Isomerization with camphorsulfonic acid

The Pummerer rearrangement has been employed in tandem with other reactions to enable complex transformations to be carried out efficiently and in a one-pot manner. Studies of these have been reported mainly by Padwa who has utilized such transformations in the syntheses of natural products. A particularly intriguing cascade sequence involving the Pummerer rearrangement was employed in the synthesis of the alkaloid jamtine, 57. " Padwa et al. synthesized the bromo-enamide 55 in a 4 1 (Z/ ) mixture of isomers. Treatment of the isomeric mixture with camphorsulfonic acid caused the the sulfoxide to undergo a Pummerer/Mannich ion cyclization, which was then followed by a spontaneous Pictet-Spengler reaction to furnish the isoquinoline core. Although a 5 2 1 1 mixture of diastereoisomers was obtained, the desired diastereoisomer 56 was formed preferentially. This was attributed to a 4 r-Nazarov-type conrotatory electrocyclisation which controls the direction of closure from the a-acylthionium ion intermediate. 

The chirality of compound 16b was completely inverted, to give 2,3-0-cyclohexylidene-L-ribonolactone (19), by means of a procedure (28) that involves oxidation of the hydroxymethyl group of 16b by ruthenium te-traoxide, followed by reduction of the lactone group. The resulting intermediate 1,5-lactone (18) underwent isomerization (with cyclohexylidene migration) upon refluxing in xylene, in the presence of a catalytic amount of D-camphorsulfonic acid [in Ref. (28) the formulas for the series were erroneously depicted]. 

Similarly, on treatment with pyridine and camphorsulfonic acid (5)-( )-a-4-methylphenylsul-phinyl-a,/ -enoates, obtained by a different route, underwent isomerization and rearrangement to produce (/ )-( )- >-hydroxy-a,/ -enoates with 64-72% ee54. 

Dehydration of allylic alcohols with strong acid leads to allenes if an isomerization to an alkyne is not possible. This method has been used for the synthesis of tetraarylallenes and tetra-r-butylallene. This method was also used for the first synthesis of an optically active allene by an enantioselective dehydration using (-l )-camphorsulfonic acid. ... 

A significant improvement in the catalyst adds further utility to this strategy. Using 1-5 mol% of indenyl complex 15 along with indium triflate and camphorsulfonic acid (CSA) as cocatalysts, the redox isomerization in Equation 1.19 was completed in 20 min compared with 1.5 h under the indium chloride cocatalyst conditions. 

Tetraiupine, m.p. 81-83°, [ ] 4.63° (water), is an alkaloid isomeric with lupinine found in Lujnnus palmen W. (78). The d-camphorsulfonic acid salt, m.p. 164-165°, [a]J, 18.18° (water), depressed the melting point of ( — ) lupinine d-camphorsulfonate. Tetraiupine depressed the melting point of (-t-)epf-lupinine (120, 127). The alkaloid is therefore not identical with either lupinine or cpt-lupinine. Couch (78) suggested that it might be the allo-lupinine (LX) postulated as an alkaloidal contaminant of lupinine (120). 

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