Preparation of 2‐substituted 1,3,4‐thiadiazoles and mesoionic 1,3,4‐thiadiazole[3,2‐c]quinazolines from 4H‐3,1‐benzothiazine‐4‐thiones or 3‐amino‐4(3H)‐quinazolinethiones

Abstract

Reaction of 4H-3.1-benzothiazine-4-lhiones 1 with S-methylisothiosemicarbazide hydroiodide yields bamino-1.3.4-thiadiazoles 2 insteud of the espected 1.2,4-triazolo[3,2-c]quinazolines. Structures of compounds 2 have been established by mcans of '>C-N M R analysis and X-ray crystallography. 3-Arnino-4(3/f )-quinazolinethiones 4 react with ketcne dithioacetals 5 to give the new mesoionic t,3,~thiadiazolo[3.2-c]quinazolinylium-2-methanides 6 which undergo ring opening by the action or hydrochloric acid to givc z-methylene-1,3.4-thiadiamles 7. Compounds 2 are convcrted into the corresponding oxygen analog 9 by treatment with hydrochloric acid.The reaction of primary amines with 4H-3,1-benzothiazine-4-thiones to give 4(3H)-quinazolinethiones is well-documented. In this context, we have studied the reaction with several bifunctionalized nitrogen nucleophiles giving fused quinazoline derivatives'). On the other hand, we have reported the preparation of mesoionic 1,2,4-triazolo[3,2-c]-quinazolines from 3-amino-4(3H)-quinazolinethiones and aromatic isothiocyanates" or diaryl~arbodiimides~). We report here attempts to synthesize 2-substituted S-aryl-l,3,4-thiadiazoles by two approaches: a) from 4H-3,l-benzothiazine-4-thiones and S-methylisothiosemicarbazide to give 2-amino-l,3,4-thiadiazoles and b) via mesoionic 1,3,4-thiadiazolo[3,2-c]quinazolines to give 2-methylene-l,3,4-thiadiazoles.In general, the most common procedure for the synthesis of 5-substituted 2-amino-l,3,4-thiadiazoles is the acylation of thiosemicarbazide or derivatives followed by dehydration under acidic conditions4). However, no general useful procedure for the preparation of 2-methylene-l,3,4-thiadiazoles has hitherto been reported, it has only been briefly mentioned 5,6) the preparation of mesoionic heptafulvenes derived from the 1,3,4-thiadiazole ring system.Our approach to the preparation of 5-aryl-2-amino-1,3,4-thiadiazoles is based on the ring opening of 4H-3,l-benzothiazine-Cthiones by the action of thiosemicarbazide derivatives. Thus, the reactions between 1 and S-methylisothiosemicarbazide hydroiodide7) in ethanolic solutions in the

Synthesis, Antibacterial, and Antioxidant Evaluation of Novel Series of Condensed Thiazoloquinazoline with Pyrido, Pyrano, and Benzol Moieties as Five- and Six-Membered Heterocycle Derivatives

“…The 2,2-diphenyl-1-picryl-hydrazyl (DPPH) radical scavenging activity (RSA) evaluation is a standard assay in antioxidant activity studies and widely used as a rapid technique for assess the ability of compounds or extracts as scavengers of free radicals and hence evaluate the antioxidant activity of synthetics compounds [ 36 , 37 , 38 ]. In this work, the interaction of the synthesized compounds with stable DPPH free radical indicates their free radical scavenging ability.…”

“…However, α,α dioxoketen dithioacetals are a pivotal starting material in synthetic chemistry due to their synthetic precursors that generate various heterocycle compounds [ 34 , 35 ]. The high potential ability of these intermediates to behave as polarized push-pull interacting systems on C–C double bonds makes them adaptable starting materials for the synthesis of a variety of novel heterocyclic compounds [ 36 ]. Based on the above discussion and in connection with our previous works on α,α dioxoketen dithioacetals as an efficient starting material [ 37 , 38 ], herein we report the novel convenient synthetic method for some new thiazolo[2,3-b]quinazolines 4 ( a – e ), pyrido[2′,3′:4,5]thiazolo[2,3-b]quinazolines 5 ( a – e ), 6 ( a – e ), and 7 ( a – e ), pyrano[2′,3′:4,5]thiazolo[2,3-b]quinazolines 8 ( a – e ), and benzo[4,5]thiazolo[2,3-b]quinazoline 9 ( a – e ) derivatives.…”