Carvon camphor, synthesis

The photochemical [2 + 2] cycloaddition reaction of alkenes, the light-induced cycloaddition of two carbon-carbon ir-bonds to produce a cyclobutane, is a potentially highly useful reaction in organic synthesis since two new carbon-carbon bonds are formed and a maximum of four new stereocenters are introduced in the process. Ciamician reported the first example in 1908 when he observed the formation of carvone camphor (2) on prolonged exposure of carvone (1) to Italian sunlight (equation 1). ... 

Both a- and P-pinenes are popular starting materials for the synthesis of other monoterpene chiral synthons such as carvone, terpineol, and camphor (vide infra). Reactions leading to other monoterpenes are briefly summarized in Figure 5.1. Treatment of a-pinene with lead tetraacetate followed by rearrangement gives trans-verbenyl acetate (7), which is hydrolyzed to yield trans-verbenol (8) 8 Subsequent oxidation of 8 gives verbenone (9), which can be reduced to give cw-verbenol... 

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. 

Another important problem in 1973 when I started my pheromone synthesis was the relationship between the absolute configuration of a pheromone and its bioactivity. At that time there was no experimental result about the relationship, just because no one synthesized both the pure enantiomers of a pheromone. I really wanted to know the relationship, because among other bioactive compounds, there were several known cases, as shown in Figure 4.2. Only a single enantiomer of glutamic acid and also that of estrone are bioactive, while both the enantiomers of carvone as well as those of camphor were odoriferous. 

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