Intramolecular cycloaddition reactions of two furfuryl fumarates yield, after crystallization, pure diastereomers of oxabicyclo[2.2.1]heptene derivatives which are versatile starting points for the total synthesis of natural products. The crystal structures of the adducts, menthyl (3aS,6R,7R,7aR)-1,3,3a,6,7,7a-hexahydro-3,3-dimethyl-1-oxo-3a,6-epoxyisobenzofuran-7-carboxylate, C₂₁H₃₀O₅, and methyl syn-1,3,3a,6,7,7a-tetrahydro-3-(1,1-dimethylethyl)-1-oxo-3a,6-epoxyisobenzofuran-7-carboxylate, C₁₄H₁₈O₅, establish the configuration at the chiral centers and provide insights into the factors controlling the diastereomeric differentiation.

In the crystal structure of the title complex, [Pt(C₉H₆NS)₂], although the aromatic ligands are coordinated to a central heavy metal atom, T-shaped and shifted π-stacked arrangements of the aromatic moieties are preferred, leading to a sandwich herring-bone type of crystal-packing motif. The crystal structure is therefore consistent with the view that the arene–arene interactions are determined by electrostatics (multipole–multipole).

Racemic azomethine ylides [(I) or (IV)] have been found to undergo cycloaddition to a trans-enamine [(II)] in a nonstereospecific way to yield mixtures of racemic diastereoisomeric 1:1 adducts: 5-dimethylamino-1,1,6-trimethyl-2,6a-di-p-tolyl-1,1a,5,6,6a,6b- hexahydro-4H-3,3a-diazacyclopropa[e]indene-4,4-dicarbonitrile, C₂₉H₃₃N₅ [(IIIa) and (IIIb)], or 2-dimethylamino-1,5,5-trimethyl-6-(4-methylbenzoyl)-7a-p-tolyl- 2,3,5,7a-tetrahydro-1H-pyrrolizine-3,3-dicarbonitrile (pyrrolizine is pyrrolo[1,2-a]pyrrole), C₂₉H₃₂N₄O [(Va) and (Vb)]. Crystal structure analyses prove the structures and configurations for three diastereoisomeric adducts (IIIa), (IIIb) and (Vb) unequivocally. In all three cases, the dimethylamino group appears endo at the five-membered ring formed in the cycloaddition step. In (cis-1a-cis-4a-transoid-8a,8b-transoid-8b,8c)-1,1-dimethyl- 4a-(pyrrolidin-1-yl)-2,8b-di-p-tolyl-1,1a,4a,5,6,7,8,8a,8b,8c-decahydro-4H-3,3a-diazacyclopropa[c]fluorene-4,4-dicarbonitrile, C₃₄H₃₉N₅, (VII), the cycloadduct formed using an azomethine ylid [(I)] and a cyclic enamine [(VI)], the aminofunction is again found in an endo position.

The azomethine ylide moieties of dicyano(7,7-dimethyl-2,5-di-p-tolyl-3,4-diazabicyclo[4.1.0]hepta-2,4-diene- 3-ium-3-yl)methanide, C₂₄H₂₂N₄, and (4-benzoyl-5,5-dimethyl-2-phenyl-1-azacyclopenta-1,3-diene-1-ium-1-yl)dicyanomethanide, C₂₂H₁₇N₃O, show normal bond distances and angles in comparison with related structures reported previously. The dipolar N-C distances of 1.430 (4) and 1.401 (3) Å are appreciably shortened compared with the C-N⁺ bond lengths [1.485 (9) Å] found in N-substituted pyridinium compounds, which is consistent with previous observations. Both title compounds pack in such a way that the ylide dipoles are arranged above each other but pointing in opposite directions.

Despite the demands of the stronger hydrogen-bonding inter­actions present in the crystal structures of the title compounds, C₁₆H₁₀N₈ and C₁₆H₁₂N₆O.H₂O, T-shaped and shifted π-stacked arrangements of aromatic moieties are preferred, leading to herringbone and/or π-stacked crystal-packing motifs. Therefore, in both these compounds the crystal packing is in accordance with the theory of arene–arene interactions being dominated by electrostatics.