Reactions and rearrangements of chroman-4-ones and related compounds

Abstract

The synthesis of a range of novel 2,2-disubstituted chroman-4-ones has been accomplished by the pyrrolidine catalysed condensation of a 2-hydroxyacetophenone with aldehydes and ketones. The reaction permits access to both symmetrically and unsymmetrically substituted compounds. The reaction has been extended to the preparation of a novel furochromanone, an analogue of the natural product khellin.
The Schmidt reaction of 2,2-disubstituted chroman-4-ones in an acetic-sulphuric acid mixture affords 2,3,4.5-tetrahydro-1.4-
benzoxazepine-5(411)--ones, resulting from alkyl migration of the iminodiszonium intermediate. Chroman-4-ones possessing a carbocation stabilising function show anomalous behaviour, the initially formed 1,4-benzoxazepinone being particularly
susceptible to protolysis. 2-Methyl-2-phenylchromanone affords a 2-oxazoline as the major product, whilst 2-cyclopropyl-2-
methylchromanone gives a mixture of five components which have been separated and characterised. The major products result from hydration and acetolysis of the cyclopropyl carbenium ion.
Products resulting from aryl migration and anti-Markovnikov addition of acetic acid to the cyclopropane ring are also formed.
Carbanions derived from chromanones have been generated and used to prepare a range of 3-hydroxymethylenechroman-4-ones. Conjugate reduction of the corresponding chromone by sodium borohydride in pyridine provides a simple entry to 3-acetyl and 3-aroyl-2-methylchromanones. These dicarbonyl compounds are versatile intermediates and have been used to prepare a range of benzopyrano isoxazoles, pyrazoles. pyrroles and pyrimidines.
3-Hydroxymethylenespiro[chroman-2. 1 '-cyclopentan]-4-one is particularly unstable to acid-base conditions. Attempts to
prepare this compound resulted only in the formation of 3-(1 cyclopentenyl)chromone, arising from a bond-switch operation
initiated by fragmentation of the pyranone ring in a retro-Michael reaction. Conditions suitable for extension of this reaction to other 3-hydroxymethylenechromanones have been developed, affording access to a range of hitherto inaccessible 3-alkenylchromones.
The regio- and stereo-chemical outcome of the reaction has been elucidated.
Some preliminary investigations of the behaviour of 3-alkenyl chromones in Diels-Alder reactions are described.