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Medium-ring diphosphines offer unique opportunities to study the interactions between phosphorus atoms. In addition, monocyclic medium-ring diphosphines may be very useful ligands in transition metal complexes. A number of useful synthetic routes to these compounds have been developed, but further efforts in this direction might be rewarding.

Transketalization reactions between the diketals of 1,4-cyclohexanedione and the diketonides of pentaerythritol yield a spiropolymer which is insoluble in all solvents tried, but which can be hydrolysed to 1,4-cyclohexanedione and pentaerythritol. At short reaction times, oligomers such as 3,3,21,21-tetramethyl-1,5,10,14,19,23,26,29-octaoxapentaspiro[5.2.2.2.2.5.2.2.2.2.]untriacontane can be isolated. However, all attempts to achieve a genuine equilibrium between the monomers, oligomers and polymer were fruitless. The 13 C cross-polarization/magic angle spinning (CP/MAS) n.m.r. spectrum of the polymer shows extra lines relative to those of soluble model compounds; this is probably due to a slow ring inversion of the 1,3-dioxane rings in the solid polymer.

Eschweiler-Clarke methylations of both acyclic and cyclic polyamines can lead to methylation products of fragments of the original polyamine; thus 1,5,9,13-tetra-azatridecane yields 1,1,3,3-tetramethylpropanediamine exclusively and 1,5,9-triazacyclododecane gives 45% 1,5,9-trimethyl-1,5,9-triazacyclodecane and 45% 2,6,10-trimethyl-2,6,10-triazaundecane.

Reaction of two equivalents of 1-lithio-3,4-dihydronaphthalene with acenaphthenequinone between 0°C and 20°C leads to a derivative of tricyclo[4.3.0.05,9]nonane by di-oxy-Cope rearrangement followed by the unprecedented ”criss-cross“ 2π + 2π cycloaddition of the two enolate ions formed.

Quaternary centres impose a particularly simple conformational regime on adjacent acyclic bonds; this is shown by force field calculations on alkanes and confirmed by crystal structure database studies on quaternary ammonium ions; possible implications are considered.

Cyclonona-1,2,4,5,7,8-hexaene (2) and cyclonona-1,2,4,8-tetraen-6-yne (4) are relatively strain-free according to MINDO/3 and MNDO calculations. The valence isomeric structures (1) and (3) with trefoil aromaticity are less stable by ∼100 and 37 kcal mo1-1 respectively.

1,6-Diazabicyclo[4.3.3]dodecane ( 7 ), 1,6-diazabicyclo[4.3.2]undecane ( 8 ) and 1,8-diazabicyclo[6.3.3]tridecane ( 9 ) have been made by reduction of tricyclic α-aminoammonium salts with LiAlH 4 ; the protonation and oxidation of 8 and 9 are discussed.

Condensation of 4-chlorobutanal and 5-chloropentanal with six cyclic diamines gave twelve α-aminoammonium salts; cleavage of these with LiAlH 4 gave medium-ring bicyclic diamines.

1,5-Shifts of vinyl and related groups occur by a mechanism in which σ-bond-making is almost complete before bond-breaking begins; in at least one case (the rearrangement of cis-9,10-dihydronaphthalenes) a discrete intermediate is involved.