Recebido em 18/4/06; aceito em 10/11/06; publicado na web em 17/7/07Ethanolic extracts of the leaves of Casearia sylvestris yielded a novel clerodane diterpene, 15-hydroxy-3-cleroden-2-one, together with the known diterpenes (-)-hardwickiic acid, reported for the first time from this species, and casearins B and G, previously isolated from C. sylvestris. The structures of all four compounds were determined by spectrometric analysis. The new clerodane diterpene and (-)-hardwickiic acid contain structural features that are completely different from the highly oxygenated casearins and casearvestrins isolated from C. sylvestris.
An EtOH extract of the leaves of Casearia sylvestris afforded new clerodane diterpene, casearin X, together with the known compounds casearins B, D, L, and O, and caseargrewiin F. Casearin X degraded to the corresponding dialdehyde when stored in CDCl(3). The diterpenes isolated were cytotoxic to human cancer cell lines, with caseargrewiin F being the most active and the new clerodane, casearin X, the second active compound with IC(50) values comparable to the positive control doxorubicin. All isolated diterpenes showed lower activities against normal human cells than against cancer cell lines, which might indicate a possible selective action on cancer cells. Casearin X dialdehyde was not cytotoxic to cancer cells indicating that the occurrence of these CO groups at C(18) and C(19) is incompatible with the cytotoxic activity.
Two Alteromonas sp. strains isolated from deep seawater were grown to promote the production of exopolysaccharides (EPS, E611 and E805), which were incorporated into chitosan solutions to develop films. The combination of the major marine polysaccharides (chitosan and the isolated bacterial EPS) resulted in the formation of homogenous, transparent, colorless films, suggesting good compatibility between the two components of the film-forming formulation. With regards to optical properties, the films showed low values of gloss, in the range of 5–10 GU, indicating the formation of non-glossy and rough surfaces. In addition to the film surface, both showed hydrophobic character, with water contact angles higher than 100 º, regardless of EPS addition. Among the two EPS under analysis, chitosan films with E805 showed better mechanical performance, leading to resistant, flexible, easy to handle films.
A highly stereoselective synthesis of the C4–C17 fragment of saliniketals A and B was completed. The key steps in this synthesis included a syn‐aldol reaction mediated by a boron enolate and a double diastereodifferentiating aldol reaction mediated by a titanium enolate. Moreover, a substrate‐controlled Grignard reaction, an intramolecular Wacker‐type cyclization and a Seyferth–Gilbert homologation provided the C4–C17 fragment of saliniketals A and B in 16 steps and with a 7 % overall yield.
Aliltricloroestananas quirais e aquirais reagem com α-alcóxi aldeídos quirais para fornecer alcoóis homoalílicos com moderados a bons níveis de diastereosseletividade 1,4-syn.Achiral and chiral allyltrichlorostannanes reacted with chiral α-alkoxy aldehydes to give the corresponding homoallylic alcohols with moderate to good levels of 1,4-syn-diastereoselection.
Keywords: allyltrichlorostannanes, allylsilanes, homoallylic alcohols
IntroductionAllylsilanes and allylstannanes are among the most important groups of organometallic-type reagents available for the control of acyclic stereochemistry and their reaction with aldehydes in the presence of Lewis acids is an important procedure for the preparation of homoallylic alcohols.
1,2The addition of allylstannanes bearing a stereogenic center to chiral aldehydes is particularly interesting in organic synthesis. We recently communicated that in situ prepared chiral allyltrichlorostannanes react with chiral aldehydes to give 1,4-syn homoallylic alcohols with high levels of diastereoselectivity. [3][4][5][6][7][8][9][10][11] We wish to describe here a stereocontrolled reaction between achiral and chiral allyltrichlorostannanes with chiral lactate-derived aldehydes to give fragments which can be found in a large variety of naturally-occurring products with promising biological activities. 12 This study details our efforts to understand the double stereodifferentiating stereocontrol elements involved in chiral allyltrichlorostannane additions to chiral aldehydes.
13
Results and DiscussionAchiral and chiral allylsilanes 1-4 were prepared from the corresponding easily available methyl esters, as described in previous papers from this laboratory (Scheme 1). [3][4][5][6][7][8][9][10][11]14 According to previously established experimental procedures, the allylsilanes were mixed with SnCl 4 (1.0 equiv. in CH 2 Cl 2 ) before the addition of a solution of the aldehyde in order to promote the SiMe 3 /SnCl 3 exchange leading to the corresponding allyltrichlorostannanes 5-8 (Scheme 1).
5To the best of our knowledge, the first spectroscopic information available on exchange reactions involving allylsilanes and SnCl 4 was reported by Denmark and co-workers in 1988.13 In 1999, we described the first direct evidence for interaction between SnCl 4 and chiral allylic silane 3 bearing an ether functionality that generated a new species by means of NMR spectroscopy.5 In a continuation of these initial studies we have done a spectroscopic study ( For allyltrimethylsilane 1 the SiMe 3 /SnCl 3 exchange producing allyltrichlorostannane 5 and Me 3 SiCl is complete after 2 h at room temperature (Scheme 1). 5 For allylsilane 2 the SiMe 3 /SnCl 3 exchange to give 6 and Me 3 SiCl is faster, as expected for a 1,1-disubstituted electron-rich olefin, being complete after 10 minutes at room temperature. o C showed formation of Me 3 SiCl and complete consumption of both allylsilanes within less than 1 minute to give allyltrichlorostannanes (R)-7 and (S)-8, respectively. It appears that the oxygen function...
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