Cyclopent 4,5-fused

A cycloisomerisation of the c -2,4-dienal 19 to 4,6,7,8-tetrahydro-3//-isochromene is promoted by Pd catalysts, whereas the Pt-catalysed reaction affords fused cyclopent-3-enones <06OL3153>. 

W-Cyclopent[c]isoxazoles, 3a,4,5,6-tetrahydro-, 56, 264, 60, 310, 311 Cyclopropa-fused uracils, formation and rearrangement, 55, 196 Cyclopropanation, of... 

There are several examples known where methylenecyclopropanes have been employed as cosubstrates in reactions of the Khand—Pauson type. Generally, this cobalt-mediated reaction is performed with hexacarbonyldicobalt complexes of alkynes, preferably strained alkenes such as norbornene or cyclobutene, and carbon monoxide to produce cyclopent-2-enones. Stoichiometric amounts of cobalt (in the form of the alkyne complexes) are normally needed. Methylenecyclopropane (1) can be employed as the strained alkene and reacts with a variety of alkynes to yield spiro-fused products. Silica gel or zeolites function as promoters of the reaction, giving rise to improved yields. ... 

The phosphonates (124), prepared from diazoalkanephosphonates by rhodium catalysed O-H insertion, undergo olefination to provide a synthesis of cyclic vinyl ethers (125).64 l-(Cyclopent-l-enylcarbonyl)vinyl-phophonates (126) have been used to synthesize fused ring systems containing two or three five-membered rings. The conversion of (126) into the aldehyde (127) followed by intramolecular olefinations leads ultimately to tricyclo[6.3.0.03.7]undecenone derivatives, e.g. (128).65... 

Photochemistry, in the shape of the de Mayo reaction, also features in a new synthesis of the linear fused triquinane hirsutene (20). Thus, photocycloaddition between dimedone and 2-methyl-cyclopent-2-enol was accompanied by spontaneous retro-aldolization of the intermediate adduct (17), leading to (l8a). McMurry coupling of the dione (l8b), derived from (l8a), followed by manipulation of the functional groups in (19) then led to (+)-hirsutene... 

Several syntheses of aldehydes and ketones based on the insertion of carbon monoxide have been reported. Reaction of norbornadiene with monosubstituted acetylenes in the presence of octacarbonyldicobalt gives the fused cyclopen-tenone (35) in high yield conjugate addition of a cuprate and vacuum pyrolysis completes a simple synthesis of 2,3-disubstituted cyclopent-4-enones (36) (Scheme 30)." Conjugated dienes with methyl iodide in the presence of the same cobalt reagent yield conjugated dienones (37) [equation (8)]." In a review of carbonyl insertion reactions, the same reagent is reported to catalyse the reaction... 

Photocycloadditions are also exploited in the synthesis of fused polycyclic compounds. Tetracyclic (4-hetera)cyclopent[, c]acenaphthylenes can be synthesized by light-induced cycloaddition of 4-alk-l-ynylcoumarins to 2,3-dimethylbut-2-enes (Scheme 6.33). Addition of triplet-excited 4-alk-l-ynylcoumarin to the alkene affords the triplet biradical, which undergoes 1,5-cyclization to cyclopentenylcar-bene. The latter undergoes electrocyclic ring closure and [1,9]-H shift to yield the product [37]. 

Fussed Five-monbered Rings.—comprehensive review of the chemistry of 4-acetoxycyclopent-2-en-l-one includes some approaches to fused cyclopent-anoids. The vinylcyclopropane rearrangement has been extended into a novel three-carbon annulation reaction by using the (1-trimethylsiIylcycIopropyI) cyclohexene (60), which gives 80% of the rearranged product (61). Obviously,... 

Figure 2.1 Selected cyclopenta-fused polycyclic arenes with subunit in bold (a) Cyclopent[h/]aceanthrylene, (b) dibenzopentalene, and (c) indeno[1,2-6]fluorene.

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