Synthesis of calix[4]naphthalenes and their properties

Ashram, Muhammad (1997) Synthesis of calix[4]naphthalenes and their properties. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The calixarenes 2-4 are well-known and well-studied cyclic oligomers formed by the base-induced condensation of formaldehyde with p-tert-butylphenol. They are potentially a very versatile class of host molecules. -- This thesis describes the syntheses and some properties of the calixnaphthalenes, which are a new class of cyclic formaldehyde-naphthol tetramers which are analogous with the calixarenes. These calixnaphthalenes were prepared by either one-pot or convergent procedures. -- Calixnaphthalenes offer some advantages over the calixarenes. For example, since the naphthalene unit is larger than benzene the cavity of cone conformations of the corresponding calixnaphthalenes should be deeper. Also, the presence of a B ring in naphthalene provides a site for the addition of different functional groups, which allow calixnaphthalenes to be modified. Furthermore, due to the low symmetry of 1–naphthol and 3-hydroxy-2-naphthoic acid, some calixnaphthalenes can be inherently chiral. They therefore have potential applications as chiral hosts, or chiral ligands. -- Calix[4]naphthalenes 10-12 were synthesized first by the direct condensation of 1-naphthol and formaldehyde under basic conditions. Due to the difficulty in the separationand purification of these compounds, a convergent approach was used to synthesize these compounds as well as 13 in larger amounts for further investigations. The calix[4]-naphthalenes are conformationally flexible at room temperature even after their derivatization as tetraester derivatives. -- In order to synthesize calixnaphthalenes which are closer to calixarenes in the location of the hydroxyl groups within the lower rim of the calixnaphthalene basket, a retrosynthetic analysis of compounds 57 and 62 showed that 3-hydroxy-2-naphthoic acid (9) would be a suitable starting material. Syntheses of compounds 57 and 62 were effected by self-condensation reactions of 3-(hydroxymethyl)-2-naphthol and 6-tert-butyl-3-(hydroxymethyl)-2-naphthol, respectively, using TiCl4/dioxane conditions. -- Variable temperature ¹H NMR analyses showed that compounds 57 and 62 are conformationally flexible at room temperature, but the methylene protons are split into doublets at -10°C and the molecules are locked in the cone conformation as its preferred conformation at -20°C. X-ray analysis showed that in the solid state 57 adopted a pinched-cone conformation. -- In order to modify calixnaphthalenes 57 and 62, they were converted into their ester derivatives by reaction with ethyl bromoacetate. Calix[4]naphthalene 57 produced two tetraester derivatives in the cone and partial-cone conformations, 70b and 70a, respectively. Calix[4]naphthalene 62 produced mono- and diester derivatives 62a and 62b, respectively. ¹H NMR and molecular modeling analyses revealed that 62b exists preferrentally in the 1,3-alternate conformation. -- Using a modified oxidation procedure, the hydroxyl groups of 57 and 62 were oxidized to produce bis(spirodienone) derivatives. Compound 57 afforded only one somer, 78, having C2 symmetry as revealed by NOED experiments and confirmed by x- ray analysis. Compound 62 produced two isomers, 81 and 80, having C, and C2 symmetries, respectively. Mild oxidation of 62 produced one monospirodienone isomer. -- Dithiadihomocalix[4]naphthalenes 82 and 83 were synthesized from l-naphthol and 3-hydroxy-2-naphthoic acid using procedures commonly employed in cyclophane chemistry. These compounds are found to be conformationally flexible at room temperature. Photolytic sulfur extrusion of 83 produced the corresponding dihomocalix[4]naphthalene 83a while such conditions employed with 82 did not produce the corresponding 82a. -- In order to enlarge the annulus of calix[4]naphthalenes derived from 3-hydroxy-2- naphthoic acid, approaches to the tetrahomocalix[4]naphthalene isomers 85a and 91a-93a were attempted. The synthetic approach employed was the base-mediated coupling of 97 and 98 to produce, in principle, tetrathianaphthalenophanes 91-94, which are potential precursors of the corresponding tetrahomocalix[4]naphthalenes after sulfur extrusion. Instead, four isomeric dithianaphthalenophanes 99-102 were produced from the above coupling reaction. Photolysis of these dimers produced two isomeric tetrahydrodibenzo-pyrenes, 107 and 108, instead of [2.2](l,3)naphthalenophanes, 111 and 112. This type of sulfur extrusion with concomitant transannular cyclization appears to be general one, which could offer some advantages for the synthesis of tetrahydropyrenes and tetrahydrodibenzopyrenes.

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