Enmein into Gibberellins

The chemical conversion of enmein into natural gibberellins was subsequently achieved by two groups Okamoto and coworkers (107) converted enmein into gibberellin A15, while E. Fujita and coworkers (108) converted enmein into gibberellins A15 and A37. 

The proposed mechanism for functionalization of the methyl group by photolysis of the nitrone is shown in Fig. 4. 

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The foregoing chemical conversions of enmein into gibberellins A15 and A37 correspond to a formal total synthesis of the latter compounds, because the total synthesis of enmein has been accomplished. A more direct total synthesis was also carried out. Compound (137) was synthesized from 5-methoxy-2-tetralone and converted into (138) as shown in Scheme 25. 

In 1966, when the structure of enmein (62) was elucidated, the total synthesis of natural gibberellins had not yet been reported. Hence, Okamoto and co workers attempted the chemical transformation of enmein into a gibberellane derivative (106) and reported the formation of such compound by a benzilic acid rearrangement-like reaction of keto hemi-acetal (128), an oxidation product of acyloin (110). This is shown in Scheme 20. 

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