The First Enantiomerically Pure [n]Triangulanes and Analogues: σ-[n]Helicenes with Remarkable Features

(M)‐(−)‐ and (P)‐(+)‐Trispiro[2.0.0.2.1.1]nonanes [(M)‐ and (P)‐3] as well as (M)‐(−)‐ and (P)‐(+)‐tetraspiro[2.0.0.0.2.1.1.1]undecanes [(M)‐ and (P)‐4]‐enantiomerically pure unbranched [4]‐ and [5]triangulanes–have been prepared starting from racemic bicyclopropylidenecarboxylic [(1RS)‐12] and exo‐dispiro[2.0.2.1]heptane‐1‐carboxylic [(1RS,3SR)‐13] acids. The optical resolutions of rac‐12 and rac‐13 furnished enantiomerically pure acids (S)‐(+)‐12, (R)‐(−)‐12, (1R,3S)‐(−)‐13, and (1S,3R)‐(+)‐13. The ethyl ester (R)‐25 of the acid (R)‐(−)‐12 was cyclopropanated to give carboxylates (1R,3R)‐26 and (1R,3S)‐26. The ester (1R,3S)‐26 and acids (1R,3S)‐13 and (1S,3R)‐13 were converted into enantiomerically pure methylene[3]triangulanes (S)‐(−)‐ and (R)‐(+)‐28. An alternative approach consisted of an enzymatic deracemization of endo‐[(1SR,3SR)‐dispiro[2.0.2.1]heptyl]methanol (rac‐20) or anti‐[(1SR,3RS)‐4‐methylenespiropentyl]methanol (rac‐18). This afforded (S)‐(−)‐ and (R)‐(+)‐28 (starting from rac‐20), as well as enantiomerically pure (M)‐(−)‐ and (P)‐(+)‐1,4‐dimethylenespiropentanes [(M)‐ and (P)‐23] starting from rac‐18. The methylenetriangulanes (S)‐(−)‐ and (R)‐(+)‐28 were cyclopropanated furnishing (M)‐ and (P)‐3. The rhodium‐catalyzed cycloaddition of ethyl diazoacetate onto (S)‐(−)‐ and (R)‐(+)‐28 yielded four diastereomeric ethyl trispiro[2.0.0.2.1.1]nonane‐1‐carboxylates in approximately equal proportions. The enantiomerically pure esters (1R,3S,4S)‐ and (1S,3R,4R)‐30 were isolated by careful distillation and then transformed into [5]triangulanes (M)‐ and (P)‐4 using the same sequence of reactions as applied for (M)‐ and (P)‐3. The structures of the key intermediates (R)‐12 and rac‐31 were confirmed by X‐ray analyses. Although [4]‐ and [5]triangulanes have no chromophore which would lead to any significant absorption above 200 nm, they have remarkably high specific rotations even at 589 nm with [α]$\rm{_{D}^{20}}$=−192.7 [(M)‐3, c=1.18, CHCl3)] or +373.0 [(P)‐4, c=1.18, CHCl3)]. This remarkable optical rotatation is in line with their helical arrangement of σ bonds, as confirmed by a full valence space single excitation configuration interaction treatment (SCI) in conjunction with DFT computations at the B3LYP/TZVP//B3LYP/6‐31+G(d,p) level of theory which reproduce the ORD very well. Thus, it is appropriate to call the helically shaped unbranched [n]triangulanes the “σ‐[n]helicenes”, representing the σ‐bond analogues of the aromatic [n]helicenes. High