Furan, Siloxy

Modes of cycloaddition of alkylideneallyl cation are also controlled by the reaction conditions. [4 + 3] Cycloaddition occurs in the reaction with furan. The [4 + 3] cycloaddition with furan was observed for the siloxy-substituted allyl cation 5S, but not for the methoxy-substituted allyl cation 5M. The lower electrophilicity of 5S may prefer the concerted pathway of [4 + 3] cycloaddition in competition with the stepwise pathway to yield a [3 + 2] cycloadduct and an electrophilic substitution product. 

An elegant application of the [3 + 4] cycloaddition methodology was showcased in model studies directed toward the synthesis of the core skeleton of CP-263114 115 (Scheme 14.12) [103]. The key step is the intramolecular [3 + 4] cycloaddition of furan with the siloxy-substituted vinyldiazoacetate in 112. The oxabicychc system 113 was obtained in 68% yield and was converted in eight steps to furnish the core structure 114 [103]. 

G. Casiraghi, L. Colombo, G. Rassu, and P. Spanu, Synthesis of enantiomerically pure 2,3-dideoxy-hept-2-enono-1,4-lactone derivatives via diastereoselective addition of 2-(trimethyl-siloxy)furan to D-glyceraldehyde and D-serinal-based three-carbon synthons, Tetrahedron Lett. 30 5325 (1989). 

The synthetic scheme of Suzuki et al. [59] was based on the fact that adducts resulting from furan addition often rearrange to phenols (compare with [61]). In our group, we also investigated the reactions of naphthoquinones with siloxy-furans such as 64. However, instead of the expected [4 + 2] cycloaddition, a Michael addition that proceeded without catalyst occurred in the reaction with 3-chlorojuglone (63) [39]. Interestingly, both the Michael acceptor 63 and the donor 64 reacted in a 1,4-reaction mode. The regio- and stereochemistry of the product 65 were confirmed by x-ray analysis (Scheme 18). 

Heteroatom-based siloxy dienes have received attention in recent years as an efficient method for building up ring frameworks. The subject has been reviewed, and only a few applications are illustrated here [135], This is a powerful process, since in a relatively small number of steps several rings can be joined together. The hrst two equations [136,137] of Scheme 80 show how a single butenolide unit can be attached, while the last sequence illustrates an iterative methodology [138]. It should he noted that in the addition of trimethylsiloxy furan to the anomeric acetate. 

The reaction of vinylcarbenoids with furans can lead to two products, the [3+4] annulation product and a triene, in which the fiiran ring has been unraveled. The product ratio is sensitive to both the structure of the fiiran and the vinylcarbenoid. In the reaction of 4 with furans, the ratio of [3+4] annulation product 67 to triene 68 varies greatly in changing from furan to 2-methylfium to 2-methoxyfiu (Scheme 26). In the case of 2,5-dimethylfiiran, the reaction with 4 results in clean formation of the [3+4] annulation product 69. The reaction of the vinyldiazomethane 15 with fiiran results in a product ratio similar to that obtained in the reaction of fiiran with 4. However, the siloxy-substituted vinyldiazoacetate 70 results in the clean formation of the [3+4] annulation product 71 in 90% yield, without any evidence for formation of the triene 72. 

X0 compds. 44, 861 Fulvenes, 6-acoxy- 44, 777 -, 6-siloxy- 44, 777 -, 8-oxy-44, 556 Furan-4-carboxylic acid esters,... 

Another cycloaddition, in which this time the silyl enol ether functions as a dienophile, is the SnCl4-catalysed addition of butadiene to (180 R = Me) giving (181). If R = H in the starting material a 4 + 3 reaction takes place, producing the seven-membered ring (182). Flash-vacuum thermolysis of P-keto-trimethylsilyl enol ethers has been used in a substituted furan synthesis, and the same process has been used to prepare a-allenic acids (183) from siloxy-dienes. ... 

Moreover, Palombi showed that 2-trimethylsiloxyfuran was a good candidate, making it feasible to access 5-hydroxy butenoUdes stereoselectively (Scheme 7.9) [14], To further expand the scope, Curti demonstrated that N-Boc 2-siloxypyrrole was amenable [15] and also developed 2-catalyzed hyper-vinylogous reactions using extended furan-based siloxy polyenes [16]. 

Homoquinones are also prepared by a variety of methods that involve the free-radical reactions of propellanes with benzoquinones, ° the Michael addition of suUbxonium methyhde" and ethyl cyanoac-etate anion to naphthoquinone and of sulfonium methyhdes to benzoquinones, the photooxygenation of norcaradienes, the reaction of diethoxyphthalans with quinones, and the Diels-Alder reactions of siloxy- and alkoxy-substituted furans with cyclopropenes. ... 

Oda, K., Sakai, M., and Machida, M., Photochemistry of the nitrogen-thiocarbonyl systems. 30. Intermolecular photoaddition reaction of arenecarbothioamides to 2-methoxy- and 2-(trimethyl-siloxy)furans. Facile synthesis of arene-fused aminobenzoates by novel photoinduced benzannulation, Chem. Pharm. Bull, 45, 584, 1997. 

---