endo-6-(Hydroxymethyl)bicyclo[3.1.0]hept-3-en-2-one esters and the photochemical challenge: [2+2] cycloaddition versus skeletal rearrangement

The photochemical reactivity of enantiomerically pure bicyclo[3.1.0]hept-3-en-2-ones has been examined to determine whether a [2+2] cycloaddition with an alkene could be achieved without interference from a photochemically induced skeletal rearrangement. The formation of a tricyclo[5.2.0.02,5]nonan-6-one was indeed the major pathway, exclusively providing the cis-anti-cis isomer. However, the formation of norbornenones was also observed, as was stereochemical scrambling at C6 in the starting material.

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((1S,5R,6R)-2-Oxobicyclo[3.2.0]hept-3-en-6-yl)methyl acetateC10H12O3[α]D23=+400 (c 1.00, CHCl3)Source of chirality: (+)-(1R,3S)-cyclopent-2-en-1,3 diol monoacetateAbsolute configuration: (1S,5R,6R)

((1S,5R,6R)-2-Oxobicyclo[3.2.0]hept-3-en-6-yl)methyl pivaloateC13H18O3[α]D23=+280 (c 0.25, CHCl3)Source of chirality: (+)-(1R,3S)-cyclopent-2-en-1,3 diol monoacetateAbsolute configuration: (1S,5R,6R)

((1S,5R,6R)-2-Oxobicyclo[3.2.0]hept-3-en-6-yl)methyl 3-butenoateC12H14O3[α]D23=+343 (c 1.00, CHCl3)Source of chirality: (+)-(1R,3S)-cyclopent-2-en-1,3 diol monoacetateAbsolute configuration: (1S,5R,6R)

((1S,5R,6R)-2-Oxobicyclo[3.2.0]hept-3-en-6-yl)methyl 4-pentenoate((1S,5R,6R)-2-oxobicyclo[3.2.0]hept-3-en-6-yl)methyl 4-pentenoateC13H16O3[α]D23=+661 (c 0.40, CHCl3)Source of chirality: (+)-(1R,3S)-cyclopent-2-en-1,3 diol monoacetateAbsolute configuration: (1S,5R,6R)

(2aR,3aS,5aS,5bR)-Hexahydro-1-oxacyclobuta[cd]pentalen-4(2H)-oneC8H10O2[α]D23=+50 (c 0.150, CHCl3)Source of chirality: (+)-(1R,3S)-cyclopent-2-en-1,3 diol monoacetateAbsolute configuration: (2aR,3aS,5aS,5bR)