Novel synthesis of degradation products of carotenoids, megastigmatrienone analogues and blumenol-A

Synthesis of 4-alkylidene-3,5,5-trimethylcyclohex-2-enones 7 has been achieved utilising 1,4-conjugate dehydrobromination of allylic bromides 5 as a key step. This chemical transformation is applied to the synthesis of degradation products of carotenoids: megastigmatrienones 7e/1–4, 4-methylene-3,5,5-trimethylcyclohex-2-enone 7a, 4-(3-hydroxybutylidene)-3,5,5-trimethylcyclohex-2-enone 9, 1,3,7,7-tetramethyl-2-oxabicyclo[4.4.0]dec-5-en-9-one 10a–b and 3,4,7,8-tetrahydro-4,4,7-trimethylnaphthalen-2(6H)-one 15. A novel photoisomerisation of 4-[(Z )-3-acetoxybut-2-enyl]-4-hydroxy-3,5,5-trimethylcyclohex-2-enone 19 to 4-[(E )-3-acetoxybut-2-enyl]-4-hydroxy-3,5,5-trimethylcyclohex-2-enone 20 enables us to synthesise blumenol-A 21.