Dehydration of dimeric 6-hydroxycyclohexa-2,4-dienones–an alternative course for these reactions and a model for the maytenone to anhydromaytenone transformation

Abstract

Periodate oxidation of thymol gave the dimeric 6-hydroxycyclohexa-2,4-dienone (12), 3,10-dihydroxy-3,10-di-isopropyl-6,11-dimethyltricyclo[6.2.2.0^2,7]dodeca-5,11-diene-4,9-dione, and dehydration of this yields 4,11-di-isopropyl-7,12-dimethyl-13-oxapentacyclo[7.2.1.1^4,12.0^2,11]tridec-6-ene-5,10-dione (13). The structure of (13) rests upon its ozonolysis to yield anti-7-acetyl-4,9-di-isopropyl-1-methyl-5-oxo-10-oxatetracyclo[4.4.0.0^2,4.0^3,8]decane-9-carboxylic acid (15) which can be isomerised to give 1-hydroxy-2,7-di-isopropyl-5-methyl-10-oxo-6-oxapentacyclo[7.2.1.0^2,4.0^3,8.0^5,12]dodecane-7-carboxylic acid (21) with dilute aqueous sodium hydroxide. Alternatively acid catalysed isomerisation of (15) gives the syn isomer (19) and the X-ray crystal structure of the corresponding methyl ester syn-7-acetyl-4,9-di-isopropyl-1-methyl-5-oxo-10-oxatetracyclo[4.4.0.0^2,4.0^3,8]decane-9-carboxylic acid methyl ester (20) is reported. The dehydration of (12) to give (13) is considered to be a model for the transformation of the bis-diterpene maytenone into anhydromaytenone.