Synthesis of Aristotelia‐Type Alkaloids,Part XVI

Two independent total syntheses of the Aristotelia alkaloid (−)‐serratenone ((−)‐ 1 ) are disclosed, one starting with (−)‐α‐pinene, the other one with (S)‐α‐terpineol. These correlations led to a revision of the originally proposed absolute configuration of the natural product. In the course of systematic investigations of the behavior of the indole alkaloids (+)‐makomakine ((+)‐ 18 ) and (−)‐hobartine ((−)‐ 22 ) towards oxidizing reagents, it was found that treatment with I₂ leads to no less than five different products. Depending on the exact reaction conditions, each of them can be obtained as the major component in yields between 40 and 60%. One of these compounds was shown to be identical with the natural product (+)‐11,12‐didehydromakonin‐10‐one ((+)‐ 28 ).     

The Total Synthesis and Biological Assessment of trans‐Epothilone A

The total synthesis of (12S,13S)‐trans‐epothilone A ( 1a ) was achieved based on two different convergent strategies. In a first‐generation approach, construction of the C(11) C(12) bond by Pd⁰‐catalyzed Negishi‐type coupling between the C(12)‐to‐C(15) trans‐vinyl iodide 5 and the C(7)‐to‐C(11) alkyl iodide 4 preceded the (nonselective) formation of the C(6) C(7) bond by aldol reaction between the C(7)‐to‐C(15) aldehyde 25 and the dianion derived from the C(1)‐to‐C(6) acid 3 . The lack of selectivity in the aldol step was addressed in a second‐generation approach, which involved construction of the C(6) C(7) bond in a highly diastereoselective fashion through reaction between the acetonide‐protected C(1)‐to‐C(6) diol 31 (‘Schinzer's ketone') and the C(7)‐to‐C(11) aldehyde 30 . As part of this strategy, the C(11) C(12) bond was established subsequent to the critical aldol step and was based on B‐alkyl Suzuki coupling between the C(1)‐to‐C(11) fragment 40 and C(12)‐to‐C(15) trans‐vinyl iodide 5 . Both approaches converged at the stage of the 3‐O, 7‐O‐bis‐TBS‐protected seco acid 27 , which was converted to trans‐deoxyepothilone A ( 2 ) via Yamaguchi macrolactonization and subsequent deprotection. Stereoselective epoxidation of the trans C(12) C(13) bond could be achieved by epoxidation with Oxone ^® in the presence of the catalyst 1,2 : 4,5‐di‐O‐isopropylidene‐L ‐erythro‐2,3‐hexodiuro‐2,6‐pyranose ( 42a ), which provided a 8 : 1 mixture of 1a and its (12R,13R)‐epoxide isomer 1b in 27% yield (54% based on recovered starting material). The absolute configuration of 1a was established by X‐ray crystallography. Compound 1a is at least equipotent with natural epothilone A in its ability to induce tubulin polymerization and to inhibit the growth of human cancer cell lines in vitro. In contrast, the biological activity of 1b is at least two orders of magnitude lower than that of epothilone A or 1a .     

Film flow over strongly undulated bottom profiles at low Reynolds numbers – Experimental study

We study gravity driven films of a Newtonian fluid flowing down sinusoidal bottom profiles. We give experimental evidence of vortices in the film flow under creeping flow conditions down to Reynolds numbers of the order of 10^–5. The vortices are created in the valleys of the undulated bottom profile. They are visualized employing a particle image velocimeter with fluorescent tracers. It turns out that the vortices are generated beyond a critical film thickness.     

Thermally induced Lengthening of weak Dative Bonds in Donor Functionalized Organoaluminum Alkoxides observed by X‐ray Single Crystal Diffraction

The length of weak dative bonds in donor functionalized organo aluminum alkoxides [R₂Al—O—R'—Y]₂ (Y = NR₂, OR, SR) show a temperature dependence in the solid state that is distinctively different from the common effect of libration. This observation is interpreted as being caused by thermal excitation. The possible general relevance of this phenomenon for the study of very weak bonds in molecular systems is discussed.