Abstract Photochemical [2+ 2] cycloaddition reactions under high pressure of cyclopentenone 1a or cyclohexenone 15, as well as of 1a or 3-substituted cyclopentenones 1b–d with cyclopentene (2), and of 1b with 3, 3-dimethyl-1-butene (18) are described. Whereas the pressure dependence on the regioselectivity in the photoaddition of 1a and 15 is very small, δδV≠ values up to 2.0±0.3 cm 3· mol-1 have been found for the ...
