Oxonium ylide approach

Most recently, we have investigated the use of iterative oxonium ylide [1,2]- or [2,31-shifts as a convenient approach to the polypyran domains often found in the marine polyether ladder toxins (Scheme 18.8) [21]. Initial studies indicated that [l,2]-shifts of O-benzyl oxonium ylides such as 19 a or 19 b were inefficient. Alternative metallocarbene processes including C-H insertion and dimerization were found to predominate in these cases, again suggesting that carbene-ylide equilibration may occur [21b]. On the rationale that concerted [2,3]-shifts of the corresponding O-allyl oxonium ylides might occur more readily, the allyl ethers 19 c, 19 d were then examined. These examples were much more effective, especially in conjunction with the optimized catalyst Cu(tfacac)2 [21a]. However, rhodium(II) triphenylacetate (Rh2(tpa)4) [22] was found to... 

Sulfonium ylides generated through base-promoted deprotonation of sulfonium salt have been extensively studied. The reaction of sulfides with a diazo carbonyl compound in the presence of a transition metal catalyst is an alternative approach to obtain sulfonium ylides. Sulfonium ylides are more stable than the corresponding oxonium ylides. Stable sulfonium ylides generated by the reaction of an Rh(ii) carbene complex with thiophene have been reported (Figure 5). ... 

Two different approaches to the core structure of the zaragozic acids are based on the generation of oxonium ylides. 

Scheme 16.25 Iterative approach to polypyrans via oxonium ylide [2,3]-shifts.

Scheme 16.37 Dactylol approach using oxonium ylide [l,2]-shift.

Olah approached the problem by attempting to synthesize the ylide directly, via three routes deprotonation of TMO ion, desilylation of dimethyl((trimethyl si lyl )methyl )oxonium ion, and reaction of photolytically-generated methyl-carbene with DME [26,29]. 

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