Thiazolium-4-olates cycloaddition

Dipolar cycloaddition reactions of thioisoraunchnones (l,3-thiazolium-4-olates) have not been as extensively studied as those of munchnones (l,3-oxazolium-5-olates) despite offering rapid access to novel heterocyclic compounds. The cycloaddition of the thioisomunchnone (52) with trans-P-nitrostyrene results in the formation of two diastereoisomeric 4,5-dihydrothiophenes (53) and (54) via transient cycloadducts. These cycloadducts then undergo rearrangement under the reaction conditions <96JOC3738>. 

Reaction with acetylenic dipolarophiles represents an efficient method for the preparation of 2,5-dUiydrothiophenes. These products can be either isolated or directly converted to thiophene derivatives by dehydration procedures. The most frequently used dipolarophile is dimethyl acetylenedicarboxylate (DMAD), which easily combines with thiocarbonyl yhdes generated by the extrusion of nitrogen from 2,5-dihydro-1,3,4-thiadiazoles (8,25,28,36,41,92,94,152). Other methods involve the desUylation (31,53,129) protocol as well as the reaction with 1,3-dithiohum-4-olates and l,3-thiazolium-4-olates (153-158). Cycloaddition of (5)-methylides formed by the N2-extmsion or desilylation method leads to stable 2,5-dUiydrothiophenes of type 98 and 99. In contrast, bicyclic cycloadducts of type 100 usually decompose to give thiophene (101) or pyridine derivatives (102) (Scheme 5.37). 

In contrast to thiomiinchnones, thioisomiinchnones (5) (l,3-thiazolium-4-olates) have received considerable attention over the past three decades, and a rich array of 1,3-dipolar cycloaddition chemistry is described in Section 10.2.5. These heterocycles were initially constructed by Potts (1,53,57) from thioamides and a-halo acid chlorides, an example of which is shown in Scheme 10.17 (58). 

Somewhat earlier, Souizi and Roberts (63) reported mesoionic heterocycle interconversions leading to l,3-thiazolLum-4-thiolates, l,3-thiazolium-4-olates, and l,3-dithiolium-4-thiolates from l,3-dithiolium-4-olates. This elegant chemistry, which involves cycloaddition reactions, is presented in Section 10.3.8. 

Avalos and co-workers (220-228) extensively investigated the 1,3-dipolar cycloaddition chemistry of 2-aminothioisomiinchnones with both acetylenic and olefinic dipolarophiles. For example, sugar derivatives of the mesoionic imi-dazo[2,l-Z7]thiazolium-3-olate system react regioselectively with a variety of acetylenic dipolarophiles [DMAD, diethyl azodicarboxylate (DEAD), methyl propiolate, ethyl phenylpropiolate] to give the corresponding imidazo[l,2-a]pyr-idin-4-ones (e.g., 323) following sulfur extrusion from the not isolable cycloadducts (220). Similarly, these thioisomtinchnones react with diethyl azodicarboxylate and arylisocyanates in the expected fashion (221), and also with aryl aldehydes to form episulfides (222). 

Addition of l,3-thiazolium-4-olates to (l-alkynyl)carbene tungsten complexes lb,n affords thiophene and/or pyridone complexes, 64 and 65, by extrusion of sulfur and isocyanate, respectively, from the [3+2] adduct formed initially (Scheme 21).97 [3+2] cycloaddition reactions of azomethine... 

Mesoionic thiazoles represent masked ylides which can undergo cycloaddition reactions. 1,3-Thiazolium-5-olate (73) and thioisomunchnone (74) can be considered as an azomethine ylide dipole and a thiocarbonyl ylide dipole, respectively. The cycloaddition chemistry of these compounds have been studied and their utility for the synthesis of heterocycles demonstrated <94S993>. 

Imidazo[2,l-h]thiazolium-3-olate 160 (Section VI,C Scheme 47) underwent rapid cycloaddition with acetylene mono- and dicarboxylic esters to give the imidazo[l,2-a]pyridin-3-yl acyclo C-nucleoside 208 through sulfur extrusion from the intermediate 207 (91MI7) (Scheme 62). 

Dimethyl-thiopropanal geht mit 2-Methyl-3-phenyl-l,3-thiazolium-5-olat eine Cycloaddition und anschlieBende Cycloreversion ein, wodurch unter formaler Ringoffnung des Aus-gangs-l,3-Thiazols Thioessigsaure-[N-(3,3-dimethyl-l-butenyl)-anilid] entsteht1621 ... 

Andercrseits reagieren l,3-Thiazolium-4-olate als Thiocarbonyl-ylidc und bilden mit Hetero-cumulenen die entsprechenden [3 + 2]-Cycloaddukte die Reaktion laBt sich auch als [4 + 2J-Cycloaddition eines Azabutadiens verstehen970 ... 

The dipolar cycloaddition of sydnones (l,2,3-oxadiazolidin-5-one derivatives) (108) with benzocyclobutene (109) has been used to synthesize 3 f-2,3-benzodiazepines (110). This synthesis involves a cycloaddition-extrusion-ring-expansion sequence, and with other mesionic compounds, for example the thiazolium-4-olate (111), primary cydoadducts (112) were isolated (Scheme 13)... 

The regiospedfic [3+2] 1,3-dipolar cycloaddition of 2-amino-1,3-thiazolium-4-olates (2-aminothioisomunchnones) to methyl propiolate to form pyridone derivatives has been investigated. In this study a dipolarophile, 13, was tethered to 1-amino-1-deoxy-D-glucitol pentaacetate and formed pyridone 14 upon 1,3-dipolar cycloaddition to but-2-yndioic acid dimethyl ester. ... 

Meso-lonlc Thiazolo[2,3-6]thiazoles.—Thiazoline-2-thione (179) reacts readily with 2-bromo-2-phenyIacetyl chloride in the presence of base (Scheme 16) to produce 2-phenylthiazolo[2,3-6]thiazolium-3-olate (180), which undergoes cycloaddition reactions with acetylenic and olefinic dipolarophiles in much the same way as (175). With dimethyl acetylenedicarboxylate, for example, the thiazolo(3,2-a]pyridin-5-one (181) is formed. ... 

The synthesis of imidazo[2,l-h]thiazolium-3-olate zwitterions such as (100) from sugar thiourea derivatives, and their cycloaddition reactions to form imidazo[l,2-fl]pyridin-4-ones such as (101) (Scheme 10) have been detailed. ... 

---