Substituent Effects on the Photorearrangements of 4-Alkoxy-4-carbomethoxy-3-methoxy-2,5-cyclohexadien-1-ones

Article abstract of DOI:10.1021/jo00022a033

Enantiomerically enriched 4-carbomethoxy-3,4-dimethoxy-2,5-cyclohexadien-1-one (1) undergoes a completely diastereoselective (but not enantiospecific) type A photorearrangement at 366 nm to give 6-carbomethoxy-4,6-dimethoxybicyclo<3.1.0>hex-3-en-2-one (2) in 81 percent isolated yield.A chiral NMR shift study indicates that 1 undergoes partial loss of absolute configuration at C(4) during irradiation.Bicyclohexenone 2 is photostable at 366 nm but rearranges to its diastereoisomer 3 and phenols 4 and 5 at > 300 nm.It is proposed that ground-state conformational effects control the type A photoreactivity of 1 and that rearrangement to 2 occurs in accord with the principle of least motion in the transition states for rearrangements to diastereomerically related zwitterions 1a and 1b (Scheme III).The intramolecular 2 + 2 photocycloaddition of 4-carbomethoxy-3-methoxy-4-(1'-oxa-3'-butenyl)-2,5-cyclohexadien-1-one (9) gives a mixture of 7-oxatricyclodecenones 10 (58percent) and 11 (18percent), without interference from type A photoreactivity.