O-Galactal

An early study of the allylation reactions of oxocarbenium systems has been described by Danishefsky for synthesis efforts directed towards indanomycin. The reactions of ( )-and (Z)-2-butenylsilanes silanes are the focus for this report. However, the study also examined reactions of the (E,Z)-mixture of 2-butenyl-tri-n-butylstannanes with tri-o-acetyl-o-galactal 132 at -30 °C, in the presence of BF3 OEt2... 

Balasubramanian et al. [42] accomplished microwave-assisted glycosylations in solution by irradiation of a mixture of 49a and tri-O-acetyl-o-galactal 49b with an appropriate alcohol or substituted phenol in the presence of montmorillonite K-10 as catalyst they obtained solely the corresponding a-anomers of unsaturated glycosides 50 and 51 in much shorter times and with better yields than those obtained by conventional heating (Scheme 12.25). Silica gel and ferric chloride were also used as catalysts, but either no reaction occurred or extensive decomposition occurred when these acids were used. 

The formation of 0-glycoside 23 was accomplished by reacting tri-O-acetyl-o-galactal with methyl 2,3,6-tri-O-benzyl-a-D-glucopyranoside in the presence of Pd(0) or Lewis acid catalysts. ... 

The rule can lead to names which are less than user friendly. o-Galactal, for example, is 2,6-anhydro-5-deoxy-D-nrti6 io-hex-5-enitol rather than 1,5-anhydro-2-deoxy-D-/yxo-hex-l-enitol which is more intuitive and would be the choice of workers in the field (in the Dictionary all three names are quoted). 

The D-g/wco-pentitol-l-yl substituted uracil 96 and its D-galacto-isomer were synthesized from tri-O-benzyl-D-glucal and -o-galactal, respectively, by [2+2]cy-cloaddition of trichloroacetyl or chlorosulfonyl isocyanate, cleavage of the p-lactam rings in the products with methanol after AT-carbamoylation, and cycliza-tion of resulting glycosylureas. Tosylation of 4-(D-ga/acro-pentitol-l-yl)-2-phenyl-2/f-l,2,3-triazole led to the 3,6-anhydride 97 and two partially tosylated derivatives. ... 

Scheme 31 Ferrier rearrangement of D-glycal 64 with D-/yxo configuration (o-galactal, not recommended name) via transient formation of a 3-O-diethoxyphosphanyl intermediate 65.

Subsequently this route to 3-deoxy sugars was developed by further work in our laboratory by Ferrier and Sankey (23). They demonstrated that although tetra-O-acetyl-2-hydroxy-D-galactal undergoes rearrangement in boiling acetic acid, and that reaction is speeded up and goes to completion if catalytic amounts of methanesulfonic acid are added, the... 

From the rearrangement of tetra-O-acetyl-2-hydroxy-D-galactal in boiling acetic acid it was possible to isolate l,2,4,6-tetra-0-acetyl-2,3-dide-h.ydro-3-deoxy-a-jy-threo-hexose (32) (58%) and a small amount of 1,2,3,4,6-penta-O-acetyl-jS-D-galactopyranose. In the reaction mixture the presence of some a-pentaacetate was demonstrated chromatographically but NMR spectroscopy indicated no resonances corresponding to the / anomer of compound 32. These spectral measurements indicate that compound 32 constituted 80% of the mixture of products. 

Deoxy-2-[ F]fluoro-D-galactose can be prepared through an addition reaction to tri-O-acetyl-D-galactal (465), but better through Sn2 reaction (K F-Kryptofix 222 in MeCN) of methyl 3,4-0-isopropylidene-2-0-triflyl-6-0-trityl-y -D-talopyranoside (220 see Section 11,2), according to the cold synthesis. 

Abstract. In this contribution we present the results based on high-resolution spectra of 45 clump stars of the Galactic field. The main atmospheric parameters and abundances of 12C, 13C, N, O and other mixing sensitive chemical elements were investigated. Elemental ratios in the sample of field stars are compared to the results available for evolved stars in open clusters and to the theoretical prediction of extra mixing in stellar interiors. 

In addition to the indication of pollution from SNela (i.e. the knee ) found in the [Mg/Fe] and [O/Fe] trends,for kinematically selected thick disk samples, there is also evidence that thick disk stars well above the galactic plane show the same trend. In Fig. 2 we show the first results of a study of in situ metal-rich thick disk stars. It appears that there might be a downward trend in [Mg/Fe] as a function of [Fe/H] also well above the thin disk. This must, however, be further established before any firm conclusions can be drawn. 

Abstract. We present recent advances in the determination of chemical abundances of galactic Planetary Nebulae and discuss implications resulting from the comparison with theoretical predictions. From the analysis of diagrams of abundances of N/O vs He/H, N/O vs N/H and N/O vs O/H we argue that very likely the often used solar photospheric abundance of oxygen of 8.9, in usual units, is overestimated by a factor of 2-3, as suggested by very recent work in the Sun. This would solve an astrophysical problem with the measured abundances in planetaries. 

Abstract. The astrophysical origins of the element fluorine remain uncertain due in part to the availability of just a small number of abundance results for this element, that has readily observable transitions only in the infrared via vibration-rotation lines of HF. In this paper, we discuss all the available Galactic fluorine abundances to date, and add results for field stars with metallicities between [Fe/H] = -0.5 and -1.0, plus two stars that are members of the Orion association. The fluorine abundances obtained for the young Orion members are found to be in agreement with the trend of [F/O] versus O observed for the disk and they are a good representation of the present day value in the Galactic disk. 

Fig. 1. Oxygen abundances as a function of the activity index, Rx, derived from X-ray data (left-hand panels) and the excitation temperature Texc (right-hand panels). The bottom panels show the difference between [O/Fe] yielded by the OI triplet at about 7774 A and the [OI] A6300 line. Filled circles RS CVn binaries ([2] and [3]), filled squares field subgiants [3], filled triangles Pleiades stars, open triangles Hyades stars, open circles, squares and hexagons disk dwarfs. The source of the literature data for the open cluster and Galactic disk stars can be found in [4].

Table 1 lists our abundances of Fe, C, O and a-elements for the observed clusters. An overall excess of a-elements is shared by all the clusters up to solar metallicity, consistent with SNell being responsible for the gas enrichment. Our findings are also in good agreement both with all previous high resolution studies (see [2], [3], [6]) and with recent abundance determinations for field stars in the Galactic Bulge (see [10]). 

The chemical analysis has revealed that rather low C/O ratios are found in metal-poor extragalactic carbon stars, as found for galactic carbon stars of the solar vicinity. Furthermore, the three analyzed stars show similar s-elements enhancements [ls/Fe]=0.8-1.3 and [hs/Fe]=l.l-1.7. This leads to new constraints for evolutionary models. For instance, the derived C/O and 13C/12C ratios are lower than model predictions at low metallicity. On the contrary, theoretical predictions of neutrons exposures for the production of the s-elements are compatible with observations (see Fig. 1). Finally, from their known distances, we have estimated the luminosities and masses of the three stars. It results that SMC-B30 and Sgr-C3 are most probably intrinsic carbon stars while Sgr-Cl could be extrinsic. 

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