Camphor rearrangement

In further studies on thiabenzenes, Mislow and co-workers (170) demonstrated that deprotonation of optically active 2-chloro-lO-(2,5-xylyl)-10 thioxanthenium perchlorate 132, resolved via the (+> camphor-10-sulfonate salt, affords optically active 2-chloro-10-(2,5-xylyOthiaanthracene 133, which subsequently rearranges to the corresponding 9-substituted thioxanthene. 

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

Recently, Gartner and colleagues have reported the [1,2]-Wittig rearrangement of camphor-derived acetal 39 to 40 (equation 21) . 

Another enantiospecific synthesis of longifolene was done starting with camphor, a natural product available in enantiomerically pure form (Scheme 13.26). The tricyclic ring system is formed in step C by an intramolecular Mukaiyama reaction. The dimethyl substituents are formed in the first step of sequence E by hydrogenolysis of the cyclopropane ring. The final step of the synthesis involves a rearrangement of the tricyclic ring system that is induced by solvolysis of the mesylate intermediate. 

Beckmann rearrangement of 6-endo-chlorocamphor oxime (Vol. 7, p. 43) yields (220) and the cis- and frans-ring-fused (221), and camphor oxime rearranges with hydrazoic acid in polyphosphoric acid to yield (222). ... 

Langlois and co-workers ° developed a stereoselective hetero-Claisen rearrangement of camphor-based oxazoline A -oxides 300, available from hydroxylami-noisobomeol 298 and orthoesters 299. The rearrangement, initiated by acylation of... 

Bomeol is oxidized to camphor with chromic or nitric acid dehydration with dilute acids yields camphene. Bomeol is readily esterified with acids, but on an industrial scale bornyl esters are prepared by other routes. For example, levorotatory bomeol is synthesized industrially from levorotatory pinenes by Wagner Meerwein rearrangement with dilute acid, followed by hydrolysis of the resulting esters [86]. 

Exercise 30-7 Camphor can be made on an industrial scale from a-pinene (turpentine) by the following reactions, some of which involve carbocation rearrangements of a type particularly prevalent in the bicyclic terpenes and the scourge of the earlier workers in the field trying to determine terpene structures. 

The 3 + 2-cycloaddition of commercially available Me3SiCHN2 with camphor sultam-derived dipolarophiles produces 3-trimethylsilyl-substitutcd-A1 -pyrazolincs which on acid treatment convert into optically active A2-pyrazolines.60 The nucleophilic addition of ethyl diazoacetate with /V-cthoxycarbonyl-/V-(2,2,2-trichlorocthylidcne)a-mine produces a new diazo intermediate (35), which by 1,3-dipolar cycloaddition followed by a sigmatropic rearrangement of the cycloadduct (36) furnishes a substituted pyrazole (37) (Scheme 13).61. 

As early as 1893, it was discovered795 that camphor 185 rearranges to 3,4-dimethylacetophenone 186 in concentrated H2SO4 and the mechanism of the reaction... 

Camphor is of considerable importance technically, being used in the manufacture of celluloid and medicinal products. It is manufactured industrially from a-pinene, obtained from turpentine, by several processes (66-107) which differ mainly in detail. Synthetic camphor is usually obtained as the racemic modification. The formation of camphor involves the Wagner-Meerwein rearrangements, e.g. ... 

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... 

The diastereomerically pure camphor-based 1,3-oxathiane 36, formed by the reaction of a,(S-unsaturated aldehydes with the hydroxythiol 37, undergoes a stereospecific ketene Claisen rearrangement with dichloroketene to give the stereo-pure 10-membeted thiolactone <02CC2534>. 

The rearrangements of carbocations also play a role in the formation of ter-pene natural products. Menthol and camphor are examples of terpenes derived from plant oils. They are constructed using a common building block, as shown in Section 25-8, and undergo a series of rearrangements in the course of construction to generate the most stable carbocation. 

There is a better route. The more likely cation formed on the way to pinene could rearrange to the camphor cation. This is a known chemical reaction and is a simple 1,2-shift of the kind discussed in Chapter 37. However the new cation is formed, addition of water and oxidation would give... 

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