The decarboxylation of N-carboxy-2-imidazolidinone has previously been established as a model for the transfer of carbon dioxide from N(1‘)-carboxybiotin. The present paper reports the pH-dependence of the reaction as well as the acceleration of the reaction in methanol and in acetonitrile. These results suggest that enzymic reactions of N(1‘)-carboxybiotin in a hydrophobic active site with decreased hydrogen bonding can be rapid if the energy of desolvation is compensated by the energy made available by association of the substrate and protein. In addition, a report on the decarboxylation of N-carboxy-2-imidazolidinone in organic solvents containing macrocycles (Kluger, R.; Tsao, B. J. Am. Chem. Soc. 1993, 115, 2089−90) must be reinterpreted on the basis of the inherent instability of the substrate under the reaction conditions.

The Cu"-induced oxidative coupling of a,w -di(cyclopentadienyl)alkane-diides 6 (n = 2-5) has been shown to proceed mainly by an intermolecular pathway to give polymers 8, while the yield of intramolecular coupling 6 +7 strongly decreases with increasing number n of C-atoms of the alkyl chain (Scheme 3 ) . For 11 = 2, intramolecular coupling may be considerably enhanced by replacing the H-atoms of the CH,CH, bridge of 6a (n = 2) by Me groups. In this case, intramolecular couplings 11 + 20 (Scheme 7) and 22 +23 + 24 (Scheme 8) are accomplished with a total yield of 59% and 54%, respectively. All the intramolecular couplings investigated so far proceed stereoselectively to give the C2-symrnetrical cyclohexanes 7a, 20 and 23 with a fixed chair conformation. These results are easily explained, if a conformational equilibrium E F?. F is operative in which large substituents R are assumed to enhance the gauche-conformation F which is the favored conformation for intramolecular couplings. Bridged dihydropentafulvalenes 20 and 23 are quantitatively rearranged to the thermodynamically favored bridged pentafulvenes 27 and 28 under base or acid catalysis, respectively (Scheme 9).

New Pathways to Precursors of PentalenePentalene dimers 2 and 3 are easily available in moderate yields by CuCI,-induced oxidative coupling of dilithium-pentalenediide (5) (Scheme f). On the other hand, NBS bromination of 1,5-dihydropentaIene (4) or of 1.2-dihydropentalene (8) gives unstable 1 h o m o -I ,2-dihydropentalene (9), while subsequent in-siru elimination with Et,N exclusively gives syn-cis-pentalene dimer 2 in moderdle yields (Scheme 3). NMR-Spectroscopic evidence for compounds 2,3, and 9 is presented, and mechanistic alternatives for the formation of pentalene dimers 2 and 3 are discussed. 1.Einleitung. -Pentalen (1)2)3) hat sowohl die synthetischen als auch die theoretischen organischen Chemiker seit mehr als vier Jahrzehnten fasziniert. Trotz zahlreicher Syntheseversuche konnte jedoch der Grundkorper 1 bisher spektroskopisch nicht charakterisiert werden, und alle bis heute isolierten substituierten Pentalene waren entweder sterisch oder elektronisch stabilisiert. Hexaphenylpentalen war das erste einfache Pentalen, welches 1962 durch Le Gofjrsynthetisiert wurde [4]; spater konnten die elektronisch stabilisierten Pentalene 1,3-Bis(dimethylamino)pentalen [5] und 1,4-Diamino-3,6-dime-

~ ~_ _ _ _ _ _CuC1,-Induced oxidative coupling of 2-(rert-butyl)-6,6-dimethylpentafulvenyl anion 9 predominantly takes place at C(7) and C(5) to give [7-71 and (7-51 coupling products 15 and 16 in 35 and 47% yields, respectively (Scheme 3 ) whose structures are elucidated from ID-and 2D-NMR analysis. Compared with the product distribution observed for 6,6-dimethylpentafulvenyl anion 2 (Scheme I ) , no coupling at C(2)/C(3) of 9 is observed. This means that, besides electronic effects, steric effects are also important in oxidative couplings of fulvenyl anions. The same couplings occur in the case of 2,3-bis(6,6-dimethylfulven-2-yl)-2,3-dimethylbutane dianion 10 as well, but, due to electronic as well as conformational effects (Scheme 5), intermolecular coupling (to give polymers 17, Scheme 4 ) is strongly favored over intramolecular coupling. Mechanisms explaining base-catalyzed isomerization 15a + 15b e 1% (Scheme 6 ) as well as isomerization 16a e 16b (Scheme 7 ) are proposed.

1997other polycyclic compounds other polycyclic compounds Q 1150 -106Coupling Reactions.Part 17. New Pathways to Precursors of Pentalene.-Two new routes to the pentalene dimes (II) and (III) are described starting from the 1,5-dihydropentalene (I). The yields for these onepot reactions are moderate. The unstable compound (IV) is NMR-spectroscopically characterized. Mechanisms for the formation of (II) and (III) are discussed. - (YOU, S.; CHAI, S.; SCHWARZ, N.; NEUENSCHWANDER, M.; Helv. Chim. Acta 80 (1997) 5, 1627-1638 Inst. Chem. Biochem., Univ. Bern, CH-3012 Bern, Switz.; DE)