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Irradiation of crystalline (+)-2,4:3,5-di-0-methylene-D-mannitol 1,6-di-Znm-cinnamate (1) induces intermolecular cyclobutane formation leading to chiral oligomers of increasing length. The dimer, trimer, tetramer, and pentamer have been isolated and characterized. The intramolecular distance between the olefinic residues within the molecules of 1 is larger than in any previous examples of solid-state photodimerization or oligomerization and, in spite of the large, bulky tetraoxa-c/Tdecalin group separating the reaction centers, the cinnamate groups of the monomer are properly aligned for intermolecular photocycloaddition. Crystals of 1 are monoclinic, a = 15.218 (6) Á, b = 13.217 (7) Á, c = 5.882 (3) Á, ß = 76.95 (3)°; the space group is P2|, with two molecules in the cell. The structure has been solved by direct methods and refined to an R factor of 0.043 on 2270 nonzero reflections. The crystal structure analysis establishes the correct molecular structure of the sugar alcohol derivative, previously in doubt, and elucidates the molecular packing which is responsible for the observed products. The chemical consequences of solid-state irradiation of compound 12, a monothiophene analogue of 1, have been considered; 12, however, shows an unexpected reluctance to crystallize in the same structure as 1.

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The two analogs, (-)-2,3-di-O-methyl-L-erythritol 1,4-di-transcinnamate, 1, and (-)-2,3-0-isopropylidene-L-erythritol 1,4-di-trans-cinnamate, 2, were irradiated in benzene solution. Intramolecular 2 + 2 photocycloaddition occurs in both derivatives and the stereochemistry of the cyclobutane products, after removal of the L-erythritol, was established in both cases as f3 (symmetry m)-and li (symmetry 2)-truxinate. While the asymmetric induction was 85% in the synthesis of ( + )-dimethyl li-truxinate from the open-chain dimethoxy derivative 1, the more rigid isopropylidene ring derivative led to an asymmetric induction of only 6% and afforded the opposite enantioner, (-)-dimethyl li-truxinate, in excess. Computer simulation of the diastereomeric ground state conformations which are believed to lead to the observed products from 1 was carried out to enable better understanding of the factors which determine the asymmetric synthesis. The simulations afford geometries and relative energies of the product precursor conformations. The results are in qualitative agreement with the experimental observations in predicting an excess of one of the two possible li -diastereorners. Since the geometry is also obtained by the simulation, it allows one to predict the absolute configuration of the predominant chiral product.• Present address: The Hebrew University of Jerusalem.

The relative importance of polar and steric influences on the course of the acid-catalysed opening of several epoxycholestanes bearing a vicinal cis-hydroxy-or -acetoxy-group has been investigated.