Furans, reactions with vinylcarbenoids

The nature of the dipolar intermediate is crucial in the reaction with furans, since the product distribution is greatly influenced by the substituents present on the furan and vinylcarbenoid (Tab. 14.10) [84—86]. Electron-rich 2-methoxyfuran leads to exclusive formation of triene 89 with vinyldiazoacetate 42, whereas the corresponding reaction with 2,5-dimethylfuran affords only the [3+4] cycloaddition product 88 in 70% yield. 

The reaction with furan forms a mixture of the two products with a preference for the [3+4] cycloadduct 88. Similarly, the reaction of furan with the vinylcarbenoid of si-loxy-substitilted vinyldiazoacetate 87 yields 90% of the [3+4] cycloaddition product with no triene formation. 

Reaction of vinylcarbenoids with furans offers another level of complexity because the furanocyclo-propanes in this case would be divinylcyclopropanes capable of a Cope rearrangement as well as electro-cyclic ring opening to trienes.27b c As illustrated in Scheme 42, the product distribution was dependent on the furan structure. With 2,5-disubstituted furans [3.2.1 ]-bicyclic systems (202) were exclusively formed, but with furan or 2-substituted furans, trienes (203) were also produced. 

Davies et al. have reported that the formal [4 + 3] cycloaddition reaction of vinylcarbenoids (38) with furan to give (39) via (40) proceeds satisfactorily (Scheme 10). ... 

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. 

Carbenoid Reactions. Other synthetically useful reactions include carbenoid addition to form 2-oxabicyclo[3.1.0]hex-3-ene systems. 1 The reaction of vinylcarbenoids with furan provides two products (eq 22). These results may be rationalized invoking a nons3tnchronous cyclopropanation mechanism that leads to two possible dipolar transition states. - ... 

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