ALPHA-HETERO-SUBSTITUTED FURANS AS DIENES IN [4-DIMETHYLOXYALLYL CATION FOR THE PREPARATION OF NEW VERSATILE CYCLOHEPTANE SYNTHONS - A STUDY OF THE FACTORS CONTROLLING THE DIASTEREOSELECTIVITY(3]CYCLOADDITION WITH 1,3)

[4+3] Cycloaddition reactions of 2-functionalized furans with 1,3-dime thyloxyallyl cation, afford versatile polyfunctionalized cycloheptane synthons. Increasing electron-donating properties and/or decreasing si ze of the substituent at C-2 of furan improves yield but decreases ste reoselectivity. The opposite effect is also observed, so, by designing a bulky enough alpha-substituent it is possible to obtain endo stereo specificity. cis Stereospecificity is observed in all studied cases an d it is probably determined by the type of reaction mechanism.