3‐Alkylthio and 3‐aminopyrazolo[3,4‐D]pyridazines. Ring contraction of pyridazino[4,5‐e][1,3,4]thiadiazines via extrusion of sulfur

Abstract

Reaction of 4‐chloro‐2‐methyl‐5‐(1‐methylhydrazino)‐3(2H)‐pyridazinone (1) with carbon disulfide followed by alkylation yielded 2‐alkylthio‐4H‐pyridazino[4,5‐e][1,3,4]thiadiazine derivatives 2. Oxidative cyclization of 5‐(4‐substituted 1‐methylthiosemicarbazido)‐3(2H)‐pyridazinone derivatives 4 with N‐bromosuccinimide also gave 2‐substituted amino‐4H‐pyridazino[4,5‐e][1,3,4]thiadiazine derivatives 5. Heating of 2 and 5 resulted in ring contraction to afford the corresponding pyrazolo[3,4‐d]pyridazine derivatives 6, 7 via sulfur extrusion. A possible mechanism for the desulfurization reaction is discussed, comparing with a structural difference between a type of pyridazino[4,5‐e][1,3,4]thiadiazine (2,5) and another one (9,11,13,15).

Synthesis of Pyridothiazepines via a 1,5‐Dipolar Cycloaddition Reaction between Pyridinium 1,4‐Zwitterionic Thiolates and Activated Allenes

“…Based on previous reports [11] and our experimental results, we proposed a possible mechanism for the [5 + Scheme 2. [5 + 2]-Dipolar cycloaddition reaction using activated allenes.…”

Synthesis of 1,3,4-Thiadiazines and 1,3,4-Oxadiazines

“…The reaction of 6-chloroquinoline-5,8-dione hydrochloride (12) with thiosemicarbazide or semicarbazide gave 2-amino-4H-1,3,4-thiadiazino[6,5-s]quinoline-5,lO-dione (13) or 2amino-4H-l,3,4-oxadiazino[6,5-~quinoline-5,1O-dione (14), respectively (Scheme 6).8 The reaction of 2,3-dichloronaphthoquinone (15) (27), which was cyclized into the imidazo (31), while the reaction of compound (30) with benzyl isothiocyanate afforded the adduct (32), whose reaction with sodium hydroxide …”

An Efficient Route to Pyridazino[4, 3‐e][1, 3, 4]thiadiazines

“…These compounds have been described as being potential inhibitors of cyclic nucleotide phosphodiesterase , dyestuff , and precursors of herbicides . Despite their importance from pharmacological and synthetic point of views, comparatively few methods for their preparation have been reported , and among those structural isomer, pyridazino[4,3‐ e ][1,3,4]thiadiazine ( 3 ) has been largely overlooked. The only undisputed example of this heterocyclic compound has been synthesized in two steps by Oda and his co‐workers through cyclocondensation of 6‐chloro‐2‐methyl‐5‐(1‐methylhydrazino)‐3(2 H )‐pyridazinone with benzyl isothiocyanate in the presence of sodium hydroxide .…”