Patchouli alcohol, structure

Occasionally, however, things can go awry and examples exist in the chemical literature (albeit very few in number) of natural products whose structures, even after the "structure confirmation" by total synthesis, were shown to be incorrect in the light of the results obtained by X-ray crystallographic analysis. Patchouli alcohol, a natural sesquiterpene of some interest in the perfumery industry, provides an illuminating example. 

Later on, however, X-ray crystallographic analysis by Dunitz of a single crystal of the corresponding chromic diester [11] showed that the actual structure of patchouli alcohol was that of a bridged tricyclic compound (4) (Scheme 1.1), with all its rings having six carbon atoms. 

Draw the structures of twistane, tropinone, exo-brevicomin, patchouli alcohol, longifolene, sativene, luciduline and porantherine and search bifunctional relationships, rings, synthetically significant rings, bridgeheads, core bonds and strategic bonds of each one of them. 

A useful synthesis (ref.ll)of patchouli alcohol, an important fragrant constituent of patchouli oil, from (+)-camphor, that onetime important natural product which was employed as a plasticiser for nitrocellulose (itself a semi-synthetic polymer), was complicated by structural revision of the sesquiterpene alcohol. Dihydrocarvone (14) obtained by saturation of the ring double bond in carvone, a major constituent of oil of spearmint has been employed for two very different sesquiterpenes, the ketone campherenone (15) and the alcohol, occidentalol (16). In the first case an enol acetate was converted to a bicyclic intermediate by earlier established methodology and the route emulated a plausible biogenetic sequence giving racemic campherenone (ref.12) as shown. Any chirality in (14) is apparently lost. 

Ourisson and co-workers have published full details of the isolation and structural elucidation of both y-gurjenene (360) and seychellene (361, R = Seychellene, which can formally be derived from patchouli alcohol (362) by a... 

The sesquiterpenoids that are important in odour terms mostly have complex cyclic structures. The problem of elimination of alcohols to produce olefins on attempted isolation is even more acute with sesquiterpenoids than with monoterpenoids because of their higher boiling points, which require more vigorous distillation conditions. The sesquiterpenoids responsible for the odours of vetiver and patchouli oils have complex structures which can only be reached by lengthy and hence uneconomic syntheses. Some of these components are shown in Figure 4.33. The vetivones are the major components of vetiver oil but contribute little to the odour. Minor components such as zizanal and khusimone are much more important. Similarly in patchouli oil, minor components such as nor-patchoulenol and nor-tetrapatchoulol are more important than the major constituent, patchouli alcohol. 

The final proof of the structure of a natural product after the latter has also been synthesized in the chemist s lab was, for a long time, common procedure [156], In a few cases, disagreement raised a few eyebrows. This was the case for patchouli alcohol and for a molecule called hexacyclinol [157]. Quinine is an example of the difficulties associated with the notion of a total synthesis. Shouts [35, 37,158] and murmurs [lib,159] have been expressed to comment on the wealth of total syntheses of natural products performed in the second half of the twentieth century. 

Synthetic Lesson from Patchouli Alcohol The Trouble with the Last Structural Proof [160]... 

The bicyclooctane core of the structure of patchouli alcohol suggests the Diels-Alder reaction as an approach to the synthesis and this was... 

The possibilities of using two-bond disconnections (i.e., the Diels-Alder transform) are easy to appreciate with respect to patchouli alcohol (50), a bridged tricyclic target. It is a historically amusing compound, as its initial (wrong) structure was proven by synthesis [163] (a proof that later had to be revised) [164] The correct structure of 50 possesses three... 

Vetiver and patchouli are two oils of great importance in perfumery (Williams, 1996, 2004). Both contain complex mixtures of sesquiterpenoids, mostly with complex polycyclic structures (Sell, 2003). The major components of vetiver oil are a-vetivone (136), b-vetivone (137), and khusi-mol (119), but the most important components as far as odor is concerned are minor constituents such as khusimone (151), zizanal (152), and methyl zizanoate (153). Nootkatone (154) is an isomer of a-vetivone and is an important odor component of grapefruit. Patchouli alcohol (145) is the... 

The synthesis of natural products of ever increasing complexity has been a perennial task of preparative organic chemistry. However, over time, the motivation underlying this activity has changed in a characteristic manner. Up to the 1960s the synthesis of a natural product constituted the sole - and later on, the most important - proof of its structure. The reliability of such structural proof was not 100% - as can be seen from the structural assignment of patchouli-alcohol 1, a case in which the wrong structure has been proven by synthesis [1]. The correct structure was finally established by X-ray analysis [2]. 

The first synthesis of patchouli alcohol was actually directed towards an tv-patchoulane skeleton (A), the structure then believed to represent the natural product. It was only by a fortuitous rearrangement during the final stages of the synthesis that Buchi arrived at patchouli... 

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