Cycloaddition-elimination reactions

The reaction of 5(4H)-oxazolones (32) and miinchnones with triphenylvinylphos-phonium bromide (33) provides a mild synthesis of substituted pyrroles (34) (Scheme 11). The cycloaddition-elimination reactions of 5-imino-l,2,4-thiadiazolidin-3-ones with enamines and ester enolates produce 2-iminothiazolidines. " Chiral isomtinchnone dipoles show jr-facial diastereoselectivity with IV-phenyl- or A -methyl-maleimide in refluxing benzene. ... 

A related cycloaddition/elimination reaction occurs when 5-imino-1,2,4-dithiazolidin-3-ones (300) or -3-thiones (306) are treated with phenylisocyanate to afford 2-phenyl-5-imino-1,2,4-thia-diazolidin-3-ones (307) (Equation (45)) <92JPR685>. 

In contrast, the 5-phenyliminothiatriazoline (323) reacts as a masked 1,3-dipole with a variety of electrophilic nitriles. Tosyl cyanide and ethyl cyanoformate both react with (323) in refluxing chloroform to give initially (324) which then isomerizes to (325) as the reaction proceeds (Scheme 71) <91JHC333>. When the solvent is changed to acetone the reaction with the nitriles proceeds faster due to the formation of the adduct (326) which is capable of undergoing cycloaddition/elimination reactions at 20 °C. Trichloroacetonitrile does not react with (323) in chloroform solution in acetone,... 

Oxathiazolidinimine (180) containing the S—C=N thioamidate grouping can enter the cycloaddition-elimination reaction sequence with iso thiocyanates to give 1,2,4-dithiazolidindiimine (181) which in turn adds-eliminates RNCS to produce diimine (184) which isomerizes into thia-diazolidine (183). Another thiadiazolidine (182) is also formed directly from (180) which apparently... 

Similar facile cycloaddition-elimination reactions are observed when the nitrile group is replaced by an alkyne or alkene group. Thus, thermolysis of thiatriazolines (56) in refluxing benzene for 3 days provided the intramolecular adducts (57) (Equation (5)) <9314439 >. 

R ) (85d). The isomers (86) and (87) were obtained in 47% and 32% yields respectively, establishing the consecutive cycloaddition elimination reactions shown in path (b). 

Oxathiazolidin-3-imine (93) possesses a thioimidate moiety similar to 4-methyl-5-phenyl-imino-l,2,3,4-thiatriazoline (70) and is capable of undergoing cycloaddition-elimination reactions with isothiocyanates to give identical products, however at a much higher rate. Acetone can therefore be used as solvent to catalyze this kind of reaction of thiatriazolines. 

Analogous cycloaddition-elimination reactions have been reported for mesoionic imidazolium oxides, which are obtained by the reaction of disubstituted amidines with a-bromoacyl halides (equation 125) (77JOC1639). 

Electron-poor nitriles react with compound 87 and its derivatives to form the 5-amino-l,2,4-thiadiazole derivatives 104 <1985JOC1295>. Therefore, the formation of product 94 (see Scheme 21) may be explained alternatively by the addition of amidonitrile 93 to compound 90. The mechanism of the formation of product 104 was discussed in detail in CHEC-II(1996) <1996CHEC-II(4)691> but most probably the steps involved are (1) reaction of the electrophilic nitrile with the exocyclic nitrogen of compound 87 or its derivatives (2) loss of nitrogen similarly to the previous reactions and formation of an imine 103 (3) masked 1,3-dipolar cycloaddition/elimination reaction of the nitrile to the imine 103. Since the same nitrile is expelled in the elimination step, only 1 equiv of reagent is needed (Scheme 24). 

In the case of the attacks of neutral molecules on alkynes, nucleophilic attack is often difficult to distinguish from molecular cycloaddition or electrophilic initiation. Reaction (7) is typical of many which could equally as well be formulated as beginning with a dipolar cycloaddition or an acyclic zwitterion Detailed mechanism of these cycloaddition-elimination reactions remains to be explored... 3 . 

Recent developments in the chemistry of mesoionic compounds85 include cycloaddition-elimination reactions, which afford novel synthetic routes to a variety of heterocyclic systems. These reactions may be seen as involving 1,3-dipolar cycloadditions, following Huisgen,86 or alternatively as 1,4-cycloadditions to heterodiene systems,87 depending on the choice of canonical structure to represent the mesoionic compound. Benzyne has been employed in such reactions less frequently than more stable acetylenic or ethylenic dipolarophiles. 

By an apparent cycloaddition elimination reaction with dimethyl acetylenedicarboxylate, ethyl propiolate, or benzyne, 1,4,2-benzodithiazines afford 1,4-benzodithiins 1 and nitriles derived from the N2-C3 part of the 1,4,2-dithiazine ring (see Vol. E9a, p 270).70... 

Recent developments in the chemistry of mesoionic compounds include cycloaddition-elimination reactions, which afford novel synthetic routes to a variety of heterocyclic systems. These reactions may be seen as involving... 

S2Sa. Since the imine 525b was not a precursor of the cyclazine 524, it was concluded that C-S bond breaking must precede C-C and C-N bond formation, as would be the case if 3,5-didehydroimidazo[l,5-a]pyridine (525c) were an intermediate. An alternative explanation involving a [2-h8] cycloaddition of the reactants followed by elimination of lithium thioethoxide was also considered, however, but has not yet been disproven. Precedent for such cycloaddition-elimination reactions exists in the closely related pyrrocoline system. ... 

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