Oxazolium salts 2 + 3 cycloadditions

The photocycloaddition of aliphatic and aromatic aldehydes with 2,4,5-trimethyloxazole (131) gave bicyclic oxetanes 132 in almost quantitative yields hydrolitic cleavage led selectively to erytro a-amino-P-hydroxy methyl ketones 133 <00CC589>. The oxazolium salt 134 was converted to the azomethine ylide 136 via electrocyclic ring opening of the oxazoline 135. Intramolecular cycloaddition afforded 137 in 66% overall yield which was transformed into the aziridinomitosene derivative 138 . 

A series of publications by Vedejs et al. (17-19) outlined the use of oxazolines in the generation of azomethine ylides. Reduction of the oxazolium salts 71 (R = Ph or Me) with PhaSiH/CsF led to formation of the unstable 4-oxazolines (72) and their valence bond azomethine ylide tautomers (17), which could be trapped in situ with DM AD to afford the bicyclic adducts 73, where R and R = Ph or Me, in 75% yield via [2 + 2] cycloaddition (Scheme 3.18). 

This French group (145) has also been able to divert the usual Diels-Alder cycloaddition pathway of Reissert salts with oleftnic esters to a 1,3-dipolar cycloaddition pathway by the addition of triethylamine. In addition, mtinchnone imine 243 can be trapped with dipolarophiles to furnish 244 (146). No Diels-Alder cycloadducts derived from the oxazolium salt were detected. In contrast, fumarate... 

Cycloaddition of 1050 with DMAD gave the tetrasubstituted bispyrrole 1058. In this case, loss of 2 mol HCIO4 from the bis-oxazolium salt was presumed to generate a mesoionic species (not shown) that underwent cycloaddition with DMAD, followed by a retro Diels-Alder reaction and loss of 2 mol of HNCO to yield 1058. 

This group " also diverted the usual Diels-Alder cycloaddition pathway of Reissert salts with olefinic esters to a 1,3-dipolar cycloaddition pathway by the addition of triethylamine. Thus treatment of munchnone imine 364 with ethyl acrylate and triethylamine affords the 1,3-dipolar cycloaddition product 366 (30%) as the major product, formed by fragmentation of cycloadduct 365 (Fig. 4.121). The Diels-Alder product (not shown) is formed in 15% yield. Similar products to 366 are formed with dimethyl and diethyl maleate and fumarate. Laude and coworkers " also were able to trap munchnone imine 367 with dipolarophiles to furnish 368 (Fig. 4.122). No Diels-Alder cycloadducts derived from the oxazolium salt were detected. In contrast, fumarate and acrylate esters give only Diels-Alder cycloadducts from the tautomeric oxazolium salt (not shown). However, benzo-quinones and 1,4-naphthoquinone react in a 1,3-dipolar fashion with munchnone imine 372 derived from Reissert compound 369 to give 373 (Scheme 4.11). " Diels-Alder cycloadducts derived from oxazolium salt 371 were not observed. In a... 

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