The characteristic structural feature of polyenemacrolide antibiotics includes a hydrophobic conjugated polyene part and a hydrophilic 1,3-polyol part. The stereochemical determination of such a polyol chain is a difficult task. There are two factors underlying the difficulties; these compounds exhibit poor crystalline properties for X-ray analysis and give a complex overlapping NMR signals which prevent assignment of signals. To date, there are several strategies for the stereochemical determination of these compounds. We now report an efficient stereochemical determination of RK-397 (1) based on combined use of two useful methods; acetonide-^<13>C-NMR analysis developed by Rychnovsky and lactone-^1H-NMR analysis developed by us. RK-397 (1) was converted into tetraacetonide 7. The ^<13>C-NMR spectrum of 7 showed three syn acetonides and one anti acetonide. The relative configurations at C13, 15 and C25, 27 were assigned as syn by analysis of 2D-NMR spectrum of 7. Acetylation of 1 followed by ozonolysis, reduction and acetylation provided 8 and 9. The absolute stereochemistry of 8 was determined as 30S, 31S by converting into diol 10. Decaacetate 9 was converted into acetoxy lactone 15 and unsaturated lactone 16. The ^1H-NMR analysis of both compounds showed 23,25-anti and 21,23-syn relationships. CD spectrum of 16 showed positive Cotton effect, which indicates an R configuration at C23. The ^<13>C-NMR spectrum of tetraacetonide 18, prepared from 16, showed two syn acetonides and one anti acetonide. To determine the relative configuration at C19, 21, lactone 16 was converted into acetoxy lactone 21. The ^1H-NMR analysis of 21 showed 19, 21-anti. configuration. Thus, all chiral centers of RK-397 (1) were assigned efficiently by combined use of acetonide-^<13>C-NMR analysis and lactone-^1H-NMR analysis. Interestingly, the configurations at C19 and C21 of 1, corresponding to 14-desmethyl mycoticin A, are different from those of mycoticin A.
