Polycyclic nitrogen compounds. Part iii. Synthesis of 3,3a‐dihydrothiazolo[3,4‐α]quinoxalin‐4‐ones
New tricyclic quinoxalinone skeletons with bridge‐head nitrogen atoms and containing sulphur in a fully‐reduced five‐membered ring C were obtained. 3,3a‐Dihydrothiazolo[3,4‐α]quinoxalin‐4‐ones I‐III were prepared by metal‐acid reductive cyclisation of N‐(nitrophenyl)‐ and N‐(dinitrophenyl)thiazolidine‐4‐carboxylic acids IVa,b,c. Attempts to obtain the skeleton by selective hydrogen transfer reductive cyclisation of the corresponding esters Va,b,c were unsuccessful.