O-to-C rearrangement

The utility of glycosyl phosphates in the synthesis of C-glycosides was demonstrated in the synthesis of the natural product 8,10-di-O-methyIbergenin 39. Utilizing the O-to-C rearrangement, the key C-aryl linkage was installed and further elaboration afforded the product in good yield. 

Deprotection of O-benzyl and N-BOC groups.2 This combination of a soft nucleophile and a hard electrophile cleaves O-benzyltyrosine without O to C rearrangement, and also cleaves the benzyloxycarbonyl group at 25°. It was used in a synthesis of the pentapeptide Met-enkephalin, Tyr-Gly-Gly-Phe-Met. It was also used for cleavage of BOC groups protecting the NE-amino group of lysine in a synthesis of a snake venom mastoparan.3... 

In a study aimed at probing the reactivity of pyranosyl oxocarbenium ions, She-noy and Woerpel reported on the diastereoselectivity associated with the intramolecular O to C rearrangement of 4-alkoxy 1-vinyl ethers 17 (Scheme 29).58... 

This reagent has been shown to be particularly effective for the net O to C rearrangement of alkynyl tributylstannane derivatives from mixed acetals derived from furanyl and pyranyl lactols (eq 61). Attempts to accomplish the same transformation with other Lewis acids (TiCLj, SnCLj, EtAlCl2, Me2AlCl) were not successful. 

Stabilized carbanions (acetoacetates, malonates, Meldrum acid, nitroderivatives, and the like) are efficiently aUylated by allylic carbonates under Pd catalysis without added base (Scheme 1). Apart from the cases reported up to 1987 (vide supra), many references witness the possibilities of this type of reaction. " Attention has been paid to asymmetric versions (see Sect Y2.4), " " to several stereochemical and mechanistic aspects, and to remote asymmetric induction " (Scheme 33). New O- to C-rearrange-ments similar to those of Scheme 5 have been published. " Cyanide should be considered also as stabilized C-nucleophile despite its overall inversion stereochemical behavior " (Scheme 10 and Table 1). 

Alkylidenetetrahydrofurans such as (30) undergo palladium (0)-catalysed rearrangement to cyclopentanones, in contrast to the thermal rearrangement, which leads to cycloheptanones [equation (38)]. Palladium-catalysed alkylation of /3-keto-esters and allylic ethers can be carried out in an intramolecular fashion to synthesize cyclopentanones [equation (39)].The O- to C-rearrangement of intermediates closely related to (30) was also noted in this reaction. 

In accordance with this working hypothesis, the chiral ammonium betaine 16 proved to be a highly efficient catalyst for the O- to C-rearrangement of azlactone carbonates (Scheme 40.23). 

Reaction of 1-phosphinylimidazoles with methanesulphonic acid at — 60 °C gives the phosphinyl sulphonate esters 54. These compounds are unstable at ambient temperatures and undergo a [1,2] — O to S rearrangement of the phosphinyl moiety to yield phos-phinoyl sulphones 55105. Little is known about this reaction except that, if both R1 and R2 are alkoxy groups, the rearrangement is not observed. 

The reverse process of a Brook rearrangement, the O- to C-trialkylsilyl group shift from a silyl ether to a carbanion terminus, is well-known and has been intensively studied [Eq. (22)] [25]. 

In some epoxide ring-opening reactions C—O bond cleavage is accompanied by the development of enough carbocation character at carbon —O) to allow rearrangement to occur. These reac-... 

Alkyl transfers from O to C (Stevens rearrangement), carbenes and methyl carbonium ions have all been postulated to explain the formation of lower olefins from methanol and dimethyl ether over heterogeneous acid catalysts the reaction is autocatalytic, e.g. 

Scheme 40.18 Isothiourea 12 promoted O- to C

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