Preparation of 6‐demethoxy‐N‐formyl‐N‐northebaine and its Diels‐Alder reactions with methyl vinyl ketone and nitroethene; novel 8‐nitro‐substituted 6α,14α‐ethenoisomorphinans and 6β,14β‐ethenomorphinans (chemistry of opium alkaloids, part XXVIII)