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We present an innovative approach that simultaneously enables direct antenna frequency-hopped M-ary frequency shift keying (DAFH-MFSK) modulation and beamsteering through the use of time-modulated arrays (TMAs). The distinctive feature of our approach lies in the modulation of the TMA excitations with binary periodic sequences, which can be easily frequency-adjusted and time-delayed to simultaneously allow for DAFH-MFSK direct antenna modulation and beamsteering. Notably, our TMA proposal offers a distinct advantage over conventional architectures in terms of performance metrics, including reduced insertion losses and enhanced phase resolution for beamsteering, while also simplifying hardware complexity.

Efficient resource allocation strategies are pivotal in vehicular communications as connected devices steeply increase in scenarios with much more stringent requirements. In this work, we propose a deep reinforcement learning (DRL)-based sequential scheduling approach for sum-rate maximization in the uplink of intelligent reflecting surface (IRS)-assisted multi-user (MU) multiple-input multiple-output (MIMO) vehicular communications. We formulate the scheduling task as a partially observable Markov decision process (POMDP) and propose a novel stream-level sequential solution based on the proximal policy optimization (PPO) algorithm. We consider a realistic imperfect channel state information (ICSI) model and assess the proposal in several communication setups comprising both spatially uncorrelated and correlated links. Simulation results show that the proposed DRL-based sequential scheduling approach is a robust alternative to more computationally demanding benchmarks.

This work focuses on wideband intelligent reflecting surface (IRS)-aided multiuser MIMO systems. One of the major challenges of this scenario is the joint design of the frequency-dependent base station (BS) precoder and user filters, and the IRS phase-shift matrix which is frequency flat and common to all the users. In addition, we consider that the channel state information (CSI) is imperfect at both the transmitter and the receivers. A statistical model for the imperfect CSI is developed and exploited for the system design. A minimum mean square error (MMSE) approach is followed to determine the IRS phase-shift matrix, the transmit precoders, and the receiving filters. The broadcast (BC)- multiple access channel (MAC) duality is used to solve the optimization problem following an alternating minimization approach. Numerical results show that the proposed approach leads to substantial performance gains with respect to baseline strategies that neglect the inter-user interference and do not optimize the IRS phase-shift matrix. Further performance gains are obtained when incorporating into the system design the statistical information of the channel estimation errors.

An innovative approach to hybrid analog-digital precoding for the downlink of wideband massive MIMO systems is developed. The proposed solution, termed Rank-Constrained Coordinate Ascent (RCCA), starts seeking the full-digital precoder that maximizes the achievable sum-rate over all the frequency subcarriers while constraining the rank of the overall transmit covariance matrix. The frequency-flat constraint on the analog part of the hybrid precoder and the non-convex nature of the rank constraint are circumvented by transforming the original problem into a more suitable one, where a convenient structure for the transmit covariance matrix is imposed. Such structure makes the resulting full-digital precoder particularly adequate for its posterior analog-digital factorization. An additional problem formulation to determine an appropriate power allocation policy according to the rank constraint is also provided. The numerical results show that the proposed method outperforms baseline solutions even for practical scenarios with high spatial diversity.

We propose an algorithm for joint precoding and user scheduling in multiple-input multiple-output systems with extremely-large aperture arrays, assuming realistic channel conditions and imperfect channel estimates. The use of statistical channel state information (CSI) for user scheduling, and a proper selection of the set of users for which CSI is updated allow for obtaining an improved achievable sum spectral efficiency. We also confirm that the effect of imperfect CSI in the precoding vector design and the cost of training must be taken into consideration for realistic performance prediction.

The combination of multiple-input multiple-output (MIMO) systems and intelligent reflecting surfaces (IRSs) is foreseen as a critical enabler of beyond 5G (B5G) and 6G. In this work, two different approaches are considered for the joint optimization of the IRS phase-shift matrix and MIMO precoders of an IRS-assisted multi-stream (MS) multi-user MIMO (MU-MIMO) system. Both approaches aim to maximize the system sum-rate for every channel realization. The first proposed solution is a novel contextual bandit (CB) framework with continuous state and action spaces called deep contextual bandit-oriented deep deterministic policy gradient (DCB-DDPG). The second is an innovative deep reinforcement learning (DRL) formulation where the states, actions, and rewards are selected such that the Markov decision process (MDP) property of reinforcement learning (RL) is appropriately met. Both proposals perform remarkably better than state-of-the-art heuristic methods in scenarios with high multi-user interference.

