Acylation of p-toluidine (I) with 3-chloropropionyl chloride afforded the beta-chloroamide (II), which was cyclized to dihydroquinolinone (III) by treatment with AlCl3 at 120 C. After protection of the N atom of (III) as the tert-butyl carbamate (IV), reduction of the 2-oxo group by NaBH4 in ethanol at -25 C, followed by acidic quench with ethanolic HCl, afforded the N-Boc-2-ethoxy derivative (V). The tandem Mannich-Michael cyclization of the alpha-ethoxycarbamate (V) with the (silyloxy)diene (VII) (derived from 1-penten-3-one (VI) in the presence of trimethylsilyl triflate) furnished the benzoquinolizinone (VIII) as a mixture of diastereoisomers. Finally, oxidation of this mixture with mercuric acetate afforded the desired delta-4 unsaturated compound.
The reaction of 6-methyl-1,2,3,4-tetrahydroquinolin-2-one (I) with Lawesson's reagent in refluxing toluene gives 6-methyl-1,2,3,4-tetrahydroquinolin-2-thione (II), which is condensed with ethyl vinyl ketone (III) by means of K2CO3 or DBU in THF to yield the addition product (IV). The reaction of (IV) with dimethyl sulfate in refluxing toluene affords the thioiminium ion (V), which, without isolation, is cyclized to the target compound by reaction with DBU in refluxing toluene.