Thia helicene

The axial-to-helical configuration transfer was more recently applied to [7]thia-helicenes 29 and 33, using McMurry reaction of dialdehyde or diketone in the annelation step, to form the central benzene ring [78, 79]. Both enantiomers of helicenes could be obtained. 

Fig. 15.31 Space-filling plot for [7]thia-helicene 33 obtained from the X-ray structure of the racemic crystal.

The synthetic methodologies for the preparation of these pecuhar thia-helicenes are therefore quite different with respect to those of the thiaheH-cenes described above in which thiophene alternates with benzene rings. 

A representative example of such thiahelicenes is compound 42 prepared through an oxidative coupling between two appropriately functionalized P-trithiophene units 43 to give 44 which with LDA followed by annelation in the presence of (PhS02)2 affords thia[7]helicene 42 (Scheme 18). 

The thia[7]helicene 54 is prepared as the pure (f)-(+) enantiomer via an asymmetric Ulhnann coupling of the enantiomerically pure iodo derivative 55 which gives the intermediate 57. Conversion to the dialdehyde 56 is followed by intramolecular McMurry coupling then gives product 54 (Scheme 23) (1997TL457, 1997J04465). 

Ben Hassine et al. prepared [6]helicene derivatives (23)-(25) and discussed their characterization. Diastereomers of the (M/P)-[6]helicenyl (5)-camphanoate (25) were readily isolated by silica gel chromatography. In addition, novel thia[6]helicenes (26) and (27), thia[5]helicene (28), and aza[5]helicene (29) were synthesized and characterized by his group. Because the presence of heteroatoms in such heterohelicenes can tune helicity parameters, many novel heterohelicenes have been designed and prepared. 

Heterodiarylethenes (38)-(40) upon UV irradiation in the presence of iodide and/or propylene oxide led to heterohelicenes. " Dehaen et al. prepared some thia[n]helicenes ( = 5, 6, 7, 9, and 11) upon visible ir-radiation. The compound (41) underwent double oxidative photocyclization to form the thia[ll]helicene (42) in 33% yield. The photocyclization of the L-menthyl-substituted compound (43) gave the (i p,P)-phospha[6]helicene (44) in 25% 3deld. The resulting (44) further underwent the intermolecular [2 -I- 2] cycloaddition to form the dimeric phospha[6]helicene (45) in 75% yield. ... 

A variety of aza-, oxa- thia-, phospha-helicenes were prepared by... 

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