We introduce Distance-Based Scheduling (DBS), a new technique for user selection in downlink multi-user communications with extra-large (XL) antenna arrays. DBS categorizes users according to their equivalent distance to the antenna array. Such categorization effectively accounts for inter-user interference while largely reducing the computational burden. Results show that ( ${i}$ ) DBS achieves the same performance as the reference zero-forcing beamforming scheme with a lower complexity; (ii) a simplified version of DBS achieves a similar performance when realistic spherical-wavefront (SW) propagation features are considered; (iii) SW propagation brings additional degrees of freedom, which allows for increasing the number of served users.

This paper presents an indoor person localization and tracking system that uses multiple smart cameras equipped with artificial intelligence (AI) accelerators serving as edge-computing nodes. Our main contributions are as follows: (a) the development of a new multicamera tracking system for indoor scenarios; (b) the release of a multitarget multicamera tracking dataset; and (c) the development of an annotation mechanism based on waypoints. The system can simultaneously track several individuals while preserving their privacy and anonymity, because no images or sensitive data are transmitted outside the edge nodes. Only the position and appearance of each person were transmitted to the central server. In addition, a multitarget multicamera tracking dataset was released. The dataset contains recordings from five cameras in an indoor scenario and is annotated with the real-world coordinates of individuals. Ground-truth annotations were semiautomatically generated using a mechanism in which people equipped with mobile phones followed specific paths with predefined waypoints. Software related to the ground-truth annotation mechanism has also been released as open source.

The design and synthesis of two α,γ‐cyclic octapeptides decorated with one and two Zn–porphyrin units in their periphery is described. In nonpolar organic solvents the α,γ‐cyclic octapeptides quantitatively self‐assemble into Zn–bis‐ or –tetraporphyrin architectures that could act as molecular tweezers. The self‐assembly process, however, is not regioselective and affords a mixture of different regioisomers that are involved in chemical exchange processes. The regioisomers with the Zn–porphyrin units positioned in register with respect to each other are proposed to be the less abundant species in the solution mixture. It has been demonstrated that the coordination of 1,4‐diazabicyclo[2.2.2]octane (DABCO) to the supramolecular bis‐ or tetraporphyrin tweezers is an effective way to achieve regioisomeric control of the self‐assembled mixture of dimers. Thus, DABCO functions as an external molecular trigger and, when used under strict stoichiometric control with respect to the Zn–porphyrin units, provokes the exclusive formation of self‐assembled dimers with a cofacial arrangement of Zn–porphyrin units through the formation of sandwich‐type complexes. The use of excess DABCO fragments the sandwich complexes and affords open dimers of high stoichiometry with DABCO molecules axially monocoordinated to the Zn–porphyrin units, probably as a regioisomeric mixture. In the case of Zn–tetraporphyrin tweezers, the ditopic coordination of DABCO at the two binding sites shows a moderate positive cooperativity factor, αP=5. These assemblies have potential applications as light‐induced energy and electron‐transfer switches regulated by DABCO coordination; such applications would require the introduction of additional chromophores in the cyclic peptide scaffold.

Cyclic octapeptides composed of α‐amino acids alternated with cis‐3‐aminocycloalkanecarboxylic acids, self‐assemble as drumlike dimers through β‐sheet‐like, backbone‐to‐backbone hydrogen bonding. Heterodimerization appears to be significantly more favored than homodimerization, and this represents a novel approach for the design and fabrication of highly stable heterodimeric assemblies. A multicomponent equilibrium network based on fluorescently derivatized self‐assembling α,γ‐cyclic octapeptides has been successfully used to form light‐harvesting/light‐converting ensembles with a distinctive organization of donor and acceptor units able to act as efficient artificial photosystems.

