Our synthesis involves three steps for building the C17 skeleton, permits the preparation of all eight possible pairs of isomers, and allows variations to be introduced at C-3, C-7, and C-11 of the juvenile hormone. Stereospecific syntheses, on the other hand, do not, of course, allow preparation of the other isomer pairs, nor can other substituents be introduced at all the positions named above.
Our synthesis starts from simple materials. From y-butyrolactone and ethyl acetate or propionate are prepared two aacyl-y-butyrolactones which give the a-bromo ketones on ketone fission in the presence of hydrogen bromide. The carbonyl group is then protected as ethylene ketal, and the bromo ketals are treated with triphenylphosphine, which gives the triphenylphosphonium salts needed for the first two carbonyl-olefinations.
The phosphorane (I) is obtained from the phosphonium bromide and sodamide in liquid ammonia and gives a 69 % yield in the Wittig reaction with 2-butanone. According to gas-chromatographic analysis the resulting mixture of cistrans-isomers of the ethylene ketal contains 63 % of the cisand 37 % of the trans-compound; 42 % of the cis-Clo ketal