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An investigation of routes towards naturally occurring naphthopyrans

Gruchlik, Yolanta (1998) An investigation of routes towards naturally occurring naphthopyrans. PhD thesis, Murdoch University.

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Many natural products possessing the 3,4-dihydro-1H-naphtho[2,3-c ]pyran ring system occur as 5,10-quinones. Owing to their significant biological activity as antibiotics or potential antineoplastic agents, the 3,4-dihydro-lH-naphthopyran-5,10-quinones have attracted considerable interest and many have been synthesised by several research groups. However, the corresponding non-quinonoid naphthopyrans have received little attention. The investigations described in this thesis are directed towards a synthesis of glucoside B 7, the non-quinonoid cleavage product of the naturally occurring aphid pigments protoaphin-jb, protoaphin-sl and deoxyprotoaphin, as well as related naphthopyrans.

In the first chapter, the isolation and identification of the aphid pigments is presented. This is followed by a brief review of previous syntheses of some compounds related to the aphid pigment derivatives, focusing on the syntheses of the pyran ring system. The racemic synthesis of the aphid pigment derivatives quinones A and A' and deoxyquinone A and previous efforts involving models for the synthesis of glucoside B are reviewed in the latter part of the chapter. The first asymmetric approach towards the quinonoid derivatives of the aphid pigments is also discussed.

Several approaches for the preparation of glucoside B are described. In Chapter 2, the first approach towards a racemic synthesis of glucoside B is described through a key stereoselective isomerisation of 4-aryl-2,5-dimethyl-1,3-dioxolanes into pyrans using titanium tetrachloride, an intramolecular version of the Mukaiyama reaction.

A possible approach towards the first asymmetric synthesis of glucoside B, involving an intramolecular reaction between metal phenolates and chiral aldehydes is proposed in Chapter 3. Since glucoside B lacks an oxygen in the 5-position, the possibility of cleaving this oxygen from an appropriate precursor is investigated first.

A highly promising synthetic strategy is detailed in Chapter 4. It involves intramolecular cyclisation of asymmetric (3'-hydroxyphenyl)aldehydes under mildly acidic conditions. These are prepared by the initial transformations of commercially available 2,3-dihydroxybenzaldehyde into 1-(3'-benzyloxy-2'-methoxyphenyl)ethanol. The aliphatic side chain is then extended by formation of a benzylic ether with (S)-ethyl lactate. Ultimately, two diastereomeric optically pure phenols, each bearing an aldehydic side chain derived from (S)-ethyl lactate, are prepared.

Thus, the first chiral synthesis of a benzopyran having the same absolute stereochemistry as the enantiomer of glucoside B, i.e., benzo[c]pyran 246, is presented.

Item Type: Thesis (PhD)
Supervisor(s): Giles, Robin
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