Now with nickel:[3C+2C+2C] cycloadditions involving non‐activated alkylidenecyclopropanes provide a practical entry to a variety of interesting 6,7‐fused bicyclic systems (see scheme; cod=1,5‐cyclooctadiene). DFT calculations, combined with experimental data, suggest that the catalytic cycle involves the initial formation of 1‐alkylidenenickelacyclobutane intermediates, such as 1.

Halogen‐free CC coupling: Catalytic amounts of [Pd2(dba)3] (dba=dibenzylideneacetone) and a sterically encumbered phosphite (L1) promote the addition of CH bonds of terminal alkynes (2) to alkylidenecyclopropanes (1). The reaction gives rise to 1,4‐enynes (3) in good yields. The catalytic cycle probably consists of a Pd‐catalyzed CH activation of the terminal alkyne, a regioselective hydropalladation followed by distal bond cleavage of the alkylidenecyclopropane, and a final reductive elimination.

Efficient at room temperature: The Au complex generated in situ from [(IPr)AuCl] and AgSbF6promotes the [4C+3C] intramolecular cycloaddition of allenes and dienes at room temperature, and in a particularly efficient and versatile manner. A DFT study on dimethylallenyl precursors agreed with the formation and cycloaddition of a metal–allyl cation intermediate, and points to the 1,2‐hydride shift as the key rate‐limiting step.

“Formal” and standard RuII‐catalyzed [2+2+2] cycloaddition of 1,6‐diynes to alkenes gave bicyclic 1,3‐cyclohexadienes in relatively good yields. When terminal 1,6‐diynes 1were used, two isomeric bicyclic 1,3‐cyclohexadienes 4or 6were obtained, depending on the acyclic or cyclic nature of the alkene partner. When unsymmetrical substituted 1,6‐diynes 7were used, the reaction with acyclic alkenes took place regio‐ and stereoselectively to afford bicyclic 1,3‐cyclohexadienes 8. A cascade process that behaves as a “formal” RuII‐catalyzed [2+2+2] cycloaddition explained these results. Initially, a Ru‐catalyzed linear coupling of 1,6‐diynes 1and 7with acyclic alkenes occurs to give open 1,3,5‐trienes of type 3, which after a thermal disrotatory 6e−π‐electrocyclization led to the final 1,3‐cyclohexadienes 4and 8. When disubstituted 1,6‐diyne 10was used with electron‐deficient alkenes, new exo‐methylene cyclohexadienes 12arose from a competitive reaction pathway.

Four polyhydroxylated γazido and three γamino acids 1–4have been synthesized from –quinic acid. The nitrogen functionality in the reported derivatives was introduced through regioselective ringopening of cyclic sulfates with azide anion. The synthesis of the 4 and 5epimers of γazido acid 1a– derivatives 2and 3a, respectively – was achieved by regioselective oxidation of dibutylstannylene acetals derived from 1,2diols, followed by diastereoselective reduction of their corresponding αhydroxy ketones. The presence of the azide functional group in the reported γazido acids was exploited to permit interlinking of bioactive molecular entities.© WileyVCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008

Peptide foldamers constitute a growing class of nanomaterials with potential applications in a wide variety of chemical, medical and technological fields. Here we describe the preparation and structural characteristics of a new class of cyclic peptide foldamers (3α,γ‐CPs) that, depending on their backbone N‐methylation patterns and the medium, can either remain as flat rings that dimerize through arrays of hydrogen bonds of antiparallel β‐sheet type, or can fold into twisted double reverse turns that, in the case of double γ‐turns, associate in nonpolar solvents to form helical supramolecular structures. A 3α,γ‐CP consists of a number of multiples of a repeat unit made up of four amino acid residues of alternating chirality: three corresponding to α‐amino acids and one to a γ‐amino acid (a cis‐3‐aminocycloalkanecarboxylic acid).

