Alkaloid sulfoxides, isomerization

It is reasonable to explain the isomerization of alkaloid sulfoxides in the same terms. 

Opposite to many other examples studied earlier in case of alkaloid sulfoxides in question the reaction does not show high stereospecificity and both possible isomers ars formed. They could be separated and assignments of the configurations to both isomers could be given by interpretation of NMR spectra since in each pair of isomers obtained from neothiobinupha-ridine, thiobinupharidine and thionuphlutine only one has the acetoxy group very close to the furan protons which causes different values for the observed chemical shifts in isomeric acetoxyderivatives. 

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. 

Pummerer reaction. The hydrogen atoms in the sulfinylmethyline group in the sulfoxides of three isomeric sulfur alkaloids neothiobinupharidine, thiobinupharidine, and thionuphlutine are acidic enough for Pummerer rearrangement. 

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