Allyl propynyl ethers of general formula RCECCHZOCHZCH=CHR (1) undergo cycloremangement to 3-oxabicyclo[4.l.0lhept-4-enes (2) in oxygen-free benzene upon brief treatment a t room temperature with catalytic amount of PtC14. The transformation of 1 to 2 is assumed to involve platinumallene intermediates. The structure of 7-(l-naphthyl)-6-phenyl-3-oxabicyclo[4.l.0lhept-4-ene (2c) has been determined by X-ray diffraction analysis. The naphthyl and phenyl groups were shown to be oriented cis to each other. In the presence of [(CO2)Rh(~-C1)12 the oxabicycloheptenes 2 undergo cyclopropane-ring cleavage. 6,7-Diphenyl-3-oxabicyclo[4.l.0]hept-4-ene (2b) forms the rhodocyclic complex 3b. In the absence of air, the oxygen-free analog of la, PhC=C(CHz)&H=CHz (4), rearranges to the unstable 7-phenylbicyclo[3.2.Olhept-1(7)-ene (51, which can be trapped by oxygen as stable 2-(2-oxo-2-phenylethyl)cyclopentanone (6).
Treatment of PtCl(4) with CO at 40-110 degrees C forms a powerful alkyne hydration catalyst that operates both under homogeneous conditions in wet THF and under phase-transfer conditions in (CHCl(2))(2)/H(2)O in the presence of tricaprylmethylammonium chloride (Aliquat 336). Complex HPtCl(CO)(2) is regarded as the active hydration catalyst. It is assumed to be formed by initial transformation of PtCl(4) to H(2)[Pt(3)(CO)(6)](n) (n = 5, 6) followed by reaction with HCl (generated by decomposition of the starting platinum salt).
We have demonstrated recently that separate entrapment in sol ± gel matrices of a catalyst and of a reagent which poisons it enables the simultaneous use, in one pot, of these chemicals, which otherwise must be used in consecutive steps. [1] Here we extend the methodology of changing ªchemical hostilityº into ªchemical friendlinessº and compatibility, to the classical family of opposing reagents namely acids and bases.We routinely teach at elementary-level chemistry that since acids and bases annihilate each other when brought together, one needs to separate acidic steps from basic ones in reaction sequences. We show here that sol ± gel entrapment [2] solves this problem, and makes it possible to place in one-pot acids and bases without their mutual destruction, while still allowing these reagents to activate or participate in desired reactions. In numerous studies it has been shown that molecules entrapped within sol ± gel matrices retain their chemical and physical properties [3] and that external substrate molecules can enter the pore network, react with the dopant, and emerge from the pores as products. Thus, the only events. The results of all the inhibition tests are found in Table 1. In conclusion, by a mix of accident, serendipity and rational design, we have discovered compound 3 which is the most potent small-molecule E-selectin inhibitor to date. Compound 3 can be prepared in ten steps in b 25 % overall yield.
We extend our sol-gel methodology of one-pot sequences of reactions with opposing reagents to an enzyme/metal-complex pair. Sol-gel entrapped lipase and sol-gel entrapped RhCl[P(C(6)H(5))(3)](3) or Rh(2)Co(2)(CO)(12) were used for one-pot esterification and C-C double bond hydrogenation reactions, leading to saturated esters in good yields. When only the enzyme is entrapped, the homogeneous catalysts quench its activity and poison it. Thus, when 10-undecenoic acid and 1-pentanol were subjected in one pot to the entrapped lipase and to homogeneously dissolved RhCl[P(C(6)H(5))(3)](3) under hydrogen pressure, only 7% of the saturated 1-pentyl undecanoate was obtained. The yield jumped 6.5-fold when both the enzyme and the catalyst were immobilized separately in silica sol-gel matrixes. Similar one-pot esterifications and hydrogenations by sol-gel entrapped lipase and heterogenized rhodium complexes were carried out successfully with the saturated nonoic, undecanoic, and lauric acids together with several saturated and unsaturated alcohols. The use of (S)-(-)-2-methylbutanol afforded an optically pure ester. The heterogenized lipase is capable of inducing asymmetry during esterification with a prochiral alcohol. Both the entrapped lipase and the immobilized rhodium catalysts can be recovered simply by filtration and recycled in further runs without loss of catalytic activity.
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