Methylenecyclopentanes 2-vinyl

These methylenecyclopentane annulations may also be applied to conjugated dienones, e.g., 4-methyl-4-[2-(trimethylsilylmethyl)-2-propeny[]-3-vinyl-2-cyclohexenones43 44. Again, fluoride ion was used successfully to generate either octahydro-5//-inden-5-ones or hexahydro-2( 1H (-pentalenones. 

Functionalized exo-methylenecyclopentanes can also be obtained by ruthenium-catalyzed intramolecular C-H bond activation [15]. l-(2-Pyridyl)-, l-(2-imidazolyl)-, and l-(2-oxazolyl)-l,5-dienes proceeded in a regiospecific manner to give five-membered ring products (Eq. 10). The proposed mechanism initially involves the activation of the vinylic C-H bond of the exocyclic C=C bond assisted by preliminary coordination of the nitrogen atom, followed by intramolecular insertion of the other C=C bond (see Eq. 6). 

Asymmetric [3 + 2] cycloaddition of 2-(sulfonylmethyl)-2-propenyl carbonate 56 with methyl acrylate or methyl vinyl ketone is also catalyzed by the palladium-ferrocenylphosphine 8b complex to produce methylenecyclopentane derivatives with up to 78% ee (Scheme 2-36) [52]. 

The [3-1-2] methylenecyclopentane annulation of [(trimethylsilyl)methylene]-cyclopropane dicarboxylates with unactivated and electron-rich alkenes (vinyl ether, vinyl thioether, or vinyl silyl ether) are efficiently photocatalyzed by butyl disulfide or bis(tributyltin) [78]. 

Other 1,3-dienes, such as 1,3-butadiene, isoprene and methyl-2,4-pentadienoate, either do not react with methylenecyclopropanes or yield only 3-vinylmethylene-cyclopentane derivatives exclusively (Table 10 and Eq. 114). Quite unexpectedly, methyl-2,4-pentadienoate reacts only at the terminal C=C bond, giving a vinyl-methylenecyclopentane in poor yield 224) (Eq. 114). 

The carbene arising from 4-vinylcyclohexene was considered to be a (3-vinylcyclopen-tyl)carbene which can give rise to only one methylenecyclopentane and four isomeric vinyl-bicyclo[3.1.0]hexanes. The absence of products which correspond to the internal addition of the carbene to the vinyl group may be attributed to the slowness in the rotation of the substituent group to a favorable geometry in the lifetime of the excited state. 

Cycloaddition.2 In the presence of this nickel catalyst, methylenecyclo-propanes undergo an unusual cycloaddition across carbon-carbon double bonds. Thus methylenecyclopropane (1) when heated in a sealed tube (60°, 48 hrs.) with excess methyl acrylate in the presence of bis(acrylonitrile)nickel(0) gives the 1 1 adduct, methyl 3-methylenecyclopentanecarboxylate (2), in 82% yield. Methyl vinyl ketone or acrylonitrile is also a suitable substrate. The reaction provides a useful synthesis of methylenecyclopentane derivatives. 

The atom-transfer cyclizations of 2-iodo-6-heptyne amides substituted with a chiral sultam derivative yield E Z mixtures of vinyl iodides38. The reductive dehalogcnation with tributyltin hydride and subsequent desilylation gives two diastereomeric methylenecyclopentanes in a ratio of 90 10. 

The corresponding methylenecyclopentanes are readily formed from a wide variety of activated alkenes, such as unsaturated esters, lactones, and nitriles, enones, vinyl sulfones, and nitro-aUcenes.4 The breadth of utility of this reaction is exemplified below by its use as key steps in syntheses of albene (eq 3), brefeldin A (eq 4), hirsutene (eq 5), the core spirocarbocyclic rings of the ginkgolides (eq 6), cephalotaxine (eq 7), and kem-panes (eq 8). The use of doubly activated alkenes, as in eq 8, is of particular advantage in the annulation of 2-cycloalkenones. ... 

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