Carene isomerization

Menthol from (+)-3-Carene. An Indian manufacturing process for (—)-menthol starts from 3-carene, the major component of Indian turpentine oil (55-65%). (+)-3-Carene isomerizes to (+)-2-carene, which can be pyrolyzed to (+)-tra s-2,8-/ -menthadiene. Isomerization of the latter yields (+)-isoterpi-nolene, which is hydrogenated to give >50% (+)-3-/ -menthene. Epoxidation and subsequent rearrangement lead to a menthone isomenthone mixture,... 

The oxides show catalytic activities for alkylation of phenol with methanol, butene isomerization, and carene isomerization. The alkylation activity correlates with acidity, while butene isomerization activity correlates with basicity. 3-Carene undergoes selective double bond migration to 2-carene on the binary oxide containing 90% ZnO, while menthadiene and cymene are produced by a three-membered ring opening over the catalyst containing small amounts of ZnO, acidic binary oxides. 

Sabinene and carene are isomeric natural products with the molecular formula CjoHig (a) Ozonolysis of sabinene followed by hydrolysis in the presence of zinc gives compound A What IS the structure of sabinene" What other compound is formed on ozonolysis" (b) Ozonoly SIS of A carene followed by hydrolysis in the presence of zinc gives compound B What is the structure of A carene" ... 

Carene has also been isomerized over an S-alumina catalyst to a 50 50 mixture of dipentene (15) and carvestrene (30). The cmde mixture can be readily polymerized to a terpene resin or copolymerized with piperjiene (63,64). 

The turpentine fraction is gasified during the process, and valuable chemicals, such as a-pinene and carene can be separated by distillation. They can be further processed by catalytic treatment, such as isomerization and oxidation [6-8]. 

Metals (Raney nickel or Ni/Si02, Pd/C,...) can easily catalyze the isomerization of 3-carene into 2-carene (Scheme 44), but the selectivity is low, due to the hydrogenation of the two isomers into carane. 

A new isomerization catalyst can be prepared by the modification of silica-supported nickel with tetrabutyltin. This catalyst is capable of the selective isomerization of 3-carene to 2-carene.269... 

In the isomerization of (+)-3-carene into (+)-2-carene or the dehydrogenation of 2-butanol into 2-butanone, the selectivity into the desired product is also increased by the introduction of small amounts of Sn, which will form adatoms poisoning unselective sites.324... 

Several photochemically induced vinylcyclopropane to cyclopentene rearrangements of nor-carene derivatives to form bicyclo[3.2.0]heptenes can be understood as ring contractions of cyclohexenes to cyclobutanes. Upon direct irradiation of norcar-2-ene (bicyclo[4.1.0]hept-2-ene) at 214 nm (pentane solution), however, complex product mixtures were obtained containing only small amounts of bicyclo[3.2.0]hept-2-ene, while toluene sensitized photolysis in 50 millimolar solution in degassed pentane at 254 nm gave mainly the cyclobutane derivative 13 in addition to EjZ-isomeric hepla-l,3,6-trienes.72... 

Impregnation of cobalt and molybdenum (without sodium) increases largely the isomerizing activity of the catalyst the /3-pinene is then completely converted. The catalysts prepared with sodium molybdate and sodium hydroxide (Co-Mo-Na and Na-Co-Mo-Na) have lower isomerizing activities while their HDS activities are significantly increased. As in the case of alumina supported catalysts the sulfided CoMo phase protected by a double layer of alkaline ions on the carbon support gives the best results in HDS of /3-pinene. The behaviour of this catalyst was examined in desulfurization of the turpentine oil (40% a-pinene, 25% /3-pinene, 25% A -carene and 10% camphene + dipentene + myrcene, 1500 ppm S). The results are recorded in Table 6. 

In the course of our earlier studies on terpenylboranes we developed a simple transformation of a- into 3-pinene," and a stereoselective synthesis of allylic alcohols by the reduction of vinylic epoxides.12 An extension of these studies to contrathermo-dynamic isomerization of a-thujene, 2- and 3-carene,13 and kinetic resolution of vinylic epoxides by the reduction with terpenylboranes,14 is described. 

Several years ago, we developed a contrathermodynamic isomerization of a- to 3-pinene via diethylmyrtenylborane obtained from a-pinene by metalation-transmetalation.11 Later, (+)-p-pinene of > 99 % ee was prepared by this method.22 Extending this work to other bicyclic monoterpenes, the isomerization of (+)-2-, (+)-3-carene, and (-)-a-thujene, was studied.13 Among bicyclic monoterpene olefins with an exocyclic double bond, 3(10)-carene is a rare compound reported only a few times in the long history of terpene chemistry, and its rotation is still controversial.23,24 Sabinene is isolated from the oil of savin, and no convenient synthesis of an optically active sabinene is known. 

Preparative Methods both reagents are derived from the corresponding B-methoxydiisocaranylborane which is prepared in two steps from (+)-3- or (-b)-2-carene (eq 1 and eq 2). Note only the (+) isomer of 3-carene occurs naturally (+)-2-carene is obtained by base-catalyzed isomerization, and both are commercially available. 

Bicyclo[4,l,0]heptanes.—The hydrogenation, hydroboration, thermolysis, amination, oxidation, and photochemical isomerization of various carane derivatives has been reviewed by Cocker. Another case of the repetition of earlier work has appeared, this time concerning the catalytic reduction of car-3-ene (307). Further work on the acid- and base-catalysed ring-opening of carene epoxides is reported (Scheme 17). ... 

Derivatives of (-l-)-chrysanthemic acid could in principle behave like d" -carene but as they bear a carbonyl group as an additional functionality, the question is whether the 7t-system of the carbonyl group will participate in a (competing) homo [1,5] hydrogen shift, even more so since carbonyl-variants of the 1 -> 3 isomerization are known. An interesting dichotomy has been observed for the derivatives 69. Whereas the ethyl ester of chrysanthemic acid (69, R = OEt) isomerizes to 70 exclusively at 500°C/50 Torr, i.e. with participation of its... 

The products are crystalline and can be stored at 0 "C under nitrogen for prolonged periods of time without any appreciable loss of hydride activity or isomerization. (lS)-Di-4-isocaranyl-borane is sensitive to thermal isomerization and its use above room temperature should be avoided. The opposite enantiomeric boranes have not been prepared since (—)-2- and (—)-3-carene are not readily available. 

The isomerization of many terpene oxides (see Section 5.4) has been investigated using a wide range of solid acids (and bases). Arata and Tanabe are responsible for much of this work which includes d-limonene oxide [16], 2- and 3-carene oxide [17], and carvomenthene oxide [18]. 

Several natural products contain the norcarane (bicyclo[4.1.0]heptane) structure. The isomeric carenes arc comparatively inexpensive compounds occurring in almost enantiomerically pure form, but only as the (-r)-isomers [configuration (15)]. It is possible to convert ( + )-3-carene to ( + )-2-carene by base-induced isomerization31-38, without loss of optical purity. 

While Ni/Si02 catalysts isomerize 3-carene into 2-carene (Scheme 76) with only a 30% yield along with >30% of byproducts, [Nis(Sn)o.o4], prepared by the addition of 0.04 M of Bu4Sn/Nis on the Ni catalyst, increases the yield of 2-carene to 37% and decreases the amount of byproducts to <10%. For this catalyst, tin is present as adatoms on the most hydrogenating sites (very likely those situated on the faces). 

Scheme 76 Isomerization of 3-carene into 2-carene on [Nig(Sn)o,o4]/Si02 catalyst.

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