Studies in Naphthopyran chemistry

Li, Jianchang (1991) Studies in Naphthopyran chemistry. Doctoral thesis, Lancashire Polytechnic.

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A series of derivatives of naphtho[1,2-b]pyran and the [2,1-b] and [2,3-b] isomers has been prepared. Those compounds with aryl substituents adjacent to the hetematom have been obtained by an acid catalysed reaction of a naphthol with a 1,1-
diarylprop-2yn-l-ol and thermal cyclisation of the intermediate ether. Alkyl and spiro-cycloalkyl derivatives have been synthesized by the reduction of the related naphthopyranones and dehydration of the resulting pyranols. The latter reaction was accompanied by dimerised and polymerised material in the [1,2-b] and [2,3-b] series.
Some reactions of the pyran double bond have been studied in order to assess the influence of the position of fusion of the naphthalene unit on to the pyran ring. Based on the reaction with hypobromous acid and on direct epoxidation of the
double bond, the naphthopyrans are more reactive than the corresponding benzopyrans. Particular reactivity was exhibited by the [1,2-b] and [2,3-b] systems, which were also prone to electrophilic attack in the naphthalene moiety.
Ring opening reactions of the epoxynaphthopyrans have been investigated and found to be attractive precursors for benzo-fused derivatives of chroman-3-ones and chroman-3,4-diones.
The 9-hydroxy derivatives of the naphtho[2,1-b]pyrans have been etherified with o-halogenoalcohols and these compounds together with their 9-hydroxy precursors have been esterified with methacryloyl chloride. A range of novel polymers has been prepared by the free radical solution polymerisation of these methacrylate esters. The resulting naphtho[2,1-b]pyran based polymers varied in the substitution at C-3 and in the length of the group separating the naphthopyran unit
from the polymer backbone. Macromolecular weight and its distribution have been measured for all of the polymers by gel permeation chromatography. Increase in the length of the spacer group led to a higher molecular weight and to a narrower distribution for the polymer. Conversely, increasing the size of the substituents at C-3 resulted in a decrease in molecular weight.
The photochromic properties of the monomers and polymers have been investigated. Alkyl and spiro-cycloalkane substituted naphtho[2,1-b]pyrans showed no photochromism at 0 °C, but their derived polymers become pale yellow on irradiation. Diaryl derivatives exhibited red to brown photochromism both as monomers and polymers. Polymerisation resulted in a decreased rate of fading which is influenced by the length of the spacer.

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