A new synthetic approach to the bicyclo[3.2.1]octane ring system, based on intramolecular photocycloaddition involving cyclopentane-1,3-dione enol esters

Abstract

Irradiation of 1:1 mixtures of enol acetates derived from 4-prop-2-enylcyclopentane-1,3-diones [i.e.(9), (23), (27), (32a), (32b)], leads to 6-acetoxytricyclo[3.2.1.0^3,6]octan-2-ones [i.e.(12), (24), (28), (33a), (33b)] in high yield (>70%). The adducts result from regioselective intramolecular [2 + 2] cycloaddition in the 5-prop-2-enyl enol acetate isomers [e.g.(10)], suggesting that equilibration between the isomeric enol acetates [e.g.(10) and (11)] is rapid during the irradiations. Fragmentation of the 6-acetoxytricyclo[3.2.1.0^3,6]octan-2-ones, either directly [e.g.(12)→(16); KOH–EtOH] or via the corresponding acetoxy-mesylates [e.g.(24)→(25)→(26)] provides a facile route to a range of substituted bicyclo[3.2.1]octane ring systems.

Irradiation of the isomerically pure enol acetates (39), containing a methyl substituent at C-2, by contrast produce largely (ratio 3:2) 3-acetoxytricyclo[3.2.1.0^3,6]octan-7-one adducts (43) resulting from the alternative mode of intramolecular cycloaddition in the enol acetate (see Scheme 1); these adducts are also elaborated to bicyclo[3.2.1]octenones [e.g.(45)].