The problem of constructing diastereoselectively the C/D ring system of stemarane diterpenes from a bicyclo[2.2.2]octane intermediate was solved resulting in very simple synthesis of (+)-13-stemarene 1. The obtaining of the latter represents also a formal synthesis of (+)-18-deoxystemarin 2. In the key step, the epimeric mixture 10, dissolved in toluene, was converted by the action of TsOH into (+)-stemar-13-en-15-one 28.
(+)-2-Deoxyoryzalexin S (1), the nominal enantiomer of a diterpenoid isolated in Chile from Calceolaria species, was regio- and diastereoselectively synthesized from (+)-podocarpic acid. (+)-2-Deoxyoryzalexin S (1) was characterized also as its acetyl derivative, (+)-2, whose structure was confirmed by X-ray crystallographic analysis. Surprisingly, comparison of the data recorded for (+)-1 and (+)-2 and those reported in the literature for the Calceolaria isolated diterpenoid 1 and its derivative (-)-2 showed some differences, suggesting that the latter do not possess the proposed structures.
The d and l amino acid mediated enantioselective intramolecular aldol reaction of 4‐((1‐methyl‐2,6‐dioxo‐cyclohexyl)methyl)‐pent‐4‐enal 1 leading, after dehydration, to (‐)‐(R) and (+)‐(S) 4a‐methyl‐3‐methylene‐5‐oxo‐2,3,4,4a,5,6,7,8‐octahydro‐naphthalene‐1‐carbaldehyde 2 was explored. It was found that (‐)‐(R) carbaldehyde 2 is enantioselectively formed in the presence of l‐amino acids while (+)‐(S) carbaldehyde 2 is enantioselectively formed in the presence of d‐amino acids. (‐)‐(R) Carbaldehyde 2 was then transformed into (+)‐22. The absolute configuration and relative stereochemistry of the latter was established by single‐crystal X‐ray diffraction analysis of p‐iodobenzoate (+)‐23.
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