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In view of refro-Diels-Alder reactions (RDA reactions), the calicene precursor 9 has been synthesized in a comparably simple four-step synthesis by dibromocarbene addition at dibenzobarrelene (10-11, 44%), halogenlithium exchange followed by reaction with cyclopentenone (11-+12, 91 %) and H,O as well as HBr elimination (12-14-9, 43%) (Scheme 5 ) . First experiments with respect to the thermal behavior of 9 show that, although RDA reaction seems to be relatively easily occurring according to the results of 'Curie-Point' pyrolysis, only anthracene and no calicene 2 has been detected so far.

Synthesis of Triafulvalene Precursors by ‘Carbene‐Dimerization’ of 1‐Halogeno‐1‐lithiocyclopropanes Bi(cyclopropylidenes) 7a, 7c, and 7e are available in a simple one‐pot reaction by treating 1,1‐dibromocyclo‐propanes 5 at −95° with BuLi and CuCl2. Attempts towards triafulvalene precursors with good leaving groups are reported. The most promising attempt makes use of 2,2′‐bis(phenylthio)‐3,3′‐bis(trimethylsilyl)‐1,1′‐bi(cyclopropylidene) (7c) which has been oxidized to give the bis(phenylsulfonyl) derivative 7g. So far, F−‐induced elimination experiments with 7g failed.

The chemotherapeutic agent Taxol (3, Scheme 1), originally obtained from Pacific yew tree bark, has shown remarkable promise in the treatment of both breast and ovarian cancer. 2 As part of our ongoing research in this area, we have developed a strategy for the synthesis of taxanes based on pinene (1), 3 which provides concise, enantiomerically controlled access to the tricarbocyclic taxane core (e.g., 2a in six steps), structural analogs, and taxol itself. 4 A recently introduced C9-C10 linker variant of this strategy has also provided tricycle 2b in only seven steps from pinene (1). 5 Herein we describe the next phase of this program, the elaboration of 2b toward both taxol and its analogs entailing concise solutions for the functionalization of the B and C rings.The elaboration of 2b began with protection of its C13 hydroxyl group as the TBS ether (4, Scheme 2). It is a noteworthy consequence of the striking interconnectedness of functional group reactivities in this structural series that 4 failed to undergo C1 deprotonation and oxidation under conditions in which the closely related silyl ether derivative of 2a reacted readily. 3 At the core of this reactivity difference is the C4 substituent (H vs MeO), which restricts 4 to a conformation in which the C1-CH bond is improperly aligned with the C2 carbonyl for deprotonation. Removal of this conformational constraint through saturation of the C9 center was found to resolve this problem, as demonstrated by the highly efficient conversion of the hydroboration product 5 (with t-BuOK/DMSO/O 2 in THF at -25°C) to the C1-oxidized product 6 in 96% yield. The primary alcohol of 6 was subsequently converted to the C9 ketone 7 (82%) by oxidation with Dess-Martin's periodinane 6 to an aldehyde (84%) followed by oxidative decarbonylation usingFurther elaboration of diketone 7 was initially accomplished by Birch reduction. However, complications with over-reduction of the aromatic ring prompted our selection of the C2 monoketone 8 in order to achieve better control over this reduction. For this purpose, ketone 8 (mp 149-150°C) was prepared by double reduction of 7 with sodium, followed by selective protection of the C9 alcohol and reoxidation of the C2 alcohol (53% yield over three steps). When 8 was treated with K/NH 3 /THF, the R-and -C3-stereoisomers 9a and 9b/ 9c were obtained in a ratio that was dependent on the reaction conditions and workup. Although it was possible to obtain either C3 epimer, compounds possessing the -CH configuration were selected for study since they were expected to afford greater stereocontrol over the introduction of groups at C8 and to be amenable to epimerization at a later synthetic stage. Ketone 8 was therefore selectively reduced to the -CH derivatives 9b and 9c (77% yield, along with a trace of 9a). Silyl ether 9b was readily converted to 9c upon treatment with acid.Access to our multipurpose analog precursor, enone 13, entailed initial reduction of the C2 ketone of 9c with Na in NH 3 /THF to the desired C2 alcohol stereoisomer 10 (85% y...

(1 3. IV. 93) Synthesis and Pyrolysis of a Triafulvene PrecursorIn view of retro-Dirk-Alder reactions (RDA reactions), the triafulvene precursor 3 has been prepared in a simple three-step synthesis by dibromocarbene addition at dibenzo-barrelene (11 -+ 12; a%), halogen-Li exchange followed by methylation (12 + 14, 100%) and HBr elimination (14 + 3,62%) (Scheme 3). Reactivity of the so far unknown bridged 1, I-dibromocyclopropane 12 has been explored, including reductions, allylic rearrangements, and 'carbene dimerizations' (Scheme 4 ) . First experiments with respect to the thermal behavior of 3 show that RDA reaction, although occurring in most cases, is not the predominant pathway. When 3 is heated in a sealed tube without solvent, two dimers 26 and 27 are isolated in a total yield of 55% (Scheme 6). On the other hand, gas-phase pyrolysis of 3 at 400' mainly produces rearranged 28 (56%; Scheme 7). It is assumed that bridged trimethylenemethane 29 is an essential intermediate in thermal rearrangements of 3 (Scheme 8).1. Einleitung. -Seit mehr als 20 Jahren wird in verschiedenen Laboratorien versucht, zwei der einfachsten Vertreter der Fulvene und Fulvalene, Triafulven (1) und Calicen (2) zu synthetisieren. Uberraschendenveise blieb Calicen (2) bis heute unbekannt, obwohl es zwei Ringe enthalt, welche sich gegenseitig elektronisch stabilisieren sollten. Andererseits wurden schon recht friih Hinweise darauf erhalten, dass Triafulven als reaktive Zwischenstufe durchlaufen wird [3]. 1981 gelang der erste direkte Nachweis des Grundkor-