Definition of Several Control Elements Relevant to the Stereodefined Serial Elaboration of Belted Poly(spirotetrahydrofurans) Fitted with a Cyclohexane Core

Diels-Alder Reaction of Photochemically Generated (E)-Cyclohept-2-enones: Diene Scope, Reaction Pathway, and Synthetic Application

Upon irradiation at λ = 350 nm, cyclohept-2-enone undergoes an isomerization to the strained (E)-isomer. The process was studied by XMS-CASPT2 calculations and found to proceed by two competitive reaction channels on either the singlet or the triplet hypersurface. (E)-Cyclohept-2-enone is a reactive dienophile in thermal [4 + 2] cycloaddition reactions with various dienes. Ten different dienes were probed, most of which─except for 1,3-cyclohexadiene─underwent a clean Diels-Alder reaction and gave the respective trans-fused six-membered rings in good yields (68-98%). The reactions with furan were studied in detail, both experimentally and by DLPNO-CCSD(T) calculations. Two diastereoisomers were formed in a ratio of 63/35 with the exo-product prevailing, and the configuration of both diastereoisomers was corroborated by single crystal X-ray crystallography. The outcome of the photoinduced Diels-Alder reaction matched both qualitatively and quantitatively the calculated reaction pathway. Apart from cyclohept-2-enone, five additional cyclic hept-2-enones and cyclooct-2-enone were employed in their (E)-form as dienophiles in the Diels-Alder reaction with 1,3-cyclopentadiene (80-98% yield). The method was eventually applied to a concise total synthesis of racemic trans-α-himachalene (four steps, 14% overall yield).

Reactions of Allylmagnesium Reagents with Carbonyl Compounds and Compounds with C═N Double Bonds: Their Diastereoselectivities Generally Cannot Be Analyzed Using the Felkin-Anh and Chelation-Control Models

This review describes the additions of allylmagnesium reagents to carbonyl compounds and to imines, focusing on the differences in reactivity between allylmagnesium halides and other Grignard reagents. In many cases, allylmagnesium reagents either react with low stereoselectivity when other Grignard reagents react with high selectivity, or allylmagnesium reagents react with the opposite stereoselectivity. This review collects hundreds of examples, discusses the origins of stereoselectivities or the lack of stereoselectivity, and evaluates why selectivity may not occur and when it will likely occur.

Synthesis of, and Structural Assignments to the Stereoisomers of Bis (2,2‘)- and Tris (2,2‘,2‘ ‘)-Tetrahydrofurans: Conformational Features and Ionic Binding Capacities of These Gateway Polycyclic Networks

Construction of the polytetrahydrofuranyl building blocks 6-10 from the common bissiloxyacetone precursor 11 is detailed. The approach is concise and, for the bis-(THF) pair, capitalizes on the full retention of configuration observed during the rhodium-promoted decarbonylation of aldehydes 18 and 19. The capability of the title compounds to associate with alkali metal ions in solution and the gas phase has demonstrated a preference for Li+ over Na+ and K+ in all cases, with 6 and 7 exhibiting somewhat higher binding selectivities than 8-10. The relative energy orderings of attainable conformations with the bis-THF and tris-THF series were explored computationally. The various envelope arrangements present in the individual THF units are shown to play a significant role alongside prevailing gauche interactions. The "gauche effect" is shown computationally not to be an accurate predictor of the lowest energy conformer.

Practical designed syntheses of all stereoisomeric bis(2,2')- and tris(2,2',2' ')-tetrahydrofurans

A convenient synthetic entry to the entire group of bis(2,2')- and tris(2,2',2' ')-tetrahydrofurans has been developed. The method is concise and relies on chromatographic separation, decarbonylation, and annulation to arrive at a specific product of defined relative configuration. [reaction: see text]

Selective lithium ion binding involving inositol-based tris(spirotetrahydrofuranyl) ionophores: formation of a rodlike supramolecular ionic polymer from a homoditopic dimer

The stereoselective replacement of all three hydroxyl groups in myo-inositol orthoformate by spirotetrahydrofuran rings in that manner which projects the C--O bonds in the molecular interior has been examined. The heterocyclic components were introduced sequentially, a protocol that demonstrated the utility of precomplexation to LiClO(4) as a stereocontrol tactic. The capability of 3 to coordinate to alkali metal ions was quantified. The conformationally restricted nature of this ligand conveys high selectivity for binding to lithium ion. Beyond that, the ionophore prefers to form 2:1 complexes with Li(+) and exhibits little tendency for 1:1 stoichiometry. These properties are shared by the "dimer" 36, in which two building blocks of type 3 have been conjoined by a 1,3-butadiyne tether positioned at the ortho ester terminus. This bifacial ligand reacts with one equivalent of LiClO(4) or LiBF(4) to form rodlike ionic polymers. Alternative recourse to lithium picrate results in production of the doubly capped homoditopic complex 41. Various other aspects of the chemistry peculiar to these systems are discussed.

