Total synthesis and NMR investigations of cylindramide

Cylindramide (I) was built up from three components: a hydroxyornithine deriv. II, a tetrazolylsulfone III, and a substituted pentalene subunit IV. Deriv. II was prepd. in a six-step reaction sequence involving the Wittig reaction and a Sharpless asym. dihydroxylation starting from N-Boc-3-aminopropanal. Tetrazolylsulfone III was accessible in four steps from dioxinone V. The synthesis of the pentalene fragment IV started from cycloocta-1,5-diene, that was converted into an enantiopure bicyclo-[3.3.0]octanedione. The latter was functionalized to give deriv. IV. The total synthesis was accomplished by inducing C-C bond formation by Sonogashira coupling of derivs. IV and II followed by olefination with tetrazolylsulfone III under Julia-Kocienski conditions, macrocyclization, and subsequent Lacey-Dieckmann condensation to form the tetramic acid unit. As indicated by extensive 1H and 13C NMR spectroscopic investigations (DQF-COSY, ROESY spectra), the stereochem. of synthetic I corresponds with that of the naturally occurring product. ROE data were used for mol. modeling of the lowest-energy structures for cylindramide