Use of arene 1,3-substituents to control cyclopropane ring formation during meta photocycloaddition of ethenes to the benzene ring

Abstract

The 3-trifluoromethyl derivatives of benzyl alcohol, benzyltrimethylsilane and phenoxytrimethylsilane undergo 2,6-photocycloaddition to cyclopentene to give the 1,11-disubstituted-tetracyclo[6.3.0.0 2,11 .0 3,7 ]undec-9-enes 7, 10, 11 and 12. This specificity in isomer formation is considered to originate from an intramolecular attractive interaction between the 1,3-substituents on the arenes during the addition of the ethene and this causes an asymmetric distortion of the C 6 ring which then controls the cyclopropane ring formation in the adduct. Similar reaction of 3-trifluoromethyl-phenol, -benzyl alcohol and -benzyltrimethylsilane, with vinyl acetate yields the 2-trifluoromethyl-endo-6-acetoxytricyclo[3.3.0.0 2,8 ]oct-3-enes 16, 17 and 18 respectively, showing that the directing influence of substituents on the arene dominates any effect of the acetoxy group on the photocycloaddition process. The control by Si ؒ ؒ ؒF interaction is not, however, significant during the intramolecular photocycloaddition of the bichromophores 19 and 20 and only the 1,6-bridged dihydrosemibullvalene 24 and products derived from ortho cycloaddition are respectively formed.

Arene–Alkene Cycloaddition

“…For example, a trifluoromethyl group and a hydroxymethyl group influenced the meta cycloaddition outcome by hydrogen bonding. 122 This reaction was exploited in the total synthesis of natural products. Wender and Dreyer 123 converted a meta cycloadduct 168 into a [3.3.3]propellane core 169, en route to the synthesis of (±)-modhephene 171 (Scheme 52).…”

“…Substituents on the aromatic ring with a 1,3-relationship may mutually interact at the transition state, thus influencing the overall regioselectivity. For example, a trifluoromethyl group and a hydroxymethyl group influenced the meta cycloaddition outcome by hydrogen bonding …”

A Photochemical Approach to the Gelsemine Skeleton

“…The desired meta photoadduct ( 7 ) was isolated in 38% yield after chromatographic separation, in addition to two other minor components ( 8 and 9 ). We assume that these minor components are derived from an initial ortho photocycloaddition process, a result which was also reported to have occurred during the irradiation of 3-(3‘-trifluoromethylphenoxydimethylsilyl)prop-1-ene . It is likely that both of the minor components are derived from a common cycloocta-1,3,5-triene intermediate ( 11 ), which forms as the result of a facile thermally induced electrocyclic ring opening of the initial ortho photoadduct ( 10 ).…”

“…originally reported the intramolecular meta photocycloaddition of 3-(phenoxydimethylsilyl)prop-1-ene ( 6 ), but unfortunately described the wrong (1,7-bridged) photoadduct. A recent publication by Gilbert, which investigated the mechanistic properties of silicon-tethered meta photocycloaddition reactions, confirmed the correct structure to be the result of 2‘,6‘ cycloaddition and gave 1,6-bridged adduct 7 . In attempting the photolysis reaction of 3-(phenoxydimethylsilyl)prop-1-ene, we isolated three different components (Scheme ).…”