Novel addition reactions of some benzopyrans

Abstract

Several chroman-4-ones were prepared by the pyrrolidine catalysed reaction of 2'-hydroxyacetophenone with carbonyl compounds. These were converted to 4-oxochroman-3-carbonitriles by a sequence of reactions which involved the preparation of 3-hydroxymethylenechroman-4-ones, their reaction with hydroxylamine to give benzopyrano[3,4-d]isoxazoles and base catalysed ring opening of the isoxazoles. The reaction of 2-isopropylchroman-4-one with ethyl formate yielded a rearrangement product, 3-(2-methylprop-l-enyl)chromone. The reaction of 2-cyclopropyl-2-methyl-3- hydroxymethylenechroman-4-one with hydroxylamine yielded 5-(2-hydroxyphenyl)-4-(4-hydroxy-1-methylbut-1-enyl)isoxazole as a major product. These difficulties limited the availability of benzopyrano[3,4-d]isoxazoles for the preparation of 4-oxochroman-3-carbonitriles.
A new rearrangement was discovered which accompanies the reduction of 4-oxochroman-3-carbonitriles with sodium cyanoborohydride and yields of 2-amino-3-alkylchromones in admixture with 3-hydroxy-3-(2-hydroxyphenyl)-2-
alkylpropanonitriles.
Lithium dialkynylcuprates were prepared and were successfully added, in a 1,4-fashion, to chromone-3-carbonitrile yielding 2-(2-hydroxybenzoyl)-2-penten-4-ynonitriles. The addition of lithium dipent-1-ynylcuprate to chromone-3-carbonitrile gave a remarkable new compound Z-2-[6-propyl-8-oxo-6H,8H-bis[1]benzopyrano[2,3-b:3.4'e]pyridin-6-yl]oct-2-en-4-ynonitrile. The 1,4-addition of dialkynylcuprates to chromone-3-carboxaldehyde afforded 2-ethynyl-3-hydroxymethylenechroman-4-ones
which suffered acid catalysed rearrangement to Z-2-hydroxy-3-(2-propynylidene)chroman-4-ones. Several chromones and coumarins bearing electronwithdrawing groups were prepared and gave high yields of 1,4-addition products with
lithium bis(trimethylsilylethynyl)cuprate. -Alkynyllithiums gave mainly 1,2-addition products with chromone-3-
carboxaldehyde in ether. 3-( 1 -Hydroxy-3-trimethylsilylprop-2-ynyl)chromone underwent an Ireland-Claisen rearrangement with triethyl orthoacetate yielding ethyl 5-(3-chromonyl)-3-trimethylsilyl-3,4-pentadienoate which rearranged further to give ethyl 5-(3-chromonyl)-3-trimethylsilyl-4-pentynoate. Besides the Ireland-Claisen rearrangement 3-(l-hydroxy-4,4-dimethylpent-2-ynyl)chromone also suffered an allylic Claisen rearrangement which afforded the byproduct ethyl 2-[2-Z-(4,4-dimethylpent-2-ynylidene)chroman-4-one]ethanoate.
The phosphorus(Ill) esters of the chromonylpropargylic alcohols underwent facile rearrangement to chromones bearing allene functions in position 3. The Diels-Alder adducts of these allenylchromones with 4-methyl-1,2,4-triazoline-3,5-dione were prepared and characterised.
2-(Trimethylsilylethynyl)chromone-3-carboxaldehyde was obtained from 3-hydroxymethylene-2-(trimethylsilylethynyl)chroman-4-one by oxidation with triphenylcarbenium tetrafluoroborate. 1,2-Addition of lithium acetylides afforded mixtures of 2-trimethylsilylethynyl-3-( 1 -hydroxy-2-propynyl)chromones and the desilylated 2-ethynyl-3-( I -hydroxy-2-propynyl)chromones. 3-( 1 -Hydroxy-4,4-dimethylpent-2-ynyl)-2-(trimethylsilylethynyl)chromone was
reacted with diethyl chlorophosphite giving diethyl 1-[3-(2-trimethylsilylethynyl)chromonyl]-5,5-dimethyl- I ,2-pentadien-3-phosphonate. The reaction took a different course with 3-(1-hydroxy-3-phenylprop-2-ynyl)-2- ethynylchromone affording diethyl 2-[2-(3-phenylprop-2-ynyl)chromonyl]ethynylphosphite through an apparent [2,5] sigmatropic rearrangement.
Diethyl 1-[3-(2-trimethylsilylethynyl)chromonyl]-4,4-dimethyl- 1 ,2-pentadien-3-phosphonate underwent Myers cyclisation on heating affording diethyl 2,2-dimethyl-1 -(3-trimethylsilyl-4-xanthonyl)propanephosphonate, a new method of preparation of
xanthones.