Vinyl tetrahydrofurans

The influence of the size and configuration of various cyclic vinyl carriers (Scheme 6.26) on the stereoselectivity of cycloaddition was studied, and included epoxides, (3-lactams, dioxaborolanes, and dioxans (22). Although the anti preference was maintained in all cases, the conformation of the carrier ring must also be taken into account in order to rationalize the stereoselections observed. The highest diastereomeric ratio was observed with the vinyl-tetrahydrofuran derived from glucose, where the conformational mobility of the carrier ring is substantially locked by an acetonide clamp and one face of the C=C bond is effectively shielded (22,165,215). 

Vinyl tetrahydrofurans 214 were initially synthesized by the IMSC-methodology in 1993 by Mohr [51], The reaction was catalyzed by p-toluenesulfonic acid (0.3 equivalent), and five equivalents of acetals 213 were required (Scheme 13.78). 

Oriyama et al. [52] and Ito et al. [78] published independently the preparation of similar tetrahydrofurans 216. In these cases, TMSOTf is used as the promoter and the desired tetrahydrofurans 216 are produced in excellent yields and stereoselectivity. In all cases, the cis-2,3-disubstituted vinyl tetrahydrofuran 216 is formed as the major adduct. In contrast, the selectivity in the formation of the new carbon-carbon double bond remains unsatisfactory (Scheme 13.79). 

Cis- and trans 5-[2-hydroxyisopropyl]-2-methyl-2-vinyl-tetrahydrofuran (c/s/frans-furan-linalool oxides) 171/II and rosefuran 18 ... 

C/s- and trims 5-isopropyl-2-methyl-2-vinyl-tetrahydrofuran 19I/II (cis/fra/is-anhydrofuran-linalool oxides) (80MI698, 77ZLUF98) ... 

The ease of these cyclization reactions may be due to the fact that, in the transition state, anionic character can be partially displaced onto oxygen. Independently of the mechanism of these processes which also could be considered as intramolecular Sw cyclization reactions, this useful trick has been used sometimes for the synthesis of vinylcyclopentanes, vinyl tetrahydrofurans and pyrrolidines, as described above. 

Aryl vinyloxiranes cycloadd to electron-deficient alkenes photolytically in the presence of Ph2S2 and AIBN to afford m-2-aryl-5-vinyl tetrahydrofuran derivatives via a radical mechanism... 

A soln. of startg. ester in dichloromethane treated with 2 eqs. SnC at —70° for 15 min, and the mixture allowed to warm slowly to room temp, over 2 h - product. Y 58%. Reaction is thought to proceed via a cationic oxa-Cope rearrangement. F.e, inch oxepin analogs, also satd. O-heterocyclics from ethylenic derivs., and 3-vinyl-tetrahydrofuran-2-carboxylic acid esters with an allylsilane group as internal nucleophile, s. L.D.M. Lolkema et al.. Tetrahedron Letters 29, 6365-8 (1988). 

In 2006, Widenhoefer reported an effective gold(I)-catalyzed protocol for the exo-hydroalkoxylation of y- and 6-hydroxy allenes to form 2-vinyl tetrahydrofurans and 2-vinyl tetrahydropyrans, respectively [104]. For example, treatment of 1-phenyl-5,6-heptadienol with a catalytic 1 1 mixture of [P(f-Bu)20-biphenyl]AuCl and AgOTs in toluene at room temperature led to isolation of 2-phenyl-6-vinyltetrahydropyran in 96% yield as a 7.2 1 mixture of diastereomers (Eq. (12.33)). This gold(I)-catalyzed hydroalkoxylation protocol tolerated substitution at the terminal allenyl carbon atoms and along the alkyl chain that tethered the hydroxy group to the allenyl moiety and was also effective for the S-exo hydroalkoxylation of y-hydroxy allenes. Alcaide and Almendros have shown that gold(III) also catalyzes the S-exo hydroalkoxylation of y-allenyl alcohols in modest yields (Eq. (12.34)) [105]. 

The reaction of pent-4-en-l,3-diol with aldehydes in the presence of SnCl2 afforded a mixture of 1-substituted ( )-hex-3-en-l,6-diol, 1-substituted 2-vinylbutan-l,4-diol, and trans-2-substituted-3-vinyl tetrahydrofuran (eq 48). The same reaction of ( )-hex-4-en-l,3-diol afforded fran5-2-substituted-3-(prop-l-enyl) tetrahydrofuran as the only product (eq 49). ... 

Mischitz, M. and Faber, K. (1996) Chemo-enzymatic s5mthesis of (2R,5S)- and (2R,5R)-5-(1-hydroxy-1-methylethyl)-2-vinyl-tetrahydrofuran ("linalool oxide") preparative application of a highly selective bacterial epoxide hydrolase. Synlett, 978-980. 

Mechanistic investigation of the Au -catalysed SiO)"-exo-trig hydroalkoxylation of allene (240) revealed a rapid and reversible C-0 bond formation to generate (241), followed by the turnover-limiting protodeauration producing the vinyl tetrahydrofuran (242). This pathway competes with catalyst aggregation and formation of an off-cycle bis(gold) vinyl complex (243). ... 

There are limited data available regarding the atmospheric behavior of three unsaturated furan derivatives, 2,5-dihydrofuran (Adeniji et al., 1981), 5-methyl-2,3-dihydrofuran (Martin et al., 2002), and 5-methyl-5-vinyl-tetrahydrofuran-2-ol (Calogirou et al., 1999) ... 

The 2,3-dihydro-5-methylfuran species is formed in the atmosphere at low humidity via the cyclization and subsequent dehydration of 5-hydroxy-2-pentanone, itself an important product in the oxidation of pentane (Martin et al., 2002). Similar species are expected to be formed in the oxidation of other alkanes (C4 and larger), when isomerization of the initially formed alkoxy radical is of importance. Similarly, the species 5-methyl-5-vinyl-tetrahydrofuran-2-ol is a by-product of linalool oxidation, formed via the cyclization of an initially formed unsaturated hydroxycarbonyl species (Calogirou et al., 1999). The available data are summarized in table III-E-30. 

For 5-methyl-5-vinyl-tetrahydrofuran-2-ol, the data of Calogirou et al. (1999) suggest that reaction with OH will dominate the daytime loss (lifetime 1.5 h). The reaction with NO3 will play a role at night in heavily polluted conditions (lifetime 5 h for [NO3] = 3 X 10 molecule cm ). The major product of the OH-initiated oxidation is 4-oxopentanal. Calogirou et al. (1999) postulate that this species is formed following OH addition to the acyclic isomer of 5-methyl-5-vinyl-tetrahydrofuran-2-ol, namely, 4-methyl-4-hydroxy-hex-5-enal, with glycolaldehyde as a likely co-product. 

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