Hydroquinazolines Preparation and Properties

2-Dihydroquinazoline is not known but derivatives of 1,2-dihydro-4(3H)quiriazolinones with a substituent on N-3 have been prepared 

4- and 3,4-Dihydroquinazolines are tautomeric but any attempts to prepare the former w ithout a 1-substituent have led to the latter. The greater stability to proto tropic change of 1,2-dihydronaphthalene over 1,4-dihydronaphthalene is also found in 3,4-dihydroquinazoline. Earlier claims to the preparation of l,4-dihydroquinazolines ° were erroneous and based on incomplete experimental data. The first 1,4-dihydroquinazoline was prepared as recently as 1961. 1-Methyl and l-benzyl-l,4-dihydroquinazolines were obtained from o-methylamino-and o-benzylamino-benzylamines (42) by formylation and ring closure. Attempts to remove the benzyl group gave 3,4-dihydroquinazoline. These 1,4-dihydro compounds are susceptible to oxidation, and attempts made to prepare 1,2-dimethyl-1,4-dihydroquinazoline from o- 

4-Dihydroquinazolines are stable and a large number have been prepared. They have been synthesized by reductive cyclization of acyl derivatives of o-nitrobenzylamines, and by acylation of o-amino-benzylamines followed by ring closure. The ring closure can be effected by heating with anhydrous zinc chloride ° or by distillation.  

Catalytic reduction of quinazolines unsubstituted in position 4 using palladium-charcoal, palladium on calcium carbonate, Raney nickel, or Adam s platinum has been used for preparing 3,4-dihydro- 

4- Dihydroquinazolines are also products of the reaction of quin-azoline with nucleophilic reagents (see Section II,B). 

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