THE DIELS-ALDER REACTIVITY OF 2,2'-ETHYLIDENEBIS[3,5-DIMETHYLFURAN] AND EXPLORATORY CHEMISTRY OF THE MONO-ADDUCTS

In the presence of Me(3)Al, 1-cyanovinyl acetate added to 2,2'-ethylid enebis[3,5-dimethylfuran] (1) to give a 20:10:1:1 mixture of mono-addu cts 4, 5, 6, and 7 resulting from the same regiocontrol ('para' orient ing effect of the 5-methyl substituent in 1). The additions of a secon d equiv. of dienophile to 4-7 were very slow reactions. The major mono -adducts 4 (solid) and 5 (liquid) have 2-exo-carbonitrile groups. The molecular structure of 4 ,5-dimethyl-7-oxabicyclo[2.2.1]hept-5-en-2-en do-yl acetate) was determined by X-ray single-crystal radiocrystallogr aphy. Mono-adducts 4 and 5 were saponified into the corresponding 7-ox anorbornenones 8 and 9 which were converted with high stereoselectivit y into -1,5-endo-dimethyl-7-oxabicyclo[2.2.1]heptan-2-one dimethyl ace tal (12) and its (1'RS)-stereoisomer 12a, respectively. Acetal hydroly sis of 12a followed by treatment with (t-Bu)Me(2)SiOSO(2)CF(3) led to silylation and pinacol rearrangement with the formation of -6-methyl-3 -methylidene-2-oxabicyclo[2.2.1]heptane (16). In the presence of Me(3) Al, dimethyl acetylenedicarboxylate added to 12 giving a major adduct 19 which was hydroborated and oxidized into (1RS,1'RS,2 '' RS,3 '' RS, 4SR,4 '' l)ethyl]-7-oxabicyclo[2.2.1]hept-2,3-dicarboxylate (20). Acet ylation of alcohol 20 followed by C=C bond cleavage afforded (1'RS,1 ' ' SR,2RS,2 '' SR,3RS,3 '' SR,4RS,4 '' SR,5Rs)-dimethyl .1]hept-1-yl)-e thyl]furan-2,5-diyl}bis[glyoxylate] (24).