No Abstract

The mechanism of the palladium‐catalysed [3+2] intramolecular cycloaddition of alkylidenecyclopropanes to alkynes has been computationally explored at DFT level. The energies of the reaction intermediates and transition states for different possible pathways have been calculated in a model system that involves the use of PH3as a ligand. The results obtained suggest that the most favourable reaction pathway involves the initial CC oxidative addition of the cyclopropane to a Pd0complex to give an alkylidenepalladacyclobutane, which isomerises to a methylenepalladacyclobutane intermediate. Subsequent cyclisation by alkyne carbometallation, followed by reductive elimination affords the final product. An alternative mechanism consisting of a palladaene‐type rearrangement is less probable in terms of Gibbs energy, but cannot be fully discarded because it is competitive if one considers electronic energies. For substrates that present an ester group at the terminal position of the triple bond we have found an alternative, more favourable mechanistic route that explains why the [3+2] cycloaddition of these types of systems does not lead to the expected cycloadducts.

No Abstract

A cascade dienyne ring‐closing metathesis approach has been applied to the synthesis of the tetracyclic carbon framework of a new class of hybrid compounds—the taxosteroids—possessing carbon frameworks incorporating moieties characteristic of both taxanes (such as AB rings) and steroids (i.e., CD system and side chain). This tandem cyclization is highly stereoselective, allowing the one‐step formation of the bicyclo[5.3.1]undecene system characteristic of taxol. In this work we describe the scope and limitations of such cyclizations.

No Abstract

Allenes are useful and versatile two-carbon partners in palladium catalyzed [3C+2C] intramolecular cycloadditions with alkylidenecyclopropanes enabling the preparation of dienyl bicyclo[3.3.0]octane adducts with good yields and high diastereoseletivity.

Abstract: 5,6-Dimethoxy-2,3-dihydro-7H-dibenzo[de,h] quinolin-7-one has been synthesized by cyclization of phenylethylaminophtalides with polyphosphoric acid and characterized by X-ray diffraction and NMR assignments. A theoretical study by using DFT hybrid method B3LYP and the 6-311++G(2d,p) basis set has been carried out. The local reactivity descriptors such as Fukui functions, local softness and local electrophilicity were calculated on the isolated molecule. The results of the calculations were compared with experimental data for understanding the aromatic demethoxylation to afford 5-methoxy-2,3-dihydro-7H-dibenzo[de,h]quinolin-7-one as final product.

The intrinsic recognition code associated with dsDNA allows either accelerating or retarding of a native chemical ligation reaction between tripyrrole ligands. The rate changes most probably stem from the sequence-dependent characteristics of the dsDNA−ligand complexes.

Six conformationally restricted carba-sugars have been synthesized by ring-closing metathesis of a readily available diallyl keto derivative of (–)-quinic acid. The rich functionality of the resulting spiro ketone was exploited for the diastereoselective synthesis of various spiro carba-sugars: four of them polyhydroxylated and two aminopolyhydroxylated. The results of the testing of these compounds against various commercially available glycosidases (amyloglicosidase, α- and β-galactosidase, α- and β- glucosidase, α-mannosidase, trehalase and neuraminidase) are provided. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

The synthesis of ortho-(trimethylsilyl)vinyl triflates is described. Fluoride-induced decomposition of these triflates leads to the generation of strained cycloallenes and cycloalkynes under mild reaction conditions. The generation rate of these highly reactive intermediates can be modulated by modifying the solubility of the fluoride source. The slow generation of cyclic allenes and alkynes is a prerequisite for their use in homogeneous catalysis.

Here we report the preparation and structural characteristics of self‐assembling peptide tubelets composed of 32‐membered rings formed of alternating α‐amino acids and cis‐3‐aminocyclohexanecarboxylic acids. The tubelets possess a partial hydrophobic core environment, provided by the projection of the cyclohexane C2 methylene moiety into the lumen, and a Van der Waals pore diameter of about 7 Å.

No Abstract

Linking the basic region of a bZIP transcription factor to a distamycin‐like tripyrrole peptide by means of a nitrogen‐containing tether produces a hybrid capable of high‐affinity recognition of specific, designated DNA sequences. The importance of the nitrogen in the tether is shown by the considerable reduction in affinity (more than 10‐fold) caused by its replacement with an ether linkage. Attachment of an aminopropyl chain on the pyrrole adjacent to the pyrrole bearing the nitrogen‐containing tether increases affinity approximately one order of magnitude. These results confirm that a suitable location of protonated amine groups on designed DNA‐binding peptides provides for higher affinities, most probably because of the generation of salt bridged contacts with the phosphodiester backbone.

