Improved procedure for the preparation of 3,4‐dihydro‐2,2‐dimethyl‐2H‐1‐benzopyran‐3‐ones

Abstract

Treatment of 2,2‐dimethyl‐2H‐1‐benzopyrans 5 with a m‐chloroperoxybenzoic acid‐trifluoroacetic acid mixture and subsequent short path bulb to bulb vacuum distillation of the crude 3,4‐hydroxyesters 6 formed afforded title compounds in good yields. Suppression of trifluoroacetic acid was required when using 2,2‐di‐methyl‐2H‐1‐benzopyrans with electron donating substituents such as precocenes, as starting compounds.

Comment on Cubé Resin Insecticide: Identification and Biological Activity of 29 Rotenoid Constituents

“…The rationale for the proposed structure 19 was based (in part) on the presence of NMR absorptions for a trisubstituted double bond (1H absorption at δ 6.84, s, H-4‘; δ 100.8, d, C-4‘; and δ 160.2, s, C-5‘), implying an almost quantitative presence of the enol form 1 . However, the reported spectroscopic evidence for simple models of 3,4-dihydro-2,2-dimethyl-2 H -1-benzopyran-3-ones from our past experience ( , ) and other literature reports ( , ) describes the presence (for the corresponding H-4‘ signal) of a 2H absorption at δ ∼3.5 and points out an almost quantitative presence of the keto form in the plausible keto−enol equilibrium. Because from these literature reports the keto form must be expected to be the preferred one, the question arose whether the enol tautomer was really so much favored in this particular case.…”