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Distortion, Tether, and Entropy Effects on Transannular Diels-Alder Cycloaddition Reactions of 10-18-Membered Rings. - The Journal of organic chemistry
Density functional theory calculations were performed on a set of 13 transannular Diels-Alder (TADA) reactions with 10-18-membered rings. The results were compared with those for bimolecular and intramolecular Diels-Alder reactions in order to investigate the controlling factors of the high TADA reactivities. The effects of tether length, heteroatoms, and alkynyl dienophiles on reactivity were analyzed. We found a correlation between tether length and reactivity, specifically with 12-membered macrocycles undergoing cycloaddition most readily. Furthermore, modifying 12-membered macrocycles by heteroatom substitution and utilizing alkynyl dienophiles enhances the reaction rates up to 10(5)-fold.
Transition State Gauche Effects Control the Torquoselectivities of the Electrocyclizations of Chiral 1-Azatrienes. - The Journal of organic chemistry
Hsung et al. have reported a series of torquoselective electrocyclizations of chiral 1-azahexa-1E,3Z,5E-trienes that yield functionalized dihydropyridines. To understand the origins of the torquoselectivities of these azaelectrocyclizations, we modeled these electrocyclic ring closures using the M06-2X density functional. A new stereochemical model that rationalizes the observed 1,2 stereoinduction emerges from these computations. This model is an improvement and generalization of the "inside-alkoxy" model used to rationalize stereoselectivities of the 1,3-dipolar cycloaddition of chiral allyl ethers and emphasizes a stabilizing hyperconjugative effect, which we have termed a transition state gauche effect. This stereoelectronic effect controls the conformational preferences at the electrocyclization transition states, and only in one of the allowed disrotatory electrocyclization transition states is the ideal stereoelectronic arrangement achieved without the introduction of a steric clash. Computational experiments confirm the role of this effect as a stereodeterminant since substrates with electropositive groups and electronegative groups have different conformational preferences at the transition state and undergo ring closure with divergent stereochemical outcomes. This predicted reversal of stereoselectivity for the ring closures of several silyl substituted azatrienes have been demonstrated experimentally.
Transannular [6 + 4] and Ambimodal Cycloaddition in the Biosynthesis of Heronamide A. - Journal of the American Chemical Society
The transannular [6 + 4] cycloaddition proposed as a step in the biosynthesis of heronamide A has been modeled using density functional theory. The proposed cycloaddition is highly stereoselective, affording a single product. The reaction proceeds through an ambimodal transition state that directly leads to a [4 + 2] adduct in addition to the observed [6 + 4] adduct. Interconversion of these adducts is possible via a facile Cope rearrangement. The [6 + 4] adduct is thermodynamically more stable than the [4 + 2] adduct by 5.2 kcal mol(-1) due to a combination of the ring and steric strain in the [4 + 2] product. The results strongly support the plausibility of the proposed transannular [6 + 4] cycloaddition in the biogenesis of heronamide A and may provide insights to designing substrates that selectively undergo [6 + 4] cycloaddition to form unbridged 10-membered rings.
Synthesis of ent-ketorfanol via a C-H alkenylation/torquoselective 6Ï€ electrocyclization cascade. - Angewandte Chemie (International ed. in English)
The asymmetric synthesis of ent-ketorfanol from simple and commercially available precursors is reported. A Rh(I) -catalyzed intramolecular Cï£¿H alkenylation/torquoselective 6Ï€â€…electrocyclization cascade provides a fused bicyclic 1,2-dihydropyridine as a key intermediate. Computational studies were performed to understand the high torquoselectivity of the key 6Ï€â€…electrocyclization. The computational results demonstrate that a conformational effect is responsible for the observed selectivity. The ketone functionality and final ring are introduced in a single step by a redox-neutral acid-catalyzed rearrangement of a vicinal diol to give the requisite carbonyl, followed by intramolecular Friedel-Crafts alkylation.Â© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Involvement of Lipocalin-like CghA in Decalin-Forming Stereoselective Intramolecular [4+2] Cycloaddition. - Chembiochem : a European journal of chemical biology
Understanding enzymatic Diels-Alder (DA) reactions that can form complex natural product scaffolds is of considerable interest. Schâ€‰210972 1, a potential anti-HIV fungal natural product, contains a decalin core that is proposed to form through a DA reaction. We identified the gene cluster responsible for the biosynthesis of 1 and heterologously reconstituted the biosynthetic pathway in Aspergillus nidulans to characterize the enzymes involved. Most notably, deletion of cghA resulted in a loss of stereoselective decalin core formation, yielding both an endo (1) and a diastereomeric exo adduct of the proposed DA reaction. Complementation with cghA restored the sole formation of 1. Density functional theory computation of the proposed DA reaction provided a plausible explanation of the observed pattern of product formation. Based on our study, we propose that lipocalin-like CghA is responsible for the stereoselective intramolecular [4+2] cycloaddition that forms the decalin core of 1.Â© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Prenatal diagnosis and telemedicine consultation of fetal urologic disorders. - Journal of telemedicine and telecare
In Arkansas, telemedicine is used commonly in obstetrics through Antenatal and Neonatal Guidelines, Education and Learning System (ANGELS), the existing statewide telemedicine network. This network is used primarily for tele-ultrasound and maternal-fetal medicine consultation. This study is a retrospective case series, describing all the patients who had a prenatally diagnosed urologic anomaly that required prenatal urologic consultation. From 2009-2013, approximately 1300 anomalies were recorded in the Arkansas Fetal Diagnosis and Management (AFDM) database, 14% of which were urologic anomalies. Twenty-six cases required prenatal urologic consultation, 25 of which were conducted via telemedicine. Teleconsultation allowed patients to combine maternal-fetal medicine and urologic consultations in one visit, saving time and effort and ultimately, for most patients, providing reassurance that delivery could be accomplished locally with postnatal follow-up already arranged. While there are several studies reporting the use of telemedicine for various subspecialty consultations, to our knowledge, this is the first to describe the use of telemedicine for prenatal urology consultation. Future research could randomize patients prospectively to allow comparison of both the outcomes as well as the patient experience.Â© The Author(s) 2015.