Efficient syntheses of two novel positional stereoisomers of valiolamine, a potent glycosidase inhibitor, and two polyhydroxy γ-amino acids are reported. The syntheses involve regio- and stereoselective azidolysis of epoxides by trimethylsilyl azide with BF3·OEt2 as Lewis acid. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

In this paper we present a new technique for separating convolutive mixtures of statistically independent non-Gaussian temporally-white signals. The technique is termed blind because the separation is performed without resorting to an a priori knowledge of the sources or the mixing system. The blind source separation (BSS) problem is solved by interpreting a time domain convolution as several instantaneous mixtures in the frequency domain. The sources at each frequency bin are recovered using an extension of the adaptive algorithm proposed by Dapena and Castedo in Signal Process. 75 (1999) 11–27. By extending the stability analysis presented there, we prove that the only attractors of the algorithm correspond to the points where perfect separation is achieved. We also propose novel strategies to remove the permutation and the amplitude indeterminacies that appear when the sources are recovered in a different order or with different amplitude in some frequency bins. The permutation problem is solved by clustering the outputs according to their second- or fourth-order cumulants. Afterwards, the amplitude indeterminacy is corrected taking into account the values predicted by the stability analysis. [Copyright &y& Elsevier]

The palladium(0)-catalyzed reaction of benzyne with suitably functionalized benzodiynes to afford benzo[b]fluorenones is described. The outcome of the reaction is affected by the steric and electronic properties of the diynes and proved to be regioselective when an unsymmetrically substituted aryne is used. This work represents the first example of a partially intramolecular [2+2+2] cycloaddition involving arynes. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Ring-closing metathesis (RCM) of cis-2,6-dialkenyl-2-hydroxy-1-cyclohexanones affords bicyclo[3.n.1]alkenones that are easily converted into eight- or nine-membered carbocycles by oxidative cleavage of the keto-bridging tether. Since the starting cyclohexanones are readily assembled from commercially available 1,2-cyclohexanedione, the overall process constitutes a rapid and versatile route to medium-sized carbocycles, which are otherwise difficult compounds to assemble using currently available procedures. If one of the alkenes of the cyclohexanone chains is replaced by an alkyne, the subsequent RCM produces 1,3-diene systems capable of undergoing stereoselective Diels–Alder reaction with activated dienophiles. Oxidative cleavage of the keto bridge of the resulting tricycles leads to 8–6 and 9–6 fused bicarbocycles with up to four stereocenters. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2111/2002/f3848_s.pdf or from the author.

This paper presents a new Maximum Likelihood (ML) based approach to the separation of convolutive mixtures of unobserved sources in the presence of Additive Gaussian Noise (AGN). The proposed method proceeds in two steps. First, the mixing system coefficients are estimated in the ML sense and, afterwards, this information is employed to attain source separation according to either the ML or the linear Minimum Mean Square Error (MMSE) criteria. System coefficient estimation is carried out in a block-iterative way using an extension of the Expectation Maximization (EM) method. Both deterministic and stochastic (Monte Carlo) implementations of the resulting estimation algorithm are considered. The proposed algorithms rely on the knowledge of the sources joint probability density function (p.d.f.). This is a fairly realistic assumption in applications such as digital communications but computer simulations reveal that it is not an stringent requirement. The proposed estimation algorithm can be successfully used with a tentative p.d.f. when this is not known a priori.

Highly diastereoselective thermal [5C+2C] intramolecular pyrone–alkene cycloadditions can be achieved by introducing a homochiral p‐tolylsulfinyl group at a suitable position of the alkene. The resulting adducts can be readily desulfinylated to give optically active 8‐oxabicyclo[3.2.1]octane derivatives. Interestingly, switching from a sulfinyl to a sulfonimidoyl group allows one to reverse the direction of the diastereofacial selectivity and thereby produces oxa‐bridged carbocyclic systems enantiomeric to those obtained from the sulfinyl precursors. Cleavage of the oxa‐bridge on the desulfurated adducts yields highly functionalized seven‐membered carbocyclic derivatives in enantiopure form. Alternative cleavage of the seven‐membered carbocycle provides enantiomerically enriched tetrahydrofurans. We have exploited this reaction pathway for the synthesis of the naturally occurring enantiomer of nemorensic acid.

