 2 interessiered". Neueste Versuche, die beiden Dialkylaminogruppen aus den als Vorstufen geeignet erscheinenden 4,7-Bis(dialkylamino)tricyclo[5.2.l.O4~'O]deca-l(l0),2,5,8-tetraenen131 reduktiv zu eliminieren, fuhrten lediglich zu 1,4,7-Tris(di-alky1amino)-substituierten M o n~a n i o n e n l~.~~. Um so iiberraschender ist es, wie einfach sich 22e direkt aus Tri-*