Spirolactones brominated

Oxidative laetonization. A recently discovered group of mycotoxins known as tryptoquivalineS has a novel spirolactone unit derived from N-acyltryptophans. A typical member of the group, tryptoquivaline G (1), has now been synthesized by Biichi and co-workers and has been shown to be derived from D-tryptophan. A key step involves oxidation of a derivative (2) of L-tryptophan with DMSO and methanesulfonic anhydride (2 equiv.) to 3 and 4. NBS has been used for such a reaction, but it can also effect undesired nuclear bromination. The main product 3 was converted in several steps into tryptoquivaline L, the C,9 epimer of 1. Contra-thermodynamic epimerization (KH, then protonation) converted this isomer into 1 and thus proved the absolute configuration. 

A similar oxidation of tyrosine derivative 244 with an excess of (diacetoxyiodo)benzene in cold acetonitrile followed by quenching with aqueous sodium bromide furnished brominated spirolactone 246 in 80% overall yield. A plausible mechanism for this reaction involves the iodonium derivative245 as an intermediate product and subsequent substitution of each phenyliodonium group with a bromide anion (Scheme 3.100) [311]. 

Another different C2-symmetric iodoarene system, based on an all-carbon and quite rigid a f -dimethanoanthracene core, was recently described and tested by Ibrahim for catalytic asymmetric Kita s spirolactonization [82]. For example, the para-methylated variant of the iododimethanoanthracene 113 in a 2 1 CHCI3/ CH3NO2 solvent mixture at -20°C converted the 4-brominated variant of naphthol 105 (i.e., 105b) into the corresponding spirolactone 106b in 63% yield with 67% ee (best ee) (Fig. 28). The use of the para-methoxylated variant of 113 under the same... 

The most plausible mechanism put forward to account for the formation of brominated spirolactones of oxindoles is shown below (97- 104) (175, 295). According to this mechanism, up to 3 equiv. of oxidative bromine participate in the modification-cleavage reaction (295,... 

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