ASYMMETRIC INDUCTION IN THE ELECTROCYCLISATIONS OF 1,3 DIPOLAR INTERMEDIATES - THE 1.7 CYCLIZATION OF DIENE-CONJUGATED DIAZO-COMPOUNDS TO GIVE 1H-2,3-BENZODIAZEPINES

In the cyclisation of the diene-conjugated diazo compounds 4 to give the diastereoisomeric pair of 1H-2,3-benzodiazepines 7 and 8, alkyl and silyl ethers when present as the 'medium' sized group R(3), showed the opposite effect in controlling face selectivity to that of alkyl groups and the alkoxide anion. Thus, for example, when R(3) = OMe the diastereoisomer ratio (7:8) was 8:92 while in contrast when R(3) = O- Li+ the ratio was 85:15. The relative configurations of the chiral centres in the products 7 and 8 were determined by X-ray crystallography and their ratio in the cyclisations was measured by H-1 NMR spectroscopy and by HPLC. The results are rationalised in terms of a helical transition state for the cyclisation and the steric and polar effects of the substituents.