Verbenone, rearrangement

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. 

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). 

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... 

Rearrangements 1,3-rearrangements take place over three carbon atoms. Examples are the Eries rearrangement and 1,3-alkyl shift of verbenone to chrysanthenone. 

Hydroboration of (-f)-verbenone (202 X = 0) and treatment with hydroxyl-amine-O-sulphonic acid results in a 65% yield of (-l-)-cw-S-pinene (213) by elimination, with no resultant amine. Dehydrated neutral alumina may be used for the room-temperature concerted 1,2-elimination from 10-pinanyl tosylate to yield /3-pinene (70%) and some a-pinene (14%) with little rearrangement to camphene/ The addition of diethyl N,7V-dichlorophosphoroamidate to (-)-a-pinene yields (-t-)-(214) and the structure of the adduct (-)-(13) from MePCl2-AICI3 addition to (-)-a-pinene has been confirmed by X-ray analysis. The addition of HCNO to a- and / -pinene was reported earlier. 

It then was rearranged to the corresponding chrysanthenone derivative by a photo-chemically induced allylshift (cf. arrows in verbenone derivative 26), [8b-d] and further functionalized at C9 to yield Michael acceptor... 

The synthesis depends on some key rearrangement chemistry of verbenone (14.4.1), the oxidation product of the abundant natural product pinene. Verbenone provided 10 of the 20 carbons of the baccatin III ring system and also its chirality. Verbenone (14.4.1) was converted to the chrysanthenone derivative 14.4.3 by photorearrangement of... 

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