(3Z)-2-azahexa-1,3,5-trienes: Generation and regioselectivity of 1,5- and 1,6-cyclizations

Unlike electron-rich analogs, electron-deficient azaand oxazapolyenes readily undergo thermal and catalytic 1,4-, 1,5-, and 1,6-cyclizations to produce various 4-, 5-, and 6-membered N-, N,N-, and N,O-heterocycles [1–10]. Such cyclizations have been reported for 2-aza-, 3-aza-, 1-oxa-4-aza-, 1-oxa-5-aza-, and 1,4diazahexa-1,3,5-trienes, some of which are fairly stable, while the others are reactive intermediates generated by either aza-Wittig reaction or carbenoidinitiated ring opening of 2H-azirines or isoxazoles. Only 1,6-cyclizations to pyridine derivatives have been reported for 2-azahexa-1,3,5-trienes generated by reaction of phosphazenes with carbonyl compounds [9, 10]. Herein we describe the first example of generation of electron-deficient (3Z)-2-azahexa-1,3,5-trienes (4) via carbenoid-initiated azirine ring expansion and their 1,5-cyclization; stereochemical factors determining the cyclization regioselectivity have also been determined. Initial azirines E-1 and Z-1 were synthesized according to the procedure reported in [11]. The reaction of E-1 with diazo ester 2 in boiling 1,2-dichloroISSN 1070-4280, Russian Journal of Organic Chemistry, 2016, Vol. 52, No. 12, pp. 1851–1853. © Pleiades Publishing, Ltd., 2016. Original Russian Text © K.V. Zavyalov, M.S. Novikov, A.F. Khlebnikov, N.V. Rostovskii, 2016, published in Zhurnal Organicheskoi Khimii, 2016, Vol. 52, No. 12, pp. 1857–1859.