Preisocalamendiol cyclization

In order to explain the stereospecificity of the thermally induced cyclization of preisocalamendiol (315) to give dehydroisocalamendiol (316), Terada and Yamamura have carried out molecular mechanics calculations on the ground-state conformers and probable transition states of (315). The computed values of the steric energies of the possible transition states are in good agreement with the observed experimental facts. Other studies related to the conformational aspects of germacrane sesquiterpenoids include an examination of the NOE effects in the isofuranodiene (317), which shows the 1,5-diene system to be in a crossed... 

The dihydroxy-ketal (240), previously prepared from ( —)-santonin, has been used to synthesize a number of related sesquiterpenoids. Thus the diacetate of (240) was converted in six steps into (241), which was then treated with iso-propenyl acetate-sulphuric acid the derived enol-acetate was cleaved to the triol (242) by ozonolysis and lithium aluminium hydride reduction. The triol (242) was then converted into the di-iodo acetate (243) in a number of steps and thence to shyobunone (244) by dehydroiodination, reduction, and oxidation. Thermolysis of shyobunone at 160—180 °C gave preisocalamendiol (245) in about 30% yield. More recently, Iguchi et al. have shown that preisocalamendiol (245) can be cyclized to isocalamendiol (246) in aqueous acetic acid no trace of calamendiol (247) was found. A number of other interesting acid-catalysed cyclizations have been observed in this area, e.g. the formation of (248 R = OH) and (248 R = OAc) from (249) and the formation of (250) from (251). Finally, e-cadinene (252) has been obtained from (253), the lithium aluminium hydride product of preisocalamendiol (245). 

The homoallylic alcohol (90) is cyclized to s-cadinene (91) by reaction with methanesulfonyl chloride in pyridine 145). An intramolecular Prins reaction of the corresponding ketone (preisocalamendiol, 92) is effected by treatment with aqueous acetic acid, giving isocalamen-diol (93) 146). 

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