2-oxonia Cope rearrangement

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 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... 

In the laboratory of R.D. Rychnovsky, the segment-coupling Prins cyciization was utilized for the total synthesis of (-)-centrolobine. This approach avoided the common side reactions, such as side-chain exchange and partial racemization by reversible 2-oxonia Cope rearrangement, associated with other Prins cyciization reactions. The substrate -acetoxy ether was subjected to SnBr4 in DCM, which brought about the formation of the all-equatorial tetrahydropyran in good yield. 

Tributyltin hydride has been used as a hydride donor to produce stabilized car-benium ions in 2-oxonia Cope rearrangement use of the less nucleophilic triethyl-silane resulted in poor chemical and optical yields (Scheme 12.129) [232]. 

Loh et al. found a tritlic acid-catalyzed 2-oxonia Cope rearrangement, which was used in the stereocontrolled synthesis of linear 22i -homoallylic sterols (eq 48). Interestingly, poor stereoselectivity was observed when In(OTf)3 was employed as the catalyst for this reaction. Stereoselective Mannich-type reaction of chiral aldimines with 2-silyloxybutadienes in the presence of triflic acid gives the corresponding products with 70-92% de in 62-74% chemical yield, which are not obtained by general Lewis acid-promoted methods (eq 49). ... 

Intermediate 1-6 can evolve by metal loss to give tricyclic compound 1-2, or by fragmentation to form methyl ketone 1-3 (Scheme 2.2). The competitive 2-oxonia-Cope rearrangement of intermediate 1-6 via 1-7 forms 1-8, which results in the minor epimer 1-2 of the tricychc compotmd. As other gold(I)-catalyzed reactions of enynes, this reaction is stereospecific [Refs. 4, 8, 154 in Chap. 1] [1]. 

To minimize the undesirable reversibility of the 2-oxonia Cope rearrangement and preserve the optical purity of the cyclization products, many modified reaction... 

Mixed acetals (6), bearing allyl and silyl enolate moieties, are efficiently cyclized to tetrahydropyranones (7) in the presence of (la) and DTBMP [52]. A plausible mechanism is as follows (i) the treatment of (6) with (la) forms oxocarbenium ion (8), (ii) (8) is reversibly converted into another oxocarbenium ion (9) by the 2-oxonia Cope rearrangement, and (iii) intramolecular addition of the silyl enolate moiety to the carbenium ion center in (9) gives (7). The moderate stereoselectivity concerning the substituent R is attributable to competing cyclizations from chair and boat conformations in the last step (Scheme 9.11). 

Certain O-homoallyl-oxocarbenium ions are known to undergo 1,3-allyl transfer by the 2-oxonia Cope rearrangement as described in Scheme 9.11. Recently, Rychnovsky and coworkers have demonstrated that the (la)-promoted reaction of enantioenriched mixed acetal (42) with Bu3SnH affords the rearranged product (43) with a high level of chirality transfer [84]. The key step of this reaction is the 2-oxonia Cope rearrangement of oxocarbenium ion (44)-(45) (Scheme 9.27). Under similar conditions, acetal (46) is efficiently transformed into the bicyclic product (47) by intramolecular trap of the rearranged carbenium ion intermediate (48) with the... 

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