Oxidopyrylium betaine

The cobalt(I)-catalysed 6 + 2-cycloadditions of cyclooctatetraene with monosubsti-tuted alkynes produced monosubstituted bicyclo[4.2.2]deca-2,4,7,8-tetraenes in good yields.185 The 6 + 3-cycloaddition of fulvenes (145) with 3-oxidopyrylium betaines (144) formed 5-8 fused oxa-bridged cyclooctanoids (146,147), which can be manipulated by cycloaddition reactions to produce key intermediates (148,149) for the synthesis of fused cyclooctanoid natural products e.g. lancifodilactones (Scheme 38).186,187... 

The [6+3] cycloaddition of 6,6-diarylfiilvenes to 3-oxidopyrylium betaines, derived by the action of base on 6-acetoxy-6//-pyran-3-ones, gives access to fused oxa-bridged cyclooctanoids (Scheme 23) <06T5952>. 

Krishnan and coworkers [50] treated 6,6-adamantylidenefulvene 216 with various two-substituted 3-oxidopyrylium betaines, for example, 215, providing a mixtures of [6+3]- and [3+2]adducts 217 and 218 in the ratio of 2 1 to 4 1 (Scheme 7.50). Similarly, reaction of 6,6-dithiodimethylenefulvene 219 with oxidopyrylium betain at room temperature gave 76% of a mixture of [6+3]-and [3+2]adducts 220 and 221. The formation [3+2]adducts, in addition to the [6+3] adducts, which is different from the predominant formation of the [6+3] adduct in the reaction with fulveneketene acetal 22, was traced back to a decrease in the electron density of 6,6-dithiodimethylenefulvene 219, arising from the hyperconjugation of the d-orbital of sulfur and the fulvene 7c-electrons. 

Oxidopyrylium betaines from the 1,6-intramolecular addition of a carbene to the oxygen of a carbonyl group are presented along with other carbonyl ylides in the next section. 

Porco et al. reported the synthesis of ( ) methyl rocaglate using [3 + 2] dipolar photocycloaddition reaction of an oxidopyrylium betaine derived from excited state intramolecular proton transfer reaction of 3-hydroxyflavin and methyl cinnamate [144]. Methyl rocaglate was obtained by a base-mediated a-ketol rearrangement followed by hydroxy-directed reduction sequence. They subsequently succeeded in the asymmetric synthesis of methyl rocaglate using functionalized TADDOL derivative (34) (Figure 2.30) as a chiral Bronsted acid (Scheme 2.77) [145]. 

Porco and coworkers used taddol 10 as a chiral H-bond donor in the asymmetric photochemical [3-f2] cycloaddition between oxidopyrylium betaine generated from 11 and methyl cinnamate 12 (Scheme 10.11) [72]. (-)-Methyl rocaglate was obtained... 

Oxidopyridiniums (26 Z = NR, Y = O) are converted photochemically into the bicycle (27) corresponding 3-oxidopyryliums and especially 4-oxidoisochromyliums isomerize more easily (cfi 44 — 45). 5-Oxidopyridazinium betaines are isomerized photochemically into corresponding pyrimidin-4-ones by a similar path (Scheme 6). 

Analogously, fulvene n-systems can act as a triene unit for enamines (Scheme 6.46a) [103], 2-oxyallyl cations (Scheme 6.46b) [104], chromium alkenyl carbene complexes (Scheme 6.46c) [105], 3-oxidopyrylium (Scheme 6.46d) [106], 3-oxidipyridinium betaines (Scheme 6.46e) [107], dienolates (Scheme 6.46f) [108], azirines (Scheme 6.46g) [109], and azomethine ylides (Scheme 6.46h) [110]. 

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