A one‐step, regioselective synthesis of annelated symmetric and asymmetric 3,3′‐disubstituted 2,2′‐bipyridines by cobalt(I)‐catalyzed [2+2+2] cycloadditions between 5‐hexynenitrile and 1,3‐diynes is described. In the symmetric case, the total regioselectivity of the first cycloaddition is ensured electronically by the conjugation of the triple bonds, and for aminomethylated diynes that of the second is ensured by the cobalt coordinating to the aminomethyl rather than to the hexynenitrile nitrogen. In the asymmetric case, the first cycloaddition took place chemoselectively, which in the case of bis(trimethylsilyl)‐1,3,5‐hexatriyne (viewed as a 1,3‐diyne) is explained by semiempirical calculation of LUMO coefficients. The copper(I) complex of 6 b, constituting the first reported complex of the form ML2(L is a symmetric 3,3′‐disubstituted 2,2′‐bipyridine), has been prepared. It had UV/Vis and NMR spectra reflecting the 3‐substituent‐induced mutual torsion of the bipyridine rings in the cisconformation, as was confirmed by x‐ray diffractometric determination. The bipyridine 6 cforms the dinuclear complex [Cu2(6 c)2(CH3CN)2]2+in the solid state.

We describe two total syntheses of 1-methyl-1,2,3,4-tetrahydronaphtho[1,2-f]isoquinolines based on free radical cyclization. One of them includes the cyclization of N-{2-[2-(2-bromophenyl)-1-methoxyethyl]phenylethyl}acetamides and subsequent transformation of the resulting N-[2-(10-methoxy-9,10-dihydro-1-phenanthryl)ethyl]acetamides. The second is based on the known cyclization of 1-(2-bromobenzyl)isochroman-3-ones to 4,5,6a,7-tetrahydrodibenzo[de,g]chroman-3-ones followed by transformation of the resulting 2-(1-phenanthryl)acetamides.

Two new total syntheses of 1-methyl-1,2,3,4-dihydronaphtho[1,2-f]isoquinolines are based on the construction of their phenanthrene ring system by photochemically induced cyclization of N-{2-[(E)-2-phenyl-1-ethenyl]phenylethyl}acetamides or 1-methyl-5-[(E)-2-phenyl-1-ethenyl]-1,2,3,4-tetrahydroisoquinolines.

No Abstract

Synthetic approaches to enamides, intermediates of use in the synthesis of natural products, were studied as well as their 7-endo-trigradical cyclization to 3-benzazepines.

Non-conjugated benzotriynes 2undergo a novel radical cycloaromatization to provide the benzo[ b]fluorene core of the kinamycins. The overall process involves a thermal intramolecular cyclization to the non-benzenoid biradicals 4followed by radical cyclization and hydrogen abstraction.

A practical method for synthesizing orthogonally connected peptide-oligopyrrole conjugates by coupling the oligopyrrole fragment to a selectively deprotected glutamic acid side-chain of a resin-bound peptide is described.

A practical method for the synthesis of medium-sized peptides incorporating several N-methylpyrrole units is described. These peptides might prove useful as novel sequence-specific DNA binders.

Intramolecular Diels-Alder reactions of 1-aminoisobenzofurans give benzo[c]phenanthridines. The reactive intermediates are generated from o-(diazomethyl)benzamides.

1α,24(R)-Dihydroxyvitamin D3(5e) was synthesized. The key step in the preparation of the side-chain was opening of the chiral epoxide 10by the α-anion of the nitrile 9. The triene system was synthesized by the dienyne route.

Deoxykojic acid (1) reacts with two benzyne molecules to afford 9-methyl-7-phenoxy-5,9-dihydro-5,9-epoxy-6H-benzocyclohepten-6-one (2) in 33% yield. Maltol (5) reacts with three molecules of benzyne to afford (4bα,5β,10β,11aα)-10-methyl-11a-phenoxy-4b,5,10,11a-tetrahydro-5,10-epoxy-11H- benzo[a]benzo[3,4]cyclobuta[1,2-d]cyclohepten-11-one (6a) in 36% yield.