Highly torquoselective electrocyclizations and competing 1,7-hydrogen shifts of 1-azatrienes with silyl substitution at the allylic carbon. - Organic letters
Highly torquoselective electrocyclizations of chiral 1-azatrienes are described. These 1-azatrienes contain an allylic stereocenter that is substituted with a silyl group and are derived in situ from condensation of Î³-silyl-substituted enals with vinylogous amides. The ensuing stereoselective ring closures are part of a tandem sequence that constitutes an aza-[3 + 3] annulation method for constructing 1,2-dihydropyridines. Several mechanisms for the formal 1,7-hydrogen shift of these 1-azatrienes were evaluated computationally.
Reactivity and stereoselectivity of 6Ï€ and nazarov electrocyclizations of bridged bicyclic trienes and divinyl ketones. - The Journal of organic chemistry
The 6Ï€ electrocyclizations and Nazarov cyclizations of a series of bridged bicyclic substrates were modeled with the M06-2X density functional and the def2-TZVPP basis set, and the factors responsible for the reactivities of these substrates and the stereoselectivities of their ring closures were identified. The ring closures of these bridged bicyclic trienes are up to a million-fold faster (Î”Î”G(â§§) = 10 kcal mol(-1)) than that of 1,3,5-hexatriene, despite the absence of any activating functional groups. Three effects, preorganization, predistortion, and a CH Ï€ interaction, are responsible for this sizable difference in reactivity. Stereoselectivity is partially controlled by torsional effects, but for highly exo selective electrocyclizations, it is reinforced by a second effect (either a CH Ï€ interaction or a steric clash). The absence of this second effect in the ring closures of several divinyl ketones explains the reduced selectivity of these ring closures. In one case, a divinyl ketone (ketone 6) undergoes Nazarov cyclization to yield the endo product preferentially. For this example, through-space interaction of a nonconjugated alkene with the divinyl ketone Ï€ system in the endo transition state and a steric effect override the intrinsic exo selectivity.
Terminal substituent effects on the reactivity, thermodynamics, and stereoselectivity of the 8Ï€-6Ï€ electrocyclization cascades of 1,3,5,7-tetraenes. - The Journal of organic chemistry
M06-2X/6-31+G(d,p) computations are reported for the 8Ï€-6Ï€ electrocyclization cascades of 1,3,5,7-tetraenes. The rate-determining step for these cascades is typically the second (6Ï€) ring closure. According to experiment and theory, un- and monosubstituted tetraenes readily undergo 8Ï€ electrocyclic ring closure to form 1,3,5-cyclooctatrienes; however, the 6Ï€ electrocyclizations of these cyclooctatriene intermediates are slow and reversible, and mixtures of monocyclic and bicyclic products are formed. Computations indicate that di- and trisubstituted tetraenes undergo facile but less exergonic 8Ï€ electrocyclization due to a steric clash that destabilizes the 1,3,5-cyclooctatriene intermediates. Relief of this steric clash ensures the subsequent 6Ï€ ring closures of these intermediates are both kinetically facile and thermodynamically favorable, and only the bicyclic products are observed for the cascade reactions of naturally occurring tri- and tetrasubstituted tetraenes (in agreement with computations). The 6Ï€ electrocyclization step of these cascade electrocyclizations is also potentially diastereoselective, and di- and trisubstituted tetraenes often undergo cascade reactions with high diastereoselectivities. The exo mode of ring closure is favored for these 6Ï€ electrocyclizations due to a steric interaction that destabilizes the endo transition state. Thus, theory explains both the recalcitrance of the unsubstituted 1,3,5,7-octatetraene and 1-substituted tetraenes toward formation of the bicyclo[4.2.0]octa-2,4-diene products, as well as the ease and the stereoselectivity with which terminal di- and trisubstituted tetraenes are known to react biosynthetically.
A pilot study of telemedicine for post-operative urological care in children. - Journal of telemedicine and telecare
We conducted a retrospective study of paediatric urological surgery patients over a 12-month period. We compared patients followed up by telemedicine with those who had post-operative follow-up on site at the Arkansas Children's Hospital (ACH) in Little Rock. All pre-operative patients living in northwest Arkansas were given the opportunity to use telemedicine from a satellite clinic at Lowell, 328â€‰km from the hospital. Of 61 patients, 10 chose telemedicine and 51 chose to be evaluated at the ACH clinic. All telemedicine visits were completed successfully, but in four cases, the video clarity of the telemedicine images was not sufficient for decision-making, and a digital photograph was sent by email to the physician at the ACH. There were no post-operative surgical complications in either patient group. In the telemedicine group, the median distance to the ACH was 330â€‰km, and the median distance to the remote clinic was 35â€‰km. In the on-site group, the median distance to the ACH was 293â€‰km, which was significantly less (P=0.03). In the on-site group, the median travel time to the ACH was 174â€‰min. If the telemedicine group had driven to the hospital, the median travel time would have been 192â€‰min. Logistic regression showed that for every 37â€‰km increase in distance to ACH, patients had a 111% increase in the odds of receiving telemedicine compared to receiving on-site care (OR=2.1, 95% CI: 1.0, 4.4). The pilot study supports the use of telemedicine for the post-operative evaluation of paediatric urology surgery patients and suggests that substantial travel distance and time savings can be made.Â© The Author(s) 2014 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav.
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