Allene + enone, photocycloaddition

When the stereogenic center is a chiral allene, intramolecular 12 + 2 photocycloaddition produces optically active adducts [117]. A recent reinvestigation of the photoreactivity of optically active allenic enones (156) has shown that the adducts 157a and 158a are obtained with 100% asymmetric induction [146] (Scheme 24). 

Allenes behave as somehow special olefins in such photocycloadditions 476a,b,477) Additions of enones to allene have been used as key step in the synthesis of the alkaloids annotinine 478a) (4.65) and chasmanine (4.66) 478b). 

Photocycloaddition of allene to cyclohexenone (341) gave the (3,y-enone (342), which reacted with vinyl magnesium bromide to produce the tertiary alcohol (343) in 79% yield. When the compound (343) was treated with KH and 18-crown-6 in THF at room temperature for two hours and quenched with aq. NH4C1, the cyclobutene (344) was obtained. The thermal ring opening of the cyclobutene (344) proceeded in toluene in a sealed-tube at 180 °C for twelve hours to give a readily separable 5 1 mixture of the civ-olefin (345), and the trans-olefin (346) respectively in 95 % yield. Moreover, (345) could be converted to a mixture of (346) and (345) in the ratio of 10 1 by irradiation. The compounds (345) and (346) possess the skeleton of the germacranes (347), (348) and (349) 122). 

The [2 + 2] photocycloaddition reaction of enones with allenes was first reported in 1966. A diradical intermediate is formed from a triplet enone via an exciplex. The triplet diradical cyclizes to the product after spin inversion to the singlet state [31,32]. 

Irradiation of cyclopent-2-enones in the presence of allenes affords mixtures of 6- and 7-methylenebicyclo[3.2.0]heptan-2-ones, the latter regioisomers being formed preferentially.4,5 From cyclohex-2-enones and allene, 8-methylenebicyclo[4.2.0]octan-2-ones are formed regiose-lectively.6 In contrast, six-membered a,/i-unsaturated lactones and allene afford mixtures of regioisomers.7 Mechanistic arguments for the stereochemical outcome of allene photocycloadditions to steroidal enones have been discussed.8... 

Recently, Carreira and coworkers126 obtained very high asymmetric induction (83-100%) in intramolecular [2 + 2] photocycloadditions of 1,3-disubstituted allenes with enones 289 and 290 (Scheme 63). 

The use of silica gel surfaces to modify photoprocesses continues to be of interest. The present report deals with photoaddition of alkenes to the steroidal enone (44).It is presumed that, in the adsorbed state on silica gel, the steroid will present the normally hindered side, i.e., the g-face is attacked by the alkene and the a-face is adsorbed on to the gel. The photoadditions were compared with those carried out in methanol. Thus with allene the two photoproducts (45) and (46) were obtained in 90 and 10% yields, respectively, at -78 °C in methanol, while on silica gel, both at -78 °C and at ambient temperature, the same two products were obtained in 46 and 53% yield. Similar results were obtained using ethene and cyclopentene. The photocycloaddition of ethyne and propyne to progesterone and testosterone in solution yields the cycloadducts (47) and (48), with a preference for the former mode of addition being observed. These results are in conflict with those of de Mayo and co-workers. ... 

Enones also undergo cycloadditions to allene under conditions of surface photochemistry using silica gel as a medium. Several examples have been reported, among them is the photocycloaddition of cyclohexenone to allene63. 

The photocycloaddition of allene-tethered enones, like 344, affords the expected adducts 345 in 67-90 % yields. The reaction proceeds with an enantioselectivity of > 99... 

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