Oxonia

Azonia substitution at a naphthalene bridgehead position gives the quinolizinium ion (16). Oxonia substitution, elsewhere, forms the 1- and 2-benzopyrylium ions (17) and (18). The two most well-known monoaza systems with three aromatie fused rings are aeridine (19), derived structurally from anthraeene, and phenanthridine (20), an azaphenanthrene. The better-known diaza systems inelude phenazine (21) and 1,10-phenanthroline (22), while systems with three linearly fused pyridine rings are ealled anthyridines, e.g. the 1,9,10-isomer (23). 

Decahydro-7-oxonia-2-phosphaphenalen. 2-ide Cdecahydro, because after replacement, the maximum number of noncumulative double bonds is only five)... 

Dissolve 35 mg diphenylboric anhydride (DBA) and 25 mg salicylaldehyde (= 2-hydroxybenzaldehyde) in 100 ml chloroform. This results in the formation of 2,2-diphenyl-l-oxa-3-oxonia-2-boratanaphthalene (DOOB). 

Rychnovsky and coworkers very recently described a so-far unknown domino Michael addition/2 -oxonia Cope/aldol-type reaction to give tetrahydropyran rings [475]. The transformation has as its origin an annulation process (see Section 2.1) which was discovered by the same group. 

An example of an oxonia-Cope Prins cascade involving a dioxin moiety was reported by Dalgard and Rychnovsky. Treatment of compound 198 with Lewis acids allows the cascade to proceed to give tetrahydropyranone 199 as final product (Scheme 85) <20050L1589>. 

Replacement of CH in benzene by an oxonia group (0+) gives the pyrylium cation, but no neutral oxygen analogue of pyridine is possible. Both 2H- and 4//-pyran contain -hybridized carbon atoms. Many trivial names exist for oxygen heterocycles and the more important of these are shown in Scheme 3. 

The substitution of a CH unit in benzene by 0+ (the oxonia group) gives rise to the pyrylium cation (3). Since this ring still possesses 6 ir-electrons, it may be expected to exhibit aromatic properties. As the oxygen is primarily tricovalent, the pyrylium ring may be formally regarded as a cyclic oxonium ion. However its enhanced stability relative to aliphatic and alicyclic oxonium salts is doubtless due to its aromatic nature. 

The cleaning regime specified for the containers will depend on their initial level of contamination. Normally heat (steam or hot air) or chemical means (hydrogen peroxide or oxonia) are used sometimes these are used in combination with ultraviolet radiation. 

Bottle systems are more varied, whether for glass, polyethylene terephtha-late (PET) or other plastic. Bottles are rinsed with oxonia solution and then sterile water prior to filling. The filler is generally of a non-contact type (it does not touch the bottles) and product is either weighed in or measured volumetrically. Caps are also chemically sterilised (unless a foil closure is used) and applied on a capper monoblocked with the filler, enclosed in a high efficiency pure air (HEPA) filtered enclosure. The filler and final rinser are in a class 100 room and file operator wears full protective clothing to prevent infection of the product. 

The allyl-transfer reaction based on 2-oxonia Cope rearrangement allows highly stereocontrolled chirality transfer. Triflic acid has been shown to induce the rearrangement of the 251 allyl sterols into 22-homoallylic sterols with high stereoselectivity without side reactions86 [Eq. (5.316)]. The protocol, however, is not effective for syn substrates (for example, 251, R = H, R = COOEt). 

The third group (class 3 of the above classification) of the tetrapyrrole ring localized photoreactions can be exemplified [231] by the formation of oxonia-chlorins from chlorophyll derivatives (X = Cl, CF3COO)... 

The mechanism involves 2-oxonia-Cope rearrangements via (Z)-oxocarbenium ion intermediates, as shown in Scheme 7. It has been shown that racemization can be suppressed by conditions or structural features which slow the Cope rearrangement in either forward or reverse directions, since the [3,3]-sigmatropic rearrangement must occur many times before reaction via the less favoured (Z)-oxocarbenium ion becomes important.20... 

The absence of products derived from the oxonia-Cope rearrangement in the Panek study [64] can be explained by the presence of the C02Me group, which would destabilize the oxonia-Cope product 137 (R = C02Me) because the electron-withdrawing group is directly attached to the oxonium ion carbon. 

As early as 1997, Hiemstra and Speckamp postulated the participation of an oxonia-Cope rearrangement as a crucial step during the cyclization of vinyl silane 146 (Scheme 13.50) [75]. Both ( )- and (Z)-vinylsilanes, (E)-146 and (Z)-146 respectively, were used in this study. The cyclization proceeded in both cases in good to excellent yields, furnishing the 2,6-disubstituted dihydropyrans 147. Surprisingly, the cyclization of (E)-vinylsilanes ( )-146 gave anti-2,6-dihydropyran anti-147 as the major stereoisomer, whilst in the case of (Z)-vinylsilane (Z)-146, the syn-dihydropyran syn-147 was formed as the major product. 

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