A new approach to helical primary structures of four-membered rings: (P)- and (M)-tetraspiro[3.0.0.0.3.2.2.2]hexadecane

Three-Membered-Ring-Based Molecular Architectures

“…Comparison of the values of for the now known five enantiomerically pure σ-[ n ]helicenes  ( M )-(−)- 434 (−192.7), ( P )-(+)- 438 (+373.0), ( P )-(+)- 461 (+672.9), ( P )-(+)- 463 (+909.9), and ( M )-(−)- 468 (−1302.5) 320,321  indicates a drastic and continuous increase of the specific rotation with an increasing number of three-membered rings (cf. ref ). This increase goes beyond that to be expected with increasing molecular weights (Figure ).…”

Polytriangulane

“…In constructing triangulanes beyond [3]­triangulane, , linear, helical, and branched isomers ensue (Scheme ). The helical [ n ]­triangulanes belong to the σ-helicene family, which also encompasses the spiro-cyclobutanes, , spiro-cyclopentanes, , oligotwistanes, , and helical diamondoids such as [1(2)­3]­tetramantane (Scheme ). The [ n ]­triangulanes, oligotwistanes, and diamondoids have no internal rotational degrees of freedom and therefore occur as highly rigid, solitary structures.…”

“…The σ-helicenes are inherently chiral, and their optically rotatory powers mirror their strain energies; thus, the helical [ n ]­triangulanes display the largest optical rotations of any alkane of comparable molecular weight. ,,,, This increase in the Cotton effect has been rationalized on the basis of the Walsh orbital model of the bonding in cyclopropane, in which the strained carbon–carbon bonds exhibit enhanced C methylene p -orbital contributions. , Remarkably, the rotatory power of rigid σ-helicenes , (as well as π-helicenes) increases monotonically with helix length, while it uniformly decreases in this manner for the conformationally flexible polyspirocyclobutanes and polyspirocyclopentanes. − …”

Pseudohelical and Helical Primary Structures of 1,2-Spiroannelated Four- and Five-Membered Rings: Syntheses and Chiroptical Properties

“…However, it was only in 1999 3 that the first enantiomerically pure helical hydrocarbon was reported. Today, five examples 3-6 for helices of three-membered rings are known, among them trispiro[2.0.0.2.1.1]nonane ( 1 ), , tetraspiro[2.0.0.0.2.1.1.1]undecane ( 2 ), and octaspiro[2.0.0.0.0.0.0.0.2.1.1.1.1.1.1.1]nonadecane ( 3 ). , In comparison, only two examples of helices of four-membered rings have been reported: trispiro[3.0.0.3.2.2]tridecane ( 4 ) and tetraspiro[3.0.0.0.3.2.2.2]hexadecane ( 5 ) . Of the systems cited, 1 − 3 are rigid and exist as single minimum structures, − while 4 and 5 are flexible and adopt different conformations …”

“…Of the systems cited, 1 − 3 are rigid and exist as single minimum structures, − while 4 and 5 are flexible and adopt different conformations = −192.7, c = 1.2, CHCl 3 ), , which doubles in ( M )- 2 ( = −381.2, c = 1.2, CHCl 3 ) 4 and again in ( M )- 3 ( = −890.5, c = 1.01, CHCl 3 ), , and to a low specific rotation of ( M )- 4 ( = −63.3, c = 1.1, CHCl 3 ) 7 which is further diminished in ( M )- 5 ( = −24.2, c = 1.2, CHCl 3 ) . The latter observation is especially surprising.…”