Dedicated to Professor Yoshito Kishi on the occasion of his 70th birthdayCytostatin (1) is a potent and selective inhibitor of protein phosphatase 2A that was isolated from the cultured broth of Streptomyces sp. by Ishizuka and co-workers (Scheme 1).[1] It inhibits lung metastasis of melanoma cells in mice and displays potent cytotoxic activity toward leukemia cell lines (inhibitory concentration; IC 50 = 42-65 nm). Ishizuka and coworkers assigned the two-dimensional structure (constitution) of cytostatin (1) by analysis of 1D and 2D NMR spectra. The relative configurations of C4-C6 and C9-C11 of cytostatin were assigned as syn at C4/C5 and C9/C10 and as anti at C5/C6 and C10/C11 by the research groups of Waldmann and Boger by comparison of key features of the 1 H NMR spectra of cytostatin with those of relevant models. [2,3] This approach lowered the number of structure candidates down to four, namely: 1 ss, 1 rs, 1 sr, and 1 rr.[4]Based on an analogy to fostriecin 2 [5] and related compounds (which share three stereocenters with 1), the research groups of Waldmann and Boger independently synthesized (4S,5S,6S,9S,10S,11S)-1 (hereafter called 1 ss), and concluded, by comparison of spectroscopic, physical, and biological properties, that this was the natural product.We have recently commented on the logic of proof and disproof of stereostructures by comparison of synthetic and natural samples.[6] If two or more stereoisomers of a natural product can reasonably be expected to have substantially identical spectra, then a rigorous assignment of the structure should include proof that the candidate isomer matches the natural product and, more importantly, proof that the other isomers do not. Acetogenins, such as the murisolins have very remote groups of stereocenters (10 or more methylene groups apart), so it can be expected that diastereomers might exhibit substantially identical spectra.[6] In contrast, the two groups of stereocenters in cytostatin (1) are only insulated by an ethylene group. Will enantiomers 1 ss and 1 rr have different spectra from their diastereomers 1 rs and 1 sr? To answer this question, and thereby to prove which three isomers were not cytostatin, we undertook the fluorous mixture synthesis [7,8] of all four isomers of 1.The fluorous mixture synthesis (FMS) strategy to make the four isomers of cytostatin is briefly outlined in Scheme 2. Late introduction of the triene is dictated by its chemical instability, so we followed the strategy of Bialy and Waldmann [2d] by planning to prepare four individual vinyl iodides 3 by coupling with alkenyl stannane 4.These four isomers are made over several steps from a single four-compound mixture of fluorous-tagged quasiisomers M-5 [9] ("quasi" because the compounds have different fluorous tags and are not true isomers [10] ). In turn, M-5 is made by coupling between fluorous-tagged quasiracemic aldehydes M-6 with quasiracemic keto phosphonates M-7 by a Horner-Wadsworth-Emmons (HWE) reaction.From the FMS standpoint, the configurations of the stereocenters ...

Imino sugars of the piperidine and pyrrolidine types can be specifically obtained when protected 5-amino-5-deoxyfuranopentoses, 5-amino-5-deoxyfuranohexoses, 6-amino-6-deoxyfuranohexoses, and 6-amino-6-deoxypyranohexoses undergo a tandem alkoxy radical beta-fragmentation-intramolecular cyclization reaction. The reaction is promoted by the system: iodosylbenzene-iodine under mild conditions. The tert-butoxycarbonyl, benzyloxycarbonyl, and diphenylphosphinoyl radicals have been studied as amino-protecting groups. Using this methodology, polyhydroxylated pyrrolidines of the D-erythrofuranoses 34 and 35, D-threofuranose 36, L-xylofuranose 42, and D-arabinofuranose 43 series and polyhydroxylated piperidines of the D-arabinopyranose type 37 and 38 were obtained.

The synthesis of highly functionalized nitriles by an alkoxyl radical fragmentation of cyclic beta-hydroxy azides is described. The alkoxyl radicals were generated by reaction of the alcohols with (diacetoxyiodo)benzene and iodine under mild conditions compatible with the presence of sensitive substituents and the protective groups most frequently used in carbohydrate chemistry. To explore the scope and limitations of this methodology, experiments were carried out using a variety of beta-hydroxy azides of the carbohydrate (1-6, 33, and 41), monoterpenoid (21 and 22), and steroid (23-25) families of natural products. Of special interest are the aldopentonitriles (15-18, 34, and 42) and aldotetrononitriles (19 and 20) synthesized from the corresponding 2-azido-2-deoxycarbohydrates. To demonstrate the versatility of these aldononitriles as chiral synthons, 1,4-imino-1-deoxysugar (37) and 1,5-imino-1-deoxysugar (43) analogues of the polyhydroxypyrrolidine and -piperidine types were prepared.

Öffnen und wieder schließen: Die Photoanregung von 2,3‐Diulosen I mit sichtbarem Licht löst eine ungewöhnliche sequenzielle Umlagerung aus: eine Norrish‐II‐Photoeliminierung unter Bildung der isolierbaren acyclischen Photoenol‐Zwischenstufe II, an die sich eine intramolekulare Enolexoaldolisierung anschließt. Die Kontraktion des Pyranoserings führt zu neuartigen Cyclopentitolen III. R=Acyl, Alkyl, Silyl.

Opened and closed: Visible‐light photostimulation of 2,3‐diuloses I triggers an unprecedented sequential rearrangement: A Norrish type II photoelimination to give an isolable acyclic photoenol intermediate II is followed by an intramolecular enolexo aldolization. The contraction of the pyranose ring in this process leads to a new type of cyclopentitol derivative III. R=acyl, alkyl, silyl group.