Azo bridges from azines .21. Carbonyl and enol functionalities in parallel positions to azo groups

The synthesis of compounds of type A3, carrying both, a silylenol and an azo grup in parallel rigid positions, and the syntheses of ketone and diketone of type B and C, respectively, are reported. The crucial precursors, the mono- and dihydroxy acetals 2 and 3, both obtained from 1, are transformed by the already reported route to azo compounds 4 and 5. The latter compound is also prepared by dihydroxylation of the olefinic bridge in 7. Swern oxidation of alcohols 4, 6, and 5 yields ketones 12, 13, and 14. Enol silyl ether 15 (type A3) is obtained from 12 via its enolate 17 together with small amounts of diazetidine 16 which may arise from the equilibrium 17 reversible arrow 18 reversible arrow 19. By contrast, cyano ketone 20 yields the enol silyl ether 21 quantitatively.