Preparative photolyses were generally carried out in benzene (50-100 mL) by using 0.05 M irans-stilbene and 0.5 M ester. Samples were degassed prior to irradiation through Pyrex with a 450-W medium-pressure mercury lamp by bubbling dry nitrogen through the solution for 10-15 min. Reaction progress was monitored by GC. Yields were not optimized. The isolated yield of cycloadducts from the trans-stilbene-dimethyl ethylidenemalonate reaction mixture is representative of the reactions studied.Methyl 4,4-Dimethyl-c-2,t-3-diphenylcyclobutane-lcarboxylate (la) and Methyl 4,4-Dimethyl-t-2,c-3-dicyclobutane-l-carboxylate (lb). The irans-stilbene-methyl 3methylcrotonate reaction mixture was chromatographed on alumina with 0-1 % ethyl acetate in hexane as eluant. The major adduct, la, eluted after the stilbenes and was isolated as a 6:1 mixture of la and lb. lb followed and was isolated as a 1:1 mixture of la and lb: the NMR data (Table II) are consistent with those reported by Kirsch;1 23 IR (CCI4, mixture of isomers) 5.75, 7.91 µ .Methyl t-4-Methyl-c-2, -3-dipheny lcyclobutane-1carboxylate (2a) and Methyl t-4-Methyl-t-2,c-3-diphenylcyclobutane-1 -carboxylate (2b). The irans-stilbene-methyl crotonate reaction mixture was chromatographed on alumina with 2% ethyl acetate in hexane as eluant. Attempted crystallization of the isomeric mixture was unsuccessful: the NMR data in Table II is consistent with that reported by Retjo;4 5IR (CC14, mixture of isomers) 5.76, 8.55 µ .Dimethyl r-2,t-3-Diphenyl-f-4-methylcyclobutane-l,ldicarboxylate (3a) and Dimethyl r-2,i-3-DiphenyI-c-4-methylcyclobutane-l.l-dicarboxylate (3b). A solution of irans-stilbene (0.45 g) and dimethyl ethylidenemalonate (4.0 g) in 50 mL of benzene was irradiated 38 h. The solvent was removed, the unreacted ester distilled bulb-to-bulb, and the residue chromatographed on alumina (50 g) with 0-4% ethyl acetate in hexane as eluant. Early product-containing fractions were enriched in the major isomer, 3a; later fractions were enriched in 3b. Fractional crystallization afforded no further enrichment of either isomer: yield 0.46 g (54%); NMR (see Table II); IR (CC14, mixture of isomers) 5.76, 7.84, 7.97 µ .Diethyl 4,4-Dimethyl-1-2,3-diphenylcyclobutane-1,1 -dicarboxylate (4). The irans-stilbene-diethyl isopropylidenemalonate reaction mixture was chromatographed on silica gel with 5% ethyl acetate in hexane as eluant. Stilbenes eluted first, followed by 4. The crude product was recrystallized from hexane: mp 124.5-125 °C; NMR (see Table II).Dimethyl 1,2-Dimethyl-1-3,4-diphenylcyclobutane-c-l,2dicar boxy late (5). The trans-stilbene-dimethyl dimethylmaleate reaction mixture was chromatographed on alumina with 0-2% ethyl acetate in hexane as eluant. The crude product was recrystallized from hexane: mp 104.9-105.2 °C; NMR (see Table II); IR (KBr) 5.74, 5.83 µ .
