Part I

TERPENOIDS

Monoterpenoids

BY A. F. THOMAS

There has been little increase in the volume of work published this year, but the space available for this Report is slightly reduced, so economy has been achieved in two ways. Papers not requiring any discussion, either because they are repetitive or because the minor point they make is evident from little more than the title, are placed at the end of each section. The number of formulae has been reduced, and more extensive use is made of names in the text. With these limitations, every effort has been made to quote all papers relevant to monoterpenoids.

1 Physical Measurements: Spectra etc.; Chirality

Titanium tetrachloride is recommended as a useful shift reagent in assigning 13C n.m.r. frequencies, particularly in αβ-unsaturated ketones such as carvone (1). which has a shift of -5.83 Hz for the β-carbon frequency, compared with -1.51 Hz using [Eu(fod)3].The C signals of the bridge methyl groups of camphor (2) (C-9 and C-10) have been assigned using another new shift reagent, tris-[4,4,4-trifluoro-1-(2-thienyl) -1,3-butadi-ene]europium(III), and [sup.13]C chemical shifts of substituted tricyclenes are discussed. The importance of non-axial symmetry in interpreting lanthanide-induced shifts in ketones has special relevance for monoterpenoids, and Newman discusses the case of camphor (2). Shifts induced by [Eu(dpm)3] in saturated o- and p-menthones, and its effect on the rotation of the isopropyl group in menthone and menthol have been measured.

Some well known mass spectra of monoterpenoid alcohols have been published again.

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The adsorption on a mercury electrode of borneol and adamantan-1-01 has been compared with that of camphor because of similar polarographic behaviour.'

The chirality of alcohols, notably (-)-linalool, (-)-menthol, and cis-menth-2-enol (3) (the name in the text is different), is rapidly established by measuring the c.d. of the complex with copper hexafluoroacetylacetonate. Photodecomposition of racemic camphor with circularly polarized light occurs enantiomerically, the optical purity of recovered camphor theoretically rising to 100 % at the end of the reaction. After 99% destruction of the ([+ or -])-camphor, the remainder has 20% optical activity.

Notable examples of the induction of asymmetry by complexing with monoterpenoids are the resolutions of the iron complex (4) and a titanium complex. The menthyl-(5;R = PPh,) and neomenthyl-(6; R = PPh2)diphenylphosphinesare epimeric, chiral ligands, suitable for asymmetric syntheses.' Another account has appeared of an attempt to induce asymmetry by cyclization of homogeranic (-)-menthyl ester.

Various micro-organisms (Tridioderrnu, Absidia, etc.) hydrolyse some racemic acetates chirally; thus a mixture of ([+ or -])-isopulegyl acetate [([+ or -])-(7; R = COMe)] and ([+ or -])-neoisopulegyl acetate [([+ or -])-(8; R = COMe)] is converted into a separable mixture of (-)-isopulegol [(-)-[7; R = H)], (+)-isopulegyl acetate [(+)-(7; R = COMe)], and ([+ or -])-neoisopulegyl acetate. Since interconversion with citronella1 (9) is easy, this represents a practical resolution of ([+ or -])-citronellal. The acetates of menthol and carvo-menthol undergo this reaction, but not those of the stable axial alcohols, neomenthyl acetate (5; R = OCOMe) and neocarvomenthyl acetate (10). The further the acetate group is from the asymmetric centre, the lower is the optical yield.

New separation techniques for monoterpenoids are liquid chromatography on porous polymer (Hitachi gel 3010) and gas chromatography on graphitized carbon black for the notoriously delicate separation of the menthol isomers (although neo-menthol and menthol are not cleanly separated).

2 General Chemistry

Acid-catalysed isomerization of terpenoid hydrocarbons occupies much space in the literature, usually without the emergence of great novelty (see, however, pinenes). The use of mentha-2,8-diene as substrate and other hydrocarbons on TiO2–H2SO4 catalysts is described. Liquid-phase rearrangements of pinene and limonene give results varying with acid strength, and similar variations occur with basic strength in the base-catalysed rearrangements. By judicious choice of base, it is possible to prepare a particular menthene from one more accessible. Rearrangement of limonene (11) in phosphoric acid was known to yield a bicyclic hydrocarbon; the latter is shown to be a mixture of three isomers (Scheme 1). Isomerization of α-pinene over ferric phosphate at 180 — 560 °C leads to rearrangements and ring-opening to menthanes, and heating terpenes with diethyl hydrogen phosphite yields phosphonates, also with rearranged skeletons; pinenes give menthenes, camphene gives isocamphenyl ethylphosphonate, and limonene (11) gives a mixture containing a small amount of a bornyl phosphonate.

Another paper on the hydration of monoterpenoids in the presence of an ion-exchange resin has appeared (cf. Vol. 4, p. 13). Treatment of linalool (12) with chloranil results a series of dehydrations and hydrations; myrcene (13), ocimene (14), and their hydra-tion products are formed, but cyclization to menthadienes and subsequent hydration also occur. Geraniol (15), nerol (16), and linalool (12) are interconverted, and give similar products, nerol favouring the cyclized alcohol (17). The claim that β-pinene is among the dehydration products of linalool (l2) could not be confirmed using boron trifluoride or iodine as catalyst.

Of particular relevance to monoterpenoids is the comparison of reaction parameters for triphenyl phospite ozonide (TPPO) formation and those of photosensitized oxy-genation, where it has been shown that singlet oxygen cannot be a common active species for both types; limonene (11), for example, shows a very different product distribution in the two cases. TPPO oxidation occurs at lower temperatures than ozonide decomposition, and is in some cases, e.g. α-terpinene (18), more selective. A general study of epoxidation of methylenecyclohexanes, closely related to monoterpenoids, includes a discussion of the epoxide conformations. Epoxidation of ally1 alcohols with t-nutyl hydroperoxide catalysed by vanadium or molybdenum complexes has enabled the new epoxides of geraniol (19) and linalool (20) to be prepared. Oxidation of alcohols to ketones (menthol to menthone, borneol to camphor, etc.) generally occurs with N-chlorosaccharin, but limonene (11) gives a 4-chloro insertion product.

Details of the highly...