The Fráter's synthesis is the only one stereoselective. The cis-configuration as well as the exocyclic position of the double bond were secured by the thermal ene reaction of the b-(alkenoxy)-acrylate yielding the 3-oxabicyclo[3.3.1] nonane derivative A.

Reagents : a) CH₃MgI; b) CH₂=CHCHO; c) NaBH₄; d) methyl propiolate; e) toluene, sealed tube, 300°C; f) NaOH, H₂O, MeOH; g) 2 LDA, THF puis H₃O+; h) MsCl, py; i) Zn NaI, DME, j) 2 MeLi, Et₂O.
 

The global yield is 5%, because there are more 50 % of side product formed during the thermal H-ene reaction.

 

If you desired to see the Fráter's key step amelioration, proposed by our team, click here.