Greener media in chemical synthesis and processing.
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Biomedical subjects
Juan C Palacios
Publications and source records attributed to Juan C Palacios.
Exploiting synthetic chemistry with mesoionic rings: improvements achieved with thioisomünchnones.
The chemistry of mesoionic rings, especially their use as masked dipoles, has been a fruitful area of research since the late 1950s. With recent advances in the control of regio- and stereochemistry, the dipolar cycloaddition led to widespread application. In this Account, we illustrate our contribution employing thioisomünchnones, a class of mesoionics that has found use in creating a series of heterocyclic systems hitherto inaccessible by conventional 1,3-dipolar cycloaddition. We also provide some rationalizations from computations and comment on future directions.
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Symmetry breaking by spontaneous crystallization--is it the most plausible source of terrestrial handedness we have long been looking for?--A reappraisal.
In the light of recent and controversial findings on spontaneous resolution of racemates and their implications in the origin of homochirality on earth, we present here a detailed review of this important topic. Although spontaneous resolution cannot at this moment be reliably predicted, there has also been considerable progress in crystal structure prediction and, not only thermodynamic factors, but also kinetic ones, play important roles in the efficiency of packing and crystallization. In addition, self-association and supramolecular control phenomena may be identified in cases where spontaneous resolution of enantiomers is actually occurring. While this contribution summarizes our current understanding of this intriguing phenomena, it is hoped that future work on crystalline conglomerates (or homochiral crystals) of prebiotic importance will be of further help to understand the general problem of terrestrial chirogenesis.
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Thermal and sonochemical studies on the Diels-Alder cycloadditions of masked o-benzoquinones with furans: new insights into the reaction mechanism.
What does a Diels-Alder cycloaddition look like? This question is here addressed in the case of the increasingly significant cycloadditions of masked o-benzoquinones (MOBs), which serve as versatile dienes for the construction of complex and functionalized structures. So first, what the mechanism is not: It is not (by and large) a classical, concerted [4 + 2] cycloaddition. Experimental evidence is now supported by sonochemical studies, which were instrumental in elucidating the pathway. Reactions with furans are accelerated and improved under sonication, even when conducted at -10 degrees C. Variation of the acoustic energy, temperature, and solvent composition allows us to optimize the yields and provide insights into the mechanism. Ultrasound does not cause sonochemical switching, as an alternative radical pathway should be ruled out. Results are consistent with a polar mechanism as claimed recently in a theoretical study. Moreover, this also does justice to a series of seminal papers, largely ignored, that gave a clue to the crucial issue of furan regioselectivity based on a nucleophilic addition. Most effects caused by ultrasonic agitation are of mechanochemical nature and suggest the existence of a perfectly stirred reactor with enhanced mass transfer.
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1,3-dipolar cycloaddition of 2-dialkylaminothioisomünchnones with aliphatic aldehydes: synthesis of beta-lactams and thiiranes, structure elucidation, and rationale for chemoselective fragmentation of cycloadducts.
A series of highly funtionalized beta-lactams and thiiranes can be generated on treatment of 1,3-thiazolium-4-olates (thioisomünchnones) with aliphatic aldehydes. Although in some cases a variety of products have been obtained, the present paper now provides a mechanistic rationale to explain the product distribution based on stereoelectronic effects. Thus, ring fragmentation of the initial [3+2] cycloadduct is essentially dictated by the electronic character of the aryl substituent on the nitrogen atom of the parent thioisomünchnone. However, further evolution of such cycloadducts into beta-lactams or thiiranes is governed by steric effects to a large extent. Evidence for such interactions has been obtained by computing PM3-optimized diastereomeric transition structures in the reaction of a thioisomünchnone with a chiral aliphatic aldehyde.
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Conformation of secondary amides. A predictive algorithm that correlates DFT-calculated structures and experimental proton chemical shifts.
The magnetic deshielding caused by the amido group on CON-CHalpha protons of secondary amides can easily be correlated with DFT-based structures at the B3LYP/6-31G level of theory via a novel algorithm that refines previous models, such as the classical McConnell equation. The shift is given by delta = a + 2.16 cos2(alpha - 35)/d, where alpha denotes the virtual dihedral angle resulting from linking the carbonyl and the alpha-carbons and d is the distance (A) between the shifted proton and the carbonyl oxygen. Notably, in this equation a is a parameter that can be optimized for different solvents, namely, CDCl3, DMSO-d6, and D2O. For the development of these correlations, the preferential conformation of amides is taken from the optimized structures in the gas phase obtained at the DFT level. The deshielding on anti and gauche protons in both rotamers of (Z)-acetamides and E/Z isomers of formamides has been evaluated. This methodology has proved to be highly reliable, allowing us to discard ab initio or DFT conformational arrangements when shifts calculated by the above-mentioned equation differ from the experimental values. Thus, the anti disposition between the CHalpha proton and the N-H bond appears to be the more stable conformation of simple amides. For amides bearing only one proton at Calpha, a local syn minimum can equally be characterized. The rotational barriers around the CON-alkyl bond along with the pyramidalization of the amido group have also been reassessed. As the conformation is taken away from anti or local syn minima, the nonplanarity of the amido group appears to increase.
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What does elementary chirality have to do with neutrinos?
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Experimental and theoretical insights regarding the cycloaddition reaction of carbohydrate-based 1,2-diaza-1,3-butadienes and acrylonitrile. A model case for the behavior of chiral azoalkenes and unsymmetric olefins.
A series of carbohydrate-based tetrahydropyridazines are prepared by the hetero-Diels-Alder reaction of the chiral 1,2-diaza-1,3-butadienes 1 and 2 with acrylonitrile. Reactions are regiospecific, and the observed diastereoselection is consistent with a preferred attack to the Re face of the heterodiene unit, as the chiral sugar placed at C4 does largely protect the opposite Si face. The stereochemistry of the major cycloadduct 4 has been firmly established by an X-ray crystallographic study that, in addition, reveals a conformation placing the cyano group in axial orientation. Cycloadducts such as 9 and 11, in which the axial cyano group and the carbohydrate moiety exhibit a cis relationship, undergo a facile E2 elimination that relieves the steric congestion. A detailed computational study is reported to provide better insight into the factors that influence this asymmetric cycloaddition. A DFT study (B3LYP/6-31G) on a reduced model does correctly predict the regiochemistry observed experimentally, while the facial diastereoselection is modeled at a semiempirical (PM3) level on the parent reagents, thereby accounting for the steric factor provided by the chiral substituent. The calculations also indicate that the axial orientation of the cyano group can be rationalized in terms of a stabilizing anomeric effect.
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Novel acid-catalyzed rearrangement of tetrahydro-1,2,3,4-tetrazines: unexpected formation of glycosazones.
The present contribution discloses a simple and unexpected acid-catalyzed cleavage of tetrahydrotetrazines leading to 1,2-bis(hydrazones). Incorporation of a chiral fragment derived from carbohydrates enables the rapid preparation of glycosazones, a family of compounds employed by Emil Fischer to elucidate the configuration of sugars. In addition, a mechanistic proposal accounts for experimental observations.
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