Oxidative photocyclization

A fully stereocontrolled preparation of 499 has recently been completed by Paquette and coworkers When triene 508 was treated with /7-toluenesulfonyl-acetylene, highly stereoselective addition from the endo surface occurred to deliver an adduct which was directly epoxidized (Scheme XLI). The proximity of the two jc bonds in 509 allows for ready photocyclization. Oxidative cleavage of 510 afforded... 

An explanation for the non-formation of aromatic products under entry 4 in Table 1 is that hydrogen atom abstraction from the corresponding dihydro derivative would lead to a /3-thio substituted radical, which would rapidly eliminate a thiyl radical, which in turn would readily polymerize. The last entry in the table is also surprising in that the cyclization takes place on the naphthalene /3-position when an a-position is free. The use of this photocyclization-oxidation sequence for the construction of heterohelicenes has been reviewed (71ACR65). 

This electrocyclization leading to a 1,6-fused cyclohexadiene also takes place with polyenes. A photochemical example from the vitamin A field is exemplified by the conrotatory phot clization of the (7Z)-isomer (IW) of retinal (185) to the bicyclic derivative (187). The photocyclization procedure has also been used in the aromatic series. - Thus the photocyclization-oxidation reaction of l-phenyl-4-(2 -thienyl)-1,3-butadiene (188) gave 4-phenylbenzo[b]thiophene (189). Similarly, the 3 -thienyl analogue (190) afforded 7-phenylbenzo[b]thiophene (191), the reaction exhibiting high selectivity for cyclization to the logically more reactive thiophene nucleus. ... 

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