Secondary orbital interactions determining regioselectivity in the Diels-Alder reaction. 5. Thio-substituted 1,3-butadienes

Chromium photocatalysis: accessing structural complements to Diels-Alder adducts with electron-deficient dienophiles

A chromium-catalyzed, visible light-activated net [4 + 2] cycloaddition between dienes and electron-deficient alkenes is described. Gathered evidence, via control experiments, isolated intermediates, and measured redox potentials, points to several converging reaction pathways that afford the cyclohexene adducts, including a photochemical [2 + 2] cycloaddition/vinylcyclobutane rearrangement cascade and a substrate excitation/oxidation sequence to a radical cation intermediate. Notably, the accompanying mechanistic stipulations result in a process that yields regioisomeric compounds from those generated by traditional Diels-Alder cycloadditions.

Studies toward the total synthesis of pluraflavin A

A synthetic strategy towards the potent cytostatic agent pluraflavin A has been developed. Formation of the enantioenriched anthrapyran core bearing a halogen atom enabled the introduction of the α C-aryl glycoside by Stille cross-coupling and subsequent hydrogenation of the aryl glycal. Chemo- and stereoselective O-glycosylations of α oliose and β 3-epi vancosamine residues afforded a fully glycosylated aromatic core. Attempts to install the dimethylamino group of the C-disaccharide suggest that introduction of an azide group by displacement and subsequent reduction may pave the way to the total synthesis of pluraflavin A.

Pattern recognition analysis in complex molecule synthesis and the preparation of iso-Diels-Alder motifs

The identification of synthesizable substructural domains within more complex structural targets is of significant value in designing a workable plan of synthesis. We term this process "pattern recognition analysis" (PRA). In this paper we continued to build on the theme of PRA as a potential resource in retrosynthetic blueprints to reach highly challenging targets. The paper operates at two levels. First, there is provided a clear sense of definitions of categories by which patterns are related to hypothetical reaction types. Although the required reaction type may for the moment not exist, we believe that this method of analysis is likely to promote innovation that identifies unmet needs and opportunities to advance the cause of complex target synthesis. In addition, we describe reductions to practice in expanding the menu of achievable patterns. It is likely that the future value of PRA will be associated with its utility in leading the way to new and exploitable chemical innovation.

Control elements in dynamically determined selectivity on a bifurcating surface

The mechanism and the nature of the dynamically determined product selectivity in Diels-Alder cycloadditions of 3-methoxycarbonylcyclopentadienone (2) with 1,3-dienes was studied by a combination of product studies, experimental kinetic isotope effects, standard theoretical calculations, and quasiclassical trajectory calculations. The low-energy transition structures in these reactions are structurally balanced between [4pi(diene) + 2pi(dienone)] and the [2pi(diene) + 4pi(dienone)] modes of cycloaddition. The accuracy of these structures and their bispericyclic nature is supported by the experimental isotope effects. Trajectories passing through these transition structures can lead to both [4pi(diene) + 2pi(dienone)] and [2pi(diene) + 4pi(dienone)] cycloadducts, and the mixture of products obtained varies with the structure of the diene. The factors affecting this selectivity are analyzed. The geometry of the transition structure is a useful predictor of the major product, but the selectivity is also guided by the shape of the energy surface as trajectories approach the products and by how trajectories cross the transition state ridge.

Dynamic effects on the periselectivity, rate, isotope effects, and mechanism of cycloadditions of ketenes with cyclopentadiene

The cycloadditions of cyclopentadiene with diphenylketene and dichloroketene are studied by a combination of kinetic and product studies, kinetic isotope effects, standard theoretical calculations, and trajectory calculations. In contrast to recent reports, the reaction of cyclopentadiene with diphenylketene affords both [4 + 2] and [2 + 2] cycloadducts directly. This is surprising, since there is only one low-energy transition structure for adduct formation in mPW1K calculations, but quasiclassical trajectories started from this single transition structure afford both [4 + 2] and [2 + 2] products. The dichloroketene reaction is finely balanced between [4 + 2] and [2 + 2] cycloaddition modes in mPW1K calculations, as the minimum-energy path (MEP) leads to different products depending on the basis set. The MEP is misleading in predicting a single product, as trajectory studies for the dichloroketene reaction predict that both [4 + 2] and [2 + 2] products should be formed. The periselectivity does not reflect transition state orbital interactions. The (13)C isotope effects for the dichloroketene reaction are well-predicted from the mPW1K/6-31+G** transition structure. However, the isotope effects for the diphenylketene reaction are not predictable from the cycloaddition transition structure and transition state theory. The isotope effects also appear inconsistent with kinetic observations, but the trajectory studies evince that nonstatistical recrossing can reconcile the apparently contradictory observations. B3LYP calculations predict a shallow intermediate on the energy surface, but trajectory studies suggest that the differing B3LYP and mPW1K surfaces do not result in qualitatively differing mechanisms. Overall, an understanding of the products, rates, selectivities, isotope effects, and mechanism in these reactions requires the explicit consideration of dynamic trajectories.

Theoretical Revisit of Regioselectivities of Diels−Alder Reactions: Orbital-Based Reevaluation of Multicentered Reactivity in Terms of Reactive Hybrid Orbitals

When reactions proceed under kinetic control, stereoselectivities are determined from energetic properties of the transition state (TS). Orbital interaction at the TS plays a significant role in determining reactivity and selectivity, because it is directly related to lowering of the activation barrier. However, the orbital interaction at the TS cannot be represented only in terms of the frontier orbitals at the reactant state. A reactive hybrid orbital (RHO) that is localized at the reaction site is used to represent the orbital around the relevant TS in terms of a combination of the canonical molecular orbitals (MOs). This representation can make comparison of reactivities and selectivities among molecules of different sizes feasible. In this paper, the regioselectivities of Diels-Alder cycloaddition reactions of a monosubstituted butadiene and a dienophile (a monosubstituted ethylene) are investigated in terms of several parameters such as reactivity index and stabilization index, which are obtained on the basis of the newly developed multicentered version of the RHO method. These values can also be compared with the corresponding parameters defined on the basis of the frontier orbitals. Predictions of the regioselectivities based on the RHOs are consistent with experimental observations, while those based on the frontier orbitals are unreliable. This is because the RHO is superior to the frontier orbital as a descriptor of the orbital participating in orbital interactions around the TS.