Steviol rearrangement

Upon utilization of 50% aq. acetone in the presence of 2,6-lutidine as a solvolytic medium, the mesylate (230) underwent rearrangement to give the methyl ester of (+)-steviol (231) in only 3 % yield 77). 

The reverse rearrangement, involving the conversion of the kaurene skeleton to that of the beyerene series, is found in the steviol-isosteviol and gibberellin-8,13-isogibberellin rearrangements (see Scheme 22). Apart from their mechanistic interest, these rearrangements were used in partial syntheses directed at structural correlations. 

The Chemistry of the Tetracyclic Diterpenoids.—The reaction of cnt-kaur-16-ene with thallium(lli) nitrate affords cnt-kaur-16-en-15j8-ol nitrate which undergoes a ready [3,3] sigmatropic rearrangement to cnt-kaur-15-en-17-ol nitrate. The reactions of phyllocladene and of labda-8(17)-en-13-ol with sodium azide and iodine chloride have been examined. ° The synthesis of 13-hydroxylated cnt-kaur-16-ene derivatives such as steviol using an acyloin-like cyclization of keto-esters has been developed. A detailed analysis was made of the products arising from the use of sodium in liquid ammonia in this reaction. 

Rearrangements have been widely used for angular alkylation reactions, as demonstrated by the synthesis of 3 240 or 5. a precursor of steviol (a compound with gibberellin-like activity) or kaurenoic acid (C-13 hydrogen instead of OH)421. 

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