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The First Enantiomerically Pure [ n ]Triangulanes and Analogues: σ‐[ n ]Helicenes with Remarkable Features
Author(s) -
de Meijere Armin,
Khlebnikov Alexander F.,
Kozhushkov Sergei I.,
Kostikov Rafael R.,
Schreiner Peter R.,
Wittkopp Alexander,
Rinderspacher Christopher,
Menzel Henning,
Yufit Dmitrii S.,
Howard Judith A. K.
Publication year - 2002
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20020215)8:4<828::aid-chem828>3.0.co;2-y
Subject(s) - diastereomer , ethyl diazoacetate , chemistry , stereochemistry , methylene , cycloaddition , heptane , methanol , nonane , organic chemistry , cyclopropanation , catalysis
( 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 [(1 RS )‐ 12 ] and exo ‐dispiro[2.0.2.1]heptane‐1‐carboxylic [(1 RS ,3 SR )‐ 13 ] acids. The optical resolutions of rac ‐ 12 and rac ‐ 13 furnished enantiomerically pure acids ( S )‐(+)‐ 12 , ( R )‐(−)‐ 12 , (1 R ,3 S )‐(−)‐ 13 , and (1 S ,3 R )‐(+)‐ 13 . The ethyl ester ( R )‐ 25 of the acid ( R )‐(−)‐ 12 was cyclopropanated to give carboxylates (1 R ,3 R )‐ 26 and (1 R ,3 S )‐ 26 . The ester (1 R ,3 S )‐ 26 and acids (1 R ,3 S )‐ 13 and (1 S ,3 R )‐ 13 were converted into enantiomerically pure methylene[3]triangulanes ( S )‐(−)‐ and ( R )‐(+)‐ 28 . An alternative approach consisted of an enzymatic deracemization of endo ‐[(1 SR ,3 SR )‐dispiro[2.0.2.1]heptyl]methanol ( rac ‐ 20 ) or anti ‐[(1 SR ,3 RS )‐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 (1 R ,3 S ,4 S )‐ and (1 S ,3 R ,4 R )‐ 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, CHCl 3 )] or +373.0 [( P )‐ 4 , c =1.18, CHCl 3 )]. 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.