Epoxy silyl ethers rearrangement

Sigmatropic rearrangement 1.12.42 Epoxy silyl ether rearrangement 1.1.2.43 Criegee rearrangement... 

Maruoka K, Hasegawa M, Yamamoto H, Suzuki K, Shimazaki M, Tsuchihashi G. Epoxy silyl ether rearrangements a new, stereoselective approach to the synthesis of P-hydroxy carbonyl-compounds. J. Am. Chem. Soc. 1986 108 3827-3829. 

Rearrangement of epoxy silyl ethers. When treated with 1 equiv. of TiCl4, a-silyloxy epoxides rearrange to p-hydroxy carbonyl compounds. 

Table II. MABR-Catalyzed Rearrangement of Chiral Epoxy Silyl Ethers... 

Acylation of hydroxylamine with 8-2 Rearrangement of epoxy silyl ethers... 

The exceptional bulkiness of 2,6-di-tert-butyl-4-broniophenoxy ligands in MABR is essential here for the smooth rearrangement of epoxy silyl ethers, and the less bulky methylaluminum bis(4-bromo-2,6-diisopropylphenoxide) (MAIP) was found to be totally ineffective in the rearrangement of the tert-butyldimethylsilyl ether of epoxy-geraniol (139). BF3-OEt2 as an ordinary Lewis acid gave fluorohydrin 140 as sole isol-able product (Sch. 114). 

The Lewis acid mediated rearrangement of epoxy silyl ether (69) affords the corresponding P-hydroxycarbonyl compound (70 Scheme 26). ... 

A pinacol-type rearrangement of epoxy silyl ethers has been used to prepare 0-hydroxy ketones, the migratory aptitudes observed... 

SUylated aldol adducts can be reached by using a nonaldol rearrangement promoted by treatment of bulky epoxy bis-silyl ethers with TMSOTf/i-Pr2NEt in methylene chloride at —50 °C (eq 95).Bulky mesylated epoxy silyl ethers also undergo this transformation however, a silyl triflate-promoted Payne rearrangement was observed as a side reaction depending on the stere ochemistry of the starting epoxide. 

The non-aldol aldol reaction of the isomeric epoxy silyl ethers has been reported to be controlled by the conformation of the transition states leading to an internal hydride shift. One isomer rearranges to the /1-silyloxy ketone whereas the other isomer gives a /I-elimination product. Substrates with substituents that favour formation of the chairlike transition state rearrange normally while those that do not undergo elimination (Scheme 59). 

A novel silyl triflate-promoted Payne rearrangement of silyloxy epoxides was reported by Jung et al When the ethyl substituted epoxy silyl ether 13 was treated with silyl triflate in the presence of a base, a mixture of four ketones, 16a-d and four aldehydes 17a-d were obtained. It has been assumed that two ketones and two aldehydes could be formed via a non-aldol process and an epoxide rearrangement, whereas the other four products through 14 and 15, a silyl triflate promoted Payne rearrangement. 

FIGURE 18.1. Stracture of tertiary carbocations with ring tension. The rearrangement of epoxy silyl ethers can be per-... 

In a formal synthesis of fasicularin, the critical spirocyclic ketone intermediate 183 was obtained by use of the rearrangement reaction of the silyloxy epoxide 182, derived from the unsaturated alcohol 180. Alkene 180 was epoxidized with DMDO to produce epoxy alcohol 181 as a single diastereoisomer, which was transformed into the trimethyl silyl ether derivative 182. Treatment of 182 with HCU resulted in smooth ring-expansion to produce spiro compound 183, which was subsequently elaborated to the desired natural product (Scheme 8.46) [88]. 

While allyl and glycidyl ethers are converted into a mixture of oxetane and oxepine products with xec-butyllithium, Mordini and co-workers reported that allyl, benzyl, and propargyl epoxy ethers can be regioselectively converted into 2-vinyl, 2-phenyl, or 2-aIkynyl-3-(hydroxyalkyl) oxetanes upon treatment with either Schlosser s base or other mixed metal bases. Some of the best results were obtained with the LDA/potassium ferf-butoxide mixture (LIDAKOR, ref 194). While rearrangement of propargylic or benzylic epoxide ethers formed exclusively the four-membered oxetanes, rearrangements of allyl oxiranyl ethers show a selectivity for cyclization to the seven-membered ring. Trialkylsilyl-substituted epoxide allyl ethers also show a preference for the oxepine, and mixtures are obtained as the size of the silyl substituents is increased (Scheme 17). 

Silyl enol ethers were intermediates in the reduction of y epoxy ketones by addition of methyldiphenylsilyllithium. The process involved addition to the carbonyl group, followed by 1,2-Brook rearrangement and epoxide opening. With excess of the silyllithium, the 0-Si bond of the resulting silyl enol ether 44 was cleaved, providing the yS-hydroxyketone product 45. ... 

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