Oxazolium salts reduction

Azomethine ylides. Reduction of oxazolium salts (1) with phenylsilane and cesium fluoride provides an unstable 4-oxazoline (2), which can react as an azomethine ylide with a dipolarophile such as DM AD to give a dihydropyrrole (3). ... 

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). 

The oxazolium salt (160), formed from the oxazole (159) by reaction with methyl triflate in acetonitrile, may be reductively ring opened by treatment with phenylsilane in the presence of cesium fluoride to give the azomethine ylide (161) (presumably this species is in tautomeric equilibrium with the corresponding oxazoline). The azomethine ylide can be trapped as an adduct with a suitable dipolarophile, such as dimethyl acetylenedicarboxylate (DMAD). In the case of this reagent the adduct (162) can be ox-... 

The authors also reported two distinct pathways for reduction of oxazolium salts with a metal hydride (Scheme 1.277). In both cases, only ring-opened products were observed. Thus treatment of an oxazolium salt with excess sodium borohy-dride or lithium aluminum hydride gave 1039a-c in good yield. Mechanistically, it was proposed that hydride addition at C(2) produced 1040, which was then cleaved in the presence of excess hydride to afford 1039a-c upon workup. In contrast, treatment of an oxazolium salt with one equivalent of lithium tri-terf-butoxyalu-minum hydride afforded 1042a,b in modest yield. In this case, the authors proposed that, in the absence of excess hydride, 1040 was hydrolyzed with concomitant ring... 

Vedejs and co-workers " ° applied CeHsSiHa/CsF reduction methodology of oxazolium salts as a means to access the indoloquinone ring present in aziridino-mitosene 1109. In a model smdy," the oxazolium salt 1111 was readily prepared... 

The reduction of a fuUy elaborated oxazolium salt and subsequent trapping of the azomethine ylide allowed Vedejs and co-workers" ° to complete their synthesis of aziridinomitosene. Alkylation of 1121 with 2-(phenylsulfonyl)ethyl triflate produced the oxazolium salt 1122, which was converted to the key bicyclic... 

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