Pinenes, rearrangement

Silyl substituted carbon-centred radicals, which are produced when adding RsSi to unsaturated bonds can participate in consecutive reactions other than cyclization. A simple example is given in Reaction (7.66) where the adduct of silyl radical to (3-pinene rearranged by opening the four-membered ring prior to H atom transfer [33,77],... 

Epoxy-a-pinene, produced by epoxidation of (+)-a-pinene, rearranges in the presence of Lewis acids to (-)-campholenealdehyde which is not only one of the constituents of various juniper species, but also the starting reagent for the synthesis of fragrances with the odor of sandalwood. Sandalore, for example, is produced by Knoevenagel aUcenylation of campholenealdehyde with propanal followed by reduction of the aldehyde function with sodium borohydride... 

Alpha- and beta-pinene Rearrangements Zr(IV) phosphate polymer 176... 

Thermal isomerization of a-pinene, usually at about 450°C, gives a mixture of equal amounts of dipentene (15) and aHoocimene (16) (49,50). Ocimene (17) is produced initially but is unstable and rearranges to aHoocimene, which is subject to cyclization at higher temperatures to produce a- and P-pyronenes (18 and 19). The pyrolysis conditions are usually optimized to give the maximum amount of aHoocimene. Ocimenes can be produced by a technique using shorter contact time and rapid quenching or steam dilution (51). 

Alloocimene Manufacture. a-Pinene (8) is converted thermally first to cis-ocimene (17), which rearranges to give about 40—50 wt % of the two alloocimene isomers, ie, alloocimene [7216-56-0] (53) and 4-trans-(i-trans-2S[oo[Pg.417]

Acid-catalysed rearrangement of epoxides is another widely used reaction in the fine chemicals industry. Here again the use of solid acid catalysts such as zeolites is proving advantageous. Two examples are shown in Fig. 2.25 the isomerization of rsophorone oxide (Elings et al., 1997) and the conversion of a-pinene oxide to campholenic aldehyde (Holderich et al., 1997 Kunkeler etal., 1998). Both products are fragrance intermediates. 

The synthesis of the Taxol in Scheme 13.56 by P. A. Wender and co-workers at Stanford University began with an oxidation product of the readily available terpene pinene. One of the key early steps was the photochemical rearrangement in Step B. 

Olefinic compounds such as a-pinene (309) and /Dpi none (312) undergo hydrogen abstraction followed by rearrangement and amino-oxyl insertion (310, 313) and addition reactions (311, 314, 315), as shown in reactions 43 and 44627. Other olefinic compounds such as norbomadiene, cyclo-octene and cyclo-octa-1,5-diene gave analogous results628. 

The reaction sequence Is based on the readiness with which a-pinene undergoes oxidation predominantly to the tertiary acetate with Pb(OAc)4.3 4 Dichromate oxidation of the derived alcohol proceeds by way of a second allylic rearrangement to give verbenone without affecting the neighboring stereogenic centers. 

More recently spinning disc reactors have been used by Wilson et alP0) to carry out catalytic reactions using supported zinc triflate catalyst for the rearrangement of a-pinene oxide to yield campholenic aldehyde. The results of this study, presented in Table 20.1, suggest that by using a supported catalyst on a spinning disc reactor it is possible to... 

Rearrangement of a-pinene oxide to campholenic aldehyde Batch Reactor Catalysed SDR... 

Wilson, K., Renson, A. and Clark, J. H. Catalysis Letters 61 (1-2) (1999) 51-55. Novel heterogeneous zinc triflate catalysts for the rearrangement of alpha-pinene oxide. 

Reaction conditions a-pinene 3.97 mL (25 mmol) 1,4-dioxane 40 mL oxidant air (15 mL/min) reaction time = 24h. Conversion and product composition were determined by GC and identified by GC-MS, Minor amounts of rearranged products of a-pinene oxide also formed. 

The catalysts Co-CMS5 and C0-CMS6 have also been tested for the air oxidation of a-pinene. In both the cases air oxidation commences quickly with the formation of a-pinene oxide, verbenol and verbenone as the main products along with other rearranged products as observed in the reactions described above. A conversion of 50.6% and 48.0% are observed for Co-CMSS and C0-CMS6 respectively after 24h (Table 6). 

Carbocation rearrangements synthesis of camphor from a-pinene... 

The carbocyclic 434, precursor of nucleoside analogues, was synthesized from the commercially available (—)-/3-pinene 433. The Beckmann rearrangement was used to cleave the carbocyclic ring and cis stereochemistry of the ring substituents is guaranteed (equation 183). 

The driving force in some isomerization polymerizations is relief of steric strain. Polymerization of P-pinene proceeds by the first-formed carbocation XVI rearranging to XVII... 

SCHEME 75. Epoxidation of a-pinene with Mo(CO)6/TBHP leading to a rearrangement product... 

Much effort this year has been expended on chrysanthemic acid syntheses. Aratani et al. have extended earlier work on asymmetric synthesis (Vol. 6, p. 21) by decomposing various alkyl diazoacetates in 2,5-dimethylhexa-2,4-diene in the presence of chiral copper complexes to yield up to 92% of rrans-chrysanthemic acid in 88% dextrorotatory enantiomeric excess. Mitra has used ozonolysis of (+)-a-pinene to obtain, stereospecifically, the bromo-ketone (104) which undergoes Favorskii rearrangement to yield the anticipated ester (105) from which (+)-trans-chrysanthemic acid is readily obtained a second paper reports another route from (+)-car-3-ene initially to methyl (—)-c/s-chrysanthemate or to (—)-dihydro-chrysanthemolactone (106), both of which are convertible into (+)-rra s-chrysan-... 

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