Borneol, synthesis

Thin-Layer Chromatography TLC) The function of TLC in organic synthesis is primarily one of allowing the experimenter to follow the progress of the reaction without actually interrupting the reaction. Since successful TLC can be carried out on a minute scale, only a very small fraction of the reaction mixture need be withdrawn and subjected to analysis. The following example of the TLC analysis of the chromic acid oxidation of borneol, described by Davis (3), is a useful model. 

A vast array of chiral auxiliaries have been derived from naturally occurring compounds containing the bicyclo[2.2.1]heptane unit (for review articles, see refs 1 -3). In all cases, the ultimate sources of these auxiliaries are the ketones camphor and fenchone, and the alcohols borneol and fenchol, as at least one enantiomer of each compound is provided in enantiomericaUy pure form by nature. Thus. ( + )-camphor [( + )-2], (-)-borneol [(-)- ], and (+)-fenchonc [( + )-5] are enan-tiomerically pure, convenient and inexpensive starting materials for organic synthesis and deriva-tization to give chiral auxiliaries. Most other compounds of this series are also commercially available, but can be prepared by oxidation or reduction of inexpensive precursors by standard methods. The evo-alcohols, such as the enantiomeric isoborneols, are accessible by standard complex hydride reductions of the ketones. The interconnection between these compounds is shown diagrammatically. 

Bromocamphor has been employed as the chiral starting material in an enantiospecific synthesis (ref. 146) which involved the formation of an indanone (CD moiety) related structurally to that used by Hoffmann-La Roche and led finally to (-)-estrone. Attempts to alkylate the a,p-unsaturated ketone with 2-(3-methoxyphenyl)ethyl iodide failed, not surprisingly, due to an elimination rather than the desired substitution reaction and accordingly the use of an a-methylenic derivative of the ketone and 3-methoxybenzyl chloride were obligatory. By the use of (-)-3-bromocamphor, which is readily available from (-)-borneol by oxidation to (-)-camphor and bromination, natural (+)-estrone could be obtained. 

Esters of 2-iodoxybenzoic acid (IBX-esters) 489 have been prepared by the hypochlorite oxidation of the readily available 2-iodobenzoate esters 488 (Scheme 2.139) and isolated in the form of stable microcrystalline solids [657,658], This procedure has been used for the synthesis of IBX-esters 489 derived from various types of alcohols, such as primary, secondary and tertiary alcohols, adamantanols, optically active menthols and borneol. Single-crystal X-ray data on products 489 revealed a pseudo-benziodoxole stmcture in which the intramolecular L--0 secondary bonds partially replace the intermolecular I - O secondary bonds, disrupting the polymeric structure characteristic of Phl02 and other previously reported iodylarenes [658], This stmctural feature substantially increases the solubility of these compounds in comparison to other iodine(V) reagents and affects their oxidizing reactivity. 

The first attempts to synthesis asymmetric polyisocyanides are recent ones [30]. Asymmetric polymerization of racemic isocyanide (Xllla) with nickel chloride or nickel acetylacetonate catalyst was attempted in the presence of chiral solvents, (-)-borneol or (+)-sec-butyl-alcohol, or in the presence of (+)-nickel alaninate with the racemic a-phenyl-ethylisocyanide (Xlllb), but the polymers did not show any activity. 

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