Treatment of dl‐1‐formyl[6]helicene (1) with the ylid of (EtO)2P(O)CH2CO2Et in boiling C6H6 gave a mixture of two dl epimeric cycloaddition products: trans‐2a (80%) and cis‐2b (12%) An X‐ray diffraction study (Van Meerssche et al. [2]) fully confirmed the structure and the stereochemistry assigned to the most abundant stereoisomer 2a by UV., 1H‐INDOR, NOE and catalytic hydrogenation of the non‐benzenoid double bonds.
A systematic investigation of chemically induced asymmetric photosyntheses has been carried ou in the helicene series. The following points have been examined: the influence of the nature and of the position of the chiral inducing group in the precursor, the influence of the structure of the precursor and the effect of a double induction. The best asymmetric stereoselectivities were obtained: (a) when the photocyclisation gave rise to a helicene skeleton substituted at position 1-; (b).at position 2-, when an ester of optically pure 2-hydroxymethyl[6]helicene was used as the chiral inducing group.
e n t h y l e s t e r s , have b e e n s t u d i e d I I a + I I a and I b -c I I b l . The o p t i c a l y i e l d of t h e t w o r e a c t i o n s i s of t h e same m a g n i t u d e /5-6%), b u t t h e s u b s t i t u t e d
Die Menthylester (Ia) ergeben nach 2stündiger Bestrahlung in Cyclohexan in Gegenwart von katalytischen Mengen Jod die cyclischen Menthylester (IIa) (Ausbeute 6%), die nach Hydrierung mit LiAlH4 in die chromatographierbaren Alkohole (IIb) übergehen.
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