Intramolecular Diels-Alder reactions, 2 - Studies on the synthesis of natural products with bicyclo[4,4.0]decene skeleton: Synthesis and uncatalysed intramolecular Diels-Alder reactions of the decatrienone substrates

Starting from enones 7 and 8 siloxydienes 9 and 12 were synthesized. Cyclop ropanation and subsequent alkylation with sorbyl bromide provided tetrasubs tituted siloxy-cyclopropanes 15 and 16 in good overall efficiency. Their ri ng cleavage with fluoride reagents gave 1,7,9-decatrienones 5 and 17; the l atter was converted into the protected compound 6. The thermal intramolecul ar Diels-Alder reactions of 5 and 6 were compared with those of related tri enones 3 and 4. Octalones 1, 2, 21, and 22 were formed as mixtures of three or four diastereomers. Thus, for the stereoselective construction of natur al products such as alpha-eudesmol or dihydromevinolin alternative reaction conditions for the cycloaddition step have to be developed.