Preorganized ligand arrays based on spirotetrahydrofuranyl motifs. Synthesis of the stereoisomeric 1,8,14-trioxatrispiro[4.1.4.1.4. 1]octadecanes and the contrasting conformational features and ionic binding capacities of these belted ionophores

The cis,trans trispiro ether 4 is accessible from several synthetic directions as a consequence of a crossover in reaction selectivity when proceeding from nucleophilic attack on the cis dispiro ketone to oxygenation of the alpha,beta-unsaturated ester 17. Its cis,cis isomer 3 was obtained in 17 steps and 14.6% overall yield from 3, 5-dimethoxybenzoic acid by making use of the alicyclic side chain in N as a "conformational lock". Although 4 shows no measurable tendency to complex with alkali metal ions, 3 binds strongly to Li(+) and Na(+) ions, as well as to CH(3)NH(3)(+). Whereas the 3eq conformation is populated in the solid state and in solution, complex formation occurs readily. (13)C NMR studies have defined slow exchange limits as the 2:1 sandwich complex with lithium ion is initially formed and transformed progressively into a 1:1 species upon the addition of more LiClO(4). Only the 2:1 complex with sodium ion is formed during comparable titration with NaClO(4). Association constants, molecular mechanics calculations, and X-ray crystallographic studies provide insight into the binding capacity of this belted tridentate ionophore.

Synthesis and conformational properties of several maximally substituted hexa(spirotetrahydrofuranyl)cyclohexanes. Assessment Of the pronounced bias of the all-trans D(3)(d)()-symmetric isomer for total equatorial oxygen occupancy

The poly(spirotetrahydrofuranyl)cyclohexanes 1-4 were prepared in a series of steps that began with oxidative demercuration of pentaspirocyclic chloromercurials. Once the resulting alcohols were transformed into their ketones, it proved possible to cap this center with the Normant reagent and introduce the final heterocyclic ring. The cyclohexanones and the title compounds showed a strong tendency to project their C-O bonds equatorially to the maximum extent possible. The reluctance of these systems to participate in chair-to-chair conformational equilibration was made apparent during measurements to assess their coordination capability toward alkali metal ions. Although 3 was superior to its isomers, this polyether was overshadowed by 15 to an extent in excess of two powers of 10 in their relative capability to coordinate lithium cations. The synthesis, conformation, and low-level dynamic character of 5 are also detailed. To set the stereochemistry of the sixth spirotetrahydrofuranyl ring properly in this case, it was necessary to implement a novel strategy involving late-stage introduction of the oxygen atom. This protocol required intermediate formation of homoallylic alcohol 39, the epoxidation of which proceeded principally in the desired direction. X-ray crystallographic analysis of 5 established that the chair conformation which is adopted has all six C-O bonds projected equatorially. The total inability of 5 to bind to Li(+), Na(+), and K(+) denotes the existence of a substantial barrier to ring inversion. DNMR studies undertaken to assess the magnitude of this barrier demonstrated no change in high-field (1)H and (13)C line shapes up to 573K in 1, 3-([D(3)]methoxy)benzene. Consequently, 5 may qualify as the cyclohexane having the highest chair-chair conformational inversion barrier to the present time.

Vicinal tetrahydrofuranyl substitution of alkyl chains. Tetra-, penta-, and hexafunctionalized arrays

The tetrakis (tetrahydrofuranyl) dialdehydes 14 and 19 are accessible by oxidative cleavage of extensively substituted cyclohexenes, which have in turn been assembled in a stereocontrolled manner. Reaction of 14 with an excess of the Normant reagent followed by ring closure resulted in conversion to the hexafunctionalized polyether 15. Allylindation of the same dialdehyde gave rise to lactol 16, a reactivity pattern that was essentially duplicated when 19 was treated with the Normant reagent. Attempts to add allylmagnesium bromide to 19 resulted in operation of a Tishchenko reaction with formation of lactone 23. By means of X-ray diffraction analysis, it was possible to ascertain the conformation adopted by 20 and 23 in the solid state. In addition, 15 was shown to populate a conformation in which the oxygen are very predominantly in gauche arrangements, this structural preorganization taking place in the absence of any metal ions.

Competitive Intramolecular/Intermolecular Chelation Options Operative during Indium-Promoted Additions to Pyridyl Aldehydes and to Glyoxylic Acid under Aqueous Conditions

The stereochemical course of the 1,2-addition of six allylindium reagents to 2- and 3-pyridinecarboxaldehyde and to glyoxylic acid has been investigated in order to assess the level and direction of diastereoselectivity in these coupling reactions. When 2-PyCHO is involved, the results strongly suggest that the ring nitrogen becomes chelated to the indium atom in the aqueous environment. One striking observation is the crossover in stereoselectivity seen relative to the use of 3-PyCHO. A second revealing fact is the significantly faster rate of reaction of 2-PyCHO, as long as steric effects are not allowed to interfere. The varying product distributions observed in the latter experiments are attributed to other control elements such as intramolecular chelation within the indium reagent and nonbonded steric restrictions resident in either or both reaction partners. In the absence of extramolecular chelating events (e.g., when 3-PyCHO is involved), adherence to Felkin-Anh transition-state alignments is presumably exercised. The previous working assumption that indium(III) is capable of chelation to flanking heteroatomic centers in water is supported by the present investigation.

trans,anti,trans-Tetra(spirotetrahydrofuranyl)cyclohexane-1,2-dione. Stereocontrolled Synthesis and Definition of Its Susceptibility to Photoisomerization

The title alpha-diketone (18) has been synthesized in stereocontrolled fashion. The ability to introduce the four contiguous spirocyclic ether oxygens in extended trans fashion rests on the ability of the Normant reagent (ClMgCH(2)CH(2)CH(2)OMgCl) to engage in chelation control during 1,2-addition to an alpha-oxy substituted cyclohexanone. The successful pathway is dependent on the ability of osmium tetraoxide to add (slowly) across the double bond of the cyclohexene precursor. The highly substituted 1,2-cyclohexanedione is quite sensitive to light and rearranges by means of an interesting photoisomerization process to a laterally fused pyran system. A likely mechanistic pathway for this intramolecular isomerization is presented.