Asymmetric induction in the electrocyclisations of 1,3 dipolar intermediates: the 1.7 cyclisation of diene-conjugated diazo-compounds to give 1H-2,3-benzodiazepines

Abstract

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 R³, showed the opposite effect in controlling face selectivity to that of alkyl groups and the alkoxide anion. Thus, for example, when R³= OMe the diastereoisomer ratio (7:8) was 8:92 while in contrast when R³= 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 ¹H 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.