Bicyclo hexen-3-ones

Some research groups have exploited the intrinsic reactivity of the bicycloadducts formed in the pyran-2-one Diels-Alder reactions. An example of this strategy is shown in Scheme 33, where the pyran-2-one decorated with indole functionality 255 was reacted with an assortment of electron-rich and electron-poor dienes <2000T5205>. The richly functionalized bicyclo[2.2.2]adduct 256 was then subjected to mild aminolysis to produce tetrasubstituted hexene 257 as single diastereoisomers. 

Cyclopropene can also be used as the aUcene component and affords bicyclo[3.1.0]hexen-2-ones upon reaction with alkyne dicobalt octacarbonyl complexes in the presence of NMO (Scheme 250). Vinyl ethers and vinyl esters serve as ethene equivalents in Pauson-Khand reactions. For example, reaction of vinyl benzoate with complex (169) furnished cyclopentenone (170) (Scheme 251). This reaction was used in a synthesis of (-l-)-taylorine and nortaylorine. Allenes participate in intermolecular Pauson-Khand reactions affording alkylidene-substituted cyclopentenones (Scheme 252). ... 

Jullien and coworkers studied the cyclic voltammograms of variously substituted bicyclo[3.1.0]hexen-3-ones (Table 9) and found that the half-life times (ti/2 )are generally very short ( < lO"" s) except when the substrates were phenyl-substituted at C(4). In such cases the radical anions are very stable( 6 s) owing to the greater charge delocalization. A similar trend was found in some non-fused cyclopropyl ketones systems studied by House and coworkers. ... 

TABLE 9. Reduction potentials of substituted bicyclo [3.1.0]hexen-3-ones ... 

The method was used by Meinwald for the synthesis of the highly strained bicyclo[2.1.1]hexene-2 from the tosylhydrazone of bicycio[2.1.1]hexane-2-one (4). In this case the Hofmann-elimination sequence failed. 

The thermal reaction, at room temperature, of A -bicyclo[2A0]hexene (277) gave, besides polymeric material, a dimer now identified as 2,5-dimethylenetricyclo-[4,2,2,0 ]decane (278). Reaction of (277) with irons, frans-dimethyl-2,5-hexadiene gave the olefin (279) and reduction of both (278) and (279) gave cis-2,5-dimethyltri-cyclo[4,2,2,0 ]decane. The reactive double bond in (277) differs from that usually associated with thermal olefin dimerizations. In most cases, such reactivity has been associated with double bonds which are torsionally strained. In (277) the torsion angle about the double bond must be zero and the reactivity must be associated with angular deformation at the olefinic carbons. Addition of ketene to (277) gave 3-methyl-6-methylenecyclohex-2-ene-l-one. In both reaction with ketene and dimerization, [2,2,2]propellanes would appear to be reasonable intermediates. 

In 2006, Gagosz reported the cycloisomerisation of 5-en-2-yn-l-yl acetates to acetojy bicyclo[3.1.0]hexenes at room temperature. Propargylic acetates bearing different substitutions were successfully cyclised in the presence of 1 mol% of complex XXX in dichloromethane. The reaction proceeded smoothly at room temperature and short reaction times were required. The authors showed that these complexes could be used as precursors to synthesise cycloenones when they were treated with catalytic amounts of base in methanol. In addition, when the solvent was changed from dichloromethane to methanol a cyclohexene molecule was obtained, which could be further converted to 2-cyclohexen-l-one. This product is a regioisomer of the cyclohexenones obtained using the first route (Scheme 16.66). 

A further alternative route to the intermediate (8 R = H) has been outlined in a patent by the Upjohn group. This synthesis starts with the bicyclo[3,l,0]hexenal (36), which is converted into (8 R = H) via the key intermediate (37) as outlined in Scheme 6. The route is closely similar to one of Corey s earlier schemes to total PG synthesis and has been applied to the synthesis of the and Faa PG s in addition to the Eg- and F2 -series. 

Buzas, A., Gagosz, F. (2006). Gold(I) catalyzed isomerization of 5-en-2-yn-l-yl acetates an efficient access to acetoxy bicyclo[3.1.0]hexenes and 2-cycloalk en-l-ones. Journal of the American Chemical Society, 128,12614-12615. 

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